Myxofibrosarcoma harboring an MLH1 pathogenic germline variant associated with Muir-Torre syndrome: a case report.

Background: Muir-Torre syndrome (MTS), which accounts for a small subset (1-3 %) of Lynch syndrome (LS), is an autosomal dominant genetic disorder characterized by sebaceous gland or keratoacanthoma associated with visceral malignancies. Most families with MTS have pathogenic germline variants (PGV) in MSH2. Sarcomas are not common on the LS tumor spectrum, and sarcomas associated with MTS are extremely rare.

Case Presentation: Here we report a myxofibrosarcoma of the abdominal wall in a 73-year-old man with a sebaceoma that occurred synchronically, leading to a diagnosis of MTS. The loss of MLH1 and PMS2 protein expression was detected in immunohistochemistry, and high-frequency microsatellite instability (MSI-H) was also confirmed. A germline genetic analysis revealed that he harbored the MLH1 PGV.

Conclusions: This is the first case of MSI-H myxofibrosarcoma with MTS in an MLH1 PGV carrier. Although rare, we should recognize that sarcomas can be part of the spectrum of LS and MTS.