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Mol Cell Biol, July 1998, p. 4385-4390, Vol. 18, No. 7
Department of Biology, University of
California, San Diego, La Jolla, California
92093-03221;
Department of Biology,
Massachusetts Institute of Technology, Cambridge, Massachusetts
021392;
and California Institute of
Technology, Pasadena, California 911253
Received 10 March 1998/Accepted 13 April 1998
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Involvement of p53 and p21 in Cellular Defects and
Tumorigenesis in Atm
/ Mice
SUMMARY
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGMENTS
REFERENCES
SUMMARY
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Disruption of the mouse Atm gene, whose human counterpart is
consistently mutated in ataxia-telangiectasia (A-T) patients, creates
an A-T mouse model exhibiting most of the A-T-related systematic and
cellular defects. While ATM plays a major role in signaling the p53
response to DNA strand break damage, Atm
/
p53/ mice develop lymphomas earlier than
Atm/ or p53/ mice, indicating that
mutations in these two genes lead to synergy in tumorigenesis. The cell
cycle G1/S checkpoint is abolished in Atm/
p53/ mouse embryonic fibroblasts (MEFs) following
-irradiation, suggesting that the partial G1 cell cycle
arrest in Atm/ cells following -irradiation is due
to the residual p53 response in these cells. In addition, the
Atm/ p21/ MEFs are more severely
defective in their cell cycle G1 arrest following
-irradiation than Atm/ and p21/
MEFs. The Atm/ MEFs exhibit multiple cellular
proliferative defects in culture, and an increased constitutive level
of p21 in these cells might account for these cellular proliferation
defects. Consistent with this notion, Atm/
p21/ MEFs proliferate similarly to wild-type MEFs and
exhibit no premature senescence. These cellular proliferative defects
are also rescued in Atm/ p53/ MEFs and
little p21 can be detected in these cells, indicating that the abnormal
p21 protein level in Atm/ cells is also p53 dependent
and leads to the cellular proliferative defects in these cells.
However, the p21 mRNA level in Atm/ MEFs is lower than
that in Atm+/+ MEFs, suggesting that the higher level of
constitutive p21 protein in Atm/ MEFs is likely due to
increased stability of the p21 protein.
INTRODUCTION
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Ataxia-telangiectasia (A-T) is an autosomally recessive human genetic disease characterized by pleiotropic defects in multiple systems. Affected patients suffer from growth retardation, neuronal degeneration in the cerebellum leading to ataxia, dilated blood vessels in the eye and facial area, gonadal defects, immunodeficiency, a high incidence of cancer, and hypersensitivity to ionizing radiation (20). Cells derived from A-T patients are defective in their checkpoint responses to ionizing radiation and are hypersensitive to ionizing radiation (22, 27). Following the induction of strand break damage induced by ionizing radiation, normal cells arrest their cell cycle at three cell cycle checkpoints: at the G1/S border, at S phase, and at the G2/M border (12). However, all three cell cycle checkpoints in A-T cells are defective in response to ionizing radiation. The cell cycle checkpoint defects of A-T cells have been suggested to account for the cellular hypersensitivity of these cells to ionizing radiation (22, 27).
A gene consistently mutated in A-T patients, denoted ATM, has been identified through linkage mapping and positional cloning (9, 26). The ATM gene encodes a large kinase which is similar to a family of kinases involved in DNA metabolism and cell cycle checkpoint control in response to DNA damage (26, 34). While the ATM kinase family members contain a kinase domain similar to that of phosphatidylinositol 3-kinase (PI-3 kinase), none of them have been shown to have any lipid kinase activity (13). Instead, a number of ATM family members, including FRAP, DNA-PK, and ATM, display protein kinase activity (3, 4, 11, 16). In addition, immunofluorescence studies using an anti-ATM antibody have shown that ATM is ubiquitously expressed in all murine tissues and is mainly localized in the nucleus, consistent with the notion that ATM may be involved in the detection of DNA strand break damage and in the activation of cell cycle checkpoints following DNA strand break damage (6).
To clarify the function of ATM and create a mouse model to study the
basis of the pleiotropic defects in A-T patients, we disrupted the Atm
gene in mice through homologous recombinations (33). Mice
homozygous for this mutation express most of the A-T phenotypes,
including neural degeneration, growth retardation, abolished germ cell
development, immune defects, and a high incidence of thymic lymphomas
(19, 31). Furthermore, primary cells derived from the
Atm/ mice displayed cellular defects characteristic of
A-T, including hypersensitivity to -irradiation and defective cell
cycle G1/S and S-phase checkpoint control following
-irradiation (3, 33). In addition, Atm/
mouse embryonic fibroblasts (MEFs) exhibit defective cellular proliferation, inefficient G1- to S-phase cell cycle
progression and premature senescence in culture (33).
