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Genetic relationship between pilar cysts, pilar tumors and pilar carcinomas.

e21063 Background: Pilar cysts, also known as trichilemmal cysts, are common cutaneous nodules that occur sporadically or an autosomal dominant inheritance pattern. A pilar cyst can transform into a proliferating pilar tumor (PPT) or pilar carcinoma. This study aimed to determine the genetic relationships between the malignant variants and the precursor pilar cysts. Methods: We performed whole exome (WES) and Sanger sequencing of pilar cysts and matched blood (or other non-lesional tissue representing systemic DNA) from 17 subjects with multiple familial pilar cysts and 15 with a single, apparently sporadic, pilar cyst. We then performed WES on four subjects with PPTs, one with matched blood, and one subject with a pilar carcinoma matched with blood. Results: We identified a c.2234G > A somatic mutation in phospholipase C delta 1 ( PLCD1), a tumor suppressor gene, in all 21 familial pilar cysts sequenced. In addition, 16 of the 17 subjects with familial pilar cysts were hemizygous for a c.1379G > A germline variant in PLCD1. By contrast, neither of these two mutations were found in subjects with PPTs or the subject with a pilar carcinoma. A potential loss-of-function somatic mutation of the tumor suppressor gene p53 was identified in the subject with a pilar carcinoma. Conclusions: Our results indicate that hereditary pilar cysts are an autosomal dominant tumor syndrome resulting from two hits to the PLCD1 tumor suppressor gene. However, these somatic PLCD1 mutations were not present in the four PPTs or the pilar carcinoma. This suggests that the more aggressive cyst variants originate from sporadic pilar cysts. In addition, our results are consistent with loss of p53 being a key event that causes pilar cysts to evolve into carcinomas as previously proposed.