Cobalamin (vitamin B12) is a complex organometallic molecule that is synthesized by many bacteria and is obtained in the human diet from meat, fish, and dairy products. It is not present in plant foods, so strict vegetarians are at risk for dietary deficiency. Derivatives of cobalamin are required for the activity of 2 enzymes: methylcobalamin and 5′-deoxyadenosylcobalamin. Methylcobalamin is generated during the catalytic cycle of methionine synthase, a cytoplasmic enzyme that catalyzes methylation of homocysteine to methionine (see Chapter 133). 5′-Deoxyadenosylcobalamin (AdoCbl) is required for the mitochondrial enzyme methylmalonyl-coenzyme A (CoA) mutase to catalyze the conversion of methylmalonyl-CoA, formed during catabolism of branched-chain amino acids and odd-chain fatty acids, to succinyl-CoA (see Chapter 132). Therefore, inborn errors of cobalamin metabolism result in isolated methylmalonic aciduria, isolated homocystinuria, or combined methylmalonic aciduria and homocystinuria, depending on which step in cobalamin metabolism is affected. Without treatment, hyperhomocysteinemia is usually accompanied by hypomethioninemia. Significantly elevated homocysteine levels may be associated with an increased risk of thrombosis. Decreased methionine is associated with abnormalities of the white matter of the nervous system. Elevated levels of methylmalonic acid can lead to metabolic acidosis.
Cobalamin consists of a planar corrin ring with a central cobalt atom, a 5,6-dimethylbenzimidazole base, and an upper axial ligand attached to the cobalt atom that varies in different forms of cobalamin. Physiologically important cobalamins include hydroxycobalamin (OHCbl), methylcobalamin (MeCbl), and adenosylcobalamin (AdoCbl). The most common commercially available form of vitamin B12 contains a cyano group in the upper axial position (CNCbl). The central cobalt can exist in the oxidized Co3+ state (cob[III]alamin), the Co2+ state (cob[II]alamin), or the fully reduced Co1+ state (cob[I]alamin). Converting exogenous cobalamin, typically in the form of cob(III)alamin, to its biologically active coenzyme forms involves reducing the central cobalt to cob(I)alamin and then adding the appropriate upper axial ligand (Fig. 142-1).
Metabolic pathway of cobalamin. The steps affected by inborn errors of cobalamin metabolism are shown by the red bars. AdoCbl, 5’-deoxyadenosylcobalamin; cblA, cobalamin A deficiency; cblB, cobalamin B deficiency; cblC, cobalamin C deficiency; cblD, cobalamin D deficiency; cblD v1, cobalamin D deficiency variant 1; cblD v2, cobalamin D deficiency variant 2; cblE, cobalamin E deficiency; cblF, cobalamin F deficiency; cblG, cobalamin G deficiency; cblJ, cobalamin J deficiency; cblX, cobalamin X deficiency; MeCbl, methylcobalamin; mutase, methylmalonyl-CoA mutase; synthase, methionine synthase; TCII, transcobalamin; TCBLR, transcobalamin receptor.
Absorption of dietary cobalamin is a complex process that is dependent on several cobalamin-binding proteins. Cobalamin is released from food in the acidic environment of the stomach, where it becomes bound to transcobalamin I (TCN1) found in saliva and gastric juice. Transcobalamin I is broken down in the intestine by proteolytic enzymes, and cobalamin binds the gastric intrinsic factor (GIF). The GIF-cobalamin complex is taken up by enterocytes in the distal ileum in a process mediated by a receptor, cubam, composed of 2 proteins, cubilin (CUBN) and amnionless (AMN). After uptake by enterocytes, cobalamin is released into the circulation, where it binds the transport protein transcobalamin II (TCN2).
