A Comprehensive Survey of the Mutagenic Impact of Common Cancer Cytotoxics

Background: Genomic mutations caused by cytotoxic agents used in cancer chemotherapy may cause secondary malignancies as well as contribute to the evolution of treatment resistant tumour cells. The stable diploid genome of the chicken DT40 lymphoblast cell line, an established DNA repair model system, is well suited to accurately assay genomic mutations. Results: We used whole genome sequencing of multiple DT40 clones to determine the mutagenic effect of eight common cytotoxics used for the treatment of millions of patients worldwide. We defined the spontaneous mutagenesis rate at 2.3x10-10 per base per cell division, and found that cisplatin, cyclophosphamide and etoposide induced extra base substitutions with distinct spectra. After four cycles of exposure, cisplatin induced 0.8 mutations per Mb, equivalent to the median mutational burden in common leukemias. Cisplatin-induced mutations, including short insertions and deletions, were mainly located at sites of putative intrastrand crosslinks. We found two of the newly defined cisplatin-specific mutation types as causes of the reversion of BRCA2 mutations in emerging cisplatin-resistant tumours or cell clones. Gemcitabine, 5-fluorouracil, hydroxyurea, doxorubicin and paclitaxel had no measurable mutagenic effect. The cisplatin -induced mutation spectrum showed good correlation with cancer mutation signatures attributed to smoking and other sources of guanine-directed base damage. Conclusion: This study provides support for the use of cell line mutagenesis assays to validate or predict the mutagenic effect of environmental and iatrogenic exposures. Our results suggest genetic reversion due to cisplatin-induced mutations as a distinct mechanism for developing resistance.