Prader-Willi syndrome: current understanding of cause and diagnosis

Characteristics of the eating disorder in Prader-Willi syndrome: implications for treatment

Over-eating and resultant obesity is well recognized as a central feature of the Prader-Willi Syndrome (PWS). The eating behaviour of 13 subjects with PWS was been studied retrospectively over a 28-day period and also by direct observation when given free access to food. Changing cognitions normally associated with food intake (e.g. changes in hunger) were assessed using visual analogue scales (VAS) and by asking subjects to rate photographs of particular foods. Eight out of 13 subjects (61%) with PWS had to have their access to food severely restricted. Ten (77%) ate excessive amounts when given free access to food, and although feelings of 'hunger', 'desire to eat' and 'fullness' changed in the expected direction, these changes were delayed, compared to a control group, and only occurred after eating a significantly greater amount of food. Ratings of 'hunger' and 'fullness' started to return to pre-meal levels sooner than in the controls. The present authors consider that PWS is an example of genetic obesity secondary to an impaired satiety response. These observations have important implications for treatment.

Investigations with fluorescence in situ hybridization (FISH) demonstrate loss of the telomeres on the reciprocal chromosome in three unbalanced translocations involving chromosome 15 in the Prader-Willi and Angelman syndromes

Two patients with classical features of Angel-man syndrome (AS) and one with Prader-Willi syndrome (PWS) had unbalanced reciprocal translocations involving the chromosome 15 proximal long arm and the telomeric region of chromosomes 7, 8 and 10. Fluorescence in situ hybridization (FISH) was used for the detection of chromosome 15(q11-13) deletions (with probes from the PWS/AS region) and to define the involvement of the telomere in the derivative chromosomes (with library probes and telomere-specific probes). The 15(q11-13) region was not deleted in one patient but was deleted in the other two. The telomere on the derivative chromosomes 7, 8 and 10 was deleted in all three cases. Thus, these are true reciprocal translocations in which there has been loss of the small satellited reciprocal chromosome (15) fragment.

Frequency of the Prader-Willi syndrome in the San-in district, Japan

Nineteen cases of the Prader-Willi syndrome were collected in the San-in district, in the western part of Japan, and an epidemiological study was carried out. The male to female ratio was 1:1.11. Deletion of chromosome 15q11-13 was found in seven of 16 patients examined by means of high-resolution chromosome banding. The incidence among live-births was estimated to be 6.64 x 10(-5) (1 in 15,060 live-births) in 1980-1989, and the mutation rate was 3.32 x 10(-5) per gene per generation, as determined by a direct method. The prevalence was estimated to be 5.72 x 10(-5) (1 in 17,483) in patients under 15 years old, which was higher than that in other autosomal dominant diseases (tuberous sclerosis and congenital myotonic dystrophy) also studied in the San-in district. A birth-order effect was revealed and a slightly significant difference was found as to the fathers' age between the patients and their normal siblings. The results of these two tests are in accord with the recent discovery that this syndrome is caused by paternal chromosomal aberrations.

Photoanthropometric study of craniofacial traits of individuals with Prader-Willi syndrome

A photoanthropometric method, which enables an objective description of facial structures, was used to better delineate the craniofacial characteristics of 37 individuals with Prader-Willi syndrome (PWS; 21 males and 16 females; 22 with 15q11q13 deletions and 15 with normal-appearing chromosome 15s) between the ages of 0 to 12 years. 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 the protocols established by Stengel-Rutkowski et al. [1984: Hum Genet 67:272-295] and Butler et al. [1988: Am J Med Genet 30:165-168]. Based on our measurements of 37 Prader-Willi syndrome individuals, none of the parameters were consistently outside of the normal range when compared with photoanthropometric index standards for age established from white control children [Stengel-Rutkowski et al., 1984]. However, several suggestive findings were documented by our analysis including: narrow palpebral fissure width [particularly in older children (6-12 years)], high midface, broad interalar distance, short back of the nose, prominent high chin, and broad low-set ears. No significant differences were found in craniofacial parameters between deletion or nondeletion Prader-Willi syndrome patients with this methodology. These craniofacial parameters (many not previously evaluated in PWS patients) may become useful for early detection, and aid in the diagnosis and the study of the development of the characteristic face seen in Prader-Willi patients.

Molecular characterization of two proximal deletion breakpoint regions in both Prader-Willi and Angelman syndrome patients

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct mental retardation syndromes caused by paternal and maternal deficiencies, respectively, in chromosome 15q11-q13. Approximately 70% of these patients have a large deletion of approximately 4 Mb extending from D15S9 (ML34) through D15S12 (IR10). To further characterize the deletion breakpoints proximal to D15S9, three new polymorphic microsatellite markers were developed that showed observed heterozygosities of 60%-87%. D15S541 and D15S542 were isolated from YAC A124A3 containing the D15S18 (IR39) locus. D15S543 was isolated from a cosmid cloned from the proximal right end of YAC 254B5 containing the D15S9 (ML34) locus. Gene-centromere mapping of these markers, using a panel of ovarian teratomas of known meiotic origin, extended the genetic map of chromosome 15 by 2-3 cM toward the centromere. Analysis of the more proximal S541/S542 markers on 53 Prader-Willi and 33 Angelman deletion patients indicated two classes of patients: 44% (35/80) of the informative patients were deleted for these markers (class I), while 56% (45/80) were not deleted (class II), with no difference between PWS and AS. In contrast, D15S543 was deleted in all informative patients (13/48) or showed the presence of a single allele (in 35/48 patients), suggesting that this marker is deleted in the majority of PWS and AS cases. These results confirm the presence of two common proximal deletion breakpoint regions in both Prader-Willi and Angelman syndromes and are consistent with the same deletion mechanism being responsible for paternal and maternal deletions. One breakpoint region lies between D15S541/S542 and D15S543, with an additional breakpoint region being proximal to D15S541/S542.

