A translocation breakpoint cluster disrupts the newly defined 3 ' end of the SNURF-SNRPN transcription unit on chromosome 15

Balanced translocations affecting the paternal copy of 15q11-q13 are a rare cause of Prader-Willi syndrome(PWS) or PWS-like features, Here we report o n the cytogenetic and molecular characterization of a de novo balanced reci procal translocation t(X;15)(q28;q12) in a female patient with atypical PWS , The translocation breakpoints in this patient and two previously reported patients map 70-80 kb distal to the SNURF-SNRPN gene and define a breakpoi nt cluster region, The breakpoints disrupt one of several hitherto unknown 3' exons of this gene. Using RT-PCR we demonstrate that sequences distal to the breakpoint, including the recently identified C/D box small nucleolar RNA (snoRNA) gene cluster HBII-85 as well as IPW and PAR1, are not expresse d in the patient. Our data suggest that lack of expression of these sequenc es contributes to the PWS phenotype.