Chapter 36: Inborn Errors of Metabolism

Disorders in which single-gene defects cause clinically significant blocks in metabolic pathways are called inborn errors of metabolism. Once considered rare, the number of recognized inborn errors has increased dramatically, and they are now recognized to affect 1:1500 children. Many of these disorders can be treated effectively. Even when treatment is not available, correct diagnosis permits parents to make informed decisions about future offspring.

Pathology in metabolic disorders usually results from accumulation of enzyme substrate behind a metabolic block, or from deficiency of a reaction product. In some cases, the accumulated enzyme substrate is diffusible and has adverse effects on distant organs; in other cases, as in lysosomal storage diseases, the substrate primarily accumulates locally. The clinical manifestations of inborn errors vary widely with both mild and severe forms of virtually every disorder. Phenotypes vary from classic to more rare clinical presentations based on residual enzyme activity, which is in large part determined by specific mutations in a common gene.

A first treatment strategy is to enhance the reduced enzyme activity. Gene replacement is a long-term goal. Previous problems with gene delivery to target organs and control of gene action made this clinically unavailable; however, numerous clinical research trials are now occurring and offer hope for success. Enzyme-replacement therapies using intravenously, intrathecally, or intraventricularly administered recombinant enzymes have been developed as effective strategies in many lysosomal storage disorders and more continue to be developed. Subcutaneous enzyme replacement therapy is also under development. Enzyme substitution therapy via subcutaneous injection with a modified bacterial enzyme is also now available for at least one disorder. Organ transplantation (liver, kidney, heart, or bone marrow) can provide a source of enzyme for some conditions. Pharmacologic doses of a cofactor such as a vitamin can sometimes be effective in restoring enzyme activity. Residual activity can be increased by pharmacologically promoting transcription (transcriptional upregulation) or by stabilizing the protein product through therapy with chaperones. Alternatively, some strategies are designed to cope with the consequences of enzyme deficiency. Strategies used to avoid substrate accumulation include restriction of precursor in the diet (eg, low-phenylalanine diet for phenylketonuria), avoidance of catabolism (fasting or vomiting illnesses), inhibition of an enzyme in the synthesis of the precursor (eg, the NTBC enzyme in tyrosinemia type I) (see section Hereditary Tyrosinemia), or removal of accumulated substrate pharmacologically (eg, glycine therapy for isovaleric acidemia) or by dialysis. An inadequately produced metabolite can also be supplemented (eg, glucose administration for glycogen storage disease type I).

Inborn errors can manifest at any age, affect any organ system, and mimic many common pediatric problems. This chapter focuses on when to consider a metabolic disorder in the differential diagnosis of common pediatric problems. A few of the more important disorders are then discussed in detail.

SUSPECTING INBORN ERRORS

Inborn errors must be considered in the differential diagnosis of critically ill newborns, children with seizures, neurodegeneration, recurrent vomiting, Reye-like syndrome, parenchymal liver disease, cardiomyopathy, rhabdomyolysis, renal insufficiency, unexplained metabolic acidosis, hyperammonemia, and hypoglycemia. Intellectual disability, developmental delay, and failure to thrive are often present but have little specificity. Inborn errors should be suspected when (1) degree of illness appears out of proportion to history, (2) symptoms accompany changes in diet, (3) the child’s development regresses, (4) the child shows specific food preferences or aversions, or (5) the family has a history of parental consanguinity or problems suggestive of inborn error such as intellectual disability or unexplained deaths in first- and second-degree relatives.

Physical findings associated with inborn errors include alopecia or abnormal hair, retinal cherry-red spot or retinitis pigmentosa, optic atrophy, cataracts or corneal opacity, hepatomegaly or splenomegaly, coarse features, skeletal changes (including gibbus), neurologic regression, progressive changes in white matter, and intermittent or progressive ataxia or dystonia. Other features that may be important in the context of a suspicious history include failure to thrive, microcephaly, rash, jaundice, hypotonia, and hypertonia. Finally, more common clinical presentations that appear to be nonaccidental trauma or poison ingestion may actually be inborn errors such as glutaric acidemia type 1 or methylmalonic acidemia, respectively.

LABORATORY STUDIES

Laboratory studies are almost always needed for the diagnosis of inborn errors. Serum electrolytes and pH should be used to estimate anion gap and acid-base status. Urine ketones and blood glucose values are readily available at the bedside. Serum lactate, pyruvate, and ammonia levels are available in most hospitals, but care is needed in obtaining samples appropriately. Amino acid, acylcarnitine, and organic acid studies must be performed at specialized facilities to ensure accurate analysis and interpretation. An increasing number of inborn errors are diagnosed with DNA sequencing, but interpretation of private mutations, that is, mutations only seen in a single family, can be problematic. Knowing the causative mutation in the family allows prenatal diagnosis to be done by molecular analysis. This can be done on any material that contains fetal DNA such as chorionic villi, amniotic cells, or fetal blood obtained through umbilical cord blood sampling. Large-scale next-generation sequencing such as exome or genome sequencing has been very useful in identifying disorders with nonspecific symptoms that are not readily recognized by routine metabolite screening. Specific metabolite or enzyme assays are used for confirmation.

The physician should know what conditions a test can detect and when it can detect them. For example, urine organic acids may be normal in patients with medium-chain acyl-CoA dehydrogenase deficiency or biotinidase deficiency; glycine may be elevated only in cerebrospinal fluid (CSF) in patients with nonketotic hyperglycinemia. A result that is normal in one physiologic state may be abnormal in another. For instance, ketone production (identified easily in urine) in a child who is hypoglycemic is expected. The absence of ketones in such a child would suggest a defect in fatty acid oxidation. Conversely, a newborn does not naturally have the capacity to produce ketones. Neonatal ketosis suggests an organic acidemia.

Samples used to diagnose metabolic disease may be obtained at autopsy. Samples must be obtained in a timely fashion and may be analyzed directly or stored frozen until a particular analysis is justified by the results of postmortem examination, new clinical information, or developments in the field. Studies of other family members may help establish the diagnosis of a deceased patient. It may be possible to demonstrate that parents are heterozygous carriers of a specific disorder or that a sibling has the condition.

COMMON CLINICAL SITUATIONS

1. Intellectual Disability

Some inborn errors can cause intellectual disability without other distinguishing characteristics. Measurements of plasma amino acids, urine organic acids, and serum uric acid should be obtained in every patient with nonspecific intellectual disability. Urine screens for mucopolysaccharides and succinylpurines and serum testing for carbohydrate-deficient glycoproteins are useful because these disorders do not always have specific physical findings. Absent speech can point to disorders of creatine metabolism and transport. Abnormalities of the brain detected by magnetic resonance imaging (MRI) can suggest specific groups of disorders (eg, cortical migrational abnormalities in peroxisomal biogenesis disorders).

2. Acute Presentation in the Neonate

Acute metabolic disease in the neonate is most often a result of disorders of energy metabolism and may be clinically indistinguishable from sepsis. Prominent symptoms include poor feeding, vomiting, altered mental status or muscle tone, jitteriness, seizures, and jaundice. Acidosis, alkalosis, or altered mental status out of proportion to systemic symptoms should increase suspicion of a metabolic disorder. Laboratory measurements should include electrolytes, ammonia, lactate, glucose, blood pH, urine ketones, and urine carbohydrate analysis. Amino acids in CSF should be measured if nonketotic hyperglycinemia is suspected. Plasma and urine amino acid, urine organic acid, and serum acylcarnitine analysis should be performed urgently. Neonatal cardiomyopathy or ventricular arrhythmias should be investigated with serum acylcarnitine analysis and carnitine levels.

3. Vomiting & Encephalopathy in the Infant or Older Child

Electrolytes, ammonia, glucose, urine pH, urine carbohydrate analysis, and urine ketones should be measured in all patients with vomiting and encephalopathy before any treatment affects the results. Samples for plasma amino acids, serum acylcarnitine profile, and urine organic acid analysis should be obtained early. In the presentation of a Reye-like syndrome (ie, vomiting, encephalopathy, and hepatomegaly), amino acids, acylcarnitines, carnitine levels, and organic acids should be assessed immediately. Hypoglycemia with inappropriately low urine or quantitative serum ketones suggests the diagnosis of fatty acid oxidation or ketogenesis defects.

4. Hypoglycemia

Duration of fasting, presence or absence of hepatomegaly, and Kussmaul breathing provide clues to the differential diagnosis of hypoglycemia. Serum insulin, cortisol, and growth hormone should be obtained on presentation. Urine ketones, urine organic acids, plasma lactate, serum acylcarnitine profile, carnitine levels, ammonia, triglycerides, and uric acid should be measured. Ketone body production is usually not efficient in the neonate, and ketonuria in a hypoglycemic or acidotic neonate suggests an organic acidemia. In the older child, inappropriately low urine ketone levels suggest an inborn error of fatty acid oxidation. Assessment of ketone generation requires simultaneous measurements of quantitative serum 3-hydroxybutyrate, acetoacetate, and free fatty acids in relation to a sufficient duration of fasting and age. Metabolites obtained during the acute episode can be very helpful and avoid the need for a formal fasting test.

5. Hyperammonemia

Symptoms of hyperammonemia may appear and progress rapidly or insidiously. Decreased appetite, irritability, and behavioral changes appear first with vomiting, ataxia, lethargy, seizures, and coma appearing as ammonia levels increase. Tachypnea inducing respiratory alkalosis due to a direct effect on respiratory drive is characteristic. Respiratory alkalosis is usually present in urea cycle defects and transient hyperammonemia of the newborn, while acidosis is characteristic of hyperammonemia due to organic acidemias. Physical examination cannot exclude the presence of hyperammonemia, and serum ammonia should be measured whenever hyperammonemia is possible. Severe hyperammonemia may be due to urea cycle disorders, organic acidemias, or fatty acid oxidation disorders (such as carnitine-acylcarnitine translocase deficiency). Hyperammonemia may also present as transient hyperammonemia of the newborn in the premature infant, or as valproate toxicity in an older child. Genetic etiologies can usually be suggested by measuring quantitative plasma amino acids (eg, citrulline), plasma carnitine and acylcarnitine esters, and urine organic acids and orotic acid.

