Bibliographie

Tyrosinase-positive oculocutaneous albinism (ty-pos OCA) is an autosomal recessive disorder of the melanin pigmentary system. South African ty-pos OCA individuals occur with two distinct phenotypes, with or without darkly pigmented patches (ephelides, or dendritic freckles) on exposed areas of the skin. These phenotypes are concordant within families, suggesting that there may be more than one mutation at the ty-pos OCA locus. Linkage studies carried out in 41 families have shown linkage between markers in the Prader-Willi/Angelman syndrome (PWS/AS) region on chromosome 15q11-q13 and ty-pos OCA. Analysis showed no obligatory crossovers between the alleles at the D15S12 locus and ty-pos OCA, suggesting that the D15S12 locus is very close to or part of the disease locus, which is postulated to be the human homologue, P, of the mouse pink-eyed dilution gene, p. Unlike caucasoid "ty-pos OCA" individuals, negroid ty-pos OCA individuals do not show any evidence of locus heterogeneity. Studies of allelic association between the polymorphic alleles detected at the D15S12 locus and ephelus status suggest that there was a single major mutation giving rise to ty-pos OCA without ephelides. There may, however, be two major mutations causing ty-pos OCA with ephelides, one associated with D15S12 allele 1 and the other associated with D15S12 allele 2. The two loci, GABRA5 and D15S24, flanking D15S12, are both hypervariable, and many different haplotypes were observed with the alleles at the three loci on both ty-pos OCA-associated chromosomes and "normal" chromosomes. Published erratum appears in Am J Hum Genet 1994 Sep;55(3):602

The Angelman syndrome (AS) is a neurological disorder characterized by severe mental retardation, absent speech, seizures, gait disturbances, and a typical age-dependent facial phenotype. Most cases are due to an interstitial deletion on the maternally inherited chromosome 15, in the critical region q11-q13. Rare cases also result from paternal uniparental disomy of chromosome 15. In a group of 14 patients with sporadic AS diagnosed in Switzerland, we found 2 unrelated females with paternal isodisomy for the entire chromosome 15. Their phenotypes were milder than usually seen in this syndrome: one girl did not show the typical AS facial changes; both patients had late-onset mild seizures; as they grew older, they had largely undisturbed gross motor functions, in particular no severe ataxia. Both girls were born to older fathers (45 and 43 years old, respectively). The apparent association of a relatively milder phenotype in AS with paternal uniparental disomy will have to be confirmed by detailed clinical descriptions of further patients.

Twenty-seven cases of inverted duplications of chromosome 15 (inv dup [15]) were investigated by FISH with two DNA probes specific for the Prader-Willi syndrome/Angelman syndrome (PWS/AS) region on proximal 15q. Sixteen of the marker chromosomes displayed two copies of each probe, while in the remaining 11 markers no hybridization was observed. A significant association was found between the presence of this region and an abnormal phenotype (P < .01). This is the largest study to date of inv dup(15) chromosomes, that uses molecular cytogenetic methods and is the first to report a significant association between the presence of a specific chromosomal region in such markers and an abnormal phenotype.

The D15S9 and D15S63 loci in the Prader-Willi/Angelman syndrome region on chromosome 15 are subject to parent-of-origin-specific DNA methylation. We have found two Prader-Willi syndrome families in which the patients carry a maternal methylation imprint on the paternal chromosome. In one of these families, the patients have a small deletion encompassing the gene for the small nuclear ribonucleoprotein polypeptide N, which maps 130 kb telomeric to D15S63. Furthermore, we have identified a pair of nondeletion Angelman syndrome sibs and two isolated Angelman syndrome patients who carry a paternal methylation imprint on the maternal chromosome. These Angelman and Prader-Willi syndrome patients may have a defect in the imprinting process in 15q11-13. We propose a model in which a cis-acting mutation prevents the resetting of the imprinting signal in the germ line and thus disturbs the expression of imprinted genes in this region.

The authors report a 9-year-old girl with mid-facial hypoplasia, maxillary hypoplasia, prognathia, microbrachycephaly, mouth opening and protruding tongue. She also had psychomotor retardation such as mental retardation and speech delay. Frequent laughter fits and seizure disorder was also noted. Although the high resolution chromosome study failed to demonstrate any deletion of chromosome 15q, the clinical picture was compatible with Angelman syndrome. Breast development at the age of six and rapid progression of bone age was noted at follow up. After a series of examinations, the diagnosis of gonadotropin-dependent precocious puberty was made. MRI of brain revealed an intermediate cyst in the pituitary gland and slightly enlarged pineal gland. However, serum alpha-fetoprotein and beta-HCG were undetectable and the size of the pineal gland remained the same at the 1-year follow-up. She was treated with long-acting GnRH analogue and valproic acid. The combination of precocious puberty and Angelman syndrome has not been reported before and such association needs further experience for clarification.

A 29-year-old male with Angelman syndrome and an unbalanced reciprocal translocation, 45,XY,-8, -15, +der(8),t(8;15)(p23.3;q11)pat, was evaluated with DNA studies. These showed the underlying mechanism to be paternal uniparental disomy. This is the second case reported of Angelman syndrome that has resulted from a familial unbalanced reciprocal translocation.

This review provides a discussion of behavioral neurogenetics' contribution to understanding neurodevelopmental pathways in learning and developmental disabilities. A brief overview is given of several common neurogenetic disorders with various genetic etiologies including Down syndrome, Turner syndrome, Prader-Willi syndrome, Angelman syndrome, and Tourette's syndrome. Special emphasis is placed on fragile X syndrome as representative of a newly-discovered class of genetic conditions characterized by an unstable trinucleotide repeat. A spectrum of cognitive, behavioral, and social-emotional phenotypic features associated with fragile X syndrome is examined. Also included are findings from recent neuroimaging research and a discussion of the need for the classification of symptoms on the basis of underlying genetic/medical conditions.

