ABSTRACT Genetically modified mouse models of breast cancer have been used for decades as premier basic science tools for mechanistic discovery. However, the successful implementation of mouse models as surrogates of therapeutic efficacy and translational research has been challenging. One major challenge for status quo approaches is their limited ability to model the genetic heterogeneity observed in breast cancers. Metastatic and treatment resistant HER2+ breast cancers are incurable largely due to this heterogeneity, the source of which may stem from the competition and evolution of multiple oncogenic isoforms of the driver gene HER2. The objective for this proposal is to recapitulate the genetic heterogeneity of HER2 oncogenes in a genetically tractable model more closely resembling the human condition – including an intact immune system and stromal network. Published preliminary data recently described a Cancer rainbow (Crainbow) modeling system for fluorescently barcoding and expressing multiple tumor driver genes in a single immune intact mouse. The fluorescent barcode is retrieved by multispectral imaging and single-cell “omics” techniques providing a simple solution for inducing intratumor heterogeneity and visualizing its evolution. Any tumor driver gene can be incorporated into Crainbow mice. Therefore, this proposal will test the central hypothesis that modeling the oncogenic heterogeneity of HER2 in a Cancer rainbow mouse recapitulates the phenotypic heterogeneity found in treatment resistant and metastatic HER2+ breast cancers. The central hypothesis will be tested by completing four specific aims seeking to: (Aim 1) Validate a HER2 Crainbow mouse model of tumor heterogeneity, (Aim 2) Demonstrate heterogeneity within the tumor epithelium, (Aim 3) Demonstrate heterogeneity of the tumor microenvironment and its contribution to tumor biology, and (Aim 4) Demonstrate heterogeneity and differential response to therapy. HER2 Crainbow mice will provide an autochthonous mouse model of the genetic heterogeneity found in HER2+ breast cancer, all while maintaining the endogenous contributions of the tumor microenvironment to invasion and metastasis. Completing this proposal is expected to validate the HER2 Crainbow mouse as a shareable resource strain for more predictive preclinical trials and a framework for illuminating the molecular and cellular ontogeny of invasive breast cancer.

PROJECT NARRATIVE Tumor heterogeneity is a well-known property of treatment resistant, recurrent, and metastatic breast cancer. The proposed research project is relevant to public health because it will provide a genetically tractable Cancer rainbow mouse model for inducing and visualizing tumor heterogeneity in an immune intact system. The proposed studies will credential the use of a novel Crainbow mouse model that more accurately recapitulates the heterogeneity found in treatment resistant HER2+ breast cancer patients. Our proposal will significantly improve upon the translational utility of mouse models for preclinical trials and at the same time provide a reliable tool for reconstructing the molecular and cellular interactome controlling tumor heterogeneity.