6. Acidosis

Inborn errors may cause chronic or acute acidosis at any age, with or without an increased anion gap. Inborn errors should be considered when acidosis occurs with recurrent vomiting or hyperammonemia and when acidosis is out of proportion to the clinical status. Acidosis due to an inborn error can be difficult to correct, while physiologic ketoacidosis in fasting is appropriate and corrects readily. The main causes of anion gap metabolic acidosis are lactic acidosis, ketoacidosis (including abnormal ketone body production such as in β-ketothiolase deficiency), methylmalonic aciduria or other organic acidurias, intoxication (ethanol, methanol, ethylene glycol, and salicylate), and uremia. Causes of nonanion gap metabolic acidosis include loss of base in diarrhea or renal tubular acidosis (isolated renal tubular acidosis or renal Fanconi syndrome). If renal bicarbonate loss is found, a distinction must be made between isolated renal tubular acidosis and a more generalized renal tubular disorder or renal Fanconi syndrome by testing for renal losses of phosphorus and amino acids. Inborn errors associated with renal tubular acidosis or renal Fanconi syndrome include cystinosis, tyrosinemia type I, carnitine palmitoyltransferase I, galactosemia, hereditary fructose intolerance (HFI), Lowe syndrome, lysinuric protein intolerance, and mitochondrial diseases. Tests indicated for anion gap metabolic acidosis include urine organic acids, serum lactate and pyruvate, serum 3-hydroxybutyrate and acetoacetate, and plasma amino acids, in addition to a toxicology screen.

Patients with severe acidosis, hypoglycemia, and hyperammonemia may be very ill; initially mild symptoms may worsen quickly, and coma and death may ensue within hours. With prompt and vigorous treatment, however, patients can recover completely, even from deep coma. All oral intakes should be stopped. Sufficient glucose should be given intravenously to avoid or minimize catabolism in a patient with a known inborn error who is at risk for crisis. Most conditions respond favorably to glucose administration, although a few (eg, primary lactic acidosis due to pyruvate dehydrogenase deficiency) do not. After exclusion of fatty acid oxidation disorders, immediate institution of intravenous fat emulsions (eg, intralipid) can provide crucial caloric input. Severe or increasing hyperammonemia should be treated pharmacologically or with dialysis (see section Disorders of the Urea Cycle), and severe acidosis should be treated with bicarbonate. More specific measures can be instituted when a diagnosis is established.

Criteria for screening newborns for a disorder include its frequency, its consequences if untreated, the ability of therapy to mitigate consequences, the cost of testing, and the cost of treatment. With the availability of tandem mass spectrometry, newborn screening has expanded greatly to now include 35 core conditions and multiple secondary conditions screened by most states. In general, amino acidopathies, organic acidurias, and disorders of fatty acid oxidation are the disorders for which screening now occurs. Most states also screen for hypothyroidism, congenital adrenal hyperplasia, hemoglobinopathies, biotinidase deficiency, galactosemia, cystic fibrosis, and severe combined immune deficiency. Point-of-care screening occurs for hearing loss and congenital heart disease. An increasing number of states have begun screening for some of the lysosomal and peroxisomal disorders, and a few states have begun screening for spinal muscular atrophy (SMA). The Recommended Uniform Screening Panel (RUSP) from the Secretary of the Department of Health and Human Services provides guidance for the addition of new disorders to state newborn screening panels. Screening should occur for all infants between 24 and 72 hours of life or before hospital discharge.

Some screening tests measure a metabolite (eg, phenylalanine) that becomes abnormal with time and exposure to diet. In such instances, the disease cannot be detected reliably until intake of the substrate is established. Other tests measure enzyme activity and can be performed at any time (eg, biotinidase deficiency). Transfusions may cause false-negative results in this instance, and exposure of the sample to heat may cause false-positive results. False positives also result from prematurity, parenteral nutrition, hyperbilirubinemia, and liver or renal disease. Technologic advances have extended the power of newborn screening but have brought additional challenges. For example, although tandem mass spectrometry can detect many more disorders in the newborn period, consensus on diagnosis and treatment for some conditions is still under development.

Screening tests are not diagnostic, and diagnostic tests must be undertaken when an abnormal screening result is obtained. Because false-negative results occur, a normal newborn screening test does not rule out a condition, and some common disorders (eg, ornithine transcarbamylase deficiency) are not detectable in the screening tests performed in every state.

The appropriate response to an abnormal screening test depends on the condition in question and the predictive value of the test. For example, when screening for galactosemia by enzyme assay, complete absence of enzyme activity is highly predictive of classic galactosemia. Failure to treat may rapidly lead to death. In this case, treatment must be initiated immediately while diagnostic studies are pending. In phenylketonuria, however, a diet restricted in phenylalanine is harmful to the infant whose screening test is a false-positive, while diet therapy produces an excellent outcome in the truly affected infant if treatment is established within the first weeks of life. Therefore, treatment for phenylketonuria should only be instituted when the diagnosis is confirmed. Physicians should review American College of Medical Genetics and Genomics recommendations, state laws, and regulations, and consult with their local metabolic center to arrive at appropriate strategies for each hospital and practice.

GLYCOGEN STORAGE DISEASES

Glycogen is a highly branched polymer of glucose that is stored in the liver and muscle. Different enzyme defects affect its biosynthesis and degradation. The hepatic forms of the glycogenoses cause growth failure, hepatomegaly, and severe fasting hypoglycemia. They include glucose-6-phosphatase deficiency (type I; von Gierke disease), debrancher enzyme deficiency (type III), hepatic phosphorylase deficiency (type VI), and phosphorylase kinase deficiency (type IX), which normally regulates hepatic phosphorylase activity. Glycogen synthase deficiency (type 0) causes hypoglycemia, usually after about 12 hours of fasting, and can cause mild postprandial hyperglycemia and hyperlactatemia. There are two forms of glucose-6-phosphatase deficiency: In type Ia, the catalytic glucose-6-phosphatase is deficient, and there is pronounced lactic acidosis, hyperuricemia, and hyperlipidemia in addition to hypoglycemia, and in type Ib, the glucose-6-phosphate transporter is deficient and neutropenia also occurs. Glycogenosis type IV, brancher enzyme deficiency, usually presents with progressive liver cirrhosis, as do some rare forms of phosphorylase kinase deficiency. The gluconeogenesis disorder fructose-1,6-bisphosphatase deficiency presents with major lactic acidosis and delayed hypoglycemia on fasting.

The myopathic forms of glycogenosis affect skeletal muscle. Skeletal myopathy with weakness or rhabdomyolysis may be seen in muscle phosphorylase deficiency (type V), phosphofructokinase deficiency (type VII), and acid maltase deficiency (type II; Pompe disease). The infantile form of Pompe disease also has hypertrophic cardiomyopathy and macroglossia.

Diagnosis

Initial tests include glucose, lactate, triglycerides, cholesterol, uric acid, transaminases, and creatine kinase. Functional testing includes responsiveness of blood glucose and lactate to fasting; for myopathic forms, an ischemic or nonischemic exercise test is helpful. Most glycogenoses can now be diagnosed by molecular analysis, including next-generation panels. Other diagnostic studies include enzyme assays of leukocytes, fibroblasts, liver, or muscle. Disorders diagnosable from analysis of red blood cells include deficiency of phosphorylase kinase (type IX) in half the cases. Pompe disease can usually be diagnosed by assaying acid maltase in a blood spot with confirmation in fibroblasts.

Treatment

Treatment is designed to prevent hypoglycemia and avoid secondary metabolite accumulations such as elevated lactate in glycogenosis type I. In GSD1, the special diet must be strictly monitored with restriction of free sugars and measured amounts of uncooked cornstarch, which slowly releases glucose in the intestinal lumen. Good results have been reported following continuous nighttime carbohydrate feeding or uncooked cornstarch therapy. Late complications even after years of treatment include focal segmental glomerulosclerosis, hepatic adenoma or carcinoma, and gout. Enzyme-replacement therapy in Pompe disease corrects the cardiomyopathy, but the response in skeletal myopathy is variable with optimal results seen in patients treated early and who have mutations that allow formation of some residual protein which is detected as cross-reacting material on Western blotting. Immunomodulation is used for patients whose treatment response declines due to antibodies to the recombinant enzyme. In glycogen storage diseases with intact gluconeogenesis (GSD III, VI, IX), a high-protein diet improves glucose control and reduces late complications.

GALACTOSEMIA

Classic galactosemia is caused by almost total deficiency of galactose-1-phosphate uridyltransferase. Accumulation of galactose-1-phosphate causes hepatic parenchymal disease and renal Fanconi syndrome. Onset of the severe disease is marked in the neonate by vomiting, jaundice (both direct and indirect), hepatomegaly, and rapid onset of liver insufficiency after initiation of milk feeding. Hepatic cirrhosis is progressive. Without treatment, death frequently occurs within 1 month, often from Escherichia coli sepsis. Cataracts usually develop within 2 months in untreated cases but usually reverse with treatment. With prompt institution of a galactose-free diet, the prognosis for survival without liver disease is excellent. Even when dietary restriction is instituted early, patients with galactosemia are at increased risk for speech and language deficits and ovarian failure. Some patients develop progressive intellectual disability, tremor, and ataxia. Milder variants of galactosemia with better prognosis exist.

The disorder is autosomal recessive with an incidence of approximately 1:40,000 live births.

Diagnosis

In infants receiving foods containing galactose, laboratory findings include liver dysfunction, particularly PT prolongation, together with proteinuria and aminoaciduria. Galactose-1-phosphate is elevated in red blood cells. When the diagnosis is suspected, galactose-1-phosphate uridyltransferase should be assayed in erythrocytes or GALT sequencing pursued. Blood transfusions give false-negative results and sample deterioration false-positive results.

