Download Radiation-induced genetic changes: potential implications of

"Radiation-induced genetic changes: potential
implications of recent data"
Bryn A. Bridges
Genome Damage and Stability Centre, University of Sussex,
Falmer, Brighton, BN1 9RQ, Gt. Britain
Changing paradigms
The last decade has seen the publication of numerous studies that have challenged
some of the paradigms that underly much of our thinking about radiation genetics
(reviewed by, eg [1]). Many of these studies have involved tandemly repeated DNA
loci (TRDLs) such as microsatellites and expanded simple tandem repeats (ESTRs),
minisatellites, and gene duplications in mammalian genomes. Such data as exist
indicate that the radiation sensitivity of these sequences for mutation (sequence
change) as measured by the doubling dose is comparable to that of other radiation
endpoints. However, because some of them have high rates of spontaneous instability,
it means that they also have high rates of radiation-induced sequence change. There is
also only limited evidence of dose dependence. These more recent data and their
implications have been set out at length in a recent review by my colleagues and I [2];
detailed discussion may be found there and I shall here point out only one or two
important conclusions.
Firstly, the high absolute radiation sensitivity of some of these sites means that the
mutations must be untargeted, that is, most of the effective energy loss events occur
elsewhere in the cell and not in the sequence studied. This is in some respects
paralleled by the bystander phenomena in which effects are seen in neighbouring cells
that have not been irradiated. The bystander effect has been shown for chromosome
damage in both mice and humans and has been shown to occur not only in cell
cultures but in normal human skin tissue [3]. However, whereas a convincing in vivo
demonstration of the bystander effect has yet to be made, the untargeted effects at
TRDLs are established in vivo effects. The untargeted nature of these radiation
effects means that one can no longer rely on the linear induction of complex damage
due to energy loss events to justify the linear induction of genetic damage in all cells.
This should not, however, be taken as an excuse to assume that concave, convex, or
other dose response curves are the norm. Some recent analyses with domestic radon
have shown very clearly that at low and environmentally relevant doses, the data for
lung cancer comply well with a linear dose response model [4], and it seems sensible
to continue to regard this as the norm unless there is compelling evidence to the
contrary.
The second new challenge to the existing paradigms is the evidence for
transgenerational mutagenesis. This has been observed in mice but not (so far) in
humans, and the mechanism driving such mutation transmission is not known. By
transgenerational mutagenesis is meant the appearance of mutations in the second
generation offspring of irradiated parents. (Mutations appearing in the first generation
offspring cannot be unambiguously regarded as transgenerational since they may have
occurred during sperm maturation in the irradiated father.) One is therefore dealing
with something that looks very much like a signal transduction process, the dynamics
of which may be (expected to be) far from linear with dose. A word of caution is
appropriate here. Mouse ESTRs are very different from human minisatellites and the
mechanisms for their mutation (though poorly understood) seem to be very different.
Nevertheless the principle of transgenerational DNA sequence changes now has to be
considered a possibility and should be investigated further. At the cellular level,
damage leading to chromosome aberrations and mutations persisting through many
cell generations has also been observed [5] although not all studies confirm this effect
in normal cells (Dugan and Bedford, 2003; Bouffler et al., 2001; Griffin et al., 2000).
Implications for carcinogenesis and genetic risk
What are the implications of untargeted mutations and transgenerational mutation for
radiation risk in humans? Risk factors for cancer induction are derived from
epidemiology, particularly the effects seen in the A-bomb survivors. The mechanisms
by which the carcinogenic changes are brought about are essentially irrelevant to
these risk factors except in so far that they might influence the models used for
extrapolation to low doses. At present a linear no-threshold model is assumed at low
doses and, as stated above, probably the best evidence from actual studies on
populations exposed to differing levels of radon is consistent with this model [4].
Nothing that is currently known about repeat sequence mutations seriously threatens
this model.
