De novo truncating mutations in E6-AP ubiquitin-protein ligase gene (UBE3A) in Angelman syndrome

Genomic imprinting: genetic mechanisms and phenotypic consequences in Prader-Willi and Angelman syndromes

Chromosomal 15q11-q13 region is of great interest in Human Genetics because many structural rearrangements have been described for it (deletions, duplications and translocations) leading to phenotypes resulting in conditions such as the Prader-Willi (PWS) and Angelman (AS) syndromes which were the first human diseases found to be related to the differential expression of parental alleles (genomic imprinting). Contrary to Mendelian laws where the parental inheritance of genetic information does not influence gene expression, genomic imprinting is characterized by DNA modifications that produce different phenotypes depending on the parental origin of the mutation. Clinical manifestation of PWS appears when the loss of paternally expressed genes occurs and AS results from the loss of a maternally expressed gene. Different genetic mechanisms can lead to PWS or AS, such as deletions, uniparental disomy or imprinting mutation. In AS patients an additional class occurs with mutations on the UBE3A gene. Studies of PWS and AS patients can help us to understand the imprinting process, so that other genomic regions with similar characteristics can be located, and different syndromes can have their genetic mechanisms elucidated.

Human scribble (Vartul) is targeted for ubiquitin-mediated degradation by the high-risk papillomavirus E6 proteins and the E6AP ubiquitin-protein ligase

The high-risk human papillomavirus (HPV) E6 proteins stimulate the ubiquitination and degradation of p53, dependent on the E6AP ubiquitin-protein ligase. Other proteins have also been shown to be targeted for degradation by E6, including hDlg, the human homolog of the Drosophila melanogaster Discs large (Dlg) tumor suppressor. We show here that the human homolog of the Drosophila Scribble (Vartul) (hScrib) tumor suppressor protein is also targeted for ubiquitination by the E6-E6AP complex in vitro and that expression of E6 induces degradation of hScrib in vivo. Characterization of the E6AP-E6-hScrib complex indicated that hScrib binds directly to E6 and that the binding is mediated by the PDZ domains of hScrib and a carboxyl-terminal epitope conserved among the high-risk HPV E6 proteins. Green fluorescent protein-hScrib was localized to the periphery of MDCK cells, where it colocalized with ZO-1, a component of tight junctions. E6 expression resulted in loss of integrity of tight junctions, as measured by ZO-1 localization, and this effect was dependent on the PDZ binding epitope of E6. Thus, the high-risk HPV E6 proteins induce the degradation of the human homologs of two Drosophila PDZ domain-containing tumor suppressor proteins, hDlg and hScrib, both of which are associated with cell junction complexes. The fact that Scrib/Vart and Dlg appear to cooperate in a pathway that controls Drosophila epithelial cell growth suggests that the combined targeting of hScrib and hDlg is an important component of the biologic activity of high-risk HPV E6 proteins.

Small evolutionarily conserved RNA, resembling C/D box small nucleolar RNA, is transcribed from PWCR1, a novel imprinted gene in the Prader-Willi deletion region, which Is highly expressed in brain

Prader-Willi syndrome is a complex neurodevelopmental disorder caused by the inactivation or deletion of imprinted, paternally expressed genes in chromosome band 15q11.2. We report the identification and characterization of PWCR1, a novel imprinted gene within that region, and its mouse orthologue, Pwcr1, which was mapped to the conserved syntenic region on mouse chromosome 7. Expressed only from the paternal allele, both genes require the imprinting-center regulatory element for expression and are transcribed from the same strand. They are intronless and do not appear to encode a protein product. High human/mouse sequence similarity (87% identity) is limited to a 99-bp region called "HMCR" (for "human-mouse conserved region"). The HMCR sequence has features of a C/D box small nucleolar RNA (snoRNA) and is represented in an abundant small transcript in both species. Located in nucleoli, snoRNAs serve as methylation guidance RNAs in the modification of ribosomal RNA and other small nuclear RNAs. In addition to the nonpolyadenylated small RNAs, larger polyadenylated PWCR1 transcripts are found in most human tissues, whereas expression of any Pwcr1 RNAs is limited to mouse brain. Genomic sequence analysis reveals the presence of multiple copies of PWCR1 and Pwcr1 that are organized within local tandem-repeat clusters. On a multispecies Southern blot, hybridization to an HMCR probe encoding the putative snoRNA is limited to mammals.

Differential display and gene arrays to examine auditory plasticity

Differential gene expression forms the basis for development, differentiation, regeneration, and plasticity of tissues and organs. We describe two methods to identify differentially expressed genes. Differential display, a PCR-based approach, compares the expression of subsets of genes under two or more conditions. Gene arrays, or DNA microarrays, contain cDNAs from both known genes and novel genes spotted on a solid support (nylon membranes or glass slides). Hybridization of the arrays with RNA isolated from two different experimental conditions allows the simultaneous analysis of large numbers of genes, from hundreds to thousands to whole genomes. Using differential display to examine differential gene expression after noise trauma in the chick basilar papilla, we identified the UBE3B gene that encodes a new member of the E3 ubiquitin ligase family (UBE3B). UBE3B is highly expressed immediately after noise in the lesion, but not in the undamaged ends, of the chick basilar papilla. UBE3B is most similar to a ubiquitin ligase gene from Caenorhabditis elegans, suggesting that this gene has been conserved throughout evolution. We also describe preliminary experiments to profile gene expression in the cochlea and brain with commercially available low density gene arrays on nylon membranes and discuss potential applications of this and DNA microarray technology to the auditory system.

Familial Prader-Willi syndrome: case report and a literature review

Prader-Willi syndrome (PWS) is a neurobehavioural disorder arising through a number of different genetic mechanisms. All involve loss of paternal gene expression from chromosome 15q11q13. Although the majority of cases of PWS are sporadic, precise elucidation of the causative genetic mechanism is essential for accurate genetic counselling as the recurrence risk varies according to the mechanism involved. A pair of siblings affected by PWS is described. Neither demonstrates a microscopically visible deletion in 15q11q13 or maternal disomy. Methylation studies at D15S63 and at the SNRPN locus confirm the diagnosis of PWS. Molecular studies reveal biparental inheritance in both siblings with the exception of D15S128 and D15S63 where no paternal contribution is present indicating a deletion of the imprinting centre. Family studies indicate that the father of the siblings carries the deletion which, he has inherited from his mother. The recurrence risk for PWS in his offspring is 50%.