Newborn screening by demonstrating enzyme deficiency in red cells or by demonstrating increased serum galactose allows timely institution of treatment. Some patients identified on newborn screening have a genotype that results in sufficient residual activity (Duarte allele) and treatment is not always required.

Treatment

A galactose-free diet should be instituted as soon as the diagnosis is made or suspected. Compliance with the diet can be monitored by following galactose-1-phosphate levels in red blood cells or urinary galactitol levels. Appropriate diet management requires not only the exclusion of milk but also an understanding of the galactose content of foods. Avoidance of galactose should be lifelong with appropriate calcium and vitamin D replacement, intake of which tends to be low due to the restriction of dairy products. Dual-energy X-ray absorptiometry (DEXA) scans are recommended for monitoring of bone health. All children should be monitored for appropriate development with special attention to speech and language development, and girls should routinely be screened for hypergonadotropic hypogonadism during adolescence.

HEREDITARY FRUCTOSE INTOLERANCE

Hereditary fructose intolerance (HFI) is an autosomal recessive disorder in which deficient activity of fructose-1-phosphate aldolase (aldolase B) causes hypoglycemia and tissue accumulation of fructose-1-phosphate with fructose ingestion. This is genetically and clinically different from fructose malabsorption or “dietary fructose intolerance,” which is similar in mechanism to lactose intolerance. Onset of HFI is typically in infancy when solid foods are introduced, but the disorder may go undiagnosed for years despite recurrent vomiting symptoms. Other abnormalities include failure to thrive, vomiting, jaundice, hepatomegaly, acute liver failure, proteinuria, renal Fanconi syndrome, and acute renal failure. Hypoglycemia directly follows fructose ingestion, and lactic acidosis, hypophosphatemia, and hyperuricemia may be significant. The untreated condition can progress to death from liver failure. Acute infusion of fructose may also result in death. Chronic liver disease, and more rarely chronic kidney disease, may occur even after treatment is implemented and strictly followed.

Diagnosis

The diagnosis is suggested by finding fructosuria or an abnormal transferrin glycoform in the untreated patient. Although targeted testing of common mutations is available, diagnosis is best made by full sequencing of ALDOB. Alternatively, enzyme assay in liver tissue is available.

Treatment

Treatment consists of strict dietary avoidance of fructose, sucrose, sorbitol, and related sugars. Vitamin supplementation is usually needed. Drugs and vitamins dispensed in a sucrose base should be avoided. Treatment monitoring can be done with transferrin glycoform analysis. If diet compliance is poor, physical growth retardation may occur. Growth should resume when more stringent dietary restrictions are reinstituted. If the disorder is recognized early, the prospects for normal development and life expectancy are good. As affected individuals grow up, intentional avoidance of fructose-containing foods, and resultantly good dentition, is common.

The most common disorders of central mitochondrial energy metabolism are pyruvate dehydrogenase deficiency and deficiencies of respiratory chain components. Disorders of the Krebs cycle include deficiencies in fumarase, 2-ketoglutarate dehydrogenase, malate dehydrogenase, aconitase, and succinyl-CoA ligase. In many, but not in all, patients lactate is elevated in either blood or CSF. In pyruvate dehydrogenase deficiency, the lactate-pyruvate ratio is normal, whereas in respiratory chain disorders the ratio is often increased. Care must be taken to distinguish an elevated lactate level that is due to these conditions (called primary lactic acidoses) from elevated lactate that is a consequence of hypoxia, ischemia, or sampling problems. GDF15 is a recently described improved biomarker of mitochondrial diseases.

Patients with a defect in the pyruvate dehydrogenase complex often have agenesis of the corpus callosum or Leigh syndrome (lesions in the basal ganglia, dentate nucleus, and periaqueductal gray matter). They can have mild facial dysmorphism. Recurrent altered mental status, recurrent ataxia, and recurrent acidosis are typical of many disturbances of pyruvate metabolism. The most common genetic defect is in the X-linked E₁α component, with males carrying milder mutations and females carrying severe mutations leading to periventricular cystic brain lesions. The molecular heterogeneity is large as defects in each of the subunits, in the synthesis of the cofactors lipoate and thiamine, and in the transporters for thiamine and pyruvate are described.

The respiratory chain disorders are frequent (1:5000) and involve a heterogenous group of genetic defects that produce a variety of clinical syndromes (now > 50) of varying severity and presentation. The disorders can affect multiple organs. The following set of symptoms (not intended as a comprehensive listing) can indicate a respiratory chain disorder:

1. General: Failure to thrive

2. Brain: Progressive neurodegeneration, Leigh syndrome, myoclonic seizures, brain atrophy, movement disorders, cerebellar atrophy, and leukodystrophy

3. Eye: Optic neuropathy, retinitis pigmentosa, progressive external ophthalmoplegia, and cataracts

4. Ears: Sensorineural hearing loss

5. Muscle: Myopathy with decreased endurance or rhabdomyolysis

6. Kidney: Renal Fanconi syndrome, proteinuria (in coenzyme Q deficiency)

7. Endocrine: Diabetes mellitus and hypoparathyroidism

8. Intestinal: Pancreatic or liver insufficiency, or intestinal pseudoobstruction

9. Heart: Cardiomyopathy, conduction defects, and arrhythmias

Respiratory chain disorders are among the more common causes of progressive neurodevelopmental problems in children. Patients may present with nonspecific findings such as hypotonia, failure to thrive, or renal tubular acidosis, or with more specific features such as ophthalmoplegia or cardiomyopathy. Symptoms are often combined in recognizable clinical syndromes with ties to specific genetic causes (Table 36–1). Thirteen of the more than 100 genes that control activity of the respiratory chain are part of the mitochondrial genome. Therefore, inheritance of defects in the respiratory chain may be maternal as well as Mendelian. The genetic cause of mitochondrial disease is extremely heterogenous, and over 200 different genetic causes have already been described. Mitochondrial biology is a complex system involving the maintenance of mitochondrial DNA and its transcription and translation machinery, the assembly of the complexes including cofactors, the import and processing of nuclear encoded components and the maintenance of the mitochondrial membrane and structural environment, with defects described at every step (Figure 36–1). Some clinical presentations such as MNGIE have a specific genetic cause, but other clinical presentations such as Leigh disease and multisystem presentation have many causes.

Diagnosis

Pyruvate dehydrogenase deficiency is diagnosed by enzyme assay in leukocytes or fibroblasts. Confirmation can be obtained by molecular analysis. Diagnosis of respiratory chain disorders is based on a convergence of clinical, biochemical, morphologic, enzymatic, and molecular data. Classic pathologic features of mitochondrial disorders are the accumulation of mitochondria, which produces ragged red fibers in skeletal muscle biopsy, and abnormal mitochondrial shapes and inclusions inside mitochondria on electron microscopy. However, these findings are only present in 5% of children.

Sometimes a clinical presentation is associated with a specific genetic cause (eg, Alpers syndrome caused by mutations in POLG), and directed genetic testing is possible. Most commonly, genetic testing starts with sequencing of mitochondrial DNA. Mitochondrial DNA abnormalities decrease in blood with age, and sometimes tissue samples (muscle) are needed. Next, large-scale genetic testing using either a large next-gen panel or whole exome sequencing is usually used.

Functional testing such as enzyme assays, nondenaturing blue native gel assays to analyze the assembly of complexes, and oxygen consumption-based tests in relevant tissues such as muscle and liver remain important for patients with genetic variants of unknown significance or for those where genetic testing did not yield a diagnosis. Tissue heterogeneity of expression of the functional defect may require the analysis of multiple tissues, and an overlap between normal and affected ranges makes interpretation often complex. Diagnostic criteria aid in appropriate clinical recognition. Advances in diagnostics now allow clinical diagnosis with a genetic cause to be ascertained in most cases. In some instances, the genetics and prognosis may be clear, but in other cases neither prognosis nor genetic risk can be predicted. Because of the high complexity of this group of disorders, many patients require a high degree of expertise and multiple studies to arrive at a final diagnosis.

Treatment

A consensus statement on the treatment of mitochondrial diseases from the Mitochondrial Medicine Society has been published. Patients should first avoid situations that further compromise mitochondrial capacity. Medications that impair mitochondrial translation (eg, certain antibiotics) or mitochondrial replication (eg, AZT/zidovudine) or exhibit mitochondrial toxicity (eg, valproate and propofol) should be avoided where possible. Catabolism due to prolonged fasting should be avoided by provision of sufficient calories. The best evidence for improvement in mitochondrial function is for a regimen of regular exercise (eg, 20 minutes of exercise daily). Certain conditions have specific treatments. A ketogenic diet is effective in pyruvate dehydrogenase deficiency for Leigh disease presentation. Coenzyme Q treatment is very effective in patients with primary coenzyme Q deficiency. A few conditions are responsive to riboflavin (ACAD9) or thiamin (TPK, SLC19A3). Stroke-like episodes in the MELAS syndrome should be treated with acute intravenous arginine, and either oral arginine or citrulline can be used for prevention of stroke-like episodes. Treatment with taurine improves the tRNA biochemistry in the m.3243A>G mutation and reduces stroke-like episodes. Liver transplantation has been important in the treatment of disorders with biochemical imbalance causing mitochondrial diseases such as ETHF1 and MNGIE. Other treatments are of theoretical value, with little data on efficacy. Antioxidants such as coenzyme Q are often used. Clinical trials of new, improved medications affecting mitochondrial function such as next-generation antioxidants, mitochondrial biogenesis upregulators, and molecular protectors offer new hope for defining proven therapeutic interventions.

