Mouse models have an essential role in cancer research, yet little is known about how various models resemble human cancer at a genomic level. Here, we complete whole genome sequencing and transcriptome profiling of two widely used mouse models of breast cancer, MMTV-Neu and MMTV-PyMT. Through integrative in vitro and in vivo studies, we identify copy number alterations in key extracellular matrix proteins including collagen 1 type 1 alpha 1 (COL1A1) and chondroadherin (CHAD) that drive metastasis in these mouse models. In addition to copy number alterations, we observe a propensity of the tumors to modulate tyrosine kinase-mediated signaling through mutation of phosphatases such as PTPRH in the MMTV-PyMT mouse model. Mutation in PTPRH leads to increased phospho-EGFR levels and decreased latency. These findings underscore the importance of understanding the complete genomic landscape of a mouse model and illustrate the utility this has in understanding human cancers.
Breast tumor heterogeneity has been well documented through the use of multiplatform –omic studies in human tumors. However, there is no integrative database to capture the heterogeneity within mouse models of breast cancer. This project identifies genomic copy number alterations (CNAs) in 600 tumors across 27 major mouse models of breast cancer through the application of a predictive algorithm to publicly available gene expression data. It was found that despite the presence of strong oncogenic drivers in most mouse models, CNAs are extremely common but heterogeneous both between models and within models. Many mouse CNA events are largely conserved in human tumors and in the mouse we show that they are associated with secondary tumor characteristics such as tumor histology, metastasis, as well as enhanced oncogenic signaling. These data serve as an important resource in guiding investigators when choosing a mouse model to understand the gene copy number changes relevant to human breast cancer.
The E2F transcription factors control key elements of development, including mammary gland branching morphogenesis, with several E2Fs playing essential roles. Additional prior data has demonstrated that loss of individual E2Fs can be compensated by other E2F family members, but this has not been tested in a mammary gland developmental context. Here we have explored the role of the E2Fs and their ability to functionally compensate for each other during mammary gland development. Using gene expression from terminal end buds and chromatin immunoprecipitation data for E2F1, E2F2 and E2F3, we noted both overlapping and unique mammary development genes regulated by each of the E2Fs. Based on our computational findings and the fact that E2Fs share a common binding motif, we hypothesized that E2F transcription factors would compensate for each other during mammary development and function. To test this hypothesis, we generated RNA from E2F1-/-, E2F2-/- and E2F3+/- mouse mammary glands. QRT-PCR on mammary glands during pregnancy demonstrated increases in E2F2 and E2F3a in the E2F1-/- mice and an increase in E2F2 levels in E2F3+/- mice. During lactation we noted that E2F3b transcript levels were increased in the E2F2-/- mice. Given that E2Fs have previously been noted to have the most striking effects on development during puberty, we hypothesized that loss of individual E2Fs would be compensated for at that time. Double mutant mice were generated and compared with the single knockouts. Loss of both E2F1 and E2F2 revealed a more striking phenotype than either knockout alone, indicating that E2F2 was compensating for E2F1 loss. Interestingly, while E2F2 was not able to functionally compensate for E2F3+/- during mammary outgrowth, increased E2F2 expression was observed in E2F3+/- mammary glands during pregnancy day 14.5 and lactation day 5. Together, these findings illustrate the specificity of E2F family members to compensate during development of the mammary gland.
Advances in cancer genomics and transcriptomics have provided a better understanding of the diverse genetic landscape driving breast cancer pathogenesis. Despite these advances, the molecular drivers of triple negative breast cancer, an aggressive and heterogenous disease, remains highly elusive. Utilizing a genomic approach, we examined genetic differences across mouse models of breast cancer and identified a gene that was consistently deleted in basal-like mouse models, the predominant intrinsic subtype of triple negative breast cancer. Specifically, we identified a consistent deletion in the transcription factor E2F5. In human breast cancer, E2F5 activity and expression is associated with better overall survival and relapse free survival. To further study the role of E2F5 in breast cancer, E2F5 was deleted in the mammary gland of FVB mice (E2F5cKO) through the use of a cre-lox system driven by the MMTV promoter. E2F5cKO mouse demonstrated a delay a mammary outgrowth, suggesting a potential role in mammary gland development. E2F5cKO mice were aged and observed for tumor development. This revealed mammary tumor formation after an extended latency of 19 months. The resulting mammary tumors demonstrated diverse histological patterns. E2F5cKO tumors also demonstrate high metastatic potential with 70% of tumor bearing mice developing metastasis to the lung. In addition, E2F5cKO tumors have a propensity to metastasize to axillary lymph nodes when transplanted into MMTV-Cre mice, a unique feature that is not commonly observed in other mouse models of breast cancer. To begin to understand the mechanism of tumor progression in E2F5cKO mice, differential gene expression was performed on gene expression data derived from E2F5 or GFP overexpression Human Mammary Epithelial Cells. The resulting differentially expressed genes were further filtered based on fold change, established role in cancer, potential E2F binding site and gene ontology. One of the leading candidates that arose from this analysis was KRas, an oncogene that is highly implicated in tumorigenesis. KRas expression negatively correlated with E2F5 expression. Using a bioinformatic approach, we demonstrated that this inverse relationship between KRas and E2F5 appears to be conserved in human breast cancer. Given this inverse relationship between E2F5 and KRas, we postulate that E2F5 deletion in the mammary gland of mice leads to increased KRas activity. Consistent with our hypothesis, western blot analysis revealed high activity in two major KRas signaling pathways, Akt and Erk, in the majority of E2F5cKO tumors. Together, this preliminary data suggest that E2F5 may behave as a tumor suppressor by negatively regulating the oncogene KRas. Citation Format: Briana To, Eran Robert Andrechek. The role of E2F5 in breast cancer progression and metastasis [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-03-09.
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