The Exon Junction Complex Core Represses Cancer-specific Mature mRNA Re-splicing: A Potential Key Role in Terminating Splicing

Using the TSG101 pre-mRNA, we previously discovered cancer-specific re-splicing of mature mRNA that generates aberrant transcripts/proteins. The fact that mRNA is aberrantly re-spliced in various cancer cells implies there must be an important mechanism to prevent deleterious re-splicing on the spliced mRNA in normal cells. We thus postulated that the mRNA re-splicing is controlled by specific repressors and we searched for repressor candidates by siRNA-based screening for mRNA re-splicing activity. We found that knock-down of EIF4A3, which is a core component of the exon junction complex (EJC), significantly promoted mRNA re-splicing. Remarkably, we could recapitulate cancer-specific mRNA re-splicing in normal cells by knock-down of any of the core EJC proteins, EIF4A3, MAGOH or RBM8A (Y14), implicating the EJC core as the repressor of mRNA re-splicing often observed in cancer cells. We propose that the EJC core is a critical mRNA quality control factor to prevent over-splicing of mature mRNA. Previously, it was demonstrated that the induction and overexpression of the tumor suppressor gene TP53 (p53) prevents cancer-specific aberrant splicing of TSG101 pre-mRNA, i.e., generation of the re-spliced TSG101Δ154-1054 mRNA. Therefore, this mRNA re-splicing event is under the control of TP53, presumably indirectly through other regulators; i.e., upregulation of an mRNA re-splicing repressor and/or downregulation of an mRNA re-splicing activator. To identify such repressor or activator candidates, we activated TP53, using highly selective MDM2 inhibitor RG7388 to repress aberrant re-splicing, and searched for up- or down-regulated genes on a microarray. This microarray data were deposited in this GEO database. Using siRNA-based screening for mRNA re-splicing activity, we have identified EIF4A3, one of core factors of EJC, as the repressor of mRNA re-splicing (see above Summary), however, EIF4A3 expression was not up-regulated, but rather down-regulated, under the RG7388-mediated activation of TP53. Therefore, EIF4A3 was not a TP53-dependent repressor of mRNA re-splicing. There could be another unidentified regulator of mRNA re-splicing, which is under the control of the tumor suppressor gene TP53. Reference: bioRxiv 2021.04.01.438154; doi: https://doi.org/10.1101/2021.04.01.438154