Review
Irradiation induced foci (IRIF) as a biomarker for radiosensitivity

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Abstract

It has long been known that the level of radiosensitivity between individuals covers a considerable range. This range is reflected in analysis of patient cell lines with some cell lines showing significantly reduced sensitivity to in vitro radiation exposure. Our increased exposure to radiation from diagnostic medical procedures and other life style changes has raised concerns that there may be individuals who are at an elevated risk from the harmful impact of acute or chronic low dose radiation exposure. Additionally, a subset of patients show an enhanced normal tissue response following radiotherapy, which can cause significant discomfort and, at the extreme, be life threatening. It has long been realised that the ability to identify sensitive individuals and to understand the mechanistic basis underlying the range of sensitivity within the population is important. A reduced ability to efficiently repair DNA double strand breaks (DSB) and/or activate the DSB damage response underlies some, although not necessarily all, of this sensitivity. In this article, we consider the utility of the recently developed γH2AX foci analysis to provide insight into radiation sensitivity within the population. We consider the nature of sensitivity including the impact of radiation on cell survival, tissue responses and carcinogenesis and the range of responses within the population. We overview the current utility of the γH2AX assay for assessing the efficacy of the DNA damage response to low and high dose radiation and its potential future exploitation.

Highlights

Radiosensitivity in humans can arise from defects in DNA double strand break repair and damage response signalling. ► Gamma H2AX foci analysis as a diagnostic procedure to identify patients with marked DSB repair defects. ► IRIF analysis has the potential to identify individuals with more subtle radiosensitivity.

Introduction

We are increasingly exposed to radiation during the course of our lives from routine medical usage, such as diagnostic X-rays and computed tomography (CT) scanning, from the increased frequency of flying, and, for the unfortunate few, from radiotherapy regimes [1], [2]. The elevated usage of radiation for medical purposes and nuclear power for energy enhances the risk of accidental exposure from industrial accidents and for workers in the radiation industry. Further, the risk of its usage as a weapon of terrorism remains a realistic possibility. In addition to these risks that are arguably avoidable, there is an increasing awareness of the risk of significant radiation exposure from houses built in radon-rich areas, particularly given the enhanced efficiency of insulation [3]. For many, these exposures may be of little consequence. However, there is mounting evidence that the impact of radiation exposure differs between individuals. The range in sensitivity between individuals is, perhaps, best illustrated by the response to radiotherapy where 1–5% of treated patients have a more dramatic response compared to the average individual. This represents the response to high dose exposure but current evidence suggests that there will also be individuals with enhanced sensitivity to low dose exposure. Further, analysis of cell lines derived from “normal” individuals has provided evidence for a significant range of sensitivities. Accepting the notion that the range of sensitivity between individuals is significant, it would clearly be highly beneficial to have a diagnostic test or biomarker to identify sensitive individuals. Indeed, this has been a “holy grail” of radiation protection for many years.

Section snippets

What do we mean by radiosensitive individuals?

The nature or impact of radiosensitivity depends upon the exposure situation. A number of rare individuals with substantial genetic defects in DNA damage response (DDR) genes have been described [4]. These patients normally manifest with defined syndromic clinical features that can include immunodeficiency, microcephaly, growth delay or progressive ataxia. Cells derived from such patients display pronounced radiosensitivity in clonogenic survival assays and exposure of such patients to

The underlying causes of radiation sensitivity

The use of simple organisms, such as yeast and bacteria, has been valuable in identifying pathways that protect against radiation damage [8]. Undoubtedly, the most significant pathways identified have been those involved in protecting the DNA against radiation damage. However, recent studies using highly radiation resistant organisms such as Deinococcus radiodurans have led to the suggestion that protection against protein damage might also be significant [9]. DNA is clearly the most

DNA damage responses relevant to human radiosensitivity

The DDR represents two mechanistically distinct processes: namely, pathways of DNA repair and signal transduction pathways [2], [8] (Fig. 1). They interface, however, since aspects of DNA repair are regulated by the signalling response [11].

Techniques for visualising IRIF after radiation exposure

Since H2AX is only phosphorylated following activation of the DDR, γH2AX provides a powerful tool to monitor the presence of DNA damage. γH2AX antibodies have been generated and are highly specific. However, it is important to appreciate that ATR activation occurs at ss regions of DNA generated during replication and thus γH2AX can be observed during normal replication [2]. Nevertheless, following radiation exposure the major lesion that induces γH2AX formation in G0/G1 or G2 cells is a DSB

Diagnosis of human syndromes with associated radiation sensitivity

Several syndromic disorders caused by mutations in NHEJ components have now been identified [2], [4]. Since NHEJ functions during the development of the immune response, most particularly during the process of V(D)J recombination, patients with significantly impacting mutations in NHEJ proteins display severe combined immunodeficiency (SCID) or combined immunodeficiency (CID) [41]. NHEJ also plays a critical role during neuronal development and microcephaly and/or growth delay are additional

Sensitivity to radiation induced carcinogenesis

The study of exposed populations (e.g. Japanese Atomic bomb survivors and those exposed following the Chernobyl accident) has clearly demonstrated that radiation exposure enhances cancer incidence though radiation represents a mild carcinogen [55]. There is accumulating awareness and concern that even the low doses to which we are exposed following CT scanning can contribute to the background level of carcinogenesis [56]. Given our increased use of CT scanning and other forms of radiation

Summary

γH2AX foci analysis represents a highly sensitive technique to monitor DSB induction and repair following radiation exposure, provided care is taken to consider cell cycle stage. The technique is currently used successfully for diagnosis of human syndromes conferring significant defects in DSB repair. The technique is also capable of identifying individuals with marked DSB repair defects that are insufficiently severe to confer syndromic characteristics. The assay may additionally be able to

Conflict of interest statement

There are no conflict of interest.

Acknowledgements

We thank Dr. J. Bedford for discussions. The Jeggo laboratory is supported by the Medical Research Council, the Association for International Cancer Research, the Wellcome Research Fund and the Department of Health (UK).

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