Methylmalonic acidemia, mutase deficiency
Mutase deficiency MMA. Same screening signal as PA, distinguished by methylmalonic acid in urine. The renal trajectory is what makes mut-deficiency a different long-term clinical problem.
The most common primary methylmalonic acidemias are caused by deficiency of methylmalonyl-CoA mutase, the enzyme that converts methylmalonyl-CoA to succinyl-CoA. The condition shares the screening signal, the metabolic biology, and most of the management infrastructure with propionic acidemia, but with a distinct mutase deficiency genetic basis, a related but separate cobalamin biology, and a renal complication that PA does not produce. Renal disease is the long-term complication that makes mutase-deficiency MMA a different clinical problem from PA on a long enough timeline.
What MMA-mut is
Methylmalonic acidemia from mutase deficiency, MMA-mut, is an autosomal recessive organic acid disorder caused by variants in MUT on chromosome 6p12. The mutase enzyme requires adenosylcobalamin, the activated form of vitamin B12, as a cofactor. The mutase converts methylmalonyl-CoA to succinyl-CoA, downstream of propionyl-CoA carboxylase. When mutase activity is absent (mut0) or reduced (mut-), methylmalonyl-CoA and methylmalonic acid accumulate, the resulting organic acidemia drives metabolic decompensation, and secondary inhibition of the urea cycle produces hyperammonemia.
The clinical presentation in mut0 disease is severe and resembles classical propionic acidemia: neonatal-onset metabolic crisis with poor feeding, vomiting, lethargy, hypotonia, tachypnea, hyperammonemia, and progression to coma if untreated. mut- disease has a milder course with later presentation in some cases. Cobalamin-responsive forms of MMA, caused by defects in cobalamin metabolism rather than in mutase itself (cblA, cblB, cblC, and others), respond at least partially to high-dose vitamin B12 and have different clinical phenotypes; those forms are covered separately.
The chronic course in mut0 and severe mut- disease includes recurrent metabolic decompensations, failure to thrive, developmental delay, basal-ganglia injury producing movement disorders, optic atrophy, pancreatitis, and progressive renal disease. Chronic kidney disease, often presenting in childhood or adolescence and progressing to dialysis or transplant, is a defining late complication of MMA-mut and distinguishes it clinically from propionic acidemia.
Reported live-birth incidence in US and European newborn screening programs for the combined methylmalonic acidemias runs roughly 1 in 50,000 to 1 in 100,000.
Detection
Newborn screening uses the same C3 and C3/C2 ratio elevation as for propionic acidemia. Second-tier testing distinguishes MMA from PA by urine organic acid analysis: MMA shows methylmalonic acid, methylcitrate, and 3-hydroxypropionate. The cobalamin-responsive forms are distinguished from mutase-deficiency MMA by total plasma homocysteine measurement (elevated in cblC and cblD with combined methylmalonic acidemia and homocystinuria) and by the cobalamin response challenge.
Confirmation uses plasma acylcarnitines, plasma amino acids, urine organic acids, plasma total homocysteine, and MUT sequencing for mutase deficiency. Cobalamin pathway gene panels (MMAA, MMAB, MMACHC, and others) confirm cobalamin-responsive forms.
What management looks like
Standard of care for MMA-mut is lifelong protein-restricted diet using medical formula free of isoleucine, valine, methionine, and threonine, paired with measured natural protein, calorie support, and L-carnitine supplementation. The diet is similar to that used in propionic acidemia. Cobalamin is given to all newly diagnosed cases pending the response challenge that distinguishes mutase deficiency (no clinical or biochemical response) from cobalamin-responsive forms (substantial response).
Acute decompensations are managed with intravenous dextrose, fluid resuscitation, ammonia scavengers, carglumic acid for hyperammonemia, and hemodialysis for severe acidosis or refractory hyperammonemia. Carglumic acid was approved by the FDA in 2021 for adjunctive treatment of acute hyperammonemia in MMA and PA.
Renal surveillance is part of routine follow-up. Serum creatinine, glomerular filtration rate, and urinary protein and methylmalonic acid measurements track renal function over time. Progressive chronic kidney disease frequently develops in MMA-mut, particularly in mut0 disease, and the timing of nephrology referral, dialysis, and transplant becomes part of the long-term clinical course.
Liver transplantation reduces the frequency and severity of metabolic crises, similar to its role in propionic acidemia. Combined liver and kidney transplantation is performed in selected cases with both severe metabolic disease and advanced renal failure. The decision to transplant in MMA-mut is more complex than in PA because the renal trajectory matters. Some centers preserve kidney function with isolated liver transplant earlier in the course; others wait until both organs require replacement.
mRNA-3705, a Moderna program targeting MMA-mut with hepatic delivery of MUT mRNA, entered clinical trials in 2022 with interim safety and efficacy data published in 2024. AAV-delivered gene therapy programs are in earlier stages.
What this looks like for a family
A baby is born and the heel-prick is sent. On day 4, the state lab reports an elevated C3 and C3/C2 ratio. On day 5, urine organic acids show methylmalonic acid and the metabolic team starts protein restriction, carnitine, and cobalamin. The cobalamin challenge over the following days shows no biochemical response. The diagnosis is confirmed as mut0 disease by MUT sequencing.
The neonatal period is managed without the catastrophic crisis that historically defined the disease. The first decade involves recurrent metabolic decompensations during illness, dietary management with a metabolic dietitian, and ongoing developmental and cognitive surveillance. In adolescence, renal function begins to decline. The nephrology team joins the metabolic team in the management plan. The conversations about liver transplant, then about combined liver-kidney transplant, and now about mRNA-based therapy in clinical trials are part of the medical plan from the late teens through young adulthood.
That is what MMA-mut care looks like in practice. The screen prevents the neonatal death. The metabolic management controls the acute course. The renal complication is the long-term clinical anchor, and the field is now experimenting with whether mRNA replacement therapy can change that trajectory.