High frequency of tetraploidy detected in malignant melanoma of Japanese patients by fluorescence in situ hybridization

Aneuploidy and hyperploidy are often detected in malignant melanoma by cyto genetic analysis and flow cytometric analysis of DNA content. To determine the ploidy of cells in surgical specimens of melanin-producing tumors of Ja panese patients, we performed fluorescence in situ hybridization (FISH) usi ng touch smear technique to count the number of chromosomes 18 and X + Y in interphase nuclei using a-satellite DNA probes, D18Z1, DXZ1 and DYZ3. A no rmal melanocyte strain showed two D18Z1 and two [DXZ1+DYZ3] signals per nuc leus, indicating 2N, and a malignant melanoma cell line showed 4 per nucleu s, indicating 4N, consistent with results of cytogenetic and flow cytometri c analyses. Therefore we employed this FISH method to analyze ploidy of sur gical specimens. Specimens obtained from 8 patients with nevus cell nevus s howed 2 FISH signals per nucleus. On the other hand, in all specimens obtai ned from 8 patients with malignant melanoma (6 primary and 2 metastatic mel anoma), 65-90% of cells exhibited 4 signals per nucleus, indicating 4N. His topathologically, 50-70% of cells were identified as malignant melanoma cel ls, indicating that our FISH method is effective to detect melanoma cells i n tissue. We also analyzed allelic loss of the p53 gene by FISH with a p53 locus-specific probe and mutation of the p53 gene by immunostaining since m utation and deletion of the p53 gene may cause hyperploidy. All specimens e xcept one obtained from a case with young-onset metastatic melanoma exhibit ed no allelic losses or negative p53 staining, showing the p53 gene was int act. These results indicate that tetraploidy, not caused by p53 mutation or deletion, is commonly found in malignant melanoma of Japanese patients. It is also suggested that there is no positive relationship between tetraploi dy and poorer prognosis, and mutation and allelic loss of the p53 gene migh t be markers of aggressive form of malignant melanoma.