In blood, transcobalamin II–bound cobalamin is available for uptake by most cell types. Following endocytosis, mediated by the transcobalamin receptor (TCBLR encoded by CD320), the cobalamin–transcobalamin II complex dissociates in lysosomes, and free cobalamin is transferred to the cytoplasm with the participation of at least 2 proteins LMBRD1 (cblF) and ABCD4 (cblJ). Upon egress of the cobalamin from the lysosome, the vitamin begins the cytosolic pathway of metabolism. This process involves MMACHC (cblC), which removes axial cobalamin moieties, reduces Co3+ to Co2+, and facilitates the transfer of cobalamin to MMADHC (cblD). Subsequently, cobalamin becomes associated with methionine synthase in the cytoplasm, or it may be further transported to the mitochondria, where it is converted to 5’-deoxyadenosylcobalamin, the active cofactor for methylmalonyl-CoA mutase. The steps of cellular cobalamin metabolism common to synthesis of both coenzyme derivatives remain under study.
The catalytic cycle of methionine synthase involves transfer of a methyl group from 5-methyl-THF to an enzyme-bound cob(I)alamin molecule, forming methylcobalamin (MeCbl). The methyl group is then transferred from MeCbl to homocysteine, forming methionine and regenerating cob(I)alamin. Occasionally the cob(I)alamin is oxidized to cob(II)alamin; when this occurs, regeneration of MeCbl requires the activity of a second protein, methionine synthase reductase, which uses adenosylmethionine as its methyl group donor. Remethylation of homocysteine to form methionine thus requires the activity of both methionine synthase (cblG) and methionine synthase reductase (cblE).
The system by which cobalamin enters the mitochondria is unknown, as is the form of cobalamin that crosses the mitochondrial membrane. Mitochondrial reductases that can support reduction of cob(II)alamin to cob(I)alamin have been identified in vitro, but the enzyme that catalyzes this reaction in vivo is not yet known. Cob(I)alamin receives the adenosine base from ATP to form 5’-deoxyadenosylcobalamin (AdoCbl) in a reaction catalyzed by cob(I)alamin adenosyltransferase, encoded by the MMAB gene (cblB). Another protein, the product of the MMAA gene (cblA), appears to play a role in supporting the formation of holo-methylmalonyl-CoA mutase and in maintaining the enzyme-bound cobalamin in its active form. The presence of methylmalonyl-CoA mutase, cobalamin adenosyltransferase, and the MMAA protein, possibly in association with a cob(II)alamin reductase, is required for conversion of methylmalonyl-CoA to succinyl-CoA in vivo.
ISOLATED METHYLMALONIC ACIDURIA
Isolated methylmalonic aciduria is seen in patients with methylmalonyl-CoA mutase deficiency (Online Mendelian Inheritance in Man [OMIM] no. 251000), cblA deficiency (OMIM no. 251100), cblB deficiency (OMIM no. 251110), and cblD deficiency variant 2 (OMIM no. 277410). Methylmalonyl-CoA mutase deficiency, also referred to as the mut disorder is caused by mutations in the MUT gene on chromosome 6p12.3. The mut disorder has been subdivided into 2 classes: (1) in mut0, there is no stimulation of mutase activity in cultured cells on incubation with hydroxycobalamin (OHCbl), and there is often no detectable mutase protein; (2) in mut–, mutase activity is stimulated by addition of OHCbl. Deleterious mutations in the MMAA gene on chromosome 4q31.21 result in decreased synthesis of adenosylcobalamin (cblA deficiency). The cblB deficiency is caused by mutations in the MMAB gene on chromosome 12q24.11. Patients with mutations resulting in the loss of function of cblD required for the AdoCbl synthesis also experience isolated methylmalonic aciduria and are referred to as having cblD deficiency variant 2.
