Imprinting-mutation mechanisms in Prader-Willi syndrome

Epilepsy in Prader-Willi syndrome: clinical, diagnostic and treatment aspects

BACKGROUND

Epilepsy associated with Prader-Willi syndrome (PWS) represents an early and important complication, often not clearly reported and described in the literature. Consequently, there are controversial data about the clinical characteristics of epilepsy and electroencephalographic (EEG) abnormalities found in these patients.

DATA SOURCES

Based on recent original publications, we have reviewed the different types of seizures and EEG findings in PWS patients, the response to antiepileptic treatment, and the prognosis of epilepsy.

RESULTS

The frequency of epilepsy in PWS patients ranges from 4% to 26%. The types of seizure include generalized tonic-clonic seizures, complex partial seizures, atypical absence, staring spells, and myoclonic, tonic and hemiclonic seizures, but the most frequent type is focal epilepsy. Status epilepticus has never been reported. EEG abnormalities are not typical but variable in different patients. However, generalized and focal discharges are the most frequently reported findings. There is no evidence of relationship between the course of epilepsy and frequency, morphology and spread of EEG discharges. However, epilepsy in PWS patients is usually responsive to antiepileptic monotherapy with rapid seizure control and a good outcome.

CONCLUSIONS

The frequency of epilepsy is higher in PWS patients than in general populations and this complication can be a challenge for the clinicians of these patients. Prospective studies are needed to confirm the good long-term prognosis.

Differential gene expression reveals mitochondrial dysfunction in an imprinting center deletion mouse model of Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a genetic disorder caused by deficiency of imprinted gene expression from the paternal chromosome 15q11-15q13 and clinically characterized by neonatal hypotonia, short stature, cognitive impairment, hypogonadism, hyperphagia, morbid obesity, and diabetes. Previous clinical studies suggest that a defect in energy metabolism may be involved in the pathogenesis of PWS. We focused our attention on the genes associated with energy metabolism and found that there were 95 and 66 mitochondrial genes differentially expressed in PWS muscle and brain, respectively. Assessment of enzyme activities of mitochondrial oxidative phosphorylation complexes in the brain, heart, liver, and muscle were assessed. We found the enzyme activities of the cardiac mitochondrial complexes II+‫III were up-regulated in the PWS imprinting center deletion mice compared to the wild-type littermates. These studies suggest that differential gene expression, especially of the mitochondrial genes may contribute to the pathophysiology of PWS.

Mouse models of Prader-Willi Syndrome: a systematic review

Prader-Willi Syndrome (PWS) is a neurodevelopmental genetic disorder caused by loss of expression of imprinted, paternally inherited genes on chromosome 15q11q13. This imprinted gene cluster has its homologous region on mouse chromosome 7C. The extremely well conserved synteny between the human and the murine regions gave origin to the generation of mouse models for PWS, which facilitated investigations of the role and function of single genes or gene clusters in the pathogenesis of this disease. In this review we will describe which mouse models have been generated so far and how they were developed; we will focus on the consequences of single genes' (or gene clusters') loss of expression on the phenotype, highlighting the similarities to the human PWS features. PWS mouse models have brought major improvements in our knowledge about this complex condition, although the mechanisms implicated in its pathogenesis still remain not fully understood.

DNA methylation program in developing hippocampus and its alteration by alcohol

During hippocampal development, the Cornus Ammonis (CA) and the dentate gyrus (DG) undergo waves of neurogenesis and neuronal migration and maturation independently. This stage is widely known to be vulnerable to environmental stresses, but its underlying mechanism is unclear. Alcohol exposure has been shown to alter the expression of genes that regulate the fate, survival, migration and differentiation of pyramidal and granule cells. Undermining this process might compromise hippocampal development underlying the learning and memory deficits known in Fetal Alcohol Spectrum Disorders (FASD). We have previously demonstrated that DNA methylation was programmed along with neural tube development. Here, we demonstrated that DNA methylation program (DMP) proceeded along with hippocampal neuronal differentiation and maturation, and how this DMP was affected by fetal alcohol exposure. C57BL/6 mice were treated with 4% v/v ethanol through a liquid diet along with pair-fed and chow-fed controls from gestation day (E) 7 to E16. We found that a characteristic DMP, including 5-methylcytidine (5mC), 5-hydroxylmethylcytidine (5hmC) and their binding proteins, led the hippocampal neuronal differentiation and maturation spatiotemporally as indicated by their phenotypic marks in the CA and DG pre- and post-natally. Alcohol hindered the acquisition and progression of methylation marks, and altered the chromatin translocation of these marks in the nucleus, which was correlated with developmental retardation.

Sex bias in copy number variation of olfactory receptor gene family depends on ethnicity

Gender plays a pivotal role in the human genetic identity and is also manifested in many genetic disorders particularly mental retardation. In this study its effect on copy number variation (CNV), known to cause genetic disorders was explored. As the olfactory receptor (OR) repertoire comprises the largest human gene family, it was selected for this study, which was carried out within and between three populations, derived from 150 individuals from the 1000 Genome Project. Analysis of 3872 CNVs detected among 791 OR loci, in which 307 loci showed CNV, revealed the following novel findings: Sex bias in CNV was significantly more prevalent in uncommon than common CNV variants of OR pseudogenes, in which the male genome showed more CNVs; and in one-copy number loss compared to complete deletion of OR pseudogenes; both findings implying a more recent evolutionary role for gender. Sex bias in copy number gain was also detected. Another novel finding was that the observed sex bias was largely dependent on ethnicity and was in general absent in East Asians. Using a CNV public database for sick children (International Standard Cytogenomic Array Consortium) the application of these findings for improving clinical molecular diagnostics is discussed by showing an example of sex bias in CNV among kids with autism. Additional clinical relevance is discussed, as the most polymorphic CNV-enriched OR cluster in the human genome, located on chr 15q11.2, is found near the Prader–Willi syndrome/Angelman syndrome bi-directionally imprinted region associated with two well-known mental retardation syndromes. As olfaction represents the primitive cognition in most mammals, arguably in competition with the development of a larger brain, the extensive retention of OR pseudogenes in females of this study, might point to a parent-of-origin indirect regulatory role for OR pseudogenes in the embryonic development of human brain. Thus any perturbation in the temporal regulation of olfactory system could lead to developmental delay disorders including mental retardation.

