Inborn errors of vitamin B(12) (cobalamin) metabolism are characterized by decreased production of active cobalamin cofactors and subsequent deficiencies in the activities of methionine synthase and methylmalonyl-CoA mutase. With the recent discovery of the cblJ defect in two patients with phenotypes mimicking the cblF defect, there are nine genes known to be involved in cobalamin metabolism. The new defect is caused by mutations in the ABCD4 gene, encoding an ABC transporter. At the moment, there is no clear distinction between the cblJ and cblF defects either clinically or biochemically, and both defects result in blocks in the transport of cobalamin from the lysosome to the cytoplasm. A patient was diagnosed with hyperhomocysteinemia and methylmalonic aciduria at the age of 8 years. Incorporations of both [(14)C]propionate and [(14)C]methyltetrahydrofolate in cultured fibroblasts were within reference ranges and thus too high to allow for complementation analysis. We observed decreased synthesis of both adenosylcobalamin and methylcobalamin and accumulation of unmetabolized cyanocobalamin. Exome sequencing was performed to identify causative mutation(s) and Sanger re-sequencing was performed to validate segregation of mutation in the family. By this approach, a homozygous mutation, c.423C>G, in the ABCD4 gene was identified. Here, we report the successful application of exome sequencing for diagnosis of a rare inborn error of vitamin B(12) metabolism in a patient whose unusual presentation precluded diagnosis using standard biochemical and genetic approaches. The patient represents only the third known patient with the cblJ disorder.

译文

维生素b (12) (钴胺素) 代谢的先天性错误的特征是活性钴胺素辅因子的产生减少,随后蛋氨酸合酶和甲基丙二酰辅酶a变位酶的活性不足。随着最近在两名表型模仿cblF缺陷的患者中发现cblJ缺陷,已知有9个基因与钴胺素代谢有关。新的缺陷是由编码ABC转运蛋白的ABCD4基因突变引起的。目前,无论是在临床上还是在生化上,cblJ和cblF缺陷之间都没有明确的区别,这两个缺陷都会导致钴胺素从溶酶体到细胞质的转运受到阻碍。一名患者在8岁时被诊断出患有高同型半胱氨酸血症和甲基丙二酸尿症。培养的成纤维细胞中 [(14)C] 丙酸酯和 [(14)C] 甲基四氢叶酸的加入都在参考范围内,因此太高,无法进行互补分析。我们观察到腺苷钴胺和甲钴胺的合成减少以及未代谢的氰钴胺的积累。进行外显子组测序以鉴定致病突变 (s),并进行了Sanger重新测序以验证家族中突变的分离。通过这种方法,在ABCD4基因中发现了一个纯合突变,c.423C>G。在这里,我们报告了外显子组测序在诊断罕见的先天性维生素b (12) 代谢错误中的成功应用,该患者的异常表现无法使用标准的生化和遗传学方法进行诊断。该患者仅代表第三位已知的cblJ疾病患者。

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