Patients with isolated methylmalonic aciduria are prone to episodes of life-threatening acidosis, often in response to infection or to increased protein intake. Initial presentation is most frequently during infancy, with vomiting, hypotonia, irritability, and lethargy, which can progress to coma and death if not treated. In the most severely affected individuals, there may be intractable acidosis during the neonatal period that leads to death, but in many patients, the disorder is characterized by recurrent acidotic crises characterized by high anion gap metabolic acidosis, ketoacidosis, lactic acidosis, and hyperammonemia. Anemia, neutropenia, thrombocytopenia, failure to thrive, optic nerve atrophy, acute and chronic kidney disease, pancreatitis, and basal ganglia injury occur variably, but are well-recognized complications of the disorder. Studies of a series of patients have shown that presentation is typically most severe in individuals with mut0. The clinical course of patients with cblB deficiency may resemble that of mut0 patients. cblA, cblD variant 2, mut–, and rare OHCbl-responsive cblB patients have the least severe presentation and the best prognosis. Clinically unstable patients with severe methylmalonic acidemia may be candidates for an elective liver transplantation. Progression to end-stage renal failure will necessitate kidney or combined liver-kidney transplantation.
Analysis of urinary organic acids from these patients reveals elevated levels of methylmalonic acid and propionyl-CoA derived metabolites such as methylcitric acid, 3-hydroxypropionic acid, and tiglylglycine. Amino acid analysis usually reveals elevated glycine in blood and urine, while the acylcarnitine carnitine profile shows elevated propionylcarnitine and methylmalonylcarnitine. Accumulation of CoA conjugates may result in secondary carnitine deficiency necessitating carnitine supplementation.
Elevation of methylmalonic acid levels may also be seen in individuals with methylmalonyl-CoA epimerase (racemase) deficiency and in patients with mutations affecting either subunit of succinyl-CoA ligase; however, in these orders, the levels are lower than in mut, cblA, and cblB patients. Mild elevations of methylmalonic acid need to be distinguished from dietary vitamin B12 deficiency and other forms of abnormal cobalamin absorption in the gastrointestinal tract. Elevations of both malonic and methylmalonic acids on the urine organic acid assay can be seen in the combined malonic and methylmalonic aciduria (CMAMMA; OMIM no. 614265) due to mutations in ACSF3. Differentiation between the different forms of methylmalonic aciduria depends on molecular and biochemical methods. DNA-based molecular genetic testing has become the initial confirmatory step. If DNA analysis does not reveal the diagnosis, cellular biochemical testing using cultured fibroblasts becomes necessary.
All 4 disorders are inherited as autosomal recessive traits. Mutations in most families are private, but several mutations are relatively common within a specific ethnic group. Most pathogenic variants are missense or nonsense mutations, but copy number variants (deletions and insertions) have also been described. Genotype-phenotype correlations are difficult, but in general, patients with 2 nonsense, larger copy number variants and known severe missense mutations have a more severe course of the disease and should be considered for transplantation sooner. Patients with cblD deficiency variant 2 appear to have mutations in exon 3 and 4 of the MMADHC gene encoding a protein domain required for the AdoCbl synthesis.
Patients with isolated methylmalonic aciduria require life-long monitoring and management. Episodes of acute metabolic decompensation are treated with intravenous fluids containing glucose, appropriate electrolytes, and if necessary, bicarbonate replacement. Dietary goals include a brief restriction of protein intake and supplementation of calories through carbohydrate and lipids. OHCbl responsiveness needs to be established in all patients. Acute mental status changes and new neurologic findings, even in the absence of acidosis, should be treated as metabolic emergencies. Chronic dietary management may require the judicious use of medical foods deficient in amino acids (ie, isoleucine, valine, threonine, and methionine), the precursors of methylmalonic acid. Due to poor palatability of protein-modified diet, frequent nausea, vomiting, and poor appetite, most patients need a gastrostomy tube to facilitate enteral feeding. Carnitine supplementation is used to prevent and treat secondary carnitine deficiency. Patients who experience frequent and severe decompensations may need to be evaluated for liver or combined liver-kidney transplantation. Liver and liver-kidney transplantation helps stabilize patients metabolically and reduce the number of hospitalizations secondary to acidotic decompensations and hyperammonemia. Even after liver and liver-kidney transplantation, patients need to continue a protein-restricted diet and should be monitored for secondary carnitine deficiency. Any changes in the neurologic status should prompt proactive management of metabolic status, as vision loss and brain injury can occur even after liver transplantation. The decision to undergo organ transplantation requires a consideration of many factors including the severity of the disorder, organ availability, rate of surgical complications, and the need for life-long immunosuppression.