Comparison of high resolution cytogenetics, fluorescence in situ hybridisation, and DNA studies to validate the diagnosis of Prader-Willi and Angelman's syndromes

Eighty seven referrals with Prader-Willi syndrome and 49 with Angelman's syndrome were studied. High resolution cytogenetics was performed on all probands. Molecular studies, performed on the proband and both parents in each case, utilised multiple probes from within and distal to the 15(q11-13) region in order to establish the presence of DNA deletion or uniparental disomy. In addition, FISH, with probes at D15S11 and GABR beta 3 from the Prader-Willi syndrome/Angelman's syndrome region, was performed on a subset of 25 of these patients. In the referral group with Prader-Willi syndrome, 62 patients had a normal karyotype and 25 were deleted on high resolution cytogenetics. Twenty nine were found to be deleted with DNA techniques. In the Angelman's syndrome group, 37 had a normal karyotype and 12 were deleted on high resolution cytogenetics while 26 were deleted on molecular studies. The diagnosis was reassessed in 35 referrals with Prader-Willi syndrome and 11 with Angelman's syndrome following a non-deleted, non-disomic result. Of individuals who were neither deleted nor disomic on DNA studies, a false positive rate of 11.4% (4/35) for Prader-Willi syndrome and 16.7% (2/12) for Angelman's syndrome was found for a cytogenetically detected deletion. The false negative rate for deletion detected on high resolution cytogenetics was 19.5% (12/62) for Prader-Willi syndrome and 35% (13/37) for Angelman's syndrome. Thus high resolution cytogenetics was shown to be unreliable for deletion detection and should not be used alone to diagnose either syndrome. There were no discrepancies with FISH in 25 cases when FISH was compared with the DNA results, indicating that FISH can be used reliably for deletion detection in both syndromes.

Prenatal diagnosis of chromosome 15 abnormalities in the Prader-Willi/Angelman syndrome region by traditional and molecular cytogenetics

With improvements in culturing and banding techniques, amniotic fluid studies now achieve a level of resolution at which the Prader-Willi syndrome (PWS) and Angelman syndrome (AS) region may be questioned. Chromosome 15 heteromorphisms, detected with Q- and R-banding and used in conjunction with PWS/AS region-specific probes, can confirm a chromosome deletion and establish origin to predict the clinical outcome. We report four de novo cases of an abnormal-appearing chromosome 15 in amniotic fluid samples referred for advanced maternal age or a history of a previous chromosomally abnormal child. The chromosomes were characterized using G-, Q-, and R-banding, as well as isotopic and fluorescent in situ hybridization of DNA probes specific for the proximal chromosome 15 long arm. In two cases, one chromosome 15 homolog showed a consistent deletion of the ONCOR PWS/AS region A and B. In the other two cases, one of which involved an inversion with one breakpoint in the PWS/AS region, all of the proximal chromosome 15 long arm DNA probes used in the in situ hybridization were present on both homologs. Clinical follow-up was not available on these samples, as in all cases the parents chose to terminate the pregnancies. These cases demonstrate the ability to prenatally diagnose chromosome 15 abnormalities associated with PWS/AS. In addition, they highlight the need for a better understanding of this region for accurate prenatal diagnosis.

Tentative classification of neuropsychiatric disturbances in Prader-Willi syndrome

Prader-Willi syndrome presents behavioural characteristics at the temperamental level which can be described as oppositional, explosive and at times antisocial. These traits may fluctuate and be driven by unknown biological anxiety symptoms are also frequent. Delusional psychotic thinking is manifest in some cases, but may be latent in several such patients. A third set of manifestations is a 'refusal-lethargy' syndrome of akinesis, refusal of food and drink, and soiling, which seems to be triggered by environmental circumstances but resembles the hypersomnic, lethargic depressions. These three sets of phenomena are documented through the clinical observation of nine cases and may be useful in the study of the genotype-phenotype relationship in this and other syndromes, particularly those in which similar manifestations are observed and cyclic changes are seen. The use of drugs in this syndrome can also become more rational if this classification is used to identify clearer targets for treatment. The possibility that most manifestations of the syndrome may be an expression of a hyposerotonergic defect is suggested.

Differential expression of the neuroendocrine polypeptide 7B2 in hypothalami of Prader-(Labhart)-Willi syndrome patients

Prader-(Labhart-)Willi syndrome (PWS) is characterized by infantile hypotonia, early childhood obesity, mental deficiency, short stature, small hands and feet and hypogonadism. In 70% of the cases this syndrome is associated with a defect of chromosome 15 at 15q11-q13, close to the location of the 7B2 gene (15q13-q14). The majority of the remaining PWS patients display maternal uniparental disomy on chromosome 15. Since the 7B2 gene products are expressed in neuroendocrine cells that are probably affected in PWS, e.g. by a pleiotrophic influence of the neighboring deletion, the presence of 7B2 was studied in the supraoptic and paraventricular nucleus of the hypothalamus of five subjects clinically diagnosed as PWS patients using five antibodies against various parts of the 7B2 precursor polypeptide. Three of the five PWS patients studied showed no reaction to the 7B2 antibody MON-102, whereas all 30 control patients did. However, one of the three MON-102 non-reacting PWS patients reacted to other 7B2 antibodies. In conclusion, the vanishing of 7B2 gene products is not obligatory for PWS, possibly due to the variable genetic background of PWS patients. However, in most patients there is a clear modification of 7B2 expression, pointing to altered neuroendocrine functions.