Three genes (Gabrg3, Gabra5, and Gabrb3) encoding the gamma 3, alpha 5, and beta 3 subunits of the type A gamma-aminobutyric acid receptor, respectively, are known to map near the pink-eyed dilution (p) locus in mouse chromosome 7. This region shares homology with a segment of human chromosome 15 that is implicated in Angelman syndrome, an inherited neurobehavioral disorder. By mapping Gabrg3 on a panel of p-locus deletions, we have determined that the order of genes within this cluster is centromere-p(D15S12h)-Gabrg3-Gabra5-Gabrb3-telom ere. Like Gabrb3, neither the Gabra5 nor Gabrg3 gene is functionally imprinted in adult mouse brain. Mice deleted for all three subunits die at birth with a cleft palate, although there are rare survivors (approximately 5%) that do not have a cleft palate but do exhibit a neurological abnormality characterized by tremor, jerky gait, and runtiness. We have previously suggested that deficiency of the beta 3 subunit may be responsible for the clefting defect. Most notably, however, in this report we describe mice carrying two overlapping, complementing p deletions that fail to express the gamma 3 transcript, as well as mice from another line that express neither the gamma 3 nor alpha 5 transcripts. Surprisingly, mice from both of these lines are phenotypically normal and do not exhibit any of the neurological symptoms characteristic of the rare survivors that are deleted for all three (gamma 3, alpha 5, and beta 3) subunits. These mice therefore provide a whole-organism type A gamma-aminobutyric-acid receptor background that is devoid of any receptor subtypes that normally contain the gamma 3 and/or alpha 5 subunits. The absence of an overt neurological phenotype in mice lacking the gamma 3 and/or alpha 5 subunits also suggests that mutations in these genes are unlikely to provide useful animal models for Angelman syndrome in humans.

Electroencephalography plays a very important role in the diagnosis and course monitoring of epilepsy. The EEG is also able to give decisive clues in diseases other than epilepsy. It is important to know these specific EEG patterns which may assume a key role in the diagnosis of rare neuropediatric diseases. Familiarity with such specific patterns will allow to select only specific investigations for diagnostic confirmation. We report six guiding EEG patterns and the associated disorders:
1. Re-build up phenomena for Moyamoya syndrome.
2. High amplitude alpha-beta activity for lissencephaly type I.
3. Positive spikes during low frequency photostimulation for late infantile neuronal ceroid lipofuscinosis.
4. Periodic high amplitude discharges for subacute sclerosing panencephalitis (SSPE).
5. Burst suppression pattern in non-asphyctic mature newborns for Ohtahara syndrome, non ketotic hyperglycinemia or molybden cofactor deficiency.
6. High amplitude 3-4 HZ activity with small spikes for Angelman syndrome.

BACKGROUND. Type II (tyrosinase-positive) oculocutaneous albinism is an autosomal recessive disorder that has recently been mapped to chromosome segment 15q11-q13. The frequency of this disorder is greatly increased in patients with Prader-Willi or Angelman syndrome, both of which involve deletions of chromosome 15q. The P protein is a transmembrane polypeptide that may transport small molecules such as tyrosine, the precursor of melanin. The P gene is located in chromosome segment 15q11-q13.
METHODS. We studied the tyrosinase and P genes in three patients with type II oculocutaneous albinism, one of whom also had Prader-Willi syndrome, and in one patient with a milder syndrome known as autosomal recessive ocular albinism. Individual exons of these genes were amplified from the DNA of each patient by the polymerase chain reaction and screened for mutations by simultaneous analyses of single-stranded conformation polymorphisms and heteroduplexes and subsequent DNA sequencing.
RESULTS. Mutations of the P gene were identified in all four patients. These included one frame shift, three missense mutations that result in amino acid substitutions, and one mutation that affects RNA splicing. The patient with Prader-Willi syndrome plus albinism had a typical deletion of the paternal chromosome 15, rendering him hemizygous for a maternally inherited mutant allele of the P gene. The child with ocular albinism was heterozygous for two different mutations in the P gene.
CONCLUSIONS. Abnormalities of the P gene are associated with a wide range of clinical phenotypes, including type II oculocutaneous albinism, albinism associated with the Prader-Willi syndrome, and at least some cases of autosomal recessive ocular albinism.

In order to identify genes in the Prader-Willi/Angelman syndrome critical region, radiolabeled cDNA probes from poly(A)+ RNA from mouse tissues were used to identify potential exon-containing genomic DNA fragments in cosmid or phage clones from appropriate yeast artificial chromosomes, and these fragments were subsequently used to screen human cDNA libraries. A mouse brain cDNA probe was effective in detecting control genes of various abundance including small nuclear ribonucleoprotein polypeptide N (SNRPN), hypoxanthine-guanine phosphoribosyl transferase, glyceraldehyde-3-phosphate dehydrogenase, and beta-actin. Two genes mapping within the Angelman syndrome critical region were isolated. One gene was found to encode the E6-associated protein (E6-AP; gene symbol HPVE6A), a protein which interacts with the E6 protein of human papilloma virus. The other gene is previously uncharacterized and is designated PAR-2 (D15S225E) for Prader-Willi and Angelman region-gene 2. Imprinting analysis using reverse transcription-polymerase chain reaction of RNA from fibroblasts and lymphoblasts of deletion Prader-Willi and Angelman patients demonstrated imprinting of SNRPN with exclusive expression from the paternal allele, but E6-AP and PAR-2 were not imprinted in these cultured human cells. The ability to analyze for imprinting and expression of SNRPN and other genes in this region in cultured human cells will be a valuable tool for analyzing the molecular basis of the Prader-Willi and Angelman syndromes, although imprinting may differ between cultured cells and tissues.

The Angelman syndrome (AS) and Prader-Willi syndrome (PWS) loci have been mapped to chromosome 15q11-q13. Chromosomal deletions of differing parental origin in the two syndromes have been interpreted as being due to genetic imprinting. Molecular analysis of patients with varying deletions has localized the AS locus to the interval between D15S113 and GABRB3 and the PWS locus between D15S13 and D15S113. In the present study, DNA cloning and physical mapping techniques have been used to characterize the AS/PWS chromosome region in the vicinity of D15S10, a locus that is telomeric to D15S113 and centromeric to GABRB3. A CpG island near TD3-21 at D15S10 has been cloned, allowing the identification of a widely expressed 4.5-kb transcript and providing a novel DNA marker, OP3, at this locus. OP3 and TD3-21 have been used to construct a long-range physical map extending over approximately 2800 kb. Clusters of rare-cutting restriction sites on this map locate four other CpG islands. Since these CpG islands lie within the minimum deletion intervals for AS and PWS, they mark the possible locations of candidate genes for the two syndromes.

