Imprinting Center Research Articles

Paradoxical leanness in the imprinting-centre deletion mouse model for Prader-Willi syndrome.

Prader–Willi syndrome (PWS), a neurodevelopmental disorder caused by loss of paternal gene expression from 15q11–q13, is characterised by growth retardation, hyperphagia and obesity. However, as single gene mutation mouse models for this condition display an incomplete spectrum of the PWS phenotype, we have characterised the metabolic impairment in a mouse model for ‘full’ PWS, in which deletion of the imprinting centre (IC) abolishes paternal gene expression from the entire PWS cluster. We show that PWS-ICdel mice displayed postnatal growth retardation, with reduced body weight, hyperghrelinaemia and marked abdominal leanness; proportionate retroperitoneal, epididymal/omental and inguinal white adipose tissue (WAT) weights being reduced by 82%, 84% and 67%, respectively. PWS-ICdel mice also displayed a 48% reduction in proportionate interscapular brown adipose tissue (isBAT) weight with significant ‘beiging’ of abdominal WAT, and a 2°C increase in interscapular surface body temperature. Maintenance of PWS-ICdel mice under thermoneutral conditions (30°C) suppressed the thermogenic activity in PWS-ICdel males, but failed to elevate the abdominal WAT weight, possibly due to a normalisation of caloric intake. Interestingly, PWS-ICdel mice also showed exaggerated food hoarding behaviour with standard and high-fat diets, but despite becoming hyperphagic when switched to a high-fat diet, PWS-ICdel mice failed to gain weight. This evidence indicates that, unlike humans with PWS, loss of paternal gene expression from the PWS cluster in mice results in abdominal leanness. Although reduced subcutaneous insulation may lead to exaggerated heat loss and thermogenesis, abdominal leanness is likely to arise from a reduced lipid storage capacity rather than increased energy utilisation in BAT.

Prader-Willi syndrome and atypical submicroscopic 15q11-q13 deletions with or without imprinting defects

We report a 20 year follow up on a Caucasian female, now 26 years of age, with Prader-Willi syndrome (PWS) harboring an atypical 15q11-q13 submicroscopic deletion of 100–200 kb in size first detected in 1996 involving the imprinting center, SNRPN gene and surrounding region. PWS is a rare complex disorder caused by the loss of paternally expressed genes in the 15q11-q13 region. With high resolution chromosomal microarray and methylation – specific MLPA analysis, we updated the genetic findings on our patient and found a 209,819bp deletion including the SNURF-SNRPN gene complex which includes the imprinting center and the SNORD116 region. We compared with four other similarly reported individuals in the literature with atypical submicroscopic deletions within this region but without imprinting center involvement to better characterize the specific genetic lesions causing PWS clinical findings. Clinically, our patient met the diagnostic criteria of PWS including infantile hypotonia, a poor suck with feeding difficulties, global developmental delays and later food foraging, childhood obesity, small hands and skin picking. Small atypical deletions of comparable sizes were seen in the 15q11-q13 region in all five cases and similar behavioral/physical characteristics were found despite an imprinting defect in our patient. These results further support an overlapping critical deletion region involving the non-coding snoRNA SNORD116 in common in the five individuals playing a key role in contributing to the PWS phenotype.

Epigenotype, genotype, and phenotype analysis of patients in Taiwan with Beckwith-Wiedemann syndrome.

