The imprinted domain on human chromosome 15 consists of two oppositely imprinted gene clusters, which are under the coordinated control of an imprinting center (IC) at the 5' end of the SNURF-SNRPN gene. One gene cluster spans the centromeric part of this domain and contains several genes that are transcribed from the paternal chromosome only (MKRN3, MAGEL2, NDN, SNURF-SNRPN, HBII-13, HBII-85 and HBII-52). Apart from the HBII small nucleolar RNA (snoRNA) genes, each of these genes is associated with a 5' differentially methylated region (DMR). The second gene cluster maps to the telomeric part of the imprinted domain and contains two genes (UBE3A and ATP10C), which in some tissues are preferentially expressed from the maternal chromosome. So far, no DMR has been identified at these loci. Instead, maternal-only expression of UBE3A may be regulated indirectly through a paternally expressed antisense transcript. We report here that a processed antisense transcript of UBE3A starts at the IC. The SNURF-SNRPN sense/UBE3A antisense transcription unit spans more than 460 kb and contains at least 148 exons, including the previously identified IPW exons. It serves as the host for the previously identified HBII-13, HBII-85 and HBII-52 snoRNAs as well as for four additional snoRNAs (HBII-436, HBII-437, HBII-438A and HBII-438B), newly identified in this study. Almost all of those snoRNAs are encoded within introns of this large transcript. Northern blot analysis indicates that most if not all of these snoRNAs are indeed expressed by processing from these introns. As we have not obtained any evidence for other genes in this region, which, from the mouse data appears to be critical for the neonatal Prader-Willi syndrome phenotype, a lack of these snoRNAs may be causally involved in this disease.

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct neurogenetic disorders caused by the loss of function of imprinted genes in 15q11-q13. The maternally expressed UBE3A gene is affected in AS. Four protein-encoding genes (MKRN3, MAGEL2, NDN and SNURF-SNRPN) and several small nucleolar (sno) RNA genes (HBII-13, HBII-436, HBII-85, HBII-438A, HBII-438B and HBII-52) are expressed from the paternal chromosome only but their contribution to PWS is unclear. To examine the role of the HBII-52 snoRNA genes, we have reinvestigated an AS family with a submicroscopic deletion spanning UBE3A and flanking sequences. By fine mapping of the centromeric deletion breakpoint in this family, we have found that the deletion affects all of the 47 HBII-52 genes. Since the complete loss of the HBII-52 genes in family members who carry the deletion on their paternal chromosome is not associated with an obvious clinical phenotype, we conclude that HBII-52 snoRNA genes do not play a major role in PWS. However, we cannot exclude the possibility that the loss of HBII-52 has a phenotypic effect when accompanied by the loss of function of other genes in 15q11-q13.

We investigated the gene encoding the β subunit of cGMP phosphodiesterase (PDE6B) as a candidate for generalized progressive retinal atrophy (gPRA), an autosomal recessively transmitted eye disease in dogs. The PDE6B gene was isolated from a genomic library. Single-strand conformation polymorphism analysis revealed eight intronic variations in different subsets of the 14 dog breeds investigated. In addition, we identified an 8-bp insertion after codon 816 in certain Sloughi dogs. Analysis of PRA-affected and obligatory carrier Sloughis showed that this mutation cosegregates with disease status in a large pedigree. All other exchanges identified were not located in functionally relevant parts of the gene (e.g., in the splice signal consensus sites). In most dog breeds (Labrador retriever, Tibetan mastiff, dachshund, Tibetan terrier, miniature poodle, Australian cattle dog, cocker spaniel, collie, Saarloos wolfhound, Chesapeake Bay retriever, and Yorkshire terrier), PDE6B was excluded as a candidate gene for gPRA because heterozygous allele constellations were detected in diseased animals. Therefore, the PDE6B sequence variations did not segregate together with the mutation(s) causing gPRA. Direct and indirect DNA tests concerning gPRA can be offered now for a variety of different dog breeds.