Leveraging Tumor Suppressor Inactivation for Osteosarcoma Therapy

PROJECT SUMMARY Osteosarcoma (OS) is the most common malignant bone tumor, found in the pediatric population and often leads to death due to tumor progression and metastasis. Despite intensive, multimodality treatment protocols with cytotoxic chemotherapy and aggressive surgery, 30-40% of patients still succumb to relapsed and metastatic disease. Current treatment options for these patients are extremely limited. More importantly, the clinical outcome for OS patients has not improved at all during the last four decades. Our long-range goals are to reduce OS disease burden and improve long-term survival by developing molecular-based therapies that target mechanisms of tumor growth and metastasis. The F-box protein s-phase kinase-associated protein-2 (Skp2) is the substrate recognition component of the Skp1-Cullin1-F-box (SCF) complex and considered a key oncogene through its ability to target cell cycle regulators for ubiquitin degradation, regulates cell migration and metastasis. Skp2 interacts with the accessory protein Cks1 to ubiquitinate p27 for degradation, thus promoting cell cycle progression and cancer growth. Currently, the oncogenic mechanisms and therapeutic potential of Skp2 in OS is largely unknown. There are specific and rational reasons for targeting the SCF-Skp2 axis in OS, based upon preliminary data generated by the PI and his colleagues: (1) A high Skp2 expression in OS tissues portends a poor prognosis in patients; (2) High levels of Skp2 are observed in many OS cell types, including established cell lines, patient-derived xenografts, and primary cultures from OS patients; (3) Skp2 deletion effectively blocks pituitary and prostate tumorigenesis in Rb1/p53-deficient mouse models. In OS, genomic sequencing studies reveal a high rate of Rb1/p53 co-inactivation, suggesting that OS may be susceptible to Skp2 blockade; (4) Genetic and drug-induced depletion of Skp2 reduces the proliferation and invasion of OS cell lines and; (5) Mouse embryonic fibroblasts (MEF) and OS tumor cells with Rb1/p53 deletion are more sensitive to SCF-Skp2 inhibition than wild-type and Skp2-deleted cells. Taken together, these data suggest that the SCF-Skp2 complex may offer a unique therapeutic opportunity for treating a tremendously challenging cancer such as OS. We hypothesize that blockade of the SCF-Skp2 complex in the context of Rb1/p53 inactivation will reduce OS progression. This hypothesis will be addressed by the following specific aims: Specific Aim 1 (Oncogenic Mechanisms): To define the pro-oncogenic mechanisms of SCF-Skp2 in Rb1/p53-inactivated OS. Specific Aim 2 (Translational Therapeutics): To determine the effect of inhibiting SCF-Skp2 in preclinical models of Rb1/p53- inactivated OS. Finally, since cancer organoids represent an emerging technology that faithfully replicates the tumor's genetic landscape, we will examine the effect of inhibiting SCF- Skp2 in mouse OS organoids as a proof-of-concept to translate findings into patient-specific therapeutics in the future. The successful completion of this study will be translatable to target the SCF-Skp2 complex to block osteosarcoma progression.