CCSG Pilot: Targeting Merkel Cell Carcinoma by Natural Product-Based Degraders of MCV Small T Antigen

Project Abstract Merkel cell carcinoma (MCC) is one of the most aggressive human skin cancers with a mortality rate exceeding that of melanoma, and its incidence is steadily increasing. Currently, there is no FDA-approved targeted therapy for MCC. Merkel cell polyomavirus (MCV) was discoved to be clonally integrated in at least 80% of MCC cases and has been identified as the etiological agent of most MCC. The MCV small T (sT) antigen plays a critical role in MCV-positive MCC (VP-MCC) oncogenesis. Through our collaborative screening campaign with NCI, we identified a set of novel natural products known as pyranonaphthoquinones (PNQs) that act as uniquely selective cytotoxic agents toward VP-MCC. Using unbiased chemoselectively-tagged analogues and proteomic approaches, we further identified the MCV sT antigen as a selective target protein of a PNQ analogue, b63. Additionally, we discovered that b63 acts as a molecular glue, which also binds to the E3 ubiquitin ligase UBE3B, leading to ubiquitination of MCV sT antigen for proteasomal degradation. Consistent with the reported functional importance of MCV sT antigen in promoting mTORC1-mediated hyperphosphorylation of the translational repressor 4E-BP1, leading to dysregulated cap-dependent translation for MCV transformation and MCC oncogenesis, b63-induced MCV sT antigen degradation resulted in marked inihbition of 4E-BP1 phosphorylation and subsequently restored 4E-BP1’s ability to repress cap-dependent mRNA (e.g., survivin) translation in polysomes. Furthermore, b63-induced inhibition of 4E-BP1 phosphorylation correlated with b63 cytotoxicity toward VP-MCC cells in vitro. Notably, treatment with the lactone-reduced analogue c16, reminiscent of b63, resulted in MCV sT antigen degradation associated with inhibition of 4E-BP1 phosphorylation in VP-MCC xenograt tumors in vivo. Accordingly, our central hypothesis is that the direct targeting of MCV sT antigen for degradation and inhibition of 4E-BP1 phosphorylation-mediated oncoprotein translation represents a novel strategy for the development of anti-MCC drugs. The primary goal of the proposed studies is to determine the fundamental mechanism of b63-targeted MCV sT antigen degradation associated with translational repression of VP-MCC tumorigenesis, along with the near-term goal to identify optimized analogues with suitable in vitro and in vivo potency and selectivity. Successful completion of the project will not only provide new insights into the biology and therapeutic relevance of MCV sT antigen/4E-BP1-driven translational control of MCC progression but will also facilitate rational approaches for the development of new molecular probes and early stage leads for first-in-class targeted therapies to treat MCV-driven MCC. The project will be conducted collaboratively by the members of the Markey Cancer Center between MCO and TO programs. The Shared Resource Facilities, including Redox Metabolism (proteomics), Flow Cytometry and Immune Monitoring, and Biostatistics and Bioinformatics, will be utilized in the proposed studies.