Regulatory elements associated with paternally-expressed genes in the imprinted murine Angelman/Prader-Willi syndrome domain

The Angelman/Prader-Willi syndrome (AS/PWS) domain contains at least 8 imprinted genes regulated by a bipartite imprinting center (IC) associated with the SNRPN gene. One component of the IC, the PWS-IC, governs the paternal epigenotype and expression of paternal genes. The mechanisms by which imprinting and expression of paternal genes within the AS/PWS domain – such as MKRN3 and NDN – are regulated by the PWS-IC are unclear. The syntenic region in the mouse is organized and imprinted similarly to the human domain with the murine PWS-IC defined by a 6 kb interval within the Snrpn locus that includes the promoter. To identify regulatory elements that may mediate PWS-IC function, we mapped the location and allele-specificity of DNase I hypersensitive (DH) sites within the PWS-IC in brain cells, then identified transcription factor binding sites within a subset of these DH sites. Six major paternal-specific DH sites were detected in the Snrpn gene, five of which map within the 6 kb PWS-IC. We postulate these five DH sites represent functional components of the murine PWS-IC. Analysis of transcription factor binding within multiple DH sites detected nuclear respiratory factors (NRF's) and YY1 specifically on the paternal allele. NRF's and YY1 were also detected in the paternal promoter region of the murine Mrkn3 and Ndn genes. These results suggest that NRF's and YY1 may facilitate PWS-IC function and coordinately regulate expression of paternal genes. The presence of NRF's also suggests a link between transcriptional regulation within the AS/PWS domain and regulation of respiration. 3C analyses indicated Mkrn3 lies in close proximity to the PWS-IC on the paternal chromosome, evidence that the PWS-IC functions by allele-specific interaction with its distal target genes. This could occur by allele-specific co-localization of the PWS-IC and its target genes to transcription factories containing NRF's and YY1.

The Human Obesity Gene Map: The 2003 Update

This is the tenth update of the human obesity gene map, incorporating published results up to the end of October 2003 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. Transgenic and knockout murine models relevant to obesity are also incorporated (N = 55). As of October 2003, 41 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. QTLs reported from animal models currently number 183. There are 208 human QTLs for obesity phenotypes from genome-wide scans and candidate regions in targeted studies. A total of 35 genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 272 studies reporting positive associations with 90 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, more than 430 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

De novo interstitial duplication of 15q11.2-q13.1 with complex maternal uniparental trisomy for the 15q11-q13 region in a patient with Prader-Willi syndrome

Prader-Willi syndrome is caused by the lack of paternal contribution for the imprinted 15q11-q13 region that originates through a number of mechanisms such as paternal deletion of 15q11-q13, maternal uniparental disomy, or by an imprinting defect due to epimutations in the paternal imprinting center. In the present report, we describe a female patient with complex maternal uniparental trisomy for the 15q11-q13 Prader-Willi syndrome critical region due to a de novo interstitial duplication of 15q11-q13 region that is present in one of the maternal homologs. As a result, the patient has three maternally derived copies of the Prader-Willi syndrome critical region and absence of paternal 15 contribution and thus, presents with a Prader-Willi syndrome phenotype with risk for developing additional phenotypes (e.g., autism and psychiatric phenotypes) characteristic of maternally derived duplications of this region. We suggest that this is a rather unique mechanism leading to Prader-Willi syndrome that has not been previously reported.

Visualization of the spatial positioning of the SNRPN, UBE3A, and GABRB3 genes in the normal human nucleus by three-color 3D fluorescence in situ hybridization

The three-dimensional (3D) structure of the genome is organized non-randomly and plays a role in genomic function via epigenetic mechanisms in the eukaryotic nucleus. Here, we analyzed the spatial positioning of three target regions; the SNRPN, UBE3A, and GABRB3 genes on human chromosome 15q11.2–q12, a representative cluster of imprinted regions, in the interphase nuclei of B lymphoblastoid cell lines, peripheral blood cells, and skin fibroblasts derived from normal individuals to look for evidence of genomic organization and function. The positions of these genes were simultaneously visualized, and all inter-gene distances were calculated for each homologous chromosome in each nucleus after three-color 3D fluorescence in situ hybridization. None of the target genes were arranged linearly in most cells analyzed, and GABRB3 was positioned closer to SNRPN than UBE3A in a high proportion of cells in all cell types. This was in contrast to the genomic map in which GABRB3 was positioned closer to UBE3A than SNRPN. We compared the distances from SNRPN to UBE3A (SU) and from UBE3A to GABRB3 (UG) between alleles in each nucleus, 50 cells per subject. The results revealed that the gene-to-gene distance of one allele was longer than that of the other and that the SU ratio (longer/shorter SU distance between alleles) was larger than the UG ratio (longer/shorter UG distance between alleles). The UG distance was relatively stable between alleles; in contrast, the SU distance of one allele was obviously longer than the distance indicated by the genome size. The results therefore indicate that SNRPN, UBE3A, and GABRB3 have non-linear and non-random curved spatial positioning in the normal nucleus, with differences in the SU distance between alleles possibly representing epigenetic evidence of nuclear organization and gene expression.

Mechanisms of activation of the paternally expressed genes by the Prader-Willi imprinting center in the Prader-Willi/Angelman syndromes domains

The Prader-Willi syndrome/Angelman syndrome (PWS/AS) imprinted domain is regulated by a bipartite imprinting control center (IC) composed of a sequence around the SNRPN promoter (PWS-IC) and a 880-bp sequence located 35 kb upstream (AS-IC). The AS-IC imprint is established during gametogenesis and confers repression upon PWS-IC on the maternal allele. Mutation at PWS-IC on the paternal allele leads to gene silencing across the entire PWS/AS domain. This silencing implies that PWS-IC functions on the paternal allele as a bidirectional activator. Here we examine the mechanism by which PWS-IC activates the paternally expressed genes (PEGs) using transgenes that include the PWS-IC sequence in the presence or absence of AS-IC and NDN, an upstream PEG, as an experimental model. We demonstrate that PWS-IC is in fact an activator of NDN. This activation requires an unmethylated PWS-IC in the gametes and during early embryogenesis. PWS-IC is dispensable later in development. Interestingly, a similar activation of a nonimprinted gene (APOA1) was observed, implying that PWS-IC is a universal activator. To decipher the mechanism by which PWS-IC confers activation of remote genes, we performed methylated DNA immunoprecipitation (MeDIP) array analysis on lymphoblast cell lines that revealed dispersed, rather than continued differential methylation. However, chromatin conformation capture (3c) experiments revealed a physical interaction between PWS-IC and the PEGs, suggesting that activation of PEGs may require their proximity to PWS-IC.

