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Retrospective diagnosis of trisomy 15 in formalin‐fixed, paraffin‐embedded placental tissue in a newborn girl with Prader–Willi syndrome

Paternal deletion of 15q11‐q13 and maternal uniparental disomy (UPD) of chromosome 15 are the main causes of Prader–Willi syndrome (PWS).The finding of an UPD(15) is associated with increased maternal age. We present a retrospective diagnosis of a trisomy 15 mosaicism confined to the placenta (CPM) after birth of a girl with clinical features of PWS born to a 43‐year‐old mother. Chromosome analysis after amniocentesis, performed because of advanced maternal age, had shown a normal female karyotype. In peripheral blood cells molecular studies showed the absence of the paternal allele at the SNRPN locus and fluorescence in situ hybridization (FISH) analysis excluded a deletion of the SNRPN locus on both chromosomes 15. Trisomic cells were detected by FISH on nuclei isolated from formalin‐fixed, paraffin‐embedded placental tissue using a DNA‐probe specific for the centromeric region of chromosome 15. Copyright © 2000 John Wiley & Sons, Ltd.