Recruitment of BRCA1 limits MYCN-driven accumulation of stalled RNA Polymerase in neuroblastoma

Deregulation of the MYCN gene drives the development of neuronal and neuroendocrine tumors and is often associated with a particularly poor prognosis. Here we show that activation of MYCN in human neuroblastoma cells induces promoter escape of RNAPII. If pause release of RNAPII fails, MYCN recruits the BRCA1 protein to promoter-proximal regions. Recruitment of BRCA1 prevents MYCN-dependent accumulation of stalled RNAPII and enhances transcriptional activation by MYCN. Mechanistically, BRCA1 stabilizes mRNA de-capping complexes and enables MYCN to suppress R-loop formation in promoter-proximal regions. Recruitment of BRCA1 is mediated by the ubiquitin-specific protease, USP11, which binds specifically to MYCN that is de-phosphorylated at Thr58. Thr58 phosphorylation promotes proteasomal turnover of MYCN, hence BRCA1 stabilizes chromatin association of MYCN. Since BRCA1 is highly expressed in neuronal progenitor cells during early development and since MYC is less efficient than MYCN in recruiting BRCA1, our findings argue that a cell lineage-specific stress response enables MYCN-driven tumors to cope with deregulated RNAPII function. Overall design: ChIP-seq for RNAPII, BRCA1, NMYC and DCP1A and corresponding changes in gene expression were measured by mRNA-seq in SH-EP cells harbouring a NMYC-ER chimeric protein that can be activated by 4-hydroxytamoxifen (4-OHT). To identify genes that are critical for growh of these cells we performed an shRNA screen using a pGIPZ-based shRNA library. Nascent RNA was measured with GRO-seq in the same cell line.