Disequilibrium between BRCA1 and BRCA2 Circular and Messenger RNAs Plays a Role in Breast Cancer

SIMPLE SUMMARY: Breast cancer is a common disease that is responsible for the deaths of approximately 700,000 women per year i.e., 15.5% of all female deaths from cancer. Significant progress has been made in the management of breast cancer but new biomarkers for early detection and prognosis have yet to be identified. Circular RNAs are a class of transcripts that are still poorly understood and share their mode of production with messenger RNAs. We have studied the circular RNA/messenger RNA pair in BRCA1- and BRCA2-related breast tumorigenesis using an innovative dedicated technique and have shown for the first time that there is an imbalance in the circular RNA/messenger RNA ratio between healthy and tumor breast tissues. This very new observation in breast cancer opens up new perspectives in terms of new mechanisms and the associated biomarkers. ABSTRACT: Breast cancer is a frequent disease for which the discovery of markers that enable early detection or prognostic assessment remains challenging. Circular RNAs (circRNAs) are single-stranded structures in closed loops that are produced by backsplicing. CircRNA and messenger RNA (mRNA) are generated co-transcriptionally, and backsplicing and linear splicing compete against each other. As mRNAs are key players in tumorigenesis, we hypothesize that a disruption of the balance between circRNAs and mRNAs could promote breast cancer. Hence, we developed an assay for a simultaneous study of circRNAs and mRNAs, which we have called splice and expression analyses by exon ligation and high-throughput sequencing (SEALigHTS). Following SEALigHTS validation for BRCA1 and BRCA2, our hypothesis was tested using an independent research set of 95 pairs from tumor and adjacent normal breast tissues. In this research set, ratios of BRCA1 and BRCA2 circRNAs/mRNAs were significantly lower in the tumor breast tissue compared to normal tissue (p = 1.6 × 10(−9) and p = 4.4 × 10(−5) for BRCA1 and BRCA2, respectively). Overall, we developed an innovative method to study linear splicing and backsplicing, described the repertoire of BRCA1 and BRCA2 circRNAs, including 15 novel ones, and showed for the first time that a disequilibrium between BRCA1 and BRCA2 circRNAs and mRNAs plays a role in breast cancer.