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

Role of ubiquitin-mediated proteolysis in the pathogenesis of neurodegenerative disorders

Intraneuronal inclusions containing ubiquitylated filamentous protein aggregates are a common feature of many of the major human neurodegenerative disorders, including Alzheimer's and Parkinson's disease. Loss of function mutations in enzymes of the ubiquitin conjugation/deconjugation pathway are sufficient to cause familial forms of neurodegenerative diseases, suggesting that failure of ubiquitin-mediated proteolysis could also be central to inclusion formation in the more common sporadic cases. Examination of ubiquitin-positive inclusions at the protein level provides evidence of attempted proteasomal proteolysis, however close inspection of the temporal aspects of inclusion formation indicates that ubiquitylation is probably a late event. In this regard, the presence of ubiquitin within inclusions of idiopathic neurodegenerative disorders may indicate not a primary dysfunction of ubiquitin-mediated proteolysis, but rather a secondary, presumably protective cellular response. Within this model, other factors are likely to be initiating in inclusion biogenesis. Consistent with these proposals, non-ubiquitylated forms of the principal ubiquitylated components of Alzheimer's disease neurofibrillary tangles and Parkinson's disease Lewy bodies, tau and alpha-synuclein proteins, respectively, can be degraded by proteasomes in a pathway which does not have an absolute requirement for ubiquitylation. Inhibition of proteasome function in the pathological state, as has been reported in both Alzheimer's and Parkinson's disease, could therefore contribute both to accumulation of non-ubiquitylated forms of aggregation-prone neuronal proteins, as well as impaired clearance of ubiquitylated aggregates.

Penetrance and expressivity in the molecular age

Penetrance and expressivity have been defined through clinical experience. Although penetrance is often seen as the end of the spectrum of expressivity, penetrance and expressivity are considered as distinct phenomena. A review of the known mechanisms underlying either penetrance or expressivity reveals that in most of the cases the same explanation is true for both phenomena. Some of the known mechanisms include modifier genes, the influence of the allele in trans, sex, and environmental factors. Although rapid progress has been made in understanding of the basis of incomplete penetrance and the differences of expressivity, they still remain unknown for most of the genetic disorders. In recent years, it has become evident that there is much in common between the classical Mendelian traits in which the inheritance has been seen as "simple" and most of the common diseases in which the inheritance is "complex." In both cases genetic and/or environmental factors are acting in a complex way.

The HECT ubiquitin-protein ligase (UPL) family in Arabidopsis: UPL3 has a specific role in trichome development

Attachment of one or more ubiquitins (Ubs) to various intracellular proteins has a number of roles in plants including the selective removal of regulatory proteins by the 26S proteasome. The final step in this modification is performed by ubiquitin-protein ligases (E3s) that promote Ub transfer to appropriate targets. One important family of E3s is defined by the presence of a HECT domain, an active site first found at the C-terminus of the human E3 (E6-AP). Using a consensus HECT domain as the query, we identified a family of seven HECT-containing ubiquitin-protein ligases (UPL1-UPL7) in Arabidopsis thaliana that can be grouped into four subfamilies. The UPL3 and UPL4 subfamily encodes approximately 200-kDa proteins with four Armadillo repeats similar to those in the nuclear pore protein importin-alpha, suggesting that these E3s identify their targets through binding to nuclear localization sequences. Although T-DNA disruptions of the UPL3 locus do not affect overall growth and development of Arabidopsis, the mutants show aberrant trichome morphology. Instead of developing three branches, many upl3 trichomes contain five or more branches. The upl3 trichomes also often undergo an additional round of endoreplication resulting in enlarged nuclei with ploidy levels of up to 64C. upl3 plants are hypersensitive to gibberellic acid-3 (GA3), consistent with the role of gibberellins in trichome development. The phenotype of upl3 mutants is similar to that of kaktus, a previously described set of trichome mutants with supernumerary branches. Genetic analyses confirmed that upl3 mutants and kaktus-2 are allelic with kaktus-2 plants harboring a splice-site mutation within the UPL3-transcribed region. Collectively, the data indicate that the ubiquitination of one or more activator proteins by UPL3 is necessary to repress excess branching and endoreplication of Arabidopsis trichomes.

How clinicians add to knowledge of development

Studies of human birth defects and developmental disorders have made major contributions to our understanding of development. Rare human syndromes have allowed identification of important developmental genes, and revealed mechanisms such as uniparental disomy and unstable trinucleotide repeats that were not suspected from animal studies. Some aspects of development, in particular cognitive development, can only be studied in human beings. Basic developmental mechanisms are very highly conserved across a very wide range of animals, making for a rich interplay between animal and human studies. Often, clinical studies identify a gene, or suggest a hypothesis, that can then be investigated in animals.