The gamma-aminobutyric acid (GABAA) receptor beta 3 (GABRB3) and alpha 5 (GABRA5) subunit genes have been localized to the Angelman and Prader-Willi syndrome region of chromosome 15q11-q13. GABRB3, which encompasses 250 kb, is located 100 kb proximal of GABRA5, with the two genes arranged in head-to-head transcriptional orientation. In screening 135 kb of cloned DNA within a 260-kb interval extending from within GABRB3 to the 5' end of GABRA5, 10 new (CA)n repeats have been identified. Five of these have been analyzed in detail and found to be highly polymorphic, with the polymorphism information content (PIC) ranging from 0.7 to 0.85 and with heterozygosities of 67 to 94%. In the clones from GABRB3/GABRA5 region, therefore, the frequency of (CA)n with PICs > or = 0.7 is 1 per 27 kb. Previous estimates of the density of (CA)n with PICs > or = 0.7 in the human genome have been approximately 10-fold lower. The GABRB3/GABRA5 region appears, therefore, to be enriched for highly informative (CA)n. This set of closely spaced, short tandem repeat polymorphisms will be useful in the molecular analyses of Prader-Willi and Angelman syndromes and in high-resolution studies of genetic recombination within this region.

Evidence has been accumulating from various fields of research that genomic imprinting, defined as the differential modification of genetic material depending on whether inheritance is from the male or female parent, occurs in mammals as well as in man. Human genetic diseases such as complete hydatidiform moles, triploidy, Prader-Willi syndrome, Angelman syndrome, and various cancers provide strong support for the important role of genomic imprinting in human development and represents a new mechanism for disease.

DNA replication within chromosome 15q11-q13, a region subject to genomic imprinting, was examined by fluorescence in situ hybridization. Asynchronous replication between homologues was observed in cells from normal individuals and in Prader-Willi (PWS) and Angelman syndrome (AS) patients with chromosome 15 deletions but not in PWS patients with maternal uniparental disomy. Opposite patterns of allele-specific replication timing between homologous loci were observed; paternal early/maternal late at D15S63, D15S10 and the gamma-aminobutyric acid receptor beta 3 subunit gene (GABRB3); and maternal early/paternal late at the more distal gamma-aminobutyric acid receptor alpha 5 subunit gene (GABRA5). At the most distal locus examined, D15S12, both patterns of allele-specific replication timing were detected.

The phenomenon of parental imprinting has become increasingly important in disciplines such as evolution, genetics, molecular biology, embryology and pathology. Principally, parental imprinting refers to a parent-of-origin dependent expression of a subset of autosomal loci, independent of the sex of the offspring. Today, at least seven such loci have been identified, including the human IGF2 gene. It appears that the set of imprinted genes is not always identical between the species, although the importance of maintaining this kind of gene regulation is evolutionarily conserved. It is particularly interesting from the clinical point of view that a number of human diseases, such as the Beckwith-Wiedemann and Prader-Willi/Angelman syndromes, appear to involve unbalanced parental contributions of imprinted loci. We show here that the four different human IGF2 promoters are expressed mono- and/or biallelically in complex patterns in postnatal liver specimens.

Since a previous report of a partial YAC contig of the Prader-Willi/Angelman chromosome region (15q11-q13), a complete contig spanning approximately 3.5 Mb has been developed. YACs were isolated from two human genomic libraries by PCR and hybridization screening methods. Twenty-three sequence-tagged sites (STSs) were mapped within the contig, a density of approximately 1 per 200 kb. Overlaps between YAC clones were identified by Alu-PCR dot-blot analysis and confirmed by STS mapping or hybridization with ends of YAC inserts. The gene encoding small nuclear ribonucleoprotein-associated peptide N (SNRPN), recently identified as a candidate gene for Prader-Willi syndrome, was localized within this contig between markers PW71 and TD3-21. Loci mapped within and immediately flanking the Prader-Willi/Angelman chromosome region contig are ordered as follows: cen-IR39-ML34-IR4-3R-TD189-1-PW71-SNRPN -TD3-21- LS6-1-GABRB3,D15S97-GABRA5-IR10-1-CMW1+ ++-tel. This YAC contig will be a useful resource for more detailed physical mapping of the region, for generation of new DNA markers, and for mapping or cloning candidate genes for the Prader-Willi and Angelman syndromes.

The SNRPN gene encodes a small nuclear ribonucleoprotein subunit, SmN, thought to be involved in splicing of pre-mRNA. A closely related protein, SmB/B', is constitutively expressed in all tissues except the brain, where SmN is predominantly expressed. The mouse homolog of the SNRPN gene has been shown to be functionally imprinted in mouse brain, being expressed only from the paternally derived chromosome. SNRPN has been mapped to human chromosome 15q11-q13 within the shortest region of deletion overlap for the Prader-Willi syndrome. We have now demonstrated functional imprinting of the human SNRPN gene using reverse transcription followed by the polymerase chain reaction (RT-PCR). No expression was observed in cultured skin fibroblasts of Prader-Willi patients, but was found in all Angelman patients and normal controls examined. We have also demonstrated a parent-specific DNA methylation imprint within intron 5 of the SNRPN gene, which suggests an epigenetic mechanism by which parent-specific expression of this gene might be inherited. Our findings indicate that SNRPN is expressed only from the paternally derived chromosome 15 in humans and therefore may fulfill one major criterion for being involved in the pathogenesis of the Prader-Willi syndrome.

The Prader-Willi syndrome and the Angelman syndrome are caused by the loss of function of distinct but closely linked genes on human chromosome 15. Based on a yeast artificial chromosome restriction map and two key patients we have determined that the shortest region of deletion overlap in the Prader-Willi syndrome comprises 320 kb. The region includes the anonymous DNA marker PW71 (D15S63) and the gene for the small nuclear ribonucleoprotein N (SNRPN). The SNRPN gene maps 130 kb distal to PW71 and is transcribed from centromere to telomere.

A molecular-genetic study was performed in 61 patients with Angelman syndrome (AS) and 14 patients with Prader-Willi syndrome (PWS). Southern blot analyses and/or PCR-mediated dinucleotide repeat polymorphism (DNRP) analyses revealed that 67% of AS patients have DNA deletions ranging from D15S9 to D15S12 loci. An exception was 3 sib cases whose deletion involved only 2 loci, D15S10 and GABRB3. The parental origin of the deletions in AS patients were exclusively maternal. No uniparental disomy (UPD) was found in our AS patient series, suggesting that UPD in AS is infrequent than that in PWS. Molecular deletions were observed in 6 of the 14 PWS patients. In order to develop a simple, reliable DNA-based diagnostic method, I adopted PCR-mediated DNRPs as genetic markers for the detection of deletions and/or parental origin of chromosomes 15 in AS and/or PWS patients. This method gave useful diagnostic information in 33 (89%) of 37 AS patients and 12 (86%) of 14 PWS patients, indicating no big difference from the information obtained with Southern blot analysis. Furthermore, since this DNRP method dose not require radioisotopes, it may be a first-choice, alternative way when diagnosing AS or PWS patients.

