Loss of Brca2 and p53 synergistically promotes genomic instability and deregulation of T-cell apoptosis

Abstract

BRCA2 is a breast cancer susceptibility gene of which the product is thought to be involved in monitoring genome integrity and cell cycle progression. Brca2-null mice have a defect in embryonic cellular proliferation and die in utero. Here we report the generation of T-cell lineage-specific Brca2-deficient (tBrca2-/-) mice using the Cre-loxP system. Mice with a flanked by loxP allele of Brca2 were crossed to transgenic mice bearing Cre recombinase driven by the T cell-specific promoter Lck. Thymic cellularity and distribution of subset populations were normal in tBrca2-/- mutants. Thymocytes from tBrca2-/- mice underwent normal apoptosis in response to a variety of stimuli, and activated tBrca2-/- T cells had normal proliferative capacity. tBrca2-/- T cells were more likely than wild-type cells to undergo spontaneous apoptosis, but apoptosed normally in response to restimulation or DNA-damaging stress signals. Examination of metaphase spreads of tBrca2-/- T cells revealed that the chromosomes often exhibited aberrations such as breaks and tri-radial structures. The level of chromosomal abnormalities was enhanced in T cells from tBrca2-/-; p53-/- double-mutant mice. However, tBrca2-/-; p53-/- T cells did not show the enhanced level of spontaneous apoptosis demonstrated by tBrca2-/- T cells, a difference that likely accounts for an increase in cell number and 3[H]thymidine incorporation of double-mutant T cells in culture compared with either single mutant. Despite this increased T-cell number, the onset of T-cell lymphomas was only marginally accelerated in tBrca2-/-; p53-/- mice compared with p53-/- mice. Our results support a role for Brca2 in repairing spontaneous DNA lesions, and suggest that loss of Brca2 enhances the susceptibility of mouse T-lineage cells to chromosomal aberrations and deregulation of apoptosis in the absence of p53.