Abstract

We report a patient with Prader-Willi syndrome (PWS) and mosaicism for a de novo jumping translocation of distal chromosome 15q, resulting in partial trisomy for 15q24-qter. A maternal uniparental heterodisomy for chromosome 15 was present in all cells, defining the molecular basis for the PWS in this patient. The translocated distal 15q fragment was of paternal origin and was present as a jumping translocation, involving three different translocation partners, chromosomes 14q, 4q, and 16p. The recipient chromosomes appeared cytogenetically intact and interstitial telomere DNA sequences were present at the breakpoint junctions. This strongly suggests that the initial event leading to the translocation of distal 15q was a non-reciprocal translocation, with fusion between the 15q24 break-point and the telomeres of the recipient chromosomes. These observations are best explained by a partial zygotic trisomy rescue and comprise a previously undescribed mechanism leading to partial trisomy.