Methylmalonic Acidemia, Cobalamin Disorders · MMA-Cbl

Description

Methylmalonic acidemia, cobalamin disorders (MMA-Cbl) is an autosomal recessive group of organic acid disorders caused by defects in the intracellular processing of vitamin B12 to its active coenzyme form, adenosylcobalamin. The MMAA gene encodes a mitochondrial protein required for adenosylcobalamin synthesis (cblA complementation group). MMAB encodes cob(I)alamin adenosyltransferase, the enzyme that completes adenosylcobalamin assembly (cblB). MMADHC encodes a cobalamin-trafficking protein; biallelic MMADHC variants in the C-terminal region produce the cblD-MMA variant with isolated methylmalonic acidemia. Without adenosylcobalamin, methylmalonyl-CoA mutase activity collapses, methylmalonic acid accumulates, and the same organic-acidemia cascade follows as in MMA-mut.

Affected infants present with feeding intolerance, vomiting, lethargy, hypotonia, and metabolic acidosis, often in the first weeks to months of life. The cblA group typically responds well to pharmacologic hydroxocobalamin and has the most favorable long-term outlook within MMA. The cblB group responds less reliably. Long-term complications across responsive and partially responsive children include developmental delay, basal-ganglia injury, and chronic kidney disease, although the trajectory is generally milder than MMUT-related disease when cobalamin response is robust.

Detection is by newborn screening on the dried blood spot using tandem mass spectrometry, which flags elevated propionylcarnitine (C3). Confirmation uses urine organic acids, plasma acylcarnitines, total plasma homocysteine to separate isolated MMA from combined MMA with homocystinuria (the cblC, cblD-combined, and cblF groups, caused by other genes), cobalamin-responsiveness testing, complementation analysis where available, and sequencing of MMAA, MMAB, and MMADHC. Combined incidence of isolated MMA across all causes is roughly 1 in 50,000 to 1 in 100,000 live births. Within that total, the cobalamin-responsive groups account for a substantial subset, with cblA the most common of the three.

Treatments to date

Standard of care is lifelong intramuscular hydroxocobalamin in pharmacologic doses, paired with a moderately protein-restricted diet using medical formula free of isoleucine, valine, methionine, and threonine when needed for biochemical control, calorie support, and L-carnitine supplementation. Children with strong cobalamin response on cblA can sometimes maintain control on hydroxocobalamin and modest protein restriction. Sick-day protocols and emergency management for metabolic decompensation follow the same template used in MMA-mut, with intravenous dextrose, ammonia scavengers, and dialysis for severe acidosis or hyperammonemia.

No FDA-approved disease-specific small-molecule or biologic therapy exists for the cobalamin disorders beyond hydroxocobalamin itself. Liver or combined liver-kidney transplantation has been performed in selected children with refractory disease, primarily within the cblB group, with the same caveat that residual extrahepatic disease persists. Gene therapy and mRNA-replacement programs for isolated MMA have focused on MMUT to date; the cobalamin-disorder genes have not been the lead targets in industry pipelines, although academic preclinical work on MMAA and MMAB exists.