Lingering single-strand breaks trigger Rad51-independent homology-directed repair of collapsed replication forks in polynucleotide kinase/phosphatase mutant of fission yeast
The DNA repair enzyme polynucleotide kinase/phosphatase (PNKP) protects genome integrity by restoring ligatable 5-phosphate and 3-hydroxyl termini at single-strand breaks (SSBs). In humans, PNKP mutations underlie the neurological disease known as MCSZ, but these individuals are not predisposed for cancer, implying effective alternative repair pathways in dividing cells. Homology-directed repair (HDR) of collapsed replication forks was proposed to repair SSBs in PNKP-deficient cells, but the critical HDR protein Rad51 is not required in PNKP-null (pnk1{Delta}) cells of Schizosaccharomyces pombe. Here, we report that pnk1{Delta} cells have enhanced requirements for Rad3 (ATR/Mec1) and Chk1 checkpoint kinases, and the multi-BRCT domain protein Brc1 that binds phospho-histone H2A ({gamma}H2A) at damaged replication forks. The viability of pnk1{Delta} cells depends on Mre11 and Ctp1 (CtIP/Sae2) double-strand break (DSB) resection proteins, Rad52 DNA strand annealing protein, Mus81-Eme1 Holliday junction resolvase, and Rqh1 (BLM/WRN/Sgs1) DNA helicase. Eliminating Pnk1 strongly sensitizes mre11{Delta} pku80{Delta} cells to DNA damaging agents that collapse replication forks, indicating a requirement for Mre11-Rad50-Nbs1 (MRN) protein complex that cannot be efficiently replaced by Exo1 5-3 exonuclease. Coupled with increased sister chromatid recombination and Rad52 repair foci in pnk1{Delta} cells, these findings indicate that lingering SSBs in pnk1{Delta} cells trigger Rad51-independent homology-directed repair of collapsed replication forks.\n\nAUTHOR SUMMARYDNA is constantly damaged by normal cellular metabolism, for example production of reactive oxygen species, or from exposure to external DNA damaging sources, such as radiation from the sun or chemicals in the environment. These genotoxic agents create thousands of single-strand breaks/cell/day in the human body. An essential DNA repair protein known as polynucleotide kinase/phosphatase (PNKP) makes sure the single-strand breaks have 5 phosphate and 3 hydroxyl ends suitable for healing by DNA ligase. Mutations that reduce PNKP activity cause a devastating neurological disease but surprisingly not cancer, suggesting that other DNA repair mechanisms step into the breach in dividing PNKP-deficient cells. One popular candidate was homology-directed repair (HDR) of replication forks that collapse at single-strand breaks, but the crucial HDR protein Rad51 was found to be non-essential in PNKP-deficient cells of fission yeast. In this study, Sanchez and Russell revive the HDR model by showing that SSBs in PNKP-deficient cells are repaired by a variant HDR mechanism that bypasses the requirement for Rad51. Notably, Mus81 endonuclease that resolves sister chromatid recombination structures formed during HDR of collapsed replication forks was found to be essential in PNKP-deficient cells.