Angelman syndrome: difficulties in EEG pattern recognition and possible misinterpretations

PURPOSE

This study aimed to evaluate the sensitivity of the EEG in Angelman syndrome (AS), to verify the age at onset of suggestive EEGs and to study EEG patterns, analyzing variations and comparing our findings with nomenclature previously used.

METHODS

Seventy EEG and 15 V-EEGs of 26 patients were analyzed. Suggestive EEG patterns of AS were classified in delta pattern (DP), theta pattern (TP), and posterior discharges (PDs). Generic terms were used to simplify the analysis.

RESULTS

Suggestive EEGs were observed in 25 (96.2%) patients. DP occurred in 22 patients with four variants-hypsarrhythmic-like: irregular, high-amplitude, generalized delta activity (DA) with multifocal epileptiform discharges (EDs); slow variant: regular, high-amplitude, generalized DA with rare EDs; ill-defined slow spike-and-wave: regular, high-amplitude, generalized DA with superimposed EDs characterizing a slow wave, with notched appearance; triphasic-like: rhythmic, moderate-amplitude DA over anterior regions with superimposed EDs. TP was observed in eight patients, as generalized or over the posterior regions. PDs were seen in 19 patients as runs of sharp waves or runs of high-amplitude slow waves with superimposed EDs. TP was the only age-related pattern (younger than 8 years) and observed only in patients with deletion. In 15 patients who had an EEG before the clinical diagnosis, 60% had a suggestive tracing.

CONCLUSIONS

Although some EEG descriptions are not very detailed, and every author describes findings in a slightly different manner, obviously a common denominator must exist. In this context, EEG seems to be a very sensitive method for the diagnosis of AS, offering an opportunity to corroborate this etiologic diagnosis. Conversely, we do not believe that these patterns may be accounted as specific, except for the delta pattern, which seems to be extremely unusual in other syndromes. Other EEG patterns observed in AS, such as theta activity and PDs, occur in a wide variety of disorders. Nonetheless, their importance for the EEG diagnosis of AS is supported by the fact that they are associated with other features and may be helpful in a proper clinical setting.

Synaptic plasticity: importance of proteasome-mediated protein turnover

The ubiquitin-proteasome system regulates protein degradation in every eukaryotic cell. Recent work has shown that protein turnover mediated by the ubiquitin-proteasome system plays a key role in synaptic plasticity.

Angelman syndrome associated with oculocutaneous albinism due to an intragenic deletion of the P gene

Angelman syndrome (AS) is a neurodevelopmental disorder characterized by mental retardation, speech impairment, ataxia, and happy disposition with frequent smiling. AS results from the loss of expression of a maternal imprinted gene, UBE3A, mapped within 15q11-q13 region, due to different mechanisms: maternal deletion, paternal UPD, imprinting center mutation, and UBE3A mutation. Deletion AS patients may exhibit hypopigmentation of skin, eye, and hair correlating with deletion of P gene localized in the distal part of Prader-Willi (PWS)/AS region. Our patient presented developmental delay, severe mental retardation, absence of speech, outbursts of laughter, microcephaly, ataxia, hyperactivity, seizures, white skin, no retinal pigmentation, and gold yellow hair. His parents were of African ancestry. The SNURF-SNRPN methylation analysis confirmed AS diagnosis and microsatellite studies disclosed deletion with breakpoints in BP2 and BP3. All of the 25 exons and flanking introns of the P gene of the patient, his father, and mother were investigated. The patient is hemizygous for the deleted exon 7 of the P gene derived from his father who is a carrier of the deleted allele. Our patient manifests OCA2 associated with AS due to the loss of the maternal chromosome 15 with the normal P allele, and the paternal deletion in the P gene. As various degrees of hypopigmentation are associated with PWS and AS patients, the study of the P gene in a hemizygous state could contribute to the understanding of its effect on human pigmentation during development and to disclose the presence of modifier pigmentation gene(s) in the PWS/AS region.

