BRCA1 levels and DNA damage response are controlled by the competitive binding of circHIPK3 or FMRP to the BRCA1 mRNA

Circular RNAs (circRNAs) are a class of lncRNAs composed of a single-stranded RNA molecule which forms a covalently closed circular structure. CircRNAs can influence gene expression regulation exerting a variety of functions, as they can interact with DNA, RNA and proteins. Moreover, circRNAs have been implicated in various pathological conditions, such as cancer. CircHIPK3 is a circRNA deregulated in several cancer types, usually being upregulated, and it has been regarded primarily as a miRNA sponge. This perspective fails to consider the intricate miRNA signatures that distinguish different tumour types. Therefore, investigating alternative molecular mechanisms for the widespread involvement of circHIPK3 in cancer onset and progression becomes interesting. In this context, the present study explores the possibility of circRNA-mRNA interactions as a novel model of RNA-RNA interaction. A circHIPK3 psoralen-crosslinking RNA pulldown has been performed in RD cells, a cellular model for rhabdomyosarcoma, allowing the identification of a variety of mRNAs that interact in-vivo with circHIPK3. Among them, BRCA1 mRNA was selected due to its pivotal role in cancer, being traditionally described as a tumour suppressor gene involved in DNA damage response. CircHIPK3-BRCA1 mRNA functional interaction has been validated in RD cells and in several other cancer cell lines, pointing out how circHIPK3 can widely regulate BRCA1 RNA and protein levels. Additionally, the study investigates the potential therapeutic implications of targeting circHIPK3-mediated BRCA1 mRNA regulation, particularly in sensitizing cancer cells to DNA damage-inducing drugs and PARP inhibitors. In fact, BRCA1 depletion caused by circHIPK3 knock-down through siRNA approach or by LNAs that disrupt circHIPK3-BRCA1 mRNA interaction, leads to DNA damage accumulation, thus showing a synergistic effect in combination with DNA damage-inducing drugs and a synthetic lethality effect with PARP inhibitors. In conclusion, this research aims to shed light on the complex interplay between circRNAs and mRNAs, with the hope of uncovering novel therapeutic targets and strategies for a wide range of pathological conditions, including cancer.