Occupational Medicine 2006 56(3):156-161; doi:10.1093/occmed/kql010
© The Author 2006. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
New advances in radiation biology
Kevin M. Prise
Cell and Molecular Radiation Biology Group, Gray Cancer Institute, PO Box 100, Mount Vernon Hospital, Northwood, Middlesex, HA6 2JR, UK
Correspondence to: Kevin M. Prise. Tel: +44 1923 828611; fax: +44 1923 835210; e-mail: prise{at}gci.ac.uk
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Abstract
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Current understanding of risk associated with low-dose radiation
exposure has for many years been embedded in the linear-no-threshold
(LNT) approach, based on simple extrapolation from the Japanese
atomic bomb survivors. Radiation biology research has supported
the LNT approach although much of this has been limited to relatively
high-dose studies. Recently, with new advances for studying
effects of low-dose exposure in experimental models and advances
in molecular and cellular biology, a range of new effects of
biological responses to radiation has been observed. These include
genomic instability, adaptive responses and bystander effects.
Most have one feature in common in that they are observed at
low doses and suggest significant non-linear responses. These
new observations pose a significant challenge to our understanding
of low-dose exposure and require further study to elucidate
mechanisms and determine their relevance.
Keywords Adaptive response; bystander effect; genomic instability; radiation biology; transgenerational
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Introduction
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Humans are exposed to multiple physical, chemical and biological
agents during their lifetime. Of these, ionizing radiation(s)
has long been known to be deleterious after high-dose exposure
(>100 mSv) predominantly due to cancer induction although
very high dose exposures yield tissue damage and ultimately
death (see [
1] for a general textbook). Ionizing radiations
are widely used in society, play a key role in the treatment
of cancer and are an important diagnostic tool. For radiation
protection purposes, despite a century of study, the risk estimates
for cancer induction in humans are extrapolated from the Japanese
atomic bomb survivors, who were exposed to relatively high dose
and high dose rates. Several studies of radiation workers have
been undertaken as these populations were exposed to protracted
low-dose exposures [
2,
3]. From these epidemiological data, there
has been a simple extrapolation of risk to low doses generally
found in environmental and most occupational exposures. This
has been the linear-no-threshold (LNT) model, which assumes
a linear dose-response relationship between dose and risk. Currently,
with the exception of radiotherapy, the doses that members of
the population can be typically exposed to are lower than the
doses typically received by the bomb survivors and are therefore
in regions where little epidemiological data are available.
Against a typical background dose of
3 mSv/year, examples of
routine medical exposures include 3 mSv for a breast mammogram
and 0.7 mSv for a dental x-ray [
4].
The LNT model has been an acceptable compromise with experimental data from radiation biology studies to some extent agreeing with it, although not exclusively. The relevance of the LNT approach has recently been sharply brought into debate with the observation of ‘non-targeted responses’. These are responses which do not follow the standard model of radiation effects. The standard model has been based on direct damage to DNA, leading especially to the production of DNA double-strand breaks and the downstream biological consequences of these [5] (see Figure 1). Non-targeted responses include a range of effects such as the adaptive response, genomic instability and the bystander effect. The aim of this short review is to highlight the key aspects of these new findings.
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Figure 1. Standard model for radiation effects in cells based on direct DNA damage leading to downstream biological responses.
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Advances in molecular and cell biology
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With sequencing of the human genome and technological advances
in molecular biology, many studies focus on radiation effects
on gene expression using high throughput profiling approaches.
The availability of array technology makes screening of individual
cells for gene expression changes after irradiation possible,
allowing these to be related to cellular responses even at low
dose [
6]. What has become clear is that the types of genes expressed
at low dose (<0.2 Gy) may vary substantially from those expressed
at higher doses and that there may be important time- and tissue-dependent
differences [
7,
8]. These approaches open up the possibility
of relating whole genome responses to tissue and disease responses
under radiation-protection conditions in the future. From the
human genome project, it is clear that much of the biological
response of cells and tissues is not driven by gene expression
changes but by alterations at the protein level. Proteomics
aims to study these, using technological approaches such as
mass spectrometer–based systems and automated 2D gel array
analysis [
9]. Coupled with increasing knowledge of gene expression
changes and understanding of mechanisms at low doses, these
new approaches have the potential to start to contribute to
the development of biological-based models of radiation response
to allow predictions to be made of tumour induction at low doses
below the resolution of the existing epidemiological data [
10].