Derangements of hippocampal calcium/calmodulin-dependent protein kinase II in a mouse model for Angelman mental retardation syndrome

Angelman syndrome (AS) is a disorder of human cognition characterized by severe mental retardation and epilepsy. Recently, a mouse model for AS (Ube3a maternal null mutation) was developed that displays deficits in both context-dependent learning and hippocampal long-term potentiation (LTP). In the present studies, we examined the molecular basis for these LTP and learning deficits. Mutant animals exhibited a significant increase in hippocampal phospho-calcium/calmodulin-dependent protein kinase II (CaMKII), specifically at sites Thr(286) and Thr(305), with no corresponding change in the levels of total CaMKII. In addition, mutants show a reduction in CaMKII activity, autophosphorylation capability, and total CaMKII associated with postsynaptic density. These findings are the first to implicate misregulation of CaMKII as a molecular cause for the neurobehavioral deficits in a human learning disorder.

p53 inactivation leads to impaired motor synchronization in mice

We have combined genetic and pharmacological approaches to investigate the behavioural consequences of inactivation of the murine p53 protein. Our behavioural analysis revealed that p53-null mice (p53KO) exhibit a very specific and significant motor deficit in rapid walking synchronization. This deficit, observed using the rotarod test, was the only behavioural defect of p53KO mice. We demonstrated that it was not due to an increase in neuronal number or abnormal connectivity in the olivo-cerebellar system, thought to control motor synchronization. In order to test the role of p53 in the central nervous system, we injected a pharmacological inhibitor of p53 activation, pifithrin-alpha, into the cerebellum of wild-type mice. This treatment mimicked the walking synchronization deficit of p53KO mice, suggesting that presence of p53 protein in the cerebellum is necessary to execute this synchronization of walking. Our investigation reveals a functional role of cerebellar p53 protein in adult walking synchronization.

Case report: Angelman syndrome in an individual with a small SMC(15) and paternal uniparental disomy: a case report with reference to the assessment of cognitive functioning and autistic symptomatology

The case of a 15-year-old male with Angelman syndrome, paternal uniparental disomy of chromosome 15, and a small supernumerary marker chromosome is discussed. Assessment of cognitive functioning revealed an uneven profile of ability across different domains; in particular, receptive language ability was found to be superior to expressive language ability, whilst both gross and fine motor skills were found to be relatively well developed. Assessment using the Autism Diagnostic Observation Schedule showed very little evidence of autistic symptomatology. The patient showed an interest in social interaction and used a variety of methods to communicate, including some gestures and several single words. A clinical history revealed febrile convulsions during childhood but an absence of seizures in the previous 5 years. The patient was not hypopigmented, and height, weight, and head circumference were within the normal range for his age. The implications of these features are discussed in the context of previous work describing a milder phenotype in nondeletion cases of Angelman syndrome and work that has examined the prevalence of autism spectrum disorders amongst individuals with Angelman syndrome.

Genome-wide analysis of epigenetics in cancer

Human cancers are caused by multiple mechanisms. Research in the last 30 years has firmly established the roles of a group of genes including oncogenes, tumor suppressor genes, and DNA repair genes in human cancers. The activation and inactivation of these cancer genes can be caused by genetic mutations or epigenetic alterations. The epigenetic changes in cancers include methylation of CpG islands, loss of imprinting, and chromatin modification. The completion of the genome sequences of many organisms including the human has transformed the traditional approach to molecular biology research into an era of functional genome research. Traditional research usually involves the study of one or a few genes (proteins) in a particular biological process in normal physiology or disease. Functional genome research takes advantage of newly available genome sequences and high-throughput genome technologies to study genes and/or proteins to inform the perspective of entire biological processes. I will focus on recent progress in the identification of imprinted genes and methylation of CpG islands through genome-wide analysis.

Angelman syndrome: a review of the clinical and genetic aspects

Angelman syndrome (AS) is a neurodevelopmental disorder characterised by severe learning difficulties, ataxia, a seizure disorder with a characteristic EEG, subtle dysmorphic facial features, and a happy, sociable disposition. Most children present with delay in developmental milestones and slowing of head growth during the first year of life. In the majority of cases speech does not develop. Patients with AS have a characteristic behavioural phenotype with jerky movements, frequent and sometimes inappropriate laughter, a love of water, and sleep disorder. The facial features are subtle and include a wide, smiling mouth, prominent chin, and deep set eyes. It is caused by a variety of genetic abnormalities involving the chromosome 15q11-13 region, which is subject to genomic imprinting. These include maternal deletion, paternal uniparental disomy, imprinting defects, and point mutations or small deletions within the UBE3A gene, which lies within this region. UBE3A shows tissue specific imprinting, being expressed exclusively from the maternal allele in brain. The genetic mechanisms identified so far in AS are found in 85-90% of those with the clinical phenotype and all interfere with UBE3A expression.