DISORDERS OF THE UREA CYCLE

Ammonia is derived from the catabolism of amino acids and is converted to an amino group in urea by enzymes of the urea cycle. Patients with severe defects (often those enzymes early in the urea cycle such as ornithine transcarbamylase or argininosuccinic acid synthetase deficiency [citrullinemia]) usually present in infancy with severe hyperammonemia, vomiting, and encephalopathy, which is rapidly fatal. Patients with milder genetic defects may present with vomiting, encephalopathy, or liver failure after increased protein ingestion or infection. Although late defects such as deficiency in argininosuccinic acid lyase (argininosuccinic acidemia) or arginase may cause severe hyperammonemia in infancy, the usual clinical course is chronic with intellectual disability without hyperammonemia. Ornithine transcarbamylase deficiency is X-linked; the others are autosomal recessive. Age at onset of symptoms varies with residual enzyme activity, protein intake, growth, and stressors such as infection. Even within a family, patients with ornithine transcarbamylase deficiency may differ by decades in the age of symptom onset. Many female carriers of ornithine transcarbamylase deficiency have protein intolerance. Some develop migraine-like symptoms after protein loads, and others develop potentially fatal episodes of vomiting and encephalopathy after protein ingestion, infections, or in the postpartum period. Trichorrhexis nodosa is common in patients with argininosuccinic aciduria. Arginase deficiency usually presents with spastic diplegia rather than hyperammonemia.

Diagnosis

Blood ammonia should be measured in any acutely ill newborn in whom a cause is not obvious and any child with unexplained encephalopathy. In urea cycle defects, early hyperammonemia is associated with hyperventilation and respiratory alkalosis. Plasma citrulline is low or undetectable in carbamoyl phosphate synthetase and ornithine transcarbamylase deficiency, high in argininosuccinic acidemia, and very high in citrullinemia. Large amounts of argininosuccinic acid are found in the urine of patients with argininosuccinic acidemia. Urine orotic acid is increased in infants with ornithine transcarbamylase deficiency. Prenatal diagnosis is most commonly done by molecular methods. Other causes of severe neonatal hyperammonemia, which have a different prognosis and treatment, include liver failure; blood shunting to bypass the liver as seen in transient hyperammonemia of the neonate; and the metabolic disorders pyruvate carboxylase deficiency and mitochondrial carbonic anhydrase deficiency.

Treatment

During treatment of acute hyperammonemic crisis, protein intake should be stopped, and glucose and lipids should be given to reduce endogenous protein breakdown from catabolism. Careful administration of essential amino acids facilitates protein anabolism. Arginine is given intravenously (except in arginase deficiency). It is an essential amino acid for patients with urea cycle defects and increases the excretion of waste nitrogen in citrullinemia and argininosuccinic acidemia. Sodium benzoate and phenylacetate are given intravenously to increase excretion of nitrogen as hippurate and phenylacetylglutamine. Additionally, hemodialysis or hemofiltration is indicated for severe or persistent hyperammonemia, as is usually the case in the newborn. Peritoneal dialysis and exchange transfusion are ineffective. Long-term treatment includes low-protein diet, oral administration of arginine or citrulline, and sodium benzoate or sodium phenylbutyrate (a prodrug of sodium phenylacetate). Symptomatic heterozygous female carriers of ornithine transcarbamylase deficiency should also receive such treatment. Liver transplantation may be curative and is indicated for patients with severe disorders. For arginase deficiency, enzyme replacement therapy is being developed to normalize arginine levels. Treatment with carbamylglutamate is effective for N-acetylglutamate synthase deficiency and, to some extent, mitochondrial carbonic anhydrase deficiency.

The outcome of urea cycle disorders depends on the genetic severity of the condition (residual activity) and the severity and prompt treatment of hyperammonemic episodes. Brain damage depends on the duration and the degree of elevation of ammonia and glutamine. Prolonged hyperammonemia causes permanent neurologic and intellectual impairments, with cortical atrophy and ventricular dilation seen on brain imaging. Rapid identification and treatment of the initial hyperammonemic episode is critical in improving outcome, and hyperammonemia constitutes a metabolic emergency.

PHENYLKETONURIA & THE HYPERPHENYLALANINEMIAS

Probably the best-known disorder of amino acid metabolism is the classic form of phenylketonuria caused by decreased activity of phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine. In classic phenylketonuria, there is little or no phenylalanine hydroxylase activity. In less severe hyperphenylalaninemia there may be significant residual activity. Rare variants can be due to deficiency of dihydropteridine reductase, defects in biopterin synthesis, or mutations in DNAJC12.

Phenylketonuria is an autosomal recessive trait, with an incidence in Caucasians of approximately 1:10,000 live births. On a normal neonatal diet, affected patients develop elevated phenylalanine levels (hyperphenylalaninemia). Patients with untreated phenylketonuria exhibit severe intellectual disability, hyperactivity, seizures, a light complexion, and eczema.

Success in preventing severe intellectual disability in children with phenylketonuria by restricting phenylalanine starting in early infancy led to screening programs to detect the disease early. Because the outcome is best when treatment is begun in the first month of life, infants should be screened during the first few days. A second test is necessary when newborn screening is done before 24 hours of age, and should be completed by the second week of life.

Diagnosis & Treatment

The diagnosis of phenylketonuria is based on finding elevated plasma phenylalanine and an elevated phenylalanine/tyrosine ratio in a child on a normal diet. The condition must be differentiated from other causes of hyperphenylalaninemia by examining pterins in urine and dihydropteridine reductase activity in blood. The diagnosis of hyperphenylalaninemia secondary to mutations in DNAJC12 can only be made by molecular analysis. Determination of carrier status and prenatal diagnosis of phenylketonuria or pterin defects is possible using molecular methods.

A. Phenylalanine Hydroxylase Deficiency: Classic Phenylketonuria and Hyperphenylalaninemia

In phenylketonuria, plasma phenylalanine levels are persistently elevated above 1200 μM (20 mg/dL) on a regular diet, with normal or low plasma levels of tyrosine, and normal pterins. Poor phenylalanine tolerance persists throughout life. Treatment to decrease phenylalanine levels is always indicated. Hyperphenylalaninemia is diagnosed in infants whose plasma phenylalanine levels are usually 240–1200 μM (4–20 mg/dL), and pterins are normal while receiving a normal protein intake. Treatment to reduce phenylalanine levels is indicated if phenylalanine levels consistently exceed 360 μM (6 mg/dL). In contrast, in the rare case of transient hyperphenylalaninemia, plasma phenylalanine levels are elevated early but progressively decline toward normal. Dietary restriction is only temporary, if required at all.

Treatment of all forms of phenylketonuria is aimed at maintaining phenylalanine levels less than 360 μM (6 mg/dL). Treatment can consist of dietary restriction of phenylalanine, increasing enzyme activity with pharmacologic doses of R-tetrahydrobiopterin, or new methods to interfere with phenylalanine absorption or to breakdown phenylalanine.

Dietary restriction of phenylalanine intake to amounts that permit normal growth and development is the most common therapy and results in good outcome if instituted in the first weeks of life and carefully maintained. Metabolic formulas deficient in phenylalanine are available but must be supplemented with normal milk and other foods to supply enough phenylalanine to permit normal growth and development. Plasma phenylalanine concentrations must be monitored frequently while ensuring that growth, development, and nutrition are adequate. This monitoring is best done in experienced clinics. Children with classic phenylketonuria who receive treatment promptly after birth and achieve phenylalanine and tyrosine homeostasis will develop well physically and can be expected to have normal or near-normal intellectual development. Subtle changes in executive function may be apparent.

Phenylalanine restriction should continue throughout life. Patients who discontinued diet after treatment for several years have developed subtle changes in intellect and behavior, and risk neurologic damage. Counseling should be given during adolescence particularly to girls about the risk of maternal phenylketonuria (see as follows), and women’s diets should be monitored closely prior to conception and throughout pregnancy. Late treatment may still be of benefit in reversing behaviors such as hyperactivity, irritability, and distractibility, but it does not reverse the intellectual disability.

Treatment with R-tetrahydrobiopterin results in improved phenylalanine tolerance in up to 50% of patients with a deficiency in phenylalanine hydroxylase. The best results and the most frequent responsiveness are seen in patients with hyperphenylalaninemia. Provision of high doses of large neutral amino acids results in a moderate reduction in phenylalanine and is used as an adjunctive treatment in some adults with phenylketonuria. Treatment with subcutaneous administration of pegylated phenylalanine ammonia lyase to decrease phenylalanine levels has recently been approved for adults with phenylketonuria.

B. Biopterin Defects: Dihydropteridine Reductase Deficiency and Defects in Biopterin Biosynthesis

In these patients, plasma phenylalanine levels vary. The pattern of pterin metabolites is abnormal. Clinical findings include myoclonus, tetraplegia, dystonia, oculogyric crises, and other movement disorders. Seizures and psychomotor regression occur even with diet therapy, probably because the enzyme defect also causes neuronal deficiency of serotonin and dopamine.

These deficiencies require treatment with levodopa, carbidopa, 5-hydroxytryptophan, and folinic acid. Tetrahydrobiopterin may be added for some biopterin synthesis defects.

C. Tyrosinemia of the Newborn

Plasma phenylalanine levels are lower than those associated with phenylketonuria and are accompanied by marked hypertyrosinemia. Tyrosinemia of the newborn usually occurs in premature infants and is due to immaturity of 4-hydroxyphenylpyruvic acid oxidase, resulting in increase in tyrosine and its precursor phenylalanine. The condition resolves spontaneously within 3 months, almost always without sequelae.

D. Maternal Phenylketonuria

Offspring of mothers with phenylketonuria may have transient hyperphenylalaninemia at birth. Elevated maternal phenylalanine during pregnancy causes intellectual disability, microcephaly, growth retardation, and often congenital heart disease or other malformations in the offspring. The risk to the fetus is lessened considerably by maternal phenylalanine restriction with maintenance of phenylalanine levels below 360 μM (6 mg/dL) throughout pregnancy and optimally started before conception.