Similar systematic and cellular defects have been reported in two
independently generated Atm/ mouse strains (1,
8). Therefore, Atm plays important roles in both cellular
responses to strand break damage and normal cellular growth.
p53 is required for the cell cycle G1 arrest following
-irradiation (18). The impaired p53 response to
-irradiation in Atm/ cells could account for the
defective cell cycle G1 arrest in these cells (15, 17,
33). In addition, the increased constitutive level of
p21CIP1/WAF1 observed in Atm/
MEFs might account for the cellular proliferative defects observed in
these mutant cells because p21 is involved in the inhibition of
G1- to S-phase cell cycle progression (5, 7,
30). To test the roles of the defective p53 response and the
abnormal p21 protein level in the cellular and developmental defects
observed in Atm/ mice, Atm/
p21/ and Atm/ p53/ mice
were generated.
The majority of Atm/ p53/ mice die
embryonically. The born double mutant mice are runted and develop
lymphomas sometimes of both B and T origin by 2 months of age,
indicating a synergy of the Atm and p53 mutations in tumorigenesis
since Atm/ mice develop thymic lymphomas by 4 months of
age. Similar to p53/ cells, Atm/
p53/ cells are completely defective in their cell cycle
G1 arrest following -irradiation, indicating that the
impaired but not abolished p53 response in Atm/ cells
contributes to the partial cell cycle G1 arrest following -irradiation in these cells (33). However, this synergy
in tumorigenesis is not observed in Atm/
p21/ mice. The development of Atm/
p21/ mice is grossly similar to that of
Atm/ mice. However, when compared with wild-type and
p21/ MEFs, the Atm/
p21/ MEFs proliferate normally at both low and high
passages, consistent with the notion that the increased constitutive
level of p21 in Atm/ MEFs causes the cellular
proliferation defects observed in these cells. In addition, because
little p21 is observed in Atm/ p53/
MEFs, the increased p21 protein level in Atm/ MEFs is
p53 dependent and likely due to a more stable p21 protein, because the
p21 mRNA level in Atm/ MEFs is lower instead of higher
than that in Atm+/+ MEFs.
MATERIALS AND METHODS
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Generation of Atm/ p21/ and
Atm/ p53/ mice.
The
p21/ and p53/ mice were described
previously (5, 14). Because Atm/ mice are
sterile (31), the p21/ mice were bred with
Atm+/ mice to generate Atm+/
p21+/ mice, which were intercrossed to generate
Atm+/ p21/ mice. The Atm+/
p21/ mice were then intercrossed to generate
Atm/ p21/ mice. A similar breeding
scheme was attempted to generate Atm/
p53/ mice but attempts to intercross
Atm+/ p53/ mice have failed. So
Atm+/ p53+/ mice were intercrossed instead
to generate Atm/ p53/ mice.
Flow cytometric analysis of thymocytes. Thymi were surgically removed from mice and single-cell suspensions were prepared as previously described (31). For two-color flow cytometric analysis, one-half million cells were silmutaneously stained with phycoerythrin-conjugated anti-CD4 antibody and fluorescein isothiocyanate (FITC)-conjugated anti-CD8 antibody. After being washed with staining buffer (3% fetal bovine serum [FBS] in phosphate-buffered saline [PBS]), stained cells were analyzed using a FACScan (Becton-Dickinson) and CellQuest software. All antibodies were obtained from Pharmingen.
Generation and culture of MEFs.
MEFs were derived from day
14 or day 16 embryos as previously described (33). The
Atm/ p21/ or Atm/
p53/ MEFs were derived from embryos obtained through
the intercrosses of Atm+/ p21/ mice or
Atm+/ p53+/ mice, respectively. MEFs were
cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented
with 10% fetal calf serum, 5 mM glutamine, 50 µM 
-mecaptoethanol,
50 U of penicillin per ml, and 50 U of streptomycin per ml at 37°C
with 5% CO2.
-Irradiation treatment of MEFs and cell cycle analysis.
