F. P. HUNGATE AND B. J. MCCLANAHAN 
903 
by low LET radiations, oxygen, through the 
normally functioning oxidative metabolic sys- 
tems of the cell, enhances the fusion of broken 
ends.2*'^^ Until recently it was assumed that 
rejoining concerned the structural back bone 
of DNA. However, Wolff and Scott find that 
inhibitors of protein synthesis keep breaks open 
while known DNA repair mechanisms have lit- 
tle affect on the fusion of broken ends, empha- 
sizing the importance of the protein moiety of 
the nucleoprotein complex which makes up the 
chromosomes. 
The observation that high LET radiations are 
unaffected by oxygen poses an attractive possi- 
bility to radiologists treating solid tumors 
whose central zones remain relatively anoxic 
despite all efforts to raise their oxygen tension. 
Thus, in typical X or gamma therapy, the cells 
in the center of solid tumors are anoxic and 
thus less damaged than are the more highly 
oxygenated cells on the surface of the tumor 
and the surrounding normal tissues. High LET 
radiations capable of releasing their energy in 
the tumor region would overcome this problem. 
Other Factors 
Other features such as mitotic delay,^^ selec- 
tion against aberrations,^^ etc., might well be 
discussed but instead we will briefly refer to 
the observation of a persistent factor in plasma 
capable of increasing aberration rates in cells 
from normal individuals. Goh and Sumner 
observed such a factor in plasma of individuals 
accidentally irradiated several years previously 
and Littlefield et al.^" obtained similar results 
using plasma of patients receiving therapeutic 
irradiations. Such a factor was sought when 
aberration frequencies remained high over 
much longer post irradiation times than ex- 
pected. It is difficult to imagine an effective free 
radical persisting over such long times. One 
possibility might be that viruses were involved 
since they are known to induce at least simple 
breakage.^" Inclusion of viruses as factors im- 
portant to genetic considerations is logical since 
knowledge of transduction clearly implicates 
them as part of the cell's potential information 
system. The extent to which they may modify 
aberration data has, as yet, not been evaluated 
but we are not aware of any evidence that multi- 
break aberrations such as rings and dicentrics 
are increased by the presence of viruses. 
Neither have we seen data which would force 
rejection of the possibility that viruses were the 
persistent agents in plasma inducing aberra- 
tions frequencies above those observed in con- 
trols. The interaction of radiations, viruses, and 
host information are subjects of intense interest 
and speculation is well warranted if it leads to 
informative experimentation. 
SUMMARY 
Preliminary data from studies of chromo- 
some breakage in PHA induced leukocyte cul- 
tures suggest that the frequency of aberration 
in miniature swine is only slightly lower than 
in human cells following comparable X-ray ex- 
posures. Comparisons following 9ogj._90Y beta 
radiation are incomplete. On the basis of the 
X-ray study, it appears probable that failure to 
observe two-break aberrations, i.e. rings and 
dicentrics, in miniature swine chronically ex- 
posed to 80Sr-^°Y is more probably due to low 
dose rate than to insensitivity of the chromo- 
somes to radiation. 
A brief review of the nature of radiation dam- 
age to chromosomes includes types of aberra- 
tions induced, effects on aberration yield due 
to dose, dose rate, radiation quality, oxygen, 
and other factors. Viruses are suggested as a 
possible factor in plasma causing chromosome 
breakage in individuals exposed years earlier 
to ionizing radiation. 
REFERENCES 
1. MOORHEAD, P. S., NOWELL, P. C, MELLMAN, W. J., 
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2. Bender, M. A., and Gooch, P. C. Types and rates 
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