Murine models for studying the mutational processes of APOBEC3-driven cancers and the immune response

Abstract

Breast cancer is one of the most common occurring malignancies in women. Breast cancer is a collection of immensely heterogenous diseases, where relapse and metastasis remain a looming threat. While multimodal therapeutic approaches, such as radiation therapy, surgery, cytotoxic chemotherapies, and endocrine therapies, remain the mainstay for first- and second-line treatments, many patients do not achieve curative results. Thus, therein lays a critical unmet need for the development of novel treatments or prognostic biomarkers to predict therapeutic responses in patients with breast cancer. The APOBEC family of cytosine deaminases contribute significant mutational burden in breast cancer, fueling its genetic diversity and heterogeneity. Multiple studies have demonstrated that mutational load is one of the strongest predictors of immunotherapy benefit in cancer. This is observed in recent clinical successes in the treatment of cancers with historically high rates of mutation, like melanoma, lung, and colon cancers. However, only a fraction of breast cancer patients receives clinical benefit from immune checkpoint blockade therapies. Thus remains the question as to whether APOBEC-expressing breast tumors are candidates for immunotherapies. Despite the prevalence of APOBEC mutagenesis reported in breast tumors, there has been insufficient efforts in clinical detection, identifying how this mutational process contributes to cancer progression, and what therapeutic strategies might be best leveraged to target APOBEC-positive disease. In this thesis, we utilize two mouse models to recapitulate APOBEC activity in tumors. First, we aim to examine the underlying mechanisms of APOBEC3-driven tumorigenesis. Here, we use a hepatocellular carcinoma mouse model to validate APOBEC3A as a bona fide cancer driver, capable of driving tumorigenesis independent of other well-established drivers. We showed the catalytic activity of APOBEC3A is necessary for tumor development, where it is more than likely driven by DNA-deaminase activity. Next, we sought to evaluate the interface of the tumor-immune microenvironment in APOBEC3-expressing tumors. We developed a novel, syngeneic mammary tumor model capable of engrafting into immune competent mice. Here, we generated preliminary data to demonstrate the utility of this model, as it is the first of its kind to recapitulate APOBEC-expressing cancers in a fully immune model system. We show engrafted tumor cells express high levels of PD-L1 and manifest with significant levels of infiltrating CD8-positive T lymphocytes, suggesting it may be advantageous in evaluating the efficacy of immune checkpoint blockades. We propose this novel mammary tumor model has the potential to become a powerful pre-clinical tool that can be used for studying the tumor-immune interface in breast cancers, and for identifying novel clinical markers for treatment response, like APOBEC.