[CANCER RESEARCH 44,1337-1342,
April 1984]
DMA Damage Induced in Human Diploid Cells by Decay of Incorporated
Radionuclides1
Peter K. LeMotte2 and John B. Little3
Laboratory of Radiobiology, Harvard School of Public Health, Boston, Massachusetts 02115
ABSTRACT
MATERIALS AND METHODS
Alkaline and neutral elution techniques were used to charac
terize the production of single- and double-strand DMA breaks
in human diploid fibroblasts by incorporated radionuclides. 125I
was incorporated in DMA as [125l]iododeoxyuridine, 3H as [3H]thymidine, and 14C as [14C]thymidine. Under frozen conditions,
125Iwas 3 times as efficient as 3H per decay in inducing singlestrand breaks and 6 times as efficient as 14C. For double-strand
break production, however, 125Iwas 6 times as efficient per
decay as 3H. It was calculated that, on the average, each 125I
decay produces about one double-strand break in the frozen
state. Under nonfrozen conditions, 125Iand external X-rays were
roughly 5-fold and 3H about 3-fold more efficient in double-strand
Cell Cultures. The human diploid fibroblast strain AG1522 was ob
tained from the Institute for Medical Research, Camden, NJ. These cells
were grown at 37° in a humidifed 5% CO2 atmosphere. The growth
medium was Eagle's minimal essential medium supplemented with 10%
break induction than under frozen conditions.
INTRODUCTION
fetal calf serum and gentamicin (50 ^g/ml). For alkaline elution experi
ments, V79 cells were used as an internal standard. These cells were
grown in the same medium as AG1522.
X- Irradiation and -y-Irradiation. X-irradiation of cells for alkaline elution
was carried out with a General Electric Maximar X-ray generator oper
ating at 220 kV and 15 ma (half-value layer = 0.5 mm Cu) and yielding
a dose rate of 0.8 Gy/min to the cells. For neutral elution where higher
doses were required, the cells were irradiated with 7-rays from a cobalt
60 source (United States Nuclear GR-9) at a dose rate of 0.66 Gy/sec
(100 rads = 1 Gy). In both cases, cells were prelabeled with [14C]thymidine as described below. Irradiations were then carried out either
with the cell suspension on ice at 0°or in a frozen vial at -72° in a bath
of dry ice:ethanol. Irradiation of frozen cells was performed on the same
day that the cells were frozen.
Cellular Incorporation of Radionuclides. [3H]Thymidine (methyl la
beled), [14C]thymidine (methyl labeled), or [125l]ldUrd was incorporated
into the DNA of growing cells. [3H]Thymidine and ['4C]thymidine were
There are a number of reports describing the induction of DNA
damage by incorporated radionuclides in mammalian cells (2-6,
14). Such documentation is valuable in assessing risk from
environmental exposure to these agents and in elucidating the
mechanisms for their biological effects. These reports have
largely focused on 3H, 14C, and 125I.The induction and repair of
DNA single-strand breaks have been the most investigated end
point. Although double-strand breaks may be very important in
the biological effects of incorporated radionuclides, their induc
tion has not been well studied in mammalian cells. The induction
of double-strand breaks by [125l]ldUrd4 has been assayed in
IdUrd during a single S phase of synchronous growth achieved by release
of cells from confluence by trypsinization. The overall thymidine concen
tration in the medium was in each case adjusted to 10"6 M. For alkaline
bacteria and phage (8, 9), under frozen conditions, where it has
been shown that each decay of 125Iproduces a double-strand
elution, [3H]thymidine was used at a concentration of 0.1 /iCi/ml,
[14C]thymidine at a concentration of 0.025 /iCi/ml, and [125l]ldUrd at a
break.
This paper describes investigations in human cells of the
induction of single-strand breaks by 3H, 14C, and 125Iand of
double-strand breaks by 3H and 125I,all incorporated into DNA
concentration of 0.1 MCi/ml (the concentration of IdUrd in the growth
medium was less than 10"10 M). For neutral elution, [3H]thymidine was
used at a concentration of 1.0 /iCi/ml, [14C]thymidine at a concentration
of 0.025 fiCi/ml, and [125l]ldUrd at a concentration of 0.5 (iCi/ml. The
as labeled thymidine or IdUrd. For comparison, we have also
examined the induction of single-strand breaks by X-rays and
double-strand breaks by 7-rays. We have used alkaline and
neutral elution techniques for assaying single- and double-strand
breaks, respectively. The use of these techniques allows us to
compare the results of previous studies with alkaline and neutral
sucrose gradient centrifugation with a different assay system.
