The application of microsatellites in molecular pathology

Present within the genome are large numbers of seemingly unimportant DNA segments arranged in repetitive units. Furthermore, these stretches of DNA contain variations or polymorphisms that are characteristic for an individual and results in a unique DNA fingerprint. Approximately 30% of the DNA repeat sequences are arranged as short tandem repeat sequences, which are called microsatellites. Microsatellites may consist of 1, 2 or 3 nucleotides; dinucleotides being the commonest. Microsatellites are characterised by being: stably inherited and hence highly conserved from one generation to the next, and unique to an individual and the same in different cells from the same individual. As a result of the above features, microsatellites can be used for personal identification, population genetic analysis and construction of evolutionary trees. In addition, they are located in several important gene loci and this allows microsatellites to be used as markers of disease and to provide information about individual gene status, especially in tumors. This can be accomplished by assessing allelic imbalance or loss of heterozygosity of a particular gene by analysing microsatellites located at specific loci in the gene. Recently, mutations within microsatellites have been described as a result of defective DNA repair mechanisms, resulting in the phenomenon of microsatellite instability. This has been implicated in the aetiopathogenesis of several hereditary and non-hereditary conditions. There are several ways of analysing microsatellites, the popular using radioactively-labelled primers and autoradiography. This method has several drawbacks, especially the use of radioactivity and interpretative/technical problems. The use of fluorescently-labelled primers, automated DNA sequencing coupled with a computer software package obviates these problems. This technique has the added advantage of analysing several microsatellites in large numbers of cases, simultaneously. Thus, microsatellite analysis has become an important investigative tool for the molecular biologist and has provided new information in many diseases.