FGFR3: ONCOGENES IN IMMUNE EVASION
Just as the immune system is constantly scanning our bodies for bugs and viruses that could cause disease, it is also constantly scanning for our own cells that may become cancerous. Normally, if our immune cells (often the cytotoxic CD8+ T-cell) find a cell that could become or is cancerous, the T-cell kills the cancer cell. However, non-specific killing by T-cells will lead autoimmunity. Thus, the cells in our bodies have several mechanisms that tell a T-cell, “don’t kill me, I’m supposed to be here”. We call these mechanisms immune checkpoints, and often, tumors exploit the immune checkpoint mechanisms in order to evade destruction by the immune system. Immune checkpoint inhibitors are drugs that block this immune silencing mechanism, which allows the T-cells to perform their cancer killing duty. Immune checkpoint inhibitors have proven effective for a variety of different cancer types. Nonetheless, for reasons that are not perfectly clear, these drugs are not effective in 100% of patients. We do know that some peoples’ tumors have a lot more CD8+ T-cells than other patients, and tumors with more abundant T-cells are far more responsive to checkpoint inhibition.
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Bladder cancer is a prevalent malignancy, especially in men where it is the fourth most common cancer type and the eighth leading cause of cancer death. Patients with tumors confined to the luminal tissue layer of the bladder have a 96% 5-year survival rate. Patients with muscle invasive tumors have a 60% 5-year survival rate, and those with metastatic disease have a 5-year survival rate of 5%. Historically, development of drugs designed to treat bladder cancer lagged behind other genitourinary malignancies, but in 2016 and 2017, five drugs targeting the PD-1 and PD-L1 were approved by the FDA for bladder cancer. A randomized trial performed in 2017 with 542 patients refractory to platinum-based chemotherapy found that patients receiving Pembrolizumab (anti-PD-1 monoclonal antibody) in addition to chemotherapy had significantly higher overall survival than those receiving chemotherapy alone 1. Nonetheless, the objective response rate to Pembrolizumab and similar drugs is approximately 20% 2. By using a gene signature to classify bladder tumors based on the presence or absence of cytotoxic T-cells, previous members of our lab showed a correlation between activating mutations in the FGFR3 gene and resistance to immune checkpoint inhibition. Our hypothesis is that FGFR3 alters the gene expression of the tumor cells and thus alters the immune microenvironment to make it immune suppressive.
The goal of my work is to develop and test murine models of bladder cancer that can be used to elucidate mechanisms of resistance to checkpoint inhibition. We employ two main models of bladder cancer in mice in order to explore the effects of FGFR3. The first model involves transplanting cultured MB49 cells into mice. MB49 is a bladder cancer cell line derived from mice with the C57BL/6 genetic background. Therefore, the cells can be transferred from culture directly into immune-competent Black6 mice without graft rejection. Being a stable cell line, the MB49 cells can be easily transduced to stably express our genes of interest. To test our hypothesis that mutant FGFR3 is responsible for resistance to checkpoint blockade, the two most common FGFR3 mutations (G370C and Y373C) from the previous clinical data were transduced in the MB49 cells. When these cells are transplanted into the mice, they can be treated with checkpoint inhibitors to determine whether our model mimics the clinical observation. The second model that we are developing is the genetically engineered mouse model of tumorigenesis. Bladder cancer can be induced using specific gene promoters. They may be either induced by administration of a drug such as tamoxifen, or they may be spontaneously generated. In either case, the activation of an oncogene and the deactivation of a tumor suppressor gene are required in order to generate a tumor in these bladder models, and this is achieved using the Cre recombinase transgene. To test the effects of FGFR3 mutations, we have mice with a Cre-inducible FGFR3 mutant. The ultimate goal of this research is to determine whether the FGFR3 pathway can be inhibited in order to restore sensitivity to checkpoint blockade.