Microsatellite Instability and Colorectal Cancer

Context.-About 15% of colorectal cancers are characterized by genomic microsatellite instability, and of these, about 1 in 5 (2%-4% overall) are due to Lynch syndrome, a dominantly inherited condition predisposing the patient to cancers of multiple organ systems, including the gastrointestinal tract. Identification of individuals with Lynch syndrome allows for increased surveillance of the affected individual and of potentially affected family members. Objective.-To review the literature on microsatellite instability in colorectal cancer and current laboratory diagnostic testing strategies for the detection of Lynch syndrome. Data Sources.-This review is based on peer-reviewed literature, published guidelines from professional organizations (Evaluation of Genomic Applications in Practice and Prevention Working Group, National Comprehensive Cancer Network), and information from clinical laboratories performing microsatellite instability testing. Conclusions.-Universal screening for Lynch syndrome in all individuals affected with colorectal cancer has been recommended by the Evaluation of Genomic Applications in Practice and Prevention Working Group. Preliminary screening tests can identify individuals unlikely to be affected by Lynch syndrome, thereby reducing the need for full gene analysis. Immunohistochemistry and polymerase chain reaction-based tests for microsatellite instability have similar clinical sensitivity and specificity, and each method has advantages and limitations. BRAF and MLH1 methylation testing are useful reflex tests for those with a defect in MLH1 identified by immunohistochemistry. Emerging technologies, such as high-throughput sequencing, may substantially affect diagnostic algorithms in the future. (Arch Pathol Lab Med. 2011;135:1269-1277; doi: 10.5858/arpa.2011-0035-RA)