Circular RNAs in human biology: from splicing noise to master regulators

Circular RNAs (circRNAs) are covalently closed RNA transcripts produced by back-splicing. First identified in plant viroid in 1976, they were long regarded as "splicing noise" or transcriptional byproducts. However, high-throughput sequencing has revealed thousands of circRNAs in eukaryotic cells, indicating that they are widespread and conserved. These molecules lack 5' caps and 3' poly(A) tails, making them resistant to exonucleases. Their circular structure confers exceptional stability, and many circRNAs are expressed in a tissue-specific manner, making them attractive biomarkers. Functionally, circRNAs regulate gene expression at multiple levels. Many act as "sponges" for microRNAs or RNA-binding proteins, modulating mRNA stability and translation; others influence transcription, alternative splicing, or chromatin architecture, and some can be translated into peptides. In human biology, circRNAs have diverse roles: dysregulated circRNAs contribute to tumorigenesis and shape the tumor microenvironment; they are abundant in the brain and implicated in neurodevelopment and neurodegenerative diseases; and they modulate immune responses (for example, via T cells, NK cells, and macrophages). Experimentally, circRNAs are detected by RNase R-enriched RNA sequencing and bioinformatics pipelines that identify unique back-splice junction reads. Predicted circRNAs are then validated by RT-PCR across the circular junction. The stability and versatility of circRNAs have inspired therapeutic applications, such as engineered circRNA vaccines or miRNA sponges. Overall, these findings underscore that circRNAs constitute a powerful and versatile class of gene regulators with significant diagnostic and therapeutic potential, although translating this potential into clinical success requires overcoming current challenges. Nonetheless, significant challenges in delivery, specificity, and immunogenicity remain to be addressed before clinical translation.