DNA double-strand break end resection factors and WRN facilitate mitotic DNA synthesis in human cells

Mitotic DNA synthesis (MiDAS) serves to complete the replication of genomic loci that are not fully replicated in S phase in response to replication stress. Previous studies suggest that MiDAS might proceed via break-induced DNA replication, a sub-pathway of homologous recombination repair activated at broken or collapsed replication forks. We set out to define whether DNA double strand break end-resection factors play a role in MiDAS. Here, we show that several core end-resection factors, including MRE11, CtIP and BRCA1 are essential for MiDAS. In addition, while loss of WRN or DNA2 impairs MiDAS, there is no requirement for other known end-resection factors such as EXO1 and BLM. Moreover, both the exonuclease and the helicase activities of WRN contribute to MiDAS. Because oncogene-induced replication stress is common in cancers, targeting of WRN or other factors required for MiDAS could facilitate the development of targeted cancer therapies. Mitotic DNA synthesis (MiDAS) has been proposed to utilize break-induced DNA replication, a form of homology-directed DNA repair (HDR). Here, the authors find that core DNA end-resection factors required for HDR, including MRE11, CtIP and BRCA1, and a RecQ family helicase, WRN, are essential for MiDAS.