The odyssey of MYC transcript from the nucleus to the cytoplasm : the molecular mechanism of the gene gating in human cancer cells

Abstract: Pathological expression of the MYC oncogene is a common denominator in a wide range of cancers and is linked with abnormal cell proliferation. To achieve this status, the MYC gene benefits from being embedded in a region rich in enhancers and super-enhancers that are often absent in the normal cell counterparts. How those regions regulate MYC transcription and expression is, however, not well understood, although likely players include enhancer-binding factors, the 3D nuclear architecture and local eRNAs and ncRNAs. In this thesis, two new models governing MYC expression have been identified. The first describes a posttranscriptional mechanism that is based on the gene gating concept proposed in 1985, while the second is based on the ability of the non-coding eRNA, CCAT1 to promote MYC transcription, which paradoxically antagonizes the gating of MYC. In Paper I, a model of gene gating mechanism of MYC in human cancer cells is proposed. Briefly, the Nucleopore Complex (NPC) member ELYS (or AHCTF1) recruits MYC and its distal Oncogenic Super Enhancer (OSE) to the nuclear pore in a b-catenin -dependent manner. This principle increases MYC expression post-transcriptionally by facilitating the nuclear export of its derived mRNAs and thus enabling the escape of MYC transcripts from the faster degradation rate in the nucleus compared to the cytoplasm. In Paper II, a CTCF binding site within the non-coding gene, CCAT1, positioned within the OSE, was mutated using CRISPR technique. Expanded clones carrying the mutated CTCF binding site revealed that this site is essential for the canonical WNT-mediated gating of MYC. Normally ascribed an insulator function, this non-canonical feature of CTCF was essential for the recruitment of ELYS/AHCTF1 to the OSE, thereby effectuating its anchoring to the nuclear pore. In addition, this report shows that CTCF is essential for the WNT-mediated activation of the CCAT1 gene. In Paper III, the role of the OSE transcript CCAT1 in the gating mechanism was further analyzed. siRNA-mediated knockdown of CCAT1 eRNA expression revealed its dual function. While it promotes MYC transcription in the nuclear interior, it impedes the nuclear export of its derived mRNAs. We speculate that the CCAT1 eRNA likely indirectly alleviates transcriptional pausing of MYC transcription. Conversely, transcriptional pausing is proposed to promote the migration of the MYC gene to the nuclear pores to provide a key switch in the nuclear export pathways of MYC mRNAs. In summary, this work has identified two new models of MYC expression regulation in cancer cells, thereby providing opportunities for designing new pharmaceutical strategies targeting pathological expression of this central oncogene during cancer evolution.