Abstract 2672: Eco-evolutionary profiling of the UV-driven clonal dynamics and tumor initiation

Abstract Classically, cancer initiation is attributed to a sequential acquisition of discrete genetic events such as driver mutations. However, homeostatically-stable tissues, including skin, harbor numerous somatic mutations spread across the tissue’s clones. Yet, most of these clones never give rise to cancer. Using the model of UV-driven cuSCC, we tested our hypothesis that cancer initiation occurs in three phases: 1) tissue disruption and the emergence of unusually large “goliath” clades (ecological driver), 2) clonal selection within a subset of goliaths as evidenced by the presence of cell density hotspots and higher mutational burden (evolutionary driver), and 3) emergence of macroscopic lesions. We tracked these tissue level drivers via in-vivo serial imaging and 3-D reconstruction of fluorescently labeled keratinocyte clades, yielding 25, 085 clades from 14 mice over 3 months, and 14, 525 clades from 4 of these mice over 6 months. While median and mean clade sizes differed little across UV exposure, our ecological survey revealed the emergence of large goliath clades (> cc. 4.2×106 µm3) observed essentially only within UV-exposed skin and occupying the extreme tail of the distribution of clade sizes. Goliath clades arose as serpentine structures, intercalating between adjacent keratinocytes, some with areas of unusually high local densities (“micro-lumps”). Targeted DNA sequencing of goliath revealed Kmt2a, Kmt2d, Prex2, and SCN11A as the most mutated genes. Giant clades do not typically possess common driver mutations of SCC, e.g. TP53 and NOTCH, and display two extremes of mutational profiles, with some goliaths having dominant clones (VAF >0.75), and others having no discernable clonal structure (VAF <0.025), demonstrating that strong mutational drivers are not a prerequisite for developing goliaths. scRNAseq analysis of both bulk skin and sorted clades revealed upregulation of the fraction of cycling basal-like cells, and downregulation of Alox12e-expressing granular cells, along with increased proportions of plasmacytoid dendritic cells (pDC) and CD4+ regulatory T-cells (Tregs) in UV-exposed samples, reflecting early epidermal de-differentiation and immunosuppression. By month 7, mutational burdens were significantly larger in goliath clades, particularly for those with micro-lumps. Finally, lesions emerged between months 6 and 7, only in UV-exposed skin. Based on our adaptation of the Drake equation, we conclude that the probability of observing macroscopic lesions arising in 2 of the 21 randomly sampled goliaths we followed for 3 months to be less than 10-7. Taken together, our results support the interplay between cell autonomous and non-cell autonomous compartments during cancer initiation, the earliest phase of which presage the acquisition of driver mutations. Our data constitute an unprecedented look at clonal dynamics in UV exposed skin defining the transition from normal tissue to tumor. Citation Format: Stanislav S. Avdieiev, Leticia Tordesillas, Karol Prieto, Omar Chavez Chiang, Zhihua Chen, Nihir Patel, Sofia Cordero, Luiza Silva Simoes, Y. Ann Chen, Noemi Andor, Robert A. Gatenby, Elsa R. Flores, Christopher J. Whelan, Joel S. Brown, Kenneth Y. Tsai. Eco-evolutionary profiling of the UV-driven clonal dynamics and tumor initiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2672.