E. Hyperphenylalaninemia due to DNAJC12 Mutations

Mild, non–tetrahydrobiopterin-deficient hyperphenylalaninemia due to mutations in the gene DNAJC12 is a recently reported autosomal recessive neurotransmitter disorder. DNAJC12 functions as a co-chaperone to prevent the misfolding of proteins and interacts with neuronal phenylalanine, tyrosine, and tryptophan hydroxylases. The clinical phenotype continues to be defined but appears heterogeneous and ranges from normal to intellectual disability, autism spectrum disorder, hyperactivity, dystonia, and parkinsonism. Laboratory studies typically reveal mild, BH4-responsive hyperphenylalaninemia (< 600 mmol/L) and low CSF homovanillic acid and 5-hydoxyindolacetic acid. Urine pterin profile and dihydropteridine reductase activity are normal. Some, but not all, patients may have an abnormal newborn screen suggestive of phenylketonuria. Therapy consists of sapropterin dihydrochloride with L-dopa/carbidopa, with or without 5-hydroxytryptophan, and should be started as early as possible for best outcome. Subjective improvement in cognitive and motor function has been noted even with later therapy. All children with mild hyperphenylalaninemia and global developmental delay warrant targeted testing for DNAJC12 mutations.

HEREDITARY TYROSINEMIA

Hereditary tyrosinemia type I is an autosomal recessive condition caused by deficiency of fumarylacetoacetase (FAH). It presents with acute or progressive hepatic parenchymal damage with elevated α-fetoprotein, renal tubular dysfunction with generalized aminoaciduria, hypophosphatemic rickets, or neuronopathic crises. Patients may also have impaired cognition. Tyrosine and methionine are increased in blood and tyrosine metabolites and δ-aminolevulinic acid in urine. The key diagnostic metabolite is elevated succinylacetone in blood or urine. Liver failure may be rapidly fatal in infancy or more chronic, with a high incidence of liver cell carcinoma in long-term survivors. Tyrosinemia type II (TAT) presents with corneal ulcers, palmar/plantar keratosis, neurologic dysfunction, and very high plasma tyrosine levels (> 600 μM). Patients with tyrosinemia type III (HPD) can also have developmental delay and ataxia.

Diagnosis

Similar clinical and biochemical findings may occur in other liver diseases such as galactosemia and HFI. Increased succinylacetone occurs only in fumarylacetoacetase deficiency; an elevated level is diagnostic and is detected in newborn screening. Diagnosis is confirmed by mutation analysis or by enzyme assay in liver tissue. Prenatal diagnosis is possible. Tyrosinemia types II and III are diagnosed by gene sequencing.

Treatment

A diet low in phenylalanine and tyrosine ameliorates liver disease. Pharmacologic therapy to inhibit the upstream enzyme 4-hydroxyphenylpyruvate dehydrogenase using 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) decreases the production of toxic metabolites, maleylacetoacetate and fumarylacetoacetate. It improves the liver disease and renal disease, prevents acute neuronopathic attacks, and greatly reduces the risk of hepatocellular carcinoma. Liver transplantation is effective therapy. Treatment following diagnosis by newborn screen has an excellent outcome, but cognitive dysfunction is increasingly recognized. Tyrosinemia types II and III respond well to dietary tyrosine restriction.

MAPLE SYRUP URINE DISEASE (BRANCHED-CHAIN KETOACIDURIA)

Maple syrup urine disease is due to deficiency of the enzyme complex that catalyzes the oxidative decarboxylation of the branched-chain ketoacid derivatives of leucine, isoleucine, and valine. The complex is made up of three genetically distinct subunits. Accumulated ketoacids of leucine and isoleucine, which are converted to a compound sotolone, cause the characteristic sweet odor which may be detectable in cerumen as early as day of life 1. Only leucine and its corresponding ketoacid have been implicated in causing central nervous system (CNS) dysfunction. Many variants of this disorder have been described, including mild, intermittent, and thiamine-dependent forms. All are autosomal recessive.

Patients with classic maple syrup urine disease are normal at birth but shortly (day of life 2–3) develop irritability and feeding issues, and within 1 week progress to seizures and coma. Unless diagnosis is made and dietary restriction of branched-chain amino acids is begun, most will die in the first month of life. Nearly normal growth and development may be achieved if treatment is begun before 10 days of life, which is facilitated by newborn screening.

Diagnosis

Amino acid analysis shows marked elevation of branched-chain amino acids including alloisoleucine, a diagnostic transamination product of the ketoacid of isoleucine. Urine organic acids demonstrate the characteristic ketoacids. The magnitude and consistency of metabolite changes are altered in mild and intermittent forms. A genetic testing panel that includes the multiple subunit genes can confirm the diagnosis and allows prenatal diagnosis by molecular analysis once the mutation in a family is known.

Treatment

Dietary leucine restriction and avoidance of catabolism are the cornerstones of treatment. Infant formulas deficient in branched-chain amino acids must be supplemented with normal foods to supply enough branched-chain amino acids to permit normal growth. Plasma levels of branched-chain amino acids must be monitored frequently to deal with changing protein requirements. Acute episodes of metabolic decompensation must be aggressively treated to prevent catabolism and negative nitrogen balance. Very high leucine levels require hemodialysis. Liver transplantation corrects the disorder, and the Maple Syrup Urine Disease affected liver may then safely be used for an unaffected recipient in a “domino” transplant because the recipient has enough whole body residual enzyme activity to metabolize branched-chain amino acids.

HOMOCYSTINURIA

Homocystinuria is most often due to deficiency of cystathionine β-synthase (CBS) but may also be due to remethylation defects such as deficiency of methylenetetrahydrofolate reductase (MTHFR) or defects in the biosynthesis of methyl-cobalamin (vitamin B₁₂), the coenzyme for methionine synthase. Classic homocystinuria and most forms of inherited methyl-B₁₂ deficiency are autosomal recessive. About 50% of patients with untreated CBS deficiency have intellectual disability, and most have arachnodactyly, osteoporosis, and a tendency to develop dislocated lenses and thromboembolic phenomena. Mild variants of CBS deficiency present with thromboembolic events. Patients with severe remethylation defects usually exhibit failure to thrive and a variety of neurologic symptoms, including brain atrophy, microcephaly, hydrocephalus, and seizures in infancy and early childhood.

Diagnosis

Diagnosis is made by demonstrating elevated total serum homocysteine or by identifying homocystinuria in a patient who is not severely deficient in vitamin B₁₂. Plasma methionine levels are usually high in patients with CBS deficiency and often low in patients with inherited methyl-B₁₂ deficiency. Cystathionine levels are low in CBS deficiency. In inherited methyl-B₁₂ deficiency, megaloblastic anemia or hemolytic uremic syndrome may be present and an associated deficiency of adenosyl-B₁₂ may cause methylmalonic aciduria. Mutation analysis or studies of cultured fibroblasts can make a specific diagnosis.

Treatment

About 50% of patients with CBS deficiency respond to large oral doses of pyridoxine. Pyridoxine nonresponders are treated with dietary methionine restriction and oral administration of betaine, which increases methylation of homocysteine to methionine and improves neurologic function. Early treatment prevents intellectual disability, lens dislocation, and thromboembolic manifestations, which justifies the screening of newborn infants. Large doses of vitamin B₁₂ (eg, 1–5 mg hydroxocobalamin administered daily intramuscularly or subcutaneously) are indicated in some patients with defects in cobalamin metabolism. In remethylation defects methionine may be low, requiring oral supplementation.

NONKETOTIC HYPERGLYCINEMIA

Inherited deficiency of protein subunits of the glycine cleavage enzyme causes classic nonketotic hyperglycinemia, and deficiency of the cofactor lipoate causes variant nonketotic hyperglycinemia. These defects and a defect in the glycine transporter GLYT1 constitute the glycine encephalopathies. The pathophysiology of these disorders is poorly understood, but glycine accumulation in the brain may disturb neurotransmission of the glycinergic receptors and the N-methyl-D-aspartate type of glutamate receptor. The severe form of classic nonketotic hyperglycinemia presents in the newborn as hypotonia, lethargy proceeding to coma, myoclonic seizures, and hiccups, with a burst suppression pattern on EEG. Respiratory depression may require ventilator assistance in the first 2 weeks, followed by spontaneous recovery. Patients develop severe intellectual disability and recalcitrant seizures. Some patients have a small corpus callosum or may develop hydrocephalus. All patients have restricted diffusion on MRI in the already myelinated long tracts at birth. Patients with an attenuated form present with treatable seizures, varying developmental delay, and chorea, and one-half of these may present later in infancy or in childhood. All forms of the condition are autosomal recessive.

Diagnosis

Nonketotic hyperglycinemia should be suspected in any neonate or infant with seizures, particularly those with burst suppression pattern on EEG. Diagnosis is confirmed by demonstrating a large increase in glycine in nonbloody CSF, with an abnormally high ratio of CSF glycine to plasma glycine. Combined sequencing and exonic copy number analysis of GLDC and AMT is diagnostic in more than 98% of cases. Defects in biosynthesis of the cofactors lipoate or pyridoxal phosphate also present with epileptic encephalopathy with elevated CSF glycine. Prenatal diagnosis is possible by molecular analysis.

Treatment

In patients with mild disease, treatment with sodium benzoate (to normalize plasma glycine levels) and dextromethorphan or ketamine (to block N-methyl-D-aspartate type of glutamate receptors) controls seizures and improves neurodevelopmental outcome. Treatment of severely affected patients is generally unsuccessful. High-dose benzoate therapy can aid in seizure control but does not prevent severe intellectual disability. Ketogenic diet reduces glycine levels, but the impact on outcome is very limited.

Organic acidemias are disorders of amino and fatty acid metabolism in which nonamino organic acids accumulate in serum and urine. These conditions are usually diagnosed by examining organic acids in urine, a study usually performed only in specialized laboratories. Table 36–2 lists the clinical features of organic acidemias, together with the urine organic acid patterns typical of each. Additional details about some of the more important organic acidemias are provided in the sections that follow.

