Prader–Willi syndrome (PWS) is a complex neurogenetic disorder with considerable clinical variability that is thought in large part to be the result of a hypothalamic defect. PWS results from the absence of paternal expression of imprinted genes localized in the 15q11–q13 region; however, none of the characterized genes has so far been shown to be involved in the etiology of PWS. Here, we provide a detailed investigation of a mouse model deficient for Necdin. Linked to the mutation, a neonatal lethality of variable penetrance is observed. Viable Necdin mutants show a reduction in both oxytocin-producing and luteinizing hormone-releasing hormone (LHRH)-producing neurons in hypothalamus. This represents the first evidence of a hypothalamic deficiency in a mouse model of PWS. Necdin-deficient mice also display increased skin scraping activity in the open field test and improved spatial learning and memory in the Morris water maze. The latter features are reminiscent of the skin picking and improved spatial memory that are characteristics of the PWS phenotype. These striking parallels in hypothalamic structure, emotional and cognitive-related behaviors strongly suggest that NECDIN is responsible for at least a subset of the multiple clinical manifestations of PWS.
Prader Willi Syndrome is characterized by mental retardation or learning disability, infantile hypotonia and poor suck reflex, growth retardation, delayed sexual development, and the childhood onset of pronounced hyperphagia.
Hyperphagic short stature is one of the very few behavioural diseases associated with a pathognomonic physiological abnormality. Investigations.
The most common etiology for Prader-Willi syndrome and Angelman syndrome is de novo interstitial deletion of chromosome 15q11-q13. Deletions and other recurrent rearrangements of this region involve four common `hotspots' for breakage, termed breakpoints 1-4 (BP1-BP4). Construction of an ~4 Mb YAC contig of this region identified multiple sequence tagged sites (STSs) present at both BP2 and BP3, suggestive of a genomic duplication event. Interphase FISH studies demonstrated three to five copies on 15q11-q13, one copy on 16p11.1-p11.2 and one copy on 15q24 in normal controls, while analysis on two Class I deletion patients showed loss of approximately three signals at 15q11-q13 on one homolog. Multiple FISH signals were also observed at regions orthologous to both human chromosomes 15 and 16 in non-human primates, including Old World monkeys, suggesting that duplication of this region may have occurred ~20 million years ago. A BAC/PAC contig for the duplicated genomic segment (duplicon) demonstrated a size of ~400 kb. Surprisingly, the duplicon was found to contain at least seven different expressed sequence tags representing multiple genes/pseudogenes. Sequence comparison of STSs amplified from YAC clones uniquely mapped to BP2 or BP3 showed two different copies of the duplicon within BP3, while BP2 comprised a single copy. The orientation of BP2 and BP3 are inverted relative to each other, whereas the two copies within BP3 are in tandem. The presence of large duplicated segments on chromosome 15q11-q13 provides a mechanism for homologous unequal recombination events that may mediate the frequent rearrangements observed for this chromosome.
Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG-binding protein 2 (MeCP2). Brain samples from several related neurodevelopmental disorders, including autism, pervasive developmental disorder, Prader-Willi and Angelman syndromes showed significant differences in MeCP2 expression from age-matched controls by apparently different transcriptional and post-transcriptional mechanisms. These results suggest that multiple pathways regulate the complex developmental expression of MeCP2 and are defective in autism-spectrum disorders in addition to RTT.
Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurological disorders that map to human chromosome 15q11-q13 and involve perturbations of imprinted gene expression. PWS is caused by a deficiency of paternal gene expression and AS is caused by a deficiency of maternal gene expression. Experiments in the last year have focused on molecular analysis of the human chromosomal region as well as the homologous region on central mouse chromosome 7. New transcripts and exons have been identified and the epigenetic status of the PWS/AS region in mice and humans has been examined. The imprinting center that is hypothesized to control the switch between the maternal and paternal epigenotypes has also been characterized in greater detail and a mouse model that deletes the homologous element demonstrates a conservation in imprinting center function between mice and humans. In addition, analysis of non-deletion AS patients has revealed that UBE3A intragenic mutations are found in a significant number of cases. However, both human patients and mouse model systems indicate that other genes may also contribute to the AS phenotype. Thus, although much has been learned in the last year, considerable information is still required before these complex syndromes are fully understood.