The RS447 human megasatellite tandem repetitive sequence encodes a novel deubiquitinating enzyme with a functional promoter

We have recently identified a tandem repetitive DNA sequence that we designated the RS447 megasatellite. In this study, we describe a functional novel deubiquitinating enzyme (USP17, 60 kDa) gene that is intronless and encoded by the RS447 repeating unit. Northern blot analysis in conjunction with 5' and 3' rapid amplification of cDNA ends confirmed the presence of poly(A)(+) containing RS447 RNA in normal cells. We also identified a functional promoter sequence as well as an open reading frame within every RS447 repeat. When USP17 was expressed in Escherichia coli, it exhibited deubiquitinating activity in vivo. An antibody against USP17 detected USP17 protein in human cells. Our results indicate that the RS447 repeating unit on this megasatellite repeat codes for and actively expresses a functional deubiquitinating enzyme. Although it is expressed ubiquitously in human tissues, USP17 exhibited a unique expression pattern in that its complementary strand is transcribed as an antisense transcript that may modulate the level of USP17 expression in the human brain.

Ophthalmic manifestations of Angelman syndrome

In 1965, Angelman described 3 cases of what he called "Puppet" children, named for the characteristic signs associated with what is now known as Angelman syndrome, including mental retardation, speech impairment, easy excitability, and frequent spontaneous laughter.(1) Since that report, much progress has been made in defining the syndrome's clinical manifestations and understanding its molecular foundations, including identification of deletions of 15q11-13 in some patients. There are few reports in the ophthalmic literature regarding ocular manifestations of this syndrome. (2,3) We present the case of a child with strabismus associated with Angelman syndrome, and we review the ophthalmic and systemic findings, as well as recent advances in molecular genetics, in these patients.

The dual function steroid receptor coactivator/ubiquitin protein-ligase integrator E6-AP is overexpressed in mouse mammary tumorigenesis

Steroid receptor coactivator and corepressor proteins are important mediators of steroid receptor function. Changes in the expression or activity of these limiting cofactors can contribute to the etiology of steroidal cancers. Using a mouse mammary model of multistage tumorigenesis we have examined whether the expression of select steroid receptor coactivators is altered. The 10 kb transcript of the novel dual function steroid receptor coactivator/ubiquitin protein-ligase integrator E6-AP is overexpressed 2.5-4.5 fold in the mammary tumors but not in the precursor steps of tumorigenesis; that is, immortal ductal and alveolar hyperplastic outgrowths. The over expression is striking because the 10 kb transcript is expressed to variable levels in other wild type tissues like the uterus, ovary, testis, kidney and brain but is undetectable in normal virgin mammary gland and the prostate gland. The E6-AP overexpression in the mammary tumors is substantiated by western blot analysis and immunohistochemical analysis. Absence of ER and PR in these tumors in the presence of high levels of E6-AP could contribute to steroid receptor-independent function and tumorigenesis. There is no obvious correlation between p53 (a well-characterized substrate of E6-AP) status (wt vs. mutant) and levels of E6-AP in the mouse mammary tumors.

Human genetic diseases of proteolysis

Although in the past protein stability commonly has been considered an inherent property of a given protein, the truth is far more complex. Elaborate enzymatic systems exist in multiple intracellular compartments to hydrolyze proteins. These systems are capable of providing a sensitive mechanism to regulate protein expression, a mechanism that is complementary to the transcriptional and translational control mechanisms that influence protein synthesis. The power of regulated proteolysis has been well-demonstrated in the abrupt degradation of cyclins that underlies eukaryotic cell cycle progression. Coincidental with the recent rapid gains in understanding proteolysis at a biochemical level, several human diseases have been found to result from disordered proteolysis. This article reviews several examples of human disease resulting from mutations of genes encoding serine proteases, cysteine proteases, and their inhibitors. Examples are also presented of human diseases resulting from disorders in the highly intricate ubiquitin-proteasome pathway of protein degradation. It is certain that many more human diseases will be associated in the future with disorders of proteolysis.

Imprinting mutations on human chromosome 15

Genomic imprinting is an epigenetic process by which the male and the female germline of viviparous taxa confer a sex-specific mark (imprint) on certain chromosomal regions. The imprint is reset in the germline of each generation, inherited through somatic cell divisions during postzygotic development and used to regulate parent-of-origin specific expression of susceptible genes. Aberrant imprinting leading to aberrant gene expression patterns represents a novel class of mutations and was first identified in patients with Angelman syndrome and Prader-Willi syndrome. The finding of inherited cis-acting mutations in some of these cases has led to the identification of an imprinting center, which is involved in resetting of the imprint during gametogenesis. Other mutations may interfere with the somatic inheritance of the imprint during postzygotic development.

Paternal UPD15: further genetic and clinical studies in four Angelman syndrome patients

Among 25 patients diagnosed with Angelman syndrome, we detected 21 with deletion and 4 with paternal uniparental disomy (UPD), 2 isodisomies originating by postzygotic error, and 1 MII nondisjunction event. The diagnosis was obtained by molecular techniques, including methylation pattern analysis of exon 1 of SNRPN and microsatellite analysis of loci within and outside the 15q11-q13 region. Most manifestations present in deletion patients are those previously reported. Comparing the clinical data from our and published UPD patients with those with deletions we observed the following: the age of diagnosis is higher in UPD group (average 7 3/12 years), microcephaly is more frequent among deletion patients, UPD children start walking earlier (average age 2 9/12 years), whereas in deletion patients the average is 4 (1/2) years, epilepsy started later in UPD patients (average 5 10/12 years) than in deletion patients (average 1 11/12 years), weight above the 75th centile is reported mainly in UPD patients, complete absence of speech is more common in the deleted (88.9%) than in the UPD patients because half of the children are able to say few words. Thus, besides the abnormalities already described, the UPD patients have somewhat better verbal development, a weight above the 75th centile, and OFC in the upper normal range.

