Rise of the goliaths: Serial imaging of clonal dynamics shows 3 phases of UV-induced carcinogenesis

Summary: Abstract Body: While the genetic paradigm of cancer is powerful, it remains incomplete. Although studies demonstrate the presence of high mutational burdens in normal appearing skin, we reveal additional forces governing the eco-evolutionary dynamics of carcinogenesis. Using a UV-driven mouse model of cutaneous squamous cell carcinoma, we tested our central hypothesis that cancer initiation occurs in three phases: 1) tissue disruption and the emergence of unusually large “goliath” clades, 2) clonal selection within a subset of goliaths observed as unusually high local densities of cells (“micro-lumps”) with higher mutational burdens, and 3) emergence of macroscopic lesions. We tracked ecological and evolutionary drivers of cancer initiation via in-vivo serial 3-D reconstruction of fluorescently labeled keratinocyte clades, yielding over 25,085 clade measurements. While median and mean clade sizes differed little between UV and non-UV exposure, goliath clades emerged (> 4.2E6 µm3) almost exclusively within UV-exposed skin. Unexpectedly, targeted DNA sequencing revealed very low variant allele frequencies within clades, but substantial differences among clades, suggesting that positive selection for these mutations is superfluous to the development of goliaths early in carcinogenesis. scRNAseq revealed epidermal de-differentiation and immune suppression as early events. Lesions emerged between months 6 and 7, only in UV-exposed skin. Remarkably, 2 of 21 randomly selected goliaths developed into macroscopic lesions. Our adaptation of the Drake equation estimated the probability of this to be <10-6. Taken together, our results support the presence of 3 phases of cancer initiation, the earliest of which presage the acquisition of driver mutations and explains why cancers are rare in relation to the degree of somatic mosaicism present in UV-exposed skin. Stanislav Avdieiev¹, Leticia Tordesillas¹, Karol Prieto Sarmiento¹, Omar Chavez Chiang¹, Zhihua Chen¹, Nihir Patel², Sofia Cordero¹, Luiza Simoes¹, Ann Chen³, Robert Gatenby¹, Elsa Flores¹, Christopher Whelan¹, Joel Brown¹, Kenneth Tsai¹ 1. Moffitt Cancer Center, Tampa, FL, United States. 2. Admera Health, South Plainfield, NJ, United States. 3. University of Utah Health Huntsman Cancer Institute, Salt Lake City, UT, United States. UV Biology/Injury and Non-melanoma Cancers