SRSF2 plays an unexpected role as reader of mC on mRNA, linking epitranscriptomics to cancer.

A common mRNA modification is 5-methylcytosine (mC), whose role in gene-transcript processing and cancer remains unclear. Here, we identify serine/arginine-rich splicing factor 2 (SRSF2) as a reader of mC and impaired SRSF2 mC binding as a potential contributor to leukemogenesis. Structurally, we identify residues involved in mC recognition and the impact of the prevalent leukemia-associated mutation SRSF2. We show that SRSF2 binding and mC colocalize within transcripts. Furthermore, knocking down the mC writer NSUN2 decreases mRNA mC, reduces SRSF2 binding, and alters RNA splicing. We also show that the SRSF2 mutation impairs the ability of the protein to read mC-marked mRNA, notably reducing its binding to key leukemia-related transcripts in leukemic cells. In leukemia patients, low NSUN2 expression leads to mRNA mC hypomethylation and, combined with SRSF2, predicts poor outcomes. Altogether, we highlight an unrecognized mechanistic link between epitranscriptomics and a key oncogenesis driver.