RBM5 Acts as a Tumor Suppressor in Breast Cancer Through Binding to G-quadruplexes in the BAP1 Gene Promoter to Activate Its Expression

As a member of the RNA-binding motif protein (RBM) family, RBM5 is a characterized tumor suppressor in lung and prostate cancers, with critical roles in alternative splicing of apoptosis- and cell cycle-related genes. However, its direct capacity to regulate gene transcription remains unreported. Here, we identified that RBM5 was significantly downregulated in breast cancer cells and clinical specimens, especially in the basal-like subtype. RBM5 overexpression attenuated breast cancer cell malignancy, while RBM5 knockdown exerted opposite effects. Among genes with promoter G-quadruplex (G4) motifs, RBM5 was positively correlated with multiple tumor suppressors, including BAP1, but uncorrelated with the oncogene MYC. RBM5 bound to G4 motifs in both MYC and BAP1 promoters, but differentially modulated G4 structure stability: it destabilized MYC-G4 while stabilizing BAP1-G4. Mechanistically, either the RRM1 or the RRM2 domain was sufficient for MYC-G4 binding, whereas both domains were required for BAP1-G4 interaction. RBM5 manipulation regulated endogenous BAP1 but not MYC expression. BAP1 overexpression reversed the protumorigenic effects of RBM5 knockdown in cellulo and tumor growth in a xenograft mouse model. Collectively, we reveal that RBM5 acts as a breast cancer tumor suppressor via directly binding the BAP1 promoter G4 to transcriptionally activate BAP1 expression.