Abstract Prostate cancers (PCa) harboring CDK12 alterations exhibit aggressive clinical behavior and poor responses to current therapeutic strategies. Although CDK12 is categorized as a homologous recombination repair (HRR)–related gene, clinical evidence indicates limited benefit of poly(ADP-ribose) polymerase (PARP) inhibitor monotherapy in CDK12-defective tumors. To clarify the functional consequences of CDK12 loss and identify a mechanistically rational therapeutic approach, we generated CDK12-knockout (KO) PCa cell lines using CRISPR/Cas9 and assessed cell-cycle regulation, DNA damage repair (DDR), and replication stress by flow cytometry, γH2AX immunofluorescence, and DNA fiber assays. CDK12 knockout abrogated the G0/G1 checkpoint, permitting cell cycle progression into M phase despite the persistence of unrepaired DNA double-strand breaks (DSBs), and reduced ATM transcription, resulting in a DSB response phenotype resembling ATM deficiency. CDK12 loss also increased intrinsic replication stress and sensitized PCa cells to ATR inhibition. In patient-derived xenograft models harboring CDK12 alterations, combined PARP and ATR inhibition led to enhanced DSB accumulation and selectively suppressed tumor growth in CDK12-defective models. These findings highlight the importance of interrogating individual genes within the HRR pathway to define distinct mechanistic vulnerabilities and provide a strong rationale for combined PARP and ATR inhibition as a novel therapeutic strategy for patients with CDK12-altered prostate cancer.
Kamiyama et al. (Thu,) studied this question.