Mutant Brca2/p53 mice exhibit altered radiation responses in the developing mammary gland

Appropriate balance between proliferation and apoptosis is critical for mammary gland development and is often altered during tumorigenesis. Carcinogens like radiation induce DNA damage and activate protective responses such as cell cycle arrest and apoptosis. We used mice carrying Brca2−/− and/or p53−/− mutations to evaluate the individual and combined effects of these genes on cell proliferation and apoptosis in the developing mammary gland. Mice were exposed to 5 Gy of radiation or chamber exposure (controls) followed by injection with BrdU. Mammary glands were collected 6 h post-radiation exposure and evaluated for proliferation (BrdU) and apoptosis (TUNEL) in terminal end buds (TEB) and ducts. Under control conditions, the Brca2 mutation reduced proliferation and apoptosis in TEB but not ducts, whereas the p53 mutation reduced apoptosis in TEB and ducts but did not influence proliferation. Despite these alterations in proliferation and/or apoptosis, neither mutation, either individually or combined, significantly altered the overall balance between the two as measured by the proliferation to apoptosis ratio (growth index). Following irradiation, the Brca2 mutation had no significant effect on proliferation or apoptosis, whereas the p53 mutation resulted in reduced apoptosis in TEB and ducts but did not significantly influence proliferation. Neither mutation by itself altered the growth index in the TEB after irradiation although combined Brca2/p53 mutation caused significantly increased proliferation, reduced apoptosis, and an elevated growth index in TEB and ducts. These results reveal both independent and collaborative growth regulatory roles for Brca2 and p53 under normal and adverse environmental conditions. Additionally, we demonstrate the importance of gene-environment interactions by showing that Brca2- and p53-deficient mice can compensate for their genetic deficiencies under control conditions but not after exposure to radiation. We also demonstrate distinct spatial differences in the cellular functions of Brca2 and p53 and show that combined mutation of both genes is more detrimental than loss of either gene alone.