Genetic and Epigenetic Changes in Chromosomally Stable and Unstable Progeny of Irradiated Cells

Abstract

Radiation induced genomic instability is a well-studied phenomenon, the underlying mechanisms of which are poorly understood. Persistent oxidative stress, mitochondrial dysfunction, elevated cytokine levels and epigenetic changes are among the mechanisms invoked in the perpetuation of the phenotype. To determine whether epigenetic aberrations affect genomic instability we measured DNA methylation, mRNA and microRNA (miR) levels in well characterized chromosomally stable and unstable clonally expanded single cell survivors of irradiation. While no changes in DNA methylation were observed for the gene promoters evaluated, increased LINE-1 methylation was observed for two unstable clones (LS12 and CS9) and decreased Alu element methylation was observed for the other two unstable clones (115 and Fe5.0–8). These relationships also manifested for mRNA and miR expression. mRNA identified for the LS12 and CS9 clones were most similar to each other (261 mRNA), while the 115 and Fe5.0–8 clones were more similar to each other, and surprisingly also similar to the two stable clones, 114 and 118 (286 mRNA among these four clones). Pathway analysis showed enrichment for pathways involved in mitochondrial function and cellular redox, themes routinely invoked in genomic instability. The commonalities between the two subgroups of clones were also observed for miR. The number of miR for which anti-correlated mRNA were identified suggests that these miR exert functional effects in each clone. The results demonstrate significant genetic and epigenetic changes in unstable cells, but similar changes are almost as equally common in chromosomally stable cells. Possible conclusions might be that the chromosomally stable clones have some other form of instability, or that some of the observed changes represent a sort of radiation signature and that other changes are related to genomic instability. Irrespective, these findings again suggest that a spectrum of changes both drive genomic instability and permit unstable cells to persist and proliferate.

Cancer stem cell related markers of radioresistance in head and neck squamous cell carcinoma

“…They found differential expression of ALDH1A3/ RA inducible genes that promote or suppress tumor growth in dependency of the DNA methylation state. This supports the role of epigenetic regulatory mechanisms in the radioresponse of cells [ 37 ]. Also for Non-small-cell lung carcinoma (NSCLC) ALDH1A3 was described as pre-dominant ALDH isoform, which on one hand associated with well differentiated lung cancer with good prognosis but on the other hand drives the tumorigenic and clonogenic potential of NSCLC cells [ 38 ].…”

Dysregulation of DNA methylation induced by past arsenic treatment causes persistent genomic instability in mammalian cells

“…Indeed, similar to our demonstration here of LINE‐1 remethylation levels exceeding the original levels, some of the early work on 5‐azacytidine (5‐azaC) induced DNA demethylation also demonstrated a reacquisition of methylation levels that surpassed the original levels upon withdrawal of 5‐azaC [Bender et al, ]. Although arsenic was the focus of the work reported here, there is also evidence that other environmental exposures, for example ionizing radiation, can similarly perturb genome stability, imposing so‐called bystander effects characterized by a growing awareness of altered global genome methylation and LINE‐1 sequence methylation coordinated with evidence of delayed effects and persistence of epigenome modulation [Goetz et al, ; Baulch et al, ; Luzhna et al, ].…”

2017 Michael Fry Award Lecture When DNA is Actually Not a Target: Radiation Epigenetics as a Tool to Understand and Control Cellular Response to Ionizing Radiation

“…With a growing interest in radiation epigenetics, a number of studies have shown that radiation-induced changes in DNA methylation are not unidirectional; LINE-1 DNA hypo- and hypermethylation can be observed, as well as an absence of changes in DNA methylation, especially at doses below 1 Gy (33–37). Those differences could be explained by the different types of radiation and doses, models used, as well as different methods of DNA methylation analysis.…”