NSUN-Mediated m5C RNA Modification in Stem Cell Regulation

RNA modifications comprise a core epigenetic dimension of gene regulation; among these, N6-methyladenosine (m6A) and 5-methylcytosine (m5C) have been most intensively investigated. While the functions of m6A in stem cell biology have been well characterized, the contributions of m5C remain comparatively less well defined. This review focuses on m5C modifications catalyzed by the NSUN family of RNA methyltransferases and their roles in regulating stem cell identity, pluripotency, and differentiation. Evidence from embryonic and mesenchymal stem cells, as well as animal models, demonstrates that NSUN-mediated m5C is deposited on diverse RNA substrates, including rRNA, tRNA, mRNA, mitochondrial RNA, and enhancer RNAs, thereby influencing processes such as self-renewal, cell cycle progression, RNA stability, metabolic activation, and lineage specification. Disruption of m5C regulation often leads to developmental defects, underscoring its essential role during embryogenesis. Collectively, these findings establish m5C as a versatile and dynamic regulator in stem cell biology and underscore the need for future studies to delineate the roles of the NSUN family in stem cells and define the RNA targets of m5C. In addition, its broader implications for development, regenerative medicine, and disease, including cancer, as well as its potential interplay with other RNA modifications such as m6A and pseudouridine, remain important areas for further investigation.