We report specifically that 125Iis a much more efficient inducer
of double-strand breaks in human cells than is 3H.
cells were incubated with radioactive medium from the time of trypsini
zation until 36 hr later. Cells were then chased with medium containing
nonradioactive thymidine for 1 hr, then trypsinized, and either cooled to
0°on ice or frozen to -90° in medium containing 10% glycerol for the
1This work was supported by Contract EVO4322.A004 from the United States
Department of Energy and by Training Grant CA-09078 and Center Grant ES00002 from the NIH.
2 Present address: Biology Department, Massachusetts Institute of Technology,
Cambridge, MA 02139.
3To whom requests for reprints should be addressed.
4The abbreviation used is: IdUrd, tododeoxyuridine.
Received March 29,1983; accepted December 15,1983.
APRIL 1984
purchased from New England Nuclear Corp. (Boston, MA) at specific
activities of 40 to 60 Ci/mmol and 40 to 60 mCi/mmol, respectively.
[125l]ldUrd was purchased from New England Nuclear Corp. and from
Amersham (Arlington Heights, IL) at a specific activity of 2000 Ci/mmol.
In these experiments, AG1522 human diploid fibroblasts were incubated
in complete medium containing [3H]thymidine, [14C]thymidine, or [125I]-
accumulation of radioactive decays. We found in each case that the level
of radioactivity used in the medium produced no decrease of DNA size
detectable at the time of freezing the cells. Cells were thawed at various
times up to 6 weeks later (or removed from ice at times up to 3 days
later) for the determination of the degree of DNA breakage from the
radiation.
Radioactivity Determinations. We determined incorporated radioac
tivity for each group of cells following the method described in detail
previously (10). Routinely, the radioactivity of an aliquot of cells was
measured after trichloroacetic acid precipitation of the DNA. A liquid
scintillation counter was used to determine 3H and 14Cactivity, while an
Auto-Gamma counter was used to determine 125I activity. Known
amounts of the radionuclides were used to determine counting efficien
cies. For elution assays, the radioactivity was determined for 3H, 14C,
and '25I by liquid scintillation counting.
1337
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P. K.LeMoffeand J. ß.
L/We
Alkaline and Neutral Elution. The techniques of alkaline and neutral
elution were used to determine the relative size of cellular DNA after
decay of incorporated 3H, 14C, or 125Ior after y- or X-irradiation. We
followed the basic procedures of Kohn ef a/. (7) for alkaline elution and
of Bradley and Kohn (1) for neutral elution. Alkaline elution determines
the relative size of single-stranded DNA by measurement of the rate of
elution of DNA through a filter at alkaline pH. Neutral elution determines
the relative double-stranded size by elution at a neutral pH.
For the determination of the number of DNA breaks in a frozen sample
of cells, a frozen vial was placed on ice to thaw. The cells were then
loaded onto filters for elution (described below) and washed with icecold 0.9% NaCI solution until a lysis solution was
the time of thawing until lysis, the cells were kept
repair of DNA breaks induced by incorporated
external irradiation while frozen. Certain irradiations
0°on ice. In such cases, the cells were also kept
applied. Thus, from
cold to prevent any
radionuclides or by
were carried out at
cold until the lysing
solution was applied.
For alkaline elution, the cells were loaded onto 2-^m-pore-size
carbonate filters (Nucleopore Corp.,
balanced salt solution, and lysed with
M trisodium EDTA solution containing
After a 1-hr incubation in proteinase
poly
Pleasanton, CA), washed with a
a 0.2% Sarkosyl:2.0 M NaCI:0.02
0.5 mg of proteinase K/ml.
K at room temperature, each filter
was washed with 0.02 M trisodium EDTA, and elution was initiated with
a solution of 2.0% tetrapropyl ammonium hydroxide (RSA Corp., Ardsley,
NY) and 0.02 M EDTA (acid form) at pH 12.2. Pumps were run at a speed
of 0.035 to 0.040 ml/min. Fractions were collected every 100 min to a
total of 10 fractions. The lines were washed with 0.4 N NaOH, and the
washes were saved for radioactivity determinations. The filters used for
elution were heated for 1 hr at 70°in 1N HCI, followed by addition of 0.4
N NaOH to degrade the DNA remaining on the filter. Counting of radio
activity was done by adding Aguasol (acidified by addition of 40 ml of
acetic acid to 4 liters of Aguasol) and counting samples in a liquid
scintillation counter. Results were plotted as the percentage of the DNA
remaining on the filter versus the percentage of internal standard DNA
remaining on the filter in each fraction. We compared the size of the DNA
after different treatments using an expression of relative retention of the
experimentally treated DNA compared with the internal standard DNA.
