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

Hypoxia influences the epithelial-mesenchymal transition (EMT) through the remodeling of the chromatin structure, epigenetics, and alternative splicing. Hypoxia drives CCCTC-binding factor (CTCF) induction through hypoxia-inducible factor 1-alpha (HIF1α), which promotes EMT, although the underlying mechanisms remain unclear. We find that hypoxia significantly increases CTCF occupancy at various EMT-related genes. We present a CTCF-mediated intricate mechanism promoting EMT wherein CTCF binding at the collagen type V alpha 1 chain (COL5A1) promoter is crucial for COL5A1 upregulation under hypoxia. Additionally, hypoxia drives exon64A inclusion in a mutually exclusive alternative splicing event of COL5A1exon64 (exon64A/64B). Notably, CTCF mediates COL5A1 promoter-alternatively spliced exon upstream looping that regulates DNA demethylation at distal exon64A. This further regulates the CTCF-mediated RNA polymerase II pause at COL5A1exon64A, leading to its inclusion in promoting the EMT under hypoxia. Genome-wide study indicates the association of gained CTCF occupancy with the alternative splicing of many cancer-related genes, similar to the proposed model. Specifically, disrupting the HIF1α-CTCF-COL5A1exon64A axis through the dCas9-DNMT3A system alleviates the EMT in hypoxic cancer cells and may represent a novel therapeutic target in breast cancer.