Inhibition of CDK12 induces cancer cell dependence on activated P-TEFb via the ATM-stimulated p53 and NF-kB transcriptional programs
P-TEFb and CDK12 facilitate transcription elongation by RNA polymerase II and play prominent roles in cancer. Understanding their functional interplay could inform novel anti-cancer strategies. While inhibition of CDK12 downregulates unique sets of genes, eliciting genomic instability that is being exploited for novel therapies, little is known about the significance of transcriptional induction in CDK12-targeted cells. We show that inhibition of CDK12 in colon cancer-derived cells activates P-TEFb and induces genes of key cancer signaling pathways, including p53 and NF-kB. Mechanistically, cancer cells become exquisitely dependent on P-TEFb through activation of p53-dependent apoptosis and attenuation of NF-kB-dependent proliferation. Furthermore, we show that the DNA damage-responsive ATM kinase mediates these effects. While ATM is required for the synthetic lethality of CDK12 and P-TEFb co-targeting in p53-proficient cells, co-inhibition of ATM and CDK12 synergizes in decreasing viability of p53-deficient cells. Finally, pairwise targeting of CDK12, P-TEFb and transcription initiation kinase CDK7 stimulates p53-dependent apoptosis of cancer cell spheroids. We propose that the stimulation of Pol II pause release by P-TEFb at the signal-responsive genes underlies the dependence of CDK12-targeted cancer cells on P-TEFb. Together, our work provides a rationale for combinatorial targeting of CDK12 and P-TEFb or the induced oncogenic pathways in cancer. Overall design: Precision run-on sequencing (PRO-seq) analysis of nascent transcription in human colorectal carcinoma cell line, HCT116, during the inhibition of CDK12 and CDK9.