Homocystinuria in the absence of methylmalonic aciduria is seen in patients with the cblE (OMIM no. 236270), cblG (OMIM no. 250940), and cblD variant 1 (OMIM no. 277410) deficiencies. In all 3 disorders, methionine synthase function is impaired; in the cblG deficiency, synthase-specific activity in cell extracts is decreased under all assay conditions, while in cblE deficiency, specific activity is decreased only in the presence of limiting concentrations of exogenous reducing agent. The cblG deficiency is caused by mutations affecting the MTR gene on chromosome 1q43, which encodes methionine synthase. The cblE deficiency is caused by mutations of the MTRR gene on chromosome 5p15.31, which encodes methionine synthase reductase. cblD variant 1 deficiency is caused by deleterious mutations in the gene MMADHC on chromosome 2q23.2 encoding a protein that participates in the intracellular conversion of cobalamin to adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl).
Patients with these disorders manifest with megaloblastic anemia and neurologic problems, including developmental delay, cerebral atrophy, hypotonia, microcephaly, and seizures. Some patients may present with hemolytic-uremic syndrome, renal artery thrombosis, and pulmonary hypertension. In some patients, total plasma homocysteine can be as high as in patients with homocystinuria due to cystathionine β-synthase deficiency. Presentation is typically within the first 2 years of life, but it can also present in adulthood.
Decreased methionine synthase activity results in increased total plasma homocysteine and in homocysteine in the urine, as well as reduced levels of methionine and elevated cystathionine. Serum cobalamin levels are normal. These disorders can be differentiated from homocystinuria due to cystathionine β-synthase deficiency, which presents with generally higher levels of homocysteine, elevated methionine, and decreased cystathionine levels (see Chapter 133). The diagnosis is confirmed either by DNA analysis of the implicated genes or by biochemical studies of patient’s fibroblasts. Affected fibroblasts are characterized by decreased methyl-THF incorporation with normal propionate incorporation and by decreased methylcobalamin synthesis with normal adenosylcobalamin synthesis. The disorders can be differentiated by the response of methionine synthase–specific activity to the titration of a reducing agent, but more typically, they are differentiated by complementation analysis.
All 3 disorders are inherited as autosomal recessive traits. Genotype-phenotype correlations are difficult to establish. Most described mutations are missense or nonsense. Patients with CblD variant 1 defect tend to have mutations in exons 6 and 8 of the MMADHC gene encoding a protein domain responsible for the MeCbl synthesis.
Affected individuals can tolerate the recommended daily allowance of protein and need to avoid dietary methionine deficiency. For this reason, medical foods deficient in methionine are not recommended in this group of disorders. Patients respond to OHCbl injections. Supplementation with betaine (250 mg/kg/24 h) facilitates the conversion of homocysteine to methionine through an alternative pathway independent of methionine synthase, via hepatic betaine-homocysteine methyltransferase. To support remethylation reactions, patients with hyperhomocysteninemia are supplemented with folic or folinic acid. In patients with significant hyperhomocysteinemia, attention should be given to the management of risk factors predisposing to hypercoagulability.
COMBINED METHYLMALONIC ACIDURIA AND HOMOCYSTINURIA
Five inborn errors of metabolism that affect early steps in cobalamin metabolism result in combined methylmalonic aciduria and homocystinuria. The cblC disorder (OMIM no. 277400) is caused by mutations in MMACHC on chromosome 1p34.1. The cblD disorder (OMIM no. 277410) is caused by mutations in MMADHC on chromosome 2q23.2. Products of these genes participate in the conversion of Co3+ to Co2+ and removal of the axial ligand. Although the original patients with the cblD disorder had combined methylmalonic acid and homocystinuria, additional patients with isolated homocystinuria (cblD variant 1) and isolated methylmalonic aciduria (cblD variant 2) have subsequently been recognized. The cblF (OMIM no. 277380) deficiency and cblJ (OMIM no. 614857) deficiency result in the inability to transfer endocytosed cobalamin from the lysosome to the cytoplasm. cblX deficiency (OMIM no. 309541) is a unique form of combined methylmalonic acidemia and homocystinuria. It is caused by mutations in the gene HCFC1 on chromosome X, which encodes a transcriptional coregulator involved in expression of MMACHC (cblC).
