Interphase cytogenetic demonstration of chromosome 9 loss in thick melanomas

It has been postulated that deletion of genes on chromosome 9 is important in the development of malignant melanoma. In this study, we have investigated this hypothesis by analysing the numerical complement of chromosomes 9, 17 and X by interphase cytogenetics using peri-centromeric repeat probes on paraffin sections from 15 thick melanomas. Three cases showed no relative loss or gain of chromosomes. Two cases showed gain of chromosome 17, and one case loss of chromosome 17 relative to chromosomes 9 and X. Relative chromosome 9 loss was identified in 9 cases (60%). Two of these were monosomic for chromosome 9 with a normal complement of chromosomes 17 and X and six were tetrasomic for chromosome 17 with duplication of chromosome X: chromosome 9 was disomic in five of these cases and trisomic in one. The final case showed loss of both chromosomes 9 and 17 relative to X. The chromosome patterns obtained imply that loss of chromosome 9 frequently takes place before tetraploidisation. This is in keeping with the hypothesis that loss of chromosome 9 is not a late event in melanocyte transformation. Extension of these studies to thin melanomas, in situ melanomas and dysplastic naevi will refine further the point at which these changes occur.