Targeting CDK9 With Harmine Induces Homologous Recombination Deficiency and Synergizes With PARP Inhibitors in Ovarian Cancer.

Ovarian cancer patients with homologous recombination (HR) proficiency are more likely to develop resistance to poly (ADP-ribose) polymerase (PARP) inhibitors, resulting in poor prognosis. Harmine, a natural compound with verified clinical safety, is a potential inhibitor of HR activity. However, its effect on HR has not been tested in ovarian cancer. More importantly, its specific target involving the HR pathway is still unknown. A fluorescent reporter cell system was employed to assess the HR efficiency of cells treated by harmine. The target of harmine in the HR pathway was identified via a molecular docking screening in silico and confirmed using a cellular thermal shift assay. Evaluation of harmine treatment combined with the PARP inhibitor olaparib in BRCA1/2 wild-type ovarian cancer was performed. Treatment with harmine inhibited HR activity in a dose-dependent manner in ovarian cancer cells and selectively suppressed HR-proficient cells with high replication stress, marked with CCNE1 amplification. HR-related protein cyclin-dependent kinase 9 (CDK9) was identified as the target of harmine. Specifically, harmine diminished the interaction of CDK9 with cyclin T, resulting in impaired formation of the p-TEFb complex and subsequent transcription elongation. Hence, harmine downregulated the transcription of key HR-related genes. Moreover, harmine synergized with PARP inhibitor olaparib in BRCA1/2 wild-type ovarian cancer. Harmine inhibits HR repair by interfering with CDK9-mediated transcription elongation for HR-related genes, thus enhancing the potency of olaparib, providing a promising therapeutic strategy to improve the prognosis of HR-proficient ovarian cancer patients.