Most patients with combined methylmalonic aciduria and homocystinuria belong to the cblC class. Without treatment, individuals with this disorder typically present during the first year of life with megaloblastic anemia, failure to thrive, developmental delay, hypotonia, seizures, macrocephaly, and cerebral atrophy. Other patients present later in life or in adulthood with ataxia, dementia, or psychosis. Most patients with cblC deficiency develop a bull’s eye maculopathy and then optic nerve pallor. Retinal degeneration in cblC deficiency can be seen as early as 2 months of age and appears to be resistant to treatment with OHCbl. Noncompaction cardiomyopathy can be present in some patients, necessitating life-long surveillance of the cardiac function. Despite the presence of methylmalonic aciduria, metabolic decompensations are infrequent.
Presentation in classic cblD deficiency is similar to that of cblC. cblF and cblJ deficiencies are clinically variable but tend to show better response to treatment with parenteral hydroxocobalamin. Frequent findings have included feeding difficulties, failure to thrive, growth retardation, and persistent stomatitis. Due to the likely role of LMBRD1 (cblF) and ABCD4 (cblJ) in the intestinal uptake of dietary cobalamin, patients may present with low plasma cobalamin levels. The clinical picture of cblX disorder is dominated by developmental delays, intellectual disability, and intractable epilepsy.
Patients with these disorders have the biochemical characteristics of both methylmalonic aciduria and homocystinuria, although methylmalonic acid levels are generally lower than in patients with isolated methylmalonic aciduria. Fibroblasts show decreased function of both methylmalonyl-CoA mutase and methionine synthase, and decreased synthesis of both adenosylcobalamin and methylcobalamin (Table 142-1). Fibroblasts from patients with the cblC or cblD disorder are unable to accumulate cobalamin in cells, reflecting the inability of cells to retain cobalamin that is not bound to one of the cobalamin-dependent enzymes. In cblF and cblJ disorder, fibroblasts accumulate large amounts of cobalamin, but most of this is unmetabolized cobalamin that is trapped within the lysosomes.
TABLE 142-1CLINICAL AND LABORATORY FINDINGS IN INBORN ERRORS OF METABOLISM ||Download (.pdf) TABLE 142-1CLINICAL AND LABORATORY FINDINGS IN INBORN ERRORS OF METABOLISM
|Class ||MMA ||Hcy ||Propionate Incorporation ||Methyl-THF Incorporation ||Cellular Cobalamin Uptake ||AdoCbl Synthesis ||MeCbl Synthesis |
|mut ||↑ ||N ||↓ ||N ||N ||↓ ||N |
|cblA ||↑ ||N ||↓ ||N ||N ||↓ ||N |
|cblB ||↑ ||N ||↓ ||N ||N ||↓ ||N |
|cblC ||↑ ||↑ ||↓ ||↓ ||↓ ||↓ ||↓ |
|cblX ||↑ ||↑ ||↓ ||↓ ||↓ ||↓ ||↓ |
|cblD ||↑ ||↑ ||↓ ||↓ ||↓ ||↓ ||↓ |
|cblDv1 ||N ||↑ ||N ||↓ ||N ||N ||↓ |
|cblDv2 ||↑ ||N ||↓ ||N ||N ||↓ ||N |
|cblE ||N ||↑ ||N ||↓ ||N ||N ||↓ |
|cblF ||↑ ||↑ ||↓ ||↓ ||↑ ||↓ ||↓ |
|cblJ ||↑ ||↑ ||↓ ||↓ ||↑ ||↓ ||↓ |
|cblG ||N ||↑ ||N ||↓ ||N ||N ||↓ |
cblC, cblD, cblF, and cblJ deficiencies are inherited as autosomal recessive traits. Over 40 mutations in the MMACHC gene have been identified. The most common of these is a c.271dupA mutation that represents 40% of identified mutant alleles, primarily in patients of European or western Asian origin. It is associated with early-onset severe disease when homozygous. Several other mutations show evidence of clustering in specific ethnic groups, and genotype-phenotype correlations have been observed. A small number of mutations in the MMADHC gene have been identified in patients with