Three probands with autistic disorder and isodicentric chromosome 15

We have identified three unrelated probands with autistic disorder (AD) and isodicentric chromosomes that encompass the proximal region of 15q11.2. All three probands met the Diagnostic and Statistical Manual of Mental Disorders, fourth edition [DSM-IV; American Psychiatric Association, 1994], and International Classification of Diseases ( ICD-10) diagnostic criteria for AD, confirmed with the Autism Diagnostic Interview -Revised (ADI-R). Chromosome analysis revealed the following karyotypes: 47,XX,+idic(15)(q11.2), 47,XX, +idic(15) (q11.2), and 47,XY,+idic(15)(q11.2). Haplotype analysis of genotypic maker data in the probands and their parents showed that marker chromosomes in all three instances were of maternal origin. Comparison of the clinical findings of the three AD probands with case reports in the published literature (N = 20) reveals a clustering of physical and developmental features. Specifically, these three probands and the majority of reported probands in the literature exhibited hypotonia (n = 13), seizures (n = 13), and delayed gross motor development (n = 13). In addition, clustering of the following clinical signs was seen with respect to exhibited speech delay (n = 13), lack of social reciprocity (n = 11), and stereotyped behaviors (n = 12). Collectively, these data provide further evidence for the involvement of chromosome 15 in AD as well as present preliminary data suggesting a clustering of clinical features in AD probands with proximal 15q anomalies.

Altered activity, social behavior, and spatial memory in mice lacking the NTAN1p amidase and the asparagine branch of the N-end rule pathway

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. N-terminal asparagine and glutamine are tertiary destabilizing residues, in that they are enzymatically deamidated to yield secondary destabilizing residues aspartate and glutamate, which are conjugated to arginine, a primary destabilizing residue. N-terminal arginine of a substrate protein is bound by the Ubr1-encoded E3alpha, the E3 component of the ubiquitin-proteasome-dependent N-end rule pathway. We describe the construction and analysis of mouse strains lacking the asparagine-specific N-terminal amidase (Nt(N)-amidase), encoded by the Ntan1 gene. In wild-type embryos, Ntan1 was strongly expressed in the branchial arches and in the tail and limb buds. The Ntan1(-/-) mouse strains lacked the Nt(N)-amidase activity but retained glutamine-specific Nt(Q)-amidase, indicating that the two enzymes are encoded by different genes. Among the normally short-lived N-end rule substrates, only those bearing N-terminal asparagine became long-lived in Ntan1(-/-) fibroblasts. The Ntan1(-/-) mice were fertile and outwardly normal but differed from their congenic wild-type counterparts in spontaneous activity, spatial memory, and a socially conditioned exploratory phenotype that has not been previously described with other mouse strains.

The syndrome of inv dup (15): clinical, electroencephalographic, and imaging findings

The clinical and laboratory data of four pediatric patients and one adult patient with inverted duplication (inv dup) (15) are reported. The most evident findings were dysmorphic features with frontal bossing; genital abnormalities, such as macropenis or hypospadias; mental retardation; autistic behavior; and seizures. Two additional adults with inv dup (15) from other institutions were also diagnosed in our laboratory. Seizures and mental retardation were the reasons for their referral. The clinical picture of inv dup (15) seems to be quite variable since the phenotype can also be normal. However, karyotyping and fluorescent in-situ hybridization, focused in particular on chromosome 15, appear to be indicated in patients with dysmorphic phenotypes, such as the one present in our patients, and in subjects with early-onset seizures and psychomotor retardation with autistic features.

Identification of a testis-specific gene (C15orf2) in the Prader-Willi syndrome region on chromosome 15

Prader-Willi syndrome (PWS) results from the loss of paternal contributions for a 2-Mb imprinted region on the proximal long arm of human chromosome 15. Hitherto, five paternally active genes have been identified in this region (ZNF127, NDN, MAGEL2, SNURF-SNRPN, and IPW). Here we report the identification of a novel gene in the PWS critical region, which has been designated "chromosome 15 open reading frame 2" (C15orf2). C15orf2 is an intronless gene located between MAGEL2 and SNURF-SNRPN. It is associated with a CpG island, which is methylated in all tissues tested except for germ cells. C15orf2 is transcribed as a 7.5-kb mRNA and contains an open reading frame encoding a predicted 1156-amino-acid protein of unknown function. Transcription of C15orf2 occurs exclusively in the testis, and in adult testis samples, we observed biallelic expression. By zoo-blot analysis, we found related sequences in DNA from other primates, but not in nonprimate DNA. We conclude that C15orf2 may play a role in primate spermatogenesis.

Mutation screening in Rett syndrome patients

Rett syndrome (RTT) was first described in 1966. Its biological and genetic foundations were not clear until recently when Amir et al reported that mutations in the MECP2 gene were detected in around 50% of RTT patients. In this study, we have screened the MECP2 gene for mutations in our RTT material, including nine familial cases (19 Rett girls) and 59 sporadic cases. A total of 27 sporadic RTT patients were found to have mutations in the MECP2 gene, but no mutations were identified in our RTT families. In order to address the possibility of further X chromosomal or autosomal genetic factors in RTT, we evaluated six candidate genes for RTT selected on clinical, pathological, and genetic grounds: UBE1 (human ubiquitin activating enzyme E1, located in chromosome Xp11.23), UBE2I (ubiquitin conjugating enzyme E2I, homologous to yeast UBC9, chromosome 16p13.3), GdX (ubiquitin-like protein, chromosome Xq28), SOX3 (SRY related HMG box gene 3, chromosome Xq26-q27), GABRA3 (gamma-aminobutyric acid type A receptor alpha3 subunit, chromosome Xq28), and CDR2 (cerebellar degeneration related autoantigen 2, chromosome 16p12-p13.1). No mutations were detected in the coding regions of these six genes in 10 affected subjects and, therefore, alterations in the amino acid sequences of the encoded proteins can be excluded as having a causative role in RTT. Furthermore, gene expression of MECP2, GdX, GABRA3, and L1CAM (L1 cell adhesion molecule) was also investigated by in situ hybridisation. No gross differences were observed in neurones of several brain regions between normal controls and Rett patients.