The relative retention was expressed as the percentage of experimentally
treated DNA remaining on the filter at the point when 50% of the internal
standard DNA remained on the filter.
For neutral elution, we also used 2-^m-pore-size polycarbonate filters.
In this case, following loading and washing of cells on the filter, 3.0 ml
of a 2.0% sodium dodecyl sulfate:0.05 M Tris:0.05 M glycine:0.025 M
disodium EDTA solution, pH 9.6, with 0.5 mg of proteinase K/ml was
applied to the filters, and pumping was started. After 1 hr, an eluting
solution identical to the lysing solution except lacking proteinase K was
applied, and the elution continued. A total of ten 100-min fractions was
collected. Processing of the lines and filters and counting of radioactivity
were as described for alkaline elution. Results were plotted as the
percentage of DNA remaining on the filter versus time. We expressed
relative retention as the percentage of the DNA remaining on the filter
after 10 hr of elution.
For neutral elution, DNA breakage by 3H decay was examined by
measuring the 3H-labeled DNA in each fraction. In other words, the 3H
served both as the DNA-damaging agent and as the radioactive label for
the DNA. Similarly, 125Iwas used both to induce DNA damage and at the
same time as a label to assay the elution of the DNA. For alkaline elution,
we used MC-labeled DNA from Chinese hamster V79 cells as an internal
standard when assaying 3H-induced breaks in human DNA and 3Hlabeled V79 cell DNA as an internal standard when assaying 14C-induced
breaks in human DNA. When assaying 125l-induced breaks in human
DNA, however, we also labeled the human DNA with 14Cand used 3Hlabeled V79 cell DNA as an internal standard, in order to simplify the
liquid scintillation counting procedure. We were then able to follow the
14C counts for assaying 125l-induced breaks. We corrected for any
additional breaks caused by the "C using a control sample labeled only
with 14C.
1338
RESULTS
In order to accurately compare the efficiency of DNA singlestrand and double-strand break induction by these radionuclides
and by X-rays, we allowed radiation damage to the cells to be
induced under frozen conditions or at 0°,where no repair of
DNA damage could occur. Examples of elution are plotted in
Charts 1 and 2 after X-irradiation or after damage to frozen cells
from incorporated radionuclides. For alkaline elution (Chart 1),
the percentage of the DNA remaining on the filter is shown as a
function of the percentage of internal standard DNA remaining
on the filter in each fraction. The internal standard was Chinese
hamster V79 cells irradiated with 1.5 Gy at 0°.For neutral elution
(Chart 2), the percentage of the DNA remaining on the filter is
plotted as a function of time of elution.
The results of alkaline and neutral elution of cells X- or yirradiated at -72° are shown in Charts ~\Aand 2A. The results
of alkaline and neutral elution of cells after decay of 125Iand 3H
at -90° are shown in Charts 16 and 2B. It can be seen from
these charts that both X-rays and incorporated radionuclides
produce DNA damage under frozen conditions that results in a
nearly linear elution of the DNA through the filter. A slight
concavity does begin to appear in the curves at higher doses.
Because of the uniformity of the curves, we can use relative
retention values to compare treatments, as described in "Mate
rials and Methods." From a number of experiments such as
those shown in Charts 1 and 2, we calculated the relative
retention in order to determine the dose-response relationship
for induction of DNA breaks. Dose-response curves are shown
in Charts 3 to 6. In these graphs, each point represents an
individual elution. Dose-response curves for the alkaline elution
of X-irradiated cells under frozen and nonfrozen conditions are
shown in Chart 3. It can be seen that irradiation under frozen
conditions produced about 6 times fewer single-strand breaks
than did irradiation at 0°.In Chart 4, the relative retention of cells
7-irradiated at -72° or at 0°and eluted under neutral conditions
is plotted as a function of radiation dose. There is again a large
reduction in damage seen at -72°; double-strand breaks were
induced about 5-fold less efficiently by 7-irradiation under frozen
conditions than at 0°.Control experiments in which cells were
irradiated frozen, in the presence or absence of glycerol, showed
glycerol did not have a radioprotective effect for induction of
DNA breaks under frozen conditions (data not shown).