Beckwith-Wiedemann syndrome (BWS) is a congenital overgrowth disorder predisposing to tumorigenesis that results from abnormal expression or function of imprinted genes of chromosome 11p15.5. Forty-seven patients in Taiwan with clinical suspicion of BWS were referred for diagnostic testing based on methylation profiling of H19-associated imprinting center (IC) 1 and KCNQ1OT1-associated IC2 using high-resolution melting analysis, multiplex ligation-dependent probe amplification, or high-resolution quantitative methylation profiling. Twenty-eight patients received a clinical diagnosis of BWS (the presence of 3 major features or 2 major features and at least 1 minor feature), 18 had suspected BWS (the presence of at least 1 major feature), and 1 had isolated Wilms' tumor. Nineteen patients were identified with IC2 hypomethylation (including 1 with isolated Wilms' tumor), 1 with IC1 hypermethylation, 2 with paternal uniparental disomy, and 1 with CDKN1C mutation. Several clinical features were found to be statistically different (P<0.05) between the 2 groups-clinical diagnosis of BWS (n=28) or suspected BWS (n=18)-including macroglossia, pre- or postnatal gigantism, abdominal wall defect, ear creases, facial nevus flammeus, BWS score, and the molecular diagnosis rate. Molecular lesion was detected in 81% of patients with the presence of three major features, compared with 33% and 28% of those with two or one major feature, respectively. The mean BWS score was 5.6 for 19 subjects with "IC2 hypomethylation", compared with 3.8 for 2 subjects with pUPD. The BWS score of one subject with CDKN1C mutation and one with IC1 hypermethylation was 6 and 7, respectively. The BWS score was positively correlated with the molecular diagnosis rate (P<0.01). The BWS database of epigenotype, genotype, and phenotype is expected to promote better genetic counseling and medical care of these patients.

Genomic imbalance in the centromeric 11p15 imprinting center in three families: Further evidence of a role for IC2 as a cause of Russell-Silver syndrome.

Russell-Silver syndrome is a heterogeneous disorder characterized by intrauterine growth retardation, postnatal growth deficiency, characteristic facial appearance, and other variable features. Genetic and epigenetic alterations are identified in about 60% of individuals with Russell-Silver syndrome. Most frequently, Russell-Silver syndrome is caused by altered gene expression on chromosome 11p15 due to loss of methylation at the telomeric imprinting center. To date there have been a handful of isolated clinical reports implicating the centromeric imprinting center 2 in the etiology of Russell-Silver syndrome. Here we report three new families with genomic imbalances, involving imprinting center 2 resulting in gain of methylation at this center and a Russell-Silver syndrome phenotype, including two families with a maternally inherited microduplication and the first pediatric patient with a paternally derived microdeletion. The findings in our families provide additional evidence of a role for imprinting center 2 in the etiology of Russell-Silver syndrome and suggest that imprinting center 2 imprinting abnormalities may be a more common cause of Russell-Silver syndrome than previously recognized. Furthermore, our findings together with previous clinical reports of genomic imbalances involving imprinting center 2 serve to underscore the complexity of the epigenetic regulation of the 11p15 region making it challenging to predict phenotype on the basis of genotype alone. © 2016 Wiley Periodicals, Inc.

Angelman syndrome in Hong Kong Chinese: A 20 years’ experience

AS(OMIM #105830) is a neurodevelopmental disease that characterized by severe intellectual disability, lack of speech, happy disposition, ataxia, epilepsy and distinct behavioural profile.A tertiary wide study was performed in Hong Kong with aim to examine the clinical and molecular features, genotype-phenotype correlation of the Angelman syndrome (AS) patients. There were total 55 molecularly confirmed AS between January 1995 to September 2015 for review. 65.5% of them were caused by maternal microdeletion, 10.9% by paternal uniparental disomy, 3.6% by imprinting center defect and 14.5% by UBE3A gene mutation. Genotype-phenotype correlation showed epilepsy and microcephaly is more common in microdeletion type as compared with non-microdeletional type. We have concluded that the incidence rate, clinical features and underlying genetic mechanisms in Hong Kong Chinese were comparable with other western populations. The overall average age of diagnosis in this cohort was 6.2 years old (95% C.I was 5.0–7.5 years old). It is hope that by increasing awareness and early referral could result in early diagnosis and better management for AS patient.

Prader-Willi Syndrome: Clinical Genetics and Diagnostic Aspects with Treatment Approaches.

