Up-regulation of METTL3 promoted m6A modification and epigenetic silencing of SOCS2 in human liver cancer

Epigenetic alteration is a common trait of human cancers. Chemical modifications on DNA and core histone proteins are the major mechanisms for epigenetic regulation. Recently, emerging evidence suggested that reversible chemical modifications on RNA also play a critical role in epigenetic control of gene expression. N6-methyladenosine (m6A) is the most abundant modification found in mammalian mRNA. m6A is involved in regulating mRNA stability, splicing, and translation. However, the implications of m6A modification in human carcinogenesis remain poorly understood. In this study, we investigated the expression of m6A methyltransferases and demethylases in human hepatocellular carcinoma (HCC). We found the major m6A methyltransferase METTL3 was remarkably up-regulated in HCC. High METTL3 expression was associated with poor overall and disease-free survival in HCC patients. Consistent with the METTL3 overexpression, we found that mRNA m6A level was significantly elevated in human HCC. To investigate the roles of METTL3 in human HCC, we employed lentiviral-based shRNA and CRISPR/Cas9 systems to inactivation METTL3 in HCC cell lines. We showed that knockdown and knockout of METTL3 drastically suppressed HCC proliferation and migration in vitro and abolished HCC tumorigenicity and lung metastasis in vivo. In contrast, overexpression of METTL3 by CRISPR/dCas9 SAM system promoted HCC growth. Using RNA-seq and m6A-Seq, we identified tumor suppressor gene SOCS2 as a novel target of METTL3. We detected m6A modification in SOCS2 mRNA and the m6A modification was diminished upon METTL3 knockdown. m6A modification promoted SOCS2 mRNA degradation. We demonstrated that knockdown of METTL3, mutation of m6A modification sites, and treatment of demethylating agent augmented SOCS2 mRNA expression in HCC cells. In addition, knockdown of YTHDF2, an m6A reader protein, also rescued SOCS2 mRNA expression in HCC cells. The above findings together demonstrated that up-regulation of METTL3 lead to m6A modification of SOCS2, which in turn promotes SOCS2 mRNA degradation through YTHDF2 dependent mechanism. Our findings provided a proof-of-concept model to demonstrate the importance of aberrant m6A modification in epigenetic silencing of tumor suppressor genes. Citation Format: Mengnuo Chen, Lai Wei, Cheuk-Ting Law, Felice H. Tsang, Jialing Shen, Carol L. Cheng, Long-Hin Tsang, Daniel W. Ho, David K. Chiu, Joyce M. Lee, Carmen C. Wong, Irene O. Ng, Chun-Ming Wong. Up-regulation of METTL3 promoted m6A modification and epigenetic silencing of SOCS2 in human liver cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4321.