The efficiency of single-strand break induction at -90° by 125I,
3H, and 14Cis shown in Chart 5. The dose is represented as the
number of decays occurring per cell during the period in which
the cells were frozen. From these data, a comparison of the
efficiency of single-strand break induction shows 125Ito be about
3 times more efficient per decay in inducing DNA single-strand
breaks than 3H.
The dose-response curves for double-strand break induction
at -90° by 125Iand 3H are shown in Chart 6. It can be seen that
125Iis about 6 times more effective than 3H per decay in inducing
DNA double-strand breaks. For the induction of double-strand
breaks at 0°under nonfrozen conditions (Chart 7), we found 3H
was about 3-fold more effective at 0°and 125Ito be roughly 5fold more effective than under frozen conditions, similar to the
result for external X-rays.
The results on the efficiency of induction of DNA single- and
double-strand breaks by 1251,3H,14C,and X-rays are summarized
in Tables 1 and 2. We have not attempted
to calculate the
CANCER RESEARCH
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VOL. 44
DNA Damage by Incorporated Radionuclides
5
O
15
HOURSOF ELUTION
02
IO 0.9 OB 0.7
0.6
05
Q4
03
10
FRACTION OF INTERNAL STANDARD
DNA RETAINED ON FILTER
control
3H(l2pÃ’Odecoys)-
i 09
! 08
^IWOOOdecoys)
O
03
S
IO
B
HOURS OF ELUTION
Chart 2. A, neutral elution of human cells ^ -irradiated at -72° versus time of
elution. B, neutral elution of human cells after decay of radioisotopes at -90°
versus time of elution.
10
09
08
FRACTION
07
06
OF
05
INTERNAL
0«
OS
STANDÛRD
DMA RETAINEDON FILTER
Chart 1. A, alkaline elution of human cells X-irradiated at —72°
versus elution
of internal standard treated with 1.5 Gy at 0°.8. alkaline elution of human cells
after decay of radionuclides at -90° versus elution of internal standard treated
with1.5GyatO°.
absorbed dose per decay of radionuclide, owing to the difficulty
of defining the relevant sphere of energy absorption for each
radionuclide. We therefore make comparisons based on numbers
of decays of radionuclide. Included in Tables 1 and 2 are the
values for the number of decays of each radionuclide or the
number of rads of X-ray needed to reduce the single-stranded
or double-stranded DNA size to 37% of its initial value. This is
equal to the inverse of the slope of the lines in Charts 3 to 7.
Based on certain assumptions, we have also calculated values
for the number of single- and double-strand breaks induced per
decay. In performing these elutions, we do not have a direct
measure of DNA size and so cannot directly calculate the number
of DNA breaks. We have, however, well calibrated our elution
kinetics with respect to the dose of X-rays or 7-rays needed to
produce an effect equivalent to that produced by the radio
nuclides. We therefore applied reported values from sucrose
gradient studies of the efficiency of DNA break induction by Xrays or 7-rays to calculate the number of breaks induced per
radioactive decay by each isotope. Approximately 4 single-strand
breaks are induced under frozen conditions per 0.01 Gy of XAPRIL 1984
60
DOSE (Gy)
Chart 3. Relative retention of DNA from cells X-inradiated at 0°or -72° and
eluted under alkaline conditions. The relative retention is the percentage of the
DNA retained on the filter when 50% of the internal standard DNA has eluted. For
0°data, r = 0.99; for -72° data, r = 0.94.
rays per cell (5). The number of double-strand breaks that will
be induced will be about 20 times less than the number of singlestrand breaks (13). Based on these assumptions, we give values
for the number of single- or double-strand breaks induced per
decay for each isotope.
1339
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P. K. LeMoffe and J. B. Little
I.O
A
0.»
p
fe!
no
200
DOSE (Gy)
300
0
Chart 4. Relative retention of DNA from cells -/-irradiated at 0°or -72° and
900
'000
ISOO
2OOO
DOSE(decoys/cell)
eluted under neutral conditions. The relative retention is the percentage of the DNA
retained on the filter after 10 hr of elution. For 0°data, r = 0.99; for -72° data, r
= 0.99.
«y
DISCUSSION
0.6
We have examined the efficiency of DNA strand breakage
induced in human cells by incorporated radionuclides and by
external X-rays either under frozen conditions or at 0°.Under
0.4
B
frozen conditions, much of the indirect action of the radiation is
eliminated, owing to the lack of mobility of induced free radicals.
