Pharmacological blockade of MYCN in neuroblastoma using orally-bioavailable CDK inhibitors reveals an approach widely applicable to Myc-dependent cancers. Pharmacological blockade of MYCN in neuroblastoma using orally-bioavailable CDK inhibitors reveals an approach widely applicable to Myc-dependent cancers

Neuroblastoma (NB) is a paediatric tumor wherein amplification of the oncogenic basic helix-loop-helix (bHLH) transcription factor (TF) MYCN confers clinical and biologic features prototypical of Myc-dependent cancers. TF-dependent cancers like MYCN-amplified NB are difficult to target, but the availability of clinical-candidate transcription inhibitors makes selective blockade of oncogenic TF activity a possibility. Here we investigated whether NB could be controlled via dual targeting of the transcription elongation machinery and cell cycle progression, using an orally bioavailable and selective CDK9/2 inhibitor CYC065. CYC065 blocks nascent transcription and leads to a drastic reduction in steady-state levels of short-lived transcripts. In MYCN-amplified NB, CYC065 rapidly and completely eliminated MYCN mRNA and protein, terminating expression of a MYCN-dependent gene expression programme. This result is phenocopied by multiple clinical-candidate CDK9 chemical inhibitors and genetic manipulation of CDK9. Mechanistically, CYC065 dissociates MYCN from physical proximity of P-TEFb in cells. P-TEFb co-occupies promoters and enhancers of de-differentiating pathway genes highly-occupied by MYCN, and CDK9 inhibition terminates expression of these and other highly transcribed genes. Intriguingly, targeted inhibition of CDK9 alone is insufficient to eradicate MYCN-dependent cells lines as CDK9 loss alone is compensated by upregulation of CDK2. CYC065 targeted MYCN-driven neuroblastoma in vivo, resulted in tumor regression or eradication and prolonged survival in multiple NB models, and was effective against a wide range of Myc-dependent cancer cells lines in vitro. The data establish that combined CDK9/CDK2 inhibition blocks transcriptional dependence induced by MYCN, highlighting a potential clinical strategy by which many Myc-driven cancers could be targeted. Overall design: ChIP-Seq in KELLY neuroblastoma cell lines for H3K27ac, CDK9