EXTENSIVE DNA METHYLATION SPANNING THE RB PROMOTER IN RETINOBLASTOMA TUMORS

The retinoblastoma gene (Rb) is one of the best characterized tumor su ppressor genes, and its inactivation is associated with a number of ca ncers. Previous studies have shown, by restriction enzyme analysis, th at the promoter region of the Rb gene is methylated in a significant p roportion of primary retinoblastoma tumors. We now report the fil:st d etailed methylation sequence analysis of the CpG island spanning the r etinoblastoma promoter from hypermethylated retinoblastoma tumors, Our results show methylation is not confined to a specific CpG site, as d etected by restriction enzyme studies, but extends to essentially all 27 CpG dinucleotides spanning the retinoblastoma CpG island, including the core promoter. The methylation pattern from each tumor DNA sample is different, ranging from densely to sparsely methylated profiles, S ingle CpG sites, in particular the E2F transcription factor binding si te, as well as blocks of CpGs, were undermethylated in some tumor samp les. Possible interference of methylation could be due to the binding of sequence-specific protein factors at these sites in the tumor cells . This study highlights that the dynamics of DNA methylation in cancer cells are clearly different from normal cells and gives an insight in to the mechanism of abnormal methylation of CpG islands in cancer cell s.