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Dokuz Eylül Üniv. Tıp Fak. Tıbbi Biyoloji Anabilim Dalı, İzmir The genetic changes that occur during the formation of benign melanocytic nevi and its transformation to malignant melanoma are consistent with the model of multistep mechanism underlying the tumorigenesis. Cytogenetic studies on melanocytic nevi are scarce. Chromosomal analyses performed on malignant melanoma specimens have revealed the involvement of chromosomes 1, 6, 9 and 11 in the form of various numerical and structural abnormalities. In this study, we attempted to investigate stepwise chromosomal changes that might take place during the transformation of benign nevus to malignant melanoma. Primary cell cultures were set up from the melanocytic nevi and malignant melanoma samples, excised from a total of 15 cases, including 6 melanocytic nevi, 9 malignant melanoma cases. The tumor tissues were enzymatically disaggregated using collagenase (200 U/ml) overnight. Cultures were incubated in humidified atmosphere at 37°C 5% CO2. Primary cultures were harvested after 10-15 days of growth. Karyotypes were determined by analysing at least 20 metaphases for each case. Chromosome abnormalities were described according to the ISC (1995). All the melanocytic nevi cases displayed normal karyotype. Among the malignant melanoma cases, two cases revealed clonal chromosomal changes. The karyotype in one of these cases was 46,XX, t(7;12) (q22;q24), t(2;12) (p25;q22). A supernumerary marker chromosome was detected in the other case. The breakpoints involved in the case with two reciprocal translocations were consistent with the previously reported cases. Recent cytogenetic molecular genetic studies have revealed a putative tumor suppressor gene located at the long arm of chromosome 11. Keywords :