Comparison of the expression of vimentin and actin in spitz nevi and spitzoid malignant melanomas

The differentiation between Spitz nevi (SN) and Spitzoid malignant melanomas (SMM) represents a challenge to dermatopathologists. We recently demonstrated differential expression of vimentin and Actin in SN and SMM by mass spectrometry (MS). We sought to investigate whether this differential expression could be detected using conventional immunohistochemistry or automated quantitative analysis (AQUA) of histological sections.

Cases of SN and SMM, which were previously studied by MS and have readily available blocks and enough material in the block, were selected from the Yale Spitzoid Neoplasm Repository. The cases were stained for vimentin and muscle-specific actin using standard protocols. H-scores were calculated by multiplying the percentage of cells staining and the intensity of staining. Selected cases were also studied for quantitative immunofluorescent staining using the AQUA method.

All 21 cases of SN showed strong and diffuse staining for vimentin; 19 of 21 (91%) cases had an H-score of 300 (average, 294). Similar staining results were observed in SMM; 13 of 14 (93%) cases had an H-score of 300 (average, 297). Muscle-specific actin was weakly and focally positive in 5 of 21 (24%) SN (H-score = 3.3) and 5 of 14 (39%) SMM (H-score = 3.5). The AQUA method showed no significant difference in the staining intensity of SN and SMM for both vimentin and actin.

There was no difference in the expression of vimentin and actin in SN and SMM shown by conventional immunohistochemistry or the AQUA method. This study shows that MS has much grater sensitivity in detecting the differential expression of these proteins.