Session 4

Mouse models for studying breast cancer initiation and progression



Consequences of estrogen exposure among strains of mice: a model for gene and environment interactions

D. Joseph Jerry

Estrogens together with progestins play critical roles in development of breast tissue, but also contribute to the risk of breast cancer. Twin studies reveal that 30% of breast cancer risk can be attributed to inherited genetic variants with the remaining risk associated with the environment and include differences in reproductive factors and environmental exposures. But the mechanisms by which exposure to endogenous hormones and environmental chemicals may interact with inherited risk remains unclear. Previous studies demonstrated that mammary tumors develop spontaneously in BALB/c mice with heterozygous mutations in the p53 tumor suppressor gene (BALB/c-Trp53+/-) providing a model of Li-Fraumeni Syndrome. However, C57BL/6-Trp53+/- mice rarely develop mammary tumors. We mapped the genetic modifiers and created mice that are congenic for C57BL/6 alleles within a 20Mb interval (SM1-Trp53+/-) on mouse chromosome 7. In contrast to BALB/c-Trp53+/- mice, the C57BL/6 alleles in SM1-Trp53+/- mice were sufficient to reverse repair of DNA double strand breaks by low-fidelity pathways and enhance processivity of replication forks. These strains also differ in their susceptibility to estrogen-induced DNA damage. The results show that the combined effects of estrogen-induced DNA damage and genetic modifiers regulating the fidelity of DNA repair interact to alter the incidence of mammary tumors in mice.

Mouse-INtraDuctal (MIND): An in vivo model that recapitulates the full spectrum of human DCIS pathology

Fariba Behbod

Due to an increase in screening mammography, there has been a significant increase in the rate of ductal carcinoma in situ (DCIS) diagnosis. At the present time, there are no means by which to diagnose DCIS accurately, or predict which patients require aggressive therapy. To address this gap, we present the first in vivo model, referred to as Mouse-INtraDuctal (MIND), by which patient-derived DCIS epithelial cells are injected intraductally and allowed to progress naturally in mice. Similar to human DCIS, the cancer cells form in situ lesions inside the mouse mammary ducts and mimic all histologic subtypes including micropapillary, papillary, cribriform, solid, and comedo. After a median of 9 months, 15/35 (42%) patient samples injected into 95 xenografts remained non-invasive; 20/35 (57%) patient samples injected into 95 xenografts advanced to invasive lesions. While there was some level of discordance between patients and xenografts with regards to the expression of clinically relevant biomarkers, only progesterone showed a significant correlation to invasive progression. Targeted sequencing of LCM-captured DCIS DNA from patient/xenograft pairs found a number of common, unique and private pathogenic mutations. In summary, MIND models are valuable resources for the discovery of patient-specific molecular signatures of DCIS invasiveness through the comprehensive analysis of patient-derived xenografts with variable propensity for malignancy.

Oncogene-mediated signal transduction in transgenic mouse models of human breast cancer

William Muller

Mechanistic Target of Rapamycin Complex 1 (mTORC1) is a master modulator of cellular growth, and its aberrant regulation is recurrently documented within breast cancer. While the small GTPase Rheb1 is the canonical activator of mTORC1, Rheb1-independent mechanisms of mTORC1 activation have also been reported but have not been fully understood. Employing multiple transgenic mouse models of breast cancer, we report that ablation of Rheb1 significantly impedes mammary tumorigenesis. In the absence of Rheb1, a block in tumor initiation can be overcome by multiple independent mutations in Mtor to allow Rheb1-independent re-activation of mTORC1. We further demonstrate that the mTOR kinase is indispensable for tumor initiation as the genetic ablation of mTOR abolishes mammary tumorigenesis. Collectively, our findings demonstrate that mTORC1 activation is indispensable for mammary tumor initiation, and that tumors acquire non-canonical mechanisms of mTORC1 activation.

PTHrP overexpression in mammary tumors increases tumorigenesis and causes anorexia

(Selected from posters)

Diego Grinman

Cancer Anorexia Cachexia Syndrome (CACS) is a life-threatening complication of disordered energy metabolism that afflicts 26% of all patients with breast cancer. CACS is caused by the tumor release of soluble factors leading to pronounced loss of weight, appetite, skeletal muscle and fat mass. Importantly, breast cancer patients with weight loss are more resistant to and less tolerant of chemotherapy and surgery. One of the secreted factors implicated in CACS is PTHrP. We have developed an inducible transgenic mouse model that overexpresses PTHrP in PyMT-derived mammary tumors (Tet-PTHrP;PyMT). Administering Dox to Tet-PTHrP;PyMT mice activated human PTHLH cDNA expression in mammary tumors, leading to elevated circulating PTHrP levels and accelerated tumor growth. Tet-PTHrP;PyMT mice on Dox also experienced hypercalcemia along with a profound anorexia, rapid fat wasting and weight loss. Pair feeding experiments demonstrated that anorexia is the main driver of weight loss in our model. Moreover, we found that PTHrP overexpression caused a significant activation of the lateral parabrachial nucleus (LPBN), an area of the brain involved in appetite regulation. Finally, blocking the PTHrP induced bone resorption with Osteoprotegerin corrected the hypercalcemia and prevented the anorexia and the LPBN activation. Overall, our data suggest that mammary tumor expression of PTHrP increases tumor burden and, in parallel, causes anorexia possibly through a calcium dependent mechanism.