Angelman syndrome (AS) results from a lack of maternal contribution from chromosome 15q11-13, arising from de novo deletion in most cases or rarely from uniparental disomy. These families are associated with a low recurrence risk. However, in a minority of families, more than one child is affected. No deletion has been found in these families, except one. The mode of inheritance in these families is autosomal dominant modified by imprinting. Sporadic cases, with no observable deletion, therefore pose a counselling dilemma as there could be a recurrence risk as high as 50%. We present a series of 93 AS patients, showing the relative contribution of these different genetic mechanisms. Eighty-one AS patients were sporadic cases while 12 cases came from six families. Sixty cases had deletions in 15q11-13 detected by a set of highly polymorphic (CA)n repeats markers and conventional RFLPs. Ten sporadic cases plus all 12 familial cases had no detectable deletion. In addition, two cases of de novo deletions occurred in a chromosome 15 carrying a pericentric inversion. In one of these the AS child had a cousin with Prader-Willi syndrome (PWS) arising from a de novo deletion in an inv(15) inherited from his father. One case arose from a maternal balanced t(9;15)(p24;q15) translocation. There were three cases of uniparental disomy. Five patients were monoallelic for all loci across the minimal AS critical region, but the presence of a deletion cannot be confirmed. In familial cases, all affected sibs inherited the same maternal chromosome 15 markers for the region 15q11-13.

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.

AS is characterized by severe mental retardation, seizures, ataxic gait and easily evoked laughter. About 70 approximately 80% of AS patients have a chromosomal/molecular deletion at 15 q11-q13, occurring exclusively in the maternally-derived chromosome 15. There have been 4 AS patients whose chromosomes 15 are paternal uniparental disomy. This biased parent-of-origin suggests that genomic imprinting may play a role in the occurrence of the syndrome. GABRB3 is located at 15 q11-q13. GABAA is a main inhibitory neurotransmitter in the central nervous system (CNS) and functions through its receptor. The beta 3 subunit, one of the components of the receptor, is present in the telephalonal cortex, hippocampus, thalamus and cerebellum, and a peak GABRB3 expression is observed during embryogenesis. This indicates that GABRB3 plays a role in CNS development, suppression of seizures and behavioral control. Since GABRB3 is encompassed within the smallest deletion among AS patients, it becomes a candidate responsible for the central nerve disturbances in AS. This smallest deletion was found in 3 AS sibs, their phenotypically normal mother and maternal grandfather in a family, suggesting that the paternally-derived deletion has no phenotypical effect in the offspring but the maternally-derived one. However, recent studies demonstrated that the mouse Gabrb3 is not involved in imprinting. The confirmation of GABRB3 to be the AS gene needs to provide direct evidence of its imprinting. Our preliminary study showed that GABRB3 was not expressed in hydatidiform mole that is composed only of the paternal genome, while it was expressed fully in normal villous tissue, suggesting that GABRB3 is paternally imprinted.

A patient with Angelman syndrome and a 46,XY/47,XY,+inv dup(15)(pter-->q11: q11-->pter) karyotype and a patient with Prader-Willi syndrome and a 46,XY/47,XY,+inv dup(15)(pter-->q12: q12-->pter) karyotype were investigated with molecular markers along chromosome 15. Paternal uniparental isodisomy was found for all informative markers in the first case which indicates that this, rather than the presence of the extra chromosome, is the cause of the Angelman syndrome phenotype. Similarly, the PWS patient showed maternal uniparental distomy with absence of PWS region material on the inv dup(15) chromosome. If (1) marker chromosomes are an occasional by product of 'rescuing' a trisomic fertilisation, or (2) if duplication of the normal homologue in a zygote which has inherited a marker in place of the normal corresponding chromosome 'rescues' an aneuploid fertilisation, or (3) if the presence or formation of a marker chromosome increases the probability of non-disjunction, then uniparental disomy might be found occasionally in other subjects with de novo marker chromosomes.

Thirty-two cases of uniparental disomy (UPD), ascertained from Prader-Willi syndrome patients (N = 27) and Angelman syndrome patients (N = 5), are used to investigate the pattern of recombination associated with nondisjunction of chromosome 15. In addition, the meiotic stage of nondisjunction is inferred by using markers mapping near the centromere. Two basic approaches to the analysis of recombination are utilized. Standard methods of centromere mapping are employed to determine the level of recombination in specific pairwise intervals along the chromosome. This method shows a significant reduction in recombination for two of five intervals examined. Second, the observed frequency of each recombinant class (i.e., zero, one, two, three, or more observable crossovers) is compared with expected values. This is useful for testing whether the reduction in recombination can be attributed solely to a proportion of cases with no recombination at all (because of asynapsis), with the remaining groups showing normal recombination (or even excess recombination), or whether recombination is uniformly reduced. Analysis of maternal UPD(15) data shows a slight reduction in the multiple-recombinant classes, with a corresponding increase in both the zero- and one-recombinant classes over expected values. The majority, more than 82%, of the extra chromosomes in maternal UPD(15) cases are due to meiotic I nondisjunction events. In contrast, most paternal UPD(15) cases so far examined appear to have a postzygotic origin of the extra paternal chromosome.

A human chromosomal region, 15q11-q13, was microdissected, its DNA was amplified with the primer-linker PCR method, and the PCR products were cloned into a plasmid vector to construct a microclone library. Of 193 microclones analyzed with Southern blot hybridization on hybrid cell panels, 26 (13.5%) were either single-copy (unique) or low-repetitive fragments. By screening of a cosmid library of human genomic DNA using the 26 microclones as probes, 47 positive cosmids were obtained and underwent regional mapping with chromosome fluorescence in situ hybridization (FISH). Sixteen cosmids gave FISH signals at 15p-cen, 5 at 15q11-q13, 6 at 15q22-q26, 3 at other chromosomes, and 17 no signal. These 27 cosmids mapped to chromosome 15 are useful additions to the inventory of DNA markers of this chromosome including the much interested Prader-Willi/Angelman syndrome region.

