BACKGROUND: Prader-Willi Syndrome (PWS) is a complex human genetic disease 
arising from a loss of paternal allele expression of imprinting genes on 
chromosome 15q11-q13. Normally the CpG islands at this site are heavily 
methylated in the maternal allele, but unmethylated in the paternal allele and 
therefore activated in gene expression. only the methylated allele should 
present in pws patients when methylation-specific pcr (msp) is analyzed.
METHODS: This paper reports an analysis of PWS in Thai patients using consensus 
diagnostic criteria based on a combination of clinical data, basic G-banding and 
fluorescence in situ hybridization (FISH) cytogenetics, PCR-based methylation 
assay, and bisulfite sequencing of the CpG islands of SNRPN to confirm 15q 
deletion or the methylation pattern of the SNRPN promoter and exon 1. Lack of 
complete clinical reports or inadequacy of the minimum laboratory support 
required had made it difficult to diagnose PWS, Angelman syndrome and other 
microdeletion disorders.
RESULTS: Accuracy of 100% was obtained for diagnosis of the PWS study patients 
using the minimum requirements necessary. A total of 20 patients were diagnosed 
as PWS based on clinical criteria and the scoring tool for PWS, and the same 
approach was applied to four separate patients with some unmatched criteria but 
phenotypic similarity to PWS. Findings showed that 70% of those clinically 
diagnosed as PWS patients (14/20) had a deletion at 15q11-q13 according to FISH, 
while all 20 patients showed MSP positive of SNRPN gene. Six cases (30%) without 
a paternal deletion were confirmed to have maternal uniparental disomy (mUPD) of 
PWS by MSP and methylation sequencing approaches. Noteworthy, two of the six 
cases with mUPD were 3.5 year-old twins. None of the five cases with scores 
lower than the reported consensus criteria showed positive G-band, FISH or MSP 
results.
CONCLUSIONS: We demonstrate here the high power of combining clinical findings, 
FISH and MSP in definitive diagnosis of PWS and in distinguishing between the 
two major different types of molecular mechanisms. No false positives or false 
negatives were observed in our analysis.