The effect is thus due mainly to direct interaction of the radiation
with the DNA. Furthermore, our experimental conditions were
such that the net production of strand breaks was measured,
independent of the action of metabolic repair processes in the
cell which might eventually lead to rejoining of many or most of
these breaks. Our data (Table 1) indicate 3H to be nearly twice
as effective as 14C per decay in its capacity to induce single-
02
OJ
¡0.0*
' 004
strand breaks in DNA. This result is in agreement with published
data on 3H- and 14C-induced DNA breakage in hamster cells
studied by alkaline sucrose gradient sedimentation (5). On the
other hand, 125Iwas about 3 times as effective as 3H in inducing
single-strand breaks (Table 1).
As can be seen in Table 1, approximately 4.2 single-strand
breaks were produced per 125Idecay in frozen human cells. This
9000
IO.OOO
15,000
DOSE(decays/tell)
10
08
result is in close agreement with that of Painter ef al. (14), who
found that 4 to 5 single-strand breaks were produced per 125I
decay in Chinese hamster DNA. After correction of our data for
double-strand breaks which appear as single-strand breaks (one
double-strand break equals 2 single-strand breaks), we conclude
that 125Iinduces about 2.4 single-strand breaks per se per decay.
Krisch and Sauri (9) calculated that each 125Idecay produced 1.6
single-strand breaks in T4-phage DNA after correction for dou
ble-strand breaks appearing as single-strand breaks. Though
this difference is relatively small, the higher value which we
obtain in mammalian cells is expected, owing to the greater mass
of DNA in the mammalian nucleus and therefore the larger
amount of radiation absorbed. This explanation was pointed out
by Krisch and Sauri (9).
In contradistinction to this difference between mammalian cells
and phage in the efficiency of induction of single-strand breaks
by 125I,we have close agreement concerning the efficiency of
induction of double-strand breaks. We find that each 125Idecay
produces nearly one double-strand break in human cells, a result
1340
1 06
04
03
02
2500
5000
7500
DOSE (decays/cell)
Charts. A, relative retention of DNA from cells after decay of 1Z5Iat -90°.
Elution is under alkaline conditions, r = 0.91. B, relative retention of DNA from cells
after decay of 3H at -90°. Elution is under alkaline conditions, r = 0.82. C, relative
retention of DNA from cells after decay of 14C at -90°. Elution is under alkaline
conditions, r = 0.90.
CANCER
RESEARCH
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VOL. 44
I
DNA Damage by Incorporated Radionuclides
Table 1
Induction of single-strand breaks by incorporated [â„¢IJIdUrd,pHjthymidine,
["CJthymidine, or external X-rays as measured by alkaline elution
09,
08
Single-strand break induction at -90°
' O7
! 0.6
!: 05
[*Hl ThymkJine
decays)1260
(no. of
[125l]ldUrd
±360
[3H]Thymidine
4230 ±1800
['4C]ThymidineX-Rays6D„"
7300
±210012.2
Lu
CC
of production
(single-strand breaks/de
cay)4.3
±1.2
1.3 ±0.5
0.74 ±
0.214.4/0.01
± 0.5 GyEfficiency
Gy (4)
'' D..,,.the dose necessary to reduce the relative retention to 37% of its original
value ±95% confidence interval.
b X-irradiation was at -72°.
02
20000
40,000
60,000
DOSE(decays/cell)
Chart 6. Relative retention of DNA from cells after decay of 125Ior *H at -90°.
Elutkxi is under neutral conditions. For 3H,r = 0.95; for '"I, r = 0.93.
IX)
«»IdUrd
0,8
by 125Iin phage DNA (9). This finding implies that the doublestrand breaks are induced in the DNA very close to the site of
decay and would explain why we find similar efficiency for doublestrand break induction in human cells as in bacteria and phage,
where the quantity and concentration of DNA may be very
different. It is also apparent that any differences between human
cells and bacteria in the arrangement of the DNA, such as nuclear
proteins associated with the DNA, do not have a significant effect
on the efficiency of double-strand break induction by 12SI.