Absence of MeCP2 mutations in patients from the South Carolina autism project

The methyl-CpG binding protein 2 (MeCP2) gene has recently been identified as the gene responsible for Rett syndrome (RS), a pervasive developmental disorder considered by many to be one of the autism spectrum disorders. Most female patients with MeCP2 mutations exhibit the classic features of RS, including autistic behaviors. Most male patients with MeCP2 mutations exhibit moderate to severe developmental delay/mental retardation. Ninety nine patients from the South Carolina autism project (SCAP) were screened for MeCP2 mutations, including all 41 female patients from whom DNA samples were available plus the 58 male patients with the lowest scores on standard IQ tests and/or the Vineland Adaptive Behavior Scale. No pathogenic mutations were observed in these patients. One patient had the C582T variant, previously reported in the unaffected father of an RS patient. Two other patients had single nucleotide polymorphisms in the 3' UTR of the gene, G1470A and C1516G. These variants were seen in 12/82 and 1/178 phenotypically normal male controls, respectively. The findings from this and other studies suggest that mutations in the coding sequence of the MeCP2 gene are not a significant etiological factor in autism.

A further case of a Prader-Willi syndrome phenotype in a patient with Angelman syndrome molecular defect

Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are distinct human neurogenetic disorders; however, a clinical overlap between AS and PWS has been identified. We report on a further case of a patient showing the PWS phenotype with the AS molecular defect. Despite the PWS phenotype, the DNA methylation analysis of SNRPN revealed an AS pattern. Cytogenetic and FISH analysis showed normal chromosomes 15 and microsatellite analysis showed heterozygous loci inside and outside the 15q11-13 region. The presence of these atypical cases could be more frequent than previously expected and we reinforce that the DNA methylation analysis is important for the correct diagnosis of severe mental deficiency, congenital hypotonia and obesity.

A study of the influence of different genotypes on the physical and behavioral phenotypes of children and adults ascertained clinically as having PWS

A population-based cohort of people with a clinical diagnosis of Prader-Willi syndrome (PWS) was genetically assessed using molecular diagnostic methods and subsequently divided into the following genetic subtypes involving chromosome 15: 'deletion', 'disomy' and genetically negative (referred to as 'PWS-like'). The physical and behavioral characteristics of the three groups were compared in order to evaluate the unique characteristics of the phenotype resulting from loss of expression of imprinted genes at 15q11q13 (PWS vs. PWS-like cases), the possible effect of either haploid insufficiency of non-imprinted genes (deletion cases), or gain of function of imprinted genes (disomy cases) located within the PWS critical region at 15q11q13. In this study, the main differences between probands with either a deletion or disomy are considered, and the possible involvement of contributing genes discussed. The differences within the PWS group proved difficult to quantify. It would appear that haploid insufficiency or gain of function are more subtle contributors than gender-specific genomic imprinting in the production of the PWS phenotype.

Glucocorticoid receptors in hippocampal neurons that do not engage proteasomes escape from hormone-dependent down-regulation but maintain transactivation activity

The glucocorticoid receptor (GR) protein is subjected to hormone-dependent down-regulation in most cells and tissues. This reduction in receptor levels that accompanies chronic hormone exposure serves to limit hormone responsiveness and operates at transcriptional, posttranscriptional, and posttranslational levels. The ability of glucocorticoid hormones to trigger GR down-regulation may be not universal, particularly in mature and developing neurons in which conflicting results regarding hormone control of GR protein have been reported. We find that endogenous GR is not down-regulated in the HT22 mouse hippocampal cell line and in primary hippocampal neurons derived from embryonic rats. Because GR has the capacity to be ubiquitylated in HT22 cells, receptor down-regulation must be limited by defects in either targeting of polyubiquitylated receptor to the proteasome or processing of the targeted receptor by the proteasome. Despite the lack of GR down-regulation in the HT22 cells, glucocorticoid-induced transcription from transiently transfected templates is attenuated upon prolonged hormone treatment. This termination of GR transactivation is not due to inefficient nuclear import or nuclear retention of the receptor. Furthermore, GR efficiently exports from HT22 cell nuclei in hormone-withdrawn cells, indicating that the receptor has access to both nuclear and cytoplasmic degradation pathways. Our results suggest that appropriate maturation of proteasomal degradative or targeting activities may be required, particularly in hippocampal neurons, for hormone-dependent down-regulation of GR.

Physiological functions of imprinted genes

Genomic imprinting in gametogenesis marks a subset of mammalian genes for parent-of-origin-dependent monoallelic expression in the offspring. Embryological and classical genetic experiments in mice that uncovered the existence of genomic imprinting nearly two decades ago produced abnormalities of growth or behavior, without severe developmental malformations. Since then, the identification and manipulation of individual imprinted genes has continued to suggest that the diverse products of these genes are largely devoted to controlling pre- and post-natal growth, as well as brain function and behavior. Here, we review this evidence, and link our discussion to a website (http://www.otago.ac.nz/IGC) containing a comprehensive database of imprinted genes. Ultimately, these data will answer the long-debated question of whether there is a coherent biological rationale for imprinting.