Transcription is required to establish maternal imprinting at the Prader-Willi syndrome and Angelman syndrome locus

The Prader-Willi syndrome (PWS [MIM 17620]) and Angelman syndrome (AS [MIM 105830]) locus is controlled by a bipartite imprinting center (IC) consisting of the PWS-IC and the AS-IC. The most widely accepted model of IC function proposes that the PWS-IC activates gene expression from the paternal allele, while the AS-IC acts to epigenetically inactivate the PWS-IC on the maternal allele, thus silencing the paternally expressed genes. Gene order and imprinting patterns at the PWS/AS locus are well conserved from human to mouse; however, a murine AS-IC has yet to be identified. We investigated a potential regulatory role for transcription from the Snrpn alternative upstream exons in silencing the maternal allele using a murine transgene containing Snrpn and three upstream exons. This transgene displayed appropriate imprinted expression and epigenetic marks, demonstrating the presence of a functional AS-IC. Transcription of the upstream exons from the endogenous locus correlates with imprint establishment in oocytes, and this upstream exon expression pattern was conserved on the transgene. A transgene bearing targeted deletions of each of the three upstream exons exhibited loss of imprinting upon maternal transmission. These results support a model in which transcription from the Snrpn upstream exons directs the maternal imprint at the PWS-IC.

Prader-Willi syndrome

Prader-Willi syndrome is characterized by severe infantile hypotonia with poor suck and failure to thrive; hypogonadism causing genital hypoplasia and pubertal insufficiency; characteristic facial features; early-childhood onset obesity and hyperphagia; developmental delay/mild intellectual disability; short stature; and a distinctive behavioral phenotype. Sleep abnormalities and scoliosis are common. Growth hormone insufficiency is frequent, and replacement therapy provides improvement in growth, body composition, and physical attributes. Management is otherwise largely supportive. Consensus clinical diagnostic criteria exist, but diagnosis should be confirmed through genetic testing. Prader-Willi syndrome is due to absence of paternally expressed imprinted genes at 15q11.2-q13 through paternal deletion of this region (65-75% of individuals), maternal uniparental disomy 15 (20-30%), or an imprinting defect (1-3%). Parent-specific DNA methylation analysis will detect >99% of individuals. However, additional genetic studies are necessary to identify the molecular class. There are multiple imprinted genes in this region, the loss of which contribute to the complete phenotype of Prader-Willi syndrome. However, absence of a small nucleolar organizing RNA gene, SNORD116, seems to reproduce many of the clinical features. Sibling recurrence risk is typically <1%, but higher risks may pertain in certain cases. Prenatal diagnosis is available.

15q11.2-13.3 chromatin analysis reveals epigenetic regulation of CHRNA7 with deficiencies in Rett and autism brain

Copy number variations (CNVs) within human 15q11.2-13.3 show reduced penetrance and variable expressivity in a range of neurologic disorders. Therefore, characterizing 15q11.2-13.3 chromatin structure is important for understanding the regulation of this locus during normal neuronal development. Deletion of the Prader-Willi imprinting center (PWS-IC) within 15q11.2-13.3 disrupts long-range imprinted gene expression resulting in Prader-Willi syndrome. Previous results establish that MeCP2 binds to the PWS-IC and is required for optimal expression of distal GABRB3 and UBE3A. To examine the hypothesis that MeCP2 facilitates 15q11.2-13.3 transcription by linking the PWS-IC with distant elements, chromosome capture conformation on chip (4C) analysis was performed in human SH-SY5Y neuroblastoma cells. SH-SY5Y neurons had 2.84-fold fewer 15q11.2-13.3 PWS-IC chromatin interactions than undifferentiated SH-SY5Y neuroblasts, revealing developmental chromatin de-condensation of the locus. Out of 68 PWS-IC interactions with15q11.2-13.3 identified by 4C analysis and 62 15q11.2-13.3 MeCP2-binding sites identified by previous ChIP-chip studies, only five sites showed overlap. Remarkably, two of these overlapping PWS-IC- and MeCP2-bound sites mapped to sites flanking CHRNA7 (cholinergic receptor nicotinic alpha 7) encoding the cholinergic receptor, nicotinic, alpha 7. PWS-IC interaction with CHRNA7 in neurons was independently confirmed by fluorescent in situ hybridization analysis. Subsequent quantitative transcriptional analyses of frontal cortex from Rett syndrome and autism patients revealed significantly reduced CHRNA7 expression compared with controls. Together, these results suggest that transcription of CHRNA7 is modulated by chromatin interactions with the PWS-IC. Thus, loss of long-range chromatin interactions within 15q11.2-13.3 may contribute to multiple human neurodevelopmental disorders.

Histone H3 trimethylation at lysine 36 is associated with constitutive and facultative heterochromatin

The mammalian genome contains numerous regions known as facultative heterochromatin, which contribute to transcriptional silencing during development and cell differentiation. We have analyzed the pattern of histone modifications associated with facultative heterochromatin within the mouse imprinted Snurf-Snrpn cluster, which is homologous to the human Prader-Willi syndrome genomic region. We show here that the maternally inherited Snurf-Snrpn 3-Mb region, which is silenced by a potent transcription repressive mechanism, is uniformly enriched in histone methylation marks usually found in constitutive heterochromatin, such as H4K20me3, H3K9me3, and H3K79me3. Strikingly, we found that trimethylated histone H3 at lysine 36 (H3K36me3), which was previously identified as a hallmark of actively transcribed regions, is deposited onto the silenced, maternally contributed 3-Mb imprinted region. We show that H3K36me3 deposition within this large heterochromatin domain does not correlate with transcription events, suggesting the existence of an alternative pathway for the deposition of this histone modification. In addition, we demonstrate that H3K36me3 is markedly enriched at the level of pericentromeric heterochromatin in mouse embryonic stem cells and fibroblasts. This result indicates that H3K36me3 is associated with both facultative and constitutive heterochromatin. Our data suggest that H3K36me3 function is not restricted to actively transcribed regions only and may contribute to the composition of heterochromatin, in combination with other histone modifications.

