Imprinting in Prader-Willi syndrome

 Karin Buiting¹,  Maren Runte¹, Hülya Nazlican¹, Alexander Hüttenhofer², Stephanie Groß¹, Christina Lich¹, and Bernhard Horsthemke¹

 ¹Institut für Humangenetik, Universitätsklinikum Essen, Hufelandstrasse 55, 45122 Essen, Germany.

 ²Institut für Experimentelle Pathologie, ZMBE, Universität Münster, Germany

 In a small group of patients with Prader-Willi syndrome (PWS), the disease is due to aberrant imprinting and gene silencing. Among 56 such patients, nine patients (16%) were found to have an imprinting center (IC) deletion. In most cases the IC deletion was found to be a familial mutation, but in three cases the IC deletion was a de novo event or due to a germline mosaicism in the father. Prenatal diagnosis in a second pregnancy in one of these families revealed that the fetus inherited the same IC deletion as the index patient, indicating that the father has a germinal mosaicism for the IC deletion.

By methylation analysis of sperm cells from two IC deletion carriers we found that the incorrect maternal methylation pattern on the grandmaternal chromosome of PWS IC deletion patients must occur after fertilization. This suggests that the SNURF-SNRPN exon 1 region is not a control element for a germline methylation switch, but essential for maintaining the paternal methylation imprint during early embryonic development.

Sequence analysis in 32 PWS non-IC deletion patients did not reveal any point mutation in the critical IC element, the SNURF/SNRPN exon 1/intron 1 region. We conclude that the vast majority of imprinting defects are epimutations that occur spontaneously in the absence of DNA sequence changes. In all informative PWS non-IC deletion patients the imprinting defect always occurred on the chromosome inherited from the paternal grandmother. These data suggest that the (grand)maternal imprint was not erased in the paternal germline.

Interestingly, the IC serves as a start site for a complex, paternally expressed transcript which consists of multiple alternative splice variants. This transcript encodes two proteins (SNURF and SNRPN), an antisense RNA for the maternally expressed UBE3A gene, and multiple small nucleolar (sno)RNA genes. Unlike other snoRNAs, these snoRNA genes are probably not involved in the posttranscriptional modification of rRNA, because they lack any telltale rRNA complementarity. From mouse data at least some of these snoRNA genes appear to be critical for some aspects of the PWS phenotype and therefore a lack of these snoRNAs may be causally involved in this disease.

July 2003