Molecular and clinical study of 61 Angelman syndrome patients

We analyzed 61 Angelman syndrome (AS) patients by cytogenetic and molecular techniques. On the basis of molecular findings, the patients were classified into the following 4 groups: familial cases without deletion, familial cases with submicroscopic deletion, sporadic cases with deletion, and sporadic cases without deletion. Among 53 sporadic cases, 37 (70%) had molecular deletion, which commonly extended from D15S9 to D15S12, although not all deletions were identical. Of 8 familial cases, 3 sibs from one family had a molecular deletion involving only 2 loci, D15S10 and GABRB3, which define the critical region for AS phenotypes. The parental origin of deletion, both in sporadic and familial cases, was exclusively maternal and consistent with a genomic imprinting hypothesis. Among sporadic and familial cases without deletion, no uniparental disomy was found and most of them were shown to inherit chromosomes 15 from both parents (biparental inheritance). A discrepancy between cytogenetic and molecular deletion was observed in 14 (26%) of 53 patients in whom cytogenetic analysis could be performed. Ten (43%) of 23 patients with a normal karyotype showed a molecular deletion, and 4 (13%) of 30 patients with cytogenetic deletion, del(15) (q11q13), showed no molecular deletion. Most clinical manifestations, including neurological signs and facial characteristics, were not distinct in each group except for hypopigmentation of skin or hair. Familial cases with submicroscopic deletion were not associated with hypopigmentation. These findings suggested that a gene for hypopigmentation is located outside the critical region of AS and is not imprinted.

Molecular diagnosis of Prader-Willi syndrome: parent-of-origin dependent methylation sites and non-isotopic detection of (CA)n dinucleotide repeat polymorphisms

We describe our experience in the molecular diagnosis of 22 patients suspected of Prader-Willi syndrome (PWS) using a DNA probe PW71 (D15S63) which detects a parent-of-origin specific methylated site in the PWS critical region. The cause of the syndrome was determined as deletion or uniparental disomy according to the segregation of (CA)n dinucleotide repeat polymorphisms of the PWS/AS region and more distal markers of chromosome 15. In 10 patients the clinical diagnosis was confirmed by this approach, 6 with paternal deletion and 4 with maternal disomy. In one patient, the aberrant methylation pattern that was detected by PW71 could not be confirmed by the segregation of (CA)n, probably due to paternal microdeletion in the PWS critical region which did not include the loci D15S97, D15S113, GABRB3, and GABRA5. This case demonstrates the advantage of the DNA probe PW71 in the diagnosis of PWS.

The genetics of mental retardation

Individuals affected by mental retardation are a clinically and aetiologically heterogeneous group. This heterogeneity is particularly highlighted when we consider the genetics of mental retardation. Recent advances in molecular genetic techniques have enabled us to understand more about the molecular basis of several genetic syndromes associated with mental retardation. In contrast, where there is no discrete cause, the interplay of genetic and environmental influences remains poorly understood.

Muscle histochemistry in the Prader-Willi syndrome

In a follow-up study of 259 floppy infants of undetermined cause in my laboratory, 11 patients were later diagnosed as having the Prader-Willi syndrome (PWS). To clarify the pathogenesis of muscle hypotonia in PWS, I examined muscle biopsies by histochemical and morphometric methods and the results were compared with those obtained from floppy infants with only mental retardation but with no other features. The histochemical abnormalities of PWS included (i) fiber size variation of both type 1 and 2 fibers, (ii) type 2 fiber atrophy, (iii) increased numbers of type 2C fibers, and (iv) decreased numbers of type 2B fiber. Although muscle hypotonia in PWS has been thought to be due to central nervous system abnormality, my findings suggest that primary muscle pathology, including muscle fiber immaturity and abnormal muscle fiber type distribution, at least in part, plays a role in muscle hypotonia and weakness.

Prader-Willi-like phenotype in fragile X syndrome

A 3-year-old boy was referred to the pediatric department because of unexplained extreme obesity. Height and occipitofrontal circumference were just above the 90th centile. Endocrine studies failed to show any significant abnormality. Motor and speech development were generally delayed. On clinical-cytogenetic-molecular grounds, Prader-Willi syndrome was excluded. Fragile X syndrome was diagnosed by the presence of the classical FMR-1 mutation and confirmed by cytogenetic studies, revealing 20% fragile X positive cells. We compare the clinical features in the present patient with the nine reported patients with fra(X) syndrome and extreme obesity. In pathogenesis, hypothalamic dysregulation is hypothesized. In differential diagnosis of Prader-Willi syndrome, fragile X has to be considered, especially when laboratory workup for Prader-Willi syndrome is negative. Clinical behavior can be of help.

