Lynch syndrome with exclusive skin involvement: time to consider a molecular definition?

Muir–Torre syndrome (MTS) is clinically characterized by the occurrence of skin, usually sebaceous, and visceral tumors in the same individual. The most common underlying mechanism is a constitutional defect of the mismatch repair (MMR) genes that cause Lynch syndrome (LS). Herewithin we report on a 76 years-old male patient heterozygous for a pathogenic MSH2 missense substitution who presented with a striking cutaneous phenotype in the absence of typical LS visceral tumors. The patient developed 20 skin tumors, including sebaceous adenomas/carcinomas and keratoacanthomas. Two skin tumors showed immunohistochemical loss of MSH2 and MSH6 expression. There was no apparent family history of neoplasia. Based on the variable involvement of the skin and internal organs, we suggest that the definition of tumor associations that are often observed as variants of inherited tumor syndromes, such as MTS, should be guided by the underlying molecular bases. In addition, the presence of multiple sebaceous tumors, especially if showing MMR deficiency, appears to be a very strong indicator of a constitutional MMR gene defect. The reasons underlying the high phenotypic variability of cutaneous phenotypes associated with constitutional MMR defects are yet to be determined.