Solitary Subungual Keratoacanthoma Arising in an MSH2 Germline Mutation Carrier: Confirmation of a Relationship by Immunohistochemical Analysis

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Abstract
Background: Muir-Torre syndrome (MTS) is an autosomal dominant disorder characterized by the concurrent or sequential development of at least 1 sebaceous gland tumor or keratoacanthoma and 1 or more internal malignancies. It is actually considered as a variant of hereditary nonpolyposis colorectal cancer (HNPCC) as both MTS and HNPCC are more often associated with germline mutations in the DNA mismatch repair (MMR) gene. Objective andMethods: We report the case of MTS diagnosed after the occurrence of a solitary subungual keratoacanthoma (SKA) in a man with a well-known family history of HNPCC and who is carrying a constitutional 1–7 deletion in the MSH2 MMR gene. Results: The link between the germline mutation and the skin tumor was reinforced by immunohistochemical staining. MSH2 immunoreactivity was decreased in SKA tumoral cells when compared to normal adjacent epidermis and to 5 cases of sporadic KA used as controls. Conclusion: This observation indicates that a solitary SKA may be the first clinical manifestation of MTS and brings up the relevance for regular dermatological screening for MTS-associated skin lesions among gene carriers (and symptomatic individuals) for HNPCC syndrome.