MPV17 Loss Causes Deoxynucleotide Insufficiency and Slow DNA Replication in Mitochondria

MPV17 is a mitochondrial inner membrane protein whose dysfunction causes mitochondrial DNA abnormalities and disease by an unknown mechanism. Perturbations of deoxynucleoside triphosphate (dNTP) pools are a recognized cause of mitochondrial genomic instability; therefore, we determined DNA copy number and dNTP levels in mitochondria of two models of MPV17 deficiency. In Mpv17 ablated mice, liver mitochondria showed substantial decreases in the levels of dGTP and dTTP and severe mitochondrial DNA depletion, whereas the dNTP pool was not significantly altered in kidney and brain mitochondria that had near normal levels of DNA. The shortage of mitochondrial dNTPs in Mpv17-/- liver slows the DNA replication in the organelle, as evidenced by the elevated level of replication intermediates. Quiescent fibroblasts of MPV17-mutant patients recapitulate key features of the primary affected tissue of the Mpv17-/- mice, displaying virtual absence of the protein, decreased dNTP levels and mitochondrial DNA depletion. Notably, the mitochondrial DNA loss in the patients’ quiescent fibroblasts was prevented and rescued by deoxynucleoside supplementation. Thus, our study establishes dNTP insufficiency in the mitochondria as the cause of mitochondrial DNA depletion in MPV17 deficiency, and identifies deoxynucleoside supplementation as a potential therapeutic strategy for MPV17-related disease. Moreover, changes in the expression of factors involved in mitochondrial deoxynucleotide homeostasis indicate a remodeling of nucleotide metabolism in MPV17 disease models, which suggests mitochondria lacking functional MPV17 have a restricted purine mitochondrial salvage pathway.

MPV17是一种线粒体内膜蛋白，其功能异常可通过未知机制引发线粒体DNA异常及相关疾病。脱氧核苷三磷酸 (deoxynucleoside triphosphate, dNTP) 池的紊乱是公认的线粒体基因组不稳定诱因；因此，本研究针对两种MPV17缺陷模型，检测了其线粒体中的DNA拷贝数与dNTP水平。在Mpv17基因敲除小鼠中，其肝脏线粒体的dGTP与dTTP水平显著降低，且出现严重的线粒体DNA缺失；而肾脏与脑组织线粒体的dNTP池未发生明显改变，其DNA水平接近正常。Mpv17-/-小鼠肝脏中线粒体dNTP的不足，会减慢该细胞器内的DNA复制进程，这一点可通过复制中间体水平的升高得到证实。MPV17突变患者的静息成纤维细胞可重现Mpv17-/-小鼠主要受累组织的关键特征：几乎不表达该蛋白、dNTP水平降低以及线粒体DNA缺失。值得注意的是，通过补充脱氧核苷，可阻止并挽救患者静息成纤维细胞中的线粒体DNA丢失。综上，本研究证实线粒体dNTP不足是MPV17缺陷引发线粒体DNA缺失的原因，并确定补充脱氧核苷可作为MPV17相关疾病的潜在治疗策略。此外，线粒体脱氧核苷酸稳态相关因子的表达变化表明，MPV17疾病模型中的核苷酸代谢发生了重塑，这提示缺乏功能性MPV17的线粒体，其嘌呤线粒体补救通路受到限制。