In vivo Tracking of Clonal Dynamics in UV-Induced Skin Carcinogenesis Reveals Tumor Development from Goliath Clades

While the genetic paradigm of cancer is powerful, it remains incomplete. For example, high mutational burdens are commonplace in homeostatically-stable epithelia, but the presence of driver mutations alone does not result in cancer. Here, we revealed additional factors contributing to the eco-evolutionary dynamics of carcinogenesis. The dynamics of carcinogenesis were tracked from initiation to emergence using a UV-driven model of squamous cell carcinoma, via in vivo serial 3D reconstruction of >25,000 fluorescently labeled keratinocyte clades (cells expressing a single fluorophore and related by descent). While median and mean clade sizes (8-16 cells) differed little with UV exposure, UV exposure resulted in large goliath clades (>~4.2×106 µm3; >500 cells). Of note, goliath formation preceded the acquisition of established driver mutations, and only a subset of goliaths exhibited both greater mutational loads and dense aggregations of cells (micro-lumps). Of 21 randomly selected goliath clades in macroscopically normal-appearing skin tracked weekly, 2 developed into visible lesions, while an adaptation of the Drake equation placed the probability of this occurring at less than 10-6. The earliest molecular signatures based on single-cell RNA-sequencing of irradiated skin reflected epidermal de-differentiation and immunosuppression. Overall, this study documents skin cancer initiation due to UV exposure from changes in clade distribution through macroscopic lesion development arising from goliath clades. These data highlight these transitions in relation to known driver mutations and transcriptomic changes, suggesting a possible explanation for why cancers are rare relative to the degree of somatic mosaicism present.