Overview

The Molecular Diagnostics Laboratory provides Microsatellite Instability (MSI) analysis using the Promega kit MSI Analysis System, v1.2 and custom-based primers and targets. 

Microsatellite instability (MSI) is a phenomenon commonly observed in the DNA of tumor tissues that results from mutations causing inactivation of any of the genes in the DNA mismatch repair pathway. Mismatch repair is an important mechanism for correcting nucleotide insertions and deletions that can occur during DNA replication. Defective mismatch repair leads to genomic instability with a hypermutable phenotype. Microsatellites, which are short, tandem repeating DNA sequences of 1-6 nucleotides scattered throughout the genome, are highly susceptible to this instability in the presence of defective mismatch repair.  

To evaluate DNA for MSI, the Molecular Diagnostics Lab (MDL) utilizes the Promega MSI Analysis System, v1.2, which employs a multiplex PCR of five mononucleotide and two pentanucleotide repeats using fluorescent primers. The MDL can also test for the presence of MSI using custom DNA microsatellite markers amplified with corresponding fluorescent primers. For custom MSI analysis please contact the laboratory director Sig Verselis ([email protected]) to discuss experimental details.

The fluorescent amplicons are separated and analyzed by capillary electrophoresis on an Applied Biosystems 3130xl 16-capillary genetic analyzer. Applied Biosystems GeneMapper v4.0 software is used to detect the marker alleles and assign sizes in base pairs. Typically, the allelic profiles of amplified DNA from matching normal and test samples are compared. The detection of alleles in the test sample that are absent in the matching normal sample is an indication of MSI.
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MSI analysis serves as a useful prognostic and therapeutic predictive biomarker for several types of solid tumors. 12-15% of colorectal cancers examined exhibit MSI along with better patient survival and different therapy responses compared to microsatellite stable tumors. However, patients with certain types of MSI positive endometrial cancers have poorer outcomes. MSI-high tumors are also found in gastrointestinal cancers, genitourinary, breast and thyroid cancers among others. Recently, MSI-high status of tumors predicts higher response rates to immunotherapy with immune checkpoint inhibitors.

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