Hypoxia-induced CTCF mediates alternative splicing via coupling chromatin looping and RNA Pol II pause to promote EMT in breast cancer

Cancer cells experiencing hypoxic stress employ epithelial-mesenchymal transition (EMT) to undergo metastasis through rewiring of the chromatin landscape, epigenetics and importantly alternative splicing. However, how hypoxia influences expression and genome-wide redistribution of CTCF is still not known. Here, we performed CTCF-ChIP-seq and CUT&RUN assay to analyze differential occupancy of CTCF in normoxic and hypoxic breast cancer cell line. We discovered that hypoxia-induced gain of CTCF occupancy across the genome regulates EMT in hypoxic cells. Specifically, we did 4C-seq experiments on COL5A1 (EMT associated gene) gene loci and showed that CTCF-mediated promoter-exon upstream looping regulates epigenetic mark mainly DNA demethylation at distal alternatively spliced exon 64 and regulates CTCF-mediated RNA Pol II pausing at COL5A1exon64A that decides the outcome of splicing to favor EMT under hypoxia. Overall, we uncover that hypoxia-induced differential CTCF occupancy across the genome is associated with gene expression and alternative splicing events related to cancer progression, similarly to the proposed model.