Cdc6 Overexpression Causes Non-Classical DNA Replication Stress Leading to Rapid Senescence

Oncogene-induced senescence is an important tumor-suppressive mechanism that involves induction of DNA replication stress, prematurely terminated DNA replication fork progression and DNA double-strand breaks (DSBs). This triggers a DNA damage response, resulting in senescence of the damaged cells, thus serving as a barrier to neoplastic transformation.

Cdc6 is an essential protein for licensing of origins of DNA replication. It is overexpressed in various cancers and holds oncogenic roles through transcriptional repression of tumor suppressors and epithelial markers. Cdc6 overexpression leads to rapid senescence of human bronchial epithelial (HBE) cells. Here we studied the effect of CDC6 overexpression on replication dynamics using the high resolution DNA combing technique. The analysis shows that in pre-senescent Cdc6-overexpressing HBE cells the replication dynamics is perturbed, characterized by reduced origin firing and increased DNA replication fork rates, compared to that in naive HBEs. These results highlight a novel oncogenic role of Cdc6 that includes perturbed replication dynamics. Further analyses aimed to investigate the mechanism underlying this perturbed replication and its role in driving senescence will be discussed. The results are of high relevance to the understanding of basic mechanisms regulating DNA replication, as well as genome instability. Interestingly, HBE cells overexpressing Cdc6 for a prolonged time, show a sub-population of replicating cells emerging from senescence, while still expressing Cdc6. We are further analyzing the replication dynamics in these cells, the results of which will be discussed.