A new deletion refines the boundaries of the murine Prader-Willi syndrome imprinting center

The human chromosomal 15q11-15q13 region is subject to both maternal and paternal genomic imprinting. Absence of paternal gene expression from this region results in Prader-Willi syndrome (PWS), while absence of maternal gene expression leads to Angelman syndrome. Transcription of paternally expressed genes in the region depends upon an imprinting center termed the PWS-IC. Imprinting defects in PWS can be caused by microdeletions and the smallest commonly deleted region indicates that the PWS-IC lies within a region of 4.3 kb. The function and location of the PWS-IC is evolutionarily conserved, but delineation of the PWS-IC in mouse has proven difficult. The first targeted mutation of the PWS-IC, a deletion of 35 kb spanning Snrpn exon 1, exhibited a complete PWS-IC deletion phenotype. Pups inheriting this mutation paternally showed a complete loss of paternal gene expression and died neonatally. A reported deletion of 4.8 kb showed only a reduction in paternal gene expression and incomplete penetrance of neonatal lethality, suggesting that some PWS-IC function had been retained. Here, we report that a 6 kb deletion spanning Snrpn exon 1 exhibits a complete PWS-IC deletion phenotype. Pups inheriting this mutation paternally lack detectable expression of all PWS genes and paternal silencing of Ube3a, exhibit maternal DNA methylation imprints at Ndn and Mkrn3 and suffer failure to thrive leading to a fully penetrant neonatal lethality.

Prader-Willi Syndrome: Obesity due to Genomic Imprinting

Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder due to errors in genomic imprinting with loss of imprinted genes that are paternally expressed from the chromosome 15q11-q13 region. Approximately 70% of individuals with PWS have a de novo deletion of the paternally derived 15q11-q13 region in which there are two subtypes (i.e., larger Type I or smaller Type II), maternal disomy 15 (both 15s from the mother) in about 25% of cases, and the remaining subjects have either defects in the imprinting center controlling the activity of imprinted genes or due to other chromosome 15 rearrangements. PWS is characterized by a particular facial appearance, infantile hypotonia, a poor suck and feeding difficulties, hypogonadism and hypogenitalism in both sexes, short stature and small hands and feet due to growth hormone deficiency, mild learning and behavioral problems (e.g., skin picking, temper tantrums) and hyperphagia leading to early childhood obesity. Obesity is a significant health problem, if uncontrolled. PWS is considered the most common known genetic cause of morbid obesity in children. The chromosome 15q11-q13 region contains approximately 100 genes and transcripts in which about 10 are imprinted and paternally expressed. This region can be divided into four groups: 1) a proximal non-imprinted region; 2) a PWS paternal-only expressed region containing protein-coding and non-coding genes; 3) an Angelman syndrome region containing maternally expressed genes and 4) a distal non-imprinted region. This review summarizes the current understanding of the genetic causes, the natural history and clinical presentation of individuals with PWS.

Prader-Willi-Syndrom und Angelman-Syndrom

Das Prader-Willi-Syndrom (PWS) und das Angelman-Syndrom (AS) sind distinkte neurogenetische Erkrankungen, die durch den Funktionsverlust geprägter Gene in der Region 15q11q13 hervorgerufen werden. Ein etwa 2 Mb großer Bereich in 15q11q13 unterliegt einer elternspezifischen Prägung („genomic imprinting“). Infolge der Prägung unterscheiden sich die väterliche und mütterliche Kopie dieses Bereichs in der DNA-Methylierung und der Genexpression. Die häufigsten Ursachen für PWS und AS sind eine 5–7 Mb große interstitielle Deletion in der chromosomalen Region 15q11q13, eine uniparentale Disomie 15, ein Imprintingdefekt oder im Fall von AS eine Mutation im UBE3A-Gen. Mikrodeletionen bei einer kleinen Anzahl von Patienten mit einem Imprintingdefekt haben zur Identifizierung eines Imprinting-Centers (IC) „upstream“ vom SNURF-SNRPN-Gen geführt, das die Imprintumschaltung in cis reguliert. Das IC hat eine zweiteilige Struktur: Einen zentromerischen Teil, der bei Patienten mit AS deletiert ist und für die mütterliche Prägung notwendig ist, und einen telomerischen Teil, der bei Patienten mit PWS deletiert ist und der für die Aufrechterhaltung der väterlichen Prägung notwendig ist.

Managing Prader-Willi syndrome in families: an embodied exploration

This paper aims to explore the management of a rare chromosome disorder, Prader-Willi syndrome (PWS), within families. It is particularly concerned with developing an understanding of the management of diet and other everyday practices affecting the body. People with PWS tend to experience poor muscle tone combined with food obsession. The level of control over diet needed to prevent obesity and related health complications is often lacking as individuals also experience various forms of learning disability, autistic spectrum disorders and behavioural problems. The findings are based on data from twenty qualitative case studies of English families with a young person with PWS. Analysis of management strategies highlights the centrality of embodied agency in shaping everyday practices and interactions. The significance and influence of biology within this process is particularly evident, as people with the genetic condition PWS experience embodiment and emotion in distinct ways and differently from non-PWS family members in the research sample. Focusing on the multidimensional nature of processes surrounding body management, the paper highlights three key management practices and explores how these practices are influenced by people with PWS and interpreted by family members. Three key practices are identified as: restricting access to food, keeping occupied, and use of routine. The study represents the first UK empirical sociological study of PWS and primarily adds an insight of family management of PWS to a medically dominated literature around the disorder. The findings can sensitise health and social care professionals to some potential issues for families managing PWS, and guide and develop appropriate interventions to support young people with PWS and their carers.

Precise microdeletion detection of Prader-Willi Syndrome with array comparative genome hybridization

OBJECTIVE

Prader-Willi Sydrome (PWS) is a human disorder related to genomic imprinting defect on 15q11-13. It is characterized by a series of classic features such as hypotonia, hyperphagia, obesity, osteoporosis, typical facial and body dysmorphosis, hypogonadism, mental and behaviour disorders. Our study was designed to precisely detect the microdeletions, which accounts for 65%-70% of the PWS.

METHODS

Physical and laboratory examinations were firstly performed to diagnose PWS clinically, and to discover novel clinical features. Then the patient was screened with bisulfite-specific sequencing and precisely delineated through high-density array CGH.

RESULTS

With the bisulfite-specific sequencing, the detected CpG island in the PWS critical region was found homozygously hypermethylated. Then with array CGH, a 2.22 Mb type II microdeletion was detected, covering a region from MKRN3, MAGEL2, NDN, PWRN2, PWRN1, C12orf2, SNURF-SNRPN, C/D snoRNAs, to distal of UBE3A.

CONCLUSIONS

Array CGH, after the fast screening of Bisulfite-specific sequencing, is a feasible and precise method to detect microdeletions in PWS patients. A novel feature of metacarpophalangeal joint rigidity was also presented, which is the first time reported in PWS.

