# MSI expresso: a software for determining MSI status and detecting MSI-related transcription events from RNA sequencing data

## TL;DR

MSI Expresso is a new software that detects microsatellite instability (MSI) from RNA sequencing data and identifies related transcription events, offering insights into how MSI affects gene expression.

## Contribution

MSI Expresso is the first tool to determine MSI status and detect MSI-related transcription events using RNA-sequencing data.

## Key findings

- MSI Expresso achieved nearly 100% concordance in determining MSI status from RNA-seq data of colon, prostate, and endometrial cancer samples.
- The software identifies exon skipping, unstable coding and intronic microsatellites, and provides graphical outputs of recurrent events.
- MSI Expresso complements genomic approaches by exploring the transcriptomic consequences of MSI events.

## Abstract

Summary: Microsatellite instability (MSI) is becoming increasingly important in oncology as it has been reported across more than two dozen of solid cancer types. The MSI-high phenotype has long been used as a predictive and prognostic marker in colorectal cancer and has been recently approved by the FDA as a marker for immune checkpoint blockade therapy for solid cancers. Several bioinformatics tools have been developed to assess MSI status of a tumor sample using Next-Generation Sequencing (NGS) data mostly from whole genome, whole exome, and targeted gene sequencing data. While most tools available only infer the MSI status, none of them use RNA-sequencing (RNA-seq) data and provide per microsatellite expression and genotype results. We present MSI Expresso, a software which assesses the MSI status by testing the instability of a panel of 3′UTR microsatellites from RNA-seq data and also provides a detailed landscape of MSI-related events such as exon skipping, unstable coding and intronic microsatellites with a graphical output of the recurrent events. MSI Expresso’s ability to detect the MSI status was assessed from RNA-seq data of 228 colon, 13 prostate and two endometrial cancer samples with known MSI status and achieved almost 100% concordant results. Thus, MSI Expresso is a new tool for MSI detection from RNA-sequencing data complementary to genomic and genetic approaches allowing to explore the consequence of MSI events on transcripts/transcriptome.