the much rarer cblD disorder. The classical cblD disorder is associated with truncating mutations, while mutations at the C-terminal domain are associated with isolated homocystinuria (variant 1), and mutations in the N-terminal domain are associated with isolated methylmalonic aciduria (variant 2). Most known mutations in cblF are truncating, resulting in the loss of function of it respective protein. cblJ deficiency is very rare, and the full spectrum of mutations is under study. The HCFC1 gene implicated in the cblX disorder is located on the distal subband of chromosome Xq28 and is inherited as an X-linked trait. HCFC1 encodes a chromatin-associated protein playing a role in the transcriptional regulation of gene expression. Impaired function of HCFC1 results in dysregulated expression of MMACHC (cblC) and likely genes encoding components of the glycine cleavage complex, which may explain biochemical findings and intractable epilepsy observed in these patients.
Patients with combined methylmalonic aciduria and homocystinuria are treated with large doses of cobalamin (up to 0.35 mg/kg/24 h, up to the maximum dose of 30 mg/24 h) administered subcutaneously or intramuscularly. Higher doses require concentrated forms of OHCbl to reduce the volume. Hydroxycobalamin is recommended over more widely available CNCbl; this may be true for other cobalamin disorders as well. Patients can tolerate the recommended daily allowance of protein. Medical foods deficient in methionine are not recommended in these patients. Carnitine supplementation is used to treat secondary carnitine deficiency. Supplementation with betaine, which supports conversion of homocysteine to methionine by the liver enzyme betaine-homocysteine methyltransferase, has proved effective in patients with homocystinuria. In patients with persistent hyperhomocysteinemia, attention should be given to the management of risk factors predisposing to hypercoagulability.
INBORN ERRORS OF COBALAMIN UPTAKE AND TRANSPORT
Combined methylmalonic aciduria and homocystinuria is also seen in patients with defects affecting intestinal uptake of cobalamin, its transport in the serum, and cellular uptake. Decreased uptake occurs in individuals with intrinsic factor deficiency (OMIM no. 261000), caused by mutations in the GIF gene on chromosome 11q12.1, and in those with Imerslund-Gräsbeck syndrome (IGS; OMIM no. 261100). The latter disorder is the result of dysfunction of the vitamin B12 receptor in the distal ileum, which can be caused by mutations at the CUBN gene on chromosome 10p13 or the AMN gene on chromosome 14q32, which encode cubilin and amnionless, the respective components of the cubam complex. Additional IGS families are not linked to either chromosome and represent further genetic heterogeneity in this disorder. Patients with these disorders usually present between 1 and 5 years of age with decreased serum cobalamin levels, megaloblastic anemia, and neurologic impairment. IGS patients often have proteinuria as well since cubilin and amnionless also mediate protein homeostasis in renal tubules. These disorders show an autosomal recessive inheritance. Clusters of IGS have been identified in Finland (CUBN mutations), Norway (AMN mutations), and the eastern Mediterranean (both genes). In the absence of readily available clinical tests to assess cobalamin uptake, specific diagnosis in these patients depends on molecular analysis of the GIF, CUBN, and AMN genes. Intrinsic factor deficiency and IGS require life-long treatment with parenteral hydroxocobalamin.