Neurodevelopmental disorders involving genomic imprinting at human chromosome 15q11-q13

Human chromosome 15q11-q13 is subject to regulation by genomic imprinting, an epigenetic process by which genes are expressed in a parent-of-origin specific manner. Three neurodevelopmental disorders, Prader-Willi syndrome, Angelman syndrome, and 15q duplication syndrome, result from aberrant expression of imprinted genes in this region. Here, we review the current literature pertaining to mouse models and recently identified patients with atypical deletions, which shed light on the epigenetic regulation of the chromosome 15q11-q13 subregion and the genes that are responsible for the phenotypic outcomes of these disorders.

Atp10a, a gene adjacent to the PWS/AS gene cluster, is not imprinted in mouse and is insensitive to the PWS-IC

Mutations affecting a cluster of coordinately regulated imprinted genes located at 15q11-q13 underlie both Prader-Willi syndrome (PWS) and Angelman syndrome (AS). Disruption of the predominately maternally expressed UBE3A locus is sufficient to meet diagnostic criteria for AS. However, AS patients with a deletion of the entire PWS/AS locus often have more severe traits than patients with point mutations in UBE3A suggesting that other genes contribute to the syndrome. ATP10A resides 200 kb telomeric to UBE3A and is of uncertain imprinted status. An initial report indicated bialleleic expression of the murine Atp10a in all tissues, but a subsequent report suggests that Atp10a is predominantly maternally expressed in the hippocampus and olfactory bulb. To resolve this discrepancy, we investigated Atp10a allelic expression in the brain, DNA methylation status, and sensitivity to mutations of the PWS imprinting center, an element required for imprinted gene expression in the region. We report that Atp10a is biallelically expressed in both the newborn and adult brain, and Atp10a allelic expression is insensitive to deletion or mutation of the PWS imprinting center. The CpG island associated with Atp10a is hypomethylated, a result consistent with the notion that Atp10a is not an imprinted gene.

Protein-binding elements establish in the oocyte the primary imprint of the Prader-Willi/Angelman syndromes domain

Imprinting of the PWS/AS 2.4 Mb domain in the human is controlled by a paternally active imprinting center (PWS-IC). PWS-IC on the maternal allele is methylated and inactivated by an 880-bp sequence (AS-IC) located 30 kb upstream. In this communication, we report the identification of 7 cis acting elements within AS-IC. The elements: DMR, DNS, 2 OCTA sequences, SOX, E1, and E2 bind specific proteins that form at least 2 protein complexes. Using variants of an imprinted transgene, mutated at the elements each at a time, we show that (i) all 7 elements are involved in the methylation and inactivation of the maternal PWS-IC; (ii) the OCTA and SOX elements that bind a protein complex, and the E1 and E2 elements, function in establishing the primary imprint that constitutes an active and unmethylated AS-IC in the oocyte; (iii) DNS and DMR bind a multiprotein complex that may facilitate interaction between AS-IC and PWS-IC, mediating the inactivation in cis of PWS-IC; and (iv) all 7 elements participate in maintaining an unmethylated PWS-IC in the oocyte, which is essential for its maternal methylation later in development. Altogether, the above observations imply that the cis acting elements on AS-IC display diverse functions in establishing the imprints at both AS-IC and PWS-IC in the oocyte. A postulated epigenetic mark imprints the PWS-IC in the oocyte and maintains its inactive status during development before it is translated into maternal methylation.

Analysis of the Prader-Willi syndrome chromosome region using quantitative microsphere hybridization (QMH) array

We previously developed a novel quantitative microsphere suspension hybridization (QMH) assay for high-throughput determination of genomic copy number by direct hybridization of unique sequence probes to genomic DNA followed by flow cytometric analysis. Herein, we describe the first clinical application of this assay examining the Prader-Willi syndrome (PWS) chromosome region at 15q11-13. We designed 30 unique sequence test probes (approximately 60 nucleotides each) spanning 11.37 Mb of chromosome 15q11.2-q13.3 and a disomic reference probe (Actin Beta, chromosome 7p22.1), conjugated to spectrally distinct polystyrene microsphere levels. All probes were hybridized to biotin-labeled genomic DNA in multiplex QMH reactions, and hybridization was detected using phycoerythrin-labeled streptavidin and analyzed by dual-laser flow cytometry. Copy number differences were distinguished by comparing mean fluorescence intensities (MFI) of the test probes to the reference probe in 20 individuals with PWS and six controls. The mean MFI ratio for deleted loci was 0.56 +/- 0.09 (n = 88) as compared to the MFI ratios for normal loci, 0.96 +/- 0.06 (n = 236), and duplicated loci, 1.44 +/- 0.10 (n = 22). A multiplex QMH assay could readily distinguish type I from type II deletions in PWS subjects, as well as small (approximately 4.3 kb) imprinting center (IC) deletions, with no overlap in MFI values compared with normal loci. Using this diagnostic QMH assay, the precise deleted genomic interval could be ascertained in all PWS subjects examined in the present study.

Mechanisms of imprinting of the Prader-Willi/Angelman region

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurodevelopmental disorders, each caused by several genetic and epigenetic mechanisms involving the proximal long arm of chromosome 15. Lack of a functional paternal copy of 15q11-q13 causes PWS; lack of a functional maternal copy of UBE3A, a gene within 15q11-q13, causes AS. This region of chromosome 15 contains a number of imprinted genes that are coordinately regulated by an imprinting center (PWS/AS-IC) that contains two functional elements, the PWS-SRO and the AS-SRO. A chromosome lacking the PWS-SRO has the maternal state of gene activity and epigenetic modification after either maternal or paternal transmission; a chromosome lacking the AS-SRO but containing the PWS-SRO has the paternal state of gene activity and epigenetic modification after either maternal or paternal transmission. The maternal state of chromosome 15q11-q13 is associated with methylation of the PWS-SRO, while the paternal state is associated with lack of methylation of the PWS-SRO. Although most models of PWS/AS region imprinting assume that the PWS-SRO is methylated during oogenesis and that this methylation of the maternal PWS-SRO is maintained after fertilization, several lines of evidence suggest that the maternal PWS-SRO is in fact not methylated until after fertilization. Imprinting defects affecting the PWS/AS region can arise from failure to demethylate the PWS-SRO in the male germ line, from failure to methylate the maternal PWS-SRO, or from failure to maintain PWS-SRO methylation after fertilization.