MEFs were synchronized at the G0 phase of the cell cycle by
culturing in medium supplemented with 0.1% FBS for 96 h as
previously described (7). The G0-synchronized
cells were trypsinized and irradiated in suspension with a
137Cs -ray source. Subsequently, the irradiated and
untreated MEFs were plated in 10-cm-diameter plates at a density of
0.8 × 106 to 1 × 106 cells/plate in
normal growth medium supplemented with 10 µM bromodeoxyuridine (BrdU). After 24 h of BrdU labeling, cells were harvested, fixed in 70% ethanol, and stored at 
20°C until analysis.
Northern blot analysis. Total RNA was prepared from harvested MEFs with Trizol reagent (Sigma) according to the manufacturer's protocol. Total RNA (15 µg) was electrophoresed on a 17.5% formaldehyde-1% agarose gel and was transferred to a nylon membrane (Amersham) as previously described (32). The full-length mouse p21 cDNA was used as a probe to hybridize to the membrane as previously described (21). To standardize the amount of RNA loaded into each lane, the same filter was stripped and hybridized with a glyceraldehyde-3-phosphate dehydrogenase (GAPDH) probe as previously described (32). The hybridized filter was sequentially exposed to both X-ray films and PhosphorImager screens. The amount of p21 and GAPDH mRNA in each sample was quantitated with an ImageQuant program (Molecular Dynamics).
Assays for in vitro cellular proliferation and G1/S cell cycle progression. Cellular proliferation assays were performed as previously described (7, 33). Briefly, 105 MEFs were plated onto each 35-mm-diameter plate, and each day after plating, MEFs from three plates of each genotype were trypsinized and counted with a hematocytometer. To synchronize MEFs at G0, a subconfluent culture was washed with PBS and placed in DMEM containing 0.1% FBS for 96 h (7). The synchronized MEFs were harvested and released into DMEM supplemented with 10% FBS and 10 µM BrdU for 24 h. Cells in S phase were analyzed using flow cytometry as described above.
Western blot analysis. Western blot analysis of p21 protein levels in MEFs was performed as previously described (33). Protein extracts derived from 3 × 105 cells were loaded into each lane, separated by sodium dodecyl sulfate-12% polyacrylamide gel electrophoresis, and transferred to nitrocellulose membranes (Schleicher and Schuell). The filter was probed with a polyclonal rabbit anti-p21 antibody obtained from Santa Cruz Biochemicals, and then was incubated with horseradish peroxidase-conjugated secondary antibody and developed with enhanced chemiluminescence (Amersham). To verify that an equivalent amount of protein was loaded into each lane, the same filter was subsequently probed with a polyclonal rabbit anti-tubulin antibody (ICN Biomedicals, Inc.) and developed as described above.
RESULTS
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Developmental phenotypes in Atm/
p53/ and Atm/ p21/
mice.
Because p53/ Atm+/ mice are
not healthy and usually develop testicular sarcomas, attempts to use
these mice to generate Atm/ p53/ mice
have failed so far. (Testicular sarcomas are also a common malignancy
found in the strain of p53/ mice that we used
[14].) Therefore, Atm+/
p53+/ mice were intercrossed to generate
Atm/ p53/ mice. While the predicted
frequency of Atm/ p53/ mice in the
offspring of this intercross should be 1/16, only about 2% of over 350 offspring genotyped were Atm/ p53/
mice, suggesting that about 60% of these double mutant mice die prenatally. In addition, the born Atm/
p53/ mice are apparently runted at the weaning age and
thus the prevalence of growth retardation phenotypes is hard to judge.
/ p21/ mice,
Atm+/ p21/ mice were generated and
intercrossed. Consistent with the predicted frequency of 1/4, about 25% of the offspring from the Atm+/ p21/
crosses were double mutant animals, indicating that there is no
prenatal death during the early development of the double mutant mice
(data not shown).
Atm/ mice exhibit several developmental defects,
including growth retardation and defective T-cell development
(31). While the Atm/ p53/
mice are runted and appear unhealthy by the weaning age, the Atm/ p21/ mice are healthy at this
stage of development and were studied for this spectrum of
developmental defects. Similar to Atm/ mice
(31), the 1- to 2-month-old Atm/
p21/ mice are also growth retarded with a body weight
about 79% ± 7% of that of the p21/ control mice
(data derived from six sets of Atm/
p21/ and Atm+/+ p21/ mice).
