Alternative titles; symbols
HGNC Approved Gene Symbol: MMADHC
Cytogenetic location: 2q23.2 Genomic coordinates (GRCh38) : 2:149,569,637-149,587,775 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 2q23.2 | Homocystinuria-megaloblastic anemia, cblD type | 620952 | Autosomal recessive | 3 |
| Methylmalonic aciduria and homocystinuria, cblD type | 277410 | Autosomal recessive | 3 | |
| Methylmalonic aciduria, cblD type | 620953 | Autosomal recessive | 3 |
Vitamin B12 (cobalamin) is an essential cofactor in several metabolic pathways. Intracellular conversion of cobalamin to adenosylcobalamin in mitochondria and to methylcobalamin in cytoplasm is necessary for homeostasis of methylmalonic acid and homocysteine. The MMADHC gene encodes a protein involved in an early step of cobalamin metabolism (Coelho et al., 2008).
By RT-PCR of fibroblast mRNA, Coelho et al. (2008) obtained a full-length cDNA for C2ORF25. The deduced 296-amino acid protein has a calculated molecular mass of 32.8 kD. It has an N-terminal mitochondrial leader sequence and a putative vitamin B12-binding motif, NxHxxG, at residues 81 through 86. EST database analysis suggested that C2ORF25 is highly expressed in most tissues.
Coelho et al. (2008) determined that the C2ORF25 gene contains 8 exons and spans about 18 kb.
By genomic sequence analysis, Coelho et al. (2008) mapped the C2ORF25 gene to chromosome 2q23.2.
Homocystinuria-Megaloblastic Anemia, cblD Type
In 2 unrelated patients (P1 and P2 in Suormala et al., 2004) with homocystinuria-megaloblastic anemia belonging to complementation group cblD (HMAD; 620952), Coelho et al. (2008) identified biallelic missense mutations in the MMADHC gene: P1, born of consanguineous Irish parents, was homozygous for L259P (611935.0001), and P2, born of unrelated Italian parents, was compound heterozygous for T182N (611935.0002) and Y249C (611935.0003). Parental DNA was not available for segregation studies in either family. In vitro functional expression studies showed that wildtype MMADHC rescued the mutant cellular phenotype, but constructs containing the missense alleles did not restore methionine or methylcobalamin synthesis in cblD-homocystinuria or cblD-combined cells, indicating that the mutations were loss-of-function alleles affecting methylcobalamin synthesis, but remained intact with function for adenosylcobalamin synthesis.
Methylmalonic Aciduria, cblD Type
In an Indian boy (P3 in Suormala et al., 2004) and an unrelated Haitian patient (Cooper et al., 1990) with isolated methylmalonic aciduria complementation group cblD (MACD; 620953), Coelho et al. (2008) identified biallelic mutations in the MMADHC gene (611935.0004-611935.0006). The Indian boy was homozygous for a frameshift mutation (57_64del; 611935.0004), predicted to result in premature termination (Cys19fsTer20) near the N terminus, thus lacking the mitochondrial leader sequence and resulting in MMA. The Haitian patient was compound heterozygous for a nonsense mutation (R54X; 611935.0005) and an in-frame duplication (Leu103_Ser108dup; 611935.0006). Parental DNA was not available for familial segregation studies for either patient. In vitro complementation studies showed that 2 of the mutations (611935.0004 and 611935.0005) rescued the synthesis of methylcobalamin in isolated homocystinuria cells. Coelho et al. (2008) speculated that reinitiation of translation could occur at a downstream initiation codon (Met62), producing a shorter semi-functional protein with normal methylcobalamin synthesis and absence of homocystinuria.
Methylmalonic Aciduria and Homocystinuria, cblD Type
In 3 unrelated patients with combined homocystinuria and methylmalonic aciduria (MAHCD; 277410), Coelho et al. (2008) identified homozygous mutations in the MMADHC gene: P5 (previously reported by Goodman et al., 1970) was a Spanish-American boy with a homozygous nonsense mutation (R250X; 611935.0007); P6 was a Scandinavian girl with a homozygous frameshift (c.419dupA; 611935.0008); and P7 was an Italian boy with a homozygous splice site mutation resulting in the skipping of exon 7 and an in-frame deletion (611935.0009). All mutations occurred close to the C terminus. In vitro functional expression studies showed that wildtype MMADHC could rescue the biochemical abnormalities of cells carrying these mutations. Studies of patient cells were not performed, but all 3 were predicted to result in a defective protein that lacked both functional domains or was subject to nonsense-mediated mRNA decay and a loss of function.
