Prader-Willi syndrome: current understanding of cause and diagnosis

Reading the windows to the soul: evidence of domain-specific sparing in Williams syndrome

This study tested the hypothesis that Williams syndrome, a rare genetic neurodevelopmental disorder with an unusual cognitive phenotype, involves spared abilities in the domain of understanding other minds. A group of retarded adults with Williams syndrome was compared to an age-, IQ-, and language-matched group of adults with Prader-Willi syndrome, another genetic disorder without the cognitive characteristics of Williams syndrome, and a group of age-matched normal adults, on a task that taps mentalizing ability. The task involved selecting the correct labels to match photographs of complex mental state expressions in the eye region of the face. The adults with Williams syndrome performed significantly better than the adults with Prader-Willi on this task, and about half the group performed in the same range as the normal adults. These findings are consistent with anecdotal evidence about Williams syndrome and provide evidence that mentalizing is a distinct cognitive domain. This spared cognitive capacity may be linked to the relative sparing of limbic-cerebellar neural substrate in Williams syndrome, which is also connected to cortico-frontal regions that are known to be involved in understanding complex mental states.

The growth hormone response to hexarelin in patients with Prader-Willi syndrome

Hexarelin (Hex) is a synthetic hexapeptide with potent GH-releasing activity in both animals and men. Aim of this study was to evaluate the GH response to a maximal dose of Hex and GH-releasing hormone (GHRH) in a group of patients with Prader-Willi syndrome (PWS). Seven patients (4 boys and 3 girls, age 2.4-14.2 yr) with PWS, 10 prepubertal obese children (7 boys and 3 girls, age 7.5-12.0 yr), and 24 prepubertal short normal children (11 boys and 13 girls, age 5.9-13 yr) with body weight within +/- 10% of their ideal weight were studied. All subjects were tested on two occasions with GHRH 1-29 at the dose of 1 microgram/Kg i.v., and with Hex at the dose of 2 micrograms/Kg i.v. In the PWS patients the GH response to GHRH (peak = 6.4 +/- 2.0 micrograms/l, p < 0.0001; AUC = 248 +/- 70 micrograms min/l, p < 0.0001) was significantly lower than that observed in the short normal children and similar to that observed in the obese children. In the PWS children the GH response to Hex (peak = 7.5 +/- 1.6 micrograms/l; AUC = 309 +/- 53) was similar to that observed after GHRH and significantly lower than that observed in the obese children (p < 0.05). The results of this study show that PWS patients have a blunted GH response to the administration of a maximal dose of Hex. Whether these findings reflect a more severe pituitary GH deficiency in PWS than in obese children or a deranged hypothalamic regulation of GH secretion need further investigation.

Prader-Willi syndrome: relationship of adiposity to plasma leptin levels

OBJECTIVE

Prader-Willi syndrome (PWS) is an autosomal dominant disorder involving the proximal long arm of chromosome 15, in which obesity is common. However, there is limited information on the underlying physiological mechanisms promoting obesity in this population. We tested whether there was a significant positive association between leptin and total body fat (TBF) in subjects with PWS, and whether this association was stronger among subjects with than without PWS.

RESEARCH METHODS AND PROCEDURES

We studied 21 PWS patients and 64 non-PWS controls on whom we measured serum leptin, total body fat, glucose, insulin, and resting energy expenditure. We tested whether the slope of the regression line between leptin and TBF (in kg), measured by dual energy X-ray absorptiometry, was the same for PWS patients and non-PWS controls.

RESULTS

Regression analyses indicated that the leptin-TBF association was significantly stronger among PWS patients. In contrast, the slope of the leptin-body mass index association did not significantly differ between PWS patients and non-PWS controls. None of the other outcome variables showed associations with leptin.

DISCUSSION

Results suggest that the role of leptin in promoting obesity may be greater among subjects with PWS than among non-PWS controls.

[Genetic basis of Prader-Willi and Angelman syndromes: implications for the biologic diagnosis]

Prader-Willi and Angelman syndromes are two genetic diseases whose clinical diagnosis is often impaired by a wide variability in some clinical findings. New insights in the genetic basis of these disorders allow the proposition of a biological approach to detect almost all Prader-Willi syndrome patients and over 80% of Angelman syndrome patients. Moreover, the results of these tests are indispensable for the evaluation of the recurrence risk.

