Microsatellite instability status is determined by targeted sequencing with MSIcall in 25 cancer types

Microsatellite instability (MSI) occurs in solid tumors and is a predictive biomarker for remarkable response to immune checkpoint inhibitors. Detection of MSI status has been conventionally conducted by PCR-electrophoresis-based assay (MSI-PCR) and immunohistochemistry (IHC) of mismatch repair proteins. However, these approaches require visual confirmation and involve some difficulties in determining MSI statuses from equivocal results.

We performed amplicon-based targeted sequencing of 76 microsatellite loci (MSI-NGS) in 184 formalin-fixed paraffin-embedded (FFPE) tumor tissues and baseline control samples. A bioinformatics tool, MSIcall, was used to calculate the quantitative values based on the aligned sequence reads and evaluated MSI status. Furthermore, we examined the concordance between the results from MSI-NGS and MSI-PCR/IHC. Diagnostic accuracy, sensitivity, and specificity were estimated by receiver operating characteristic (ROC) curve analysis. For validation cohort, we studied additional 50 tumor samples to determine the MSI status.

Of 184 tumor samples, MSI-PCR and IHC analysis classified 161 tumors as MSS/pMMR and 23 as MSI-H/dMMR. Using MSI-NGS combined with MSIcall, we predicted MSI status with high accuracy (98.9%), specificity (91.3%), and sensitivity (100%) in 25 types of cancers. This method achieved an area under the ROC curve (AUC) value of 0.9986. Furthermore, we achieved the 100% concordant results using additional 50 samples for validation.

We demonstrated newly developed MSI-NGS with MSIcall accurately determines the MSI status of FFPE tumor tissues thorough sequencing of tumor samples alone without patient-matched normal controls. This approach can be applied to all types of solid tumors to determine responders to immune-oncology therapy.