Epigenetic aberrations and a CpG island methylator phenotype are associated with 
poor outcome in children with neuroblastoma (NB). Previously, we have shown that 
valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, exerts antitumor 
effects in an NB xenograft model. However, the underlying antitumor molecular 
mechanisms are largely unknown. In this study, we examined the role of HDAC in 
cell proliferation, cell cycle progression, gene expression patterns, and 
epigenome in NB. Cell proliferation, cell cycle progression, caspase activity, 
RNA and protein expression, quantitative methylation, and global DNA methylation 
were examined in NBL-W-N and LA1-55n NB cell lines. Our studies showed that 
inhibition of HDAC decreased NB proliferation, and induced caspase activity and 
G1 growth arrest. Expression patterns of cancer-related genes were modulated by 
VPA. The expression of THBS1, CASP8, SPARC, CDKN1A, HIC1, CDKN1B, and HIN1 was 
upregulated, and that of MYCN and TIG1 was downregulated. HDAC inhibition 
decreased methylation levels of THBS1 and RASSF1A promoters. Inhibition of HDAC 
increased acetylation of histone 4 and overall DNA methylation levels. Our 
studies showed that inhibition of HDAC blocked cell proliferation and cell cycle 
progression in relation to alteration in cancer-related genes, increased overall 
DNA methylation, and decreased methylation of tumor suppressor genes. Further 
studies examining the antitumor effects of VPA in NB are warranted.