Photoanthropometric study of craniofacial traits in individuals with Prader-Willi syndrome on short-term growth hormone therapy

An objective photoanthropometric method, useful for delineating craniofacial characteristics, was performed on 20 individuals with Prader-Willi syndrome (PWS; 14 males and 6 females) under 12 years of age and on growth hormone therapy (e.g. for 3-12 months) to determine the effects of therapy on craniofacial features in PWS. Facial parameters were measured from strict frontal and profile photographic 35 mm slides and compared with other facial measurements from the same face (e.g. palpebral fissure width to bizygomatic diameter). We studied 16 photoanthropometric craniofacial indices following previously established protocols. Our photoanthropometric data on 20 PWS subjects meeting diagnostic criteria further supported previous findings of a high midface, a broad interalar distance, a prominent high chin and broad ears in PWS patients without growth hormone therapy. In addition, while on growth hormone therapy, the high midface, broad interalar distance and prominent high chin appeared to accentuate over time in relationship to untreated PWS patients. Conversely, broad appearing ears were not accentuated by growth hormone therapy in the PWS subjects analyzed in this study.

The mechanisms involved in formation of deletions and duplications of 15q11-q13

Haplotype analysis was undertaken in 20 cases of 15q11-q13 deletion associated with Prader-Willi syndrome (PWS) or Angelman syndrome (AS) to determine if these deletions arose through unequal meiotic crossing over between homologous chromosomes. Of these, six cases of PWS and three of AS were informative for markers on both sides of the deletion. For four of six cases of paternal 15q11-q13 deletion (PWS), markers on both sides of the deletion breakpoints were inferred to be of the same grandparental origin, implying an intrachromosomal origin of the deletion. Although the remaining two PWS cases showed evidence of crossing over between markers flanking the deletion, this was not more frequent than expected by chance given the genetic distance between proximal and distal markers. It is therefore possible that all PWS deletions were intrachromosomal in origin with the deletion event occurring after normal meiosis I recombination. Alternatively, both sister chromatid and homologous chromosome unequal exchange during meiosis may contribute to these deletions. In contrast, all three cases of maternal 15q11-q13 deletion (AS) were associated with crossing over between flanking markers, which suggests significantly more recombination than expected by chance (p = 0.002). Therefore, there appears to be more than one mechanism which may lead to PWS/AS deletions or the resolution of recombination intermediates may differ depending on the parental origin of the deletion. Furthermore, 13 of 15 cases of 15q11-q13 duplication, triplication, or inversion duplication had a distal duplication breakpoint which differed from the common distal deletion breakpoint. The presence of at least four distal breakpoint sites in duplications indicates that the mechanisms of rearrangement may be complex and multiple repeat sequences may be involved.

Comparison of leptin protein levels in Prader-Willi syndrome and control individuals

Prader-Willi syndrome (PWS) is characterized by early childhood obesity, mental deficiency, hypogonadism, hypotonia, hypopigmentation, short stature, small hands and feet, and a characteristic face. It is the most common genetic cause of obesity and obesity is the most significant health problem for PWS patients. Ob protein (leptin), which is produced by adipose tissue, is thought to play a significant role in obesity; thus, unusually low plasma leptin levels, or relative loss of sensitivity to leptin in PWS subjects, could be an important factor in their obesity. We measured plasma leptin levels in 19 obese and 14 non-obese PWS patients [mean body mass index (BMI) 37.2 and 22.0, respectively] and compared these levels to those of 28 obese controls (mean BMI 35.5) and 16 non-obese control individuals (mean BMI 21.6). The mean plasma leptin concentration (ng/ml) for obese PWS subjects was 33.4 and 23.6 for non-obese PWS subjects. Obese control leptin was 36.2 ng/ml and non-obese control was 9.9. Among the control groups, leptin levels in females were significantly higher than those in males; the obese males and females had significantly higher leptin than their respective non-obese counterparts. These differences did not hold true for the PWS subjects. Leptin levels in obese PWS males and females were similar, and the same was true of the non-obese PWS males and females. The differences between obese and non-obese PWS subjects of both sexes were small and not significant. Comparing control groups with their PWS counterparts revealed no significant differences, with one exception: circulating plasma leptin levels in non-obese PWS males were nearly five times higher than in non-obese control males with similar BMI. This difference may reflect a more female pattern of fat distribution and hypogonadism, which are characteristic of PWS males. Leptin levels in PWS patients were not obviously correlated with the chromosome 15 finding seen in the patients.

