Microsatellite instability in malignant melanomas

Microsatellite instability (NISI) is caused by deficient DNA mismatch: repair, and results in a "mutator" phenotype. Recent studies have produced contradictory results about the frequency and significance of MSI in malignant melanomas. In this study, we therefore determined the time of onset and relative frequency of MSI during the progression of melanocytic tumours, including benign melanocytic naevi. We examined 7 different microsatellite loci in 9 melanocytic nae-vi, 25 primary malignant melanomas and 8 melanoma metastases. None of the melanocytic naevi showed MSI. In contrast, moderate frequency of NISI in 1/12 (8%) was detected in thin melanomas of <0.75mm vertical 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) and in melanoma metastases, with over 25% (2/8) of the lesions investigated. We conclude that NISI occurs in a considerable subset of malignant melanomas and that there is a pattern consistent with increasing frequency of NISI with progression of melanocytic tumours.