¡0.6
In another comparison of our data, we can calculate from the
data in Tables 1 and 2 the X-ray dose needed to produce an
equivalent effect as the decay of each radioisotope in terms of
single- and double-strand break induction. In agreement with
: 0.4
Cleaver ef al. (5), we find a value of about 0.0029 Gy per decay
of 3H in terms of single-strand break induction and about 0.0017
Gy per decay of 14C. 125Iproduced an equivalent number of
single-strand breaks per decay as about 0.01 Gy of X-rays. For
the end point of double-strand break induction, 3H has a value
400
800
I200
DOSE (decays)
IO
[*H] Thymidine
0.8
0.6
ÃœJ
ce
04
0.2
0
8000
16,000
DOSE (decays)
Chart 7. Relative retention of DNA from cells after decay of 125IW or 3H (B) at
0°.Elution is under neutral conditions. For 125I,r = 0.60; for 3H, r = 0.66.
similar to that found in bacteria and in phage (8, 9). It has been
shown that dilution of the DNA concentration makes no differ
ence in the efficiency of the induction of double-strand breaks
of 0.0066 Gy per decay versus 0.0029 Gy per decay for singlestrand break induction, showing that 3H in comparison to X-rays
has perhaps a higher efficiency of double-strand break induction
compared to single-strand break induction. 125Ihas a value for
double-strand break induction of 0.039 Gy per decay. This again
demonstrates the high efficiency of double-strand break induc
tion by 125I.
The results of double-strand break induction by 125Iand 3H at
0°,under nonfrozen conditions, show a roughly 5-fold increase
for 125Iand 3-fold increase for 3H, compared with frozen cells
(Table 2). It suggests that the number of double-strand breaks
induced indirectly by free radicals is large for these radionuclides
as well as for external X-rays. It may be, though, that the quality
of the double-strand breaks produced by indirect action, partic
ularly for 125I,is different from those produced directly by the
radiation and transmutation. The double-strand breaks produced
directly at the decay site may be less easily repaired and biolog
ically more detrimental. For example, in a prior report of cell
killing of human fibroblasts induced by radionuclide decay in
DNA, we showed 125Ito be 11-fold more effective than 3H per
decay (11). In examining malignant transformation (10) and mutagenesis (12) by 125Iand 3H, we found 125Ito be 25- to 30-fold
more effective than 3H. This exceeds the greater relative produc
tion of double-strand breaks by 125Iversus 3H measured under
our assay conditions reported here. It may point to an additional
factor, the quality of the double-strand break produced by 125I,
and its relative repairability.
In summary, our results indicate that 125Iis particularly efficient
APRIL 1984
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1341
P. K. LeMotte and J. B. Little
Table 2
Induction of double-strand breaks by incorporated fljkiUrd
and fHJthymidine or external y-rays as
measured by neutral elution
Double-strand break Induction
-90°
of produc
tion
(double-strand
breaks/decay)0.86
decays)9,000
(no. of
'»1*H0*'
-y-Rays
0°
decays)1,840±
of
±0.30
± 3,200
53,000 ±22,000Efficiency 0.14 ±0.060»(no.
350 ±
160Gy
0.22/0.01 Gy (13)
of
production
(double-strand
breaks/decay)4.2
1,300
±3.1
18,700 ±9,000Efficiency 0.42 ±0.20
65 ±
23 Gy
1.2/0.01 Gy
, dose necessary to reduce the relative retention to 37% of its original value ±95% confidence interval.
0 -^-Irradiation was at -72°.
in the induction of DNA double-strand breaks in mammalian cells,
as has been observed previously in phage and bacterial DNA.
Correcting for double-strand breaks which appear as singlestrand breaks, the decay of 125Iunder frozen conditions yields
about one double-strand break for every 2.3 single-strand
breaks, whereas for 3H and X-rays, the ratios are 1:7 and 1:20,
respectively. Although the efficiency of double-strand break pro
duction per decay by 125Iis about 6 times that of 3H, singlestrand break production for the 2 isotopes is only 2.3 versus 1.0.
Under nonfrozen conditions, the relative efficiency of doublestrand break induction by 125Icompared to 3H is approximately
10, even higher than under frozen conditions. These results
suggest that DNA double-strand breaks may represent a class
of DNA damage responsible for the enhanced biological effects
of 125Iincorporated into cellular DNA as compared with 3H or X-
4.
5.
6.
7.
8.
9.
10.
rays.
11.
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CANCER
RESEARCH
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VOL.
44
DNA Damage Induced in Human Diploid Cells by Decay of
Incorporated Radionuclides
Peter K. LeMotte and John B. Little
Cancer Res 1984;44:1337-1342.
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