Targeting BRPF3 moderately reverses olaparib resistance in high grade serous ovarian carcinoma

PARP inhibitors (PARPi) kill cancer cells by stalling DNA replication and preventing DNA repair, resulting in a critical accumulation of DNA damage. Resistance to PARPi is a growing clinical problem in the treatment of high grade serous ovarian carcinoma (HGSOC). Acetylation of histone H3 lysine 14 (H3K14ac) and associated histone acetyltransferases (HATs) and epigenetic readers have known functions in DNA repair and replication. Our objectives are to examine their expression and activities in the context of PARPi-resistant HGSOC, and to determine if targeting H3K14ac or associated proteins has therapeutic potential. Using mass spectrometry profiling of histone modifications, we observed increased H3K14ac enrichment in PARPi-resistant HGSOC cells relative to isogenic PARPi-sensitive lines. By reverse-transcriptase quantitative PCR and RNA-seq, we also observed altered expression of numerous HATs in PARPi-resistant HGSOC cells and a PARPi-resistant PDX model. Knockdown of HATs only modestly altered PARPi response, although knockdown and inhibition of PCAF significantly increased resistance. Pharmacologic inhibition of HBO1 depleted H3K14ac but did not affect PARPi response. However, knockdown and inhibition of BRPF3, a bromodomain and PHD-finger containing protein that is known to interact in a complex with HBO1, did reduce PARPi resistance. This study demonstrates that depletion of H3K14ac does not affect PARPi response in HGSOC. Our data suggest that the bromodomain function of HAT proteins, such as PCAF, or accessory proteins, such as BRPF3, may play a more direct role compared to direct HATs function in PARPi response.