T-cell developments in Atm/ p21/ mice
were analyzed using flow cytometry and cell counting. While the total
number of thymocytes in Atm/ mice is on average 40% ± 10% of that of Atm+/+ control mice (33), the
total number of thymocytes in the Atm/
p53/ and Atm/ p21/ mice
in this study averaged 50% ± 5% and 52% ± 12%, respectively, of
those of p53/ and p21/ control mice
(data derived from four sets of Atm/
p53/ and p53/ control mice and seven
sets of 1- to 2-month-old Atm/ p21/ and
Atm+/+ p21/ control mice). In addition,
when determined by flow cytometry for the expression of CD4 and CD8
surface markers, CD4+ and CD8+ single-positive
mature thymocytes were still significantly reduced in the thymi of
Atm/ p21/ and Atm/
p53/ mice compared to those of Atm+/+
p21/ and Atm+/+ p53/
control mice, respectively (Fig. 1).
Therefore, the thymocyte differentiation from the CD4+
CD8+ stage to the CD4+ or CD8+
stage is consistently defective in Atm/,
Atm/ p21/, and Atm/
p53/ mice.
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Lymphomas in Atm/ p53/ and
Atm/ p21/ mice.
While
Atm/ mice invariably develop thymic lymphomas by 4 months of age, seven Atm/ p53/ mice
developed thymic and/or peripheral lymphomas by 2 months of age. When
analyzed with flow cytometry, three of the seven lymphomas derived from
Atm/ p53/ mice appear to have been
composed of two populations of tumor cells of different sizes (Fig.
2A). When the analysis was gated on these
two populations of tumor cells, the smaller tumor cells appeared to be
mainly composed of B220+ cells, which are also
Thy-1 CD4, indicating that they are
phenotypically of B-cell origin (Fig. 2B and data not shown). The
larger tumor cells expressed some T-cell markers, including CD4 and
Thy-1, but were B220, suggesting that they were of T-cell
origin (Fig. 2B and data not shown). Therefore, the
Atm/ p53/ mice can develop lymphomas of
both B- and T-cell origin.
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/ mice, Atm/
p21/ mice invariably develop thymic lymphomas by 5 months of age, and no other malignancies have been detected in these
animals at an elevated frequency. In addition, similar to the thymic
lymphoma cells derived from Atm/ mice, the thymic
lymphoma cells observed in Atm/ p21/
mice are mostly of CD4+ CD8+ immature T-cell
origin (data not shown) (31).
Cell cycle G1 arrest following -irradiation in
Atm/ p53/ and Atm/
p21/ MEFs.
In response to -irradiation,
Atm/ MEFs are partially defective in their cell cycle
G1 arrest, presumably due to a greatly reduced and delayed
p53 upregulation in response to -irradiation (33). In
addition, the constitutively higher basal protein level of p21 in
Atm/ MEFs might also affect the cell cycle
G1 arrest of these cells in response to -irradiation
(33). To test these possibilities, the cell cycle
G1 arrest following -irradiation in Atm/
p53/ and Atm/ p21/ MEFs
was evaluated as previously described (7). Briefly, MEFs synchronized at G0 by serum starvation were treated with 0 or 10 Gy of irradiation and released into medium supplemented with 10% FBS and 10 µM BrdU. After 24 h, the percentages of S-phase cells in the irradiated and untreated MEFs were assayed with flow cytometry as previously described (31) (Fig.
3A). Following -irradiation, there is
essentially no cell cycle G1 arrest in p53/
and p53/ Atm/ MEFs while there is more
than 50% reduction of S-phase cells in wild-type MEFs (Fig. 3B). In
addition, the Atm/ p21/ MEFs are more
severely defective in their cell cycle G1 arrest following
-irradiation than p21/ and Atm/ MEFs
(Fig. 3B).
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Growth properties of Atm/ p21/ and
Atm/ p53/ MEFs.
The
Atm/ MEFs exhibit defects in cellular proliferation,
including slower proliferation rate, lower saturation density,
inefficient G1- to S-phase cell cycle progression, and
premature senescence, possibly due to the increased basal level of p21
in these mutant cells (33). Therefore, the cellular
proliferation of Atm/ p21/ and
Atm/ p53/ MEFs were examined as
previously described (33). At earlier passages,
Atm/ p21/, Atm/
p53/, p21/, and wild-type MEFs
proliferate similarly and reach similar saturation densities, while as
expected, Atm/ MEFs proliferate more slowly (Fig.
4A). In addition, Atm/
p21/ and Atm/ p53/ MEFs
are capable of proliferating at high passages (passage 6), while
Atm/ MEFs are senescent (data not shown) (26,
33).
