SILENCING OF P16 CDKN2 EXPRESSION IN HUMAN GLIOMAS BY METHYLATION ANDCHROMATIN CONDENSATION/

The product of the p16/CDKN2 locus, p16(ink4), negatively regulates th e cell cycle through binding and inactivation of cyclin-dependent kina ses (CDKs)4 and 6. This locus is frequently targeted for deletion in c ell lines and primary tumor tissues. In gliomas, although up to 50% do not have detectable expression of p16/CDKN2 protein or mRNA, often th e gene is wild type in sequence. Here, we tested the hypothesis that t ranscriptional repression of p16/CDKN2 in gliomas may be mediated by a berrant methylation of the CpG island, which is in the 5' region of th e locus, Partial rather than complete p16/CDKN2 methylation was detect ed in 24% (10 of 42) of the gliomas, regardless of tumor grade, but wa s not observed in normal brain (0 of 10). We tested whether this parti al methylation could inhibit expression in a human tumor cell line in which suppressed p16/ CDKN2 expression was associated with both methyl ation and tightly compacted chromatin around the p16/CDKN2 promoter. E xposure of these cells to 5-aza-2-deoxycytidine resulted in a dramatic increase in promoter accessibility and induction of p16/CDKN2 express ion, indicating that chromatin structure, CpG island methylation, and p16/CDKN2 expression are intimately associated. Taken together, these data suggest that methylation occurs in only a subset of cells within gliomas and that the methylation-associated inactivation of p16/CDKN2 expression observed in many common human cancers may mechanistically r esult from structural changes in the chromatin containing the p16/CDKN 2 locus.