Stucki et al. (2012) studied the effect of various MMADHC constructs on protein function in cell lines. For example, mutant alleles associated with the cblD-homocystinuria (HC) phenotype were unable to rescue MeCbl synthesis, whereas mutant alleles associated with the cblD-methylmalonic aciduria (MMA) phenotype could restore MeCbl synthesis. In combined cblD-MMA/HC cells, improving mitochondrial targeting of MMADHC increased the formation of AdoCbl with a concomitant decrease in MeCbl formation. In cblD-MMA cells, this effect was dependent on the mutation and showed a negative correlation with endogenous MMADHC mRNA levels. The findings supported the hypothesis that the MMADHC protein contains various domains for targeting the protein towards the mitochondria, MeCbl synthesis, and AdoCbl synthesis. There is a delicate balance between cytosolic MeCbl and mitochondrial AdoCbl synthesis, suggesting that the cblD protein is a branch point in intracellular cobalamin trafficking. Detailed data analysis indicated that the sequence after Met116 is sufficient for MeCbl synthesis, whereas the additional sequence between Met62 and Met116 is required for AdoCbl synthesis. The nature and location of mutations within the protein thus determines 1 of the 3 biochemical phenotypes, combined MMA/HC, isolated MMA, or isolated HC.
The Human Gene Nomenclature Committee designated this gene MMADHC (methylmalonic aciduria (cobalamin deficiency) cblD type, with homocystinuria).
In an Irish boy, born of consanguineous parents (P1 in Suormala et al., 2004), with homocystinuria-megaloblastic anemia belonging to complementation group cblD (HMAD; 620952), Coelho et al. (2008) identified a homozygous c.776T-C transition in exon 8 of the MMADHC gene, resulting in a leu259-to-pro (L259P) substitution at a highly conserved residue near the C terminus. The mutation was not identified in 100 ethnically matched control chromosomes. Parental DNA was not available for familial segregation studies. In vitro functional expression studies showed that wildtype MMADHC rescued the mutant cellular phenotype, but constructs containing the missense allele did not restore methionine or methylcobalamin synthesis in cblD-homocystinuria or cblD-combined cells, indicating that the mutation was a loss-of-function allele and caused the phenotype. The patient had developmental delay, spastic ataxia, delayed visual evoked potentials, and increased mean corpuscular volume.
In an Italian boy (P2 in Suormala et al., 2004) with homocystinuria-megaloblastic anemia belonging to complementation group cblD (HMAD; 620952) Coelho et al. (2008) identified compound heterozygosity for 2 mutations in the MMADHC gene: a c.545C-A transversion in exon 6, resulting in a thr182-to-asn (T182N) substitution, and a c.746A-G transition in exon 8, resulting in a tyr249-to-cys (Y249C; 611935.0003) substitution. Both mutations occurred at highly conserved residues near the C terminus, and neither was identified in 100 ethnically matched control chromosomes. Parental DNA was not available for familial segregation studies. In vitro functional expression studies showed that wildtype MMADHC rescued the mutant cellular phenotype, but constructs containing the missense alleles did not restore methionine or methylcobalamin synthesis in cblD-homocystinuria or cblD-combined cells, indicating that the mutations were loss-of-function alleles and caused the phenotype. The patient was diagnosed at age 3 months and showed hypotonia, nystagmus, dystonia, seizures, and megaloblastic anemia.
For discussion of the c.746A-G transition in the MMADHC gene, resulting in a tyr249-to-cys (Y249C) substitution, that was found in compound heterozygous state in a patient with homocystinuria-megaloblastic anemia belonging to complementation group cblD (HMAD; 620952) by Coelho et al. (2008), see 611935.0002.
In an Indian boy (P3 in Suormala et al., 2004) with isolated methylmalonic aciduria complementation group cblD (MACD; 620953), Coelho et al. (2008) identified a homozygous 8-bp deletion (57delCTCTTTAG) in exon 3 of the MMADHC gene, resulting in a frameshift and premature termination at codon 20 (Cys19fsTer20) near the N terminus after the mitochondrial leader sequence. Coelho et al. (2008) postulated that reinitiation of translation could occur at a downstream initiation codon (Met62), producing a shorter semi-functional protein with normal methylcobalamin synthesis and absence of homocystinuria. Parental DNA was not available for familial segregation studies. The patient was born prematurely at 32 weeks' gestation and showed severe respiratory distress syndrome, necrotizing enterocolitis, and seizures.