Familial interstitial 570 kbp deletion of the UBE3A gene region causing Angelman syndrome but not Prader-Willi syndrome

Angelman syndrome (AS) is a disorder of psychomotor development caused by loss of function of the imprinted UBE3A gene. Since the paternal UBE3A copy is regularly silent, only mutations inactivating the maternal copy cause AS. Among 1,272 patients suspected of AS, we found one with an isolated deletion of the UBE3A gene on the maternally inherited chromosome. Initial DNA methylation testing at the SNURF-SNRPN locus in the patient revealed a normal pattern. The deletion was only detected through allelic loss at microsatellite loci D15S1506, D15S122, and D15S210, and confirmed with fluorescence in situ hybridization (FISH) using bacterial artificial chromosome (BAC) probes derived from the loci. It extends approximately 570 kilobase pairs (kbp), encompassing the UBE3A locus, and is flanked by loci PAR/SN and D15S986. The deletion is familial, and haplotype studies suggest that a great grandfather of the index patient already carried this deletion, and that it causes AS when inherited through the female germline but not Prader-Willi syndrome (PWS) when paternally inherited. Our findings support the hypothesis that the functional loss of maternal UBE3A gene activity is sufficient to cause AS and that the deleted region does not contain genes or other structures that are involved in PWS. Finally, this case highlights that methylation tests can fail to detect some familial AS cases with a recurrence risk of 50%.

The human secretin gene: fine structure in 11p15.5 and sequence variation in patients with autism

Secretin is a peptide hormone involved in digestion that has been studied as a potential therapeutic agent in patients with autism. We characterized the human secretin locus to determine whether mutations in this gene might play a role in a fraction of autism patients. While the secretin gene (SCT) was not found to be mutated in the majority of autistic patients, rare heterozygous sequence variants were identified in three patients. We also investigated length variation in a variable number of tandem repeats (VNTR) immediately upstream of SCT and found no significant differences in length between patients with autism and normal controls. SCT is located on 11p15.5, adjacent to DRD4 and HRAS. This region has been reported to be associated with both autism and attention deficit hyperactivity disorder (ADHD). Although imprinting is a characteristic of some genes in the vicinity, we could find no evidence for methylation of SCT in lymphoblast cells from patients or control individuals.

Association analysis of chromosome 15 gabaa receptor subunit genes in autistic disorder

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain, acting via the GABAA receptors. The GABAA receptors are comprised of several different homologous subunits, forming a group of receptors that are both structurally and functionally diverse. Three of the GABAA receptor subunit genes (GABRB3, GABRA5 and GABRG3) form a cluster on chromosome 15q11-q13, in a region that has been genetically associated with autistic disorder (AutD). Based on these data, we examined 16 single nucleotide polymorphisms (SNPs) located within GABRB3, GABRA5 and GABRG3 for linkage disequilibrium (LD) in 226 AutD families (AutD patients and parents). Genotyping was performed using either OLA (oligonucleotide ligation assay), or SSCP (single strand conformation polymorphism) followed by DNA sequencing. We tested for LD using the Pedigree Disequilibrium Test (PDT). PDT results gave significant evidence that AutD is associated with two SNPs located within the GABRG3 gene (exon5_539T/C, p=0.02 and intron5_687T/C, p=0.03), suggesting that the GABRG3 gene or a gene nearby contributes to genetic risk in AutD.

An aminophospholipid translocase associated with body fat and type 2 diabetes phenotypes

OBJECTIVE

We have shown that a region on proximal mouse chromosome 7, near the pink-eyed (p) dilution locus, contains an ATPase (pfatp), a putative aminophospholipid translocase. Studies have suggested that this gene is a prime candidate for modulating body fat or involved in lipid metabolism in mouse and humans. Toward further analyses, our objective was to generate the complete genomic structures of mouse and human genes.

RESEARCH METHODS AND PROCEDURES

The genomic structure of mouse pfatp was deduced by comparing the full-length cDNA sequence with the genomic sequence derived from a mouse BAC. The human ortholog was identified from the National Center for Biotechnology Information database. Full-length cDNA was generated, and the corresponding genomic structure was deduced from the Human Genome Database.

RESULTS

Murine pfatp, and its human ortholog, PFATP, belong to class V of the third subfamily of P-type ATPases. The gene organization is strikingly similar in both organisms and all exon-intron junctions are conserved. A putative promoter region of PFATP contains a strong CpG island. The 5' untranslated regions of the two cDNAs have potential binding sites for multiple transcription factors, including Sp1, USF, AP1, and AP2, involved in adipogenesis and adipocyte metabolism.

