DNA damage caused by replication inhibitors
Many cytotoxic chemotherapy drugs specifically target cancer cells by interfering with DNA replication, which is essential for cancer proliferation (examples: gemcitabine, 5-fluorouracil, cisplatin). These replication inhibitors stall replication fork progression, which causes toxic double-strand breaks (DSBs). Their therapeutic action can be potentiated by cancer-specific defects in DNA repair, e.g. mutation in the homologous recombination (HR) genes BRCA1 and BRCA2.
BRCA2-mutated cells are generally more sensitive to replication inhibitors than normal cells. Strangely however, they are less sensitive to gemcitabine. We have found that gemcitabine efficiently causes DSBs in normal cells, but leads to fewer DSBs in BRCA2-mutant cells. This suggests that sometimes, DNA repair can promote rather than prevent DSB formation during replication stalling (Jones et al, 2014).
To better understand this, we are now investigating the cellular pathways that promote DSB formation during replication inhibitor treatments in more detail. DNA replication is an important target for cancer therapy. Uncovering the molecular mechanisms by which replication inhibition kills cells may help to exploit this target much more effectively in the future.
Endogenous replication stress in cancer
Faithful and complete replication of the genome is essential to maintain genomic stability and prevent cancer-promoting mutations. It has been shown that cancer cells can exhibit elevated DNA damage as a result of faulty chromosome replication, which is also known as replication stress. Replication stress may be a major cause of cancer-driving genomic instability.
De-regulation of proliferation by oncogenes is thought to be the cause of replication stress in cancer. However, very little is known about the mechanisms by which hyper-proliferation in cancer may cause replication stress. My lab is interested in finding such mechanisms. Firstly, we have shown that overexpression of the oncogene Cyclin E causes increased levels of replication initiation, which slows down of replication fork progression, thus promoting replication-associated DNA damage. Secondly, we also observed that a considerable portion of Cyclin E-induced replication stress results from interference between replication forks and the transcription machinery (Jones et al, 2013).
We are now further investigating transcription-replication interference as a potential mechanism of replication stress in cancer cells. Knowing which cellular pathways can cause replication stress will help to properly detect and exploit replication stress for cancer therapy.