DNA damage from radiation

["Karam, Andrew" <Andrew_Karam@URMC.Rochester.edu>]

Not too long ago I posted a query regarding qualitative differences between
DNA damage due to radiation (including low-LET radiation) and due to other
mutagens.  The response I received by and large said that direct DNA damage
due to even low-LET radiation does differ from other mutagens, but that
indirect damage is not necessarily unique.  In addition to the welcome
feedback I received, I have just received a copy of the latest in the Annals
of the ICRP (Publication 79, Genetic Susceptibility to Cancer).  In this,
the ICRP also notes "...a strong consensus that cellular DNA is the critical
target for radiobiological effects and the differences in radiation track
structure influence the nature of induced DNA lesions and their cellular
consequences."  This report goes on to say a few paragraphs later that
"Complex lesions involving local clustering of different forms of molecular
damage are also believed to occur and may be critical for radiation quality
effects."

In addition to the ICRP report, I have just finished reading a short paper
by Ward that is in a book titled Radiation Damage in DNA by Fuciarelli and
Zimbrick (1995).  In this paper, Ward states that a single ionization can
cause a DSB if a single radical can cause two breaks or if each ion can
cause a break.  He then cites some work that found the DSB yield to 1% of
the SSB yield from single radical attack.  Another reference I have (The
Handbook of Radiobiology, Prasad, 1995) cites work done in 1977 (Elklind)
and 1981 (Ngo, Blakely, and Tobias) indicating that damage by high-LET
radiation is repairable.  Finally, a paper in the Proceedings of the NAS
(The oncogenic transforming potential of the passage of single alpha
particles through mammalian cell nuclei, Miller et al, 1999) suggests that
damage due to exposure to low levels of alpha radiation may be repairable.
To me, this suggests that DSBs are more common than I would have thought and
that repair of DSBs is not as difficult and error-prone as I had expected.  

I'm still thinking about all of this and I am certainly no expert in the
area.  I felt it appropriate to mention these to continue the train of
thought I started in the recent past and to fill you in on what I have
discovered to date.

Andy

P. Andrew Karam, CHP
Radiation Safety Officer
University of Rochester
(716) 275-1473 (voice)
(716) 256-0365 (fax)
andrew_karam@urmc.rochester.edu 
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