More than 95% of Prader-Willi syndrome (PWS) cases are caused by either a large deletion of paternal chromosome 15q11-q13 or maternal uniparental disomy (UPD) of chromosome 15. The major gene or genes responsible for PWS are subject to genomic imprinting and exclusively expressed from paternal chromosome. For patients with a large chromosomal deletion of paternal origin, the candidate genes for PWS on the maternal chromosome are structurally intact but their expression is repressed. In patients with maternal UPD, both chromosomes are structurally normal, but they have maternal chromosome epigenotype where the PWS candidate genes are repressed. The molecular mechanism of repressed gene expression on the maternal chromosome is not completely understood, but it is likely related to the methylation of DNA and chromatin structure around the promoter region of corresponding genes. DNA methylation is a common form of reversible DNA modification that has impact on regulating gene expression. It is generally believed that more DNA methylation in promoter regions may cause the repression of gene expression and less DNA methylation may favor or increase the gene transcription. Therefore, if one can find a drug that is able to modify the DNA methylation in the 15q11-q13 region, it may activate the expression of repressed genes from maternal chromosomes and hold the promise of treating the PWS. It has been shown previously that such manipulation was experimentally feasible and effective by using the DNA methylation inhibitor such as 5-aza-cytidine (5-aza-C, Vidaza), 5-aza-deoxycytidine (5-aza-dC, Decitabine) in cultured cells. The toxicity and safety concern of 5-aza-C and 5-aza-dC were the major issue initially for their clinical application. However, results from recent human trials of treating sick cell disease, leukemia, and MDS with low dose have tempted many safety concerns although a continuing follow up is needed for evaluation of long-term toxicity. In fact, both Vidaza and Decitabine have been approved by FDA for treating MDS, leukemia, and refractory anemia. Similarly, zebularine has also shown strong inhibiting effect on the DNA methylation in cancer cells but its safety has not been fully characterized. Recently, two FDA proved cardiovascular drugs, hydralazine and procainamide, have been reported to have some degree of DNA methylation inhibiting effects and to reactivate the expression of tumor suppressor genes in cancer cells. Favorable data from a phase I clinical trial of using hydralazine in treating of cervical cancer has been reported recently. We propose to explore the potential of using these drugs of treating the PWS patients by examining the effects of these drugs on the 15q11-q13 region in cultured cells from PWS patients and in PWS mouse model. We will examine the expression of paternally expressed genes from PWS candidate region in cultured cells from PWS patients and in PWS mouse model by real time RT-PCR analysis. We will examine the level of DNA methylation of associated genes from PWS candidate region after drug treatment to determine whether the level of demethylation is correlating with the activation of the candidate genes. The significance of proposed studies is apparent because positive results would support a grant application from NIH or other sources for a more thorough evaluation of the potential and eventually lead to launch a human trial of using these drugs to treat human PWS patients.