title: Silencing of p16/CDKN2 expression in human gliomas by methylation and chromatin condensation.
abstract: The product of the p16/CDKN2 locus, p16ink4, negatively regulates the cell cycle through binding and inactivation of cyclin-dependent kinases (CDKs) 4 and 6. This locus is frequently targeted for deletion in cell lines and primary tumor tissues. In gliomas, although up to 50% do not have detectable expression of p16/CDKN2 protein or mRNA, often the gene is wild type in sequence. Here, we tested the hypothesis that transcriptional repression of p16/CDKN2 in gliomas may be mediated by aberrant methylation of the CpG island, which is in the 5' region of the locus. Partial rather than complete p16/CDKN2 methylation was detected in 24% (10 of 42) of the gliomas, regardless of tumor grade, but was not observed in normal brain (0 of 10). We tested whether this partial methylation could inhibit expression in a human tumor cell line in which suppressed p16/CDKN2 expression was associated with both methylation and tightly compacted chromatin around the p16/CDKN2 promoter. Exposure of these cells to 5-aza-2-deoxycytidine resulted in a dramatic increase in promoter accessibility and induction of p16/CDKN2 expression, 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 result from structural changes in the chromatin containing the p16/CDKN2 locus.
authors.affiliation: Ludwig Institute for Cancer Research, University of California-San Diego, La Jolla, 92093-0660, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Costello
authors.forename: M S
authors.initials: MS
authors.lastname: Berger
authors.forename: H S
authors.initials: HS
authors.lastname: Huang
authors.forename: W K
authors.initials: WK
authors.lastname: Cavenee
mesh_terms: D001254:Astrocytoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D001374:Azacitidine / Q000031:analogs & derivatives / Q000494:pharmacology; D001483:Base Sequence; D001923:Brain Chemistry; D001932:Brain Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics; D002843:Chromatin / Q000235:genetics / Q000648:ultrastructure*; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000378:metabolism; D004273:DNA, Neoplasm / Q000737:chemistry / Q000235:genetics*; D000077209:Decitabine; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D005909:Glioblastoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D005910:Glioma / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D006801:Humans; D008745:Methylation; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis; D012334:RNA, Neoplasm / Q000096:biosynthesis; D012045:Regulatory Sequences, Nucleic Acid; D014407:Tumor Cells, Cultured / Q000187:drug effects
chemical_list: D002352:Carrier Proteins; D002843:Chromatin; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004273:DNA, Neoplasm; D009363:Neoplasm Proteins; D012333:RNA, Messenger; D012334:RNA, Neoplasm; D000077209:Decitabine; D004248:DNA (Cytosine-5-)-Methyltransferases; D001374:Azacitidine