CARCINOGENIC NICKEL SILENCES GENE-EXPRESSION BY CHROMATIN CONDENSATION AND DNA METHYLATION - A NEW MODEL FOR EPIGENETIC CARCINOGENS

A transgenic gpt(+) Chinese hamster cell line (G12) was found to be su sceptible to carcinogenic nickel-induced inactivation of gpt expressio n without mutagenesis or deletion of the transgene. Many nickel-induce d 6-thioguanine-resistant variants spontaneously reverted to actively express gpt, as indicated by both reversion assays and direct enzyme m easurements. Since reversion was enhanced in many of the nickel-induce d variant cell lines following 24-h treatment with the demethylating a gent 5-azacytidine, the involvement of DNA methylation in silencing gp t expression was suspected. This was confirmed by demonstrations of in creased DNA methylation, as well as by evidence indicating condensed c hromatin and heterochromatinization of the gpt integration site in 6 t hioguanine-resistant cells. Upon reversion to active gpt expression, D NA methylation and condensation are lost. We propose that DNA condensa tion and methylation result in heterochromatinization of the gpt seque nce with subsequent inheritance of the now silenced gene. This mechani sm is supported by direct evidence showing that acute nickel treatment of cultured cells, and of isolated nuclei in vitro, can indeed facili tate gpt sequence-specific chromatin condensation. Epigenetic mechanis ms have been implicated in the actions of some nonmutagenic carcinogen s, and DNA methylation changes are now known to be important in carcin ogenesis. This paper further supports the emerging theory that nickel is a human carcinogen that can alter gene expression by enhanced DNA m ethylation and compaction, rather than by mutagenic mechanisms.