Microsatellite Instability: An Indirect Assay to Detect Defects in the Cellular Mismatch Repair Machinery

The DNA mismatch repair (MMR) pathway plays a prominent role in the correction of errors made during DNA replication and genetic recombination and in the repair of small deletions and loops in DNA. Mismatched nucleotides can occur by replication error, damage to nucleotide precursors, damage to DNA, or during heteroduplex formation between two homologous DNA molecules in the process of genetic recombination. Defects in MMR can precipitate instability in simple sequence repeats (SSRs), also referred to as microsatellite instability (MSI), which appears to be important in certain types of cancers, both spontaneous and hereditary. Variation in the highly polymorphic alleles of specific microsatellite repeats can be identified using PCR with primers derived from the unique flanking sequences. These PCR products are analyzed on denaturing polyacrylamide gels to resolve differences in allele sizes of more than 2 bp. Although (CA)_n repeats are the most abundant class among dinucleotide SSRs, trinucleotide and tetranucleotide repeats are also frequent. These polymorphic repeats have the advantage of producing band patterns that are easy to analyze and can be used as an indication of a possible MMR defect in a cell. The presumed association between such allelic variation and an MMR defect should be confirmed by molecular analysis of the structure and/or expression of MMR genes.