Molecular Insight into Function of the Evolutionarily Conserved Brd4 Extraterminal Domain (ET) and Mechanism of Brd4 Functions in Human Diseases

Abstract

Bromodomain protein 4 (Brd4) plays critical roles in development, cancer progression and virus-host pathogenesis. Papillomaviruses (PV) E2 protein associates with Brd4 and this interaction is important for transcriptional regulation of the viral oncogenes by E2 as well as viral genome maintenance in host cells for some of the PV. Brd4 is causally linked to a rare, aggressive cancer, NUT Midline Carcinoma (NMC), which is typically defined by chromosomal translocation fusing the NUT gene to the Brd4 gene. The molecular mechanism behind Brd4-NUT oncogenesis remains largely unknown. To gain mechanistic insight into the biological functions of Brd4, we performed a proteomic analysis to identify and characterize Brd4 associated cellular proteins. We discovered binding partners of the Brd4 ET domain and show that interaction of these proteins with Brd4 is conserved across the human BET proteins. The Brd4 ET interactors, NSD3, JMJD6 and GLTSCR1, were found to be important for Brd4 transcriptional activation function and are recruited to the promoters they regulate in a Brd4 dependent manner. Moreover, depletion of Brd4 or NSD3 reduced H3K36 methylation demonstrating that the Brd4/NSD3 complex regulates the chromatin microenvironment. We thus identified the ET domain as an important transcription regulatory domain for Brd4. Since the ET domain is preserved in the Brd-NUT proteins, we also investigated its contribution to Brd-NUT pathogenesis. Expression of the ET domain, which competes off the ET domain interactors from Brd4-NUT, induced squamous differentiation. More specifically, depletion of the ET domain interactor, NSD3 induced squamous differentiation by Brd4-NUT while loss of JMJD6 markedly reduced proliferation of the NMC cells. Lastly, we investigated the effect of the recently developed small molecule inhibitors of BET bromodomains on PV E2 functions and papilloma virus mediated pathogenesis. BET inhibitors blocked association of Brd4 and E2 with mitotic chromosomes without affecting Brd4 dependent E2 transcription regulation of viral promoters. This finding suggests that Brd4 affects viral genome maintenance and viral transcription regulation via different mechanisms. Overall, these studies have shed new insight into the molecular mechanism of Brd4 functions and their role in human diseases.