Methyltransferases mediate cell memory of a genotoxic insult

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 chemical genotoxins also induce heritable and transmissible genomic instability phenotypes. Although such indirect induction of genomic instability is well described, the underlying mechanism has remained enigmatic. Here, we show that mouse embryonic stem cells exposed to c-radiation bear the effects of the insult for weeks. Specifically, conditioned media from the 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 neighbouring cells, 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, Dnmt1 and Dnmt3a. Targeted disruption of these genes in exposed cells 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 thus demonstrated 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 the current understanding of risks from toxin exposures and suggest modes of intervention for suppressing genomic instability in people exposed to carcinogenic genotoxins.

Extreme anti-oxidant protection against ionizing radiation in bdelloid rotifers

“…In some cases, however, IR-induced delayed reproductive death may depend on heritable effects not requiring DNA breakage (22). Effects that have been proposed in this regard include altered gene expression resulting from aberrant DNA cytosine methylation (23), centrosome amplification leading to multipolar mitosis (24), and self-sustaining production of hydrogen peroxide and elevated oxidative stress associated with mitochondrial dysfunction (25). Indeed, the question of how IR kills cells, with its important implications for radiotherapy, is a matter of continuing debate (26,27).…”

Non-targeted effects of ionising radiation—Implications for low dose risk

“…There is also evidence of possible involvement of indirect, untargeted interactions between cells and complex cytokine-like signal transduction processes [42, 59–61]. Rugo et al [62] demonstrated the role of methylation in the transmission of GI in embryonic stem cells by showing that the elimination of DNA cytosine methylation genes, Dmmt1 and Dmmt3a , completely eliminated transmission of GI. Ilnytskyy and Kovalchuk [63] presented further evidence on the role of methylation and miRNA- short RNA molecules in eukaryotic cells that regulate gene expression [64] in radiation-induced GI and BE.…”

“…CIN in bone marrow has been demonstrated in vivo after 1 or 3 Gy total body low- linear energy transfer [62] irradiation. However, at lower doses of 0–0.5 Gy there was no evidence of CIN [76].…”

Zebrafish as an In Vivo Model to Assess Epigenetic Effects of Ionizing Radiation

“…This hinted that radiation hormesis could be related to hypermethylation upon low-dose IR exposures [128]. As regards non-targeted effects, Rugo et al [131] studied the effects of the irradiated cell conditioned medium (ICCM) on unirradiated cells. Interestingly, through comet analysis, inactive DNMT1 in mice ES cell could protect neighboring cells from indirect induction of genomic instability [131].…”