A somatic origin of homologous Robertsonian translocations and isochromosomes

One t(14q14q), three t(15q15q), two t(21q21q), and two t(22q22q) nonmosaic, apparently balanced, de novo Robertsonian translocation cases were investigated with polymorphic markers to establish the origin of the translocated chromosomes. Four cases had results indicative of an isochromosome: one t(14q14q) case with mild mental retardation and maternal uniparental disomy (UPD) for chromosome 14, one t(15q15q) case with the Prader-Willi syndrome and UPD(15), a phenotypically normal carrier of t(22q22q) with maternal UPD(22), and a phenotypically normal t(21q21q) case of paternal UPD(21). All UPD cases showed complete homozygosity throughout the involved chromosome, which is supportive of a postmeiotic origin. In the remaining four cases, maternal and paternal inheritance of the involved chromosome was found, which unambiguously implies a somatic origin. One t(15q15q) female had a child with a ring chromosome 15, which was also of probable postmeiotic origin as recombination between grandparental haplotypes had occurred prior to ring formation. UPD might be expected to result from de novo Robertsonian translocations of meiotic origin; however, all de novo homologous translocation cases, so far reported, with UPD of chromosomes 14, 15, 21, or 22 have been isochromosomes. These data provide the first direct evidence that nonmosaic Robertsonian translocations, as well as isochromosomes, are commonly the result of a mitotic exchange.

Genetic imprinting in the mouse: implications for gene regulation

Genetic imprinting specifies a germline marking that subsequently results in the repression of one or other parental allele at some point in development. Genetic manipulations to generate maternal and paternal duplications of specific chromosome regions have been used to screen almost the entire mouse genome for evidence of imprinting. As a result, 15 imprinting effects involving 10 regions on 6 different chromosomes have been detected that range from early embryonic lethalities to various growth and developmental defects seen only after birth. Genes with important roles in development therefore appear to be involved. Diverse studies have identified four imprinted genes, all of which show monoallelic expression in some, but not necessarily all, tissues. A correlation with methylation is indicated but the pattern of methylation is not consistent for each of the genes; methylation is therefore unlikely to be the imprinting signal. Methods being used to identify further imprinted genes are summarized and some of the difficulties posed are indicated.

Hypopigmentation, anomalous cerebral dominance and seasonality

This paper proposes a prenatal seasonal hypopigmentation influence associated with anomalous cerebral dominance that occurs during the winter or early spring. A possible mechanism would be seasonal changes in sex hormone levels that affect the activation and inactivation of DNA by reversible methylation. The proposed prenatal seasonal hypopigmentation effect might be relevant to dyslexia, Prader-Willi syndrome, breast cancer, Alzheimer's and Parkinson's disease. Putative chromosomal loci associated with the proposed seasonal mechanism would be 15q11-13 (dyslexia and Prader-Willi syndrome), 21q region (breast cancer and Alzheimer's disease) and 19p region (pigmentation gene).

Growth and developmental patterns in Prader-Willi syndrome

Eleven Japanese patients with Prader-Willi syndrome were studied from infancy, and examined with respect to their growth and development. The chromosomal aberration was observed in 40% of the patients using a high-resolution chromosome banding technique. The birth weight in 10 out of the 11 patients was below the mean and the rate of body weight gain was severely retarded by 6 months in all cases, which suggested an insufficient utilization of nutrients in uterine life and in early infancy. The rate of height increase as well as that of body weight increase were also transiently retarded from 10 to 18 months of age in eight cases, and the body weight gain increased dramatically after 10-18 months due to hyperphagia. The patients' mean milestones of development were delayed (the mean developmental quotient was 50) in comparison with those of control patients; there was no difference between the developmental milestones of the two groups with and without the chromosomal aberration.

Difference in methylation patterns within the D15S9 region of chromosome 15q11-13 in first cousins with Angelman syndrome and Prader-Willi syndrome

Abnormalities of chromosome region 15q11-13 are associated with Angelman syndrome (AS) and Prader-Willi syndrome (PWS). Differences between the methylation patterns of the region of chromosome 15q11-13 which hybridizes to the highly conserved DNA, DN34, in normal individuals and in patients with AS and PWS have been described. We report on a family in which first cousins are affected by AS and PWS as a result of a familial paracentric inversion of 15q11-q13. The results of the studies on this family demonstrate the differences in the methylation patterns in the 2 conditions and the phenomenon of genomic imprinting, whereby genetic information is expressed differently dependent on the parent of origin.

Searching for human epilepsy genes: a progress report

Application of new genetic techniques has brought remarkable discoveries in the study of genetic diseases. The potential benefits from applying such technology to idiopathic epilepsies include improved understanding of cellular mechanisms and potential new methods of prevention and treatment. The complex problems involved in studying the hereditary epilepsies include: defining of specific phenotypes; detecting genetic and non-genetic heterogeneity; and specifying the appropriate mode of inheritance and penetrance. The gene loci for three primary epilepsies have been localized to specific chromosomal regions, and serve to demonstrate the process used in generalized linkage studies of hereditary epilepsy syndromes. Benign familial neonatal convulsions (BFNC) and Unverricht-Lundborg progressive myoclonus epilepsy are rare single-gene disorders that are sufficiently localized to chromosomal regions that positional cloning studies are likely to succeed. Juvenile myoclonic epilepsy (JME), a common hereditary syndrome with an uncertain mode of inheritance, has been reported to be linked to chromosome 6p. JME presents a challenge for generalized linkage methodology that may be overcome by attending to potential problems reviewed here. The candidate-gene method, combined with studies using animal models, holds promise for understanding these as well as other hereditary epilepsies.

