Microsatellite instability in malignant melanomas

Microsatellite instability (NISI) is caused by deficient DNA mismatch: repa ir, and results in a "mutator" phenotype. Recent studies have produced cont radictory results about the frequency and significance of MSI in malignant melanomas. In this study, we therefore determined the time of onset and rel ative frequency of MSI during the progression of melanocytic tumours, inclu ding benign melanocytic naevi. We examined 7 different microsatellite loci in 9 melanocytic nae-vi, 25 primary malignant melanomas and 8 melanoma meta stases. None of the melanocytic naevi showed MSI. In contrast, moderate fre quency of NISI in 1/12 (8%) was detected in thin melanomas of <0.75mm verti cal thickness and in 1/8 (12%) of those with a thickness >0.75 non and <1.5 mm. The rate of NISI was increased in tumours thicker than 1.5 mm (2/5) an d in melanoma metastases, with over 25% (2/8) of the lesions investigated. We conclude that NISI occurs in a considerable subset of malignant melanoma s and that there is a pattern consistent with increasing frequency of NISI with progression of melanocytic tumours.