STAU1 promotes BRCA1 translation through circHIPK3-mRNA unwinding

Circular RNAs (circRNAs) are covalently closed RNA molecules widely expressed in eukaryotes and dysregulated in several pathological contexts, including cancer. Recent studies have highlighted direct circRNA-mRNA interactions as a key mechanism in post-transcriptional gene expression regulation. Our lab previously identified a functional interaction between circHIPK3 and BRCA1 mRNA, where circHIPK3 binding displaces the translational repressor FMRP, enhancing BRCA1 protein expression and influencing the balance between DNA damage and repair in rhabdomyosarcoma RD cells. Building on this model, we investigated which double-stranded RNA-binding proteins (dsRBPs) could modulate this interaction, and identified Staufen1 (STAU1) as the main dsRBP capable of interacting with both circHIPK3 and BRCA1 mRNA in vivo, as demonstrated by RIP and CLIP experiments. STAU1 overexpression increased BRCA1 protein levels without affecting mRNA or circHIPK3 levels, while its downregulation caused BRCA1 protein decrease, elevated DNA damage markers and enhanced sensitivity to genotoxic drugs. This regulation was conserved across multiple cancer cell lines. Notably, depletion of circHIPK3 reduced the STAU1-BRCA1 mRNA interaction, indicating that circRNA-mRNA pairing is required for STAU1 recruitment. As a further confirmation, a dsRNA-binding deficient STAU1 mutant failed to upregulate BRCA1, confirming that STAU1 dsRNA binding activity is essential to regulate BRCA1 expression. Polysome profiling revealed that STAU1 associates with translating ribosomes and that its depletion shifts BRCA1 mRNA from heavy polysomes to lighter fractions, impairing translational efficiency. RNA pulldown experiments further showed that STAU1 depletion increases circHIPK3-BRCA1 mRNA association in monosomal and polysomal fractions, suggesting that STAU1 resolves the circRNA-mRNA duplex during active translation. Together, these findings describe a dynamic regulatory mechanism in which circHIPK3 promotes BRCA1 translation initiation by blocking FMRP, while STAU1 subsequently resolves the circRNA-mRNA pairing on translating ribosomes and likely remodels secondary structures along the coding sequence to ensure efficient elongation, shedding light on novel aspects of circRNA-mediated gene expression control in cancer.