EXPRESSION OF THE TUMOR-SUPPRESSOR GENE-PRODUCT P16(INK4) IN BENIGN AND MALIGNANT MELANOCYTIC LESIONS

The gene MTS1 encodes p16(INK4), an inhibitor of cyclin-dependent kina se 4, and is frequently deleted, mutated, or silenced by promoter meth ylation in melanoma cells and in the germline of familial melanoma pat ients. Although MTS1 may thus be the candidate melanoma suppressor gen e that maps to chromosome 9p21, it is not clear how dysfunction at tha t locus temporally relates to melanoma progression. To further test it s role in sporadic melanoma, the expression of p16(INK4)-protein and - mRNA was characterized in melanomas and melanocytic nevi by immunocyto chemistry and in situ reverse transcriptase-polymerase chain reaction. Histologic tissue sections were immunolabeled with anti-p16(INR4) ant ibody for 108 melanocytic lesions, including common and atypical nevi, in situ melanomas, primary invasive melanomas, and metastatic tumors. A subset of the lesions was analyzed for expression of p16(INK4)-mRNA , employing forward and reverse intron-bridging primers for reverse tr anscriptase-polymerase chain reaction amplification of the transcript corresponding to exons 1 and 2 of MTS1. Strong immunolabeling was dete cted in the melanocytes of common nevi and of nevi with architectural disorder and cytologic atypia, By digital image analysis, in contrast, labeling intensity decreased significantly and progressively in the m elanocytes of in situ, invasive, and metastatic melanomas, Results fro m the in situ reverse transcriptase-polymerase chain reaction analysis were confirmatory, showing a strong signal in the melanocytic nevi bu t progressive signal attenuation with increasing stage of melanoma, Th ese data indicate correlation between gradual loss of expression of th e MTS1 locus and progression of melanoma, further supporting an emergi ng role for the gene in the malignant transformation of melanocytes, T he failure to demonstrate reduced expression in nevi suggests either t hat these lesions are not an early stage in melanoma development, in c ontrast to prevailing assumptions, or that loss of p16(INR4) function is not an initiating event in melanocyte transformation.