Defective ALC1 nucleosome remodeling confers PARPi sensitization and synthetic lethality with HRD.

Chromatin is a barrier to efficient DNA repair, as it hinders access and processing of certain DNA lesions. ALC1/CHD1L is a nucleosome-remodeling enzyme that responds to DNA damage, but its precise function in DNA repair remains unknown. Here we report that loss of ALC1 confers sensitivity to PARP inhibitors, methyl-methanesulfonate, and uracil misincorporation, which reflects the need to remodel nucleosomes following base excision by DNA glycosylases but prior to handover to APEX1. Using CRISPR screens, we establish that ALC1 loss is synthetic lethal with homologous recombination deficiency (HRD), which we attribute to chromosome instability caused by unrepaired DNA gaps at replication forks. In the absence of ALC1 or APEX1, incomplete processing of BER intermediates results in post-replicative DNA gaps and a critical dependence on HR for repair. Hence, targeting ALC1 alone or as a PARP inhibitor sensitizer could be employed to augment existing therapeutic strategies for HRD cancers.Work in I.A.’s group is funded by the WellcomeTrust (grant number 210634), BBSRC (BB/R007195/1), and Cancer ResearchUK (C35050/A22284). Work in D.A.’s group is funded by the Cancer ResearchUK Career Development Fellowship (grant number 16304). Work in the S.J.B.lab is supported by the Coun, which receives its core fundingfrom Cancer Research UK (FC0010048), the UK Medical Research Council(FC0010048), and the Wellcome Trust (FC0010048); a European Research Council (ERC) Advanced Investigator Grant (TelMetab); and Wellcome TrustSenior Investigator and Collaborative Grants. S.S.-B. was the recipient of an EMBO Long Term Fellowship (ALTF 707-2019) and a MSCA individual fellow-ship (grant 886577). Work in the J.R.C. group is funded by CRUK Career Devel-opment Fellowship (C52690/A19270) with infrastructural support from Well-come core award 090532/Z/09/ZS