Improved detection of aneuploidy in malignant melanoma using multiparameter flow cytometry for S100 protein and DNA...

DNA aneuploidy has been demonstrated to be an independent parameter of prognostic significance in malignant melanomas. In order to improve the detection of DNA aneuploidy in malignant melanomas, and to minimize diploid non-tumor cells in the sample, we developed a two-color staining strategy for S100 protein and DNA content in paraffin embedded samples. The ability to detect aneuploidy, defined as DNA ploidy index less than or equal to 0.90 or greater than or equal to 1.10, in 37 stage I malignant melanoma samples by flow cytometry analysis was significantly improved from 10.8% of cases using traditional one-color analysis for DNA content only (propidium iodide) to 32.4% of cases using two-color analysis for simultaneous measurement of both DNA (propidium iodide) and S100 protein (fluorescein conjugated antibody) (Chi-square with Yates' correction; p less than 0.05). The largest increase in sensitivity was found in level I and II melanomas less than or equal to 0.76 mm in thickness. In addition, we report the new observation that multiple S100 protein-positive subpopulations were found significantly more frequently in malignant melanomas (23/37 cases) than in compound melanocytic nevi (5/22 cases) (Chi-square with Yates' correction; p less than 0.01). These findings suggest that there is a previously unsuspected degree of tumor heterogeneity even in thin, presumably early, malignant melanomas.