BCL-3 loss sensitises colorectal cancer cells to DNA damage by targeting homologous recombination.

The proto-oncogene BCL-3 is upregulated in a subset of colorectal cancers (CRC), where it has been shown to enhance tumour cell survival. However, although increased expression correlates with poor patient prognosis, the role of BCL-3 in determining therapeutic response remains largely unknown. In this study, we use combined approaches in multiple cell lines and pre-clinical mouse models to investigate the function of BCL-3 in the DNA damage response. We show that suppression of BCL-3 increases γH2AX foci formation and decreases homologous recombination in CRC cells, resulting in reduced RAD51 foci number and increased sensitivity to PARP inhibition. Importantly, a similar phenotype is seen in Bcl3 mice, where Bcl3 mouse crypts also exhibit sensitivity to DNA damage with increased γH2AX foci compared to wild type mice. Additionally, Apc.Kras-mutant x Bcl3 mice are more sensitive to cisplatin chemotherapy compared to wild type mice. Taken together, our results identify BCL-3 as a regulator of the cellular response to DNA damage and suggests that elevated BCL-3 expression, as observed in CRC, could increase resistance of tumour cells to DNA damaging agents including radiotherapy. These findings offer a rationale for targeting BCL-3 in CRC as an adjunct to conventional therapies and suggest that BCL-3 expression in tumours could be a useful biomarker in stratification of rectal cancer patients for neo-adjuvant chemoradiotherapy.