Molecular Pathology Laboratory Now Offers Testing for Microsatellite Instability
This is to bring to the attention of the medical community of the University of Iowa that the Molecular Pathology division of the Clinical Laboratories has validated and introduced a new test for Microsatellite Instability.
Clinical indications for the test, based on the revised Bethesda criteria (Umar A et al.) are:
• Colorectal cancer (CC) diagnosis before age 50
• Synchronous or metachronous colorectal or other Hereditary Nonpolyposis Colorectal Carcinoma (HNPCC)-related tumors regardless of age
• MSI-H morphology diagnosed before age 60
• CC of the proband and one or more first-degree relatives with CC or other HNPCC-related tumors; one of the cancers must have been diagnosed before
• CC of the proband and two or more relatives with CC or other HNPCC-related tumors, regardless of age
Fulfillment of any one of these criteria is sufficient to order the test.
The method entails multiplex PCR amplification of five microsatellite markers recommended by the National Cancer Institute (NCI).
Mononucleotide repeats: BAT25 and BAT26
Dinucleotide repeats: D2S123, D5S346, D17S250
The amplification is performed on the patients normal and tumor tissue in parallel. The primers are specially designed to work on formalin fixed paraffin embedded tissue samples, however, they work equally well on fresh tissue. The products of the amplification are sized and compared by capillary electrophoresis using the ABI3100 sequencer. The diagnosis of Microsatellite Instability High (MSI-H) is given if 2 or more of the microsatellite markers tested show variation in size between the normal and the tumor tissue. The expected turnaround time for the test is a week.
MSI testing is not considered a genetic test. It detects a phenotype that can be the consequence of a heritable mutation or a somatic silencing event resulting in the absence of functional mismatch repair (MMR) gene products. Thus, the results need to be interpreted in the context of the patient’s family history. Additional testing aimed at differentiating between a heritable and a non-heritable cause might be necessary. Such testing includes immunohistochemical studies (IHC) and subsequent screening for mutations in the defective gene by denaturing high pressure liquid chromatography (dHPLC) and direct sequencing. These tests are planned to be brought up in the laboratory depending on demand.
Questions and comments concerning the test are to be addressed to Peter L. Nagy, M.D., Ph.D. Tel: 353-4594 or by e-mail: firstname.lastname@example.org.
Umar A et al. Revised Bethesda Guidelines for hereditary nonpolyposis colorectal cancer (Lynch syndrome) and microsatellite instability. J Natl Cancer Inst 2004; 96: 261-8.