Clinical and molecular studies in fragile X patients with a Prader-Willi-like phenotype

A special subphenotype of the fragile X syndrome is reported which is characterised by extreme obesity with a full, round face, small, broad hands/feet, and regional skin hyperpigmentation. It resembles the Prader-Willi syndrome (PWS) and might therefore be named 'Prader-Willi-like'. Unlike the PWS, these PW-like fragile X patients lack the neonatal hypotonia with feeding problems during infancy followed by hyperphagia from toddlerhood. We describe five new fragile X patients and present a clinical update of three previously described patients with the PW-like phenotype. In one family, segregation of either the classical Martin-Bell or the PW-like phenotype was observed and in another family there was repeated transmission of the PW-like phenotype. Previously, one of the patients had been misdiagnosed as having classical PWS, based on clinical findings. Molecular studies of the FMR-1 gene showed the typical full mutations as seen in fragile X syndrome males. Molecular analysis of the 15q11-13 region, which is deleted in the majority of classical PWS patients, did not show any detectable abnormalities. In a group of 26 patients with suspected Prader-Willi syndrome but without detectable molecular abnormalities of chromosome 15, one fragile X patient was found. These clinical and molecular findings illustrate the necessity to perform DNA analysis of the FMR-1 gene in mentally retarded patients presenting with a PW phenotype but without the PWS specific cytogenetic/molecular abnormalities of chromosome 15.

Variable phenotypes in velocardiofacial syndrome with chromosomal deletion

Velocardiofacial syndrome (VCF) has overlapping features with DiGeorge sequence; both result from a developmental field defect and probably represent contiguous gene deletion syndromes. The association of chromosome 22q11 deletion with DiGeorge sequence led us to do molecular analysis of chromosome 22 in 18 patients with VCF, who ranged in age from 6 to 42 years. All 18 patients had monosomy for the chromosome region 22q11. Retrospectively, we correlated the presence of the deletion with various clinical findings: 100% had cleft palate, 67% the facial phenotype, 83% cardiac disease, 94% learning disabilities, 70% ophthalmologic findings, 50% short stature, 22% psychiatric disorders, and 17% hypocalcemia. Both severely phenotypically affected and mildly affected patients had the deletion. These findings stress the importance of continued surveillance of all patients with VCF for the many medical problems that may not be present at initial diagnosis. We conclude that the presence of the gene deletion does not predict the phenotypic expression in VCF. Further studies to characterize the size of the gene deletion may facilitate better prediction of the phenotype.

True telomeric translocation in a baby with the Prader-Willi phenotype

We report on a baby with a nonreciprocal de novo unbalanced translocation between chromosomes 12 and 15. Her karyotype was 45,XX, -12, -15, +der(12)t(12;15)(pter-->qter::q13-->qter). The paternal origin of the 15q11-13 region was shown by DNA marker studies and, consistent with this, the baby had the Prader-Willi (PWS) phenotype. The breakpoint on 12q was distal to D12S11 (lambda MS43) which maps to 12q24.3-qter. Fluorescent in situ hybridization using the oligonucleotides (TTAGGG)7 and (AATCCC)7 showed that the 12q telomere was still present within the translocated chromosome. Thus, the translocation was within or onto the end of the telomere of 12q. This unusual translocation is further evidence of an unexplained instability of the 15q11-13 region.

Linkage analysis in familial Angelman syndrome

Familial Angelman syndrome (AS) can result from mutations in chromosome 15q11q13 that, when transmitted from father to child, result in no phenotypic abnormality but, when transmitted from mother to child, cause AS. These mutations therefore behave neither as dominant nor as recessive mutations but, rather, show an imprinted mode of inheritance. We have analyzed two sibling pairs with AS and a larger family with four AS offspring of three sisters with several recently described microsatellite polymorphisms in the AS region. AS siblings inherited the same maternal alleles at the GABRB3 and GABRA5 loci, and the unaffected siblings of AS individuals inherited the other maternal alleles at these loci. In one of the AS sibling pairs, analysis of a recombination event indicates that the mutation responsible for AS is distal to locus D15S63. This result is consistent with a previously described imprinted submicroscopic deletion causing AS, a deletion that includes loci D15S10, D15S113, and GABRB3, all distal to D15S63. The analysis of the larger AS family provides the first clear demonstration of a new mutation in nondeletion AS. Analysis of linkage of AS to GABRB3 in these three families, on the assumption of imprinted inheritance (i.e., penetrance of an AS mutation is 1 if transmitted maternally and is 0 if transmitted paternally), indicates a maximum lod score of 3.52 at theta = 0.

Uniparental disomy revisited: the first twelve years

Uniparental disomy (UPD), the exceptional derivation of a pair of the offspring chromosomes from one parent only, may be compatible with normal or abnormal development and can result from gamete complementation, chromosome loss in trisomy, or duplication in monosomy (with or without residual mosaicism) and somatic recombination. In isodisomy, the uniparental pair is a duplicate of a same chromosome DNA template and causes an increased risk of recessive disorder by reduction to homozygosity. In heterodisomy, the pair remains heterozygous, made up of 2 non-recombinant homologous segments. But both iso- and heterodisomy may also cause disruption of the genomic imprints needed for differential expression of some maternal and paternal genes crucial to growth and development. Pure UPD preserves euploidy and, when harmful, is best regarded as a genomic qualitative imbalance by symmetrical excess and loss of parental homologous contribution affecting zygosity and imprint content. Instances of UPD reported till the spring 1992 are reviewed and their deleterious effects are described as they carried out lethality or morbidity by altering imprinting processes, mimicking deletions, generating recessive disorders, or prompting malignant cellular growth.

