Microsatellite instability in sarcoma: fact or fiction?

Abstract

Microsatellite instability (MSI) is a unique molecular abnormality, indicative of a deficient DNA mismatch repair (MMR) system. Described and characterized in the colorectal cancer literature, the MSI-positive phenotype is predictive of disease susceptibility, pathogenesis, and prognosis. The clinical relevance of MSI in colorectal cancer has inspired similar inquisition within the sarcoma literature, although unfortunately, with very heterogeneous results. Evolving detection techniques, ill-defined sarcoma-specific microsatellite loci and small study numbers have hampered succinct conclusions. The literature does suggest that MSI in sarcoma is observed at a frequency similar to that of sporadic colorectal cancers, although there is little evidence to suggest that MSI-positive tumors share distinct biological attributes. Emerging evidence in Ewing sarcoma has demonstrated an intriguing mechanistic role of microsatellite DNA in the activation of key EWS/FLI-target genes. These findings provide an alternative perspective to the biological implications of microsatellite instability in sarcoma and warrant further investigation using sophisticated detection techniques, sensitive microsatellite loci, and appropriately powered study designs.

Figure 1

(a) Microsatellite instability was originally assessed using gel electrophoresis and autoradiographic detection. In the left panel, additional bands (black arrows) in the tumor lane illustrate multiple contracted microsatellite alleles relative to the genomic control lane. In the right panel, an information (heterozygous) microsatellite is shown in the genomic control sample, and a significant loss of signal intensity for the smaller allele is observed in the tumor sample, characteristic of allelic imbalance/loss of heterozygosity (LOH). (b) Microsatellite loci are now commonly assessed using fluorescent PCR amplifications, capillary electrophoresis, and automated sequencing techniques. Laser scanners detect fluorescent PCR products and generate a chromatogram displaying microsatellite allele frequencies. Note in the tumor panel, one of the alleles has undergone contraction, depicting MSI in this tumor specimen. (c) Subcloning and direct sequencing of microsatellite amplifications can yield high resolution of the microsatellite sequence and can detect subtle changes in microsatellite constitution. This method of analysis is more time consuming, although programmed bioinformatics software can greatly assist the interpretation of high-volume data. Panel (b) modified with permission from Vilar and Gruber [57].