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Bystander effect
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A major advance in understanding radiation effects has been
the observation that cells can respond when their neighbours
are irradiated, referred to as a bystander response (see [
11,
12]
for extensive reviews). These responses were first clearly identified
in 1992 when Nagasawa and Little [
13] observed, under conditions
where only 1% of a population of Chinese hamster ovary cells
grown in culture had been traversed by a densely ionizing
-particle,
that 30% of the population nevertheless experienced the formation
of damaged chromosomes. Further studies have shown evidence
for these effects in a range of cell types and measuring a range
of end-points, including damage to chromosomes [
14], mutations
[
15], cell death [
16] and carcinogenesis measured using
in vitro transformation assays [
17]. Many studies have shown that simply
removing the medium from irradiated cells and transferring it
to non-irradiated cells is sufficient to observe a bystander
response. Another approach is to use sophisticated microbeams
which allow individual cells within populations to be selected
and irradiated with low doses of charged particles or x-rays
[
18]. Microbeams have provided defining evidence for bystander
responses and the mechanisms underpinning them [
19]. In all
these approaches, several common features of bystander response
have been observed. Firstly, the effect is observed at low dose
(<0.2 Gy) and saturates at high dose. Secondly, two main
routes of transmittance of the effect have been found: direct
cell–cell communication via specific pores between cells
called gap junctions [
20] and release of factors from irradiated
cells into the medium [
21]. A range of factors has been observed
to play a role. These include reactive oxygen species (ROS),
which are highly reactive-free radicals produced during normal
cellular oxygen metabolism and after radiation exposure [
22],
and other molecules including reactive nitrogen species [
23],
such as nitric oxide and small proteins called cytokines [
24].
All of these are also widely reported to be key signalling molecules
in cell stress responses.
Some evidence for bystander responses has been observed in vivo. The production of clastogenic factors in the serum from irradiated patients has been studied for many years. These factors lead to cell damage when added to non-exposed lymphocytes and have been postulated to be superoxide products of lipid peroxidation or cytokines, all of which have also been implicated in experimental studies of bystander mechanisms (see [25] for a review). Clinically, abscopal effects, originally defined to describe systemic effects in non-irradiated sites after localized radiotherapy, may be manifestations of bystander responses at the tissue level and have been reported in the literature since the early 1950s [26]. These have been observed in animal models under conditions of relevance to therapy. In a recent study, Camphausen et al. [27] irradiated non-tumour baring legs of mice (five fractions of 10 Gy) which had tumours transplanted at distant sites. They observed reduced tumour growth rates when the leg was irradiated with tumour inhibition decreased when the radiation dose was reduced to 12 fractions of 2 Gy. A similar study in rats under conditions where partial irradiation of lung was undertaken showed damage in the shielded upper part of the lung when only the base was irradiated. The propagation of damage involved ROS and the induction of inflammatory cytokines, such as tumour necrosis factor and interleukin-1 by the irradiation some of which may be due to partial irradiation of the liver [28]. If these responses are proven in humans, they may require the incorporation of directional and geometrical information into calculations of normal tissue complication probabilities for lung, which are currently not considered in conventional dose-volume histograms [29]. Studies with internally deposited radioactive materials have also reported evidence for bystander effect in vivo. When hamsters were injected with the -particle emitters 239PuO2 or 230Pu citrate, which concentrate in the liver, the induction of chromosome aberrations was independent of large changes in the local dose homogeneity when this was altered by injecting a range of particle sizes, but maintaining a constant total dose to the liver [30]. A similar response was observed when the induction of liver tumours was observed [31]. The authors suggested that the liver was responding to the total energy and to total dose to the liver, not to the numbers of cells traversed by an -particle or the local dose distribution [32].
Despite advances in understanding of bystander responses, further studies on their role and relevance in vivo are required. An important issue is whether these responses are damaging or protective effects as that will ultimately determine any effect they have on dose-response curves at low dose (see Figure 2). Other studies have shown protective responses such as switching off of cell division via differentiation and the removal of potentially damaged cells by cell-death processes. What will be critical is the relative role of these effects in tissues and individuals in determining overall cancer risk.
