Many chemotherapy drugs, including cisplatin, cause damage to DNA and kill cancer cells by interfering with DNA replication and cell division. The damage activates cellular DNA repair mechanisms but, if the damage is too extensive, the cell undergoes apoptosis. Unfortunately, although the initial response to cisplatin is generally good, the majority of tumours will eventually develop resistance to the drug. Resistance can develop when the cell is able to replicate DNA through damaged regions using a translesion synthesis (TSL) DNA polymerase. This type of DNA replication is highly error-prone, introducing mutations into the DNA which can drive drug resistance. Suppressing the ability of tumour cells to replicate damaged DNA using the translesion synthesis DNA polymerase, Polζ has been shown to block resistance to cisplatin in human cancer cells grown in culture and now, in two papers published in PNAS, researchers at the Massachusetts Institute of Technology have shown that the approach also works in mice.
The first paper describes a tumour transplantation approach to examine the effect of impaired translesion DNA synthesis on cisplatin response in aggressive late-stage lung cancers. The researchers used RNA interference to reduce levels of Rev3, an essential component of Polζ, and showed that a 60-70% reduction doubled survival time in cisplatin-treated animals. The team also showed that Rev3-deficient cells showed reduced cisplatin-induced mutations which have been suggested to contribute to secondary malignancies following chemotherapy.
In the second study, the researchers used a mouse model of B-cell lymphoma to show that suppressing Rev1, an essential TSL scaffold protein and dCMP transferase, inhibits both cisplatin- and cyclophosphamide-induced mutagenesis. By performing repeated cycles of tumor engraftment and treatment, the team were also able to show that Rev1 plays a critical role in the development of acquired cyclophosphamide resistance.
The studies show that chemotherapy can not only select for drug-resistant populations of tumour cells but can also directly promote the acquisition of resistance-causing mutations, suggesting that blocking translesion DNA polymerases may have dual anticancer effects by both increasing the sensitivity of tumours to chemotherapy as well as reducing the potential for emergence of drug resistance during treatment. The next challenge will be to identify inhibitors of the translesion DNA polymerases.
The relatively rapid development of drug resistance is a major obstacle to successful chemotherapy. Resistance is frequently attributed to the outgrowth of cells within the tumour which have a genetic survival advantage in the presence of drug treatment such as enhanced drug efflux, impaired drug binding or the ability to use alternative survival pathways. More recently, it has been found that acquired drug resistance does not necessarily need a stable, heritable genetic alteration and, moreover, that response to treatment can be restored following a ‘drug holiday’. Whilst modelling the acute response to a variety of anti-cancer drugs in treatment-sensitive human tumour cell lines, researchers at Massachusetts General Hospital Cancer Center and the Dana-Farber Cancer Institute consistently found a small subpopulation of reversibly ‘drug-tolerant’ cells. They found that whereas the vast majority of EGFR mutant non-small cell lung cancer-derived cells (PC9 cells) were killed by exposure to a high concentration (100 x IC50) of EGFR tyrosine kinase inhibitors (TKIs), a small fraction of cells survived. Similar populations of ‘drug-tolerant persisters’ (DTPs) were found when PC9 cells were treated with cisplatin and also in several other cancer cell lines with established drug sensitivity, suggesting that a drug-tolerant cell subpopulation is broadly present in tumour-derived cell lines.
Although DTPs are largely quiescent, about 20% eventually resume proliferation in the presence of drug to give colonies of cells referred to as ‘drug-tolerant expanded persisters’ (DTEPs) which can propagate indefinitely in the presence of drug. DTPs rapidly regain sensitivity when grown in drug-free media whereas restoration of sensitivity in DTEPs occurs at higher passage number. The reduced drug-sensitivity of both DTPs and DTEPs was linked to increased expression of a gene that encodes a chromatin-modifying enzyme, KDM5A. Although there are, as yet, no inhibitors of KDM5A, its known association with histone deacetylases (HDACs) led the team to test the effect of HDAC inhibitors on DTPs and DTEPs. Trichostatin A, an inhibitor of class I/II HDACs was found to rapidly kill PC9-derived DTPs and DTEPs but to have no effect on parental PC9 cells or TKI-resensitised DTEPs. The team went on to show that continous treatment with HDAC inhibitors, whilst having no effect on growth and survival of parental P9 cells, can prevent the emergence of EGFR TKI resistance. As well as HDAC inhibitors, a selective inhibitor of the insulin-like growth factor 1 receptor (IGF-1R) kinase also virtually eliminated the emergence of EGFR TKI-tolerant DTEPs. IGF-1R signalling was found to be necessary for drug-tolerant phenotypes in other cancer cell lines and to be mediated by the histone-demethylating activity of KDM5A.
The team hope that the results seen in cell culture experiments will extend to cancer patients and have already begun a clinical trial to see whether a combination of a chromatin-modifying agent with the EGFR TKI, erlotinib, may prevent or delay the development of resistance. Although the trial is not yet completed, early data indicate that the inclusion of a chromatin-modifying agent can dramatically improve clinical benefit in a subset of patients demonstrating acquired TKI resistance.