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DNA double-strand breaks, p53, and apoptosis during lymphomagenesis in scid/scid mice
1Fred Hutchinson Cancer Research Center, Seattle, Washington 98109-1024, USA.
Abstract:
The tumor-suppressing phenotype of p53 is thought to be due to its accumulation in response to DNA damage and resultant cell cycle arrest or apoptosis. scid/scid mice are defective in DNA double-strand break repair due to a mutation in DNA-dependent protein kinase (DNAPK). Treatment of scid/scid mice with gamma radiation or N-ethyl-N-nitrosourea resulted in approximately 86% incidence of T-cell lymphomas, compared with <6% in wild-type mice. The incidence of other tumor types was not increased in scid/scid mice, suggesting that the types of DNA double-strand break that are unrepaired in these mice are not strongly carcinogenic. To determine whether mutations in DNAPK and p53 interact, we examined mice deficient in both genes. Both scid/scid p53-/- and scid/scid p53+/- mice spontaneously developed lymphomas at shorter latency than did mice with either defect alone. Loss of the wild-type p53 allele was observed in 100% of tumors from scid/scid p53 +/- mice, indicating strong selection against p53. In contrast, p53 was not inactivated in lymphomas from scid/scid p53+/+ mice. Exposure of these tumor-bearing mice to gamma radiation resulted in p53 protein accumulation and high levels of apoptosis in all tumors that were not observed in tumors from scid/scid p53+/- mice. Thus, there was a bifurcation of molecular pathways to tumorigenesis. When p53 was heterozygous in the germ line, loss of the wild-type allele occurred, and the tumors became apoptosis resistant. When p53 was wild type in the germ line, p53 was not inactivated, and the tumors remained highly apoptosis sensitive.
Insights
Mice with DNA repair defects (DNAPK mutation) and p53 gene mutations show accelerated lymphoma development. Tumors with p53 loss become resistant to apoptosis, while those retaining p53 remain sensitive, revealing distinct cancer pathways.
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- The tumor suppressor p53 protein accumulates upon DNA damage, inducing cell cycle arrest or apoptosis.
- Severe Combined Immunodeficiency (SCID) mice harbor a mutation in DNA-dependent protein kinase (DNAPK), impairing DNA double-strand break repair.
- SCID mice treated with genotoxic agents exhibit a high incidence of T-cell lymphomas, suggesting specific unrepaired DNA breaks contribute to lymphomagenesis.
Purpose of the Study:
- To investigate the interaction between DNA-dependent protein kinase (DNAPK) mutations and p53 gene status in tumorigenesis.
- To determine how combined defects in DNA repair and p53 influence lymphoma development and apoptosis sensitivity.
Main Methods:
- Generation and analysis of mice with combined scid/scid (DNAPK deficient) and p53-deficient (p53-/- or p53+/-) genotypes.
- Monitoring spontaneous lymphoma development and latency in these genetically modified mouse models.
- Assessing p53 gene status and apoptosis response in tumors from different mouse cohorts, including radiation exposure experiments.
Main Results:
- Mice with both scid/scid and p53 deficiencies developed lymphomas more rapidly than those with single defects.
- Tumors in scid/scid p53+/- mice consistently lost the wild-type p53 allele, indicating strong selective pressure.
- Tumors from scid/scid p53+/+ mice retained functional p53, exhibited p53 protein accumulation upon radiation, and remained sensitive to apoptosis.
Conclusions:
- A bifurcation in molecular pathways to tumorigenesis exists based on p53 functional status in mice with DNA repair defects.
- Loss of wild-type p53 in heterozygous mice leads to apoptosis-resistant tumors.
- Functional p53 in germline wild-type mice confers apoptosis sensitivity to radiation-induced lymphomas.