A Polyomavirus-Positive Merkel Cell Carcinoma Mouse Model Supports a Unified Origin for Somatic and Germ Cell Cancers

Background/Objectives: The Germ Cell Theory of cancer posits that human primordial germ cells (hPGCs) are the cells of origin for malignancies. While this theory is well established for germ cell cancers, a germ cell origin for somatic cancers has been largely overlooked despite clinical observations of malignant somatic transformation (MST), wherein germ cell cancers give rise to diverse somatic cancer phenotypes, often without additional mutations. Methods: To test the Germ Cell Theory experimentally in somatic cancer, we established a virus-driven MST model linking hPGC-like cells (hPGCLCs) to Merkel cell polyomavirus (MCPyV)-positive Merkel cell carcinoma (MCC), a highly aggressive somatic cancer with a germ cell cancer-like, low-mutation epigenetic profile. The MCPyV genome was transduced into human induced pluripotent stem cells (hiPSCs) or hPGC-like cells by lentiviral transfection, followed by xenotransplantation. Results: Virus-positive MCC (VP-MCC)-like tumors were consistently induced without additional oncogenic mutations. These tumors recapitulated VP-MCC’s high-grade neuroendocrine carcinoma histology and molecular profiles. DNA methylation analysis revealed near-complete global hypomethylation in VP-MCC-like tumors, matching the unique epigenetic state of late-stage hPGCs. Notably, pluripotent intermediates were neither necessary nor sufficient for MST; transformation required acquisition of a late-hPGC-like epigenetic state. Conclusions: This is the first MST model of a somatic cancer arising through an aberrant germline-to-soma transition. Our findings unify VP-MCC and germ cell cancer biology, challenge mutation- and soma-centric paradigms, and provide a tractable platform to investigate developmental and epigenetic mechanisms of oncogenesis. This MST model supports a unifying germ cell origin for both germ cell and non-germ cell somatic malignancies.