The Mph1 Helicase Can Promote Telomere Uncapping and Premature Senescence in Budding Yeast

Double strand breaks (DSBs) can be repaired via either Non-Homologous End Joining (NHEJ) or Homology directed Repair (HR). Telomeres, which resemble DSBs, are refractory to repair events in order to prevent chromosome end fusions and genomic instability. In some rare instances telomeres engage in Break-Induced Replication (BIR), a type of HR, in order to maintain telomere length in the absence of the enzyme telomerase. Here we have investigated how the yeast helicase, Mph1, affects DNA repair at both DSBs and telomeres. We have found that overexpressed Mph1 strongly inhibits BIR at internal DSBs however allows it to proceed at telomeres. Furthermore, while overexpressed Mph1 potently inhibits NHEJ at telomeres it has no effect on NHEJ at DSBs within the chromosome. At telomeres Mph1 is able to promote telomere uncapping and the accumulation of ssDNA, which results in premature senescence in the absence of telomerase. We propose that Mph1 is able to direct repair towards HR (thereby inhibiting NHEJ) at telomeres by remodeling them into a nuclease-sensitive structure, which promotes the accumulation of a recombinogenic ssDNA intermediate. We thus put forward that Mph1 is a double-edge sword at the telomere, it prevents NHEJ, but promotes senescence in cells with dysfunctional telomeres by increasing the levels of ssDNA.

双链断裂（Double Strand Breaks, DSBs）可通过非同源末端连接（Non-Homologous End Joining, NHEJ）或同源定向修复（Homology Directed Repair, HR）两种途径完成修复。端粒（Telomeres）的结构类似双链断裂，但其修复过程受到抑制，以此防止染色体末端融合与基因组不稳定。在极少数情况下，端粒会通过断裂诱导复制（Break-Induced Replication, BIR）——一种同源定向修复途径——来维持端粒长度，该过程发生在端粒酶缺失的情境中。本研究探讨了酵母解旋酶Mph1对双链断裂与端粒处DNA修复的调控作用。研究发现，过表达的Mph1会强烈抑制染色体内部双链断裂处的断裂诱导复制，但对端粒处的该过程无抑制效果。此外，过表达的Mph1可有效抑制端粒处的非同源末端连接，但对染色体内部双链断裂处的非同源末端连接并无影响。在端粒处，Mph1可促进端粒脱帽与单链DNA（single-stranded DNA, ssDNA）的积累，这一过程会在端粒酶缺失的细胞中引发早衰。本研究提出，Mph1可通过将端粒重塑为核酸酶敏感结构，将修复途径导向同源定向修复（从而抑制非同源末端连接），进而促进重组性单链DNA中间产物的积累。综上，我们认为Mph1在端粒处扮演着双刃剑的角色：它可抑制非同源末端连接，但通过提升单链DNA水平，在端粒功能异常的细胞中加速细胞衰老。