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The DNA Mismatch Repair System in Sebaceous Tumors: An Update on the Genetics and Workup of Muir-Torre Syndrome

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Types of MTS

For most cases, MTS is characterized by germline mutations in DNA MMR genes. The most common mutation involves MSH2 (MutS Homolog 2)—found in approximately 90% of patients—followed by MLH1 (MutL Homolog 1)—found in approximately 10% of patients.22 Other MMR genes such as MSH6 (MutS Homolog 6), PMS2 (PMS1 homolog 2, mismatch repair system component), and MLH3 (MutL Homolog 3) less commonly are reported in MTS. There is a subset of patients who lose MSH2 or MLH1 expression due to promoter hypermethylation rather than a germline mutation. Methylation results in biallelic inactivation of the gene and loss of expression.23

A new subtype of MTS has been identified that demonstrates an autosomal-recessive pattern of inheritance and is referred to as MTS type 2 (autosomal-recessive colorectal adenomatous polyposis).24 In contrast to the classic MTS type 1, MTS type 2 exhibits microsatellite stability. Recent molecular analyses revealed that type 2 is due to a mutation in a base excision repair gene called MUTYH (mutY DNA glycosylase).25 These patients are likely to develop hundreds of polyps at an early age.

Muir-Torre syndrome also can occur sporadically without inheriting a germline mutation, which has been reported in a transplant patient from de novo somatic mutations or promoter hypermethylation.26 A case report of a renal transplant patient showed that switching from tacrolimus to sirolimus halted the appearance of new sebaceous neoplasms, which suggests that patients with MTS who undergo organ transplantation should potentially avoid tacrolimus and be put on sirolimus instead.27

Visceral Malignancies in MTS

Apart from frequent skin examinations, MTS patients should have frequent and rigorous visceral malignancy screening. Patients most commonly develop colorectal adenocarcinoma, especially in the proximal parts of the colon.28 In addition, they can develop numerous premalignant tumors, especially in MTS type 2. Other common tumors include endometrial, ovarian, genitourinary, hepatobiliary, breast, lung, hematopoietic, and CNS malignancies.29

Studies showed that specific loss of certain MMR proteins predispose patients to different types of visceral malignancies.30-32 For example, loss of MSH2 predisposes patients to development of extracolonic tumors, while loss of MLH1 more strongly is associated with development of colorectal adenocarcinoma.30 Patients with MSH2 also are at risk for development of CNS tumors, while patients with MLH1 mutations have never been reported to develop CNS tumors.31 Patients with loss of PMS2 have the lowest risk for development of any visceral malignancy.32

Diagnosing MTS

Let us consider a scenario whereby a dermatologist biopsied a solitary lesion and it came back as a sebaceous tumor. What would be the next step to establish a diagnosis of MTS?

Sebaceous tumors are rare outside the context of MTS. Therefore, patients presenting with a solitary sebaceous tumor should be worked up for MTS, as there are implications for further cancer screening. One helpful clue that can affect the pretest probability for MTS diagnosis is location of the tumor. A sebaceous tumor inferior to the neck most likely is associated with MTS. On the other hand, tumors on the head and neck can be spontaneous or associated with MTS.33 Another helpful tool is the Mayo score, a risk score for MTS in patients with sebaceous tumors.34 The score is established by adding up points, with 1 point given to each of the following: age of onset of a sebaceous tumor less than 60 years, personal history of visceral malignancy, and family history of Lynch syndrome–related visceral malignancy. Two points are given if the patient has 2 or more sebaceous tumors. The score ranges from 0 to 5. A risk score of 2 or more has a sensitivity of 100% and specificity of 81% for predicting a germline mutation in MMR genes.34

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