Disrupting translesion synthesis (TLS) activity enhances cell death and prevents tumor growth

DNA-damaging chemotherapy agents such as cisplatin have been a first line approach in cancer treatment for decades; however, their long-term success is often reduced by intrinsic and acquired drug resistance. Although the mechanisms causing drug resistance are quite distinct, they are directly connected to mutagenic translesion synthesis (TLS). The TLS pathway promotes DNA damage tolerance by supporting both replication opposite to a lesion and inaccurate single strand gap filling. Inhibiting TLS reduces cisplatin resistance and secondary tumor formation. Therefore, targeting TLS is a promising strategy for improving chemotherapy. MAD2L2 (i.e. Rev7) is an essential component of the TLS polymerase zeta, and it forms a complex with Rev1 polymerase. Here we present the discovery of a small molecule c#3 that directly binds to MAD2L2 and inhibits TLS activity. Using a mouse model, we demonstrated that combined treatment with cisplatin and c#3 significantly prevented tumor growth. Moreover, c#3 sensitized various cancer cell lines to cisplatin treatment. Co-treatment caused significant elevation in DNA damage levels, explaining the sensitization effect. Using diverse biochemical methods, we demonstrated that c#3 directly binds to MAD2L2 and prevents the formation of the TLS complex in cancer cells. Importantly, the drug is not cytotoxic when administered alone. Co-treatment of cisplatin and c#3 will allow significant reduction in the needed concentration of cisplatin during treatment, reducing side effects. Therefore, c#3’s activity underlines its potential as a lead compound for developing novel TLS inhibitors for improving chemotherapy treatment.