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/, Atm/
p21/, Atm+/+ p53/, and
Atm/ p53/ MEFs synchronized at
G0 through serum starvation were serum stimulated for
24 h, and the percentages of cells in S phase were determined with
flow cytometry. Similar percentages of S-phase cells were detected in
all the MEF samples tested (Fig. 4B). Therefore, all the cellular
proliferation defects tested in Atm/ MEFs are rescued
in Atm/ p53/ and Atm/
p21/ MEFs.
p21 protein and mRNA levels in various MEFs.
To test whether
the increased basal protein level of p21 in Atm/ mice
is dependent on p53, the p21 protein level in Atm/
p53/ MEFs was analyzed by Western blotting. As
described previously, a much higher level of p21 was detected in
Atm/ MEFs than Atm+/+ MEFs (Fig.
5A) (31). However, little p21
protein was detected in both p53/ and
Atm/ p53/ MEFs (Fig. 5A). As expected,
no p21 could be detected in Atm/ p21/
and Atm+/+ p21/ MEFs (Fig. 5A).
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/ MEFs is due to a higher level of p21 mRNA in these
cells, total RNA was prepared from passage 3 Atm/ and
Atm+/+ control MEFs and analyzed by Northern blotting using
the full-length mouse p21 cDNA as a probe (21). While the
p21 mRNA could be easily identified in both Atm+/+ and
Atm/ MEF samples, a lower p21 mRNA level was detected
in the Atm/ MEF samples compared to that of
Atm+/+ MEF controls derived from the same pregnant female,
indicating that the increased level of constitutive p21 protein in
Atm/ MEFs is not due to the higher p21 mRNA level in
these cells (Fig. 5B). Consistent data were obtained from an additional
two sets of Atm+/+ and Atm/ MEFs (data not
shown).
DISCUSSION
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Atm/ cells show an impaired p53 upregulation in
response to -irradiation and an increased level of p21, either or
both of which could be responsible for their cellular defects and
increased tumorigenesis. To study this issue, we generated
Atm/ p53/ and Atm/
p21/ mice and derived MEFs from them. The cellular
proliferative defects observed in Atm/ MEFs are absent
in Atm/ p21/ and Atm/
p53/ MEFs. In addition, the cell cycle G1
checkpoint response to -irradiation is more severely defective in
Atm/ p53/ and Atm/
p21/ MEFs than in Atm/ MEFs. While the
developmental defects and tumorigenesis in the Atm/
p21/ mice are similar to those in Atm/
mice, the majority of Atm/ p53/ mice
die prenatally and the surviving Atm/
p53/ mice develop tumors earlier than the
Atm/ and p53/ mice, suggesting a
cooperation of Atm and p53 in mouse embryonic development as well as in
tumor suppression.
An increased p21 protein level can inhibit cell cycle G1/S
transition and is also correlated with cellular senescence (10, 23-25, 30). Therefore, an increased constitutive p21 protein level in Atm/ MEFs could account for the cellular
proliferative defects, including slower proliferation, inefficient cell
cycle G1/S progression, and premature senescence
(33). Consistent with this notion, none of the cellular
proliferative defects observed in Atm/ MEFs were
evident in Atm/ p21/ MEFs, indicating
that the increased constitutive p21 level is indeed responsible for the
cellular proliferative defects observed in Atm/ cells.
In addition, the proliferative defects observed in Atm/
MEFs are also rescued in Atm/ p53/
MEFs, probably due to the fact that a minimum level of p21 is expressed
in any p53/ MEFs. Thus, the p21 expression in cycling
Atm/ MEFs is apparently p53 dependent, just as it is in
wild-type cells. Since in normal-cycling MEFs, p53 is thought to
transcriptionally activate p21 mRNA expression through binding to the
two p53 binding sites in the p21 promoter region, the regulation of p21
expression by p53 in Atm/ MEFs is most likely at the
transcriptional level (21).
The increased p21 level in the Atm/ MEFs might be due
to either a higher basal activity of the p53-p21 pathway leading to an increased level of p21 mRNA or a more stable p21 protein in the Atm/ MEFs. However, a lower level of p21 mRNA was
detected in the Atm/ MEFs than in the
Atm+/+ control MEFs, indicating that the higher level of
constitutive p21 protein in the Atm/ MEFs is not due to
a higher p21 mRNA level in these cells but is likely due to the
increased stability of p21 protein in these cells. This finding is also
consistent with the notion that ATM might be involved in the
maintenance of the p53 protein level not only after excessive DNA
damage but also during normal cellular proliferation.
