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We used exome sequencing to identify the genetic basis of combined malonic and methylmalonic aciduria (CMAMMA). We sequenced the exome of an individual with CMAMMA and followed up with sequencing of eight additional affected individuals (cases). This included one individual who was identified and diagnosed by searching an exome database. We identify mutations in ACSF3, encoding a putative methylmalonyl-CoA and malonyl-CoA synthetase as a cause of CMAMMA. We also examined a canine model of CMAMMA, which showed pathogenic mutations in a predicted ACSF3 ortholog. ACSF3 mutant alleles occur with a minor allele frequency of 0.0058 in ∼1,000 control individuals, predicting a CMAMMA population incidence of ∼1:30,000. ACSF3 deficiency is the first human disorder identified as caused by mutations in a gene encoding a member of the acyl-CoA synthetase family, a diverse group of evolutionarily conserved proteins, and may emerge as one of the more common human metabolic disorders.

Methylmalonic acidemia (MMA), an autosomal recessive metabolic disorder, is most often caused by mutations in methylmalonyl-CoA mutase (MUT). Severely affected patients typically present with metabolic crisis in the early neonatal period and can perish despite intervention. Survivors follow an unstable course and can require elective liver transplantation to prevent life-threatening metabolic decompensation. Therapeutic alternatives to liver transplantation such as hepatocyte-directed gene and cell therapies lack experimental validation. We have used a murine model of mut0 MMA to assess the efficacy of virus-mediated gene therapy to rescue the neonatal lethality seen in the Mut(-/-) mice. Affected pups and control littermates received either intramuscular or intrahepatic injections of adenovirus carrying the Mut gene expressed under the control of the cytomegalovirus promoter. All of the Mut(-/-) pups injected via the intramuscular route perished within the first 48 hr of birth. However, more than 50% of the Mut(-/-) pups that received intrahepatic injections survived beyond weaning (day 15). The treated mutants expressed methylmalonyl-CoA mutase mRNA and protein, and displayed decreased metabolite levels compared with uninjected Mut(-/-) mice. The results demonstrate that adenovirus-mediated, hepatic methylmalonyl-CoA mutase expression can rescue Mut(-/-) pups from neonatal mortality and provide proof-of-principle evidence for the efficacy of liver-directed gene delivery in methylmalonic acidemia.

Methylmalonic acidemia (MMAemia) is the biochemical hallmark of a group of genetic metabolic disorders that share a common defect in the ability to convert methylmalonyl-CoA into succinyl-CoA. This disorder is due to either a mutant methylmalonyl-CoA mutase apoenzyme or impaired synthesis of adenosylcobalamin, the cofactor for this enzyme. In this article, we will provide an overview of the pathways disrupted in these disorders, discuss the known metabolic blocks with a particular focus on molecular genetics, and review the use of selected model organisms to study features of methylmalonic acidemia.