Inherited interstitial duplications of proximal 15q: genotype-phenotype correlations

We present the cytogenetic, molecular cytogenetic, and molecular genetic results on 20 unrelated patients with an interstitial duplication of the proximal long arm of chromosome 15. Multiple probes showed that the Prader-Willi/Angelman critical region (PWACR) was included in the duplication in 4/20 patients, each ascertained with developmental delay. The duplication was also found in two affected but not in three unaffected sibs of one of these patients. All four probands had inherited their duplication from their mothers, three of whom were also affected. Two of the affected mothers also carried a maternally inherited duplication, whereas the duplication in the unaffected mother and in an unaffected grandmother was paternal in origin, raising the possibility of a parental-origin effect. The PWACR was not duplicated in the remaining 16 patients, of whom 4 were referred with developmental delay. In the 14 families for which parental samples were available, the duplication was inherited with equal frequency from a phenotypically normal parent, mother or father. Comparative genomic hybridization undertaken on two patients suggested that proximal 15q outside the PWACR was the origin of the duplicated material. The use of PWACR probes discriminates between a large group of duplications of no apparent clinical significance and a smaller group, in which a maternally derived PWACR duplication is consistently associated with developmental delay and speech difficulties but not with overt features of either Prader-Willi syndrome or Angelman syndrome.

Prader-Willi syndrome

Prader-Willi syndrome is a complex disorder affecting multiple systems with many manifestations relating to hypothalamic insufficiency. Major findings include infantile hypotonia, developmental delay and mental retardation, behaviour disorder, characteristic facial appearance, obesity, hypogonadism, and short stature. Obesity and the behavioural problems are the major causes of morbidity and mortality. Prader-Willi syndrome is caused by abnormalities of the imprinted region of proximal 15q and results from absence of the normally active paternal genes in this region. Such absence results from paternal interstitial deletion, maternal uniparental disomy, or a mutation or other abnormality in the imprinting process. Diagnostic identification of all causes has become available in recent years, permitting early detection and institution of appropriate management. This testing has permitted recent identification of some phenotypic differences among affected subjects of different race and between those with deletions and uniparental disomy as a cause.

Prenatal diagnosis of a fetus with a homologous Robertsonian translocation of chromosomes 15

We present a prenatal diagnosis of a de novo homologous Robertsonian translocation involving both chromosomes 15. Amniocentesis was performed on a 36-year-old woman at 16.5 weeks of gestation. Chromosome analysis documented a 45,XX,der(15;15) (q10;q10) chromosome pattern. No evidence of a deletion was observed by FISH using a SNRPN DNA probe associated with the Prader-Willi/Angelman syndrome critical region. Molecular studies in the family using six polymorphic markers for chromosome 15 and Southern blot analysis of DNA methylation for the CpG island near the SNRPN gene showed normal biparental inheritance of chromosome 15, excluding uniparental disomy. The patient was counseled that her child would not be able to bear off-spring without clinical assistance. Otherwise the health and intellect of her child were not expected to be affected by the translocation. We consider this to be the first prenatal case identified with a balanced der(15;15)(q10;q10) Robertsonian translocation and a phenotypically normal female outcome. Prenatally identified cases of der(15;15)(q10;q10) warrant further investigation by molecular methodology.

Chromosome 1p36 deletions: the clinical phenotype and molecular characterization of a common newly delineated syndrome

Deletions of the distal short arm of chromosome 1 (1p36) represent a common, newly delineated deletion syndrome, characterized by moderate to severe psychomotor retardation, seizures, growth delay, and dysmorphic features. Previous cytogenetic underascertainment of this chromosomal deletion has made it difficult to characterize the clinical and molecular aspects of the syndrome. Recent advances in cytogenetic technology, particularly FISH, have greatly improved the ability to identify 1p36 deletions and have allowed a clearer definition of the clinical phenotype and molecular characteristics of this syndrome. We have identified 14 patients with chromosome 1p36 deletions and have assessed the frequency of each phenotypic feature and clinical manifestation in the 13 patients with pure 1p36 deletions. The physical extent and parental origin of each deletion were determined by use of FISH probes on cytogenetic preparations and by analysis of polymorphic DNA markers in the patients and their available parents. Clinical examinations revealed that the most common features and medical problems in patients with this deletion syndrome include large anterior fontanelle (100%), motor delay/hypotonia (92%), moderate to severe mental retardation (92%), growth delay (85%), pointed chin (80%), eye/vision problems (75%), seizures (72%), flat nasal bridge (65%), clinodactyly and/or short fifth finger(s) (64%), low-set ear(s) (59%), ear asymmetry (57%), hearing deficits (56%), abusive behavior (56%), thickened ear helices (53%), and deep-set eyes (50%). FISH and DNA polymorphism analysis showed that there is no uniform region of deletion but, rather, a spectrum of different deletion sizes with a common minimal region of deletion overlap.

