CircHIPK3 and FMRP compete for direct binding to BRCA1 mRNA controlling its protein levels and DNA damage response [RNA_pulldown]

Circular RNAs (circRNAs) are covalently closed RNA molecules widely expressed in eukaryotes and regulated in several pathological processes, including cancer. Many studies point to their mechanism of action as miRNA and protein sponges; however, we propose a new functionality based on circRNA-mRNA interaction to regulate mRNA fate. We show that the widely expressed circHIPK3 directly interacts in vivo with the BRCA1 mRNA through the back-splicing region. This interaction favoured the translation of BRCA1 by competing for the binding of the FMRP1 RNA-binding protein, which acts as a repressor of BRCA1 translation. CircHIPK3 depletion or disruption of the CircRNA-mRNA interaction decreased BRCA1 levels and increased DNA damage, sensitizing several cancer cell lines to DNA-damage-inducing agents and rendering them susceptible to synthetic lethality. Additionally, mimicking circHIPK3 interaction with lock-nucleic acid (LNA) oligonucleotides restores BRCA1 levels in hereditary breast cancer model cells, underscoring circRNA-mRNA interaction’s importance in regulating cell homeostasis and drug response.