A clinical, cytogenetic, FISH and molecular study of supernumerary marker 15 chromosomes

We studied 17 patients with supernumerary marker chromosomes shown by fluorescent in situ hybridization (FISH) with the 15-centromere specific probe pTRA-25 to be 15-derived. Genetic constitution of the marker chromosomes was investigated using FISH, Southern blot analysis and PCR for proximal and distal loci on 15q as well as conventional cytogenetics. Eight of the 17 patients were mentally retarded. Six of the eight carried a de novo marker 15 containing one or two doses of loci known to be in or near the Prader-Willi/Angelman (PWS/AS) region, whereas none of the nine non-retarded patients had duplications of this region, and only two of the eight whose parents were available had a de novo marker. None of the mentally retarded patients had PWS or AS. In two retarded patients (one de novo, one familial) there was no duplication of the PWS/AS region. Uniparental disomy affecting the normal 15 homologs was excluded in 10 of the patients, including all eight with mental retardation.