PROPIONIC & METHYLMALONIC ACIDEMIA (KETOTIC HYPERGLYCINEMIAS)

The oxidation of valine, odd chain length fatty acids, methionine, isoleucine, and threonine results in propionyl-CoA, which is converted into L-methylmalonyl-CoA by propionyl-CoA carboxylase, and then metabolized to succinyl-CoA via methylmalonyl-CoA mutase for entry into the tricarboxylic acid (Krebs) cycle. Gut bacteria also substantially contribute to propionyl-CoA production. Propionic acidemia is due to a defect in the biotin-containing enzyme propionyl-CoA carboxylase, and methylmalonic acidemia is due to a defect in methylmalonyl-CoA mutase, in either the mutase apoenzyme or in defects of synthesis of its cofactor, adenosyl-B₁₂. Some disorders of vitamin B₁₂ metabolism affect only the synthesis of adenosyl-B₁₂ (Cbl A or B), whereas in others (Cbl C, D, F, J, X), the synthesis of methyl-B₁₂ is also blocked leading to elevated homocysteine in addition to methylmalonic acid (see Homocystinuria).

Clinical symptoms vary according to the location and severity of the enzyme block. Children with severe blocks present with acute, life-threatening metabolic ketoacidosis, hyperammonemia, coma, and bone marrow depression in early infancy or with metabolic acidosis, vomiting, and failure to thrive during the first few months of life. Most patients with severe disease have mild or moderate intellectual disability. Other complications include pancreatitis, basal ganglia stroke, cardiomyopathy (more in propionic), and interstitial nephritis and chronic kidney disease (more in methylmalonic).

All forms of propionic and methylmalonic acidemia are autosomal recessive traits (except for X-linked Cbl X) and can be diagnosed in utero.

Diagnosis

Laboratory findings consist of increases in urinary organic acids derived from propionyl-CoA or methylmalonic acid (see Table 36–2), and elevated propionylcarnitine (easily detected by the newborn screen). Hyperglycinemia and ketosis can be present, especially in acute illness. In some forms of abnormal vitamin B₁₂ metabolism, homocysteine can be elevated. Confirmation is by molecular analysis and/or by assays in fibroblasts or lymphocytes (propionic only).

Treatment

Patients with enzyme blocks in B₁₂ metabolism usually respond to pharmacologic doses of vitamin B₁₂ (hydroxocobalamin) given subcutaneously or intramuscularly. Vitamin B₁₂ nonresponsive methylmalonic acidemia and propionic acidemia require amino acid restriction, strict prevention of catabolism, and carnitine supplementation to enhance propionylcarnitine excretion. Intermittent metronidazole can help reduce the propionate load from the gut. In the acute setting, hemodialysis or hemofiltration may be needed. Liver transplant, or combined liver-renal transplantation, is an option in severe forms of these disorders.

CARBOXYLASE DEFICIENCY

Isolated pyruvate carboxylase deficiency presents with lactic acidosis and hyperammonemia in early infancy. Even if biochemically stabilized, the neurologic outcome is poor. Isolated 3-methylcrotonyl-CoA carboxylase deficiency is frequently recognized on newborn screening using acylcarnitine analysis. It is usually a benign condition that sometimes causes symptoms of acidosis and neurologic depression. All carboxylases require biotin as a cofactor. Holocarboxylase synthetase covalently binds biotin to the apocarboxylases for pyruvate, 3-methylcrotonyl-CoA, and propionyl-CoA; biotinidase releases biotin from these proteins and from proteins in the diet. Recessively inherited deficiency of either enzyme causes deficiency of all three carboxylases (ie, multiple carboxylase deficiency). Patients with holocarboxylase synthetase deficiency usually present as neonates with hypotonia, skin problems, and severe acidosis. Those with biotinidase deficiency present later with a syndrome of ataxia, seizures, seborrhea, and alopecia. Untreated patients can develop intellectual disability, hearing loss, and optic nerve atrophy. The sequelae of the disorder in many patients are preventable if treated early.

Diagnosis

This diagnosis should be considered in patients with typical symptoms or in those with primary lactic acidosis. Urine organic acids are usually, but not always, abnormal (see Table 36–2). Diagnosis is made by enzyme assay of carboxylase activities in fibroblasts or leucocytes. Biotinidase can be assayed in serum, and holocarboxylase synthetase in leukocytes or fibroblasts. Nearly all children with the condition are now diagnosed with the newborn screen.

Treatment

Isolated carboxylase deficiencies are often unresponsive to biotin supplementation. In biotinidase deficiency and holocarboxylase deficiencies, oral administration of pharmacologic doses of biotin reverses the organic aciduria within days and the clinical symptoms within days to weeks. Hearing loss can occur in patients with profound biotinidase deficiency despite treatment.

GLUTARIC ACIDEMIA TYPE I

Glutaric acidemia type I occurs due to deficiency of glutaryl-CoA dehydrogenase. Patients have frontotemporal atrophy with enlarged sylvian fissures and macrocephaly. Sudden or chronic neuronal degeneration in the caudate and putamen causes an extrapyramidal movement disorder in childhood with dystonia and athetosis. Children with glutaric acidemia type I may present with retinal hemorrhages and intracranial bleeding, and may thus be falsely considered victims of child abuse. This is a disorder that primarily affects the central nervous system, and does not present with systemic acidosis, hypoglycemia, or primary end-organ damage elsewhere. Initial symptoms have only been reported in the first 6 years of life, which represents the vulnerable period. The condition is autosomal recessive and prenatal diagnosis is possible.

Diagnosis

Glutaric acidemia type I should be suspected in patients with acute or progressive dystonia in the first 6 years of life. MRI of the brain is highly suggestive. The diagnosis is supported by finding glutaric, 3-hydroxyglutaric acid, and glutarylcarnitine in urine or serum or by finding two mutations in the GCDH gene. Demonstration of deficiency of glutaryl-CoA dehydrogenase in fibroblasts can confirm the diagnosis. Prenatal diagnosis is by mutation analysis, enzyme assay, or quantitative metabolite analysis in amniotic fluid. This condition is detected on the newborn screen.

Treatment

Strict prevention of catabolism in fasting or illness is critically important. Supplementation with carnitine and provision of a lysine and tryptophan restricted diet reduces the risk of degeneration of the basal ganglia. Benefit of arginine supplementation is controversial. Early diagnosis does not prevent neurologic disease in all patients, but it reduces the risk, warranting newborn screening. Despite treatment, affected individuals may have deficiencies in speech and fine motor skills. Neurologic symptoms, once present, do not typically resolve. Symptomatic treatment of severe dystonia is important for affected patients.

FATTY ACID OXIDATION DISORDERS

Fatty acid oxidation disorders are disorders of the transport and catabolism of fatty acids in the mitochondria. In general, fatty acid oxidation disorders present with hypoketotic hypoglycemia and, depending on the specific disorder, may include mild hyperammonemia, hepatopathy, encephalopathy, and/or skeletal myopathy or cardiomyopathy. The long-chain defects, which include very-long-chain acyl-CoA dehydrogenase (VLCAD), long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD), carnitine palmitoyltransferase deficiency I and II, and carnitine-acylcarnitine translocase deficiency, cause episodic rhabdomyolysis, cardiomyopathy, and ventricular arrhythmias. Deficiencies of VLCAD and LCHAD cause hepatic encephalopathy (Reye-like) episodes. Sudden death in infancy is a less common presentation. Symptoms specific to LCHAD deficiency include progressive liver cirrhosis, peripheral neuropathy, and retinitis pigmentosa, and a higher than expected incidence of acute fatty liver of pregnancy and HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets) during pregnancy in carrier mothers of affected infants.

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common fatty acid oxidation disorder, occurring in perhaps 1:9000 live births. Reye-like episodes, which historically have been largely caused by undiagnosed MCAD deficiency, may be fatal or cause residual neurologic damage. Episodes tend to become less frequent and severe with time. After the diagnosis is made and treatment instituted, morbidity decreases and mortality is avoided in MCAD deficiency.

Short-chain acyl-CoA dehydrogenase (SCAD) deficiency is characterized by the presence of ethylmalonic acid in the urine. Patients are generally asymptomatic; whether this deficiency is an actual clinical disease entity is increasingly being reconsidered. Glutaric acidemia type II (or multiple acyl-CoA dehydrogenase deficiency) results from defects in the flavin-mediated transfer of electrons from fatty acid oxidation and some amino acid oxidation into the respiratory chain. Some patients with glutaric acidemia type II have a clinical presentation resembling MCAD deficiency. Patients with a severe neonatal presentation may also have renal cystic disease, dysmorphic features, and profound cardiomyopathy. The least affected patients can present with late-onset myopathy and be riboflavin responsive. Some develop cardiomyopathy or leukodystrophy. Deficiency of the ketogenic enzymes 3-hydroxymethylglutaryl-CoA synthase and lyase present with hypoketotic hypoglycemia. These conditions are all autosomal recessive. Disorders of cytoplasmic fatty acid metabolism are being newly recognized. Deficiency of lipin 1, a cytoplasmic triglyceride lipase, causes severe episodes of rhabdomyolysis starting at a very early age.

Diagnosis

All disorders of fatty acid oxidation have reduced ketogenesis in response to fasting. The analysis of acylcarnitine esters (via an acylcarnitine profile) is a first-line diagnostic test used in newborn screening because it reveals diagnostic metabolites regardless of clinical status. A typical pattern can be recognized for each disorder; for instance, MCAD deficiency is characterized by elevated octanoylcarnitine. Some disorders have elevated acylglycine esters which can be identified in urine organic acid analysis or on specific quantitative acylglycine analysis. Further confirmation can be obtained from targeted gene sequencing or analysis of fatty acid oxidation in fibroblasts; enzyme assays are only rarely available.