Identification of novel imprinted transcripts in the Prader-Willi syndrome and Angelman syndrome deletion region: further evidence for regional imprinting control

Deletions and other abnormalities of human chromosome 15q11-q13 are associated with two developmental disorders, Prader-Willi syndrome (PWS) and Angelman syndrome (AS). Loss of expression of imprinted, paternally expressed genes has been implicated in PWS. However, the number of imprinted genes that contribute to PWS, and the range over which the imprinting signal acts to silence one copy of the gene in a parent-of-origin-specific manner, are unknown. To identify additional imprinted genes that could contribute to the PWS phenotype and to understand the regional control of imprinting in 15q11-q13, we have constructed an imprinted transcript map of the PWS-AS deletion interval. The imprinting status of 22 expressed sequence tags derived from the radiation-hybrid human transcript maps or physical maps was determined in a reverse transcriptase-PCR assay and correlated with the position of the transcripts on the physical map. Seven new paternally expressed transcripts localize to an approximately 1.5-Mb domain surrounding the SNRPN-associated imprinting center, which already includes four imprinted, paternally expressed genes. All other tested new transcripts in the deletion region were expressed from both alleles. A domain of exclusive paternal expression surrounding the imprinting center suggests strong regional control of the imprinting process. This study provides the means for further investigation of additional genes that cause or modify the phenotypes associated with rearrangements of 15q11-q13.

Modes of regulation of ubiquitin-mediated protein degradation

The ubiquitin-proteasome pathway is responsible for the major portion of specific cellular protein degradation. Ubiquitin-mediated degradation is involved in physiological regulation of many cellular processes, including cell cycle progression, differentiation, and signal transduction. Here, we review the basic mechanisms of the ubiquitin system and the various ways in which ubiquitin-mediated degradation can be modulated by physiological signals.

Appetitive behavior, compulsivity, and neurochemistry in Prader-Willi syndrome

Advances in genetic research have led to an increased understanding of genotype-phenotype relationships. Excessive eating and weight gain characteristic of Prader-Willi syndrome (PWS) have been the understandable focus of much of the research. The intense preoccupation with food, lack of satiation, and incessant food seeking are among the most striking features of PWS. It has become increasingly clear that the behavioral phenotype of PWS also includes symptoms similar to obsessive compulsive disorder, which in all probability interact with the incessant hunger and lack of satiation to engender the intense preoccupation and food seeking behavior that is characteristic of this disorder. Several lines of evidence suggest that genetic material on chromosome 15 may alter synthesis, release, metabolism, binding, intrinsic activity, or reuptake of specific neurotransmitters, or alter the receptor numbers and/or distribution involved in modulating feeding. Among the likely candidates are GABAnergic, serotonergic, and neuropeptidergic mechanisms. This review summarizes what is known about the appetitive behavior and compulsivity in PWS and discusses the possible mechanisms underlying these behaviors. MRDD Research Reviews 2000;6:125-130.

Attenuation of green fluorescent protein half-life in mammalian cells

The half-life of the green fluorescent protein (GFP) was determined biochemically in cultured mouse LA-9 cells. The wild-type protein was found to be stable with a half-life of approximately 26 h, but could be destabilized by the addition of putative proteolytic signal sequences derived from proteins with shorter half-lives. A C-terminal fusion of a PEST sequence from the mouse ornithine decarboxylase gene reduced the half-life to 9.8 h, resulting in a GFP variant suitable for the study of dynamic cellular processes. In an N-terminal fusion containing the mouse cyclin B1 destruction box, it was reduced to 5.8 h, with most degradation taking place at metaphase. The combination of both sequences produced a similar GFP half-life of 5.5 h. Thus, the stability of this marker protein can be controlled in predetermined ways by addition of the appropriate proteolytic signals.

gamma2-COP, a novel imprinted gene on chromosome 7q32, defines a new imprinting cluster in the human genome

We describe a novel imprinted gene, gamma 2-COP (nonclathrincoatprotein), identified in a search for expressed sequences in human chromosome 7q32 where the paternally expressed MEST gene is located. gamma 2-COP contains 24 exons and spans >50 kb of genomic DNA. Like MEST, gamma 2-COP is ubiquitously transcribed in fetal and adult tissues. In fetal tissues, including skeletal muscle, skin, kidney, adrenal, placenta, intestine, lung, chorionic plate and amnion, gamma 2-COP is imprinted and expressed from the paternal allele. In contrast to the monoallelic expression observed in these fetal tissues, biallelic expression was evident in fetal brain and liver and in adult peripheral blood. Biallelic expression in blood is supported by the demonstration of gamma 2-COP transcripts in lymphoblastoid cell lines with maternal uniparental disomy 7. Absence of paternal gamma 2-COP transcripts during embryonic development may contribute to Silver-Russell syndrome. However, on mutation scanning the only gamma 2-COP mutation detected was maternally derived. Amino acid comparison of gamma2-COP protein revealed close relation to gamma-COP, a subunit of the coatomer complex COPI, suggesting a role of gamma2-COP in cellular vesicle traffic. The existence of distinct coatomer complexes could be the basis for the functional heterogeneity of COPI vesicles in retrograde and anterograde transport and/or in cargo selection. Together, gamma 2-COP and MEST constitute a novel imprinting cluster in the human genome that may contain other, as yet unknown, imprinted genes.

