PRMT5 regulates alternative splicing of TCF3 under hypoxia to promote EMT and invasion in breast cancer

Tumor hypoxia induced alterations in the epigenetic landscape and alternative splicing influence cellular adaptations. PRMT5 is a type II protein arginine methyl transferase that regulates several tumorigenic events in many cancer types. However, the regulation of PRMT5 and its direct implication on aberrant alternative splicing under hypoxia remains unexplored. In this study, we observed hypoxia induced upregulation of PRMT5 via the CCCTC binding factor, CTCF. Further, PRMT5 mediated symmetric arginine dimethylation H4R3me2s and H3R8me2s directly regulated the alternative splicing of Transcription Factor 3 (TCF3). Under hypoxia, PRMT5 mediated histone dimethylation at the intronic conserved region (ICR) present between TCF3 exon 18a and exon 18b recruits DNA methyl transferase 3A (DNMT3A), resulting in DNA methylation. DNA methylation at the TCF3-ICR is recognized and bound by Methyl CpG binding protein (MECP2) and results in RNA-Pol II pausing, promoting the recruitment of the negative splicing factor PTBP1 to the splicing locus of TCF3 mRNA. PTBP1 promotes the exclusion of exon 18a and inclusion of exon 18b which results in the production of the pro-invasive TCF3-18B (E47) isoform which promotes EMT and invasion of breast cancer cells under hypoxia. Collectively, our results indicate that under hypoxia, PRMT5 mediated symmetric arginine dimethylation of histones regulates alternative splicing of TCF3 gene thereby enhancing EMT and invasion in breast cancer.