Treatment of precocious puberty in a female with Prader-Willi syndrome

Prader-Willi syndrome (PWS) is characterized by neonatal hypotonia, hyperphagia, childhood obesity at an early age, short stature, varying degrees of mental deficiency, and hypogonadism. In PWS, puberty is usually delayed and fails to complete, with most females never having regular menstrual cycles. We report a female patient with uniparental disomy, who experienced precocious puberty with menarche at age 8 years. The patient was treated with luteinizing hormone releasing hormone (LHRH) analog, which suppressed pubertal development. From our search of the literature this is the first application of LHRH analog to a female PWS patient for precocious puberty. Use of LHRH analog along with recombinant human growth hormone (rhGH) permitted stature closer to target height. The clinical course of this patient with PWS underscores the need for individualized treatment.

Genomic analysis of the chromosome 15q11-q13 Prader-Willi syndrome region and characterization of transcripts for GOLGA8E and WHCD1L1 from the proximal breakpoint region

BACKGROUND

Prader-Willi syndrome (PWS) is a neurobehavioral disorder characterized by neonatal hypotonia, childhood obesity, dysmorphic features, hypogonadism, mental retardation, and behavioral problems. Although PWS is most often caused by a paternal interstitial deletion of a 6-Mb region of chromosome 15q11-q13, the identity of the exact protein coding or noncoding RNAs whose deficiency produces the PWS phenotype is uncertain. There are also reports describing a PWS-like phenotype in a subset of patients with full mutations in the FMR1 (fragile X mental retardation 1) gene. Taking advantage of the human genome sequence, we have performed extensive sequence analysis and molecular studies for the PWS candidate region.

RESULTS

We have characterized transcripts for the first time for two UCSC Genome Browser predicted protein-coding genes, GOLGA8E (golgin subfamily a, 8E) and WHDC1L1 (WAS protein homology region containing 1-like 1) and have further characterized two previously reported genes, CYF1P1 and NIPA2; all four genes are in the region close to the proximal/centromeric deletion breakpoint (BP1). GOLGA8E belongs to the golgin subfamily of coiled-coil proteins associated with the Golgi apparatus. Six out of 16 golgin subfamily proteins in the human genome have been mapped in the chromosome 15q11-q13 and 15q24-q26 regions. We have also identified more than 38 copies of GOLGA8E-like sequence in the 15q11-q14 and 15q23-q26 regions which supports the presence of a GOLGA8E-associated low copy repeat (LCR). Analysis of the 15q11-q13 region by PFGE also revealed a polymorphic region between BP1 and BP2. WHDC1L1 is a novel gene with similarity to mouse Whdc1 (WAS protein homology region 2 domain containing 1) and human JMY protein (junction-mediating and regulatory protein). Expression analysis of cultured human cells and brain tissues from PWS patients indicates that CYFIP1 and NIPA2 are biallelically expressed. However, we were not able to determine the allele-specific expression pattern for GOLGA8E and WHDC1L1 because these two genes have highly related sequences that might also be expressed.

CONCLUSION

We have presented an updated version of a sequence-based physical map for a complex chromosomal region, and we raise the possibility of polymorphism in the genomic orientation of the BP1 to BP2 region. The identification of two new proteins GOLGA8E and WHDC1L1 encoded by genes in the 15q11-q13 region may extend our understanding of the molecular basis of PWS. In terms of copy number variation and gene organization, this is one of the most polymorphic regions of the human genome, and perhaps the single most polymorphic region of this type.

Genomic imprinting and imprinting defects in humans

In placental mammals some 100-200 genes are expressed only from the paternal or the maternal allele. This peculiar expression pattern is the result of genomic imprinting, an epigenetic process by which the male and the female germ line confer a parent-of-origin specific mark (imprint) on certain chromosomal regions. The size of imprinted regions ranges from several kilobases to several megabases. The process of genomic imprinting is controlled by cis-acting imprinting centers (IC) and trans-acting factors. IC mutations affect the establishment or maintenance of genomic imprints and hence the expression of all imprinted genes controlled by this IC. Imprinting defects play a causal role in several recognizable syndromes.

Molecular Basis of Genetic Neuropsychiatric Disorders

The past decade has seen tremendous advances in our understanding of the molecular and genetic basis of many neuropsychiatric disorders. Although the genetic aberrations that lead to these syndromes have been identified in many cases, not much is known about specific gene products and their function. This article reviews the molecular basis of well-known neurogenetic disorders. The syndromes discussed here follow a Mendelian pattern of inheritance and are predominantly single-gene disorders; however, most childhood and adolescent psychiatric disorders are polygenic in nature. This genetic complexity and heterogeneity has made it difficult to identify the genes involved in their etiology. Identification of genetic and environmental risk factors involved in the etiology of complex disorders, such as autism, will help in the discovery of medications that can ameliorate the symptoms.

A targeted deletion upstream of Snrpn does not result in an imprinting defect

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) result from the disturbance of imprinted gene expression within human chromosome 15q11-q13. Some cases of PWS and AS are caused by microdeletions near the SNRPN gene that disrupt a regulatory element termed the imprinting center (IC). The IC has two functional components; an element at the promoter of SNRPN involved in PWS (PWS-IC) and an element 35 kilobases (kb) upstream of SNRPN involved in AS (AS-IC). To further understand the function of the IC, we sought to create a mouse model for AS-IC mutations. We have generated two deletions at a location analogous to that of the human AS-IC. Neither deletion produced an imprinting defect as indicated by DNA methylation and gene expression analyses. These results indicate that no elements critical for AS-IC function in mouse reside within the 12.8-kb deleted region and suggest that the specific location of the AS-IC is not conserved between human and mouse.

Mechanisms regulating imprinted genes in clusters

Clustered imprinted genes are regulated by differentially methylated imprinting control regions (ICRs) that affect gene activity and repression in cis over a large region. Although a primary imprint signal for each of these clusters is DNA methylation, different mechanisms are used to establish and maintain these marks. The majority of ICRs are methylated in the maternal germline and are usually promoters for antisense transcripts whose elongation is associated with imprinting control in the domain. In contrast, ICRs methylated in the paternal germline do not appear to act as promoters and are located between genes. At least one, at the Igf2/H19 locus, is known to function as an insulator. Analysis of ICRs suggests that maternal and paternal methylation imprints function in distinct ways.