DISCUSSION

We report the generation of the complete genomic structure of a novel aminophospholipid translocase in mice and humans. Because the exact biological role and the subsequent relevance of these ATPases to obesity and diabetes are unknown, these data help to delineate the role of these genes in lipid/adipocyte metabolism.

PJA1, encoding a RING-H2 finger ubiquitin ligase, is a novel human X chromosome gene abundantly expressed in brain

RING-finger proteins contain cysteine-rich, zinc-binding domains and are involved in the formation of macromolecular scaffolds important for transcriptional repression and ubiquitination. In this study, we have identified a RING-H2 finger gene, PJA1 (for praja-1), from a human brain cDNA library and mapped it to human chromosome Xq12 between markers DXS983 and DXS1216, a region implicated in X-linked mental retardation (MRX). Northern blot analysis indicated a 2.7-kb transcript that was abundantly expressed in the brain, including regions of the cerebellum, cerebral cortex, medulla, occipital pole, frontal lobe, temporal lobe, and putamen. Amino acid sequence analysis of the 71-kDa protein PJA1 showed 52.3% identity to human PJA2 (for praja-2, also known as NEURODAP1/KIAA0438) and also a significant identity to its homologs in rat, mouse, and zebrafish. In vitro binding and immunoprecipitation assays demonstrated that both PJA1 and PJA2 are able to bind the ubiquitin-conjugating enzyme UbcH5B. Moreover, the ubiquitination assay indicated that PJA1 and PJA2 have an E2-dependent E3 ubiquitin ligase activity. Thus our findings demonstrate that PJA1 can be involved in protein ubiquitination in the brain and is a suitable candidate gene for MRX.

Tetrasomy 15q11-q13 identified by fluorescence in situ hybridization in a patient with autistic disorder

We report a female child with tetrasomy of the 15q11-q13 chromosomal region, and autistic disorder associated with mental retardation, developmental problems and behavioral disorders. Combining classical and molecular cytogenetic approaches by fluorescence in situ hybridization technique, the karyotype was demonstrated as 47,XX,+mar.ish der(15)(D15Z1++,D15S11++,GABRB3++,PML-). Duplication of the 15q proximal segment represents the most consistent chromosomal abnormality reported in association with autism. The contribution of the GABA receptor subunit genes, and other genes mapped to this region, to the clinical symptoms of the disease is discussed.

Modeling human epilepsies in mice

Two categories of mouse models of human epilepsy are now contributing to the experimental analysis of inherited seizure disorders. The first type includes homologous genetic models arrived at in the classic way; the genes from human inherited epilepsy syndromes are cloned, and mice are recreated with functionally identical mutations. The second category involves the reverse strategy: mutating single genes in mice and determining whether the newly created nervous system develops epilepsy. These "gene-forward" models define specific candidate genes that can then be tested for possible involvement in human epilepsies. Spontaneous mutation of genes in mice and other species is also a source for candidate genes. As each of these genes and their physiologic functions is defined, the focus can shift to (a) fully characterizing the clinical epilepsy phenotype, (b) tracing the steps in the molecular pathogenesis of the disorder, and (c) pinpointing molecular targets for early intervention. Along with providing a unique opportunity to understand the mechanisms of inherited epileptogenesis, the mouse models serve as ideal biological test systems to search for novel therapeutic strategies.

Mechanisms underlying ubiquitination

The conjugation of ubiquitin to other cellular proteins regulates a broad range of eukaryotic cell functions. The high efficiency and exquisite selectivity of ubiquitination reactions reflect the properties of enzymes known as ubiquitin-protein ligases or E3s. An E3 recognizes its substrates based on the presence of a specific ubiquitination signal, and catalyzes the formation of an isopeptide bond between a substrate (or ubiquitin) lysine residue and the C terminus of ubiquitin. Although a great deal is known about the molecular basis of E3 specificity, much less is known about molecular mechanisms of catalysis by E3s. Recent findings reveal that all known E3s utilize one of just two catalytic domains--a HECT domain or a RING finger--and crystal structures have provided the first detailed views of an active site of each type. The new findings shed light on many aspects of E3 structure, function, and mechanism, but also emphasize that key features of E3 catalysis remain to be elucidated.

