To test the effect of haploinsufficiency of the
Atm gene on mammary carcinogenesis after X-irradiation in
p53
+/- mice, 55
p53
+/- Atm
+/-, 61
p53
+/- Atm
+/+, 47
p53
+/+ Atm
+/- and 53
p53
+/+ Atm
+/+ mice (for a total of 216 mice) were exposed to X-rays (5 Gy) at age 5 weeks. Only one out of 53
p53
+/+ Atm
+/+ mice and none of 47
p53
+/+ Atm
+/- mice developed mammary carcinoma, indicating that almost all
p53
+/+ mice fail to develop mammary carcinomas, despite X-irradiation and irrespective of
Atm gene status. In contrast, 32 out of 55
p53
+/- Atm
+/- mice (58%) and 19 out of 61
p53
+/- Atm
+/+ mice (31%) developed mammary carcinomas (Table
2, Fig.
2). The proportion of mice developing mammary carcinomas in the
p53
+/- Atm
+/- group was significantly greater than that in the
p53
+/- Atm
+/+ group (
P = 0.0034, by χ
2 test). A total of 52 mammary carcinomas developed in 55
p53
+/- Atm
+/- mice (average number of mammary carcinomas/mouse = 0.95), whereas 28 mammary carcinoma developed in 61
p53
+/- Atm
+/+ mice (average number of mammary carcinomas/mouse = 0.46; Table
2). The average number of mammary carcinomas per mouse in the
p53
+/- Atm
+/- group was significantly greater than that in
p53
+/- Atm
+/+ mice (
P = 0.0052, by Mann–Whitney U-test). Thus,
Atm heterozygous deficiency enhanced development of mammary carcinoma in irradiated
p53 heterozygous knockout mice. Spring and coworkers [
14] observed no tumours in
Atm knockout (
Atm
+/-) heterozygous mice. Mice bearing a knockout allele of
Atm and humans carrying a mutant allele of truncated type in the
ATM gene have been shown not to have obviously elevated susceptibility to mammary carcinogenesis. Our findings show that heterozygosity for a null knockout allele of
Atm enhances mammary carcinogenesis under
p53
+/- status, although the
Atm mutation is not a dominant-negative type. Heterozygous deficiency of
p53 might make clear the effect on mammary carcinogenesis of haploinsufficiency in the
Atm gene.
Table 2
Incidence of tumours in irradiated (BALB/cHeA × MSM/Ms) F1 female mice that were heterozygously deficient for p53 and/or Atm genes
Mammary carcinoma | 32 (58%)a* | 19 (31%) | 0 (0%) | 1 (2%) |
Lymphoma | 24 (44%)b
| 29 (48%) | 26 (55%) | 22 (42%) |
Squamous cell carcinoma | 1 (2%) | 1 (2%) | 0 (0%) | 0 (0%) |
Histiocytic sarcoma | 1 (2%)c
| 0 (0%) | 0 (0%) | 0 (0%) |
Basal cell carcinoma | 0 (0%) | 1 (2%) | 0 (0%) | 0 (0%) |
Granulocytic leukaemia | 0 (0%) | 1 (2%) | 0 (0%) | 0 (0%) |
Ovarian carcinoma | 0 (0%) | 1 (2%) | 0 (0%) | 0 (0%) |
Nonthymic lymphoma (NOS) | 0 (0%) | 1 (2%) | 3 (6%) | 0 (0%) |
Solid tumour (NOS) | 1 (2%)d
| 1 (2%)e
| 1 (2%) | 1 (2%) |
Number of mammary carcinomas/mousee
| 52/55 (0.95)** | 28/61 (0.46) | 0/47 (0.00) | 1/53 (0.02) |
These mammary carcinomas were observed significantly earlier (at 18–38 weeks after irradiation; i.e. 23–43 weeks of age) than in the nonirradiated group (age 41–75 weeks; Fig.
2). In particular, mammary carcinomas frequently developed 23–28 weeks after irradiation. The mean (± standard deviation) latency periods were 32.6 ± 4.8 and 29.8 ± 3.6 weeks in
p53
+/- Atm
+/- and
p53
+/- Atm
+/+ mice, respectively. Thus, X-irradiation at 5 Gy at age 5 weeks considerably shortened the latency period of mammary carcinoma development in these two groups with different genotypes. As shown in Tables
1 and
2, the incidences of mammary carcinoma in
p53
+/- Atm
+/- mice were 58% (32 out of 55) and 50% (14 out of 28) in irradiated and nonirradiated groups, respectively; in
p53
+/- Atm
+/+ mice the incidence in the irradiated group was 31% (19 out of 61) and that in the nonirradiated group was 32% (7 out of 22). The incidence of mammary carcinoma for each genotype was similar between irradiated and non-irradiated groups. Thus, irradiation may not elevate the incidence of the tumours.
Altogether, irradiation markedly hastened mammary carcinoma development in the
p53
+/- mice, in which mammary carcinomas developed spontaneously. Furthermore, irradiation also induced lymphomas, mainly thymic lymphomas, in all four genotypes of mice. The incidence of the lymphomas did not differ significantly among the four groups, with different genotypes for
p53 and
Atm genes. A high incidence of thymic lymphoma was observed in previous studies performed using
p53 heterozygous deficient F
1 mice [
17,
28]. In the present study an extremely high incidence of tumours, most of which were mammary carcinomas and thymic lymphomas, was observed in irradiated
p53
+/- Atm
+/- mice.