The clearest example of genomic imprinting in humans comes from studies of the Angelman (AS) and Prader-Willi (PWS) syndromes. Although these are clinically distinct disorders, both typically result from a loss of the same chromosomal region, 15q11-q13. AS usually results from either a maternal deletion of this region, or paternal uniparental disomy (UPD; both chromosomes 15 inherited from the father). PWS results from paternal deletion of 15q11-q13 or maternal UPD of chromosome 15. We have recently described a parent-specific DNA methylation imprint in a gene at the D15S9 locus (new gene symbol, ZNF127), within the 15q11-q13 region, that identifies AS and PWS patients with either a deletion or UPD. Here we describe an AS sibship and three PWS patients in which chromosome 15 rearrangements alter the methylation state at ZNF127, even though this locus is not directly involved in the rearrangement. Parent-specific DNA methylation imprints are also altered at ZNF127 and D15S63 (another locus with a parent-specific methylation imprint) in an AS sibship which have no detectable deletion or UPD of chromosome 15. These unique patients may provide insight into the imprinting process that occurs in proximal chromosome 15 in humans.

Uniparental disomy is responsible for a proportion of cases in Prader-Willi, Angelman, and Wiedemann-Beckwith syndromes. In these syndromes, the chromosomes involved are thought to contain one or more imprinted genes. When two copies of the imprinted (inactivated) gene are inherited from a single parent through uniparental disomy or the active gene is deleted, the phenotype of the syndrome results. Our goal is to identify additional syndromes caused by uniparental disomy. Our approach is to select syndromes that appear to have more than one mode of inheritance and are occasionally associated with a cytogenetic abnormality. Given this criterion, we have chosen Brachmann-de Lange Syndrome (BDLS) to investigate since the phenotype is similar to that found in patients with dup(3q). We have studied 16 probands with BDLS and their parents using a multiplex of four PCR-based polymorphic loci on chromosome 3. None of the probands studied had uniparental disomy for chromosome 3 and all demonstrated normal biparental inheritance for at least one locus. Given these results, uniparental disomy of chromosome 3 does not appear to be a major contributor to the syndrome. Additionally, both maternally and paternally derived chromosome abnormalities have resulted in the dup(3q) phenotype and dominant inheritance of BDLS from both mildly affected mothers and fathers have been reported which suggests that imprinting is not involved in these syndromes.

Prader-Willi syndrome (PWS) is a disorder characterized by neonatal hypotonia with poor suck, mild to moderate mental retardation, obesity beginning after 3 yr of age, hypogonadism and characteristic facial features. High resolution cytogenetic studies showed a deletion of the proximal chromosome 15q(q11-q13) region in approximately 50%. Interestingly, the same deletion was described in another distinct mental disorder: the Angelman syndrome (AS). This deletion was confirmed by molecular analyses, and a new mechanism was reported: uniparental disomy (maternal in PWS and paternal in AS) strongly implicate genomic imprinting in this chromosomal region. The principal aim of our group is to apply cytogenetic and molecular biology techniques to perform diagnosis and genetic counselling. Patient studies were usually based on high resolution cytogenetic analysis, quantitative Southern blotting (with D15S9, D15S11, D15S10, D15S12 loci) and dinucleotide repeat polymorphism assay by polymerase chain reaction (PCR) (IR4 .3R and GABARB3). The combination of these different methods allowed us to propose a diagnostic strategy for PWS.

The mouse pink-eyed cleft-palate (p(cp)) mutation is characterized by hypopigmentation associated with cleft palate, neurological disorders and runting. Most p(cp) homozygotes are born with cleft palate and die shortly after birth, presumably as a result of feeding problems. A few exceptional p(cp) mutants live beyond this stage but display tremor and jerky gait. We report here that the genes encoding the gamma-aminobutyric acid type A (GABAA) receptor subunits alpha 5 (originally described as alpha 4; ref. 4), beta 3 and gamma 3 are disrupted by a deletion in p(cp) mice. We also show that the alpha 5 and gamma 3 genes are located between the p and beta 3 genes on mouse chromosome 7. The p(cp) deletion leads to alterations of binding properties of the GABAA receptors in the brain, providing an in vivo model system for studying GABAA receptor function. The human homologue of the region deleted in p(cp) mice is associated with Angelman syndrome. Thus, p(cp) mice may be useful in defining the region containing the gene(s) for this syndrome.

BACKGROUND: Angelman (happy puppet) syndrome is a neuro-developmental condition characterized by an ataxic gait with puppet-like limb movements, paroxysmal bouts of laughter and severe mental retardation. Although considered a rare condition, over 140 cases have been documented since its designation in 1965. To date, only one study has been published investigating the ocular defects of Angelman syndrome.
METHODS: In this paper we report the cognitive, motor, systemic, and oculo-visual findings of a 3 year old child with Angelman syndrome.
RESULTS: The oculo-visual findings include choroidal hypopigmentation, iris hypopigmentation, strabismus, and hyperopia.
CONCLUSIONS: As in many neuro-developmental conditions, early diagnosis is crucial. The ocular findings of fundal/iris hypopigmentation and strabismus with minimal refractive error in conjunction with the cognitive, behavioral, and motoric characteristics the patient exhibits may be the first clues for the diagnosis of Angelman Syndrome in a developmentally delayed child. The initiation of individualized optometric diagnosis and treatment is important for all children with developmental disabilities. The eye care professional should work in concert with speech, occupational, and physical therapists, neurologists, and special educators in the multi-disciplinary treatment and habilitation of all children with disabilities including those with Angelman Syndrome.

A patient with typical features of Angelman syndrome--a genetically inherited disorder involving developmental delay, ataxia, episodes of paroxysmal laughter and brachiocephaly--was studied with single-photon emission tomography. Hypoperfusion found in the left frontal and left temporoparietal regions can provide insights into the functional cerebral pathology, which may be due to a disturbance of the developmental process related to a chromosomal abnormality.

