Genetic separation of Brca1 functions reveal mutation-dependent Pol vulnerabilities

Homologous recombination (HR)-deficiency induces a dependency on DNA polymerase theta (Pol theta/Polq)-mediated end joining, and Pol theta inhibitors (Pol theta i) are in development for cancer therapy. BRCA1 and BRCA2 deficient cells are thought to be synthetic lethal with Pol theta, but whether distinct HR gene mutations give rise to equivalent Pol theta-dependence, and the events that drive lethality, are unclear. In this study, we utilized mouse models with separate Brca1 functional defects to mechanistically define Brca1-Pol theta synthetic lethality. Surprisingly, homozygous Brca1 mutant, Polq-/- cells were viable, but grew slowly and had chromosomal instability. Brca1 mutant cells proficient in DNA end resection were significantly more dependent on Pol theta for viability; here, treatment with Pol theta i elevated RPA foci, which persisted through mitosis. In an isogenic system, BRCA1 null cells were defective, but PALB2 and BRCA2 mutant cells exhibited active resection, and consequently stronger sensitivity to Pol theta i. Thus, DNA end resection is a critical determinant of Pol theta i sensitivity in HR-deficient cells, and should be considered when selecting patients for clinical studies. Homologous recombination (HR) gene mutations are thought to be synthetic lethal with DNA polymerase theta (Pol theta) inhibition. Here, the authors reveal that Pol theta addiction is determined by the functional impact of gene mutations on DNA end resection activity.