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Figure 2. Potential models of non-linearity at low doses for both a damaging (–) or a protective (.....) bystander response versus the standard LNT model (—).
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Genomic instability
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For radiation-induced cancer it has been assumed that a multi-step
process occurs which starts with an initiation event, such as
a radiation-induced mutation, followed by a promotion step where
a growth advantage occurs and progression via a series of unstable
changes to a tumour phenotype. Radiation biology research has
concentrated on short-term assays of biological responses, such
as cell killing defined in survival assays as an inability to
divide and produce viable colonies. The standard approach for
measuring cell killing using
in vitro cell models has been the
clonogenic assay originally developed by Puck and Marcus [
33]
in the 1950s. Using this assay, single cells are allowed to
divide and colonies consisting of
50 cells are classified as
‘viable’. This has underpinned experimental studies
of radiation cell killing in cell and tumour models, however
it is based on a narrow range of assumptions regarding the definition
of clonogenic survival. The accepted criteria assume that a
colony derived from a single cell consists of clonal descendents
of the initial cell, each with the same proliferative potential
as the starting cell. It is clear however that replating of
cells after irradiation exposure in many cases does not lead
to control survival levels, but delayed cell killing is observed
[
34,
35], also described as lethal mutations and that these appear
randomly within the cell progeny [
36]. This is a characteristic
of the phenomenon of radiation-induced genomic instability.
During carcinogenesis, cells and tissues undergo a phase of
instability during which multiple mutations are accumulated
before a full tumourogenic phenotype is established. This is
observed after radiation exposure in cell and tissue models
and is defined as an increased rate of acquisition of delayed
changes leading to chromosomal aberrations, gene mutations and
delayed cell death in the surviving progeny after irradiation
[
37]. All the various delayed effects are induced at very high
frequency and are unlikely to be due to conventional mutational
changes. Currently, little is understood of the processes involved
in initiation of inducible instabilities and in maintenance
and transmission of phenotype over many generations of cell
replication. It is becoming evident that expression of inducible
instability has a strong dependence on type of radiation exposure
[
38], cell type irradiated and genetic predisposition of the
irradiated cell [
39,
40]. Instability is also observed in human
lymphocytes where only a single radiation track is delivered
to a cell population [
41]. Elevation of ROS levels may be involved
in propagation of the response via cytokine-dependent signalling.
At tissue level, it appears that there is a role for an inflammatory
type response driven by macrophage activation leading to apoptotic
clearance of damaged cells. Several studies have suggested that
radiation-induced genomic instability and bystander effect are
related, with instability predominantly emanating from bystander
cells in some models [
42].
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Transgenerational effects
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The question of whether radiation exposure induces heritable
effects in humans has been the subject of debate [
43]. Following
studies showing genomic instability in the progeny of irradiated
cells
in vitro and from
in vivo studies, it is clear that radiation
exposure can induce transgenerational effects (see [
44] for
a review). Much of this work has been done in mouse models following
changes in repeat sequences of DNA found throughout the genome.
These sequences, termed minisatellite DNA or extended tandem
repeat sequences, are non-coding regions of the genome which
can be analysed at the molecular level for mutations. They have
a high spontaneous mutation frequency in comparison to single
gene loci which means that they can be detected in relatively
few cells and at low dose. Dubrova has shown that germline mutations
from irradiated parents can be transmitted through sperm to
offspring and lead to mutations in both somatic and germline
tissues. Similar effects have been reported in humans, specifically
from individuals exposed during the Chernobyl accident [
45].
The underlying mechanisms of these transgenerational effects
are not fully known, however it is thought that they may be
a manifestation of radiation-induced genomic instability which
can be transmitted.