Patients with transcobalamin II deficiency (OMIM no. 275350) usually present in the first months of life with megaloblastic anemia, failure to thrive, weakness, and, frequently, immunologic dysfunction. Without treatment, neurologic impairment develops. Most cobalamin circulating in blood is bound to transcobalamin II. Cobalamin bound to transcobalamin II is not available for uptake by most types of cells; rather, transcobalamin II is the primary transporter of vitamin B12 to peripheral tissues. Thus, total serum cobalamin levels may be normal, while the unsaturated vitamin B12 binding capacity is reduced. The disorder is inherited as autosomal recessive trait. Mutations in the TCN2 gene on chromosome 22q12 have been identified in a small number of patients. Treatment involves maintaining very high serum levels of cobalamin by intramuscular route.
The uptake of cobalamin bound to transcobalamin II is mediated by the transcobalamin receptor (CD320, OMIM no. 606475). Known patients were identified on the newborn screen based on elevated propionylcarnitine (C3) in blood spots. Patients tend to have mildly elevated methylmalonic acidemia with normal total plasma homocysteine at birth and readily respond to hydroxocobalamin injections. Plasma vitamin B12 levels can be elevated even without cobalamin supplementation. Studies of cellular vitamin B12 metabolism using fibroblasts reveal normal propionate and methyltetrahydrofolate metabolism, normal conversion of cyanocobalamin to adenosylcobalamin and methylcobalamin, but reduced cobalamin binding and uptake. Methylmalonic acidemia appears to be transient; however, the long-term outcome and natural history of this condition are poorly understood.
The folates are a class of molecules comprising folic acid and its derivatives. Biologically active folates consist of folic acid reduced to a tetrahydrofolate (THF) derivative through the attachment of 1-carbon groups to N5 and N10 of the pterin ring, and by addition of up to 6 additional glutamate residues by γ-peptide linkage. Folate is synthesized by many types of bacteria and is obtained in the diet from both plant and animal foods.
Within cells, folates accept 1-carbon units from various sources, primarily serine (via the reaction catalyzed by serine hydroxymethyltransferase) but also from glycine (the glycine cleavage system), formate (formyl-THF synthetase), and histidine (breakdown mediated by glutamate formiminotransferase and formiminotetrahydrofolate cyclodeaminase). Several reactions utilize 1-carbon units derived from folates (Fig. 142-2). Methyl-THF is required for methylation of homocysteine to form methionine, catalyzed by methionine synthase (described in the section on vitamin B12; see also Chapter 133). Catalytic conversion of uridylate to thymidylate requires 5,10-methylene-THF for the activity of thymidylate synthase. Carbons 2 and 8 of the purine ring are added by the folate-dependent enzymes GAR transformylase and AICAR transformylase, both of which use 10-formyl-THF as a 1-carbon donor. Thus, synthesis of both purines and 1 of 2 pyrimidines required for DNA synthesis is dependent on folates.
Metabolic pathway of folate. The steps affected in known inborn errors of cobalamin metabolism are shown by red bars. Note that the cblE and cblG disorders were discussed earlier in the section on inborn errors of cobalamin. AICAR, amino-4-imidazolecarboxamide ribonucleotide; dTMP, thymidylate; dUMP, deoxyuridylate; FAICAR, formamido-4-imidazolecarboxamide ribonucleotide; FGAR, formylglycinamide ribonucleotide; GAR, glycinamide ribonucleotide; THF, tetrahydrofolate.
Interconversion of 1-carbon substituted folates is an important aspect of cellular metabolism. 5,10-Methylene-THF can be reduced to 5-methyl-THF by the enzyme methylene-THF reductase (MTHFR). Alternatively, it can be oxidized to form 10-formyl-THF by sequential reactions catalyzed by methylene-THF dehydrogenase and methenyl-THF cyclohydrolase. The 2 activities are catalyzed by a single multifunctional protein called C1-THF synthase (OMIM no. 172460). The reaction catalyzed by MTHFR is irreversible under physiologic conditions. This means that when activity of methionine synthase, the only reaction that utilizes methyl-THF, is impaired (as in cobalamin deficiency or in the cblC, cblD, cblF, cblJ, cblE, and cblG disorders), cellular folate is trapped in its methylated form, resulting in deficiency of other folate coenzyme forms.
