HMCES corrupts replication fork stability during base excision repair in homologous recombination deficient cells

Apurinic/apyrimidinic (AP) sites and single-strand breaks (SSB) arising from base excision repair (BER) during misincorporation of damaged nucleobases may hinder replication fork stability in homologous recombination-deficient (HRD) cells. At templated AP-sites, HMCES DNA-protein crosslinks (DPC) regulate replication fork speed while avoiding APE1-mediated cytotoxic double-strand breaks (DSB). Whereas the role of HMCES at template DNA strand is well studied, its consequences on nascent DNA are less understood. Here, we provide evidence that HMCES play detrimental roles during removal of 5- hydroxymethyl-2’-deoxycytidine (5hmdC)-derived 5-hydroxymethyl-2’-deoxyuridine (5hmdU) by BER at replication forks. HRD cells display heightened HMCES chromatin levels, which increase upon 5hmdC exposure, suggesting that HMCES binds both spontaneous and 5hmdC-induced AP-sites. HMCES depletion largely suppresses 5hmdC- mediated Fancd2 ^-/- replication fork defects, chromosomal aberrations and cell lethality, suggesting that HMCES is responsible for the replication fork impairment and lethality observed in HRD cells. Therefore, HMCES-DPCs are a novel source of BER-initiated PRIMPOL-mediated ssDNA gaps, implying endogenous DPCs as a source of DNA damage in HRD tumours.