A case report A 56-year-old patient presents with a smooth, round, slightly scaly lump on the forehead, 2mm in maximum dimension. The clinical diagnosis was query basal cell carcinoma and it was excised. Microscopic findings were of sebaceoma and following immunohistochemistry staining shows loss of nuclear positivity of the DNA mismatch repair enzymes MSH2 and MSH6, which is usually associated with high-degree microsatellite instability (MSI), and raises the possibility of Muir Torre syndrome associated tumour. The patient underwent investigations, including colonoscopy, and was found to have a fungating lesion in the sigmoid colon. The patient had a left-sided hemicolectomy and the microscopic findings were of a moderately differentiated adenocarcinoma. It also has the same loss of nuclear positivity of the DNA mismatch repair enzymes MSH2 and MSH6, which further raises the possibility of Muir Torre syndrome associated tumour. Discussion Not all skin cancers are straight-forward and some can be associated with internal malignancies or visceral cancers. Skin cancers can be syndromic; they can occur together and characterise a particular abnormality or condition and Muir Torre syndrome (MTS) is a good example. Syndromes are mostly inherited as autosomal dominant traits. MTS is characterised by the development of sebaceous tumours, often multiple, in association with visceral neoplasms, usually gastrointestinal carcinomas. MTS is found as a variant of the autosomal dominant disorder, hereditary non-polyposis colorectal cancer (HNPCC), with tumours demonstrating microsatellite instability (MSI) and germline mutations in the DNA mismatch repair genes MutS homolog MSH2 and MLH1. Muir Torre syndrome This syndrome was first noted by Muir et al in 1967 and Torre in 1968 and is defined by the occurrence of a sebaceous neoplasm and internal malignancy in the absence of other predisposing factors. Cancers of the gastrointestinal and genitourinary tracts are the most common, with colorectal cancers often occurring at or proximal to the splenic flexure, contrary to most sporadic colorectal cancers. The skin lesions which develop in MTS include sebaceous adenoma, sebaceoma and sebaceous carcinoma. Multiple keratoacanthomas (with or without areas of sebaceous differentiation) are seen in some cases and reticulated acanthoma with sebaceous differentiation. Multiple sebaceous tumours and sebaceous tumours occurring before the age of 50 years are strong indicators of the syndrome. The cutaneous tumours may precede or follow the direct manifestation of the visceral cancer and may occur sporadically in family members. The visceral tumours behave less aggressively than would be expected from the histologic findings and this is particularly true for tumours with MSI. Detection of MSI in cutaneous neoplasms may form the basis of a non-invasive screening technique for hereditary non-polyposis colon syndrome (also known as Lynch syndrome), of which the Muir Torre syndrome is regarded as an allelic variant and represents 1-2% of cases with Lynch syndrome. MTS is inherited as an autosomal dominant trait. Mutations in one of the DNA mismatch repair genes MLH1, MSH2, MSH6 and PMS2 have been found in these patients. Recommendations in Muir Torre syndrome - Consider MTS in patients presenting with a sebaceous neoplasm. Immunohistochemistry examination of tumours for MLH1 and MSH2 protein can be used as a screening test to identify patients. -Individuals with or at risk of MTS or HNPCC should have: 1) Colonoscopy every 1-2 years, beginning at age 20–25, or 10 years younger than the youngest age at diagnosis in the family is strongly recommended. 2) Annual history and physical examination, including a complete skin examination and urinalysis, as well as periodic endometrial sampling and/or transvaginal ultrasound for women.

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General Practice Pathology is a new regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial, free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.