Masahiro Shuda, Ph.D.

Tumor virus research has provided a wealth of information for cancer biology. Polyomaviruses, small DNA tumor viruses that efficiently transform normal healthy cells into cancer cells, have been central to understanding the basic mechanisms of cancer cell transformation. Studies on the animal polyomavirus SV40, for example, led to the discovery of tumor suppressor protein p53 and uncovered the functions of tumor suppressor protein pRb in cell cycle regulation. My laboratory studies Merkel cell polyomavirus (MCV), the first human oncogenic polyomavirus identified from human Merkel cell carcinoma (MCC) and high-risk human papillomavirus (HPV), a causal agent of cervical cancers and head and neck squamous cell cancers (SCC). Our research  exploits human tumor virus oncogenic proteins (large T(LT) and small T(sT) antigen proteins from MCV and E6 and E7 from HPV) to identify novel cellular pathways that lead to cancer, with a specific focus on stem cell factors.

In our search for the genes deregulated by viral T antigen in virus-positive MCC tumors, we identified that the polycomb group gene enhancer of zeste homologue 2 (Ezh2) is upregulated by MCV LT antigen. Interestingly this gene is also activated by the HPV E7 gene. Ezh2 knockdown inhibits MCV+MCC and HPV+SCC proliferation, indicating that Ezh2 is one of the critical host factors activated downstream of LT protein for MCC carcinogenesis. Ezh2 is an epigenetic host gene expression regulator that catalyzes histone protein methylation through methyltransferase activity. We also performed Bio-ID experiments to search for cellular interactors with viral oncoproteins. Our preliminary results suggest that MCV sT interacts with epigenetic factors and histones. We are currently studying the roles Ezh2 and other epigenetic regulators of gene expression in virus-induced carcinogenesis by exploiting chromatin immunoprecipitation and comprehensive host gene expression analyses.

Due to the ultrastructural and gene expression similarities between normal Merkel cells and MCC, it was long thought that MCC tumors arose from transformed mechanoreceptor Merkel cells. Recent studies, however, suggest that the cellular origin of MCC may not be Merkel cells. We recently found that MCV-positive MCC cells, growth-arrested by the T antigen knockdown, can differentiate into cell that exhibit neurite outgrowth. The excitability of the differentiated cells mimics that of neurons, not Merkel cells. Merkel cell lineage factors Sox2 and Atoh1 are upregulated by T antigen and are essential for cell proliferation in MCC. This suggests that MCV-positive MCC may arise from primogenitor/stem cells, which differentiate into non-Merkel cells, and that the viral oncoprotein exploits drivers of Merkel cell development for carcinogenesis. To characterize the MCV-transformed progenitor cells, we seek to identify the differentiation capacities of MCC cell lines after T antigen knockdown. The MCC cell culture model that we previously developed may allow us to identify the cellular origin of MCV-positive MCC and also serve as a model to study MCV-induced stem cell/progenitor cell transformation. Transformed stem cells such as cancer stem cells display resistance to chemotherapeutic agents. MCC is also known to be chemotherapy-resistant. Thus, we may need to develop specific therapies targeting host molecules that maintain stemness, such as Sox2.