Role of N6-Methyladenosine (m6A) RNA Modification in Multiple Myeloma

Multiple myeloma (MM) is considered a chronic and incurable disease due to its highly complex and heterogeneous molecular abnormalities. In recent years, integrating proteasome inhibitors and immunomodulatory drugs into MM frontline therapy has significantly improved treatment efficacy with a median overall survival (OS) being prolonged from 3-4 to 7 years. Despite this progress, patients refractory to the aforementioned agent classes display a median OS of only 9 months. Thus, the clinical necessity for developing novel therapeutic alternative approaches is self-evident.

Methylation of N6-adenosine (m6A) is known to be important for diverse biological processes including gene expression control, translation of protein, and messenger RNA (mRNA) splicing. m6A regulatory enzymes consist of "writers" METTL3 and METTL14, "readers" YTHDF1 and YTHDF2, and "erasers" FTO and ALKBH5. However, the functions of m6A mRNA modification and the specific role of these enzymes in MM remain unknown.

Here we report that METTL3, a key component of the m6A methyltransferase complex, is highly expressed in MM cell lines and in isolated patient's MM cells. In contrast, we found no significant differences in the expression of the m6A demethylases FTO and ALKBH5. Accordingly, compared to plasma cells from healthy donors, global PolyA⁺ RNA showed a significant increase in m6A content in patient's MM plasma cells. In MM cell lines, global m6A profiling by methylated RNA-immunoprecipitation sequencing revealed m6A peaks near the stop codon in mRNAs of multiple oncogenes including MAF and CCND1. Cross-linking immunoprecipitation showed that METTL3 bound to the m6A peak within MAF and CCND1 mRNA. Depletion of METTL3 by shRNA had little effect on global mRNA levels, but specifically reduced protein levels of c-Maf and Cyclin D1. Moreover, downregulation of METTL3 in several MM cell lines results in cell cycle arrest and apoptosis.

Together, these results describe a role for METTL3 in promoting translation of a subset of oncogenes in MM and identify this enzyme as a potential therapeutic target for multiple myeloma.