Balanced translocation 46,XY,t(2;15)(q37.2;q11.2) associated with atypical Prader-Willi syndrome

The lack of normally active paternal genes in 15q11-q13, as an outcome of either a paternal deletion or maternal disomy, accounts for >95% of all patients with Prader-Willi syndrome. Other mechanisms, including imprinting mutations and unbalanced translocations involving pat 15q11-q13, have been described elsewhere. In this study, we present a patient with a rare balanced, de novo translocation-46,XY,t(2;15)(q37.2;q11.2)-involving breakage within the Prader-Willi/Angelman syndrome region of the paternal homologue, without an apparent deletion. The patient demonstrated several manifestations of the Prader-Willi syndrome but was clinically atypical. Cytogenetic and molecular studies of this case demonstrated the translocation breakpoint to be between SNRPN and IPW, with mRNA expression of SNRPN and PAR-5 but absence of IPW and PAR-1 expression. These results suggest that disruption of either IPW expression or a nearby gene by an upstream break may contribute to the Prader-Willi syndrome phenotype and that expression of SNRPN or other upstream genes is responsible for other aspects of the classical Prader-Willi syndrome phenotype.

Hypopigmentation in the Prader-Willi syndrome correlates with P gene deletion but not with haplotype of the hemizygous P allele

The Prader-Willi syndrome (PWS) usually results from a paternal deletion of 15q11-q13 or maternal disomy for chromosome 15. Reduced pigmentation of skin, hair, and eyes is common in PWS and was suggested previously to be associated with the 15q11-q13 deletion. The P gene, located in this same region, is associated with OCA2, an autosomal recessive disorder that is the most frequent form of tyrosinase-positive oculocutaneous albinism. We studied 28 individuals with PWS and found that hemizygosity for the P gene was significantly correlated with the occurrence of hypopigmentation among PWS patients. However, we found little or no relationship between the occurrence of hypopigmentation and the polymorphism haplotype of the intact P allele. Thus, our results indicate that hypopigmentation is likely the result of deletion of the P gene in the context of PWS but do not support the linked hypothesis that hypopigmentation results from hemizygosity for variant P alleles with reduced function.

Mosaicism for deletion 1p36.33 in a patient with obesity and hyperphagia

We report on a 4-year-old girl with obesity and hyperphagia whose peripheral blood cytogenetic analysis showed mosaicism for a deletion of band 1p36.33. Terminal 1p deletions are rarely reported and this patient represents the first identified case of mosaicism. Given the subtlety of the cytogenetic abnormality and the possibility of mosaicism, the incidence of such deletions has probably been underestimated. While a characteristic phenotype associated with this karyotypic abnormality was described recently, the present report highlights the additional clinical findings of obesity and hyperphagia and the overlap of manifestations with Prader-Willi syndrome.

Familial translocation t(Y;15)(q12;p11) and de novo deletion of the Prader-Willi syndrome (PWS) critical region on 15q11-q13

We describe a 17-year-old girl with mild Prader-Willi syndrome (PWS) due to 15q11-q13 deletion. The deletion occurred on a paternal chromosome 15 already involved in a translocation, t(Y;15)(q12;p11), the latter being present in five other, phenotypically normal individuals in three generations. This appears to be the first case of PWS in which the causative 15q11-q13 deletion occurred on a chromosome involved in a familial translocation, but with breakpoints considerably distal to those of the familial rearrangement. The translocation could predispose to additional rearrangements occurring during meiosis and/or mitosis or, alternatively, the association of two cytogenetic anomalies on the same chromosome could be fortuitous.

Elevated plasma gamma-aminobutyric acid (GABA) levels in individuals with either Prader-Willi syndrome or Angelman syndrome

Plasma gamma-aminobutyric acid (GABA) levels were measured in 14 subjects with Prader-Willi syndrome, 9 subjects with Angelman syndrome, and matched control subjects. Mean levels in both patient groups were 2 to 3 times higher than in nonretarded moderately obese or retarded nonobese control subjects. Levels in each patient group differed significantly from both control groups. Neither the two patient groups nor the two control groups differed. GABA levels seemed unrelated to genetic status (chromosome 15 deletion or disomy). These preliminary findings of elevated plasma GABA levels possibly represent a compensatory increase in presynaptic GABA release in response to hyposensitivity of a subset of GABA receptors and could produce increased postsynaptic activation of other normal GABA receptor subtypes, resulting in complex alterations of GABAergic function throughout the brain.

