Decitabine is a potent demethylating agent that exhibits clinical activity 
against myeloid malignancies. Numerous genes silenced by hypermethylation are 
reactivated by decitabine through a mechanism involving promoter demethylation 
with subsequent release of histone deacetylases (HDACs) and accumulation of 
acetylated histones. Recent studies indicating that decitabine also induces 
regional chromatin remodeling of some unmethylated genes suggest additional 
mechanisms of action. Decitabine reactivates unmethylated p21WAF1 in some AML 
cell lines but the possible occurrence of p21WAF1 methylation in AML in vivo has 
not been studied in detail and decitabine effects on p21WAF1 chromatin 
remodeling have not been reported. We found that p21WAF1 mRNA was undetectable 
in 6 of 24 AML patient samples and 4 of 5 AML cell lines but there was no 
evidence of p21WAF1 promoter methylation. However, decitabine induced p21WAF1 in 
AML cell lines KG-1 and KG-1a in association with release of HDAC1 and increased 
acetylated histone H3 at the unmethylated p21WAF1 promoter. Decitabine effects 
on p21WAF1 histone acetylation and induction were enhanced by the HDAC inhibitor 
trichostatin A and were independent of wild type p53. Our findings indicate that 
decitabine can relieve p21WAF1 repression in AML by a mechanism that involves 
release of HDAC1 without requiring promoter demethylation. Furthermore, our 
study provides evidence that combined decitabine and HDAC inhibitor treatment 
can enhance chromatin remodeling and reactivation of an unmethylated tumor 
suppressor gene. This latter finding is of relevance to the clinical use of 
these agents in AML as we found the p21WAF1 promoter to be unmethylated in vivo.