While the cellular proliferative defects in Atm/ cells
are rescued in Atm/ p21/ cells, the
growth retardation observed in Atm/ mice is not rescued
in the Atm/ p21/ mice because these
double mutant mice are still smaller than their p21/
control littermates. Therefore, the growth retardation in
Atm/ mice cannot be due solely to the cellular
proliferative defects in Atm/ cells. Instead, other
potential defects, such as the abnormal production of growth factors
due to the neural defects in Atm/ mice, might account
for the growth retardation in Atm/ mice
(19). In addition, there is a significant reduction of the
total number of thymocytes in Atm/ mice, possibly due
to defective thymocyte proliferation or impaired V(D)J recombination or
both (31). However, in contrast to the findings obtained for
the Atm/ p21/ and Atm/
p53/ MEFs, there is no apparent rescue of thymus
cellularity in the Atm/ p21/ and
Atm/ p53/ mice, indicating that p53 and
p21 are not involved in the thymus hypoplasia in Atm/
mice. However, this does not rule out the possibility that defective thymocyte proliferation contributes to the thymus hypoplasia in Atm/ mice.
The cell cycle G1 arrest following -irradiation is
abolished in p53/ MEFs but is partially defective in
Atm/ MEFs (15, 17, 18, 33). Consistent with
the notion that the cell cycle G1 arrest in response to
-irradiation is p53 dependent (18), the p53 upregulation
in response to -irradiation in Atm/ cells is greatly
impaired and delayed (15, 17, 33). Therefore, our findings
that Atm/ p53/ MEFs are completely
deficient in their cell cycle G1 arrest in response to
-irradiation indicate that the residual p53 response to
-irradiation in Atm/ cells contributes to the
partial G1 cell cycle arrest in Atm/ MEFs.
Thus, in response to DNA strand break damage, ATM plays a major role in
signaling the p53 upregulation but there exist ATM-independent
signaling pathways that can partially compensate for this ATM activity.
While ATM plays a major role in signaling the p53 response to
-irradiation, the Atm/ p53/ mice
develop lymphomas earlier than Atm/ mice, indicating
that mutations in these two genes can cooperate in tumorigenesis. Two
observations could account for this synergy in tumorigenesis. First,
the cell cycle checkpoint defects occurring in response to DNA damage
induced by -irradiation are more severe in Atm/
p53/ cells than in Atm/ cells.
Secondly, the p53-dependent apoptosis of thymocytes in response to
-irradiation is only partially defective in Atm/
thymocytes but is completely abolished in Atm/
p53/ cells (29, 33). In addition, ATM
signals the upregulation of p53 only in response to DNA strand break
damage but is not involved in such signaling in the cellular responses
to DNA damage induced by UV or base mismatch (33, 33a).
Therefore, while only the cellular response to DNA strand break damage
is defective in Atm/ cells, cellular responses to
multiple forms of DNA damage and other cellular stresses are defective
in Atm/ p53/ cells, thus providing the
potential basis for the synergy of tumorigenesis in
Atm/ p53/ mice. The
Atm/ p53/ mice also develop lymphomas
earlier than p53/ mice (14). Therefore,
other defective but p53-independent cellular responses to strand break
damage in Atm/ cells, such as the defective S-phase
cell cycle checkpoint which occurs in response to strand break damage,
can account for the earlier onset of tumors in Atm/
p53/ mice (3).
An independently generated Atm/ p53/
mouse line was reported while this paper was in preparation
(29). The findings for our Atm/
p53/ mice are in general agreement with those for the
reported mouse line. While this paper was being reviewed, two reports
described the meiosis, tumorigenesis, and apoptosis responses in two
independently generated Atm/ p21/ mouse
lines (2, 28).
ACKNOWLEDGMENTS
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This project was partially supported by a National Institute of Health grant to D.B. and grants from AT Childrens Projects to D.B. and Y.X. Y.X. was partly supported by the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation. D.B. is an American Cancer Society Research Professor.
FOOTNOTES
* Corresponding author. Mailing address: Department of Biology, University of California, San Diego, Bonner Hall 3430, 9500 Gilman Dr., La Jolla, CA 92093-0322. Phone: (619) 822-1084. Fax: (619) 534-0053. E-mail: yangxu{at}ucsd.edu.
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Mol Cell Biol, July 1998, p. 4385-4390, Vol. 18, No. 7
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