Roles of the mitochondrial replisome in mitochondrial DNA deletion formation

Abstract Mitochondrial DNA (mtDNA) deletions are a common cause of human mitochondrial diseases. Mutations in the genes encoding components of the mitochondrial replisome, such as DNA polymerase gamma (Pol γ) and the mtDNA helicase Twinkle, have been associated with the accumulation of such deletions and the development of pathological conditions in humans. Recently, we demonstrated that changes in the level of wild-type Twinkle promote mtDNA deletions, which implies that not only mutations in, but also dysregulation of the stoichiometry between the replisome components is potentially pathogenic. The mechanism(s) by which alterations to the replisome function generate mtDNA deletions is(are) currently under debate. It is commonly accepted that stalling of the replication fork at sites likely to form secondary structures precedes the deletion formation. The secondary structural elements can be bypassed by the replication-slippage mechanism. Otherwise, stalling of the replication fork can generate single- and double-strand breaks, which can be repaired through recombination leading to the elimination of segments between the recombination sites. Here, we discuss aberrances of the replisome in the context of the two debated outcomes, and suggest new mechanistic explanations based on replication restart and template switching that could account for all the deletion types reported for patients.

摘要 线粒体DNA（mtDNA）缺失是人类线粒体疾病的常见致病原因。编码线粒体复制体（mitochondrial replisome）组分的基因发生突变，如DNA聚合酶γ（Pol γ）与mtDNA解旋酶Twinkle，已被证实与此类缺失的积累及人类病理状态的发生密切相关。近期本团队研究表明，野生型Twinkle的表达水平变化可促进mtDNA缺失的形成，这提示不仅复制体组分的编码基因突变，其化学计量比失调也可能具有致病性。目前，复制体功能异常引发mtDNA缺失的具体机制仍存在学术争议。学界普遍认为，复制叉在易形成二级结构的位点发生停滞是缺失形成的前置过程。这类二级结构可通过复制滑动机制得以绕过；反之，复制叉停滞可引发单链与双链断裂，此类断裂可通过重组途径修复，最终导致重组位点间的基因片段缺失。本文围绕两种存在争议的结局展开复制体异常相关探讨，并基于复制重启与模板转换机制提出新的阐释，该阐释可涵盖患者中已报道的所有mtDNA缺失类型。