ABSTRACT An emerging body of evidence suggests that mRNA methylation plays a key role in the post-transcriptional regulation of gene expression. Methylation can regulate mRNA splicing, stability, and translation, but the health significance of mRNA modification has not been studied in cardiovascular disease. Now, in a mouse model of cardiac hypertrophy and heart failure, we have identified the first example of mRNA 2'-O-methylation (Nm) affecting cardiovascular pathology. Based on our findings, we propose to delineate the specific mechanisms by which Nm modification of mRNA influences cardiac hypertrophy and heart failure. Our long-term goals are to determine the overall importance of mRNA Nm methylation in cardiac disease susceptibility, and to define the relevant mechanisms. The objective of this proposal is to determine specifically how loss of Rpl13a snoRNAs leads to protection from cardiac hypertrophy and heart failure. Our central hypothesis is that Rpl13a snoRNAs guide Nm modification of multiple mRNA targets in the heart, and that these modifications regulate physiologic and pathophysiologic gene expression at the level of mRNA abundance and protein translation. We propose to test our hypothesis with the following Specific Aims: (1) Define how Rpl13a snoRNAs regulate cardiac hypertrophy and heart failure. (2) Determine the in vivo network of snoRNA-guided Nm modifications on cardiac mRNA. (3) Determine how snoRNA-guided Nm modifications on mRNA alter protein expression in both normal development and pathologic hypertrophy. Successful completion of these Aims will define, for the first time, how snoRNA-guided mRNA methylation contributes to cardiovascular health and disease. As a result, we expect that our findings will open new avenues in the study of cardiac hypertrophy and heart failure. Finally, since snoRNA- guided Nm modification of mRNA has not been previously reported, these results will also be broadly informative in the field of RNA modifications.