5′ CpG island methylation of p16 is associated with absence of p16 expression in glioblastomas

Recent evidence shows that transcriptional silencing as a consequence of hypermethylation of CpG islands is an important mechanism in the inactivation of p16^INK4a tumor suppressor gene. This study is designed to clarify the significance of p16^INK4a hypermethylation in 23 cases of glioblastomas (GBMs) by methylation-spedfic polymerase chain reaction (PCR) and p16 immunostaining. Fourteen cases (60.9%) out of 23 GBMs revealed hypermethylation on p16. p16 immunostaining revealed that 13 (93%) of these 14 hypermethylation cases showed complete loss of immunoreactivity and only one (7%) case retained immunoreactivity. Among 9 methylation-negative cases, 4 were immunonegative, which might be related to mutations or deletions other than hypermethylation. The most significant finding was that of 17 cases with immunonegativity, 13 cases (76.5%) showed hypermethylation. We reconfirmed that p16 hypermethylation may be one of the major mechanisms of tumorigenesis of GBMs and the results between the methylation specific-PCR study and p16 immunostaining had a good correlation.