A number sign (#) is used with this entry because of evidence that homocystinuria-megaloblastic anemia cblD type (HMAD) is caused by homozygous or compound heterozygous mutation in the MMADHC gene (611935) on chromosome 2q23.

Biallelic mutation in the MMADHC gene can also cause isolated methylmalonic aciduria cblD type (MACD; 620953) and combined methylmalonic aciduria and homocystinuria cblD type (MAHCD; 277410), depending on the location of the mutation within the gene.

Homocystinuria-megaloblastic anemia cblD type (HMAD) is an autosomal recessive metabolic disorder of cobalamin (cbl; vitamin B12) metabolism. Affected individuals present in infancy or early childhood with neurologic abnormalities, including developmental delay, impaired gross motor skills, dystonia or spastic ataxia, poor speech, and cerebral or cerebellar atrophy on brain imaging. Additional more variable features may include nystagmus, poor eye contact, hypotonia, and seizures. Laboratory studies show elevated plasma homocystine, low methionine, and megaloblastic anemia; methylmalonic acid levels are normal. Treatment with betaine, folic acid, and OH-cobalamin results in some improvement of the clinical and biochemical abnormalities (Suormala et al., 2004).

See also HMAE (236270) and HMAG (250940).

Suormala et al. (2004) reported 2 unrelated children with isolated homocystinuria belonging to complementation group cblD, which the authors termed 'cblD variant 1.' The first child (P1), born from a consanguineous Irish family, presented at age 6 years with severe neurologic abnormalities, including global developmental delay, spastic ataxia, and no vocal skills or eye contact. Brain imaging showed cerebral and cerebellar atrophy. The second patient (P2), born of unrelated Italian parents, presented at age 3 months with severe hypotonia, nystagmus, dystonia, and seizures. Brain imaging showed reduced myelination and small cerebellar vermis. Laboratory studies in both showed elevated plasma homocystine, decreased plasma methionine, normal methylmalonic acid, megaloblastic red cells with low-normal Hb or anemia, normal or mildly reduced B12, and normal or increased folate. Both patients responded well to betaine, folic acid, and OH-cobalamin treatment, but remained neurologically impaired. At 16 years of age, P1 showed some neurologic improvement, but had severely impaired intellectual development with behavioral abnormalities and needed help with daily activities. At 4 years of age, P2 had normal gross motor and fine motor skills and brain imaging, residual mild speech delay, and an IQ of 84. In vitro studies of cells from the 2 patients showed reduced formation of methionine, indicating decreased activity of methionine synthase, and deficient MeCbl synthesis. Total cobalamin uptake was normal. Enzyme activity responded to addition of MeCbl. Cell lines from these patients complemented cblC, cblE, and cblG reference cells, but not cblD. Molecular analysis excluded cblE (236270) and cblG (250940).

The transmission pattern of HMAD in the families reported by Suormala et al. (2004) was consistent with autosomal recessive inheritance.

By complementation of cblD patient cells with somatic cell hybrids, Coelho et al. (2008) localized the defect to human chromosome 2. Fine mapping identified a 10.2-Mb regions on 2q22.1-2q23.3 between markers D2S150 and D2S2324.

In 2 unrelated patients (P1 and P2 in Suormala et al., 2004), with HMAD, 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 and caused the phenotype.

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 (HMAD) phenotype were unable to rescue MeCbl synthesis, whereas mutant alleles associated with the cblD-methylmalonic aciduria (MACD) phenotype could restore MeCbl synthesis. In combined cblD-MMA/HC cells (MAHCD), improving mitochondrial targeting of MMADHC increased the formation of AdoCbl with a concomitant decrease in MeCbl formation. In cblD-MACD 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 belonging to complementation group D: combined MMA/HC, isolated MMA, or isolated HC.