The Rare RNA Methylations m²G, Cm, m⁵U and ms²i⁶A: Roles in Disease Pathogenesis and Emerging Therapeutic...

RNA methylation modifications play a central and multifaceted role in various physiological processes by precisely regulating key steps in the RNA life cycle, including nuclear processing, nuclear export, splicing, and cytoplasmic translation. These modifications, which occur on the four nucleotides that constitute RNA strands, are tightly regulated by specific proteins known as "writers," "readers," and "erasers." Advances in high-throughput sequencing and mass spectrometry technologies have progressively unveiled the biological functions of common RNA methylation marks such as N6-methyladenosine (m⁶A), N1-methyladenosine (m¹A), and 5-methylcytosine (m⁵C). However, our understanding of how RNA modifications influence various cellular processes remains limited, and research focusing on the biological significance of rare RNA methylation modifications is particularly scarce. This review shifts the research focus toward several relatively understudied and less widely recognized RNA methylation modifications, providing an in-depth analysis of four specific modifications: N2-methylguanosine (m²G), 2'-O-methylcytidine (Cm), 5-methyluridine (m⁵U), and 2-methylthio-N6-isopentenyladenosine (ms²i⁶A). It comprehensively elucidates their molecular mechanisms, biological functions, and associations with disease. In addition, this article summarizes the current methodologies available for detecting RNA modifications and discusses the potential applications of these RNA methylation modifications in disease therapy.