Abstract 171: Cell memory of a genotoxic insult is mediated by methyltransferases

Abstract

Characterization of the direct effects of DNA damaging agents shows how DNA lesions lead to specific mutations. Yet, serum from Hiroshima survivors, Chernobyl liquidators, and radiotherapy patients can induce a clastogenic effect on naive cells, showing indirect induction of genomic instability that persists years after exposure. Such indirect effects are not restricted to ionizing radiation, as a chemical genotoxin induces a heritable and transmissible genomic instability phenotype. While such indirect induction of genomic instability is well described, the underlying mechanism has remained enigmatic. Here, we show that mouse embryonic stem (ES) cells exposed to γ-radiation remember the insult for weeks. Specifically, conditioned media from progeny of exposed cells can induce DNA damage and homologous recombination in naive cells. Notably, cells exposed to conditioned media also elicit a genome destabilizing effect on their neighbours, thus demonstrating transmission of genomic instability. Moreover, we show that the underlying basis for the memory of an insult is completely dependent on two of the major DNA cytosine methyltransferases (MTases), Dnmt1 and Dnmt3a. Targeted disruption of these genes completely eliminates transmission of genomic instability. Furthermore, transient inactivation of Dnmt1, using a tet-suppressible allele, clears the memory of the insult, thus protecting neighbouring cells from indirect induction of genomic instability. We have therefore shown that a single exposure can lead to long-term genome destabilizing effects that spread from cell to cell, and we provide a specific molecular mechanism for these persistent bystander effects. Collectively, our results impact current understanding of risks from toxin exposures and suggest specific mechanisms for suppressing genomic instability in people exposed to carcinogenic genotoxins. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 171.