Circular RNAs in cancer: Its biogenesis, functions, relationships with cancer progression, applications in immunotherapy and biomarker potentials

Circular RNAs (circRNAs) are covalently closed, single-stranded RNAs generated via backsplicing. They are highly stable and evolutionarily conserved, making them promising candidates for cancer therapy and diagnosis. CircRNAs regulate cancer progression by modulating genome instability, angiogenesis, metastasis, stemness, and chemoresistance. They do so through mechanisms including microRNA (miRNA) sponging, protein interaction, translational templating, and transcription/translation regulation. CircRNAs play a critical role in cancer immunotherapy. They modulate immune checkpoint blockade (ICB) responses and cytokine secretion to reshape the tumor immune microenvironment (TME). CircRNAs also serve as stable platforms for neoantigen-based cancer vaccines and improve in vivo chimeric antigen receptor T cell (CAR-T) therapy by replacing unstable linear mRNA. Additionally, circRNAs are potential noninvasive biomarkers due to their abundance in body fluids and differential tumor-normal expression. Despite challenges such as unclear regulatory networks, off-target effects, and inefficient delivery, this review systematically summarizes the biogenesis of circRNAs, their functional mechanisms, their roles in cancer progression, and their applications in cancer immunotherapy. The review also highlights their utility as biomarkers and future translational directions, providing a focused overview of their potential to advance cancer immunotherapy.