The clinical success of PARP 1/2 inhibitors (PARPi) prompts the expansion of their
applicability beyond homologous recombination deficiency. Here, we demonstrate that
the loss of either the chromatin remodeler ALC1 or the accessory subunits of DNA polymerase
epsilon, POLE3 and POLE4, sensitizes cells to PARPi. We show that ALC1 is important
for mobilizing PARP1 from sites of DNA damage. Upon ALC1 deletion, PARPi treatment
leads to enhanced PARP1 trapping and accumulation of double-strand breaks, which underlies
the sensitivity of ALC1 KO to PARPi. We also establish that ALC1 overexpression, a
common feature in multiple tumor types, reduces the sensitivity of BRCA-deficient
cells to PARP inhibitors. Strikingly, targeting ALC1 could bypass PARPi resistance
caused by restoration of homologous recombination. Furthermore, we demonstrate that
POLE4 loss affects replication speed leading to the accumulation of single-stranded
DNA gaps behind replication forks upon PARPi treatment, due to impaired post-replicative
repair. POLE4 knockouts elicit elevated replication stress signaling involving ATR
and DNA-PK. Importantly We find POLE4 to act parallel to BRCA1 in inducing sensitivity
to PARPi and counteracts acquired resistance associated with restoration of homologous
recombination. Altogether, our findings establish ALC1 and POLE4 as a promising target
to improve PARPi driven therapies and hamper acquired PARPi resistance.
