PARG-deficient tumor cells have an increased dependence on EXO1/FEN1-mediated DNA repair

Abstract

Targeting poly(ADP-ribose) glycohydrolase (PARG) is currently explored as a therapeutic approach to treat various cancer types, but we have a poor understanding of the specific genetic vulnerabilities that would make cancer cells susceptible to such a tailored therapy. Moreover, the identification of such vulnerabilities is of interest for targeting BRCA2;p53-deficient tumors that have acquired resistance to poly(ADP-ribose) polymerase inhibitors (PARPi) through loss of PARG expression. Here, by performing whole-genome CRISPR/Cas9 drop-out screens, we identify various genes involved in DNA repair to be essential for the survival of PARG;BRCA2;p53-deficient cells. In particular, our findings reveal EXO1 and FEN1 as major synthetic lethal interactors of PARG loss. We provide evidence for compromised replication fork progression, DNA single-strand break repair, and Okazaki fragment processing in PARG;BRCA2;p53-deficient cells, alterations that exacerbate the effects of EXO1/FEN1 inhibition and become lethal in this context. Since this sensitivity is dependent on BRCA2 defects, we propose to target EXO1/FEN1 in PARPi-resistant tumors that have lost PARG activity. Moreover, EXO1/FEN1 targeting may be a useful strategy for enhancing the effect of PARG inhibitors in homologous recombination-deficient tumors.

Intrinsic PARG inhibitor sensitivity is mimicked by TIMELESS haploinsufficiency and rescued by nucleoside supplementation

“…Several other genes emerged as possible PARGi sensitizers, including CTPS2 , TK1 , DUT (pyrimidine biosynthesis genes) as well as POLB , FEN1 , LIG1 (BER genes), although their effects were more modest and/or limited to one or two cell lines, highlighting that there may be cell-line-specific redundancies and different buffering capacities. However, TYMS , DUT , POLB , LIG1 and FEN1 have been identified by others as PARGi synthetic lethality genes ( 90 , 91 ), and these observations support the notion that inhibition of nucleotide biosynthesis and BER can sensitize cancer cells to pharmacological PARG inhibition.…”