Prader-Willi syndrome (PWS) is a neuro-developmental genetic disorder due to lack of expression of genes inherited from the paternal chromosome 15q11-q13 region with three main genetic subtypes. These include paternal 15q11-q13 deletion (about 70% of cases), maternal uniparental disomy 15 or both 15s from the mother (20-30% of cases), and defects in the imprinting center (1-3%) which controls the expression of imprinted genes in this chromosome region. Clinical manifestations include infantile hypotonia with a poor suck resulting in failure to thrive, short stature, small hands/feet and hypogonadism/hypogenitalism due to growth and other hormone deficiencies, hyperphagia and excessive weight gain with obesity and cognitive and behavioral problems including obsessive compulsions, tantrums and self-injury. The phenotype is likely related to hypothalamic dysfunction. Hyperphagia and obesity with related complications are major causes of morbidity and mortality in PWS requiring accurate diagnosis, appropriate medical management and treatment; the major objective of our report. An extensive review of the literature was undertaken including genetics, clinical and behavioral aspects, and updated health-related information addressing the importance of early diagnosis and treatment of individuals with Prader-Willi syndrome. A searchable, bulleted and formatted list of topics related to this obesity syndrome was provided utilizing a Table of Contents approach for the clinical practitioner. Physicians and other health care providers can use this review with clinical, genetic and treatment summaries divided into sections that are pertinent in the context of clinical practice. Finally, frequently asked questions by clinicians, families and other interested participants will be addressed.

Correction of aberrant imprinting by allele-specific epigenome editing.

Imprinting disorders are caused by the loss of the normal allele-specific DNA methylation at imprinting centers. Epigenetic editing is a promising approach to alter DNA methylation at defined genomic target regions. The novel development of CRISPR-Cas9-based DNA binding domains may allow for an allele-specific editing of DNA methylation at imprinted loci, for the first time offering a rational approach for correction of the molecular defects in imprinting disorders.

Sexual dichotomy of gonadal function in Prader-Willi syndrome from early infancy through the fourth decade.

At what age does the type of hypogonadism, namely hypothalamic or primary gonadal defect, become established in men and women with Prader-Willi syndrome (PWS)? The type of hypogonadism becomes established only in late adolescence and early adulthood. The etiology of hypogonadism in PWS is heterogeneous and the clinical expression is variable. Primary testicular failure is common in PWS men, while combinations of ovarian dysfunction and gonadotrophin deficiency are seen in women. This is a prospective study of a cohort of 106 PWS patients followed for a mean duration of 4.5 years. Serial blood samples were obtained and assayed for gonadotrophins, inhibin B, anti-Mullerian hormone (AMH), dehydroepiandrosterone sulfate (DHEAS), testosterone (males), and estradiol (females). Results were compared with normal reference values obtained from the literature. For the purpose of this study, we defined the following age groups: infants <1 year; children 1-10 years; adolescents 11-20 years and adults >20 years. Study participants were 49 males (aged 2 months to 36 years) and 57 females (aged 1 month to 37 years) with genetically confirmed diagnoses of PWS (deletions 60, uniparental disomy 54, imprinting center defect 2) followed in the Israel national multidisciplinary PWS clinic. Serum LH levels were in the normal range (1.0-6.0 mIU/ml) for 7/10 adult men, and high in 3, while FSH (normal range 1.0-6.1 mIU/ml) was elevated (34.4 ± 11.5 mIU/ml) in 6 and normal (3.5 ± 1.6 mIU/ml) in 4 men. Testosterone was low (5.7 ± 3.4 nmol/l) compared with the normal range of 12.0-34.5 nmol/l in the reference population in all men >20 years. AMH showed a normal decrease with age, despite low testosterone levels. Inhibin B was normal (241 ± 105 pg/ml) in infant boys, but low or undetectable in most adult men. Hormonal profiles were more heterogeneous in women than in men. Estradiol was consistently detectable in only 7/13 adult women. Inhibin B was low or undetectable in all PWS females although occasional samples showed levels within the normal range of 15-95 pg/ml. Vaginal bleeding was reported to occur for the first time in eight women at a median age of 20 years (13-34 years), but only one had regular monthly menses. The type of hypogonadism (primary or secondary) in PWS can be determined only after age 20 years. The study cohort was heterogeneous, showing variability in BMI, cognitive disability and medical treatment. Demonstration of the natural history of reproductive hormone development in PWS suggests that androgen replacement may be indicated for most PWS boys in mid-adolescence. Recommendations for hormone replacement in PWS women need to be individually tailored, serial measurements of inhibin B should be performed, and contraception should be considered in those women who may have the potential for fertility.

