Vitamin B12-responsive methylmalonic acidemia

Vitamin B12-responsive methylmalonic acidemia (also known as cobalamin-responsive methylmalonic aciduria or vitamin B12-responsive MMA) is a group of inherited metabolic disorders characterized by the accumulation of methylmalonic acid in the blood and urine due to impaired activity of the enzyme methylmalonyl-CoA mutase, which requires adenosylcobalamin (a form of vitamin B12) as a cofactor. Unlike other forms of methylmalonic acidemia, patients with this subtype show clinical and biochemical improvement when treated with pharmacological doses of vitamin B12 (hydroxocobalamin or cyanocobalamin). The condition encompasses several complementation groups, most notably cblA (due to mutations in the MMAA gene) and cblB (due to mutations in the MMAB gene), which affect the intracellular synthesis or transport of adenosylcobalamin. The disease primarily affects the metabolic and neurological systems. Key clinical features include recurrent episodes of metabolic decompensation with metabolic acidosis, hyperammonemia, lethargy, vomiting, failure to thrive, developmental delay, and hypotonia. These episodes are often triggered by illness, fasting, or high protein intake. If untreated, the condition can lead to progressive neurological damage, renal complications, and potentially life-threatening metabolic crises. Some patients may also develop chronic kidney disease over time. Treatment centers on intramuscular or oral hydroxocobalamin (vitamin B12) supplementation, which can significantly reduce methylmalonic acid levels and improve clinical outcomes, particularly in the cblA subtype. Dietary management with protein restriction is also employed to reduce the metabolic load. Carnitine supplementation may be used to prevent secondary carnitine deficiency. Early diagnosis through newborn screening and prompt initiation of treatment are critical for improving long-term outcomes. The cblA form generally has a better prognosis and greater B12 responsiveness compared to the cblB form, though both benefit from early intervention. Long-term monitoring for renal function and neurological development is essential.

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