Neurobehavioral and electroencephalographic abnormalities in Ube3a maternal-deficient mice

Angelman syndrome (AS), characterized by motor dysfunction, mental retardation, and seizures, is caused by several genetic etiologies involving chromosome 15q11-q13, including mutations of the UBE3A gene. UBE3A encodes UBE3A/E6-AP, a ubiquitin-protein ligase, and shows brain-specific imprinting, with brain expression predominantly from the maternal allele. Lack of a functional maternal allele of UBE3A causes AS. In order to understand the causal relationship between maternal UBE3A mutations and AS, we have constructed a mouse model with targeted inactivation of Ube3a. The inactive allele contains a lacZ reporter gene for analysis of brain-specific imprinting. Maternal, but not paternal, transmission of the targeted allele leads to beta-galactosidase activity in hippocampal and cerebellar neurons. Maternal inheritance of the Ube3a mutant allele also causes impaired performance in tests of motor function and spatial learning, as well as abnormal hippocampal EEG recordings. As predicted from the dependence of UBE3A-mediated ubiquitination of p53 on HPV E6 protein, our maternal-deficient mice show normal brain p53 levels.

Chromosomal abnormalities and epilepsy: a review for clinicians and gene hunters

PURPOSE

We analyzed databases on chromosomal anomalies and epilepsy to identify chromosomal regions where abnormalities are associated with clinically recognizable epilepsy syndromes. The expectation was that these regions could then be offered as targets in the search for epilepsy genes.

METHODS

The cytogenetic program of the Oxford Medical Database, and the PubMed database were used to identify chromosomal aberrations associated with seizures and/or EEG abnormalities. The literature on selected small anomalies thus identified was reviewed from a clinical and electroencephalographic viewpoint, to classify the seizures and syndromes according to the current International League Against Epilepsy (ILAE) classification.

RESULTS

There were 400 different chromosomal imbalances described with seizures or EEG abnormalities. Eight chromosomal disorders had a high association with epilepsy. These comprised: the Wolf-Hirschhorn (4p-) syndrome, Miller-Dieker syndrome (del 17p13.3), Angelman syndrome (del 15q11-q13), the inversion duplication 15 syndrome, terminal deletions of chromosome 1q and 1p, and ring chromosomes 14 and 20. Many other segments had a weaker association with seizures. The poor quality of description of the epileptology in many reports thwarted an attempt to make precise karyotype-phenotype correlations.

CONCLUSIONS

We identified certain chromosomal regions where aberrations had an evident association with seizures, and these regions may be useful targets for gene hunters. New correlations with specific epilepsy syndromes were not revealed. Clinicians should continue to search for small chromosomal abnormalities associated with specific epilepsy syndromes that could provide important clues for finding epilepsy genes, and the epileptology should be rigorously characterized.

Cooperation of HECT-domain ubiquitin ligase hHYD and DNA topoisomerase II-binding protein for DNA damage response

Ubiquitin ligases define the substrate specificity of protein ubiquitination and subsequent proteosomal degradation. The catalytic sequence was first characterized in the C terminus of E6-associated protein (E6AP) and referred to as the HECT (homologous to E6AP C terminus) domain. The human homologue of the regulator of cell proliferation hyperplastic discs in Drosophila, designated hHYD, is a HECT-domain ubiquitin ligase. Here we show that hHYD provides a ubiquitin system for a cellular response to DNA damage. A yeast two-hybrid screen showed that DNA topoisomerase IIbeta-binding protein 1 (TopBP1) interacted with hHYD. Endogenous hHYD bound the BRCA1 C-terminus domains of TopBP1 that are highlighted in DNA damage checkpoint proteins and cell cycle regulators. Using an in vitro reconstitution, specific E2 (ubiquitin-conjugating) enzymes (human UbcH4, UbcH5B, and UbcH5C) transferred ubiquitin molecules to hHYD, leading to the ubiquitination of TopBP1. TopBP1 was usually ubiquitinated and degraded by the proteosome, whereas X-irradiation diminished the ubiquitination of TopBP1 probably via the phosphorylation, resulting in the stable colocalization of up-regulated TopBP1 with gamma-H2AX nuclear foci in DNA breaks. These results demonstrated that hHYD coordinated TopBP1 in the DNA damage response.

Molecular and cytogenetic analyses on Brazilian youths with pervasive developmental disorders

The Pervasive Developmental Disorders (PDDs) constitute a group of behavioral and neurobiological impairment conditions whose main features are delayed communicative and cognitive development. Genetic factors are reportedly associated with PDDs and particular genetic abnormalities are frequently found in specific diagnostic subgroups such as the autism spectrum disorders. This study evaluated cytogenetic and molecular parameters in 30 youths with autism or other PDDs. The fragile X syndrome was the most common genetic abnormality detected, presented by 1 patient with autism and 1 patient with PPD not-otherwise specified (PPD-NOS). One girl with PDD-NOS was found to have tetrasomy for the 15q11-q13 region, and one patient with autism exhibited in 2/100 metaphases an inv(7)(p35q36), thus suggesting a mosaicism 46,XX/46,XX,inv(7)(p15q36) or representing a coincidental finding. The high frequency of chromosomopathies support the hypothesis that PDDs may develop as a consequence to chromosomal abnormalities and justify the cytogenetic and molecular assessment in all patients with PDDs for establishment of diagnosis.