P.1.a.004 The new genetic markers of development syndromes associated with intellectual disability in Armenian population

AS(OMIM #105830) is a neurodevelopmental disease that characterized by severe intellectual disability, lack of speech, happy disposition, ataxia, epilepsy and distinct behavioural profile.A tertiary wide study was performed in Hong Kong with aim to examine the clinical and molecular features, genotype-phenotype correlation of the Angelman syndrome (AS) patients. There were total 55 molecularly confirmed AS between January 1995 to September 2015 for review. 65.5% of them were caused by maternal microdeletion, 10.9% by paternal uniparental disomy, 3.6% by imprinting center defect and 14.5% by UBE3A gene mutation. Genotype-phenotype correlation showed epilepsy and microcephaly is more common in microdeletion type as compared with non-microdeletional type. We have concluded that the incidence rate, clinical features and underlying genetic mechanisms in Hong Kong Chinese were comparable with other western populations. The overall average age of diagnosis in this cohort was 6.2 years old (95% C.I was 5.0–7.5 years old). It is hope that by increasing awareness and early referral could result in early diagnosis and better management for AS patient.

Autism spectrum disorder in Prader-Willi syndrome: A systematic review.

Prader-Willi syndrome (PWS) is a rare genetic disorder that results from lack of expression of paternally-derived genes on chromosome 15q11-13; caused by a deletion (DEL), uniparental disomy (UPD), or a rare imprinting center defect. PWS is associated with a distinct behavioral phenotype that in some respects overlaps with autism spectrum disorder (ASD), a neurodevelopmental disorder characterized by restricted or repetitive behaviors (RRBs) and social-communication impairment. The goal of this review was to (i) review published literature investigating core ASD symptoms in PWS and (ii) provide a prevalence estimate of ASD in PWS. Two independent reviewers searched Medline, CINAHL, PsychINFO, Embase, and Web of Science to find studies that answered the research questions. Individuals with PWS demonstrate significant levels of RRBs and social-communication impairment, in some reports reaching similar levels to those of non-PWS ASD comparison groups. Individuals with UPD had more social-communication impairment than those with DEL. Of 786 PWS participants, 210 (26.7%) were reported as meeting criteria for ASD, either based on clinical diagnosis or by exceeding clinical cut-points on relevant ASD symptom measures. In studies that distinguished genetic subtypes, rates of ASD were higher in individuals with PWS with UPD (67 of 190; 35.3%) than those with DEL (47 of 254; 18.5%). Published data on the association of PWS and ASD to date are limited to sample means of 8 years of age and older. Further research is needed to identify early markers of ASD in PWS children, to support earlier diagnosis and intervention for this important comorbidity.

Calorie seeking, but not hedonic response, contributes to hyperphagia in a mouse model for Prader-Willi syndrome.

Prader–Willi syndrome (PWS) is a neurodevelopmental disorder caused by deletion or inactivation of paternally expressed imprinted genes on human chromosome 15q11‐q13, the most recognised feature of which is hyperphagia. This is thought to arise as a consequence of abnormalities in both the physiological drive for food and the rewarding properties of food. Although a number of mouse models for PWS exist, the underlying variables dictating maladaptive feeding remain unknown. Here, feeding behaviour in a mouse model in which the imprinting centre (IC) of the syntenic PWS interval has been deleted (PWS ICdel mice) is characterised. It is demonstrated that PWS ICdel mice show hyperghrelinaemia and increased consumption of food both following overnight fasting and when made more palatable with sucrose. However, hyperphagia in PWS ICdel mice was not accompanied by any changes in reactivity to the hedonic properties of palatable food (sucrose or saccharin), as measured by lick‐cluster size. Nevertheless, overall consumption by PWS ICdel mice for non‐caloric saccharin in the licking test was significantly reduced. Combined with converging findings from a continuous reinforcement schedule, these data indicate that PWS ICdel mice show a marked heightened sensitivity to the calorific value of food. Overall, these data indicate that any impact of the rewarding properties of food on the hyperphagia seen in PWS ICdel mice is driven primarily by calorie content and is unlikely to involve hedonic processes. This has important implications for understanding the neural systems underlying the feeding phenotype of PWS and the contribution of imprinted genes to abnormal feeding behaviour more generally.

