Genotoxic effects in cancer patients after treatment with single drug etoposide or melphalan chemotherapy




Karnaoukhova, Larissa

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One of the complications of cancer chemotherapy is an increased risk of secondary treatment-related malignancy. It is believed that secondary cancers arise as a result of mutagenic and genotoxic effects of chemotherapeutic agents. The mechanisms by which these agents induce secondary cancer are not known. This study was designed to investigate the potential genotoxic effects of two agents, etoposide and melphalan, in cancer patients receiving single drug chemotherapy. Both drugs are potent mutagens; and have been implicated as causative agents of secondary acute myeloid leukemia (sAML). Two groups of patients were studied: small cell lung cancer (SCLC) patients who received etoposide and multiple myeloma (MM) patients treated with melphalan. The induction of mutation at the hypoxanthine phosphoribosyl transferase (hprt) gene was investigated in patients' lymphocytes by using the hprt T-cell clonal assay. The results showed that the mean hprt mutant frequency (MF) did not increase after treatment with neither agent, however, a significant elevation of MF was detected in a patient who received the highest melphalan dose. The sequence analysis of mutants revealed a significant enhancement of AT > TA transversions in post-etoposide spectra compared to the control spectra, however, no mechanistic explanation for the induction of such mutation by etoposide can be proposed. Etoposide, a topoisomerase II inhibitor and an inducer of DNA strand breaks, is expected to produce large genomic deletions and rearrangements. No enhancement of large deletions or rearrangements was seen in post-etoposide spectra. The elimination of mutated cells by apoptosis is proposed to explain the negative result. In post-melphalan spectra, a significant enhancement of GC > TA transversions was detected compared to the control spectra. GC > TA transversion is an expected melphalan-induced mutation since alkylating agent melphalan preferentially causes guanine adducts and crosslinks. The results suggest that while single drug chemotherapy does not induce mutation in the majority of patients, exposure to high doses of chemotherapy agents may be associated with the induction of mutation. This correlates with the dose-dependent increase in the risk of secondary malignancy after chemotherapy. The significance of drug-specific changes in the mutational spectra seen in patients' lymphocytes can be evaluated after mutations in proto-oncogenes and/or tumor suppressor genes are identified in secondary cancer cases. Results also suggest that the hprt mutational spectrum might be a more sensitive biomarker of the genotoxic exposure than the hprt MF. To investigate the clastogenic effect of etoposide chemotherapy, blood cells from SCLC patients were screened for the appearance of multiple lineage leukemia (MLL) gene rearrangements using fluorescent in situ hybridization (FISH). MLL gene is rearranged in more than 50% of sAML cases in patients previously treated with etoposide or related compounds. Analysis of the results revealed that FISH is not sensitive enough method for the detection of rare rearrangements. The use of RT-PCR technique is proposed as a more realistic approach for monitoring patients treated with etoposide chemotherapy for the appearance of the MLL gene rearrangements.



Chemotherapy, Cancer, Drugs