Mitochondrial DNA (mtDNA) depletion/deletions syndromes (MDDS) encompass a clinically and etiologically heterogenous group of mitochondrial disorders caused by impaired mtDNA maintenance. Among the most frequent causes of MDDS are defects in nucleoside/nucleotide metabolism, which is critical for synthesis and homeostasis of the deoxynucleoside triphosphate (dNTP) substrates of mtDNA replication. A central enzyme for generating dNTPs is ribonucleotide reductase, a critical mediator of de novo nucleotide synthesis composed of catalytic RRM1 subunits in complex with RRM2 or p53R2. Here, we report 5 probands from 4 families who presented with ptosis and ophthalmoplegia as well as other clinical manifestations and multiple mtDNA deletions in muscle. We identified 3 RRM1 loss-of-function variants, including a dominant catalytic site variant (NP_001024.1: p.N427K) and 2 homozygous recessive variants at p.R381, which has evolutionarily conserved interactions with the specificity site. Atomistic molecular dynamics simulations indicate mechanisms by which RRM1 variants affect protein structure. Cultured primary skin fibroblasts of probands manifested mtDNA depletion under cycling conditions, indicating impaired de novo nucleotide synthesis. Fibroblasts also exhibited aberrant nucleoside diphosphate and dNTP pools and mtDNA ribonucleotide incorporation. Our data reveal that primary RRM1 deficiency and, by extension, impaired de novo nucleotide synthesis are causes of MDDS.

中文翻译：

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RRM1 变体通过受损的从头核苷酸合成引起线粒体 DNA 维持障碍

线粒体 DNA (mtDNA) 耗竭/缺失综合征 (MDDS) 包括一组由 mtDNA 维护受损引起的临床和病因异质性线粒体疾病。MDDS 最常见的原因是核苷/核苷酸代谢缺陷，这对于 mtDNA 复制的脱氧核苷三磷酸 (dNTP) 底物的合成和稳态至关重要。核糖核苷酸还原酶是产生 dNTP 的核心酶，它是核苷酸从头合成的关键介质，由与 RRM2 或 p53R2 复合的催化 RRM1 亚基组成。在这里，我们报告了来自 4 个家庭的 5 名先证者，他们表现出上睑下垂和眼肌麻痹以及其他临床表现和肌肉中的多个 mtDNA 缺失。我们确定了 3 个RRM1功能丧失变异体，包括显性催化位点变异体 (NP_001024.1: p.N427K) 和 p.R381 的 2 个纯合隐性变异体，其与特异性位点的相互作用在进化上是保守的。原子分子动力学模拟表明RRM1变体影响蛋白质结构的机制。先证者培养的原代皮肤成纤维细胞在循环条件下表现出 mtDNA 耗竭，表明从头合成核苷酸受损。成纤维细胞还表现出异常的核苷二磷酸和 dNTP 池以及 mtDNA 核糖核苷酸掺入。我们的数据表明，原发性 RRM1 缺陷，进而受损的从头核苷酸合成是 MDDS 的原因。