(Epi)genotype-phenotype correlations in Beckwith-Wiedemann syndrome.

Beckwith-Wiedemann syndrome (BWS) is characterized by cancer predisposition, overgrowth and highly variable association of macroglossia, abdominal wall defects, nephrourological anomalies, nevus flammeus, ear malformations, hypoglycemia, hemihyperplasia, and organomegaly. BWS molecular defects, causing alteration of expression or activity of the genes regulated by two imprinting centres (IC) in the 11p15 chromosomal region, are also heterogeneous. In this paper we define (epi)genotype-phenotype correlations in molecularly confirmed BWS patients. The characteristics of 318 BWS patients with proven molecular defect were compared among the main four molecular subclasses: IC2 loss of methylation (IC2-LoM, n=190), IC1 gain of methylation (IC1-GoM, n=31), chromosome 11p15 paternal uniparental disomy (UPD, n=87), and cyclin-dependent kinase inhibitor 1C gene (CDKN1C) variants (n=10). A characteristic growth pattern was found in each group; neonatal macrosomia was almost constant in IC1-GoM, postnatal overgrowth in IC2-LoM, and hemihyperplasia more common in UPD (P<0.001). Exomphalos was more common in IC2/CDKN1C patients (P<0.001). Renal defects were typical of UPD/IC1 patients, uretheral malformations of IC1-GoM cases (P<0.001). Ear anomalies and nevus flammeus were associated with IC2/CDKN1C genotype (P<0.001). Macroglossia was less common among UPD patients (P<0.001). Wilms' tumor was associated with IC1-GoM or UPD and never observed in IC2-LoM patients (P<0.001). Hepatoblastoma occurred only in UPD cases. Cancer risk was lower in IC2/CDKN1C, intermediate in UPD, and very high in IC1 cases (P=0.009). In conclusion, (epi)genotype-phenotype correlations define four different phenotypic BWS profiles with some degree of clinical overlap. These observations impact clinical care allowing to move toward (epi) genotype-based follow-up and cancer screening.

Parent stress across molecular subtypes of children with Angelman syndrome.

Parenting stress has been consistently reported among parents of children with developmental disabilities. However, to date, no studies have investigated the impact of a molecular subtype of Angelman syndrome (AS) on parent stress, despite distinct phenotypic differences among subtypes. Data for 124 families of children with three subtypes of AS: class I and II deletions (n = 99), imprinting centre defects (IC defects; n = 11) and paternal uniparental disomy (UPD; n = 14) were drawn from the AS Rare Diseases Clinical Research Network (RDCRN) database and collected from five research sites across the Unites States. The AS study at the RDCRN gathered health information to understand how the syndrome develops and how to treat it. Parents completed questionnaires on their perceived psychological stress, the severity of children's aberrant behaviour and children's sleep patterns. Children's adaptive functioning and developmental levels were clinically evaluated. Child-related stress reached clinical levels for 40% of parents of children with deletions, 100% for IC defects and 64.3% for UPD. Sleep difficulties were similar and elevated across subtypes. There were no differences between molecular subtypes for overall child and parent-related stress. However, results showed greater isolation and lack of perceived parenting skills for parents of children with UPD compared with deletions. Better overall cognition for children with deletions was significantly related to more child-related stress while their poorer adaptive functioning was associated with more child-related stress. For all three groups, the severity of children's inappropriate behaviour was positively related to different aspects of stress. How parents react to stress depends, in part, on children's AS molecular subtype. Despite falling under the larger umbrella term of AS, it is important to acknowledge the unique aspects associated with children's molecular subtype. Identifying these factors can lead to tailored interventions that fit the particular needs of families of children with different AS subtypes.

