<p class="MDPI12title">Simultaneous Study of Circular RNAs and Messenger RNAs in Colorectal Cancer: The Unbalanced Fate of a Couple?

Background/Objectives: Circular RNAs (circRNAs) are emerging players in human diseases, with function as part of competing endogenous networks. Given the importance of messenger RNA (mRNA) regulation in human diseases and the potential of circRNAs in this regulation, we studied the circRNA-mRNA couple in blood within a cohort of 712 patients suspected of having hereditary colorectal cancer (CRC) and 249 matched controls. Methods: The circRNA-mRNA couple was studied by SEALigHTS (Splice and Expression Analyses by exon Ligation and High-Throughput Sequencing) using a panel of 23 genes involved in CRC predisposition, i.e., 788 probes designed at exon ends, enabling the exploration of all exon-exon junctions. Following reverse transcription and probe hybridization on cDNA, nearby probes are ligated, and the number of ligations quantified using unique molecular identifiers and sequencing. Results: We described 220 circular junctions, including 47 novel ones. The circRNA/mRNA ratio was 1.93-fold higher in patients compared to controls (p&lt;2x10-16), irrespective of age of cancer onset. This increase was mainly driven by POLD1 (fold change 3.11) and a single circPOLD1 with oncogenic potential. Conclusions: This study supports the idea of a physiological balance between circRNA and mRNA that can be disrupted under pathological conditions. It rules out a competitive mechanism between circular and linear transcripts in CRC predisposition and raises questions about the role of specific circRNAs in the development of CRC, either as a cause or a consequence.