International Journal of Radiation Biology 71, 1-19, 1997
Review: proximity effects in the production of chromosome aberrations
by ionizing radiation.
Abstract
After ionizing radiation induces DNA double strand breaks (DSBs),
misrejoining produces chromosome aberrations. Aberration yields
are influenced by "proximity" effects, i.e. by the dependence of
misrejoining probabilities on initial DSB separations.
We survey proximity effects, emphasizing
implications for chromosome aberration formation mechanisms,
for chromatin geometry, and for dose-response relations.
Evidence for proximity effects comes from observed biases for
centric rings and against three-way interchanges, relative
to dicentrics or translocations. Other evidence comes from
the way aberration yields depend on radiation dose and quality,
tightly-bunched ionizations being relatively
effective. We conclude:
(1) misrejoining probabilities decrease as the
distance between DSBs at the time of their formation increases,
and almost all misrejoining occurs among DSBs initially
separated by less than 1/3 of a cell nucleus diameter;
(2) chromosomes occupy (irregular) territories during the G0/G1
phase of the cell cycle, having dimensions also roughly 1/3 of a cell
nucleus diameter; (3)
proximity effects have the potential to probe
how much different chromosomes
intertwine or move relative to each other;
(4) incorporating proximity effects
into the classic random breakage-and-reunion model
allows quantitative interrelation of
yields for many different aberration types and of
data obtained with various FISH
painting methods or whole-genome scoring.
Subjecting cells to ionizing radiation
during the G0/G1 phase of the cell cycle
causes chromosome-type aberrations, through chromosome breakage and
large-scale rearrangement of the pieces.
At various times, chromosome aberrations have been suggested as
symptoms and/or causes of most major radiobiological effects
(surveys in Cornforth and Bedford 1993).
They are of particular interest in connection with
biodosimetry (e.g. Bender et al. 1988, Lucas et al. 1992,
Bauchinger 1995,
Gebhart et al. 1996, Durante et al. 1996) or as indicators of
radiosensitivity
(e.g. Wlodek and Hittelman 1988, Russell et al. 1995, Jones et al.
1995).
Certain chromosome aberrations are strongly
linked with most hematopoietic cancers (Rabbitts 1995).
It is probable (Cornforth and Bedford 1993; see also Section 2
below) that most chromosome aberrations result from
illegitimate reunion ("misrejoining")
of free ends from different DNA double strand breaks (DSBs).
Accepting this picture, one finds that aberration formation is
influenced by "proximity" effects, i.e. effects which occur because
DSB free ends are more likely to undergo illegitimate reunion
if the DSBs are initially formed close together
than if the DSBs are formed far apart (Sax 1940, survey in Savage 1996).
Proximity effects can be inferred by analyzing aberration
yields as a function of
aberration type, of radiation quality, or of dose.
Proximity effects influence relative yields of different
types of aberrations,
because any one chromosome at one time is somewhat localized
in an irregular territory (or "domain") during G0/G1, rather
than being spread out more or less uniformly
over the whole cell nucleus (Appendix 2).
For example, compared to expectations based on complete
randomness, proximity effects bias for centric rings,
which involve two DSBs on one
(localized) chromosome, relative to
dicentrics, which involve two DSBs on two different chromosomes.
It is remarkable that chromosome aberration data suggested
reasonable models for chromosome localization (Sax 1940,
Savage and Papworth 1973) long before modern methods
gave direct confirmation that chromosomes
do occupy localized territories during G0/G1 (Appendix 2).
Proximity effects influence relative aberration yields for
radiations of different quality
inasmuch as radiations producing tightly bunched DSBs, such as high
LET (i.e. densely-ionizing) radiations, are more effective than
low LET radiations at producing aberrations (survey in Goodhead 1987).
Proximity effects
also enhance the importance of the nearby DSB pairs induced
by a single primary radiation track,
compared to the relatively distant DSB pairs
induced by different primary radiation tracks, an effect which can be
uncovered by varying the dose (survey in Kellerer 1985).
The advent of fluorescent \fIin situ\fR
hybridization (FISH) chromosome painting
has dramatically increased the
scope of aberration studies
(survey in Gray et al. 1994, Simpson and Savage 1996).
Detailed results on many types of simple or complex aberrations
have been obtained, and more sophisticated painting techniques
(Ballard and Ward 1993) promise to reveal additional aspects of
aberrations and proximity effects.
We shall here review some current ideas on proximity effects.
Damage leading to loss or transfer of chromosome portions smaller than
1 Mb (one million base pairs) will not be considered directly,
since the resolution afforded by light microscopy is not adequate
for routine detection of such "small" scale phenomena.
Moreover, to keep the review focussed, no detailed discussion of related
endpoints such as chromatid aberrations, clonogenic inactivation,
mutations, or length distributions of broken DNA fragments will
be given; some premature chromosome condensation (PCC) results
will be
discussed, but only those on the kinetics of aberration formation.
Section 2 outlines some background,
terminology, and models needed to interpret chromosome aberration data.
Section 3 concerns evidence for proximity effects,
based on the relative frequencies of particular
aberration types, on
the relative effectiveness of different qualities of radiation,
on the dose dependence of aberration formation,
and on cell-to-cell variation of aberration number.
Section 4 briefly discusses the
relations of proximity effects to the kinetics of aberration formation.
After summarizing the results, Section 5 discusses some
implications for cell nucleus ultrastructure and for applications.
Three appendices respectively discuss: (a) randomness of DSB induction
and of DSB free end illegitimate reunion; (b) current information on
large-scale chromosome geometry and motion;
and (c), dose-response relations.
Many of the basic ideas throughout this review
were first clarified by D.E. Lea, more than a half-century ago
(Lea 1946).
International Journal of Radiation Biology
71, 1-19, 1997.
R.K. SACHS & A.M. CHEN
Dept. Math., University of California, Berkeley, CA 94720.
D.J. BRENNER Center for Radiological
Research, Columbia University, New York, NY 10032.
Proximity