The majority of cases of the two distinct disorders Prader-Willi syndrome (PWS) and Angelman syndrome (AS) result from cytogenetic deletions of chromosome 15q11-q13. These deletions are exclusively of maternal origin in AS but of paternal origin in PWS indicating that the 15q11-q13 region is subject to genomic imprinting. Transmission of a submicroscopic deletion in one three generation family resulted in AS only upon maternal transmission of the deletion with no clinical phenotype associated with paternal transmission (1,2). The breakpoint of this submicroscopic deletion has been cloned and sequenced. This is the first deletion junction from the AS/PWS region which has been so characterized. The nucleotide sequence of the deletion junction revealed a 19 bp insertion of unknown origin with no evidence of repetitive elements. A probe from the proximal deletion breakpoint, PB11, lies within the currently defined minimum region of deletion overlap in PWS, which contains the SNRPN and D15S63 loci. Our results suggest that the imprinted gene(s) responsible for the PWS phenotype are proximal of pB11 in this deletion overlap region.

We report two patients who were less than two years old that were diagnosed as having Angelman syndrome. Cytogenetic confirmation of the disease was performed. We emphasize clinical and electroencephalographic features in infants that allow an early diagnosis of this syndrome and allow for prompt genetic counseling.

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.

The communication development in 11 children with Angelman's syndrome is described. The clinical observation that these children appear to have a greater ability with receptive rather than expressive language is investigated and these skills assessed using published communication schedules. In addition the understanding and the use of nonverbal communication such as natural gesture was studied. The data collected highlight the fact that these children have developed very few words and have difficulty in using gestural or sign systems. This has implications for speech and language therapists and the children's remedial programmes. Possible future longitudinal studies are suggested.

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.

The myelin membrane is essential for rapid conduction of nerve impulses through the central nervous system. Failure of myelination--dysmyelination--may arise through several mechanisms. The synthesis of a particular myelin protein can be defective, as occurs for proteolipid protein in Pelizaeus-Merzbacher disease and for myelin basic protein in the 18q- syndrome. Delay in myelination with a more generalized diminution in white matter is characteristic of many inherited metabolic diseases, including galactosemia, pyridoxine-dependent seizure disorder, glutaric aciduria type 1, and infantile Refsum disease. Demyelination or breakdown in myelin is characteristic of metachromatic leukodystrophy, Krabbe disease, mitochondrial disorders, adrenoleukodystrophy, Canavan disease, Alexander disease, and orthochromatic leukodystrophy. A fourth category is reserved for malformation syndromes. These include Cockayne, Fukuyama, Walker-Warburg, and Angelman syndromes. Demyelination also occurs in HIV-infected individuals with central nervous system findings and in multiple sclerosis. Much of the evidence for leukodystrophy in these disorders comes from neuroimaging. Some of these disorders are treatable.

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.

The Prader-Willi and Angelman syndromes are now well established as the paradigm of genomic imprinting in human disease. Over the past year, much has been learnt about the mechanisms by which these syndromes arise and molecular diagnostics for the majority of patients are now available. Mouse models for aspects of the syndromes have been established, and the first association between a gene, located in chromosome 15, at 15q11-q13, and a phenotype (albinism) has been proven. Large parts of the critical regions have been cloned and at least six genes identified. Three genes or DNA sequences may be imprinted: two of these demonstrate DNA-methylation imprints and one is functionally imprinted in mouse. While the molecular mechanism of imprinting is not yet understood, it is beginning to yield its secrets to DNA methylation, replication, and chromatin structure studies of the phenomenon.

Duplications of the proximal long arm of chromosome 15 have been seen in the Prader-Willi syndrome (PWS), and in subjects without the Prader-Willi phenotype but with other clinical features including short stature, diabetes, anal and jejunal atresia, and acanthosis nigricans. The non-PWS subjects all had different phenotypes despite the identical findings on cytogenetic analysis. A normal phenotype has also been observed in patients with similar duplications. We report a further patient with a duplication of 15q11-13 which was detected cytogenetically and confirmed on molecular genetic analysis. She has developmental delay, particularly concerning the acquisition of speech, and an ataxic gait. These are interesting clinical features in view of the association of Angelman syndrome with abnormalities of 15q11-13.

Consistent progress has been obtained in recent years in the diagnosis and understanding of genetic disease. High resolution chromosome analysis has narrowed the gap between cytogenetics and Mendelism by proving that some diseases considered to be due to monogenic inheritance result in fact from deletion of contiguous genes or breakage of a specific gene. Gene "imprinting" has been found to be the cause of some syndromes, such as the Prader-Willi, Angelman, and Beckwith-Wiedemann. Molecular cytogenetics has become a tool for analysing small rearrangements not easily seen using traditional chromosome analysis, for detecting mosaicisms, and narrowing the "critical" regions of chromosomal syndromes. "Positional cloning" techniques have improved our ability to map disease-genes. Linkage studies and direct analysis of cloned genes are now having a large impact on the analysis of genetic diseases segregating in families, and in testing for healthy heterozygotes and presymptomatic patients. Discovery of mutations in the mitochondrial genome has widened our understanding of disease inheritance. A few disorders have been shown to be due to tandem amplification of GC-rich intragenic triplets. Genetic counseling has taken advantage of these improvements which have a large impact on our knowledge of genetic heterogeneity and variability.

Joan H.M. KNOLL, Joseph WAGSTAFF and Marc LALANDE, Cytogenetic and molecular studies in the Prader-Willi and Angelman syndromes: an overview, in American Journal of Medical Genetics, Volume 46, Number 1, pages 2-6 and 47. (April 1, 1993)

The majority of patients with Angelman syndrome and Prader-Willi syndrome have a cytogenetic and molecular deletion of chromosome 15q11q13 with the primary difference being in the parental origin of deletion. Our current understanding of the cytogenetics and molecular genetics of these 2 clinically distinct syndromes will be discussed in this review.

J.L. ZACKOWSKI, Robert D. NICHOLLS, B.A. GRAY, A. BENT-WILLIAMS, W. GOTTLIEB, P.J. HARRIS, M.F. WATERS, D.J. DRISCOLL, Roberto T. ZORI and Charles A. WILLIAMS, Cytogenetic and molecular analysis in Angelman syndrome, in American Journal of Medical Genetics, Volume 46, Number 1, pages 7-11. (April 1, 1993)

We report on cytogenetic and molecular analyses of 29 Angelman syndrome (AS) individuals ascertained in 1990 through the first National Angelman Syndrome Conference. High resolution GTG- and GBG-banded chromosomes were studied. Standard molecular analysis with six 15q11q13 DNA sequences was used to analyze copy number and parental origin of 15q11q13. Concordance between molecular and cytogenetic data was excellent. The combined data showed that 23 of the 27 probands (85%) on whom we had definitive results have deletions of the chromosome 15q11q13 region. Two classes of deletion were detected molecularly: most patients were deleted for the 5 more proximal probes, but in 2 cases the deletion extended distally to include in sixth probe. In the 13 cases where the parental origin of the deleted chromosome 15 could be established, it was maternal. There were no cases of uniparental disomy. Cytological observations of the relative sizes of the heterochromatic regions of the short arm of chromosome 15 suggested that chromosomes with large heterochromatic blocks may be more prone to de novo deletion.

