Silencing of the p18INK4c gene by promoter hypermethylation in Reed-Sternberg cells in Hodgkin lymphomas

p18(INK4c) is a cyclin-dependent kinase (CDK) inhibitor that interferes with the Rb-kinase activity of CDK6/CDK4. Disruption of p18(INK4c) in mice impairs B-cell terminal differentiation and confers increased susceptibility to tumor development; however, alterations of p18(INK4c) in human tumors have rarely been described. We used a tissue-microarray approach to analyze p18(INK4c) expression in 316 Hodgkin lymphomas (HLs). Nearly half of the HL cases showed absence of p18(INK4c) protein expression by Reed-Sternberg (RS) cells, in contrast with the regular expression of p18(INK4c) in normal germinal center cells. To investigate the cause of p18(INK4c) repression in RS cells, the methylation status of the p18(INK4c) promoter was analyzed by methylation-specific polymerase chain reaction (PCR) and bisulfite sequencing. Hypermethylation of the p18(INK4c) promoter was detected in 2 of 4 HL-derived cell lines, but in none of 7 non-Hodgkin lymphoma (NHL)-derived cell lines. We also detected p18(INK4c) hypermethylation, associated with absence of protein expression, in 5 of 26 HL tumors. The correlation of p18(INK4c) immunostaining with the follow-up of the patients showed shorter overall survival in negative cases, independent of the International Prognostic Score. These findings suggest that p18(INK4c) may function as a tumor suppressor gene in HL, and its inactivation may contribute to the cell cycle deregulation and defective terminal differentiation characteristic of the RS cells. (C) 2004 by The American Society of Hematology.