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Lowdose Radiation Program Workshop V
2005 Abstract
Title: mFISH Reveals an LET-Independent Excess of Radiation-Induced
Dicentrics Involving Homologous Chromosomes
Authors: 1Y. Plan, 2L.
Hlatky, 2P. Hahnfeldt, 1R. Sachs, 3B. Loucas, and 3M. Cornforth
Institutions: 1 Department of Mathematics, University of California,
Berkeley CA; 2Department of Radiation Oncology, Dana-Farber
Cancer Institute, Harvard Medical School, Boston MA; 3Department
of Radiation Oncology, University of Texas Medical Branch,
Galveston, TX
Radiogenic chromosome aberrations are informative
about DNA repair/misrepair mechanisms, including the relative
importance of non-homologous and homology-dependent recombinational
pathways. In addition, because of chromosome localization
in the interphase nucleus and proximity effects, aberrations
are also informative about chromosome juxtapositions.
One
potentially informative aspect of aberration spectra is the
ratio of homologous to heterologous dicentrics. This ratio
is influenced by, and thus potentially informative about,
underlying DNA damage repair/misrepair processes and also
the geometry of individual chromosome domains within the
interphase nucleus at the time of irradiation. We reasoned
that if, during G1 or G0, homology plays any role in the
competition between restitution (e.g. eurepair) and visible
misrejoining, the observed ratio should differ systematically
from that expected assuming complete randomness, independent
of homology, for DSB (DNA double strand break) repair/misrepair.
For example, if eurepair of potentially clastogenic DSBs
in G1 were to occur by homologous recombination through a
conservative pathway (i.e., involving conversion without
crossover), then one might expect a systematic downward bias
in the ratio.
24-color mFISH was used to determine the ratio
of homologous (1-color) to heterologous (2-color) dicentrics
produced in human lymphocytes or fibroblasts by 137Cs γ-rays,
3.5 MeV 238Pu α-particles, and 1 GeV/nucleon 56Fe ions
at various doses.
Assuming that randomness independent of
homology holds, the expected ratio for diploid human male
cells is ~0.024, as was shown by deriving a formula applicable
to simple interchanges and then extending the results,
via Monte Carlo simulation, to the general situation where
complex aberrations are also considered. Instead, we observed
an approximate 2-3 fold excess of homologous dicentrics,
with probability of occurrence by chance less than 0.02 for
each of the three radiations. Overall, approximately 18
homologous dicentrics were expected but 47 were found.
That the observed excesses were rather similar for both
high and low LET radiations suggests to us that a homology-dependent
misrejoining process is responsible for excess frequencies
of homologous dicentrics, as opposed to proximity of homologous
chromosomes during irradiation. If this interpretation
is correct, then it is clear that such processing of DSBs
during G0/G1 phases of the cell cycle is not necessarily
desirable, since it can lead to deleterious recombination
products (dicentrics).
Support is gratefully acknowledged from NIH GM68423 (YP),
NASA 03-OBPR-07-0059-0065(LH), NASA NSCOR04-0014-0017 (RKS),
NIH 1RO1-CA78496-04 (PH), and DOE/NASA OBPR DE-FG03-02ER63442
(MC).
