The vitamin B12 processing enzyme, mmachc, is essential for zebrafish survival, growth and retinal morphology

Abstract

<jats:title>Abstract</jats:title> <jats:p>Cobalamin C (cblC) deficiency, 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 the 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 the loss of mmachc function and to develop the first viable animal model of cblC deficiency. mmachc mutants survived the embryonic period but perished in early juvenile life. The mutants displayed the metabolic and clinical features of cblC deficiency including methylmalonic acidemia, severe growth retardation and lethality. Morphologic 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 bred to express rod and/or cone fluorescent reporters, manifested a retinopathy and thin optic nerves (ON). Expression analysis using whole eye mRNA revealed the dysregulation of genes involved in phototransduction and cholesterol metabolism. Zebrafish with mmachc deficiency recapitulate the several of the phenotypic and biochemical features of the human disorder, including ocular pathology, and show a response to established treatments.</jats:p>

Versatile enzymology and heterogeneous phenotypes in cobalamin complementation type C disease

“…Notably, MeCbl had little effect on biochemical markers but was most effective to restore growth. Despite limitations owing to possibly quite different uptake, transport, and metabolism of cobalamin in fish, responses were comparable with trends seen in cblC patients ( Sloan et al., 2020 ). Quintana et al.…”

“…Work by Sloan et al. identified the first fully viable and stable animal model of cblC disease with zebrafish ( Sloan et al., 2020 ). Knockdown of the Mmachc gene by morpholino antisense oligonucleotides resulted in severe and lethal embryonic phenotype.…”

“…This time course of gene expression coincides with the death of cblC mutant fish in the early juvenile period. A protective effect of maternal MMACHC is discussed for early development ( Chern et al., 2020 ; Sloan et al., 2020 ). In adult zebrafish, Mmachc expression is highest in the liver, muscle, kidney, and spinal cord, the latter ones reflecting organs typically affected by cblC disease.…”

Mutations in Hcfc1 and Ronin result in an inborn error of cobalamin metabolism and ribosomopathy

“…Though, zebrafish studies suggested that global introduction of human MMACHC mRNA is capable of rescuing craniofacial defects observed in hcfc1b morpholino knockdowns 56 . We cannot currently explain these species-specific differences, but a recent report of mmachc frameshift mutant zebrafish does not describe any craniofacial malformations 57 .…”

A Great Catch for Investigating Inborn Errors of Metabolism—Insights Obtained from Zebrafish

“…The lack of the cofactor for MMUT, subsequently perturbates its enzyme activity. A zebrafish knockout model of the causing gene mmachc is the first viable model organism, displaying typical disorder symptoms including MMA, retinopathy, and juvenile lethality [ 76 ]. The model proved to be effective for testing therapeutic approaches, with findings showing that small molecules already used for patients, such as hydroxocobalamin and methylcobalamin, recover the disease severity in this model.…”