Parental imprinting and human disease

Parental imprinting is a process that results in allele-specific differences in transcription, DNA methylation, and DNA replication timing. Imprinting plays an important role in development, and its deregulation can cause certain defined disease states. Absence of a paternal contribution to chromosome 15q11-q13, due to hemizygous deletion or uniparental disomy, results in the Prader-Willi syndrome. The absence of a normal maternal copy of the same region causes Angelman syndrome. The Beckwith-Wiedemann syndrome is associated with the failure of normal biparental inheritance of chromosome 11p15, and loss of imprinting is observed in several cancers including Wilms' tumor. The study of the molecular basis of abnormal imprinting in these disorders will facilitate the identification and characterization of other imprinted human disease loci.

Prader-Willi syndrome--a case report of the multidisciplinary management of the orofacial problems

A 24-year-old man with Prader-Willi syndrome presented with a class III malocclusion, featuring maxillary hypoplasia and severe enamel deficiency. Treatment involved orthodontic alignment, surgical advancement of the maxilla and restorative treatment to augment vertical facial height, improve the final occlusion and increase short clinical dental crown heights. The principal features of the syndrome and the management of this case are discussed.

A 5-year-old white girl with Prader-Willi syndrome and a submicroscopic deletion of chromosome 15q11q13

We report on a 5-year-old white girl with Prader-Willi syndrome (PWS) and a submicroscopic deletion of 15q11q13 of approximately 100-200 kb in size. High resolution chromosome analysis was normal but fluorescence in situ hybridization (FISH), Southern hybridization, and microsatellite data from the 15q11q13 region demonstrated that the deletion was paternal in origin and included the SNRPN, PAR-5, and PAR-7 genes from the proximal to distal boundaries of the deletion segment. SNRPN and PW71B methylation studies showed an abnormal pattern consistent with the diagnosis of PWS and supported the presence of a paternal deletion of 15q11q13 or an imprinting mutation. Biparental (normal) inheritance of PW71B (D15S63 locus) and a deletion of the SNRPN gene were observed by microsatellite, quantitative Southern hybridization, and/or FISH analyses. Our patient met the diagnostic criteria for PWS, but has no reported behavior problems, hyperphagia, or hypopigmentation. Our patient further supports SNRPN and possibly other genomic sequences which are deleted as the cause of the phenotype recognized in PWS patients.

Relationship of sleep abnormalities to patient genotypes in Prader-Willi syndrome

To assess whether sleep abnormalities are related to the genetic abnormalities in Prader-Willi Syndrome (PWS), we performed polysomnographic studies (nighttime and daytime) and determined the chromosome 15 genotypes in eight patients with PWS. Four patients demonstrated sleep onset REM periods (SOREM), and five met the objective polysomnographic criteria for severe or moderate excessive daytime sleepiness (EDS). Three of the four patients with SOREM displayed a paternally derived deletion of chromosome 15q11-q13, whereas the fourth exhibited maternal uniparental heterodisomy in this chromosomal region (UPD). Two of the four patients that did not display SOREM carried paternally derived deletions; the remaining two demonstrated UPD. Four of the five patients with EDS displayed paternal deletions, and the fifth exhibited UPD. One of three patients without evidence of EDS demonstrated paternal deletion; the remaining two showed UPD. Although neither EDS nor SOREM was not consistently associated with a specific genetic abnormality, these phenotypes may be more common in patients with paternal deletions than in those UPD. Sleep abnormalities in PWS cannot be explained by a single genetic model.

Genetics of nonsyndromic oral clefts revisited

Nonsyndromic oral clefts are among the most common birth defects, affecting approximately 1 in 1000 Caucasian newborns. In recent decades, many investigators have used genetic and epidemiologic methods to identify etiologic factors, but results have often been inconclusive or contradictory. Etiologic heterogeneity is undoubtedly a major component in these birth defects, and there may not be a single answer to this problem. Here, we describe the main features of published studies pointing out their strengths and limitations. Additionally, we give insight into current methods for detecting the presence of interaction between genetic markers and environmental exposures in the etiology of oral clefts.

A new stress-related syndrome of growth failure and hyperphagia in children, associated with reversibility of growth-hormone insufficiency

BACKGROUND

Growth failure without organic aetiology but associated with behavioural disturbance and psychosocial stress has been termed psychosocial short stature. This condition is not a valid diagnostic entity, but encompasses failure to thrive, stunting secondary to chronic malnutrition, and idiopathic hypopituitarism. Some children show spontaneous catch-up growth when removed from the source of stress, without further treatment, but until now precise definition of this subgroup for the purpose of clinical identification has not been possible.

METHODS

Hospital-referred children with growth failure unrelated to organic pathology, who came from stressful homes, were compared with children of short-normal stature identified from an epidemiological survey (n = 31). Growth-hormone dynamics were studied in the hospital group by a combination of diurnal profiles and provocation tests. The tests were repeated after a hospital stay of 3 weeks away from familial stress. Standard behavioural measures were obtained from home and school.

