Bloom syndrome DNA helicase mitigates mismatch repair-dependent apoptosis

Generation of O6-methylguanine (O6-meG) by DNA-alkylating agents such as N-methyl N-nitrosourea (MNU) activates the multiprotein mismatch repair (MMR) complex and the checkpoint response involving ATR/CHK1 and ATM/CHK2 kinases, which may in turn trigger cell cycle arrest and apoptosis. The Bloom syndrome DNA helicase BLM interacts with the MMR complex, suggesting functional relevance to repair and checkpoint responses. We observed a strong interaction of BLM with MMR proteins in HeLa cells upon treatment with MNU as evidenced by co-immunoprecipitation as well as colocalization in the nucleus as revealed by dual immunofluorescence staining. Knockout of BLM sensitized HeLa MR cells to MNU-induced cell cycle disruption and enhanced expression of the apoptosis markers cleaved caspase-9 and PARP1. MNU-treated BLM-deficient cells also exhibited a greater number of 53BP1 foci and greater phosphorylation levels of H2AX at S139 and RPA32 at S8, indicating the accumulation of DNA double-strand breaks. These findings suggest that BLM prevents doublestrand DNA breaks during the MMR-dependent DNA damage response and mitigates O6-meG-induced apoptosis.