Organic Acid Research Section, Medical Genomics and Metabolic Genetics Branch, National Human Genome Research Institute, NIH, Bethesda, MD, 20892 USA.
Zebrafish Core Facility, Translational and Functional Genomics Branch, National Human Genome Research Institute, Bethesda, MD, 20892 USA.
Hum Mol Genet. 2020 Aug 3;29(13):2109-2123. doi: 10.1093/hmg/ddaa044.
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.
钴胺素 C（cblC）缺陷是细胞内钴胺素代谢最常见的先天性错误，由负责维生素 B12 加工和细胞内转运的 MMACHC 基因突变引起。这种隐性疾病的特征是无法将钴胺素代谢为腺苷钴胺素和甲基钴胺素，导致下游酶甲基丙二酰辅酶 A 变位酶和蛋氨酸合成酶活性受损，引起甲基丙二酸血症、高同型半胱氨酸血症和低蛋氨酸血症的生化紊乱。钴胺素 C 缺陷可伴有广泛的临床表现，包括进行性失明，在小鼠中表现为极早胚胎致死。由于斑马鱼拥有全套的钴胺素代谢酶，我们使用基因组编辑来研究 mmachc 功能丧失，并开发首个可行的 cblC 缺陷动物模型。mmachc 突变体在胚胎期存活下来，但在幼年期早期死亡。突变体表现出 cblC 缺陷的代谢和临床特征，包括甲基丙二酸血症、严重生长迟缓和致死。当突变体在补充用于治疗患者的小分子（包括羟钴胺素、甲钴胺素、蛋氨酸和甜菜碱）的水中饲养时，形态和代谢参数得到改善。此外，表达杆状和/或锥状荧光报告基因的 mmachc 突变体表现出视网膜病变和视神经变薄（ON）。使用全眼 mRNA 进行的表达分析显示，涉及光转导和胆固醇代谢的基因失调。具有 mmachc 缺陷的斑马鱼再现了人类疾病的几种表型和生化特征，包括眼部病理学，并对已建立的治疗方法有反应。