Jill CLAYTON-SMITH, Clinical research on Angelman syndrome in the United Kingdom: observations on 82 affected individuals, in American Journal of Medical Genetics, Volume 46, Number 1, pages 12-15. (April 1, 1993)

Clinical information has been obtained on 82 Angelman syndrome (AS) families in the UK. Each patient was examined by the author and a detailed clinical history taken. The findings of this study are presented.

Robert D. NICHOLLS, Genomic imprinting and uniparental disomy in Angelman and Prader-Willi syndromes: a review, in American Journal of Medical Genetics, Volume 46, Number 1, pages 16-25 and 48-49. (April 1, 1993)

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.

Christiana M. LEONARD, Charles A. WILLIAMS, Robert D. NICHOLLS, O. Frank AGEE, Kytja K.S. VOELLER, Janice C. HONEYMAN and Edward V. STAAB, Angelman and Prader-Willi syndrome: A magnetic resonance imaging study of differences in cerebral structure, in American Journal of Medical Genetics, Volume 46, Number 1, pages 26-33. (April 1, 1997)

Recent improvements in magnetic resonance imaging techniques now allow the developing brain to be visualized in sufficient detail to perform "in vivo neuropathology." In this study we compared the cortical morphology in six children with Angelman and four with Prader-Willi syndrome. These two syndromes are of special interest because, although they are both caused by deletions in the same region of chromosome 15, Angelman children are far more severely affected, and do not speak. We measured the length of the banks of the Sylvian fissure in a gapless series of thin sagittal images. Angelman children had a significantly larger proportion (75%) of anomalous fissures than the Prader-Willi children (12%). Anomalous cortical growth could result from mistimed expression and recognition of macromolecules involved in axonal guidance, target recognition, and pruning. We hypothesize that misrouting of long projection axons may be related to the Sylvian fissure anomalies and the language disorder in Angelman syndrome.

Kandace A. PENNER, Joy JOHNSTON, Barbara H. FAIRCLOTH, Patricia IRISH and Charles A. WILLIAMS, Communication, cognition and social interaction in the Angelman syndrome, in American Journal of Medical Genetics, Volume 46, Number 1, pages 34-39. (April 1, 1993)

Persons with Angelman syndrome (AS) have mental retardation, epilepsy, and a characteristic "puppet-like" gait. Behaviorally, they are distinctive because they have no speech and have excessive laughter. A speech and communication evaluation of 7 persons with AS was performed to provide improved understanding of the speech deficit. Assessments included prelanguage and language development, oral motor abilities, and cognitive and social interaction skills. Results indicate that the typical lack of speech may not be due to mental retardation alone. Oral motor dyspraxia, and deficits in social interaction and attention were characteristic of AS and contributed to the lack of speech.

Richard A. KING, Georgia L. WIESNER, DeWayne TOWNSEND and James G. WHITE, Hypopigmentation in Angelman syndrome, in American Journal of Medical Genetics, Volume 46, Number 1, pages 40-44. (April 1, 1993)

Chromosome region 15q is thought to contain one or more genes that are important for melanin pigment synthesis in the hair, skin, and eyes. Hypopigmentation has been identified in the Prader-Willi (PWS) and Angelman (AS) syndromes. We have examined 6 individuals with AS to further characterize the pigment pattern in this condition. The age of the 5 girls and one boy ranged from 2.4 to 7.0 years. None had obvious albinism. Hair color ranged from light blond to brown. Skin was type I in 3 and type II in 3. Eye changes included nystagmus in 2, strabismus in 4, and reduced retinal pigment in 5. The mean hairbulb tyrosinase activity was 0.37 +/- 0.44 pmol/hb/120 min for the individuals with AS, with a range of 0.00 to 1.13 (normal brown control 1.49 +/- 0.79, normal blond control 1.50 +/- 0.85). Electron microscopic examination of hairbulb melanocytes showed normal melanosome and melanocyte architecture and number, but reduced melanin formation, with many stage II and III premelanosomes but few stage IV fully melanized melanosomes. Hypopigmentation characterized by light skin, reduced retinal pigment, low hairbulb tyrosinase activity, and incomplete melanization of melanosomes is part of the phenotype of AS, and is similar to that found in PWS.

IR4-3R (D15S11) is an anonymous DNA sequence from human chromosome 15. Using YAC cloning and restriction enzyme analysis, we have found that IR4-3R detects five related DNA sequences, which are spread over 700 kb within the Prader-Willi/Angelman syndrome chromosome region in 15q11-q13. The RsaI and StyI polymorphisms, which were described previously, are associated with the most proximal copy of IR4-3R and are in strong linkage disequilibrium. IR4-3R represents the third DNA sequence family that has been identified in 15q11-q13.

In recent years it has become apparent that the parental origin of genetic material has an impact on gene expression and this effect has become known as genomic imprinting. The evidence for the influence of genomic imprinting on behavior and in the etiology of certain neurobehavioral disorders is discussed. The possibilities for a role for genomic imprinting in the inheritance of behaviors related to alcohol abuse and alcoholism and in the paternal alcohol syndrome are also explored.

Deletions of 15q11.2-q12 are associated with either the Prader-Willi (PWS) or Angelman (AS) syndromes. It has been suggested that excessive recombination in this region might explain the high frequency of such deletions, and the frequent involvement of chromosome 15 in translocations and nondisjunction. We have studied recombination in the PWS region by linkage analysis of non-PWS families. No recombination was found (with maximum lod scores greater than 3.0) for most pair-wise combinations of probes: 39, IR4-3R, ML34, 189-1, 3-21. A 'hotspot' of recombination is observed between loci detected by p3-21 and pIR10-1. The female recombination fraction in this region was significantly higher than that for males. Close linkage with 0.06 recombination was found for the IR10-1 and CMW-1 pair. No excess recombination was found between sites bounding common breakpoints observed in deletions associated with PWS and AS. It is suggested that these deletions form frequently because of the presence of duplicated DNA sequences and/or inversions in this region, and not because of a high rate of homologous recombination.

