Abstract A056: Lymph and liver metastasis in E2F5 conditional knockout mouse model
Authors: To Briana , Garcia-Lerena Jesus , Vusich John , Broeker Carson , Rennhack Jonathan , Hollern Daniel , Andrechek Eran 
Published: February 2024

Abstract
Alterations to normal development in mammary gland biology can result in tumor formation. Mammary development and functional processes are controlled by various transcription factors, including the E2Fs. Commonly known as cell cycle regulators, the E2Fs While the activator E2Fs are well studied, the function of the repressor E2Fs in mammary biology is not well known. This is due in part to the early lethality observed in the knockouts of E2F5. To predict a role for E2F5 in mammary developmental biology we queried scRNAseq data and generated a predictive gene signature. This demonstrated that there were functional stage specific expression patterns for E2F5, including virgin development and involution after lacation. Moreover, when we used the predictive signature for activity, we noted that low levels of E2F5 activity corresponded to a poor prognosis in human breast cancer. Based on these preliminary data we hypothesized that E2F5 played an essential role in mammary development and that loss could contribute to tumor biology. To test the premise that E2F5 had a role in mammary development, we generated a mammary epithelial specific knockout of E2F5. At 4 weeks of age, the outgrowth of the epithelium into the fat bad of the mammary gland of these mice significantly lagged behind the littermate controls. This demonstrated a clear role for E2F5 in development of the mammary gland. We also computationally predicted a role in involution and this was also confirmed through in vivo experiments. We noted accelerated involution with the loss of E2F5 following removal of pups. The second major hypothesis that we tested was that E2F5 had a role in tumor biology, and we planned to test this by interbreeding the E2F5 conditional knockout mice to various other strains. As we aged the E2F5 conditional knockout mice, we were surprised to note that mammary tumors formed after a long latency with the loss of E2F5 alone. These tumors were highly heterogeneous at both a histological and gene expression level, with samples clustering with each of the major human PAM50 subtypes of breast cancer. These tumors were also highly metastatic with lesions developing in the lungs, as many mouse models do. However, unlike most mouse models of breast cancer the E2F5 conditional knockout tumors also metastasized to the liver and lymph nodes. Given the extended latency of the model system, we transplanted the E2F5 conditional knockout tumors into syngeneic recipient mice. We repeatedly observed lymphatic metastases or liver metastases, dependent on the primary tumor. To enrich these tissue trophisms, we repeatedly implanted lymph or liver metastases into the mammary fat pad, resulting in significant enrichment. Preliminary RNAseq and Cut and Run have been completed to examine E2F5 specific gene expression alterations that lead to trophism and metastasis enrichment, with targets that are being currently validated. Since most genetically engineered mouse models have not been characterized to have lymph or liver metastasis, this work represents an important new model.
Citation Format: Briana To, Jesus Garcia-Lerena, John Vusich, Carson Broeker, Jonathan Rennhack, Daniel Hollern, Eran Andrechek. Lymph and liver metastasis in E2F5 conditional knockout mouse model [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A056.