Replication stress underlies genomic instability at CTCF/cohesin-binding sites in cancer

CCCTC-binding factor (CTCF) and cohesin play a significant role in the formation of chromatin loops and topologically associating domains (TADs), which influence gene expression and DNA replication. CTCF/cohesin-binding sites (CBSs) present at the loop anchors and TAD boundaries are frequently mutated in cancer; however, the molecular mechanisms underlying this remain unclear. Here, we investigate whether the binding of CTCF/cohesin on DNA imposes constraints on DNA replication, leading to replication stress and genomic instability. Our results reveal that CTCF and cohesin remain cobound to DNA during replication (S phase) in cancer cells (HeLa). Further, examination of replication stress through ChIP-seq of the DNA damage response/repair proteins (MRE11, STN1, FANCD2, ?H2AX, RAD51 and ATM) showed high enrichment of these proteins at CBSs (as compared to their immediate flanking regions and control sites) and positively correlated with the binding strength of CTCF/cohesin at CBSs in the S phase. Moreover, analysis of somatic mutations from cancer genomes supports that the enrichment of mutations at CBSs is significantly higher in samples harbouring somatic copy number deletion in MRE11 and STN1 compared to wild-type samples. Together, these results demonstrate that the co-binding of CTCF/cohesin on the DNA during the S phase causes replication stress and DNA strand breaks, and this could lead to genome instability at CBSs observed in cancer. Overall design: Chromatin Immunoprecipitation (ChIP-seq) for the ATM/ CTCF/ FANCD2/ gH2AX/ H2AX/ MRE11/ RAD21/ RAD51 were performed in Mid S synchronized HeLa cells (2mM Thymidine treatment for 16h followed by 4 h release) with or without DNA damage using 2mM HU (for 3h) or 25uM Etoposide (for 3h). Prolonged (ie, 16h) HU (2mM) treatment was performed in asynchronous HeLa cells