Mol. Cell https://doi.org/10.1016/j.molcel.2021.05.009 (2021)
The base excision repair (BER) pathway corrects single-base DNA lesions. Repair is initiated by a DNA glycosylase that removes the damaged base, and the abasic site is incised by the endonuclease APE1 to generate a single-strand break (SSB) that is filled by DNA Polβ and ligated by DNA ligase III (LIG3) to restore an intact strand. BER is stimulated by the poly(ADP-ribose) polymerase PARP1, which recognizes SSB intermediates to recruit additional repair factors, and by XRCC1, a DNA-binding ‘scaffold’ protein that associates with Polβ and LIG3 to assemble a BER complex. Mutations in human XRCC1 cause neurodegenerative disease, but its precise role in BER has remained elusive. Now, Takeda, Caldecott and colleagues show that XRCC1 prevents cytotoxic PARP1 accumulation at SSBs, which impedes their repair. Human cells depleted of XRCC1 or PARP1 individually or in combination unexpectedly revealed that loss of PARP1 reverses hypersensitivity to the DNA alkylating agent methyl methanesulfonate (MMS) and the associated increase in SSBs caused by XRCC1 deletion, suggesting that XRCC1 is dispensable for efficient BER in the absence of PARP1. MMS treatment resulted in the accumulation and retention of PARP1 at SSB intermediates in XRCC1-deficient cells, a phenotype similar to what is seen when wild-type cells are treated with pharmacological inhibitors that ‘trap’ PARP1 at DNA lesions. XRCC1 suppresses excessive PARP1 engagement and activity at SSB intermediates generated by APE1 in vitro, and its activity is linked to its role as a protein ‘scaffold’ because XRCC1 mutants that are unable to form a BER complex with Polβ and LIG3 fail to suppress PARP1 ‘trapping’ and toxicity in MMS-treated cells. Notably, PARP1 ‘trapping’ impedes XRCC1 and Polβ binding to SSB intermediates, and PARP1 deletion restores Polβ recruitment in XRCC1-deficient cells. These new findings establish an essential role for XRCC1 in preventing excessive PARP1 engagement at SSB intermediates, which otherwise impedes recruitment of downstream BER factors and compromises genome integrity.
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