BRD4 c.1213-1 G > T, BRD4-S loss, and NHWD-870 response: Evidence from a NUT carcinoma case and an isogenic lung squamous cell model

Responses to bromodomain and extra-terminal domain inhibitors vary among patients with NUT carcinoma (NC), but the molecular basis of this heterogeneity remains incompletely understood. Motivated by the exceptional clinical course of a patient with advanced pulmonary NC harboring both a BRD4-NUTM1 fusion and a tumor-restricted BRD4 c.1213-1 G > T splice-site variant, we examined whether altered BRD4 isoform expression contributes to drug response. In an isogenic SK-MES-1 model, homozygous introduction of c.1213-1 G > T selectively reduced BRD4-S RNA and protein while largely preserving BRD4-L abundance. Genetic back-correction and BRD4-S re-expression restored BRD4-S and MYC expression. Transcriptional-block experiments showed accelerated loss of BRD4-S RNA in mutant cells, and UPF1 depletion partially restored BRD4-S, consistent with a contribution from UPF1-dependent RNA surveillance. Mutant cells showed a left-shifted NHWD-870 dose–response, fewer colonies, and increased apoptosis; these phenotypes were attenuated by BRD4-S re-expression or back-correction. MYC mRNA and protein were both reduced in mutant cells. Cycloheximide-chase analysis further showed a shorter MYC protein half-life, while proteasome inhibition restored MYC abundance and MYC immunoprecipitation revealed increased high-molecular-weight ubiquitin signal. These findings support a model in which c.1213-1 G > T alters BRD4 isoform balance and is associated with enhanced NHWD-870 response in this engineered system.