Analysis of candidate imprinted genes in PWS subjects with atypical genetics: a possible inactivating mutation in the SNURF/SNRPN minimal promoter

Prader-Willi syndrome (PWS) is a neurodevelopmental disorder associated with abnormalities of chromosome 15q11q13. The majority of cases result either from a deletion approximately 4 Mb in size, affecting chromosome 15 of paternal origin or from UPD(15)mat; these account for approximately 70 and approximately 20-25% of PWS cases, respectively. In the remaining 3-5% of PWS cases where neither the deletion nor UPD is detectable, PWS is thought to be caused either by a defect in the imprinting centre resulting in a failure to reset the paternally inherited chromosome 15 derived from the paternal grandmother or, very occasionally, from a balanced translocation involving a breakpoint in 15q11q13. Nine probands with a firm clinical diagnosis of PWS but who had neither a typical deletion in the PWS region nor UPD(15)mat were investigated for inactivating mutations in 11 genes located in the PWS region, including SNURF and SNRPN, which are associated with the imprinting centre. Other genes studied for mutations included MKRN3, NDN, IPW, HBII-85, HBII-13, HBII-436, HBII-438a, PAR1 and PAR5. A possibly inactivating mutation in the SNRPN minimal promoter region was identified. No other inactivating mutations were found in the remainder of our panel of PWS subjects with atypical genetics. Expression levels of several of the candidate genes for PWS were also investigated in this series of probands. The results indicate that PWS may result from a stochastic partial inactivation of important genes.

Clinical-etiologic correlation in children with Prader-Willi syndrome (PWS): An interdisciplinary study

Prader-Willi syndrome (PWS) is a multisystemic disorder caused by the loss of expression of paternally transcribed genes within chromosome 15q11-q13. Most cases are due to paternal deletion of this region; the remaining cases result from maternal uniparental disomy (UPD) and imprinting defects. To better understand the phenotypic variability of PWS, a genotype-phenotype correlation study was performed in 91 children with PWS. Patients were diagnosed by Southern Blot Methylation assay and genetic subtypes were established using FISH and microsatellite analyses. Fifty-nine subjects with deletion (31/28 males/females; mean age 3.86 years), 30 with UPD (14/16 males/females; mean age 3.89 years) and 2 girls with a presumed imprinting defect (mean age 0.43 yrs) were identified. For correlation purposes patients were grouped as "deleted" and "non-deleted." An increased maternal age was found in the UPD group. Four of Holm's criteria were more frequently present in the deleted group: need for special feeding techniques, sleep disturbance, hypopigmentation, and speech articulation defects. Concerning cognitive assessments, only 9.52% of subjects with deletion had Full-Scale IQ (FSIQ) > or =70, while 61.53% of subjects without deletion had FSIQ > or =70. Similar results were found in behavioral measures. Sleep disorders and carbohydrate metabolism were systematically assessed. Polysomnoghaphic studies revealed a higher frequency of central events with desaturations > or =10% in the deleted group (P = 0.020). In summary, the phenotype was significantly different between both groups in certain parameters related to the CNS. These results might be related to the differences in brain gene expression of the genetic subtypes.

Imprinting center analysis in Prader–Willi and Angelman syndrome patients with typical and atypical phenotypes

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are genetic disorders caused by a deficiency of imprinted gene expression from the paternal or maternal chromosome 15, respectively. This deficiency is due to the deletion of the 15q11-q13 region, parental uniparental disomy of the chromosome 15, or imprinting defect (ID). Mutation of the UBE3A gene causes approximately 10% of AS cases. In this present study, we describe the molecular analysis and phenotypes of two PWS patients and four AS patients with ID. One of the PWS patients has a non-familial imprinting center (IC) deletion and displayed a severe phenotype with an atypical PWS appearance, hyperactivity and psychiatric vulnerability. The other PWS and AS patients did not present genetic abnormalities in the IC, suggesting an epimutation as the genetic cause. The methylation pattern of two AS patients showed a faint maternal band corresponding to a mosaic ID. One of these mosaic patients displayed a mild AS phenotype while the other displayed a PWS-like phenotype.

Deficiency of Rbbp1/Arid4a and Rbbp1l1/Arid4b alters epigenetic modifications and suppresses an imprinting defect in the PWS/AS domain

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are caused by deficiency of imprinted gene expression from paternal or maternal chromosome 15q11-q13, respectively. Genomic imprinting of the PWS/AS domain is regulated through a bipartite cis-acting imprinting center (PWS-IC/AS-IC) within and upstream of the SNRPN promoter. Here, we show that two Rb-binding protein-related genes, Rbbp1/Arid4a and Rbbp1l1/Arid4b, are involved in the regulation of imprinting of the IC. We recovered these two genes from gene trap mutagenesis selecting for altered expression of an Snrpn-EGFP fusion gene strategy. RBBP1/ARID4A is an Rb-binding protein. RBBP1/ARID4A interacts with RBBP1L1/ARID4B and with the Snrpn promoter, implying that both are part of a protein complex. To further elucidate their roles on regulation of imprinting, we deleted the Rbbp1/Arid4a and Rbbp1l1/Arid4b genes in mice. Combined homozygous deficiency for Rbbp1/Arid4a and heterozygous deficiency for Rbbp1l1/Arid4b altered epigenetic modifications at the PWS-IC with reduced trimethylation of histone H4K20 and H3K9 and reduced DNA methylation, changing the maternal allele toward a more paternal epigenotype. Importantly, mutations of Rbbp1/Arid4a, Rbbp1l1/Arid4b, or Rb suppressed an AS imprinting defect caused by a mutation at the AS-IC. These data identify Rbbp1/Arid4a and Rbbp1l1/Arid4b as new members of epigenetic complexes regulating genomic imprinting at the PWS/AS domain.

The human HYMAI/PLAGL1 differentially methylated region acts as an imprint control region in mice

Imprinting centers (IC) can be defined as cis-elements that are recognized in the germ line and are epigenetically modified to bring about the full imprinting program in a somatic cell. Two paternally expressed human genes, HYMAI and PLAGL1 (LOT1/ZAC), are located within human chromosome 6q24. Within this region lies a 1-kb CpG island that is differentially methylated in somatic cells, unmethylated in sperm, and methylated in mature oocytes in mice, characteristic features of an IC. Loss of methylation of the homologous region in humans is observed in patients with transient neonatal diabetes mellitus and hypermethylation is associated with a variety of cancers, suggesting that this region regulates the expression of one or more key genes in this region involved in these diseases. We now report that a transgene carrying the human HYMAI/PLAGL1 DMR was methylated in the correct parent-origin-specific manner in mice and this was sufficient to confer imprinted expression from the transgene. Therefore, we propose that this DMR functions as the IC for the HYMAI/PLAGL1 domain.

