DIFFERENTIAL S100-BETA EXPRESSION IN CHOROIDAL AND SKIN MELANOMAS - QUANTITATION BY THE POLYMERASE CHAIN-REACTION

Purpose. S100beta, a member of a calcium-binding protein family (S100s ), is an important clinical marker for skin melanoma. In contrast, uve al melanomas appeared to express S100beta protein less frequently and to a lesser degree. This study was performed to verify and extend this finding to the mRNA level. Methods. A quantitative polymerase chain r eaction (PCR)-based method was used. A ratio, comparing the S100beta P CR fragment to that of beta-actin (an internal reference gene), was ge nerated to compare S100beta mRNA expression among samples. Results. Th e ratios for skin melanomas (1.2 to 3.9; three tissues and two cell li nes) were significantly higher than that for choroidal melanomas (0.1 to 0.63; seven of eight primary tumors and four of four cell lines). O nly one choroidal melanoma biopsy had a ratio greater than 1. The PCR products from choroidal melanoma were identical in size and sequence t o the S100beta, as determined by gel electrophoresis and RNA conformat ional polymorphism. Because the ratios were also low in choroidal mela noma cell lines, the S100beta phenotype appears to be genetically stab le. Conclusion. S100beta is differentially expressed at the RNA and pr otein levels by skin and choroidal melanomas, which are derived from d istinct populations of melanocytes. However, choroidal melanomas expre ssing little or no S100beta were significantly stained by antiserum sp ecific for the S100 protein family. Taken together, these data suggest that choroidal melanocytes express another, perhaps even novel, S100 protein(s).