FINDINGS

In a distinctive subgroup (n = 29), growth-hormone insufficiency was associated with characteristic behavioural features, especially hyperphagia and polydipsia, and a normal body-mass index. When the children were removed from their stressful home circumstances, growth-hormone insufficiency spontaneously resolved only in formerly hyperphagic subjects. 74% of the non-hyperphagic cases (n = 23) were anorexic, with a low body-mass index and normal growth-hormone responses to provocation tests.

INTERPRETATION

We present explicit behavioural and developmental criteria by which the novel syndrome of hyperphagic short stature may be recognised clinically. Such children have a capacity for spontaneous recovery of growth-hormone production on removal from or reduction of stress. Discriminant and predictive validity of the core symptoms are demonstrated. Preliminary familial studies indicate a possible genetic predisposition.

Identification of a novel paternally expressed transcript adjacent to snRPN in the Prader-Willi syndrome critical region

Small nuclear ribonucleoprotein-associated polypeptide N (snRPN) and an anonymous transcript, PAR-5, are two of the paternally expressed transcripts mapped to the Prader-Willi syndrome critical region. Using long-range PCR, we have isolated the genomic interval between snRPN and PAR-5, identified a novel transcript in this region, and termed it PAR-SN. Northern analysis demonstrates that PAR-SN is expressed in brain, skeletal muscle, and heart. Like snRPN and PAR-5, PAR-SN is expressed exclusively from the paternal homolog in cultured lymphoblasts. Sequence analysis of the transcript revealed no significant open reading frame but did include a polymorphic dinucleotide repeat (CA)17.

Routine screening for microdeletions by FISH in 77 patients suspected of having Prader-Willi or Angelman syndromes using YAC clone 273A2 (D15S10)

About 70% of patients with Prader-Willi syndrome (PWS) and Angelman syndrome (AS) have a common interstitial de novo microdeletion encompassing paternal (PWS) or maternal (AS) loci D15S9 to D15S12. Most of the non-deletion PWS patients and a small number of non-deletion AS patients have a maternal or paternal uniparental disomy (UPD) 15, respectively. Other chromosome 15 rearrangements and a few smaller atypical deletions, some of the latter being associated with an abnormal methylation pattern, are rarely found. Molecular and fluorescence in situ hybridization (FISH) analysis have both been used to diagnose PWS and AS. Here, we have evaluated, in a typical routine cytogenetic laboratory setting, the efficiency of a diagnostic strategy that starts with a FISH deletion assay using Alu-PCR (polymerase chain reaction)-amplified D15S10-positive yeast artificial chromosome (YAC) 273A2. We performed FISH in 77 patients suspected of having PWS (n = 66) or AS (n = 11) and compared the results with those from classical cytogenetics and wherever possible with those from DNA analysis. A FISH deletion was found in 16/66 patients from the PWS group and in 3/11 patients from the AS group. One example of a centromere 15 co-hybridization performed in order to exclude cryptic translocations or inversions is given. Of the PWS patients, 14 fulfilled Holm's criteria, but two did not. DNA analysis confirmed the common deletion in all patients screened by the D15S63 methylation test and in restriction fragment length polymorphism dosage blots. In 3/58 non-deletion patients, other chromosomal aberrations were found. Of the non-deleted group, 27 subjects (24 PWS, 3 AS) were tested molecularly, and three patients with an uniparental methylation pattern were found in the PWS group. The other 24/27 subjects had neither a FISH deletion nor uniparental methylation, but two had other cytogenetic aberrations. Given that cytogenetic analysis is indispensable in most patients, we find that the FISH deletion assay with YAC 273A2 is an efficient first step for stepwise diagnostic testing and mutation-type analysis of patients suspected of having PWS or AS.

Annotation: behavioural phenotypes: a window onto the biology of behaviour

Characteristic behavioural patterns (including cognitive processes and social interactions) have been reported in a number of syndromes arising from genetic or chromosomal abnormalities, suggesting that molecular analysis of the underlying defect could reveal the biological basis of the behavioural phenotype. Because of the rarity of many of the syndromes, and the complexity of their genetic basis, there are great difficulties in establishing the validity of the association between syndrome and behavioural phenotype. Nevertheless, evidence from animal studies with relevance to human behavioural phenotypes shows that the pathway from genotype to phenotype may be accessible by careful delineation of behavioural phenotypes.

