BMI1 as a Novel Driver and Target in Alveolar Rhabdomyosarcoma
Despite significant efforts within oncology, effective therapies for alveolar rhabdomyosarcoma (ARMS) remain unrealized. Patients with ARMS are treated with intensive multimodal therapy, but outcomes still remain unacceptable. Hence, we are focused on better understanding the oncogenic networks underlying this extraordinarily aggressive sarcoma. ARMS displays an extremely quiet genomic landscape, primarily characterized by the PAX-FOXO1 fusion protein. However, the role of the epigenome in shaping malignancy has become increasingly appreciated and previous gene expression studies demonstrated that ARMS is specifically enriched for Polycomb target gene methylation, suggesting that Polycomb complexes may be deregulated in ARMS and play oncogenic roles. Thus, we hypothesized that BMI1, a member of the Polycomb family, would play a role in sustaining the malignant phenotype in ARMS. Consistent with this hypothesis, we have preliminary data demonstrating that BMI1 is robustly expressed in ARMS, supports ARMS proliferation and survival, and that inhibition of BMI1 using a small molecule inhibitor, PTC-209, inhibits ARMS growth. Our underlying hypothesis is that BMI1 promotes malignant characteristics in ARMS and constitutes a novel therapeutic target for this aggressive histotype. To further assess this hypothesis, we propose two related, yet independent, aims. The first aim will investigate the mechanisms by which BMI1 contributes to the malignant phenotype in ARMS. We hypothesize that BMI1 exerts its effects through increasing cell proliferation and survival in ARMS. We will evaluate the impact of BMI1 on ARMS cell proliferation, determining effects on cell cycle status and apoptosis. In addition, we will examine the effects of BMI1 on genes implicated in ARMS and described as targets of BMI1, including MYCN, CDKN1A, and PTEN. We will complement these efforts and use RNA-Seq analysis to provide an unbiased appraisal of BMI1-influenced signaling networks. The second aim will assess the preclinical rationale for BMI1 inhibition in the treatment of ARMS. We hypothesize that pharmacologic inhibition of BMI1 will result in decreased ARMS proliferation and survival. We will first carry out in vitro studies using PTC-209 and examine impact on cell proliferation and apoptosis. Next, we will carry out in vivo studies using ARMS xenografts; furthermore, we will assess the impact of BMI1 inhibition on BMI1 levels, as well as MYCN, CDKN1A, and PTEN levels. This proposal has direct relevance to the treatment of an aggressive sarcoma, as it investigates BMI1, a currently targetable epigenetic regulator with further BMI1 inhibitors being developed in both the academic and industrial pipeline. Hence, we are keenly interested in developing a comprehensive understanding of how BMI1 shapes the malignant phenotype in ARMS, laying the foundation for the clinical evaluation of BMI1 inhibition in this extremely aggressive disease.