Histopathology Department, Hospital Pathology Service, Queen Mary Hospital

Microsatellites are short tandem repeats of one to four nucleotides scattered throughout the genome. Since microsatellites are flanked by unique sequences, they can be amplified by PCR using specific primers. These features of microsatellites, together with its highly polymorphic nature, make PCR based analysis of microsatellites a very useful tool in the study of tumours. For example, microsatellite instability or replication error (RER) is a diagnostic marker for Hereditary Non-Polyposis Colorectal Cancer (HNPCC). HNPCC is a genetic disease resulted from mutations of DNA Mismatch Repair (MMR) genes. Inactivation of MMR genes will cause errors in DNA replication which may manifest as RER in the colorectal tumours of HNPCC patients. To determine whether or not a tumour has RER, DNA was extracted from both normal and tumour tissues and then PCR amplified using microsatellite specific primers. In this study, a total of 54 colorectal tumours from Queen Mary Hospital were analysed with 5 microsatellite loci, namely, TP53, D2S123, D18S58, BAT26 and BAT4O. DNA was extracted from paraffin blocks and amplified in the presence of 32P labelled nucleotides. After amplification, the PCR products were electrophoresed on denaturing polyacrylamide gel. The gel was then fixed, dried and autoradiographed. RER is interpreted when the microsatellite PCR bands from the tumour tissue showed a change in pattern compared to that of the normal tissue. Of the 54 cases analysed, 10 were shown to have RER. Some of these 10 patients were later shown to have familial history of colorectal cancer and/or germline MMR gene mutations.

Apart from RER, microsatellite analysis can also be used to detect the Loss of Heterozygosity (LOH) of nearby tumour suppressor genes. The detection of LOH in microsatellite loci can sometimes be used to confirm the presence of tumour cells in clinical specimens.

Thus, microsatellite analysis can be applied in various aspects of tumour diagnosis, clinical management, screening and monitoring of treatment.

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