Hypermethylation of the p16/CDKN2A/MTS1 gene and loss of protein expression is associated with nonfunctional pituitary adenomas but not somatotrophinomas

The cyclin-dependent kinase inhibitor 2A/multiple tumor suppressor gene I ( CDKN2A/MTSI/p16) plays an important role in the control of progression from G(1) to S-phase of the cell cycle through the inhibition of CDK4-mediated RBI phosphorylation. In this study we investigated 46 nonfunctional pituita ry tumors and 21 somatotrophinomas for aberrant methylation of the CpG isla nd contained within the CDKN2A gene as an alternative mechanism of gene sil encing. We demonstrate methylation in 32/46 (70%) of nonfunctioning tumors, in contrast to 2/21 (9.5%) somatotrophinomas and 0/15 histologically norma l postmortem pituitaries. Methylation in noninvasive and invasive nonfuncti onal tumors was approximately equal at 15/20 (75%) and 17/26 (65%), respect ively. Immunohistochemical analysis showed an absence of CDKN2A protein in 25/32 (78%) methylated nonfunctioning tumors, demonstrating a highly signif icant overall correlation (P = 0.00007) between hypermethylation of the gen e and absence of the p16 protein. The association between hypermethylation and absence of CDKN2A protein remained when the cohort of nonfunctional tum ors was further subdivided into noninvasive 12/15 (80%; P = 0.004) and inva sive 13/17 (76%; P = 0.01), suggesting this to be an early event in pituita ry tumorigenesis. In contrast, a single invasive methylated somatotrophinom a failed to express the CDKN2A protein. These data show that hypermethylati on of the CpG island within exon I, but not exon 2, of the CDKN2A gene is f requently associated with loss of protein expression in nonfunctional pitui tary tumors, but not somatotrophinomas, suggesting different tumorigenic pa thways. Genes Chromosomes Cancer 24:328-336, 1999. (C) 1999 Wiley-Liss, Inc .