For genetic risk, the situation is perhaps less clear. We know so little about the roles
of repeat sequences in vivo. I find it unlikely in evolutionary terms that they are “junk
DNA” as some have argued. Certainly a few mutations in such sequences are known
to be associated with human disease and they may also affect transcription or mRNA
splicing. However, there is no evidence currently that radiation induced changes in
DNA repeat sequences are associated with any health effect. One would expect that
were such changes to have a heritable health effect they could be regarded for risk
factor purposes as being covered by the sorts of models explored by Sankarayaryanan
and his colleagues. Of course, if transgenerational mutagenesis were to be shown to
be likely in humans then the implications would be different and would need to be
considered carefully. Overall, however, there seems no reason to expect a dramatic
revision of germline risk estimates in the immediate future.
Gene expression and pregnancy outcome
The only area of possible concern might be pregnancy outcome and the reason is that
some repeat sequence changes are known to affect gene expression, in fact the whole
area of gene expression and pregnancy outcome following irradiation seems ripe for
re-evaluation. Properly ordered differential gene expression is crucial to the proper
development of the embryo and foetus. We know from the work of David Barker and
his colleagues that dietary restriction associated with low birth weight can lead to a
variety of health problems in later life [6]. Changes in gene expression would seem
necessarily to be involved in this, and for one case recent work in rats has elucidated
the mechanism [7]. A low protein diet initiated at the beginning of pregnancy led to
modulation of gene expression in embryonic day 13 metanephroi at a time when
nephrons and glomeruli had yet to form. The offspring had fewer renal glomeruli and
a higher systemic blood pressure than controls.
That ionising radiation causes multiple changes in gene expression is well known
from studies with cellular systems, whole animals (e.g., [8], and humans (e.g., [9] [10]
but the doses employed have generally been high. Nevertheless, gene expression can
be a sensitive indicator of radiation response and in a myeloid leukaemia cell line
linear responses for induction of several genes have been reported down to doses as
low as 2 cGy [11]. In primary human fibroblasts more than 200 genes have been
reported to be transiently up-regulated by 1 cGy of X-rays [12]. There have also been
preliminary reports of changes in gene expression in the descendents of irradiated
mice [13] [14] albeit at higher doses.
A few years ago when I was Chairman of the Committee on Medical Aspects of
Radiation in the Environment (COMARE) in the UK, I was involved in a review of
pregnancy outcome following parental exposure to radiation [15]. We found that
almost all the epidemiological studies on pregnancy outcome lacked statistical power
due to the low doses to which the populations had been exposed. Only with neural
tube defects was there a suggestion (no more) of an association with fathers’
occupational exposure. Epidemiological studies of pregnancy outcome inevitably
have concerned themselves with serious effects such as miscarriage, stillbirth,
neonatal death or congenital malformation. Any health consequences resulting from
changes in gene expression might well be more subtle than these and unlikely to be
detectable at or shortly after birth. For example, the changes in gene expression
resulting from dietary restriction in humans during pregnancy have been reported to
lead in later life to such significant health problems as coronary heart disease, stroke,
hypertension, non-insulin-dependent diabetes, and depression [6] [16]. If there are
radiation-induced changes in gene expression in the embryo following exposure of
parental germ cells or of the embryo itself, the health consequences might well fall
within the normal range of variability in the human population and would certainly be
difficult to distinguish in epidemiological studies. For ionising radiation this has to be
considered in the context of the evolution of human and other life in an environment
of measurable background radiation exposure. Even with animals such effects could
not be readily studied. Nevertheless, subtle effects which fall within the range of
normal variability that exists among otherwise healthy people may still have public
health consequences.
However, the technical advances that have become available for the study of gene
expression in the last few years have been considerable and for the first time it is now
possible to look at changes in gene expression following low dose irradiation of
parental germ cells or of the embryo. Such studies would, if negative, reassure us that
there is nothing to be concerned about. If positive, however, they could give valuable
clues as to the likely health consequences in later life and suggest possible
epidemiological approaches.
Acknowledgement
My thought on the effects of radiation on repeat DNA sequences benefited greatly
from the review process initiated by the UK Committee on Medical Aspects of
Radiation in the Environment (COMARE) which led to the publication of [2]. The
views expressed here are, of course, my own.
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