Prader-Willi phenotype caused by paternal deficiency for the HBII-85 C/D box small nucleolar RNA cluster

Abstract

Prader-Willi syndrome (PWS) is caused by deficiency for one or more paternally expressed imprinted transcripts within chromosome 15q11-q13, including SNURF-SNRPN and multiple small nucleolar RNAs (snoRNAs). Balanced chromosomal translocations that preserve expression of SNURF-SNRPN and centromeric genes but separate the snoRNA HBII-85 cluster from its promoter cause PWS. A microdeletion of the HBII-85 snoRNAs in a child with PWS provides, in combination with previous data, effectively conclusive evidence that deficiency of HBII-85 snoRNAs causes the key characteristics of the PWS phenotype, although some atypical features suggest that other genes in the region may make more subtle phenotypic contributions.

Figure 1. Clinical features of the affected individual

(a,b) Individual showing morbid obesity with facial features as shown. (c) Upper extremities are notable for small hands relative to body size. (d) External genitalia after laparoscopic orchiopexy at 13 months. Parental informed consent, as approved by the Baylor College of Medicine Institutional Review Board, was obtained to publish the photographs.

Figure 2. Analysis of microdeletion causing PWS and expression studies

(a) A high-resolution oligonucleotide array-CGH plot is shown with loss of a segment in 15q11.2 from position ∼22,835,000 bp to ∼23,010,100 bp (red arrows). (b) A schematic physical map of the 15q11–q13 genomic interval is shown, highlighting the deleted segment with respect to SNRPN, UBE3A and snoRNAs within the interval. (c) Sequencing of a ∼2.6-kb PCR fragment across the breakpoint revealing a deletion of 174,584 bp with an 8 bp insertion at the breakpoint. (d) Methylation analysis by DNA blot hybridization to rule out large deletion, uniparental disomy or imprinting abnormalities. Probe corresponding to SNRPN exon 1 was used for hybridization, and a normal methylation pattern was seen. Lane 1, large deletion AS; 2, large deletion PWS; 3, normal control; 4, affected individual. M, methylated maternal allele; UM, unmethylated paternal allele. (e) Expression analysis was carried out using RT-PCR of lymphoblast RNA for SNRPN, snoRNA HBII-85, ESTs AK094315 and AB061718, and UBE3A (size of PCR products is in parentheses). GAPDH was used as internal RT-PCR control. RT+, with reverse transcriptase; RT−, without reverse transcriptase. Affected individual shows lack of HBII-85 transcript in RT+ lane with presence of transcripts for all other loci tested. Lane 1, normal control; 2, large deletion AS; 3, large deletion PWS; 4, affected individual.