Efficient genome duplication is essential for maintaining genetic stability. DNA replication can be slowed by DNA lesions, transcription and RNA-DNA hybrids, limited supplies of nucleotides, fragile sites, and oncogene expression. Slowing of replication fork progression, regardless of the source, is termed “replication stress”. When replication stress is encountered, ATR kinase, one of the master regulators of the DNA damage response, coordinates the repair of stalled or collapsed replication forks to re-start the DNA replication process.

It is now understood that endogenous replication stress is a common feature in cancer, and targeting DNA replication, or the pathways that respond to DNA replication stress, are active targets of chemotherapeutic development. Recent evidence from a number of laboratories has shown that chemical ATR inhibition (ATRi) can selectively kill cancer cells. It is expected that ATRi works by exacerbating the endogenous replication stress in cancer cells. However, the mechanism(s) of cell death due specifically to ATRi treatment, or in response to replication stress in general, remain undefined.

As presented elsewhere at this meeting (abstract from V.P. Masamsetti), our lab has identified how pharmacologically induced DNA replication stress translates to cell death through an unexpected mechanism. In brief, DNA replication stress induces Spindle Assembly Checkpoint-dependent mitotic arrest. The mitotic arrest then drives independent pathways that 1) change telomere structure, independent of telomere length, leading to DNA damage response activation at chromosome ends; and 2) cohesion fatigue resulting from microtubule pulling forces and cohesin opening by WAPL. Both pathways contribute to cell death. This discovery also identified a mechanism of cell death through mitotic catastrophe.

In this project, we are investigating how ATRi treatment translates to cell death in cancer cells. Using the ATR inhibitor VE-822, we have identified cancer cell lines that are sensitive to ATRi, and identified that ATRi sensitivity is not a function of telomere length maintenance mechanism (telomerase vs ALT) as described previously. Our data show cell death due to ATRi treatment primarily occurs during mitotsis, following mitotic arrest. We are currently in the process of identifying if ATRi treatment induces a telomere-length independent DNA damage response, and cohesion fatigue, and if sensitivity to ATRi is a consequence of endogenous levels of DNA replication stress. Progress on these experiments will be reported at the conference.