DISORDERS OF THE UREA CYCLE
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
Typical presentation is infantile encephalopathy; later onset presentations are common with cyclic vomiting or encephalopathy with illness or protein load.
Diagnosis possibly suspected with the finding of hyperammonemia frequently with minimal other laboratory findings.
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.
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.
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.
Patient and parent support group website with useful information for families: http://www.nucdf.org
NA: Inborn errors of metabolism with hyperammonemia: urea cycle defects and related disorders. Pediatr Clin North Am 2018;65(2):231–246
PHENYLKETONURIA & THE HYPERPHENYLALANINEMIAS
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
Intellectual disability, hyperactivity, seizures, light complexion, and eczema characterize untreated patients.
Newborn screening for elevated plasma phenylalanine identifies most infants.
Disorders of cofactor metabolism also produce elevated plasma phenylalanine level.
Early diagnosis and treatment with phenylalanine-restricted diet prevent intellectual disability.
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.
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.
B: DNAJC12 deficiency: a new strategy in the diagnosis of hyperphenylalaninemia. Mol Genet Metab 2018;123(1):1–5
H: Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency. J Hum Genet 2019;64(2):67–71
CR: Phenylalanine ammonia lyase (PAL): from discovery to enzyme substitution therapy for phenylketonuria. Mol Genet Metab 2018;124(4):223–229
JJ: Maternal phenylketonuria and hyperphenylalaninemia in pregnancy: pregnancy complications and neonatal sequelae in untreated and treated pregnancies. Am J Clin Nutr 2012;95(2):374
et al: The complete European guidelines on phenylketonuria: diagnosis and treatment. Orphanet J Rare Dis 2017;12(1):162
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
Consider in a child presenting with liver disease with or without accompanying renal disease or bone disease.
Elevated urinary succinylacetone is diagnostic of tyrosinemia, type 1.
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.
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.
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.
et al: Diagnosis and treatment of tyrosinemia type 1: a US and Canadian consensus group review and recommendations. Genet Med 2017;19(12): Epub 2017 Aug 3
MAPLE SYRUP URINE DISEASE (BRANCHED-CHAIN KETOACIDURIA)
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
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.
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.
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.
BH: Branched-chain amino acid metabolism: from rare Mendelian diseases to more common disorders. Hum Mol Genet 2014;23:R1–R8
AS: Living donor liver transplantation in maple syrup urine disease–case series and world’s youngest domino liver donor and recipient. Pediatr Transplant 2016;20:395–400
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
Consider in a child of any age with a marfanoid habitus, dislocated lenses, or thrombosis.
Diagnosis is suggested by elevated total homocysteine and methionine.
Newborn screening allows early diagnosis and treatment resulting in a normal outcome.
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 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.
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.
et al: Guidelines for diagnosis and management of the cobalamin-related remethylation disorders cblC, cblD, cblE, cblF, cblG, cblJ and MTHFR deficiency. J Inherit Metab Dis 2017;40(1):21–48
et al: Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency. J Inherit Metab Dis 2017;40(1):49–74
ESSENTIALS OF DIAGNOSIS & TYPICAL FEATURES
Severely affected newborns present with apnea, hypotonia, lethargy, myoclonic seizures, and hiccups, and develop severe mental and motor retardation.
Mildly affected children have developmental delay, hyperactivity, mild chorea, and seizures.
CSF glycine is elevated.
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.
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.
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.
et al: The genetic basis of classic nonketotic hyperglycinemia due to mutations in GLDC
. Genet Med 2017;19:104–111
et al: Biochemical and molecular predictors of prognosis in nonketotic hyperglycinemia. Ann Neurol 2015;78:606–618