Mechanisms of PAX3-FOXO1 and HES3 Cooperation in Rhabdomyosarcoma
Rhabdomyosarcoma is the most common soft tissue sarcoma in children and has no targeted treatment options. The most aggressive forms of rhabdomyosarcoma are caused when pieces of two chromosomes break off and fuse together and create an abnormal protein called PAX3-FOXO1. This new protein turns on and off hundreds–if not thousands–of genes, transforming a normal cell into a cancer cell. The exact mechanism of how PAX3-FOXO1 accomplishes this is unknown and insight into this process is hindered by the lack of models. I developed zebrafish models of rhabdomyosarcoma that allow me to study PAX3-FOXO1 activity in the context of vertebrate development. I found that by inserting human PAX3-FOXO1 into the zebrafish genome that zebrafish develop rhabdomyosarcoma that is consistent with the human disease. By studying my new zebrafish model, I discovered that PAX3-FOXO1 turns on an important gene, HES3, which correlates to poor prognosis in patients.
Project Goal: I will probe the important pathways and processes by which PAX3-FOXO1 and HES3 cooperate in more aggressive rhabdomyosarcoma. This includes understanding how PAX3-FOXO1 and HES3 inhibit muscle cell maturation and how this functions in more aggressive disease. I have also identified targets downstream of PAX3-FOXO1 and HES3 for drugs that are currently in clinical trials for other cancers. I will investigate the efficacy of these drugs in pediatric rhabdomyosarcoma to repurpose them for timely use in the clinic. Using short- and long-term strategies, and diverse robust model systems, my goal is to identify novel therapies or therapy combinations for children battling rhabdomyosarcoma.
Project Update 2023: The support of Alex’s Lemonade Stand Foundation is critical for our research focused on PAX3-FOXO1 fusion-positive rhabdomyosarcoma, an aggressive pediatric muscle cancer that has no targeted therapies. Our goal is to understand the biology of rhabdomyosarcoma and leverage these insights to identify therapeutic opportunities to help children. We use multiple systems; including, zebrafish, cancer cells, and patient tumor genomic data to identify the most conserved drivers of the disease. Previously, we found that a gene involved in brain development, named HES3, was highly expressed in the most aggressive form of rhabdomyosarcoma and predicted reduced overall survival in patients. This was an unexpected finding. We want to know: 1) why this neural gene is expressed in muscle tumors, and 2) how it is contributing to more aggressive disease. To do this, we used CRISPR/CAS9 technology to knock-out the zebrafish form of HES3, and remove the gene from the fish. We found that HES3 plays a role in regulating the matrix surrounding and supporting cells, and likely providing an environment supportive of metastatic disease. Our models also found that mechanistically HES3 is a direct target of PAX3-FOXO1, that has a simultaneous increase in accessible DNA in that region and in deposition of a histone modification associated with increased gene expression. A global view of gene expression changes indicates that PAX3-FOXO1 initially induces a neural program and suppresses a skeletal muscle program, of which HES3 is a potential core regulator. We are functionally confirming our findings with an engineered mammalian cell culture based system. Overall, the goal is to integrate data from zebrafish, cell models, and patient tumors to identify therapeutic opportunities and improve outcomes for children with cancer.
