Methylmalonic acidemia results from inherited pathogenic variants in genes encoding enzymes and cofactors involved in mitochondrial propionyl-CoA metabolism. Documented causative genes include MMUT, which encodes methylmalonyl-CoA mutase (mut0 and mut− subtypes); MMAB, involved in adenosylcobalamin synthesis (cblB subtype); MMAA, associated with the cblA subtype; MMADHC, responsible for the cblD-MMA subtype; and MCEE, encoding methylmalonyl-CoA epimerase. Most individuals with isolated MMA are compound heterozygotes carrying two different pathogenic variants. Precise genotype-phenotype correlations are difficult to establish given the prevalence of compound heterozygosity and the limited recurrence of individual variants across the population. The majority of subtypes follow an autosomal recessive inheritance pattern.

Methylmalonic acidemia is frequently identified through newborn screening programs using measurement of propionylcarnitine (C3) in dried blood spots. Elevated C3 values above laboratory-specific cutoffs prompt follow-up biochemical testing. Confirmatory workup typically includes measurement of plasma and urine methylmalonic acid concentrations, plasma amino acids, urine organic acids, and acylcarnitine profile. Additional assessments may include plasma ammonia, blood gas analysis, and serum vitamin B12 and homocysteine levels. Differentiation from related metabolic conditions—including propionic acidemia and combined methylmalonic acidemia with homocystinuria—relies on biochemical patterns and genetic sequencing of the relevant causative genes. Vitamin B12 responsiveness testing is performed to identify subtypes that may demonstrate a biochemical response to hydroxocobalamin supplementation. Some infants with the mut0 subtype present with clinical decompensation before newborn screening results become available.

Management of methylmalonic acidemia requires a multidisciplinary approach, typically coordinated through a specialized metabolic center. No treatments are specifically FDA-approved for MMA as an indication. Multiple investigational agents have received orphan drug designation, including hydroxocobalamin-based formulations, 2,2-dimethylbutanoic acid, and several gene therapy vectors targeting the MMUT and MMAB genes (including AAV9-MMUT and lentiviral vector approaches developed by academic and commercial sponsors). For individuals with B12-responsive subtypes, particularly cblA and certain cblB cases, intramuscular hydroxocobalamin constitutes a central element of ongoing metabolic management. Dietary approaches involve restriction of propiogenic amino acid precursors—isoleucine, valine, methionine, and threonine—balanced against sufficient caloric and protein intake for growth. Specialized metabolic formulas are used to supplement nutrition within these constraints. L-carnitine supplementation addresses secondary carnitine deficiency documented in affected individuals. Metronidazole is employed periodically in some individuals to reduce propionate generation by intestinal flora. Gastrostomy or jejunostomy has been utilized in cases with significant feeding difficulties. Liver transplantation has been undertaken in severe cases to reduce hepatic metabolic burden, though renal and extrahepatic complications may persist after transplantation. Fasting, physiologic stress, increased dietary protein intake, and nephrotoxic medications have been documented in the clinical management literature as circumstances and agents associated with adverse metabolic outcomes in MMA.

Prognosis in methylmalonic acidemia is strongly influenced by enzymatic subtype and timing of diagnosis. The mut0 and cblB subtypes are associated with the highest rates of mortality and neurologic complications. Renal disease has been documented in approximately 43–60% of individuals with the mut0 subtype, with a median age of onset between 6 and 11 years, and in approximately 66% of those with the cblB subtype. The cblA subtype, when identified early and managed consistently with injectable hydroxocobalamin, is associated with a milder disease course, slower decline in renal function, and better neurocognitive outcomes compared with the mut0, mut−, and cblB subtypes. Expanded newborn screening has been associated with decreased early mortality, less severe clinical presentation at diagnosis, and lower rates of irreversible cerebral damage in affected individuals. Disease management remains lifelong, with ongoing monitoring of renal function, metabolic stability, and neurodevelopmental progress.

Methylmalonic acidemia has a growing and active research portfolio, with particular focus on gene therapy and advanced genomic interventions. Multiple clinical trials are currently active or recruiting, including studies of liver-targeted lentiviral gene therapy, adeno-associated virus (AAV9) vectors targeting the MMUT gene, mRNA replacement therapy (mRNA-3705 by ModernaTX), and hydroxocobalamin formulations for specific subtypes including the cblC type. Long-term follow-up studies for participants who received investigational gene therapy are ongoing, including a study coordinated by Alexion Pharmaceuticals following administration of hLB-001 gene therapy. Observational cohort studies are examining long-term outcomes with carglumic acid in organic acidemia patients across adult and pediatric populations. A stable isotope 13-C-propionate breath test has been developed as a potential surrogate biomarker of disease severity and treatment response and is under evaluation in specialized centers. A prospective natural history study is recruiting pediatric participants with MMA due to MMUT gene variants. Published research spans gene therapy approaches, biomarker identification, and epidemiological investigations.