Novel patterns of inheritance of genetic disease are illustrated by the Angelman syndrome

OBJECTIVE

To characterise the molecular abnormalities present in a cohort of patients with the Angelman syndrome.

METHODS

DNA samples from 10 patients with the Angelman syndrome were investigated with molecular probes. Family studies were performed by means of DNA polymorphism analysis and densitometric estimation of allele copy number to determine the underlying mutation and its parental origin.

RESULTS

Nine probands were shown to have molecular (DNA) deletions involving chromosome 15q11-q13. Polymorphism analyses demonstrated that all deletions were maternal in origin. Five of the nine had normal karyotypes, with deletions only detected after DNA study. One patient had inherited both chromosomes 15 from her father. This represented an example of paternal uniparental disomy of chromosome 15.

CONCLUSIONS

Development of the Angelman syndrome can result from either deletion of the maternally-derived copy of chromosome 15q11-q13 or the presence of two paternally derived copies of chromosome 15, that is, uniparental disomy. DNA testing allows the identification of deletions that are not seen on cytogenetic analysis and can provide additional information regarding the parental origin of the deletion. Uniparental disomy is most readily established by DNA studies.

High-resolution mapping of the gamma-aminobutyric acid receptor subunit beta 3 and alpha 5 gene cluster on chromosome 15q11-q13, and localization of breakpoints in two Angelman syndrome patients

The gamma-aminobutyric acid (GABAA) receptors are a family of ligand-gated chloride channels constituting the major inhibitory neurotransmitter receptors in the nervous system. In order to determine the genomic organization of the GABAA receptor beta 3 subunit gene (GABRB3) and alpha 5 subunit gene (GABRA5) in chromosome 15q11-q13, we have constructed a high-resolution physical map using the combined techniques of field-inversion gel electrophoresis and phage genomic library screening. This map, which covers nearly 1.0 Mb, shows that GABRB3 and GABRA5 are separated by less than 100 kb and are arranged in a head-to-head configuration. GABRB3 encompasses approximately 250 kb, while GABRA5 is contained within 70 kb. This difference in size is due in large part to an intron of 150 kb within GABRB3. We have also identified seven putative CpG islands within a 600-kb interval. Chromosomal rearrangement breakpoints--in one Angelman syndrome (AS) patient with an unbalanced translocation and in another patient with a submicroscopic deletion--are located within the large GABRB3 intron. These findings will facilitate chromosomal walking strategies for cloning the regions disrupted by the DNA rearrangements in these AS patients and will be valuable for mapping new genes to the AS chromosomal region.

Pancreatic polypeptide infusions reduce food intake in Prader-Willi syndrome

Prader-Willi syndrome is characterized by dramatic hyperphagia and morbid obesity, and is associated with a deficiency in basal and meal-stimulated serum pancreatic polypeptide (PP) levels. Intravenous infusions of pancreatic polypeptide (90 min, 50 pmol/kg/h) restored normal serum PP levels, and a regimen of morning and afternoon PP infusions was found to significantly reduce food intake in Prader-Willi subjects. Food intake was evaluated in a 60-min free-feeding test that shows high reliability and validity. Basal food intake during saline infusions was striking (approximately 60 chicken sandwich quarters), and this intake was reduced overall by approximately 12% during PP infusions. This reduction was apparent only for female subjects, and may have reflected enhanced satiation rather than an overall suppression of food intake. No differences were apparent across subjects, in either basal food intake or the PP-related decrease in food intake, in the presence or absence of the widely recognized chromosomal marker for this syndrome [deletion of 15(q11-q13)]. More specific gene defects as recently reported in these subjects, however, suggest that the Prader-Willi syndrome may represent an important model for the study of food intake regulation.

Genomic imprinting and uniparental disomy in Angelman and Prader-Willi syndromes: a review

Although Angelman (AS) and Prader-Willi (PWS) syndromes are human genetic disorders with distinctly different developmental and neurobehavioural phenotypes, they both have abnormalities in inheritance of chromosome 15q11-q13. Whether AS or PWS arises depends on the parental origin of a deletion or uniparental disomy (the inheritance of 2 copies of a genetic locus from only one parent) for 15q11-q13. Normal development requires a genetic contribution for this genetic region from both a male and a female parent. The dependence on parental origin implies that genes in human 15q11-q13 have distinct functions depending upon epigenetic, parent-of-origin differences, known as genomic imprinting. Here, I review the role of uniparental disomy and genomic imprinting in the pathogenesis of AS and PWS, and briefly discuss phenotype-genotype correlations using candidate genes and mouse models, in particular for hypopigmentation.

Cytogenetic evidence of involvement of chromosome regions 15q12 and 12q15 in conditions with associated overgrowth

Syndromes with associated overgrowth are poorly understood. Besides their mode of inheritance, nothing is known regarding the basic genetic alterations that lead to their abnormal phenotypic manifestations. The chromosome localization of the genes involved remains unknown for this group of syndromes, with the only exception being the Wiedemann-Beckwith syndrome.

