NUT midline
carcinoma (NMC) is a rare but highly aggressive
cancer typically caused by the translocation t(15;19), which results in the formation of the BRD4-NUT fusion
oncoprotein. Previous studies have demonstrated that fusion of the
NUT protein with the double bromodomains of BRD4 may significantly alter the cellular gene expression profile to contribute to NMC
tumorigenesis. However, the mechanistic details of this BRD4-NUT function remain poorly understood. In this study, we examined the NUT function in transcriptional regulation by targeting it to a LacO transgene array integrated in U2OS 2-6-3 cells, which allow us to visualize how NUT alters the in situ gene transcription dynamic. Using this system, we demonstrated that the
NUT protein tethered to the LacO locus recruits p300/
CREB-binding protein (CBP), induces
histone hyperacetylation, and enriches BRD4 to the transgene array
chromatin foci. We also discovered that, in BRD4-NUT expressed in NMC cells, the NUT moiety of the fusion
protein anchored to
chromatin by the double bromodomains also stimulates
histone hyperacetylation, which causes BRD4 to bind tighter to
chromatin. Consequently, multiple BRD4-interacting factors are recruited to the NUT-associated
chromatin locus to activate in situ transgene expression. This gene transcription function was repressed by either expression of a dominant negative inhibitor of the p300-NUT interaction or treatment with (+)-JQ1, which dissociates BRD4 from the LacO
chromatin locus. Our data support a model in which BRD4-NUT-stimulated
histone hyperacetylation recruits additional BRD4 and interacting partners to support transcriptional activation, which underlies the BRD4-NUT oncogenic mechanism in NMC.