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Adaptive responses
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Adaptive responses are another response which has challenged
traditional thinking in radiation effects [
46]. These are observed
when a small dose of (priming) radiation reduces the effect
of a larger (challenge) dose, typically given several hours
later. The earliest studies were done in human lymphocytes [
47],
however these responses have also been measured after
in vivo exposures in mice for cancer induction [
48]. Adaptive responses
appear to be highly variable and depending on the cells system
and end-point used. Mechanistically, it has been suggested that
stimulation of repair processes and antioxidant activity play
a role, but the precise molecular mechanisms are still poorly
characterized. Adaptive effects have also been related to radiation
hormesis which is defined as responses to radiation which are
beneficial at low doses. Radiation hormesis predicts that low-dose
effects have a threshold dose above which risk increases in
contrast to the LNT approach. Most of the evidence for this
has been extrapolated from existing epidemiological data and
there is a pressing need for further experimental studies [
49].
An interesting observation of adaptive responses is that many are observed when very low priming doses are given (<0.2 Gy). These may potentially induce bystander-signalling pathways and there is a need to further study interactions between these two processes [50]. One possibility is that non-targeted responses in general are simply a stress response mechanisms of biological systems to low-dose irradiation.
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Concluding remarks
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New observations and mechanisms of response to radiation exposure
in biological systems are questioning our understanding of the
effects of low-dose exposure. These new advances in radiation
biology have evolved alongside the tremendous advances in cell
and molecular biology that have occurred in the last 10 years
and new technological approaches for studying the effects of
radiation at low doses. Together, these approaches have the
potential of impacting on understanding of the relationship
between radiation exposure and human disease.
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Conflicts of interest
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None declared.
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Acknowledgements
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The author acknowledges the support of the Department of Health
(RRX92) the European Commission (RISC-RAD), the US Department
of Energy (FG02-03ER63633, DE-FG02-02ER63305), Cancer Research
UK (C1513/A2676) and the Gray Cancer Institute.
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References
|
|---|
- Hall EJ. Radiobiology for the Radiologist, 5th edn. Philadelphia, PA: Lippincott Williams & Wilkins, 2000.
- Muirhead CR, Goodill AA, Haylock RG et al. Occupational radiation exposure and mortality: second analysis of the National Registry for Radiation Workers. J Radiol Prot 1999;19:3–26.[CrossRef][Medline]
- Cardis E, Gilbert ES, Carpenter L et al. Effects of low doses and low dose rates of external ionizing radiation: cancer mortality among nuclear industry workers in three countries. Radiat Res 1995;142:117–132.[ISI][Medline]
- Brenner DJ, Doll R, Goodhead DT et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003;100:13761–13766.[Abstract/Free Full Text]
- Bedford JS, Dewey WC. Radiation Research Society. 1952–2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells? Radiat Res 2002;158:251–291.[CrossRef][ISI][Medline]
- Amundson SA, Bittner M, Fornace AJ Jr. Functional genomics as a window on radiation stress signaling. Oncogene 2003;22:5828–5833.[CrossRef][ISI][Medline]
- Amundson SA, Do KT, Shahab S et al. Identification of potential mRNA biomarkers in peripheral blood lymphocytes for human exposure to ionizing radiation. Radiat Res 2000;154:342–346.[CrossRef][ISI][Medline]
- Fornace AJ Jr, Amundson SA, Bittner M et al. The complexity of radiation stress responses: analysis by informatics and functional genomics approaches. Gene Expr 1999;7:387–400.[ISI][Medline]
- Ahram M, Adkins JN, Auberry DL, Wunschel DS, Springer DL. A proteomic approach to characterize protein shedding. Proteomics 2005;5:123–131.[CrossRef][ISI][Medline]
- Preston RJ. Radiation biology: concepts for radiation protection. Health Phys 2004;87:3–14.[ISI][Medline]
