281 Chronic inflammatory disease alters skin keratinocyte clonal composition, mutation burden, and selection

Previous studies have identified the clonal expansion of skin keratinocytes harboring cancer driver mutations in histologically-normal, sun-exposed skin. However, whether or not the presence of chronic inflammation affects the clonal composition, mutation burden, and selection of skin keratinocytes has not been explored. We analyzed somatic mutations in skin epidermis samples collected from patients with psoriasis (Pso) or atopic dermatitis (AD). Normal skin samples were also analyzed as controls. We combined microbiopsy, laser capture microdissection, and single cell-derived culture approaches, allowing analyses of keratinocyte clones at different scales. First, we determined the rate at which age-related clonal expansion progresses in normal, non-sun-exposed (NNSE) skin, and found that patients with chronic skin inflammatory diseases tend to show larger clone sizes than NNSE skin. Second, we show that the mutation accumulation rate is constant in NNSE skin using single cell-derived colonies, and the mutation burden in keratinocyte colonies under chronic inflammation deviates from its expected values. Third, selection in Pso and AD skin are different; Pso skin frequently contained keratinocyte clones harboring known skin cancer drivers whereas clones with such mutations are largely absent in AD. Selection analysis shows neutrality in AD; immunological dysregulation in AD skin may nullify selective advantages conferred to keratinocytes by cancer drivers. Lastly, the life histories of keratinocyte clones are different between NNSE skin and AD. In contrast to NNSE, phylogenies of keratinocyte clones in AD show extensive branching, suggesting itch-scratch cycles may have altered the clonal composition of the skin in patients with life-long AD. Together, our study shows that chronic inflammatory diseases inscribe their marks on the genome of keratinocytes.