Novel microsatellite instability test of sebaceous tumours to facilitate low-cost universal screening for Lynch syndrome

BACKGROUND: One in five patients with sebaceous tumours (STs) may have Lynch syndrome (LS), an inherited disorder that increases the risk of developing cancer. Patients with LS benefit from cancer surveillance and prevention programmes and immunotherapy. While universal tumour mismatch repair (MMR) deficiency testing is recommended in colorectal and endometrial cancers to screen for LS, there is no consensus screening strategy for STs, leading to low testing rates and inequity of care. OBJECTIVES: To assess a low-cost and scalable sequencing-based microsatellite instability (MSI) assay, previously shown to enhance LS screening of colorectal cancers, for MMR deficiency detection in STs against the current clinical standard of immunohistochemistry (IHC). METHODS: Consecutive ST cases (n = 107) were identified from the records of a single pathology department. MMR protein IHC staining was interpreted by a consultant histopathologist. MSI analysis used amplicon sequencing of 14 microsatellites and a naive Bayesian classifier to calculate the sample MSI score. RESULTS: Loss of MMR protein expression was observed in 49/104 STs with interpretable IHC [47.1%, 95% confidence interval (CI) 37.3-57.2]. MMR deficiency was less frequent in carcinoma than in adenoma and sebaceoma (P = 4.74 × 10-3). The majority of MMR-deficient STs had concurrent loss of MSH2 and MSH6 expression. The MSI score achieved a receiver operator characteristic area under curve of 0.944 relative to IHC. Lower MSI scores were associated with MSH6 deficiency. CONCLUSIONS: These data support MSI testing as an adjunct or alternative to MMR IHC in STs. Integration of STs into established LS screening pathways using this high-throughput methodology could increase testing and reduce costs.