Single allele loss-of-function mutations select and sculpt conditional cooperative networks in breast cancer

The most common events in breast cancer (BC) involve chromosome arm losses and gains. Here we describe identification of 1089 gene-centric common insertion sites (gCIS) from transposon-based screens in 8 mouse models of BC. Some gCIS are driver-specific, others driver non-specific, and still others associated with tumor histology. Processes affected by driver-specific and histology-specific mutations include well-known cancer pathways. Driver non-specific gCIS target the Mediator complex, Ca++ signaling, Cyclin D turnover, RNA-metabolism among other processes. Most gCIS show single allele disruption and many map to genomic regions showing high-frequency hemizygous loss in human BC. Two gCIS, Nf1 and Trps1, show synthetic haploinsufficient tumor suppressor activity. Many gCIS act on the same pathway responsible for tumor initiation, thereby selecting and sculpting just enough and just right signaling. These data highlight ~1000 genes with predicted conditional haploinsufficient tumor suppressor function and the potential to promote chromosome arm loss in BC.

Abstract A29: Generation of a novel transgenic mouse model with regulatable β-catenin expression to examine importance of activated β-catenin signaling to long-term maintenance of mammary tumor growth

Stabilized β-catenin expression is a well described initiator of mammary tumorigenesis in the mouse and elevated nuclear expression has been observed in human triple-negative tumor samples. However, the importance of stabilized β-catenin to continued tumor growth after initiation, and in the context of other driver mutations, has yet to be elucidated.

To ascertain the importance of stabilized β-catenin after tumor initiation, we generated a novel transgenic mouse model, utilizing the tet-off system, to control expression of stabilized β-catenin. Expression was tissue restricted through insertion of a stop cassette flanked by loxP sites upstream of stabilized β-catenin and firefly luciferase, which were coexpressed to allow repeated bioluminescent imaging of experimental mice. Insertion of the expression vector was targeted to the Rosa26 locus in R1 embryonic stem cells through homologous recombination. Following aggregation and chimera generation, two founder lines were established.

Pups from early litters carrying one allele of regulatable stabilized β-catenin at the Rosa26 locus, but not targeted by Cre, were smaller in size compared with wild-type littermates. They also developed perianal skin lesions around 2-3 weeks of age, with severe necrosis evident by 3-4 weeks of age, requiring the animals to be euthanized. Backcrossing to the desired FVB strain did not alter development of skin lesions, which were observed in offspring from both founder lines. While aberrant gene expression was not observed in these animals, placing breeding females on doxycycline chow through pregnancy, up until pup weaning, eliminated the condition, suggesting a role for the tet machinery in the development of the condition. Maintenance of breeding cages on doxycycline chow allowed for healthy transgenic pups to survive past 3 weeks of age.

Initially, six animal cohorts were established, along with control animals, using two different mammary Cre strains, WAP-Cre and MMTV-NLST-Cre, crossed with regulatable, stabilized β-catenin (St-bcatFL) animals, as well as animals expressing mutated p53-R270H. While mammary tumors developed in animals expressing Cre-targeted regulatable St-bcatFL, the penetrance and growth kinetics were lower than observed with other mammary mouse models expressing stabilized β-catenin. As significantly fewer animals developed tumors, we were only able to test the effect of turning off stabilized β-catenin expression following tumor initiation in a small number of animals, with tumor stasis but not regression observed in animals expressing Cre-targeted St-bcatFL, but not in animals expressing both St-bcatFL and p53-R270H. The regulatable St-bcatFL mice were also crossed with animals expressing mutant PIK3CA*E545K and MMTV-NLST-Cre, to examine the importance of stabilized β-catenin to the rapid tumor growth we have observed in mice expressing stabilized β-catenin through deletion of exon 3, and PIK3CA*E545K. While all of the PIK3CA*E545K;Exon3 fl/+; Cre+ animals examined developed multiple mammary tumors by 5 months of age, the PIK3CA*E545K;St-bcatFL;Cre+ mice failed to develop tumors within the same time period. These results indicate that mammary tumor development, including penetrance and growth kinetics, differs significantly between our novel regulatable stabilized β-catenin mouse model and other more aggressive models of stabilized β-catenin. These differences will require alternative methods to determine whether stabilized β-catenin expression is required for the long-term maintenance of tumor growth.

Funding for this project was provided by the Terry Fox Foundation, as well as the Canadian Breast Cancer Foundation.

Citation Format: Jennifer L. Gorman, Jessica R. Adams, Emma M. Jones, Sean E. Egan, James R. Woodgett. Generation of a novel transgenic mouse model with regulatable β-catenin expression to examine importance of activated β-catenin signaling to long-term maintenance of mammary tumor growth [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr A29.

