Hyper-Active Non-Homologous End Joining Selects for Synthetic Lethality Resistant and Pathological Hematopoietic Stem Cells

The prominent role of Fanconi anemia (FA) proteins involves homologous recombination (HR) repair. Poly[ADP-ribose] polymerase1 (PARP1) functions in multiple cellular processes including DNA repair and PARP inhibition is an emerging targeted therapy for cancer patients deficient in HR. Here we show that PARP1 activation in hematopoietic stem and progenitor cells (HSPCs) in response to genotoxic or oxidative stress attenuates HSPC exhaustion. Mechanistically, PARP1 controls the balance between HR and non-homologous end joining (NHEJ) in double strand break (DSB) repair by preventing excessive NHEJ. Disruption of the FA core complex skews PARP1 function in DSB repair and led to hyper-active NHEJ in Fanca^-/- or Fancc^-/- HSPCs. Re-expression of PARP1 rescues the hyper-active NHEJ phenotype in Brca1^-/-Parp1^-/- but less effective in Fanca^-/-Parp1^-/- cells. Inhibition of NHEJ prevents myeloid/erythroid pathologies associated with "synthetic lethality" and reduces human engraftment. Our results suggest that hyper-active NHEJ may select for "synthetic lethality" resistant and pathological HSPCs.