A disease specific questionnaire for assessing behavior in individuals with Prader–Willi syndrome

ObjectivePrader–Willi syndrome (PWS) is a genetic multisystem disorder with various medical, cognitive, behavioral and psychiatric problems. PWS is caused by the lack of expression of paternal genes on chromosome 15q2-q13 due to a deletion (70–75%), uniparental disomy (25–30%) or imprinting center defect (<5%). The common PWS behavioral and psychiatric characteristics are very typical in all ethnicities and were reported worldwide. Still, each individual has a specific profile of these common traits and the severity of his or her symptoms varies over time. Behavioral problems are the most important factor affecting the quality of life of both the individuals and their families. There is a need for a standardized tool to assess the specific behavioral profile of each individual and its present severity, in order to enable physicians to tailor the specific treatment needed and assist in a more accurate clinical follow up. To the best of our knowledge no such a tool has been standardized and published. We developed, based on the literature (mainly Forster and Gourash's paradigm) and our clinical experience, a 37 item disease specific questionnaire, the “PWS Behavioral Questionnaire” (PWSBQ) for assessing behavior in PWS patients. The purpose of the present study was to validate this tool in the entire adolescent and adult PWS population in Israel. MethodsThe PWSBQ focuses on five major domains—abnormal emotional regulation, food-seeking related behavior, lack of flexibility, oppositional behavior and interpersonal problems and lastly body related behaviors.Caregivers of all Hebrew speaking individuals with PWS over the age of 12years attending the Israeli national multidisciplinary PWS clinic were recruited. Of the 54 eligible individuals, 53 participated. They were interviewed with the PWSBQ and in addition filled the “Hyperphagia Questionnaire” and the “Child Behavioral Checklist” (CBCL).After verifying the questionnaire's content validity, all items on the PWSBQ were analyzed for internal reliability by calculating Cronbach's α. Criterion validity was evaluated by correlation testing with regard to the Hyperphagia Questionnaire and CBCL. In order to assess the questionnaire's interpretability, the correlation between the PWSBQ and the “Clinical Global Impression” (CGI) scores was evaluated. ResultsThe PWSBQ total score was positively correlated with both the CBCL total score and the CGI score (0.662 and 0.549, p<0.001 respectively). Of the five domains, four had acceptable internal reliability (excluding the body related behaviors domain, which was thus removed from the total score). Criterion validity was established for the four domains remaining in the statistical analysis (abnormal emotional regulation, food seeking related behavior, lack of flexibility and oppositional behavior and interpersonal problems). ConclusionsOur findings suggest that the PWSBQ is a valid and reliable tool for the assessment of current behavioral problems among individuals with PWS. Although further research is needed in order to verify PWSBQ's ability to identify changes in the behavioral status of a given individual, it can now be used both in research and in a clinical setting, enabling the physician to plan the most suitable treatment based on the current behavioral status.

From Prader–Willi Syndrome to Psychosis: Translating Parent-of-Origin Effects into Schizophrenia Research

Prader-Willi syndrome (PWS) is a relatively rare disorder that originates from paternally inherited deletions and maternal disomy (mUPD) within the 15q11-q13 region or alterations in the PWS imprinting center. Evidence is accumulating that mUPD underlies high prevalence of psychosis among PWS patients. Several genes involved in differentiation and survival of neurons as well as neurotransmission known to act in the development of PWS have been also implicated in schizophrenia. In this article, we provide an overview of genetic and epigenetic underpinnings of psychosis in PWS indicating overlapping points in the molecular background of PWS and schizophrenia. Simultaneously, we highlight the need for studies investigating genetic and epigenetic makeup of the 15q11-q13 in schizophrenia indicating promising candidate genes.

Increased body mass in infancy and early toddlerhood in Angelman syndrome patients with uniparental disomy and imprinting center defects.