Abstract P1-07-24: Modeling breast cancer metastasis in the mouse via measurement of circulating tumor cells

Objectives: Advances in the detection of circulating tumor cells (CTC) in the blood of metastatic breast cancer patients have indicated that patients with greater than 5 CTC/7.5mL of blood have poorer prognosis; however, little is known about the biology of these cells and what characteristics are critical for breast tumor cell dissemination. We are developing and evaluating several mouse models of breast cancer to characterize important markers that distinguish CTCs from other cell types in the blood as well as to assess whether silencing of various metastasis supporting genes leads to reductions in CTC counts and formation of metastatic tumors.

Methods: This study involves two separate approaches to characterize CTCs. The first involves xenograft models using human breast cancer cells injected into the fat pad of immunocompromised mice. These cells are tagged with both mCherry, for detection in blood, and luciferase, to track metastatic spread of the primary tumor cells to other sites via intravital imaging. Blood was collected from tumor bearing mice at end point, processed and stained for characterized CTC markers. The second approach involves endogenous mouse models to characterize CTC counts and relevant markers in a variety of metastatic and non-metastatic murine breast cancer models using negative depletion to remove contaminating normal and hematopoietic cells. In both approaches CTCs have been assessed using the Imagestream by Amnis.

Results: Initial xenograft experiments with MDA-MB-231 cells injected into the mouse mammary gland fat pad revealed a low concentration of mCherry+/cytokeratin+/Hoechst+/CD45- cells in the blood of mice without confirmed metastatic dissemination. Pilot studies on endogenous tumor models have demonstrated the utility of our negative depletion approach to remove contaminating hematopoietic cells as well as the detection of a population of EpCAM+/Hoechst+/CD45- cells.

Conclusions: Initial experiments have provided feasibility for detailed characterization of mouse CTCs, which will be presented. This work complements our study being carried out on characterizing CTCs in metastatic breast cancer patients. Increasing understanding of the biology of circulating tumor cells and relating their characteristics to those of both the primary and metastatic tumors may reveal mediators common and functionally important to all 3 tumor cell populations and may therefore serve as better targets for treatments that are effective against both the primary and metastatic tumors.

Citation Format: Jennifer L Gorman, Michael Parsons, Eldad Zacksenhaus, Sean Egan, Martin Chang, Jim R Woodgett. Modeling breast cancer metastasis in the mouse via measurement of circulating tumor cells [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-07-24.

Angiotensin II evokes angiogenic signals within skeletal muscle through co-ordinated effects on skeletal myocytes and endothelial cells

Skeletal muscle overload induces the expression of angiogenic factors such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP)-2, leading to new capillary growth. We found that the overload-induced increase in angiogenesis, as well as increases in VEGF, MMP-2 and MT1-MMP transcripts were abrogated in muscle VEGF KO mice, highlighting the critical role of myocyte-derived VEGF in controlling this process. The upstream mediators that contribute to overload-induced expression of VEGF have yet to be ascertained. We found that muscle overload increased angiotensinogen expression, a precursor of angiotensin (Ang) II, and that Ang II signaling played an important role in basal VEGF production in C2C12 cells. Furthermore, matrix-bound VEGF released from myoblasts induced the activation of endothelial cells, as evidenced by elevated endothelial cell phospho-p38 levels. We also found that exogenous Ang II elevates VEGF expression, as well as MMP-2 transcript levels in C2C12 myotubes. Interestingly, these responses also were observed in skeletal muscle endothelial cells in response to Ang II treatment, indicating that these cells also can respond directly to the stimulus. The involvement of Ang II in muscle overload-induced angiogenesis was assessed. We found that blockade of AT1R-dependent Ang II signaling using losartan did not attenuate capillary growth. Surprisingly, increased levels of VEGF protein were detected in overloaded muscle from losartan-treated rats. Similarly, we observed elevated VEGF production in cultured endothelial cells treated with losartan alone or in combination with Ang II. These studies conclusively establish the requirement for muscle derived VEGF in overload-induced angiogenesis and highlight a role for Ang II in basal VEGF production in skeletal muscle. However, while Ang II signaling is activated following overload and plays a role in muscle VEGF production, inhibition of this pathway is not sufficient to halt overload-induced angiogenesis, indicating that AT1-independent signals maintain VEGF production in losartan-treated muscle.

Correspondence between students' scores on the Millon Clinical Multiaxial Inventory-II and Personality Diagnostic Questionnaire-Revised

The relation between two measures of personality disorders was examined in a nonclinical sample of 113 college students (86 women, 27 men) who completed the Millon Clinical Multiaxial Inventory-II and the Personality Diagnostic Questionnaire-Revised. Raw scores for 10 of 11 corresponding scales on the two instruments were significantly correlated (median r = .49). Compared to Millon's inventory, the Personality Diagnostic Questionnaire-Revised designated more subjects as having eccentric personality disorders but fewer as having anxious personality disorders. The significant association between scores on the inventories suggests that they assess personality traits that vary continuously in nonclinical samples. However, the finding that they differ significantly in their assignment of clinical labels shows that they should not be used to diagnose personality disorders in nonclinical populations.