Molecular Characterization ofTwo Proximal Deletion Breakpoint Regions inBothPrader-Willi andAngelman SyndromePatients

Summary Prader-Willi syndrome (PWS)andAngelman syndrome (AS) aredistinct mental retardation syndromes caused by paternal andmaternal deficiencies, respectively, inchromosome15q11-q13. Approximately 70%ofthese patients havealarge deletion of-4Mb extending fromD15S9 (ML34) through D15S12 (IR10). Tofurther characterize thedeletion breakpoints proximal toD15S9, three new polymorphic microsatellite markers weredeveloped that showed observed heterozygosities of60%-87%. D15S541 andD15S542 wereisolated from YACA124A3containing theD15S18(IR39) locus. D15S543 wasisolated froma cosmid cloned fromtheproximal right endofYAC254B5 containing theD15S9(ML34) locus. Gene-centromere mapping ofthese markers, using apanel ofovarian teratomasofknownmeiotic origin, extended thegenetic mapof chromosome 15by2-3cMtoward the centromere. Analysis ofthemoreproximal S541/S542 markers on53PraderWilli and33Angelman deletion patients indicated two classes ofpatients: 44%(35/80) oftheinformative patients weredeleted forthese markers (class I), while 56%(45/ 80)werenotdeleted (class II), with nodifference between PWSandAS.Incontrast, D15S543 wasdeleted inall informative patients (13/48) orshowed thepresence ofasingle allele (in35/48 patients), suggesting that this marker is deleted inthemajority ofPWSandAScases. These results confirm thepresence oftwocommonproximal deletion breakpoint regions inboth Prader-Willi andAngelman syndromes andareconsistent with thesamedeletion mechanismbeing responsible forpaternal andmaternal deletions. Onebreakpoint region lies between D15S541/S542 and D15S543, withanadditional breakpoint region being proximal toD15S541/S542.