Abstract 2542: MSI2 Binds LncRNAs and Promotes Self-Renewal and Oncogenesis Through MYC Expression

Abstract Backgrounds and Aims: RNA-binding protein MSI2 is elevated in several cancers and is linked to poor prognosis. Here, we sought to elucidate the role of MSI2 in regulating the expression of proto-oncogenes or tumor suppressor genes in hepatocellular carcinoma (HCC). Methods: We performed RIP-seq using anti-MSI2 antibody in tumor-initiating stem-like cells (TICs) and subsequent validation by qPCR analysis. Results: Among the MSI2-bound RNAs, MYC mRNA and long noncoding RNA (LncRNA) miR22 host gene (MIR22HG) and MARAT1 were identified by RIP-qPCR analysis. MSI2-MARAT1 binding dereppresses polycomb reppressor to recruit E2F1 in MYC promoter regions. It is known that the 5’-untranslated region of MYC contains an internal ribosome entry sequence (IRES) which allows IRES-dependent translation initiation along with canonical cap-dependent mechanisms. Our data show that MSI2 acts as an IRES trans-acting factors for MYC mRNA translation. MSI2 does not have any effect on the steady state level of MYC mRNA but significantly increases MYC protein levels, indicating that MSI2 regulates MYC at the post-transcriptional level. Further, we observed that MSI2 reduced the level of mature miR22 derived from MIR22HG processing. We showed that miR22 suppresses the expression of MYC thorough a 3’-UTR dependent process. Overexpression of MSI2 promotes TIC self-renewal and tumor initiation property in xenograft tumor mouse models, whereas silencing MSI2 reduces this occurrence. Conclusion: we demonstrate that MSI2 promotes liver tumorigenesis by maintenance of MYC expression through dereppression of polycomb repressor and an IRES-dependent translation mechanism and possibly by inhibition of miR22 processing. Thus, MSI2 may be a useful prognostic factor for HCC and MSI2 targeted therapy and thus may be beneficial in the treatment of HCC patients. Citation Format: Hifzur Siddique, Padmani Narayan, Vasu Punj, Douglas E. Feldman, Keigo Machida. MSI2 binds LncRNAs and promotes self-renewal and oncogenesis through MYC expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2542. doi:10.1158/1538-7445.AM2017-2542