Treatment

Management of all disorders of fatty acid oxidation involves prevention of hypoglycemia by avoiding prolonged fasting (> 8–12 hours). This includes vigorous treatment of fasting associated with illness with glucose. Because fatty acid oxidation can be compromised by associated carnitine deficiency, young patients with MCAD deficiency usually receive oral carnitine when carnitine levels are low. Restriction of dietary long-chain fats is not necessary in MCAD deficiency but is required for severe VLCAD and LCHAD deficiencies. Medium-chain triglycerides are contraindicated in MCAD deficiency but are a potential energy source for patients with severe VLCAD and LCHAD deficiencies or carnitine-acylcarnitine translocase deficiency. Other potential alternative fuel sources include protein and triheptanoin. Riboflavin may be beneficial in some patients with glutaric acidemia type II. Outcome in MCAD deficiency is excellent but is more guarded in patients with the other disorders.

CARNITINE

Carnitine is an essential nutrient found in highest concentration in red meat. Its primary function is to transport long-chain fatty acids into mitochondria for oxidation. Primary carnitine uptake deficiency may manifest as hepatic encephalopathy (Reye-like syndrome), cardiomyopathy, or skeletal myopathy with hypotonia. These disorders are rare compared with secondary carnitine deficiency, which may be due to diet (vegan diet, intravenous alimentation, or ketogenic diet), renal losses, drug therapy (especially valproic acid), and other metabolic disorders (especially organic acidemias). The prognosis depends on the cause of the carnitine abnormality. Primary carnitine deficiency is one of the most treatable causes of dilated cardiomyopathy in children.

Free and esterified carnitine can be measured in blood. If carnitine insufficiency is suspected, the patient should be evaluated to rule out disorders that might cause secondary carnitine deficiency.

Oral or intravenous L-carnitine is used in carnitine deficiency or insufficiency in doses of 25–100 mg/kg/day or higher. Treatment is aimed at maintaining normal carnitine levels. Carnitine supplementation in patients with some disorders of fatty acid oxidation and organic acidurias may also augment excretion of accumulated metabolites, although supplementation may not prevent metabolic crises in such patients.

Hypoxanthine-guanine phosphoribosyltransferase is an enzyme that recycles the purine bases hypoxanthine and guanine to inosine monophosphate and guanosine monophosphate, respectively. Hypoxanthine-guanine phosphoribosyltransferase deficiency (Lesch-Nyhan syndrome) is an X-linked recessive disorder. The complete deficiency is characterized by CNS dysfunction, purine wasting with compensatory increase in purine synthesis, and xanthine and hypoxanthine overproduction resulting in hyperuricemia and hyperuricosuria. Depending on the residual activity of the mutant enzyme, male hemizygous individuals may be severely disabled by choreoathetosis, spasticity, and compulsive, mutilating lip and finger biting, or they may have only gouty arthritis and urate ureterolithiasis. Adenylosuccinate lyase deficiency involves a defect in the synthesis of purines. Patients present with static intellectual disability, hypotonia, and seizures.

Diagnosis

Diagnosis of Lesch-Nyhan syndrome is made by demonstrating an elevated uric acid:creatinine ratio in urine, followed by demonstration of enzyme deficiency in red blood cells or fibroblasts or by molecular analysis. Screening for adenylosuccinate lyase deficiency is by measurement of urine succinylpurines, with confirmation by further metabolite and molecular assays.

Treatment

Hyperhydration and alkalinization are essential to prevent kidney stones and urate nephropathy. Allopurinol and probenecid may be given to reduce hyperuricemia and prevent gout but do not affect the neurologic status. Physical restraints are often more effective than neurologic medications for self-mutilation. No effective treatment exists for adenylosuccinate lyase deficiency.

Lysosomes are cellular organelles in which complex macromolecules are degraded by specific acid hydrolases. Deficiency of a lysosomal enzyme causes its substrate to accumulate in the lysosomes, resulting in a characteristic clinical picture. These lysosomal storage disorders are classified as mucopolysaccharidoses, lipidoses, or oligosaccharidoses, depending on the nature of the stored material. Two additional disorders, cystinosis and Salla disease, are caused by defects in lysosomal proteins that normally transport material from the lysosome to the cytoplasm. Table 36–3 lists clinical and laboratory features of these conditions. Most are inherited as autosomal recessive traits, and all can be diagnosed in utero.

Diagnosis

The diagnosis of mucopolysaccharidosis is suggested by certain clinical and radiologic findings (dysostosis multiplex, which includes enlarged sella turcica, scaphocephaly, broad ribs, hook-shaped vertebrae [L1 and L2 most affected], and prominent pointing of the metacarpals and broad phalanges). Urine screening tests can detect increased mucopolysaccharides and oligosaccharides, and further identify which specific species are present. Diagnosis must be confirmed by enzyme assays using leukocytes or cultured fibroblasts. Lipidoses present with visceral symptoms or neurodegeneration. The pattern of the leukodystrophy associated with many lipidoses can indicate a specific condition. Diagnosis is made by appropriate enzyme assays of peripheral leukocytes or cultured skin fibroblasts. Molecular analysis is also available for most conditions.

Treatment

Most conditions cannot be treated effectively, but new avenues have given hope in many conditions. Hematopoietic stem cell transplantation (HSCT) can greatly improve the course of some lysosomal diseases and is first-line treatment in some, such as infantile Hurler syndrome. Several disorders are treated with infusions of recombinant modified enzyme. Infusions are typically given intravenously; however, use of intrathecal infusion can allow for more effective treatment of neurologic symptoms. Treatment of Gaucher disease is very effective, and long-term data suggest excellent outcome. Similar treatments have been developed for Fabry disease, several mucopolysaccharidoses, Wolman, and Pompe disease. Substantial improvements in these conditions have been reported but with limitations. New avenues for treatment through substrate inhibition and chaperone therapy are being developed. Treatment of cystinosis with cysteamine results in depletion of stored cystine and prevention of complications including renal disease. Niemann-Pick C is being more effectively treated with cyclodextrin.

Peroxisomes are intracellular organelles that contain a large number (> 70) of enzymes. The enzyme systems in peroxisomes participate in metabolism of VLCFAs, branched chain fatty acids (phytanic and pristanic acid), bile acids, some amino acids, oxalate and plasmalogens.

In peroxisomal biogenesis disorders, multiple enzymes are deficient due to global peroxisomal dysfunction. The clinical presentations are termed Zellweger spectrum disorders. Patients present as neonates or infants with seizures, hypotonia, characteristic facies with a large forehead and fontanel, hepatopathy, feeding difficulties, retinal dystrophy, and hearing loss. At autopsy, renal cysts, brain neuronal migration abnormalities, and absent or empty peroxisomes are seen. Patients with a milder biochemical and clinical phenotype have ataxia, developmental delay, retinopathy, and hearing loss.

In other peroxisomal diseases, only a single enzyme is deficient. Patients with D-bifunctional protein deficiency or acyl-CoA oxidase deficiency have a Zellweger-like phenotype. Primary hyperoxaluria (alanine-glyoxylate aminotransferase deficiency) causes renal stones and nephropathy. Mutations in the X-linked VLCFA transporter gene, ABCD1, cause either a rapidly progressive and fatal leukodystrophy (X-linked adrenoleukodystrophy [X-ALD]) or a slowly progressive spasticity and neuropathy (adrenomyeloneuropathy). Adrenal insufficiency usually accompanies either neurologic presentation. Defective phytanic acid oxidation causes adult Refsum disease, with symptoms of ataxia, leukodystrophy, cardiomyopathy, neuropathy, and retinal dystrophy. Defects of plasmalogen synthesis cause rhizomelic chondrodysplasia punctata, with symptoms of skeletal dysplasia and neurologic disease. Except for X-ALD, all peroxisomal diseases have recessive inheritance.

Diagnosis

The best initial test for Zellweger spectrum disorders and X-ALD is assessment of VLCFA levels in plasma. Measurements of phytanic acid, pristanic acid, pipecolic acid, bile acid intermediates, and plasmalogens are also appropriate for evaluation of some peroxisome disorders. Increasingly, peroxisomal disorders are identified with genetic testing. Several states test for X-ALD with newborn screening.

Treatment

Treatment for most peroxisomal disorders is symptomatic and supportive. HSCT may be an effective treatment at the early stages of X-ALD, and close monitoring of affected males is necessary to determine optimal timing of HSCT. Corticosteroid treatment is necessary for any patients with adrenal insufficiency. Dietary treatment for avoidance of phytanic acid (mainly present in meat and dairy) is effective for adult Refsum disease. Liver transplantation is an effective treatment for primary hyperoxaluria.

Many proteins, especially extracellular and lysosomal proteins, require glycosylation for normal function. The congenital disorders of glycosylation (CDGs) are a family of over 100 different disorders that result from defects in the synthesis of glycans or in the attachment of glycans (polysaccharides) to proteins as a post-translational modification. The most common CDG is phosphomannomutase-2 deficiency (PMM2-CDG). Children with PMM2-CDG may present with diarrhea, developmental delay, abnormal subcutaneous fat distribution, inverted nipples, strabismus, cerebellar hypoplasia, liver disease, cardiomyopathy and pericardial effusions, endocrine abnormalities, and peripheral neuropathy. Patients with phosphomannose isomerase deficiency (PMI-CDG) have a combination of hepatopathy, protein-losing enteropathy, and hyperinsulinemic hypoglycemia. Glycosylation defects have also been found to underlie syndromes such as multiple exostoses syndrome, Walker-Warburg syndrome, muscle-eye-brain disease, and dystroglycan-related muscular dystrophies.