Maladaptive behaviour in Prader-Willi syndrome in adult life

Thirty adults with Prader-Willi syndrome (PWS) were compared with 30 adults with non-specific learning disability matched for age, sex and severity of mental retardation. Maladaptive behaviour was assessed with the Aberrant Behavior Checklist (ABC), a 58-item structured interview which rates behaviours from 0 (not a problem) to 3 (severe problem) and which yields five factors (I) irritability, agitation; (II) lethargy, withdrawal; (III) stereotypic behavior; (IV) hyperactivity, non-compliance; and (V) inappropriate speech). The PWS sample had significantly higher factor I (P < 0.001) and factor V (P < 0.05) scores. The PWS sample had mean scores above 1 for 17 ABC items; the contrast subjects had no mean scores above 1. The factor I scores for the PWS sample were similar to those of inpatients in hospital facilities for adults with mental retardation and mental illness or severely challenging behaviour. The results support previous work, and extend it by suggesting that temper tantrums, self-injury, impulsiveness, lability of mood, inactivity and repetitive speech are characteristic behaviours in PWS in adult life. Studies of the reasons for heterogeneity in behaviour are now needed.

Isochromosome 15q of maternal origin in a Prader-Willi patient with pituitary adenoma

We report on a Prader-Willi syndrome (PWS) patient carrier of a balanced 15q15q translocation and affected by a prolactin-secreting pituitary adenoma. Evidence provided by molecular studies indicates that the structural rearrangement is an isochromosome of maternal origin. According to the identification of isodisomy as the basis of the association of rare disorders and the recent report on chromosome 15 monosomy and nullisomy in pituitary adenoma, we suggest that in our case PWS and pituitary adenoma might be related.

Breakage in the SNRPN locus in a balanced 46,XY,t(15;19) Prader-Willi syndrome patient

A patient with Prader-Willi syndrome (PWS) was found to carry a de novo balanced reciprocal translocation, t(15;19)(q12;q13.41), which disrupted the small nuclear ribonucleoprotein N (SNRPN) locus. The translocation chromosome 15 was found to be paternal in origin. Uniparental disomy and abnormal DNA methylation were ruled out. The translocation breakpoint was found to have occurred between exon 0 (second exon) and 1 (third exon) of the SNRPN locus outside of the SmN open reading frame (ORF), which is intact. The transcriptional activities of ZNF127, IPW, PAR-1, and PAR-5 were detected with RT-PCR from fibroblasts of the patient, suggesting that these genes may not play a significant role in the PWS phenotype in this patient. Transcription from the first two exons and last seven exons of the SNRPN gene was also detected with RT-PCR; however, the complete mRNA (10 exons) was not detected. Thus, the PWS phenotype in the patient is likely to be the result of disruption of the SNRPN locus.

Respiratory sinus arrhythmia in patients with Prader-Willi syndrome

In this investigation, we sought to further test the hypothesis that parasympathetic deficiency exists among persons with Prader-Willi syndrome, by examining respiratory sinus arrhythmia. The study sample comprised two groups of patients: 14 subjects with Prader-Willi syndrome and 14 age- and sex-matched controls. Each subject's electrocardiogram was recorded in a quiet room and digitized by a personal computer during five 1-minute periods. RR intervals within each 1-minute period were converted to heart rate in 120 successive 0.5-second intervals. The resultant heart rate time series was converted to its underlying frequency composition by a fast Fourier transform and averaged across minutes. Respiratory sinus arrhythmia was defined as the variability in the time series over a frequency range (0.096 to 0.48 Hz) corresponding to a range of respiratory rates from six to 30 breaths/minute. Analysis revealed significantly less variability in the heart rates of subjects with Prader-Willi syndrome relative to age- and sex-matched controls (group x frequency bin: F = 2.26, P < .05). An analysis of covariance adjusting for body mass index differences between the groups produced identical results. These findings support the existence of a parasympathetic deficiency among subjects with Prader-Willi syndrome independent of their body mass. This is likely due to dysregulation of the central autonomic network.

Angelman syndrome in an inbred family

Angelman syndrome (AS) is characterized by severe mental retardation, absent speech, puppet-like movements, inappropriate laughter, epilepsy, and abnormal electroencephalogram. The majority of AS patients (approximately 65%) have a maternal deficiency within chromosomal region 15q11-q13, caused by maternal deletion or paternal uniparental disomy (UPD). Approximately 35% of AS patients exhibit neither detectable deletion nor UPD, but a subset of these patients have abnormal methylation at several loci in the 15q11-q13 region. We describe here three patients with Angelman syndrome belonging to an extended inbred family. High resolution chromosome analysis combined with DNA analysis using 14 marker loci from the 15ql1-q13 region failed to detect a deletion in any of the three patients. Paternal UPD of chromosome 15 was detected in one case, while the other two patients have abnormal methylation at D15S9, D15S63, and SNRPN. Although the three patients are distantly related, the chromosome 15q11-q13 haplotypes are different, suggesting that independent mutations gave rise to AS in this family.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

