An association between sebaceous carcinoma and microsatellite instability in immunosuppressed organ transplant recipients.

Sebaceous carcinomas are rare cutaneous appendageal tumors that may occur sporadically or in association with an internal malignancy in Muir-Torre syndrome. In Muir-Torre syndrome microsatellite instability can often be demonstrated in tumor DNA as a result of an inherited mutation in one of several known mismatch repair genes; however, the role of microsatellite instability in sporadic sebaceous carcinomas has not been previously studied. In this report we describe the clinicopathologic characteristics of a series of unselected sebaceous carcinomas and examine them for the presence of microsatellite instability. Of 10 consecutive tumors identified over a 10 y period, only one was from a patient known to have Muir-Torre syndrome. Of the nine presumed sporadic cases, five were from four renal transplant recipients and four from otherwise healthy individuals. Microsatellite instability was demonstrable in three cases: in the Muir-Torre syndrome-associated tumor and in two tumors from transplant patients. Microsatellite instability was subsequently also found in a sebaceous carcinoma from a further transplant patient prospectively sought from another institution. The presence of microsatellite instability in post-transplant sebaceous carcinomas was associated with loss of expression of the mismatch repair protein hMSH2. In summary, sebaceous gland carcinomas, while characteristic of Muir-Torre syndrome, are commonly found outside this context. Among presumed sporadic cases, our data suggest they may be over-represented in immunosuppressed renal transplant recipients. The presence of microsatellite instability in transplant-associated lesions, together with loss of hMSH2 expression suggests that immunosuppression might unmask a previously silent Muir-Torre syndrome phenotype in some cases. Alternatively, there is experimental evidence to suggest that immunosuppressive drugs, most plausibly azathioprine, could select for the emergence of a mutator phenotype and thus predispose to the development of sebaceous carcinomas. The role of mismatch repair defects in other post-transplant skin malignancies remains to be established.