Abstract 2675: Clonal expansion of skin keratinocyte
Authors: Ishida Yoshihiro , Kakiuchi Nobuyuki , Fujii Yoichi , Hirano Tomonori , Inoue Yoshikage , Nishimura Tomomi , Ogasawara Tatsuki , Maeda Hirona , Shiraishi Yuichi , Chiba Kenichi , Tanaka Hiroko , Miyano Satoru , Otsuka Atsushi , Kabashima Kenji , Ogawa Seishi 
Published: July 2021

Abstract
Skin is the largest organ system and is exposed to ultraviolet (UV) or sun light, a well-known mutagenic insult in various skin cancers, depending on the body site. A previous study showed that keratinocytes in sun-exposed eyelids harbor a high number of somatic mutations affecting a number of common driver mutations found in skin cancers, including NOTCH1, NOTCH2, NOTCH3, FAT1, TP53, and RBM10, suggesting pervasive expansion of clones before the development of skin cancer. However, clonal expansion in other skin sites in association not only with UV but also other insults, such as chronic inflammation is yet to be evaluated.
Thus, to fully characterize the clonal expansion in the skin, we performed sequencing analysis of apparently normal skin, as well as that affected by chronic inflammatory conditions such as psoriasis and atopic dermatitis. We analyzed small fragments (0.2mm2) of epidermal samples accrued from surplus biopsy materials using exome sequencing.
Analysis of bulk epidermal samples revealed a median of ~30 mutations/exome. Sun-exposed skin was notable for a high mutation burden (&gt;200 mutations/exome). On the basis of the analysis of their variant allele frequencies, the median clone size was estimated to be ~0.1mm2. Several cases harbored large of clones that spanned multiple samples, which were invariably found in sun-exposed skin. In one of such cases, a clone spanned a region having more than 2mm in diameter. Skin affected by chronic inflammation showed a higher number of mutations. dN/dS analysis revealed positively selected driver genes that were similar to the previous report. Diseased skin showed a similar mutation profile that is seen in apparently normal sun-exposed skin. Analysis of mutational signature disclosed signatures that are related ageing (SBS1) and UV light exposure (SBS7). The contribution of SBS7 was prominent in the sun-exposed area but also detected in areas not exposed to sun light, such as the back, buttock, or thighs. By contrast, inflammation-affected skin samples are characterized by a large contribution of the SBS1 signature, likely reflecting increased cell cycles.
In conclusion, our analysis shows UV light is a relevant driver of clonal expansion of skin keratinocytes. Chronic inflammation accelerates an accumulation of somatic mutations in and/or clonal expansion of keratinocytes.
Citation Format: Yoshihiro Ishida, Nobuyuki Kakiuchi, Yoichi Fujii, Tomonori Hirano, Yoshikage Inoue, Tomomi Nishimura, Tatsuki Ogasawara, Hirona Maeda, Yuichi Shiraishi, Kenichi Chiba, Hiroko Tanaka, Satoru Miyano, Atsushi Otsuka, Kenji Kabashima, Seishi Ogawa. Clonal expansion of skin keratinocyte [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2675.