The ubiquitin-conjugating enzymes UbcH7 and UbcH8 interact with RING finger/IBR motif-containing domains of HHARI and H7-AP1

Ubiquitinylation of proteins appears to be mediated by the specific interplay between ubiquitin-conjugating enzymes (E2s) and ubiquitin-protein ligases (E3s). However, cognate E3s and/or substrate proteins have been identified for only a few E2s. To identify proteins that can interact with the human E2 UbcH7, a yeast two-hybrid screen was performed. Two proteins were identified and termed human homologue of Drosophila ariadne (HHARI) and UbcH7-associated protein (H7-AP1). Both proteins, which are widely expressed, are characterized by the presence of RING finger and in between RING fingers (IBR) domains. No other overt structural similarity was observed between the two proteins. In vitro binding studies revealed that an N-terminal RING finger motif (HHARI) and the IBR domain (HHARI and H7-AP1) are involved in the interaction of these proteins with UbcH7. Furthermore, binding of these two proteins to UbcH7 is specific insofar that both HHARI and H7-AP1 can bind to the closely related E2, UbcH8, but not to the unrelated E2s UbcH5 and UbcH1. Although it is not clear at present whether HHARI and H7-AP1 serve, for instance, as substrates for UbcH7 or represent proteins with E3 activity, our data suggests that a subset of RING finger/IBR proteins are functionally linked to the ubiquitin/proteasome pathway.

Recent advances in the genetics of epilepsy: insights from human and animal studies

Progress in understanding the genetics of epilepsy is proceeding at a dizzying pace. Due in large part to rapid progress in molecular genetics, gene defects underlying many of the inherited epilepsies have been mapped, and several more are likely to be added each year. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, and correlate these advances with rapidly expanding information about the mechanisms of epilepsy gained from both spontaneous and transgenic animal models. We also provide practical suggestions for clinicians confronted with families in which multiple members are afflicted with epilepsy.

Ubiquitin-mediated proteolysis in learning and memory

Sensitization of defensive reflexes in Aplysia is a simple behavioral paradigm for studying both short- and long-term memory. In the marine mollusk, as in other animals, memory has at least two phases: a short-term phase lasting minutes and a long-term phase lasting several days or longer. Short-term memory is produced by covalent modification of pre-existing proteins. In contrast, long-term memory needs gene induction, synthesis of new protein, and the growth of new synapses. The switch from short-term (STF) to long-term facilitation (LTF) in Aplysia sensory neurons requires not only positive regulation through gene induction, but also the specific removal of several inhibitory proteins. One important inhibitory protein is the regulatory (R) subunit of the cAMP-dependent protein kinase (PKA). Degradation of R subunits, which is essential for initiating long-term stable memory, occurs through the ubiquitin-proteasome pathway.

Angelman syndrome: a review of clinical and genetic aspects

This paper reviews Angelman syndrome (AS) with regard to the clinical features in childhood and adulthood, epileptic seizures and EEG findings, neuroimaging studies and the present knowledge on the genetic mechanisms underlying this syndrome. Different clinical phenotypes and genotypes of AS are described, including chromosome 15q11-13 deletion, uniparental disomy, methylation imprinting abnormalities and mutations in the UBE3A gene.

GABA and epileptogenesis: comparing gabrb3 gene-deficient mice with Angelman syndrome in man

The GABAergic system has long been implicated in epilepsy with defects in GABA neurotransmission being linked to epilepsy in both experimental animal models and human syndromes (Olsen and Avoli, 1997). However, to date no human epileptic syndrome has been directly attributed to an altered GABAergic system. The observed defects in GABA neurotransmission in human epileptic syndromes may be the indirect result of a brain besieged by seizures. The use of animal models of epilepsy has sought to address these matters. The advent of gene targeting methodologies in mice now allows for a more direct assessment of GABA's involvement in epileptogenesis. To date several genes associated with the GABAergic system have been disrupted. These include the genes for glutamic acid decarboxylase, both the 65- and 67-kDa isoforms (GAD65 and GAD67), the tissue non-specific alkaline phosphatase gene (TNAP) and genes for the GABA(A) receptor subunits alpha6, beta3, gamma2, and delta (gabra6, gabrb3, gabrg2, and gabrd respectively). Gene disruptions of either GAD67 or gabrg2 result in neonatal lethality, while others, GAD65, TNAP, and gabrb3 exhibit increased mortality and spontaneous seizures. GABA receptor expression has been found to be both regionally and developmentally regulated. Thus in addition to their obvious role in controlling excitability in adult brain, a deficit in GABAergic function during development could be expected to elicit pleiotropic neurodevelopmental abnormalities perhaps including epilepsy. The GABA(A) receptor beta3 subunit gene, gabrb3/GABRB3 (mouse/human), is of particular interest because of its expression early in development and its possible role in the neurodevelopmental disorder Angelman syndrome. Individuals with this syndrome exhibit severe mental retardation and epilepsy. Mice with the gabrb3 gene disrupted likewise exhibit electroencephalograph (EEG) abnormalities, seizures, and behavioral characteristics typically associated with Angelman syndrome. These gabrb3 gene knockout mice provide direct evidence that a reduction of a specific subunit of the GABA(A) receptor system can result in epilepsy and support a GABAergic role in the pathophysiology of Angelman syndrome.

Transmission of Angelman syndrome by an affected mother

PURPOSE

To determine: 1) If a 15q11-13 deletion was transmitted from a female with Angelman syndrome to her fetus, and 2) If the UBE3A gene was functionally imprinted in fetal eye.

METHODS

Individuals were genotyped by microsatellite analysis. DNA methylation imprints were assessed by Southern blot analysis and methylation-specific PCR. Expression was analyzed by RT-PCR.

RESULTS

The mother and fetus inherited large deletions of maternal 15q11-13 and demonstrated paternal-only DNA methylation imprints along 15q11-13. UBE3A was paternally expressed in eye tissue from the fetus with Angelman syndrome.

CONCLUSIONS

We show that females with Angelman syndrome are fully capable of reproduction and that UBE3A is not imprinted in fetal eye.

