rs606231425 — MMACHC

MMACHC p.Gly155Glu — A Rare but Serious Cobalamin Processing Defect

MMACHC ¹¹ Methylmalonic aciduria and homocystinuria type C protein — the enzyme that unlocks vitamin B12 for cellular use is the gatekeeper of intracellular cobalamin metabolism. Dietary vitamin B12 arrives in cells in chemically inert forms (cyanocobalamin, hydroxocobalamin) that must be processed — decyanated and dethiolated — before they can be converted into the two active cofactors the body needs: adenosylcobalamin (for breaking down branched-chain amino acids and odd-chain fatty acids) and methylcobalamin (for the methionine synthase reaction that regenerates methionine and keeps homocysteine in check). MMACHC performs this processing step. When it fails, both downstream cofactors are depleted simultaneously, causing a dual biochemical crisis: methylmalonic acidemia and homocystinemia together.

The Mechanism

The p.Gly155Glu substitution replaces a small, flexible glycine residue at position 155 of MMACHC with the bulkier, charged glutamic acid. Position 155 lies within the TonB-like domain ²² A structural domain in MMACHC that interacts with cobalamin and facilitates its initial processing of the protein. The glycine-to-glutamate change is predicted to disrupt local protein folding and impair cobalamin binding and processing activity. In the single homozygous case reported, the child had plasma homocysteine of 123 µmol/L (normal <15 µmol/L) and methylmalonic acid of 14,424 nmol/L (normal <270 nmol/L) — a pattern consistent with complete functional loss of MMACHC.

The Evidence

The G155E variant was documented in a 2013 case series³³ 2013 case series
Kömhoff M et al. Combined pulmonary hypertension and renal thrombotic microangiopathy in cobalamin C deficiency. Pediatrics 2013 by Kömhoff et al., which reported five children with cblC deficiency presenting with the rare combination of pulmonary arterial hypertension and renal thrombotic microangiopathy. A Dutch child homozygous for c.464G>A (p.Gly155Glu) presented at 2.5 years of age and died from right ventricular failure two weeks after diagnosis despite treatment. ClinVar classifies this variant as pathogenic (RCV000148300), based on this single submission from OMIM (0-star review, reflecting a single published case report).

For heterozygous carriers, a mouse model study⁴⁴ mouse model study
Moreno-Garcia MA et al. The Mmachc gene is required for pre-implantation embryogenesis in the mouse. Mol Genet Metab 2014 demonstrated that animals with one functional copy of Mmachc show 50% protein reduction accompanied by significantly elevated homocysteine and methylmalonic acid compared to wild-type littermates. Human carriers are described as clinically asymptomatic by GeneReviews, but the mouse data suggests subclinical metabolic perturbation is biologically plausible and warrants monitoring attention.

CblC disease overall occurs at approximately 1:100,000–200,000 births⁵⁵ approximately 1:100,000–200,000 births. This specific G155E allele is one of many rare pathogenic MMACHC variants and is individually extremely uncommon, documented only in a single family. It is not present in gnomAD or other large population databases. Chip-based genotyping arrays (23andMe v3/v4/v5) do not include this variant; it is only detected by whole-exome or whole-genome sequencing.

Practical Actions

For heterozygous carriers: current evidence does not establish clinical disease in carriers, but the mouse model data supports a precautionary approach of ensuring optimal B12 status. Hydroxocobalamin is the preferred form because it enters the MMACHC processing pathway directly; cyanocobalamin requires additional decyanation steps. Monitoring serum homocysteine and methylmalonic acid provides an objective readout of functional cobalamin adequacy.

For homozygous individuals: this is a medical emergency requiring specialist management. The 2017 European guidelines⁶⁶ 2017 European guidelines
Huemer M et al. Guidelines for diagnosis and management of cblC and related remethylation disorders. J Inherit Metab Dis 2017 recommend parenteral hydroxocobalamin (1 mg/day, titrated to metabolic response) as the cornerstone of treatment, with oral betaine (250 mg/kg/day) to support remethylation via the betaine-homocysteine methyltransferase ⁷⁷ BHMT — an alternative enzyme that converts betaine + homocysteine → dimethylglycine + methionine, bypassing the MMACHC-dependent pathway pathway.

Interactions

MMACHC acts upstream of two critical enzymes: methylmalonyl-CoA mutase (which requires adenosylcobalamin) and methionine synthase (MTR, rs1805087, which requires methylcobalamin). Any MMACHC dysfunction compounds with MTR and MTRR ⁸⁸ MTRR rs1801394 encodes methionine synthase reductase, which reactivates MTR by remethylating its cobalamin cofactor variants that already reduce methionine synthase activity — the combined effect on methylation capacity and homocysteine levels will exceed either variant alone. Similarly, MTHFR (rs1801133) variants that reduce methylfolate supply will aggravate the homocysteine accumulation driven by impaired MMACHC function.