The most common inborn error of folate metabolism is MTHFR deficiency (OMIM no. 236250) due to mutations of the MTHFR gene on chromosome 1p36.22 (see Chapter 133). Decreased MTHFR activity results in deficiency of methyl-THF, the source of the methyl group used by methionine synthase. Thus, biochemically, MTHFR deficiency is characterized by hyperhomocysteinemia and homocystinuria with hypomethioninemia. Clinical presentation varies, but patients usually show symptoms during infancy or early childhood that include feeding difficulties, lethargy, hypotonia, developmental delay, and seizures. Cerebral atrophy and demyelination are often present. Patients with later onset may have intellectual disability, ataxia, or psychiatric problems. Unlike patients with inborn errors of cobalamin metabolism, in which methionine synthase activity is impaired, patients with MTHFR deficiency do not have megaloblastic anemia. Cultured fibroblasts have decreased incorporation of label from [14C]formate into cellular macromolecules, but incorporation of label from [14C]methyl-THF is normal. Diagnosis depends on molecular DNA analysis revealing deleterious mutations or enzyme assay of cell extracts demonstrating reduced MTHFR-specific activity.
MTHFR deficiency is inherited as an autosomal recessive trait. Almost all mutations have been restricted to 1 or 2 families—an exception is a c.1129C>T mutation that is present at a high frequency among the Old Order Amish.
Patients with MTHFR deficiency have been treated with a variety of agents, including methyl-THF and other folates, methionine, pyridoxine, cobalamin, carnitine, betaine, and riboflavin, individually and in various combinations. Of these, betaine appears to be the most effective. Treatment is most successful when the disease is diagnosed at an early stage, before irreversible neurologic damage occurs.
MTHFR deficiency needs to be differentiated from common polymorphism in the MTHFR gene Common polymorphisms of the MTHFR gene result in decreased enzyme activity less severe than that seen in patients with severe MTHFR deficiency. The most well studied of these is a c.665C>T (formerly c.677C>T) variant that results in “thermolability” of the enzyme. Homozygosity for the c.665C>T variant is associated with increased serum homocysteine levels in individuals with low folate intake, but the clinical significance of this variant is doubtful.
GLUTAMATE FORMIMINOTRANSFERASE DEFICIENCY
A small number of patients have been identified who have deficiency of the bifunctional enzyme that catalyzes the transfer of the formimino group from formiminoglutamate (FIGLU), generated during histidine catabolism, to THF to form 5-formimidoyl-THF, which is then converted to 10-formyl-THF. Patients with glutamate formiminotransferase deficiency (OMIM no. 229100) are characterized by elevated FIGLU levels either constitutively or in response to a histidine load. Because the affected enzyme is expressed only in liver and kidney, using enzyme assay to confirm the diagnosis is performed using molecular DNA analysis. Two classes of patients have been reported: one was characterized by intellectual disability, physical retardation, and cortical atrophy, while the second showed no intellectual disability but massive FIGLU excretion. Therefore, it has been suggested that the severe manifestations in the first group of patients were the result of ascertainment bias.
The gene encoding the bifunctional enzyme, FTCD, has been identified on chromosome 21q22.3, and causative mutations have been identified in several patients. It remains unclear whether there is any clinical phenotype beyond FIGLU excretion in this disorder. The disorder is inherited as autosomal recessive trait and is amenable to newborn screening.
HEREDITARY FOLATE MALABSORPTION
Mutations in a proton-coupled folate transporter result in reduced intestinal folate uptake (OMIM no. 229050), impaired transport across the blood-brain barrier, and ultimately intracellular folate deficiency. Therefore, presentation typically occurs in the first months of life with diarrhea, failure to thrive, developmental delay, megaloblastic anemia, and neurologic deterioration. Serum and cerebrospinal fluid folate levels are reduced. Patients with this autosomal recessive disorder have mutations in the SLC46A1 gene on chromosome 17q11.2. Optimal treatment involves maintaining adequate levels of folate in the central nervous system to avoid neurologic impairment; the appropriate level must be determined for each patient individually and may involve administration of very large doses of folate.
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