Regulation of the Src family tyrosine kinase Blk through E6AP-mediated ubiquitination

The Src family of nonreceptor tyrosine kinases are important regulators of a variety of cellular processes, including cytoskeletal organization, cell-cell contact, and cell-matrix adhesion. Activation of Src family kinases also can induce DNA synthesis and cellular proliferation; therefore, tight regulation of their kinase activities is important for the cell to maintain proliferative control. Posttranslational phosphorylation and dephosphorylation are recognized as the principle modifications by which the activities of the Src family of tyrosine kinases are regulated. We have discovered that this family of kinases also is regulated by ubiquitin-mediated proteolysis. Studies aimed at the identification of cellular targets for E6AP, an E3 ubiquitin protein ligase involved in ubquitin-mediated degradation, led us to the identification of members of the Src family kinases as potential substrates for E6AP. We have found that E6AP can bind to several of the Src family tyrosine kinases. Here we show that activated Blk is preferentially degraded by the ubiquitin-proteasome pathway and that its ubiquitination is mediated by E6AP. Identification of members of the Src tyrosine kinase family as substrates of the E6AP ubiquitin-protein ligase implicates a role for the ubiquitin pathway in regulating the activities of individual members of this important family of signaling molecules.

Ionizing radiation and genetic risks. X. The potential "disease phenotypes" of radiation-induced genetic damage in humans: perspectives from human molecular biology and radiation genetics

Estimates of genetic risks of radiation exposure of humans are traditionally expressed as expected increases in the frequencies of genetic diseases (single-gene, chromosomal and multifactorial) over and above those of naturally-occurring ones in the population. An important assumption in expressing risks in this manner is that gonadal radiation exposures can cause an increase in the frequency of mutations and that this would result in an increase in the frequency of genetic diseases under study. However, despite compelling evidence for radiation-induced mutations in experimental systems, no increases in the frequencies of genetic diseases of concern or other adverse effects (i.e., those which are not formally classified as genetic diseases), have been found in human studies involving parents who have sustained radiation exposures. The known differences between spontaneous mutations that underlie naturally-occurring single-gene diseases and radiation-induced mutations studied in experimental systems now permit us to address and resolve these issues to some extent. The fact that spontaneous mutations (among which are point mutations and DNA deletions generally restricted to the gene) originate through a number of different mechanisms and that the latter are intimately related to the DNA organization of the genes, are now well-documented. Further, spontaneous mutations include those that cause diseases through loss of function as well as gain of function of genes. In contrast, most radiation-induced mutations studied in experimental systems (although identified through the phenotypes of the marker genes) are predominantly multigene deletions which cause loss of function; the recoverability of an induced deletion in a livebirth seems dependent on whether the gene and the genomic region in which it is located can tolerate heterozygosity for the deletion and yet be compatible with viability. In retrospect, the successful mutation test systems (such as the mouse specific locus test) used in radiation studies have involved genes which are non-essential for survival and are also located in genomic regions, likewise non-essential for survival. In contrast, most of the human genes at which induced mutations have been looked for, do not seem to have these attributes. The inference therefore is that the failure to find induced germline mutations in humans is not due to the resistance of human genes to induced mutations but due to the structural and functional constraints associated with their recoverability in livebirths. Since the risk of inducible genetic diseases in humans is estimated using rates of "recovered" mutations in mice, there is a need to introduce appropriate correction factors to bridge the gap between these rates and the rates at which mutations causing diseases are potentially recoverable in humans. Since the whole genome is the "target" for radiation-induced genetic damage, the failure to find increases in the frequencies of specific single-gene diseases of societal concern does not imply that there are no genetic risks of radiation exposures: the problem lies in delineating the phenotypes of recoverable genetic damage that are recognizable in livebirths. Data from studies of naturally-occurring microdeletion syndromes in humans and those from mouse radiation studies are instructive in this regard. They (i) support the view that growth retardation, mental retardation and multisystem developmental abnormalities are likely to be among the quantitatively more important adverse effects of radiation-induced genetic damage than mutations in a few selected genes and (ii) underscore the need to expand the focus in risk estimation from known genetic diseases (as has been the case thus far) to include these induced adverse developmental effects although most of these are not formally classified as "genetic diseases". (ABSTRACT TRUNCATED)

Diagnosis of Angelman syndrome: clinical and EEG criteria

In order to evaluate which diagnostic criteria can be indicative for an early diagnosis of Angelman syndrome (AS), 144 children with severe epilepsy and mental retardation were evaluated. In 10 of them the diagnostic criteria indicated by Williams were present. Of the remaining 134 patients we were able to diagnose one 15-year-old patient with AS, on the basis of the EEG findings, even though the typical clinical features of the syndrome were absent. In all patients the diagnosis of AS was confirmed by fluorescent in situ hybridization (FISH) in 10 patients and by methylation analysis in one patient. AS is very likely when both typical clinical and EEG findings are present. Nevertheless, it must be considered in all patients affected by severe epilepsy and mental retardation, when the EEG pattern is sufficiently indicative, and FISH and/or molecular analysis should be performed even in absence of typical clinical signs.

Angelman syndrome resulting from UBE3A mutations in 14 patients from eight families: clinical manifestations and genetic counselling

Angelman syndrome (AS) is a neurological disorder with a heterogeneous genetic aetiology. It most frequently results from a de novo interstitial deletion in the 15q11-q13 region, but in a few cases it is caused by paternal uniparental disomy (UPD) or an imprinting mutation. The remaining 20 to 30% of AS patients exhibit biparental inheritance and a normal pattern of allelic methylation in the 15q11-q13 region. In this latter group, mutations in the UBE3A gene have recently been shown to be a cause of AS. Here we describe the phenotypic expression in 14 AS cases involving eight UBE3A mutations. These comprise 11 familial cases from five families and three sporadic cases. Subtle differences from the typical phenotype of AS were found. Consistent manifestations were psychomotor delay, a happy disposition, a hyperexcitable personality, EEG abnormalities, and mental retardation with severe speech impairment. The other main manifestations of AS, ataxia, epilepsy, and microcephaly, were either milder or absent in various combinations among the patients. In addition, myoclonus of cortical origin was frequently observed with severe fits inducing myoclonic seizures. The majority of the patients were overweight. This study showed that ataxia, myoclonus, EEG abnormalities, speech impairment, characteristic behavioural phenotype, and abnormal head circumference are attributable to a deficiency in the maternally inherited UBE3A allele. Furthermore, analysis of mutation transmission showed an unexpectedly high rate of somatic mosaicism in normal carriers. These data have important consequences for genetic counselling.

