HYPERMETHYLATION CAN SELECTIVELY SILENCE INDIVIDUAL P16(INK4A) ALLELES IN NEOPLASIA

Inactivation of p16(ink4A) and other tumor suppressor genes has been a ssociated with promoter region hypermethylation in neoplasia. However, direct proof for aberrant DNA methylation as an independent event for loss of gene function has been difficult to obtain. We addressed this question in the colon carcinoma cell line HCT116, which contains one allele of p16(ink4A) with a coding region frameshift mutation and one wildtype allele, Neither allele contains a mutation in the proximal pr omoter region, The promoter of the wild-type allele, but not the mutan t allele, is hypermethylated, and only the mutant allele is expressed, Transcription from the methylated/wild-type allele was restored after cell treatment with the demethylating agent 5-aza-2'-deoxycytidine. T hus, in neoplastic cells, stable allele-specific loss of transcription may arise from aberrant methylation of a nonmutated promoter region, identifying hypermethylation as a direct mechanism for tumor suppresso r gene inactivation.