Molecular diagnosis of Prader-Willi and Angelman syndromes by methylation-specific melting analysis and methylation-specific multiplex ligation-dependent probe amplification

Approximately 99% of Prader-Willi syndrome (PWS) and 80% of Angelman syndrome (AS) cases have deletions at a common region in chromosome 15q11.2-q13, uniparental disomy for chromosome 15 (UPD15), or imprinting center defects affecting gene expression in this region. The resulting clinical phenotype (PWS or AS) in each class of genomic abnormalities depends on the parent of origin. Both disorders are characterized at the molecular level by abnormal methylation of imprinted regions at 15q11.2-q13. Other rare chromosome 15 rearrangements and a few smaller atypical deletions associated with abnormal methylation patterns also have symptoms overlapping with either PWS or AS.

We designed a methylation-specific melting analysis (MS-MA) method for a rapid screening of PWS/AS and evaluated methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) for diagnosis of PWS/AS associated with deletions, UPD15, or rare duplications. Forty-nine previously genotyped samples were tested by MS-MA. We also tested 26 MS-MA genotyped samples and 1 additional sample with rare duplication of chromosome region 15q11-q12.

PWS/AS genotyping results obtained by MS-MA and by MS-MLPA were fully concordant. In addition, MS-MLPA was superior in detecting deletions/rare duplications, possible UPD15, or imprinting center defects, which were usually determined by a laborious fluorescence in situ hybridization method or by chromosomal segregation analysis for the parental-origin using short-tandem repeat makers.

MS-MA appears to be an efficient primary method to diagnose PWS/AS, and use of the quantitative MS-MLPA method provides detailed information about deletions, rare duplications, and possibly UPD.