Our work over the last decade has helped identify a new target for killing cancer cells, opening the door for potential new therapy. The target pathway is utilised by cancer cells to repair DNA double-stranded break repair and was published recently in the journal Cell Reports.
DNA replication
Topoisomerase 1-targeted chemotherapy is one of the mainstays of treating cancer cells. Currently-used anti-cancer drugs (Camptothecin, Topotecan and Irinotecan) target a molecule (the enzyme Topoisomerase 1 or Top1) involved in DNA replication. While DNA replication is essential to cell division, runaway replication characterises cancer.
However, we found that cancer cells sometimes develop resistance to Topoisomerase 1-targeted chemotherapy through their intrinsic DNA repair toolbox. Based on these insights, a combination of molecules (the protein PRMT5, and the enzyme TDP1) can be used as potential targets for developing novel anti-cancer therapeutics, thus taking us a step closer to developing precision medicine approaches for cancer patients.
Top1, an enzyme in all higher eukaryotes, is essentially responsible for relaxing DNA as it coils during replication (and transcription). The drugs directed at this pathway disrupt the activity of Top1 by changing its shape and rendering it ineffective. While these result in a significant amount of cell death, including cancer cells, natural cellular repair mechanisms (using TDP1) often kick in and counteract the action of the drug.
We developed CRISPR-mediated knock-out cells where the PRMT5 (Protein arginine methyltransferase 5) enzyme in the cells is no longer present. When challenged with a low dosage of camptothecin which is below the toleration levels used in chemotherapy, we found that the cancer cytotoxicity increased markedly. This helped confirm that PRMT5 deficiency in the cell is the target of the camptothecin.
Drugs combination
The enzyme PRMT5 is broadly overexpressed in many cancer cells. Therefore, targeting the PRMT5 enzyme with drugs in combination with low dosage camptothecin will help in killing cancer cells more effectively.
The PRMT5 enzyme, which is found in abundance in cancer cells, directly regulates the natural cellular repair mechanisms through chemical finetuning. This results in repairing of DNA breaks generated by camptothecin and thus, resistance to chemotherapy.
In the last decade, our lab has been investigating DNA repair pathways that offer resistance to camptothecin and its clinical derivatives. The objective was to uncover new avenues to kill cancer cells in target-based chemotherapy or personalised chemotherapy using breast and ovarian cancer. We have been using mouse models to further test the combination drug therapy using in vivo tumours.
Major breakthrough
In 2018, we achieved a breakthrough with the identification of the DNA repair proteins TDP1 (Tyrosyl DNA phosphodiesterase 1) and PRMT5 binding which was published in Nucleic Acids Research. But at that time, we did not understand the mechanism of action. Continuous research in the last five years helped unravel the mechanism in detail and the implications of a combinatorial chemotherapy.
PRMT5 inhibitor, GSK3326595, has been approved as a monotherapy in phase II clinical trials of cancer. Therefore, our latest work provides a new rationale for using the combination of Top1-PRMT5 inhibitors in tumorigenesis.
A personalised approach
Since the rate of proliferation is higher in the case of cancer cells, the chances of combination drug uptake are higher. The personalised approach of combinatorial chemotherapy will effectively kill cancer cells bypassing induced chemoresistance.
More studies are needed to confirm the lab results, and the end goal is to extend the basic research to human clinical trials to assess the therapeutic potential.
( Benu Brata Das is Professor at the School of Biological Sciences, Indian Association for the Cultivation of Science, Kolkata.)
