Drug-induced DNA hypermethylation was observed to constitute one component of the response of human tumor cells to toxic concentrations of commonly used cancer chemotherapy agents. In both human lung adenocarcinoma cells (HTB-54) and human rhabdomyosarcoma cells (CCI-136), pulse exposures to the topoisomerase II inhibitors etoposide and nalidixic acid; to the antibiotic doxorubicin; to the microtubule inhibitors vincristine, vinblastine, and colchicine; to the DNA cross-linking agent cisplatinum; to hydroxyurea; and to the antimetabolites 1-β-d-arabinofuranosylcytosine, 5-fluorouracil, 5-fluorodeoxyuridine, and methotrexate were associated with profound drug-induced DNA hypermethylation. Exposure of human T-lymphocytes (MOLT-4) to toxic pulse doses of 3′-azidodideoxythymidine was associated with similar drug-induced DNA hypermethylation. In every case, drug-induced DNA hypermethylation was observed only when the degree of DNA synthesis inhibition caused by the drug exceeded 90% and when drug levels or duration of exposure was sufficient to kill 90–100% of exposed cells. Drug-induced DNA hypermethylation was shown not to represent a tissue culture phenomenon, since it occurred in vivo during high-dose 1-β-d-arabinofuranosylcytosine and hydroxyurea treatments in two leukemic patients. Drug-induced alterations in DNA methylation were frequently biphasic, with hypomethylation occurring at drug concentrations which produced mild DNA synthesis inhibition and which killed less than 50% of exposed cells. Exposure to the alkylating agents 1,3-bis(2-chloroethyl)-1-nitrosourea and cyclophosphamide and to the antimetabolites 5-azadeoxycytidine and 6-thioguanine was associated with DNA hypomethylation at all studied concentrations in HTB-54 cells. Drug-induced DNA hypermethylation could be blocked by preexposure to hypomethylating agents administered at nontoxic to midly toxic concentrations. Drug-induced DNA hypermethylation may be capable of creating drug-resistant phenotypes by inactivating genes the products of which are required for drug cytotoxicity. Perhaps paradoxically, drug-induced DNA hypermethylation may also produce a second class of drug-resistant tumor cells, characterized by overexpression of particular gene products, by potentiating the process of gene amplification.

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This work was supported by National Cancer Institute Grant R29-CA47217-01, by North Carolina United Way Grant 5-880272, and by the Burroughs Wellcome fund.

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