The diagnosis of Angelman syndrome (AS) is based on clinical features and genetic testing. Developmental delay, severe speech impairment, ataxia, atypical behavior and microcephaly by two years of age are typical. Feeding difficulties in young infants and obesity in late childhood can also be seen. The NIH Angelman-Rett-Prader-Willi Consortium and others have documented genotype-phenotype associations including an increased body mass index in children with uniparental disomy (UPD) or imprinting center (IC) defects. We recently encountered four cases of infantile obesity in non-deletion AS cases, and therefore examined body mass measures in a cohort of non-deletion AS cases. We report on 16 infants and toddlers (ages 6 to 44 months; 6 female, and 10 male) with severe developmental delay. Birth weights were appropriate for gestational age in most cases, >97th% in one case and not available in four cases. The molecular subclass case distribution consisted of: UPD (n = 2), IC defect (n = 3), UPD or IC defect (n = 3), and UBE3A mutation (n = 8). Almost all (7 out of 8) UPD, IC and UPD/IC cases went on to exhibit >90th% age- and gender-appropriate weight for height or BMI within the first 44 months. In contrast, no UBE3A mutation cases exhibited obesity or pre-obesity measures (percentiles ranged from <3% to 55%). These findings demonstrate that increased body mass may be evident as early as the first year of life and highlight the utility of considering the diagnosis of AS in the obese infant or toddler with developmental delay, especially when severe. Although a mechanism explaining the association of UPD, and IC defects with obesity has not been identified, recognition of this correlation may inform investigation of imprinting at the PWS/AS locus and obesity.

Angelman syndrome imprinting center encodes a transcriptional promoter

Clusters of imprinted genes are often controlled by an imprinting center that is necessary for allele-specific gene expression and to reprogram parent-of-origin information between generations. An imprinted domain at 15q11–q13 is responsible for both Angelman syndrome (AS) and Prader–Willi syndrome (PWS), two clinically distinct neurodevelopmental disorders. Angelman syndrome arises from the lack of maternal contribution from the locus, whereas Prader–Willi syndrome results from the absence of paternally expressed genes. In some rare cases of PWS and AS, small deletions may lead to incorrect parent-of-origin allele identity. DNA sequences common to these deletions define a bipartite imprinting center for the AS–PWS locus. The PWS–smallest region of deletion overlap (SRO) element of the imprinting center activates expression of genes from the paternal allele. The AS–SRO element generates maternal allele identity by epigenetically inactivating the PWS–SRO in oocytes so that paternal genes are silenced on the future maternal allele. Here we have investigated functional activities of the AS–SRO, the element necessary for maternal allele identity. We find that, in humans, the AS–SRO is an oocyte-specific promoter that generates transcripts that transit the PWS–SRO. Similar upstream promoters were detected in bovine oocytes. This result is consistent with a model in which imprinting centers become DNA methylated and acquire maternal allele identity in oocytes in response to transiting transcription.

Influence of the Prader-Willi syndrome imprinting center on the DNA methylation landscape in the mouse brain

Reduced representation bisulfite sequencing (RRBS) was used to analyze DNA methylation patterns across the mouse brain genome in mice carrying a deletion of the Prader-Willi syndrome imprinting center (PWS-IC) on either the maternally- or paternally-inherited chromosome. Within the ∼3.7 Mb imprinted Angelman/Prader-Willi syndrome (AS/PWS) domain, 254 CpG sites were interrogated for changes in methylation due to PWS-IC deletion. Paternally-inherited deletion of the PWS-IC increased methylation levels ∼2-fold at each CpG site (compared to wild-type controls) at differentially methylated regions (DMRs) associated with 5′ CpG island promoters of paternally-expressed genes; these methylation changes extended, to a variable degree, into the adjacent CpG island shores. Maternal PWS-IC deletion yielded little or no changes in methylation at these DMRs, and methylation of CpG sites outside of promoter DMRs also was unchanged upon maternal or paternal PWS-IC deletion. Using stringent ascertainment criteria, ∼750,000 additional CpG sites were also interrogated across the entire mouse genome. This analysis identified 26 loci outside of the imprinted AS/PWS domain showing altered DNA methylation levels of ≥25% upon PWS-IC deletion. Curiously, altered methylation at 9 of these loci was a consequence of maternal PWS-IC deletion (maternal PWS-IC deletion by itself is not known to be associated with a phenotype in either humans or mice), and 10 of these loci exhibited the same changes in methylation irrespective of the parental origin of the PWS-IC deletion. These results suggest that the PWS-IC may affect DNA methylation at these loci by directly interacting with them, or may affect methylation at these loci through indirect downstream effects due to PWS-IC deletion. They further suggest the PWS-IC may have a previously uncharacterized function outside of the imprinted AS/PWS domain.

