ABSTRACT: Cobalamin C deficiency (cblC), the most common inborn error of intracellular cobalamin metabolism, is caused by mutations in MMACHC, a gene responsible for the processing and intracellular trafficking of vitamin B12. This recessive disorder is characterized by a failure to metabolize cobalamin into adenosyl- and methylcobalamin, which results in the biochemical perturbations of methylmalonic acidemia, hyperhomocysteinemia, and hypomethioninemia caused by impaired activity of the downstream enzymes, methylmalonyl-CoA mutase and methionine synthase. Cobalamin C deficiency can be accompanied by a wide spectrum of clinical manifestations, including progressive blindness, and in mice, manifests with very early embryonic lethality. Because zebrafish harbor a full complement of cobalamin metabolic enzymes, we used genome editing to study loss of mmachc function. mmachc homozygotes survived the embryonic period but perished in early juvenile life. The mutants displayed metabolic and clinical features of cblC deficiency including methylmalonic acidemia, severe growth retardation, and lethality. Clinical and metabolic parameters improved when the mutants were raised in water supplemented with small molecules used to treat patients, including hydroxocobalamin, methylcobalamin, methionine, and betaine. Furthermore, mmachc mutants, some bred to express rod and or cone fluorescent reporters, manifested a retinopathy and thin optic nerves. Expression analysis using whole eye mRNA revealed dysregulation of genes involved in phototransduction and cholesterol metabolism. Zebrafish with mmachc deficiency recapitulate the phenotypic and biochemical features of the human disorder, including ocular pathology, and a show a response to established treatments