Evaluation of potential models for imprinted and nonimprinted components of human chromosome 15q11-q13 syndromes by fine-structure homology mapping in the mouse

Prader-Willi and Angelman syndromes are complex neurobehavioral contiguous gene syndromes whose expression depends on the unmasking of genomic imprinting for different genetic loci in human chromosome 15q11-q13. The homologous chromosomal region in the mouse genome has been fine-mapped by using interspecific (Mus spretus) crosses and overlapping, radiation-induced deletions to evaluate potential animal models for both imprinted and nonimprinted components of these syndromes. Four evolutionarily conserved sequences from human 15q11-q13, including two cDNAs from fetal brain (DN10, D15S12h; DN34, D15S9h-1), a microdissected clone (MN7; D15F37S1h) expressed in mouse brain, and the gene for the beta 3 subunit of the gamma-aminobutyric acid type A receptor (Gabrb3), were mapped in mouse chromosome 7 by analysis of deletions at the pink-eyed dilution (p) locus. Three of these loci are deleted in pre- and postnatally lethal p-locus mutations, which extend up to 5.5 +/- 1.7 centimorgans (cM) proximal to p; D15S9h-1, which maps 1.1 +/- 0.8 cM distal to p and is the mouse homolog of the human gene D15S9 (which shows a DNA methylation imprint), is not deleted in any of the p-locus deletion series. A transcript from the Gabrb3 gene, but not the transcript detected by MN7 at the D15F37S1h locus, is expressed in mice homozygous for the p6H deletion, which have an abnormal neurological phenotype. Furthermore, the Gabrb3 transcript is expressed equally well from the maternal or paternal chromosome 7 and, therefore, its expression is not imprinted in mouse brain. Deletions at the mouse p locus should serve as intermediate genetic reagents and models with which to analyze the genetics and etiology of individual components of human 15q11-q13 disorders.

Paternal uniparental disomy in a child with a balanced 15;15 translocation and Angelman syndrome

Chromosome 15 (15q11-q13) abnormalities cause two distinct conditions, Angelman syndrome (AS) and Prader-Willi syndrome (PWS). We present the first case of a child with a balanced 15;15 translocation and AS in whom molecular studies were crucial in confirming a diagnosis. DNA polymorphisms demonstrated paternal uniparental disomy for chromosome 15, consistent with the diagnosis of AS. The molecular studies also showed the patient to be homozygous at all loci for which the father was heterozygous, suggesting that the structural rearrangement was an isochromosome 15q and not a Robertsonian translocation.

Prader-Willi syndrome: consensus diagnostic criteria

The diagnosis of Prader-Willi syndrome (PWS) is based on clinical findings that change with age. Hypotonia is prominent in infancy. Obesity, mild mental retardation or learning disability, and behavior problems, especially in association with food and eating, result in a debilitating physical and developmental disability in adolescence and adulthood. No consistent biological marker is yet available for PWS in spite of recent research activity in cytogenetics and molecular genetics. Diagnostic criteria for PWS were developed by consensus of seven clinicians experienced with the syndrome in consultation with national and international experts. Two scoring systems are provided: one for children aged 0 to 36 months and another one for children aged 3 years to adults. These criteria will aid in recognition of the syndrome in hypotonic infants and in obese, mildly retarded, behaviorally disturbed adolescents and adults. They will also ensure uniform diagnosis for future clinical and laboratory research in PWS.

Multiplex PCR of three dinucleotide repeats in the Prader-Willi/Angelman critical region (15q11-q13): molecular diagnosis and mechanism of uniparental disomy

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct mental retardation disorders caused by a deficiency of paternal (PWS) or maternal (AS) contributions for chromosome 15 by either deletion or uniparental disomy (UPD). To further study the molecular mechanisms involved in these disorders and to improve molecular diagnostic methods, we have isolated three dinucleotide repeat markers in the PWS/AS critical region. An Alu-CA PCR method was used to isolate CA-repeat markers directly from yeast artificial chromosome (YAC) clones identified by probes IR4-3R (D15S11), LS6-1 (D15S113), and GABAA receptor B3 (GABRB3). Three markers with 6-11 alleles and 73-83% heterozygosities were identified and analyzed by multiplex PCR. Gene-centromere mapping was performed on a panel of ovarian teratomas of known meiotic origin, and showed the most proximal marker, IR4-3R, to be 13 cM (95% confidence limits: 7-19 cM) from the centromere of chromosome 15. Molecular diagnostic studies were performed on 20 PWS and 9 AS patients. In 17 patients with deletions, the parental origin of deletion was determined. Ten PWS patients were shown to have maternal heterodisomy. Since these markers are only 13 cM from the centromere, heterodisomy indicates that maternal meiosis I nondisjunction is involved in the origin of UPD. In contrast, two paternal disomy cases of AS showed isodisomy for all markers tested along the length of chromosome 15. This suggests a paternal meiosis II nondisjunction event (without crossing over) or, more likely, monosomic conception (due to maternal nondisjunction) followed by chromosome duplication.(ABSTRACT TRUNCATED AT 250 WORDS)

Strategies for the molecular genetic analysis of obesity in humans

Studies of twins, adopted children, and some human populations indicate that body composition is significantly influenced by genetic factors. However, in no specific instance in either man or animals is the precise etiology of obesity known at the molecular level. Attempts to identify the molecular basis of obesity in humans have been hampered by difficulties in measuring food intake and energy expenditure with sufficient accuracy, as well as the apparent polygenic control of body composition in man. These constraints have stimulated interest in inbred animal strains, particularly mice, that have a genetic predisposition to obesity. Using the techniques of positional cloning, molecular markers flanking two autosomal recessive mouse obesity mutants (ob and db), which demonstrate a metabolic/behavioral phenotype similar to that observed in obese humans, have been identified. These markers are being used: (1) as starting points for chromosome walks to identify these genes, (2) as an aid in identifying genetically obese rodents prior to the development of the experimentally confounding obese phenotype, and (3) to investigate the possible contribution of the ob and db gene products to obesity in families segregating an obese phenotype. Additionally, genetic crosses segregating these obesity mutations are being used to identify "polygenes" that influence the severity of obesity and type II diabetes. Such studies may ultimately lead to the characterization of genes that influence the development and severity of obesity and non-insulin-dependent diabetes (NIDDM) in humans.

