Atm heterozygosity cooperates with loss of Brca1 to increase the severity of mammary gland cancer and reduce ductal branching

The role of homozygous ataxia telangiectasia mutated (ATM) mutations in familial and sporadic forms of cancer is well established, but the contribution of ATM heterozygosity to mammary gland and other cancers has been controversial. To test the effect of Atm heterozygosity on mammary gland cancer, mice with complete loss of exon 11 of Brca1 specifically in mammary epithelium (Brca1-MG-Δex11) were studied in either Atm heterozygous or Atm wild-type backgrounds. Targeted deletion of Brca1 in mammary epithelium resulted in carcinomas and adenocarcinomas of varying histology with long (>9 months) latency. Latency to tumorigenesis was found to be unchanged in the Brca1-MG-Δex11;Atm heterozygous mice compared with Brca1-MG- Δex11;Atm wild-type mice. However, the mice displayed variable tumor severity and differences in mammary tissue development. Mammary tumors from Brca1-MG-Δex11;Atm heterozygous mice were anaplastic and undifferentiated in all 20 tumors tested, whereas tumors from mice that were Brca1-MG-Δex11 but wild-type for Atm displayed variable histologic profiles, with some anaplastic tumors and other differentiated and less invasive tumor types. Previously reported developmental defects for Brca1-deficient mice were also observed in our model with and without Atm heterozygosity, but Brca1-MG-Δex11;Atm heterozygous mice displayed decreased ductal branching during puberty, a phenotype that was not observed in Brca1-MG-Δex11;Atm wild-type mice. Our results provide evidence that Atm heterozygosity influences severity of mammary gland tumors in the Brca1-MG-Δex11 tumor-prone mouse and suggest that this mutation leads to a newly characterized developmental defect during glandular maturation.