Defects in the glycosylphosphatidylinositol anchor system cause neurologic symptoms such as epilepsy, hypotonia, brain abnormalities, and other organ dysfunction with characteristically elevated alkaline phosphatase due to lack of anchoring of the enzyme to the endothelial cell wall. Combined deficiencies are present in Golgi disorders such as abnormalities in the COG complex that moves glycoproteins; these present with variable combinations of liver disease, neurologic symptoms, recurrent infections and hyperthermia, cardiac defects, and cutis laxa. Phosphoglucoisomerase deficiency (PGM1-CDG) causes both a glycogen storage disease (type XIV) with hypoglycemia, but also malformations such as cleft palate and liver, cardiac, and endocrine abnormalities. Finally, N-glycanase-1 (NGLY1) is the first described disorder of deglycosylation and presents with developmental delays, elevated transaminases, chorea, and alacrima.

Diagnosis

Diagnosis may be suspected in the setting of altered levels of glycosylated proteins such as transferrin, thyroxine-binding globulin, lysosomal enzymes, and clotting factors (IX, XI, antithrombin III, and proteins C and S). Diagnosis is confirmed by finding patterns of abnormal glycosylation of selected proteins. Most diagnostic laboratories examine serum transferrin to screen for N-linked CDGs and apoCIII for O-linked CDGs. In some cases, muscle biopsy with immunohistochemistry may be a diagnostic test. CDG diagnosis may be further confirmed by assaying enzyme activity in some cases. Increasingly, broad genetic testing is the initial diagnostic modality that suggests a CDG.

Treatment

Treatment is supportive, including monitoring and providing early treatment for expected clinical features. Mannose treatment is curative for patients with PMI-CDG, and galactose treatment is important for patients with PGM1-CDG, making timely recognition of these disorders important.

Several defects of cholesterol synthesis are associated with malformations and neurodevelopmental disability. Smith-Lemli-Opitz (SLO) syndrome is an autosomal recessive disorder caused by a deficiency of the enzyme 7-dehydrocholesterol D7-reductase. It is characterized by microcephaly, poor growth, intellectual disability, typical dysmorphic features of face and extremities (particularly two- to three-toe syndactyly), and often malformations of the heart and genitourinary system. It is further described in Chapter 37. Conradi-Hünermann syndrome is characterized by chondrodysplasia punctata and atrophic skin. Defects in the metabolism of cholesterol to bile acids usually cause cholestatic liver disease and failure to thrive. Cerebrotendinous xanthomatosis (CTX) manifests with progressive ataxia, spastic paraparesis, cataracts, cognitive decline, and, later, xanthomatous eruptions of the skin. Some patients with CTX may initially present with cholestatic liver disease or chronic diarrhea in infancy.

Diagnosis

In SLO, elevated 7- and 8-dehydrocholesterol in serum or amniotic fluid is diagnostic. Serum cholesterol levels may be low or in the normal range. Enzymes of cholesterol synthesis may be assayed in cultured fibroblasts or amniocytes, and mutation analysis is possible. CTX is diagnosed by detection of characteristic abnormalities of bile acids in blood and urine as well as elevated cholestanol.

Treatment

Although postnatal treatment does not resolve prenatal injury, supplementation with cholesterol in SLO may improve growth and behavior. The role of supplemental bile acids is controversial. CTX responds to treatment with chenodeoxycholic acid, which inhibits the formation of bile alcohols by suppressing the first-step cholesterol 7α-hydroxylase, with substantial functional recovery.

Abnormalities of neurotransmitter metabolism are increasingly recognized as causes of significant neurodevelopmental disabilities. These disorders impact the synthesis of the neurotransmitters dopamine and serotonin. Affected patients may present with movement disorders (especially dystonia and oculogyric crises), seizures, abnormal tone, or intellectual disability, and may initially be diagnosed with cerebral palsy. Patients may be mildly affected (eg, dopa-responsive dystonia with diurnal variation) or severely affected (eg, intractable seizures with profound intellectual disability).

Pyridoxine-dependent epilepsy manifests as a seizure disorder in the neonatal or early infantile period that responds to high doses of pyridoxine. The disorder is caused by deficient activity of the enzyme α-amino adipic semialdehyde dehydrogenase resulting from mutations in the antiquitin (ALDH7A1) gene. The enzyme is involved in lysine catabolism and dietary lysine restriction aids in treatment. Pyridoxal-phosphate–responsive encephalopathy manifests as a severe seizure disorder in infancy that responds to pyridoxal-phosphate supplementation. This disorder is caused by mutations in the PNPO gene encoding pyridox(am)ine oxidase, which is necessary for activation of pyridoxine.

Deficient serine synthesis leads to congenital microcephaly, infantile seizures, and failure of myelination. The most severe disorder of serine synthesis is Neu-Laxova syndrome, characterized by premature birth with microcephaly, skeletal abnormalities and early lethality. Classic serine deficiency presents with microcephaly, refractory epilepsy, and hypomyelination. Defects in all four genes involved in serine synthesis and transport (PHGDH, PSAT1, PSPH, SLC4A1) occur in an autosomal recessive pattern. Defects in asparagine synthase and glutamine synthase result in severe microcephaly, underdevelopment of the brain with gyral simplification, and refractory epilepsy.

Glut1 deficiency syndrome results from mutations in SLC2A1, which act in a dominant fashion. The resultant CSF glucose deficiency causes seizures as well as dystonia and other movement disorders.

Diagnosis

Although some disorders can be diagnosed by examining plasma amino acids or urine organic acids (eg, 4-hydroxybutyric aciduria), in most cases, diagnosis requires analysis of CSF. Spinal fluid samples for neurotransmitter analysis require special collection and handling, as the neurotransmitter levels are graduated along the axis of the CNS. A phenylalanine loading test can be diagnostic for mild defects in GTP-cyclohydrolase deficiency, in which neurotransmitter analysis may be insufficiently sensitive. Analysis of CSF shows elevated threonine and decreased pyridoxal-phosphate in pyridoxal-phosphate–responsive disease, and decreased serine and glycine in serine biosynthetic defects. Urine or plasma α-aminoadipic acid and piperideine-6-carboxylate best identifies infants with pyridoxine-dependent seizures. CSF analysis of serine, asparagine, or glutamine detects deficiencies in these amino acids most effectively. Glut1 deficiency syndrome can be diagnosed by demonstrating low glucose and lactate in CSF.

Treatment

Biosynthesis defects of dopamine and serotonin are usually treated with a combination of levodopa, 5-hydroxytryptophan, and carbidopa. Pyridoxine-dependent epilepsy is treated with pyridoxine in high doses and a lysine-restricted diet, whereas pyridoxal-phosphate–responsive encephalopathy requires pyridoxal-phosphate supplementation. Supplementation with serine and glycine can improve outcomes in serine deficiency. Glut1 deficiency syndrome is treatable with a ketogenic diet. For several conditions, such as pyridoxine-responsive seizures, pyridoxal-phosphate–responsive encephalopathy, or dopa-responsive dystonia, response to treatment is dramatic.

Creatine is essential for storage and transmission of phosphate-bound energy in muscle and brain. The disorders arginine:glycine amidinotransferase (AGAT) deficiency and guanidinoacetate methyltransferase (GAMT) deficiency are autosomal recessive, whereas creatine transporter (CrT1) deficiency is X-linked. Patients demonstrate developmental delay, seizures, and severe expressive language delay (particularly in CrT1 deficiency). Patients may also show developmental regression and brain atrophy. Patients with GAMT deficiency have more severe seizures and an extrapyramidal movement disorder. The seizure disorder is milder in male CrT1-deficient patients. Some female heterozygotes may show learning disabilities.

Diagnosis

Creatine and guanidinoacetate levels may be measured in blood or urine and are typically the initial diagnostic study. Creatine transporter deficiency is detected using the urine creatine:creatinine ratio. Magnetic resonance spectroscopy may demonstrate decreased creatine concentration in the brain. Sequencing of SLC6A8, GATM, and AGAT as part of a broad genetic testing approach may be the initial diagnostic study for those patients with nonspecific presentations. Pilot studies of newborn screening for some creatine deficiency syndromes are underway.

Treatment

Treatment with oral creatine supplementation is partially successful in GAMT and AGAT deficiencies. Treatment with combined arginine restriction and ornithine supplementation in GAMT deficiency can decrease guanidinoacetate concentrations and improve the clinical course. Early treatment from infancy greatly improves outcomes in GAMT. Combined therapy using arginine, glycine, and creatine supplementation in CrT1 deficiency has been tried with variable efficacy.

Expanded newborn screening has had a large impact on the field of metabolic disorders. Patients are being diagnosed earlier, and for some patients, this dramatically reduces the disease burden. Expanded newborn screening has also revealed unexpected consequences. For example, the clinical spectrum of many disorders is being expanded to include mildly affected or asymptomatic patients. Gradually, a more refined therapeutic approach is being developed for the more mildly affected patients; for some patients on the milder end of a disease spectrum who would have never become symptomatic, therapy is not needed at all. In addition, newborn screening for several conditions detects maternal disease such as maternal vitamin B₁₂ deficiency and maternal carnitine uptake deficiency; this poses new management and risk questions for this largely asymptomatic or presymptomatic population. Further, limitations in diagnostic testing create difficulty in discriminating carriers from patients affected with mild disease manifestations that still pose health risks (eg VLCAD deficiency and glutaric aciduria type I) and may detect the presence of pseudodeficiency alleles that show biochemical but not clinical enzymatic deficiency (eg, Hurler and Pompe disease). These diagnostic challenges not only add to parental anxiety, but treatment of an unaffected child may entail risk to the child or to family dynamics.

Continuous improvements in newborn screening have resulted in the addition of screening for several new conditions such as severe combined immunodeficiency (SCID) and peroxisomal and lysosomal storage disorders. Many more disorders will be considered candidates for newborn screening in the near future. Careful consideration will need to be given to the risks and benefits to screening for such conditions. The U.S. Secretary for Health and Human Services’ Advisory Committee on Heritable Disorders in Newborns and Children has established a rigorous process for review before recommending a disorder be considered for nationwide screening. Despite approval through this rigorous process, many states are struggling with implementation of new screening tests.