Molecular study of chromosome 15 in 22 patients with Angelman syndrome

DNA studies in 22 families with Angelman syndrome (AS) were performed using the chromosome 15 marker loci D15S9, D15S10, D15S11, D15S12, D15S13, D15S18, D15S24, D15S86, the alpha-actin gene and the GABA beta 3 receptor gene (GABRB3). Uniparental disomy of chromosome 15 was excluded in all patients. Eighteen AS patients (82%) showed a molecular deletion of chromosome 15q11-q13 with one or more of these markers. No duplications of junction fragments, bridging deletions or duplication breakpoints were observed. The GABRB3 gene was deleted in all deletion-positive patients tested. Analysis of maternal DNA indicated that each deletion was a de novo event. All deletions were of maternal origin; this is in agreement with genomic imprinting in AS.

Fluorescence in-situ hybridisation and molecular studies used in the characterisation of a Robertsonian translocation (13q15q) in Prader-Willi syndrome

A patient with classical Prader-Willi syndrome was found to have a Robertsonian translocation 45,XY,t(13q15q)mat. On CBG banding, the translocation chromosome had a large centromere with one primary constriction. Using fluorescence in situ hybridisation, positive signals were obtained with chromosome 13 and chromosome 15 centromere probes, proving that the translocation was dicentric. NOR banding was negative in this chromosome, suggesting that the breakpoints were at 13p11 and 15p11. DNA studies showed that, while there was no deletion involving 15(q11-13), maternal uniparental disomy for chromosome 15 was present. We compare our findings with the five other cases of familial Robertsonian translocation PWS that have been reported.

Small nuclear ribonucleoprotein polypeptide N (SNRPN), an expressed gene in the Prader-Willi syndrome critical region

Prader-Willi syndrome (PWS) is associated with paternally derived chromosomal deletions in region 15q11-13 or with maternal disomy for chromosome 15. Therefore, loss of the expressed paternal alleles of maternally imprinted genes must be responsible for the PWS phenotype. We have mapped the gene encoding the small nuclear RNA associated polypeptide SmN (SNRPN) to human chromosome 15q12 and a processed pseudogene SNRPNP1 to chromosome region 6pter-p21. Furthermore, SNRPN was mapped to the minimal deletion interval that is critical for PWS. The fact that the mouse Snrpn gene is maternally imprinted in brain suggests that loss of the paternally derived SNRPN allele may be involved in the PWS phenotype.

A candidate mouse model for Prader-Willi syndrome which shows an absence of Snrpn expression

The best examples of imprinting in humans are provided by the Angelman and Prader-Willi syndromes (AS and PWS) which are associated with maternal and paternal 15q11-13 deletions, respectively, and also with paternal and maternal disomy 15. The region of the deletions has homology with a central part of mouse chromosome 7, incompletely tested for imprinting effects. Here, we report that maternal duplication for this region causes a murine imprinting effect which may correspond to PWS. Paternal duplication was not associated with any detectable effect that might correspond with AS. Gene expression studies established that Snrpn is not expressed in mice with the maternal duplication and suggest that the closely-linked Gabrb-3 locus is not subject to imprinting. Finally, an additional new imprinting effect is described.

Psychopharmacogenetic aspects of Prader-Willi syndrome

The study of genes, drugs, and behavior in three male adolescents with Prader-Willi syndrome (PWS) revealed a clinical profile that raises questions about the indications for neuroleptic and appetite-suppressing medications in this condition. Evidence of the inadvisability of neuroleptic medication and of the pathophysiology of PWS has led to a remarkable control of violent outbursts and hyperphagia by carbamazepine in one patient afflicted with both PWS and Klinefelter's syndrome. Testosterone and behavioral therapy proved to be useful in the management of two patients. The present observations, which are supported by recent advances in the pathophysiology of satiety, suggest that PWS should be understood as a metabolic disorder and subjected to psychopharmacogenetic study.

Prader-Willi syndrome in a brother and sister without cytogenetic or detectable molecular genetic abnormality at chromosome 15q11q13

We report on a 12-year-old boy and his 7-year-old sister with the Prader-Willi syndrome. They both had severe initial hypotonia with feeding problems and later developed an increasing appetite. Both sibs have almond-shaped eyes, triangular mouth, hypogonadism, retarded growth, and mental retardation. An older brother suffered from severe hypotonia and died at 7 days of age. The children have normal chromosomes by high-resolution technique and have inherited the same chromosomes 15 short arm polymorphisms from their parents. The family was informative for one of four DNA markers specific for the 15q11q13 region. No deletion was found using this marker. The parents were healthy and unrelated. Autosomal recessive inheritance or a paternally inherited submicroscopic deletion are possible explanations for the sib occurrence in this family.

Deletion of 15q12 in Angelman syndrome: report of 3 new cases

Deletion of 15q12 has been reported in patients with Angelman syndrome (AS). We report chromosome studies showing del(15q12) in three new cases, diagnosed as having AS. We were also able to determine, through heteromorphism studies, that the origin of the deleted chromosome in all three probands is maternal. This is a consistent finding in previously reported cases of AS.