m5C Methylation of mRNA: Still More Questions Than Answers

mRNA plays a pivotal role in cellular processes of genetic information transfer, and post-transcriptional modifications of its nucleotides enable regulation of these processes with each particular mRNA. m5C methylation is a specific RNA modification that is rather common in tRNA, rRNA, lncRNA and other types of non-coding RNAs, whereas in mRNA it is found not very often. However, the functioning of m5C-methylated mRNAs differs from the non-methylated ones quite dramatically. Of the eight m5C RNA methyltransferases in humans, only two, NSUN2 and NSUN6, were found to be capable of modifying the main portion of cellular mRNAs. A deficiency of NSUN2 and NSUN6 causes global changes in the transcriptome and translatome, and a corruption of the NSUN2 gene in humans is associated with neurodegenerative diseases and intellectual disability. Despite intensive research of m5C mRNA methylation in recent years, many aspects of this phenomenon and its significance remain problematic and far from understood. In this review, we discuss the available information on mRNA methylome biogenesis, methods for its study and analysis, effects of m5C modification on mRNA life, and place it in the general context of cell biology, attempting to draw the biological relevance of m5C mRNA methylation. We highlight the main inconsistencies and difficulties that arise, analyze their possible causes, and propose potential directions for further research that could clarify the controversial issues and provide a link between the NSUN2 deficiency and neurodegenerative diseases.