Prader-Willi Syndrome: Genetics and Behavior

Since its inception, the John F. Kennedy Center has attempted to overcome developmental problems, which create restrictive barriers to the participation of individuals with specific disabilities in our broader society. Some of Nicholas Hobbs's earliest efforts involved developing strategies for preventing children's emotional and behavior problems, which interfered with their later full participation in society. Other investigators in the Kennedy Center explored ways of reducing dysfunctional repetitive movement problems and self-injury commonly associated with autism and severe mental retardation. We have become concerned about a group of people who have the potential to live largely independently (or semi-independently), to work at meaningful jobs in the community, and to make full use of the same recreational and leisure opportunities as other members of society but who are prevented from doing so because of a life-threatening behavior problem. Prader-Willi syndrome (PWS) is a genetic developmental disability characterized by a group of specific behavioral features of which an insatiable appetite is the most striking. PWS is the most commonly known genetic cause of obesity. The eating disorder associated with PWS can be so severe as to be life threatening, including eating to the point of stomach rupture and death. Though a cluster of commonly covarying clinical features are exhibited by people with this syndrome, only the eating disorder is common to all affected individuals. PWS shares behavioral features with other disorders and disabilities, such as obsessive compulsive disorder and autism, but only PWS includes the unique combination of characteristics that distinguish this syndrome. Because eating disorders such as bulimia and anorexia nervosa also share features with PWS, any light that could be shed on the causes and treatment of the eating disorder in PWS could potentially have far-reaching implications for other eating disorders as well. In this article, we review the behavioral, cognitive, and other psychological features of PWS and explore their relationships to known genetic mechanisms.

Derivative Y chromosome resulting from a t(Y;15) (q12;q11.2) in a boy with Prader-Willi syndrome

Chromosome analysis of peripheral lymphocytes from a boy with Prader-Willi syndrome showed the presence of 45 chromosomes, including der(Y) resulting from a t(Y;15) (q12;q11.2). In situ hybridization using DYZ3 and D15S11 showed a positive signal and negative signal in derivative Y chromosome, respectively. The deficiency of 15pter q11.2 may not influence the clinical manifestation of Prader-Willi syndrome.

Deficiency of the beta 3 subunit of the type A gamma-aminobutyric acid receptor causes cleft palate in mice

In addition to its function in the nervous system, gamma-aminobutyric acid (GABA) has been implicated in mouse craniofacial development by the results of both teratological, and genetic studies. We previously reported that disruption of the cleft palate 1 (cp1) locus, closely linked to the pink-eyed dilution (p) locus on mouse chromosome 7, causes a 95% penetrant, recessive, neonatally-lethal cleft palate (CP) in mice homozygous for the p(4THO-II) deletion. We proposed that the beta 3 subunit gene (Gabrb3) of the GABAA receptor might be a candidate for cp1 (ref. 4); our earlier studies had localized cp1 to an interval beginning distal to the gene for the GABAA receptor alpha 5 subunit (Gabra5) and ending within the Gabrb3 coding region. To test the hypothesis that deletion of Gabrb3, and not another gene in the interval, causes CP, we performed an experiment to rescue the CP phenotype by introducing a Gabrb3 transgene into p(4THO-II) homozygotes. We now show that such transgenic mice are phenotypically normal, indicating that Gabrb3 is indeed the cp1 locus.

Transdifferentiation of myoblasts by the adipogenic transcription factors PPAR gamma and C/EBP alpha

Skeletal muscle and adipose tissue development often has a reciprocal relationship in vivo, particularly in myodystrophic states. We have investigated whether determined myoblasts with no inherent adipogenic potential can be induced to transdifferentiate into mature adipocytes by the ectopic expression of two adipogenic transcription factors, PPAR gamma and C/EBP alpha. When cultured under optimal conditions for muscle differentiation, murine G8 myoblasts expressing PPAR gamma and C/EBP alpha show markedly reduced levels of the myogenic basic helix-loop-helix proteins MyoD, myogenin, MRF4, and myf5 and are completely unable to differentiate into myotubes. Under conditions permissive for adipogenesis including a PPAR activator, these cells differentiate into mature adipocytes that express molecular markers characteristic of this lineage. Our results demonstrate that a developmental switch between these two related but highly specialized cell types can be controlled by the expression of key adipogenic transcription factors. These factors have an ability to inhibit myogenesis that is temporally and functionally separate from their ability to stimulate adipogenesis.