Epithelial-to-mesenchymal transition transcription factors (EMT-TFs) induce genomic instability via down-regulation of DNA ligase I (LIG1) in carcinoma cell lines

Epithelial-to-mesenchymal transition (EMT) is the reversible process in which cells lose their polarity and cell-cell adhesion and acquire mesenchymal traits. EMT is induced by a network of transcription factors known as EMT-TFs. This group of proteins includes members of ZEB, TWIST and SNAIL families; they play a role in embryonic development, and also in pathological conditions, tissue fibrosis and cancer. EMT-TFs control various cancer related pathways including cell cycle control, DNA damage response and chemo- and radioresistance. Our data demonstrate that one of the genes whose expression is strongly repressed by ZEB proteins is DNA ligase 1, LIG1. LIG1 catalyses joining Okazaki fragments during DNA replication, and is implicated in Nucleotide Excision and Homologous Recombination DNA repair pathways. Although functional redundancy in LIG1 and another DNA ligase, LIG3 has been reported, down-regulation of LIG1 during an EMT results in the delay in single strand breaks repair induced by low-dose of UV.

We show that ZEB proteins arrest cells in G1 phase of the cell cycle, and reduce transcription of LIG1 in Rb-E2F-dependent manner. Cell cycle arrest largely occurs through up-regulation of the cyclin dependent kinases inhibitor p27Kip1. Knockdown of p27KIP1 uncouples EMT from cell cycle arrest and down-regulation of LIG1 resulting in LIG1-deficient actively proliferating cells. We show that these cells are characterised by a high level of DNA damage and elevated chromosomal aberrations. We also show that depletion of LIG1 is by itself sufficient for the induction of chromosomal instability.

EMT programs lead to the formation of a pool of dormant circulating tumour cells, which extravasate at distant organs and undergo mesenchymal epithelial transition (MET) to form metastases. We modelled a transient EMT in vitro, and found that cell cycle re-entry is mutagenic and associated with reduced LIG1 expression and enhanced level of fusions. Additionally, transient EMT results in a fluctuation in the average number of chromosomes and increases cell diversity with regard to the level of ploidy. We propose that transient EMT contribute to genomic instability in cancer.