Department of Cellular Biochemistry, University Medical Center Göttingen, Göttingen, D-37073, Germany.
Departamento de Bioquímica y Biología Molecular y Celular, Universidad de Zaragoza-CIBER de Enfermedades Raras (CIBERER)-Instituto de Investigación Sanitaria de Aragón (IIS Aragón), Zaragoza, 50013, Spain.
Hum Mol Genet. 2018 Dec 1;27(23):4135-4144. doi: 10.1093/hmg/ddy305.
Protein import into mitochondria is facilitated by translocases within the outer and the inner mitochondrial membranes that are dedicated to a highly specific subset of client proteins. The mitochondrial carrier translocase (TIM22 complex) inserts multispanning proteins, such as mitochondrial metabolite carriers and translocase subunits (TIM23, TIM17A/B and TIM22), into the inner mitochondrial membrane. Both types of substrates are essential for mitochondrial metabolic function and biogenesis. Here, we report on a subject, diagnosed at 1.5 years, with a neuromuscular presentation, comprising hypotonia, gastroesophageal reflux disease and persistently elevated serum and Cerebrospinal fluid lactate (CSF). Patient fibroblasts displayed reduced oxidative capacity and altered mitochondrial morphology. Using trans-mitochondrial cybrid cell lines, we excluded a candidate variant in mitochondrial DNA as causative of these effects. Whole-exome sequencing identified compound heterozygous variants in the TIM22 gene (NM_013337), resulting in premature truncation in one allele (p.Tyr25Ter) and a point mutation in a conserved residue (p.Val33Leu), within the intermembrane space region, of the TIM22 protein in the second allele. Although mRNA transcripts of TIM22 were elevated, biochemical analyses revealed lower levels of TIM22 protein and an even greater deficiency of TIM22 complex formation. In agreement with a defect in carrier translocase function, carrier protein amounts in the inner membrane were found to be reduced. This is the first report of pathogenic variants in the TIM22 pore-forming subunit of the carrier translocase affecting the biogenesis of inner mitochondrial membrane proteins critical for metabolite exchange.
蛋白质通过位于内外膜的转位酶进入线粒体，这些转位酶专门用于高度特定的一组客户蛋白。线粒体载体转位酶（TIM22 复合物）将多跨膜蛋白（如线粒体代谢物载体和转位酶亚基（TIM23、TIM17A/B 和 TIM22））插入到线粒体内膜中。这两种类型的底物对于线粒体代谢功能和生物发生都是必不可少的。在这里，我们报告了一个患者，在 1.5 岁时被诊断出患有神经肌肉疾病，包括张力减退、胃食管反流病和持续升高的血清和脑脊液中的乳酸（CSF）。患者的成纤维细胞显示出氧化能力降低和线粒体形态改变。使用跨线粒体细胞杂种系，我们排除了线粒体 DNA 中的候选变异是这些影响的原因。全外显子组测序发现 TIM22 基因（NM_013337）的复合杂合变异，导致一个等位基因的提前终止（p.Tyr25Ter）和第二个等位基因中保守残基（p.Val33Leu）的点突变，位于 TIM22 蛋白的膜间空间区域。尽管 TIM22 的 mRNA 转录物升高，但生化分析显示 TIM22 蛋白水平更低，TIM22 复合物形成的缺陷甚至更大。与载体转位酶功能缺陷一致，发现内膜中的载体蛋白含量减少。这是首例报道 TIM22 孔形成亚基的致病性变异影响关键代谢物交换的内线粒体膜蛋白生物发生的报道。
