Epigenetic inactivation of the RASSF1A tumor suppressor gene (TSG) at chromosome 
3p21.3 was examined in medulloblastoma, the most common malignant brain tumor of 
childhood. Seventy-nine % (27 of 34) of primary tumors and 100% (8 of 8) of 
medulloblastoma cell lines displayed extensive tumor-specific DNA 
hypermethylation across the RASSF1A promoter-associated CpG island. 
Hypermethylation was associated with epigenetic silencing of RASSF1A 
transcription in medulloblastoma cell lines, and RASSF1A expression in these 
lines was restored after treatment with the DNA-methyltransferase inhibitor 
5-aza-2'-deoxycytidine. No evidence was found of RASSF1A inactivation by genetic 
mechanisms (gene mutation or deletion) in either cases with no evidence of 
RASSF1A hypermethylation or paired normal/tumor cases and cell lines with 
evidence of total RASSF1A CpG island hypermethylation. Epigenetic inactivation 
by biallelic hypermethylation therefore represents the primary mechanism of 
RASSF1A gene inactivation in medulloblastoma. Furthermore, RASSF1A 
hypermethylation is a frequent event in medulloblastoma tumorigenesis detectable 
in adult (5 of 7) and pediatric patients (22 of 27) and in all histological 
variants and age and sex groupings. Importantly, these data demonstrate that 
comprehensive analysis of the genome and epigenome will be required for 
identification of the key tumor suppressor genes involved in medulloblastoma 
development.