R-loop accumulation in spliceosome mutant leukemias confers sensitivity to PARP1 inhibition by triggering transcription-replication conflicts.

RNA splicing factor (SF) gene mutations are commonly observed in patients with myeloid malignancies. Here we showed that SRSF2- and U2AF1-mutant leukemias are preferentially sensitive to PARP inhibitors (PARPi) despite being proficient in homologous recombination repair. Instead, SF-mutant leukemias exhibited R-loop accumulation that elicited an R-loop associated PARP1 response, rendering cells dependent on PARP1 activity for survival. Consequently, PARPi induced DNA damage and cell death in SF-mutant leukemias in an R-loop dependent manner. PARPi further increased aberrant R-loop levels, causing higher transcription-replication collisions and triggering ATR activation in SF-mutant leukemias. Ultimately, PARPi-induced DNA damage and cell death in SF-mutant leukemias could be enhanced by ATR inhibition. Finally, the level of PARP1 activity at R loops correlated with PARPi sensitivity, suggesting that R-loop associated PARP1 activity could be predictive of PARPi sensitivity in patients harboring SF gene mutations. This study highlights the potential of targeting different R-loop response pathways caused by spliceosome gene mutations as a therapeutic strategy for treating cancer.