Differential S100β expression in choroidal and skin melanomas: Quantitation by the polymerase chain reaction

Purpose. S100β, a member of a calcium-binding protein family (S100s), is an important clinical marker for skin melanoma. In contrast, uveal melanomas appeared to express S100β 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 reaction (PCR)-based method was used. A ratio, comparing the S100β PCR fragment to that of β-actin (an internal reference gene), was generated to compare S100β mRNA expression among samples. Results. The ratios for skin melanomas (1.2 to 3.9; three tissues and two cell lines) were significantly higher than that for choroidal melanomas (0.1 to 0.63; seven of eight primary tumors and four of four cell lines). Only one choroidal melanoma biopsy had a ratio greater than 1. The PCR products from choroidal melanoma were identical in size and sequence to the S100β, as determined by gel electrophoresis and RNA conformational polymorphism. Because the ratios were also low in choroidal melanoma cell lines, the S100β phenotype appears to be genetically stable. Conclusion. S100β is differentially expressed at the RNA and protein levels by skin and choroidal melanomas, which are derived from distinct populations of melanocytes. However, choroidal melanomas expressing little or no S100β were significantly stained by antiserum specific for the S100 protein family. Taken together, these data suggest that choroidal melanocytes express another, perhaps even novel, S100 protein(s).