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 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.

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.

We have established a probe order within the Angelman/Prader-Willi chromosomal regions by multicolor fluorescence in situ hybridization (FISH). The probe [locus] order extending distally from the centromere is 34[D15S9]-IR4-3R[D15S11]-189-1[D15S13]-PW71++ + [D15S63]-3-21[D15S10]-28 beta 3-H3[GABRB3]-IR10-1 [D15S12]. This order agrees with that recently reported (1) with the exception of PW71 [D15S63]. In addition, a second gamma-aminobutyric acid (GABAA) receptor, the alpha 5 subunit, has been localized within the reference map to between GABRB3 and D15S12. The locus order was further confirmed by DNA hybridization analysis of two patients, one with Angelman syndrome and one with Prader-Willi syndrome, with different unbalanced translocations and molecular extents of deletion. Our results provide a framework map of chromosome 15q11-q13 into which additional markers can be oriented and allow a further differentiation of the critical genetic regions of the two syndromes.

The mouse pink-eyed dilution (p) locus on chromosome 7 is associated with defects of skin, eye and coat pigmentation. Mutations at p cause a reduction of eumelanin (black-brown) pigment and altered morphology of black pigment granules (eumelanosomes), but have little effect on pheomelanin (yellow-red) pigment. We show here that the human complementary DNA DN10, linked to the p locus in mice, identifies the human homologue (P) of the mouse p gene, and appears to encode an integral membrane transporter protein. The expression pattern of this gene in various p mutant mice correlates with the pigmentation phenotype; moreover, an abnormally sized messenger RNA is detected in one mutant, p(un), which reverts to the normal size in p(un) revertants. The human P gene corresponds to the D15S12 locus within the chromosome segment 15q11-q13, which is typically deleted in patients with Prader-Willi and Angelman syndrome (see ref. 5 for review). These disorders are phenotypically distinct, depending on the parent of origin of the deleted chromosome, but both syndromes are often associated with hypopigmentation of the skin, hair and eyes (see ref. 8 for review), and deletion of the P gene may be responsible for this hypopigmentation. In addition, we report a mutation in both copies of the human P gene in one case of tyrosinase-positive (type II) oculocutaneous albinism, recently linked to 15q11-q13 (ref. 9).

Molecular and cytogenetic studies in Angelman syndrome have demonstrated that the condition is genetically heterogeneous with a recurrence risk in certain families which may be as high as 50%. In an attempt to identify such families, cytogenetic polymorphisms of chromosome 15 have been studied in both affected and unaffected siblings of Angelman syndrome patients. The results suggest that in those cases with a cytogenetically visible 15q11q13 deletion where the recurrence risk is low, normal siblings inherit either maternal chromosome 15 homologue with impunity. By contrast, in cases where the proband does not demonstrate a cytogenetic 15q11q13 deletion, unaffected siblings tend to inherit the alternative homologue to that found in their affected siblings. These findings may have importance for genetic counseling.

The Angelman syndrome or "happy puppet" syndrome is a disorder of severe mental retardation, seizure, paroxysms of laughter, absent speech, jerky movements and ataxic gait. We present two sibs, man and woman, with this disorder, fact that support the possible autosomal recessive inheritance as a cause of this pathology, which hereditary mechanism is still a controversial point. Besides, we can observe different expression, being the woman more severely affected than the man. To our knowledge, this is the first mexican family reported with this syndrome, and with a ten years follow up. Chromosomal studies, with high resolution technique, were normal, we did not find the 15 chromosomic deletion referred as a possible cause in some cases, that is why it is undeniable that genetic heterogeneity exists in this syndrome.

We report two sibs with Angelman syndrome or an apparently new syndrome. In addition to severe mental retardation and seizures, clinical examination showed an ataxic and stiff legged gait, truncal hypotonia with hypertonia of the limbs, dysmorphic facial features (brachycephaly, large mouth, pointed chin and a prominent jaws) and scoliosis. Brain CT scan and MRI revealed ventricular enlargement and squared frontal horns. Pregnancy and delivery were uneventful. Karyotypes were normal. No deletion of chromosome 15q11-13 region was shown by molecular genetic techniques. The parents who are normal are second cousins. The condition is therefore probably inherited as an autosomal recessive one.

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.

Parental ages associated with both maternal and paternal uniparental disomy (UPD) of chromosome 15 are highly elevated in comparison to Zurich population-based controls, with mean maternal and paternal ages of 35.6 and 38.1, respectively for UPD patients (diagnosed in Zurich) and 28.0 and 31.0, in controls. The parental ages are also significantly higher than observed for trisomies of other chromosomes diagnosed in Zurich. The higher age of UPD cases may be due to the fact that two errors, both a gain and a loss of a chromosome 15, are necessary. We suggest that gamete complementation, zygote formation from two gametes one of which is nullisomic and the other disomic for the same chromosome, may be a major mechanism of UPD formation, as well as secondary loss of a chromosome in a trisomic conception, and that there is an association between increased paternal age and nondisjunction.

Five patients with inv dup(15) chromosomes were investigated with molecular probes on proximal 15q to determine the parental origin and extent of the duplicated segment. Cytogenetic investigation showed that four patients carried one and a fifth patient had two extra chromosomes derived from number 15 in all cells. In situ hybridization with a chromosome 15 library and a centromere 15 probe confirmed that the entire inv dup chromosomes were derived from chromosome 15. Molecular analysis using probes mapping in the region deleted in Prader-Willi syndrome (PWS) and Angelman syndrome (AS) patients implied that in at least two patients the extra chromosomes were asymmetric with one copy of the PWS region on the extra marker chromosome but two copies of the region centromeric to the PWS region. Three other cases had an inv dup(15) with two extra copies of the PWS region, but in one of these, heteromorphisms clearly demonstrated that the two centromeres derived from two different chromosomes. The inv dup(15) presumably resulted from an illegitimate recombination event between two different chromosomes 15 in most or all of these cases. All patients showed a maternal origin of the duplicated chromosome. The clinical severity appears to be associated with dosage of the PWS/AS region rather than with differences in the extent of the duplicated segment.

Par en haut !
This way up !
Retour à la page d'accueil...
Back to Home Page...