- Morgan WF. Non-targeted and delayed effects of exposure to ionizing radiation: II. Radiation-induced genomic instability and bystander effects in vivo, clastogenic factors and transgenerational effects. Radiat Res 2003;159:581–596.[CrossRef][ISI][Medline]
- Morgan WF. Non-targeted and delayed effects of exposure to ionizing radiation: I. Radiation-induced genomic instability and bystander effects in vitro. Radiat Res 2003;159:567–580.[ISI][Medline]
- Nagasawa H, Little JB. Induction of sister chromatid exchanges by extremely low doses of a-particles. Cancer Res 1992;52:6394–6396.[Abstract/Free Full Text]
- Prise KM, Belyakov OV, Folkard M, Michael BD. Studies of bystander effects in human fibroblasts using a charged particle microbeam. Int J Radiat Biol 1998;74:793–798.[CrossRef][ISI][Medline]
- Nagasawa H, Little JB. Unexpected sensitivity to the induction of mutations by very low doses of alpha-particle radiation: evidence for a bystander effect. Radiat Res 1999;152:552–557.[ISI][Medline]
- Mothersill C, Seymour C. Medium from irradiated human epithelial cells but not human fibroblasts reduces the clonogenic survival of irradiated cells. Int J Radiat Biol 1997;71:421–427.[CrossRef][ISI][Medline]
- Sawant SG, Randers-Pehrson G, Geard CR, Brenner DJ, Hall EJ. The bystander effect in radiation oncogenesis: I. Transformation in C3H 10T1/2 cells in vitro can be initiated in the unirradiated neighbors of irradiated cells. Radiat Res 2001;155:397–401.[ISI][Medline]
- Folkard M, Prise K, Schettino G, Shao C, Gilchrist S, Vojnovic B. New insights into the cellular response to radiation using microbeams. Nucl Instrum Methods Phys Res B 2005;231:189–194.[CrossRef]
- Shao C, Folkard M, Michael BD, Prise KM. Targeted cytoplasmic irradiation induces bystander responses. Proc Natl Acad Sci U S A 2004;101:13495–13500.[Abstract/Free Full Text]
- Azzam EI, de Toledo SM, Little JB. Direct evidence for the participation of gap junction-mediated intercellular communication in the transmission of damage signals from alpha-particle irradiated to nonirradiated cells. Proc Natl Acad Sci U S A 2001;98:473–478.[Abstract/Free Full Text]
- Yang H, Asaad N, Held KD. Medium-mediated intercellular communication is involved in bystander responses of X-ray-irradiated normal human fibroblasts. Oncogene 2005;24:2096–2103.[CrossRef][ISI][Medline]
- Azzam EI, De Toledo SM, Spitz DR, Little JB. Oxidative metabolism modulates signal transduction and micronucleus formation in bystander cells from alpha-particle-irradiated normal human fibroblast cultures. Cancer Res 2002;62:5436–5442.[Abstract/Free Full Text]
- Shao C, Stewart V, Folkard M, Michael BD, Prise KM. Nitric oxide-mediated signaling in the bystander response of individually targeted glioma cells. Cancer Res 2003;63:8437–8442.[Abstract/Free Full Text]
- Iyer R, Lehnert BE. Factors underlying the cell growth-related bystander responses to alpha particles. Cancer Res 2000;60:1290–1298.[Abstract/Free Full Text]
- Emerit I. Reactive oxygen species, chromosome mutation, and cancer: possible role of clastogenic factors in carcinogenesis. Free Radic Biol Med 1994;16:99–109.[CrossRef][ISI][Medline]
- Kaminski JM, Shinohara E, Summers JB, Niermann KJ, Morimoto A, Brousal J. The controversial abscopal effect. Cancer Treat Rev 2005;31:159–172.[CrossRef][ISI][Medline]
- Camphausen K, Moses MA, Menard C et al. Radiation abscopal antitumor effect is mediated through p53. Cancer Res 2003;63:1990–1993.[Abstract/Free Full Text]
- Khan MA, Hill RP, Van Dyk J. Partial volume rat lung irradiation: an evaluation of early DNA damage. Int J Radiat Oncol Biol Phys 1998;40:467–476.[CrossRef][ISI][Medline]
- Moiseenko VV, Battista JJ, Hill RP, Travis EL, Van Dyk J. In-field and out-of-field effects in partial volume lung irradiation in rodents: possible correlation between early DNA damage and functional endpoints. Int J Radiat Oncol Biol Phys 2000;48:1539–1548.[CrossRef][ISI][Medline]
- Brooks AL, Retherford JC, McClellan RO. Effect of 239PuO2 particle number and size on the frequency and distribution of chromosome aberrations in the liver of the Chinese hamster. Radiat Res 1974;59:693–709.[ISI][Medline]
- Brooks AL, Benjamin SA, Hahn FF, Brownstein DG, Griffith WC, McClellan RO. The induction of liver tumors by 239Pu citrate or 239PuO2 particles in the Chinese hamster. Radiat Res 1983;96:135–151.[ISI][Medline]
- Barcellos-Hoff MH, Brooks AL. Extracellular signaling through the microenvironment: a hypothesis relating carcinogenesis, bystander effects, and genomic instability. Radiat Res 2001;156:618–627.[CrossRef][ISI][Medline]
- Puck TT, Marcus PI. Action of x-rays on mammalian cells. J Exp Med;103:653–669.
