Deciphering Host-Pathogen Interactions Governing Merkel Cell Polyomavirus (MCPyV) Life Cycle and Disease Pathogenesis

Abstract

This dissertation investigates critical gaps in our understanding of the biology of Merkel Cell Polyomavirus (MCPyV), a recently discovered human polyomavirus (PyV) that causes Merkel cell carcinoma (MCC), an aggressive neuroendocrine skin cancer. MCPyV infection is highly prevalent amongst the general population, yet our understanding of its life cycle and disease pathogenesis is still limited. While the host-pathogen interactions directing the viral infection pathways of most well-characterized PyVs have been uncovered, much of MCPyV's mode of entry has remained elusive. Here, I reveal a unique strategy that MCPyV uses during entry to bypass the barrier of the nuclear envelope to access the host cell's nucleoplasm for viral DNA replication. By establishing a robust cell culture model of infection, I discovered that MCPyV employs a mechanism of nuclear import that is distinct from the classical nuclear pore complex (NPC)-mediated pathway used by other PyVs. Instead, MCPyV capitalizes on the host cell division process to specifically hijack mitotic nuclear envelope breakdown (MNEB) for nuclear entry. I validated this finding by demonstrating that cell cycle arrest significantly impairs MCPyV infection, establishing the mitotic phase as the critical window for viral nuclear entry. This discovery represents a paradigm shift in our understanding of PyV nuclear entry mechanisms and suggests novel therapeutic approaches for targeting cell cycle-dependent viral entry. Next, I address the paradox that MCPyV DNA is detected highly in numerous cell types despite limited understanding of its tissue tropism and cellular reservoirs. Through comprehensive analysis of MCPyV infection patterns across human and monkey cell lines, coupled with single-cell RNA sequencing of healthy and cancerous tissues, I observed several interesting observations, including: the detection of viral capsids in cell lysates without corresponding reporter protein expression, and the selective presence of MCPyV transcripts in MCC samples but not in healthy or immunocompromised skin tissues. These results establish new paradigms for understanding PyV cellular tropism, highlighting the roles of both cell cycle dynamics and potential cytoplasmic or tissue specific persistence in viral infection. In sum, this work advances our understanding of the MCPyV life cycle and its association with MCC. Moreover, it emphasizes the crucial role of specific cellular conditions in enabling successful viral entry and replication, providing a foundation for future therapeutic interventions and research on MCPyV biology.