Identification of HHR23A as a substrate for E6-associated protein-mediated ubiquitination

The human papilloma virus E6-associated protein (E6AP) functions as a ubiquitin protein ligase (E3) in the E6-mediated ubiquitination of p53. E6AP is also an E3 in the absence of E6, but its normal cellular substrates have not yet been identified. Here we report the identification of HHR23A, one of the human homologues of the yeast DNA repair protein Rad23, as an E6-independent target of E6AP. HHR23A binds E6AP and is ubiquitinated in vitro in an E6AP-dependent manner. Ubiquitinated forms of endogenous HHR23A are detectable in mammalian cells. Overexpression of wild-type E6AP in vivo enhances the ubiquitination of HHR23A, whereas a dominant negative E6AP mutant inhibits HHR23A ubiquitination. Although HHR23A is a stable protein in non-synchronized cells, its levels are regulated in a cell cycle-dependent manner, with specific degradation occurring during S phase. The S phase degradation of HHR23A could be blocked in vivo by dominant negative E6AP, providing direct evidence for the involvement of E6AP in the regulation of HHR23A. Consistent with a role of the HHR23 proteins in DNA repair, UV-induced DNA damage inhibited HHR23A degradation. Although the precise role of HHR23 proteins in DNA repair and cell cycle progression remains to be elucidated, our data suggest that E6AP-mediated ubiquitination of HHR23A may have important implications in DNA repair and cell cycle progression.

Angelman syndrome with uniparental disomy due to paternal meiosis II nondisjunction

We report a case of Angelman syndrome (AS) with paternal uniparental disomy (pUPD) of chromosome 15. This 6-year-old girl with overgrowth had frequent, but only provoked laughter, was mildly ataxic with limb hypertonia, and had no intelligible speech. She had deep-set eyes, protruding tongue, and prominent chin. The karyotype was normal. DNA analysis with microsatellites from chromosome 15 showed no inheritance of maternal alleles both within and outside the AS critical region. Proximal markers showed reduction to homozygosity of paternal alleles, intermediate markers showed nonreduction, and distal markers reduction, thus suggesting a meiosis II nondisjunction event in the father with two crossovers. This is, to our knowledge, the first reported case of AS due to meiosis II nondisjunction. We present detailed physical measurements in this patient, adding to the clinical description of the milder phenotype in AS due to pUPD.

Human diseases and genomic imprinting

In summary, there are a number of conditions where genomic imprinting effects are recognized to be associated clinical disorders of importance in humans. There may be many more. Genomic imprinting should be suspected in any disorder with overgrowth, undergrowth, or behavior abnormalities. Disorders with unusual pattern of inheritance should be studied for the possibility that genomically imprinted gene(s) are involved. Understanding the mechanisms of genomic imprinting has major ramifications in terms of recurrence risk, prediction of whether offspring will be affected, and risk of malignancy. Of particular concern is the potential for uniparental disomy when trisomy is found during prenatal diagnosis.

A novel imprinted gene, encoding a RING zinc-finger protein, and overlapping antisense transcript in the Prader-Willi syndrome critical region

We describe a complex imprinted locus in chromosome 15q11-q13 that encodes two genes, ZNF127 and ZNF127AS. The ZNF127 gene encodes a protein with a RING (C3HC4) zinc-finger and multiple C3H zinc-finger motifs, the former being closely related to a protein from variola major virus, the smallpox etiological agent. These motifs allow prediction of ZNF127 function as a ribonucleoprotein. The intronless ZNF127 gene is expressed ubiquitously, but the entire coding sequence and 5' CpG island overlaps a second gene, ZNF127AS, that is transcribed from the antisense strand with a different transcript size and pattern of expression. Allele-specific analysis shows that ZNF127 is expressed only from the paternal allele. Consistent with this expression pattern, in the brain the ZNF127 5' CpG island is completely unmethylated on the paternal allele but methylated on the maternal allele. Analyses of adult testis, sperm and fetal oocytes demonstrates a gametic methylation imprint with unmethylated paternal germ cells. Recent findings indicate that ZNF127 is part of the coordinately regulated imprinted domain affected in Prader-Willi syndrome patients with imprinting mutations. Therefore, ZNF127 and ZNF127AS are novel imprinted genes that may be associated with some of the clinical features of the polygenic Prader-Willi syndrome.

In vivo nuclease hypersensitivity studies reveal multiple sites of parental origin-dependent differential chromatin conformation in the 150 kb SNRPN transcription unit

Human chromosome region 15q11-q13 contains a cluster of oppositely imprinted genes. Loss of the paternal or the maternal alleles by deletion of the region or by uniparental disomy 15 results in Prader-Willi syndrome (PWS) or Angelman syndrome (AS), respectively. Hence, the two phenotypically distinct neurodevelopmental disorders are caused by the lack of products of imprinted genes. Subsets of PWS and AS patients exhibit 'imprinting mutations', such as small microdeletions within the 5' region of the small nuclear ribonucleoprotein polypeptide N ( SNRPN ) transcription unit which affect the transcriptional activity and methylation status of distant imprinted genes throughout 15q11-q13 in cis. To elucidate the mechanism of these long-range effects, we have analyzed the chromatin structure of the 150 kb SNRPN transcription unit for DNase I- and Msp I-hypersensitive sites. By using an in vivo approach on lymphoblastoid cell lines from PWS and AS individuals, we discovered that the SNRPN exon 1 is flanked by prominent hypersensitive sites on the paternal allele, but is completely inaccessible to nucleases on the maternal allele. In contrast, we identified several regions of increased nuclease hypersensitivity on the maternal allele, one of which coincides with the AS minimal microdeletion region and another lies in intron 1 immediately downstream of the paternal-specific hypersensitive sites. At several sites, parental origin-specific nuclease hypersensitivity was found to be correlated with hypermethylation on the allele contributed by the other parent. The differential parental origin-dependent chromatin conformations might govern access of regulatory protein complexes and/or RNAs which could mediate interaction of the region with other genes.

Prader-Willi and Angelman syndromes: update on genetic mechanisms and diagnostic complexities

Significant advances have been made in determining the genetic basis of the Prader-Willi and Angelman syndromes; disorders in which genomic imprinting is abnormal. These advances will be instrumental in unravelling the pathogenesis that underlies these neurobehavioural disorders.

