Idylla MSI test as a new tool for microsatellite instability detection in sebaceous tumours and keratoacanthomas.

Aim Sebaceous tumours and keratoacanthomas can be associated with mismatch repair (MMR) deficiency and thus microsatellite instability (MSI). In such tumours, MSI phenotype could be an argument to search for an underlying Muir-Torre syndrome (MTS). MTS has been recognised as a variant of Lynch syndrome, characterised by a deficiency of the MMR proteins. In Lynch syndrome, the sensitivity and specificity of the techniques used to detect MSI is well described, which is not the case for skin tumours. In our hands, immunohistochemistry is a sensitive and specific method to detect MMR deficiency in those tumours. Contrasting with tumours of Lynch spectrum, sensitivity and specificity of molecular methods has not been extensively studied. This study aimed at evaluating two molecular methods to detect MSI phenotype in MTS associated tumours: a commonly used pentaplex PCR using Bethesda markers and the fully automated method using the Idylla MSI assay. Methods A comparison between PCR, and Idylla was performed on 39 DNA extracted from cutaneous tumours. Immunohistochemistry was used as the gold standard to calculate sensitivity and specificity of both molecular techniques. Results Concordant results were found in 32 cases (82%) with pentaplex PCR and in 36 cases (92%) with Idylla. The sensitivity of pentaplex PCR to detect MSI phenotype was 76% whereas Idylla sensitivity was 90%. Conclusion Idylla is more performant than PCR, for the detection of MSI in MTS-associated tumours and is a reliable additional technique to help detecting MTS in these tumours.