Imprinting analysis of the mouse chromosome 7C region in DNMT1-null embryos

The mouse chromosome 7C, orthologous to the human 15q11–q13 has an imprinted domain, where most of the genes are expressed only from the paternal allele. The imprinted domain contains paternally expressed genes, Snurf/Snrpn, Ndn, Magel2, Mkrn3, and Frat3, C/D-box small nucleolar RNAs (snoRNAs), and the maternally expressed gene, Ube3a. Imprinted expression in this large (approximately 3–4Mb) domain is coordinated by a bipartite cis-acting imprinting center (IC), located upstream of the Snurf/Snrpn gene. The molecular mechanism how IC regulates gene expression of the whole domain remains partially understood. Here we analyzed the relationship between imprinted gene expression and DNA methylation in the mouse chromosome 7C using DNA methyltransferase 1 (DNMT1)-null mutant embryos carrying Dnmt1ps alleles, which show global loss of DNA methylation and embryonic lethality. In the DNMT1-null embryos at embryonic day 9.5, the paternally expressed genes were biallelically expressed. Bisulfite DNA methylation analysis revealed loss of methylation on the maternal allele in the promoter regions of the genes. These results demonstrate that DNMT1 is necessary for monoallelic expression of the imprinted genes in the chromosome 7C domain, suggesting that DNA methylation in the secondary differentially methylated regions (DMRs), which are acquired during development serves primarily to control the imprinted expression from the maternal allele in the mouse chromosome 7C.

A novel variant in CDKN1C is associated with intrauterine growth restriction, short stature, and early-adulthood-onset diabetes.

CDKN1C, a cyclin-dependent kinase inhibitor and negative regulator of cellular proliferation, is paternally imprinted and has been shown to regulate β-cell proliferation. CDKN1C mutations are associated with growth disorders, including Beckwith-Wiedemann syndrome and IMAGe syndrome. To investigate the genetic basis for a familial disorder characterized by intrauterine growth restriction, short stature, and early-adulthood-onset diabetes. Genomic DNA samples (15 affected and 26 unaffected from a six-generation pedigree) were analyzed by genome-wide single nucleotide polymorphism arrays, whole exome and Sanger sequencing, and multiplex ligation-dependent probe amplification. Subjects were assessed for height, weight, adrenal gland size, ACTH, diabetes status, and testis volume. Linkage and sequence analyses were performed, and the identified genetic variant was functionally evaluated in reconstitution studies. The pedigree followed a paternally imprinted pattern of inheritance, and genetic linkage analysis identified a single significant 2.6-megabase locus on chromosome 11p15, within the imprinting center region 2. Multiplex ligation-dependent probe amplification did not detect copy number variants or methylation abnormalities. Whole exome sequencing revealed a single novel variant in the proliferating cell nuclear antigen-binding region of CDKN1C (c.842G>T, p.R281I) that co-segregated with affected status and, unlike variants found in IMAGe, did not entirely abrogate proliferating cell nuclear antigen binding. Clinical assessments revealed that affected individuals had low testicular volume but normal adrenal function. We report a novel CDKN1C mutation associated with features of IMAGe syndrome, but without adrenal insufficiency or metaphyseal dysplasia, and characterized by early-adulthood-onset diabetes. Our data expand the range of phenotypes observed with CDKN1C defects and suggest that CDKN1C mutations may represent a novel monogenic form of diabetes.