Genomic imprinting: implications for human disease

Genomic imprinting refers to an epigenetic marking of genes that results in monoallelic expression. This parent-of-origin dependent phenomenon is a notable exception to the laws of Mendelian genetics. Imprinted genes are intricately involved in fetal and behavioral development. Consequently, abnormal expression of these genes results in numerous human genetic disorders including carcinogenesis. This paper reviews genomic imprinting and its role in human disease. Additional information about imprinted genes can be found on the Genomic Imprinting Website at http://www.geneimprint.com.

The Angelman syndrome-associated protein, E6-AP, is a coactivator for the nuclear hormone receptor superfamily

In this study, we found that the E6-associated protein (E6-AP/UBE3A) directly interacts with and coactivates the transcriptional activity of the human progesterone receptor (PR) in a hormone-dependent manner. E6-AP also coactivates the hormone-dependent transcriptional activities of the other members of the nuclear hormone receptor superfamily. Previously, it was shown that E6-AP serves the role of a ubiquitin-protein ligase (E3) in the presence of the E6 protein from human papillomavirus types 16 and 18. Our data show that the ubiquitin-protein ligase function of E6-AP is dispensable for its ability to coactivate nuclear hormone receptors, showing that E6-AP possesses two separable independent functions, as both a coactivator and a ubiquitin-protein ligase. Disruption of the maternal copy of E6-AP is correlated with Angelman syndrome (AS), a genetic neurological disorder characterized by severe mental retardation, seizures, speech impairment, and other symptoms. However, the exact mechanism by which the defective E6-AP gene causes AS remains unknown. To correlate the E6-AP coactivator function and ubiquitin-protein ligase functions with the AS phenotype, we expressed mutant forms of E6-AP isolated from AS patients and assessed the ability of each of these mutant proteins to coactivate PR or provide ubiquitin-protein ligase activity. This analysis revealed that in the majority of the AS patients examined, the ubiquitin-protein ligase function of E6-AP was defective whereas the coactivator function was intact. This finding suggests that the AS phenotype results from a defect in the ubiquitin-proteosome protein degradation pathway.

Molecular mechanism of angelman syndrome in two large families involves an imprinting mutation

Patients with Angelman syndrome (AS) and Prader-Willi syndrome with mutations in the imprinting process have biparental inheritance but uniparental DNA methylation and gene expression throughout band 15q11-q13. In several of these patients, microdeletions upstream of the SNRPN gene have been identified, defining an imprinting center (IC) that has been hypothesized to control the imprint switch process in the female and male germlines. We have now identified two large families (AS-O and AS-F) segregating an AS imprinting mutation, including one family originally described in the first genetic linkage of AS to 15q11-q13. This demonstrates that this original linkage is for the 15q11-q13 IC. Affected patients in the AS families have either a 5.5- or a 15-kb microdeletion, one of which narrowed the shortest region of deletion overlap to 1.15 kb in all eight cases. This small region defines a component of the IC involved in AS (ie., the paternal-to-maternal switch element). The presence of an inherited imprinting mutation in multiple unaffected members of these two families, who are at risk for transmitting the mutation to affected children or children of their daughters, raises important genetic counseling issues.

Effects of a low dose of melatonin on sleep in children with Angelman syndrome

The effects of low dose melatonin therapy on sleep behavior and serum melatonin levels were studied in Angelman syndrome (AS) children suffering from insomnia. 24-hour motor activity was monitored in 13 AS children (age 2-10 yr) in their home environments for 7 days prior to melatonin treatment and for 5 days during which a 0.3 mg dose of melatonin was administered daily 0.5-1 hour before the patient's habitual bedtime. Blood samples were with-drawn at hourly intervals over two 21-hour periods in order to measure individual endogenous serum melatonin levels and the levels induced by melatonin treatment. Actigraphic recording of motor activity, confirmed by parents' reports, showed a significant improvement in the patients' nocturnal sleep pattern as a result of melatonin treatment. Analysis of the group data revealed a significant decrease in motor activity during the total sleep period following melatonin treatment, and an increase in the duration of the total sleep period. Endogenous peak nocturnal melatonin values ranged from 19 to 177 pg/ml. The administration of melatonin elevated peak serum hormone levels to 128-2800 pg/ml in children of different ages and body mass. These data suggest that a moderate increase in circulating melatonin levels significantly reduces motor activity during the sleep period in Angelman syndrome children, and promotes sleep.

Functional domains of the Rsp5 ubiquitin-protein ligase

RSP5, an essential gene of Saccharomyces cerevisiae, encodes a hect domain E3 ubiquitin-protein ligase. Hect E3 proteins have been proposed to consist of two broad functional domains: a conserved catalytic carboxyl-terminal domain of approximately 350 amino acids (the hect domain) and a large, nonconserved amino-terminal domain containing determinants of substrate specificity. We report here the mapping of the minimal region of Rsp5 necessary for its essential in vivo function, the minimal region necessary to stably interact with a substrate of Rsp5 (Rpb1, the large subunit of RNA polymerase II), and the finding that the hect domain, by itself, is sufficient for formation of the ubiquitin-thioester intermediate. Mutations within the hect domain that affect either the ability to form a ubiquitin-thioester or to catalyze substrate ubiquitination abrogate in vivo function, strongly suggesting that the ubiquitin-protein ligase activity of Rsp5 is intrinsically linked to its essential function. The amino-terminal region of Rsp5 contains three WW domains and a C2 calcium-binding domain. Two of the three WW domains are required for the essential in vivo function, while the C2 domain is not, and requirements for Rpb1 binding and ubiquitination lie within the region required for in vivo function. Together, these results support the two-domain model for hect E3 function and indicate that the WW domains play a role in the recognition of at least some of the substrates of Rsp5, including those related to its essential function. In addition, we show that haploid yeast strains bearing complete disruptions of either of two other hect E3 genes of yeast, designated HUL4 (YJR036C) and HUL5 (YGL141W), are viable.