- Seymour CB, Mothersill C. Delayed expression of lethal mutations and genomic instability in the progeny of human epithelial cells that survived in a bystander-killing environment. Radiat Oncol Investig 1997;5:106–110.[CrossRef][Medline]
- Trott KR, Hug O. Intraclonal recovery of division probability in pedigrees of single x-irradiated mammalian cells. Int J Radiat Biol Relat Stud Phys Chem Med 1970;17:483–486.[ISI][Medline]
- Thompson LH, Suit HD. Proliferation kinetics of x-irradiated mouse L cells studied with time-lapse photography. II. Int J Radiat Biol Relat Stud Phys Chem Med 1969;15:347–362.[ISI][Medline]
- Lorimore SA, Coates PJ, Wright EG. Radiation-induced genomic instability and bystander effects: inter-related nontargeted effects of exposure to ionizing radiation. Oncogene 2003;22:7058–7069.[CrossRef][ISI][Medline]
- Kadhim MA, Macdonald DA, Goodhead DT, Lorimore DT, Marsden SJ, Wright EG. Transmission of chromosomal instability after plutonium alpha-particle irradiation. Nature;355:738–740.
- Kadhim MA. Role of genetic background in induced instability. Oncogene 2003;22:6994–6999.[CrossRef][ISI][Medline]
- Watson GE, Lorimore SA, Clutton SM, Kadhim MA, Wright EG. Genetic factors influencing a-particle-induced chromosomal instability. Int J Radiat Biol 1997;71:497–503.[CrossRef][ISI][Medline]
- Kadhim M, Marsden S, Malcolmson A, Folkard M, Prise KM. Studies of targeted effects on human lymphocytes using a charged-particle microbeam. Radiat Res 2000;153:227–228.[CrossRef]
- Lorimore SA, Kadhim MA, Pocock DA et al. Chromosomal instability in the descendants of unirradiated surviving cells after alpha-particle irradiation. Proc Natl Acad Sci USA 1998;95:5730–5733.[Abstract/Free Full Text]
- United Nations Scientific Committee on the Effects of Atomic Radiation. Hereditary effects of radiation. In: Report to the General Assembly with Scientific Annex. New York: United Nations, 2001.
- Dubrova YE. Radiation-induced transgenerational instability. Oncogene 2003;22:7087–7093.[CrossRef][ISI][Medline]
- Dubrova YE, Nesterov VN, Krouchinsky NG et al. Further evidence for elevated human minisatellite mutation rate in Belarus eight years after the Chernobyl accident. Mutat Res 1997;381:267–278.[ISI][Medline]
- Wolff S. Aspects of the adaptive response to very low doses of radiation and other agents. Mutat Res 1996;358:135–142.[CrossRef][ISI][Medline]
- Olivieri G, Bodycote J, Wolff S. Adaptive response of human lymphocytes to low concentrations of radioactive thymidine. Science 1984;223:594–597.[Abstract/Free Full Text]
- Mitchel RE, Jackson JS, McCann RA, Boreham DR. The adaptive response modifies latency for radiation-induced myeloid leukemia in CBA/H mice. Radiat Res 1999;152:273–279.[ISI][Medline]
- Upton AC. Radiation hormesis: data and interpretations. Crit Rev Toxicol 2001;31:681–695.[CrossRef][ISI][Medline]
- Sawant SG, Randers-Pehrson G, Metting NF, Hall EJ. Adaptive response and the bystander effect induced by radiation in C3H 10T(1/2) cells in culture. Radiat Res 2001;156:177–180.[CrossRef][ISI][Medline]
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Abstract
Introduction