Evaluation of 3 molecular-based assays for microsatellite instability detection in formalin-fixed tissues of patients with endometrial and colorectal cancers

Microsatellite instability (MSI) status is routinely assessed in patients with colorectal and endometrial cancers as it contributes to Lynch syndrome initial screening, tumour prognosis and selecting patients for immunotherapy. Currently, standard reference methods recommended for MSI/dMMR (deficient MisMatch Repair) testing consist of immunohistochemistry and pentaplex PCR-based assays, however, novel molecular-based techniques are emerging. Here, we aimed to evaluate the performance of a custom capture-based NGS method and the Bio-Rad ddPCR and Idylla approaches for the determination of MSI status for theranostic purposes in 30 formalin-fixed paraffin embedded (FFPE) tissue samples from patients with endometrial (n = 15) and colorectal (n = 15) cancers. All samples were previously characterised using IHC and Promega MSI Analysis System and these assays set as golden standard. Overall agreement, sensitivity and specificity of our custom-built NGS panel were 93.30%, 93.75% and 92.86% respectively. Overall agreement, sensitivity and specificity were 100% with the Idylla MSI system. The Bio-Rad ddPCR MSI assay showed a 100% concordance, sensitivity and specificity. The custom capture-based NGS, Bio-Rad ddPCR and Idylla approaches represent viable and complementary options to IHC and Promega MSI Analysis System for the detection of MSI. Bio-Rad ddPCR and Idylla MSI assays accounts for easy and fast screening assays while the NGS approach offers the advantages to simultaneously detect MSI and clinically relevant genomic alterations.