Mouse imprinting defect mutations that model Angelman syndrome

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurobehavioral disorders resulting from deficiency of imprinted gene expression from paternal or maternal chromosome 15q11-15q13, respectively. In humans, expression of the imprinted genes is under control of a bipartite cis-acting imprinting center (IC). Families with deletions causing PWS imprinting defects localize the PWS-IC to 4.3 kb overlapping with SNRPN exon 1. Families with deletions causing AS imprinting defects localize the AS-IC to 880 bp 35 kb upstream of the PWS-IC. We report two mouse mutations resulting in defects similar to that seen in AS patients with deletion of the AS-IC. An insertion/duplication mutation 13 kb upstream of Snrpn exon 1 resulted in lack of methylation at the maternal Snrpn promoter, activation of maternally repressed genes, and decreased expression of paternally repressed genes. The acquisition of a paternal epigenotype on the maternal chromosome in the mutant mice was demonstrated by the ability to rescue the lethality and growth retardation in a mouse model of a PWS imprinting defect. A second mutation, an 80-kb deletion extending upstream of the first mutation, caused a similar imprinting defect with variable penetrance. These results suggest that there is a mouse functional equivalent to the human AS-IC.

The Prader-Willi/Angelman imprinted domain and its control center

The present review focuses on the recent advances towards understanding the mode of operation of the imprinting center (IC) within the Prader-Willi/Angelman syndromes (PWS/AS) domain. Special emphasis is put on the elucidation of the functional interaction between the two parts of the center, AS-IC and PWS-IC. The recent studies, on which the review is based, reveal cis-acting elements and trans-acting proteins that constitute the two parts of the IC and presumably provide the molecular mechanism for this interaction. AS-IC acquires the primary imprint during gametogenesis by establishing the maternal epigenotype. The unmethylated maternal allele of the AS-IC binds, very likely, a trans-acting factor that confers methylation on the PWS-IC maternal allele after fertilization. It is assumed that the PWS-IC paternal epigenotype, once established, spreads across the entire PWS/AS domain in the soma.

Regulation of growth and metabolism by imprinted genes

A small sub-set of mammalian genes are subject to regulation by genomic imprinting such that only one parental allele is active in at least some sites of expression. Imprinted genes have diverse functions, notably including the regulation of growth. Much attention has been devoted to the insulin-like growth factor signalling pathway that has a major influence on fetal size and contains two components encoded by the oppositely imprinted genes, Igf2 (a growth promoting factor expressed from the paternal allele) and Igf2r (a growth inhibitory factor expressed from the maternal allele). These genes fit the parent-offspring conflict hypothesis for the evolution of genomic imprinting. Accumulated evidence indicates that at least one other fetal growth pathway exists that has also fallen under the influence of imprinting. It is clear that not all components of growth regulatory pathways are encoded by imprinted genes and instead it may be that within a pathway the influence of a single gene by each of the parental genomes may be sufficient for parent-offspring conflict to be enacted. A number of imprinted genes have been found to influence energy homeostasis and some, including Igf2 and Grb10, may coordinate growth with glucose-regulated metabolism. Since perturbation of fetal growth can be correlated with metabolic disorders in adulthood these imprinted genes are considered as candidates for involvement in this phenomenon of fetal programming.

Establishment of the primary imprint of the HYMAI/PLAGL1 imprint control region during oogenesis

Imprinting within domains occurs through epigenetic alterations to imprinting centers (ICs) that result in the establishment of parental-specific differences in gene expression. One candidate IC lies within the imprinted domain on human chromosome region 6q24. This domain contains two paternally expressed genes, the zinc finger protein gene PLAGL1 (ZAC/LOT1) and an untranslated mRNAcalled HYMAI. The putative IC overlaps exon 1 of HYMAI and is differentially methylated in somatic tissues. In humans, loss of methylation within this region is seen in some patients with transient neonatal diabetes mellitus, and hypermethylation of this region is found in ovarian cancer and is associated with changes in expression of PLAGL1, suggesting that it plays a key role in regulating gene expression. Differential methylation within this region is conserved in the homologous region on mouse chromosome 10A and is present on the maternal allele. In this paper, we report that DNA methylation is established during the growth phase of oogenesis and that this coincides with the establishment of monoallelic expression from this region lending further support to the hypothesis that this region functions as an IC.

Imprinting defects on human chromosome 15

The Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurogenetic diseases that are caused by the loss of function of imprinted genes on the proximal long arm of human chromosome 15. In a few percent of patients with PWS and AS, the disease is due to aberrant imprinting and gene silencing. In patients with PWS and an imprinting defect, the paternal chromosome carries a maternal imprint. In patients with AS and an imprinting defect, the maternal chromosome carries a paternal imprint. Imprinting defects offer a unique opportunity to identify some of the factors and mechanisms involved in imprint erasure, resetting and maintenance. In approximately 10% of cases the imprinting defects are caused by a microdeletion affecting the 5' end of the SNURF-SNRPN locus. These deletions define the 15q imprinting center (IC), which regulates imprinting in the whole domain. These findings have been confirmed and extended in knock-out and transgenic mice. In the majority of patients with an imprinting defect, the incorrect imprint has arisen without a DNA sequence change, possibly as the result of stochastic errors of the imprinting process or the effect of exogenous factors.

Identification of cis- and trans-acting factors possibly modifying the risk of epimutations on chromosome 15

In the majority of patients with a chromosome 15 imprinting defect (ID) causing Prader-Willi syndrome (PWS) or Angelman syndrome (AS), the defect is a primary epimutation that occurred spontaneously in the absence of a DNA mutation. We have investigated whether common DNA sequence variants in the bipartite imprinting centre (IC) are associated with an increased susceptibility to imprinting defects. We have determined the haplotype structure of the IC and found that the two IC elements called 'PWS-SRO' and 'AS-SRO' lie on separate haplotype blocks. To identify susceptible IC sequence variants, we have used the transmission disequilibrium test. While we did not observe preferential transmission of a paternal allele or haplotype in 41 PWS-ID trios, we found a trend for preferential maternal transmission of one AS-SRO haplotype (H-AS3) in 48 AS-ID trios (P=0.058) and could identify two sequence variants in H-AS3 that are responsible for this effect. We also obtained tentative evidence that homozygosity for the 677C>T variant of the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene on chromosome 1 might increase the risk of a maternal imprinting defect: the frequency of the TT genotype was significantly higher in the mothers of the AS patients with an imprinting defect than in the patients' fathers or the general population (P=0.028). Our findings suggest that women with the IC haplotype H-AS3 or homozygosity for the MTHFR 677C>T variant may have an increased risk of conceiving a child with an imprinting defect, although the absolute risk is low.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.