In a Haitian boy (P4) previously reported by Cooper et al. (1990) with isolated methylmalonic aciduria type cblD (MACD; 620953), Coelho et al. (2008) identified compound heterozygosity for 2 mutations in exon 4 of the MMADHC gene: a c.160C-T transition, resulting in an arg54-to-ter (R54X) substitution, and a 18-bp in-frame duplication (c.307_324dup; 611935.0006), resulting in the duplication of residues Leu103 to Ser108, closer to the N terminus. Parental DNA was not available for familiar segregation studies. The patient was diagnosed at age 11 months, and had severe ketotic coma, dehydration, hyperammonemia, leukopenia, and thrombocytopenia.
For discussion of the 18-bp duplication in the MMADHC gene that was found in compound heterozygous state in a patient with isolated methylmalonic aciduria type cblD (MACD; 620953) by Coelho et al. (2008), see 611935.0005.
In a Spanish American boy (P5), born of consanguineous parents, with combined methylmalonic aciduria and homocystinuria complementation group cblD (MAHCD; 277410) originally reported by Goodman et al. (1970), Coelho et al. (2008) identified a homozygous c.748C-T transition in exon 8 of the MMADHC gene, resulting in an arg250-to-ter (R250X) substitution. The boy had an acute psychotic episode, marfanoid appearance, nystagmus, mildly impaired intellectual development, and increased mean corpuscular volume.
In a Scandinavian girl (P6) with combined methylmalonic aciduria and homocystinuria complementation group cblD (MAHCD; 277410), Coelho et al. (2008) identified a homozygous 1-bp duplication (c.419dupA) in exon 5 of the MMADHC gene, resulting in a tyr140-to-ter (Y140X) substitution. Parental DNA was not available for familial segregation studies. She presented in early infancy with developmental delay, seizures, and megaloblastic anemia.
In a boy (P7), born of consanguineous Italian parents, with combined methylmalonic aciduria and homocystinuria complementation group cblD (MAHCD; 277410), Coelho et al. (2008) identified a homozygous 4-bp deletion (c.696+1delGTGA) in intron 7 of the MMADHC gene, resulting in the skipping of exon 7 and an in-frame deletion (Phe204Ala232del). Each unaffected parent was heterozygous for the mutation. The patient was diagnosed at age 22 days and showed poor feeding, encephalopathy, seizures, and increased mean corpuscular volume.
Coelho, D., Suormala, T., Stucki, M., Lerner-Ellis, J. P., Rosenblatt, D. S., Newbold, R. F., Baumgartner, M. R., Fowler, B. Gene identification for the cblD defect of vitamin B12 metabolism. New Eng. J. Med. 358: 1454-1464, 2008. [PubMed: 18385497] [Full Text: https://doi.org/10.1056/NEJMoa072200]
Cooper, B. A., Rosenblatt, D. S., Watkins, D. Methylmalonic aciduria due to a new defect in adenosylcobalamin accumulation by cells. Am. J. Hemat. 34: 115-120, 1990. [PubMed: 2339678] [Full Text: https://doi.org/10.1002/ajh.2830340207]
Goodman, S. I., Moe, P. G., Hammond, K. B., Mudd, S. H., Uhlendorf, B. W. Homocystinuria with methylmalonic aciduria: two cases in a sibship. Biochem. Med. 4: 500-515, 1970. [PubMed: 5524089] [Full Text: https://doi.org/10.1016/0006-2944(70)90080-3]
Stucki, M., Coelho, D., Suormala, T., Burda, P., Fowler, B., Baumgartner, M. R. Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism. Hum. Molec. Genet. 21: 1410-1418, 2012. [PubMed: 22156578] [Full Text: https://doi.org/10.1093/hmg/ddr579]
Suormala, T., Baumgartner, M. R., Coelho, D., Zavadakova, P., Kozich, V., Koch, H. G., Berghauser, M., Wraith, J. E., Burlina, A., Sewell, A., Herwig, J., Fowler, B. The cblD defect causes either isolated or combined deficiency of methylcobalamin and adenosylcobalamin synthesis. J. Biol. Chem. 279: 42742-42749, 2004. [PubMed: 15292234] [Full Text: https://doi.org/10.1074/jbc.M407733200]