A necdin/MAGE-like gene in the chromosome 15 autism susceptibility region: expression, imprinting, and mapping of the human and mouse orthologues

BACKGROUND

Proximal chromosome 15q is implicated in neurodevelopmental disorders including Prader-Willi and Angelman syndromes, autistic disorder and developmental abnormalities resulting from chromosomal deletions or duplications. A subset of genes in this region are subject to genomic imprinting, the expression of the gene from only one parental allele.

RESULTS

We have now identified the NDNL2 (also known as MAGE-G) gene within the 15q autistic disorder susceptibility region and have mapped its murine homolog to the region of conserved synteny near necdin (Ndn) on mouse Chr 7. NDNL2/MAGE-G is a member of a large gene family that includes the X-linked MAGE cluster, MAGED1 (NRAGE), MAGEL2 and NDN, where the latter two genes are implicated in Prader-Willi syndrome. We have now determined that NDNL2/Ndnl2 is widely expressed in mouse and human fetal and adult tissues, and that it is apparently not subject to genomic imprinting by the PWS/AS Imprinting Center.

CONCLUSION

Although NDNL2/MAGE-G in the broadly defined chromosome 15 autistic disorder susceptibility region, it is not likely to be pathogenic based on its wide expression pattern and lack of imprinted expression.

Parent-specific complementary patterns of histone H3 lysine 9 and H3 lysine 4 methylation at the Prader-Willi syndrome imprinting center

The Prader-Willi syndrome (PWS)/Angelman syndrome (AS) region, on human chromosome 15q11-q13, exemplifies coordinate control of imprinted gene expression over a large chromosomal domain. Establishment of the paternal state of the region requires the PWS imprinting center (PWS-IC); establishment of the maternal state requires the AS-IC. Cytosine methylation of the PWS-IC, which occurs during oogenesis in mice, occurs only after fertilization in humans, so this modification cannot be the gametic imprint for the PWS/AS region in humans. Here, we demonstrate that the PWS-IC shows parent-specific complementary patterns of H3 lysine 9 (Lys9) and H3 lysine 4 (Lys4) methylation. H3 Lys9 is methylated on the maternal copy of the PWS-IC, and H3 Lys4 is methylated on the paternal copy. We suggest that H3 Lys9 methylation is a candidate maternal gametic imprint for this region, and we show how changes in chromatin packaging during the life cycle of mammals provide a means of erasing such an imprint in the male germline.

Distinct phenotypes distinguish the molecular classes of Angelman syndrome

BACKGROUND

Angelman syndrome (AS) is a severe neurobehavioural disorder caused by defects in the maternally derived imprinted domain located on 15q11-q13. Most patients acquire AS by one of five mechanisms: (1) a large interstitial deletion of 15q11-q13; (2) paternal uniparental disomy (UPD) of chromosome 15; (3) an imprinting defect (ID); (4) a mutation in the E3 ubiquitin protein ligase gene (UBE3A); or (5) unidentified mechanism(s). All classical patients from these classes exhibit four cardinal features, including severe developmental delay and/or mental retardation, profound speech impairment, a movement and balance disorder, and AS specific behaviour typified by an easily excitable personality with an inappropriately happy affect. In addition, patients can display other characteristics, including microcephaly, hypopigmentation, and seizures.

METHODS

We restricted the present study to 104 patients (93 families) with a classical AS phenotype. All of our patients were evaluated for 22 clinical variables including growth parameters, acquisition of motor skills, and history of seizures. In addition, molecular and cytogenetic analyses were used to assign a molecular class (I-V) to each patient for genotype-phenotype correlations.

RESULTS

In our patient repository, 22% of our families had normal DNA methylation analyses along 15q11-q13. Of these, 44% of sporadic patients had mutations within UBE3A, the largest percentage found to date. Our data indicate that the five molecular classes can be divided into four phenotypic groups: deletions, UPD and ID patients, UBE3A mutation patients, and subjects with unknown aetiology. Deletion patients are the most severely affected, while UPD and ID patients are the least. Differences in body mass index, head circumference, and seizure activity are the most pronounced among the classes.

CONCLUSIONS

Clinically, we were unable to distinguish between UPD and ID patients, suggesting that 15q11-q13 contains the only significant maternally expressed imprinted genes on chromosome 15.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

Chromosomal microdeletions: dissecting del22q11 syndrome

Identifying the genes that underlie the pathogenesis of chromosome deletion and duplication syndromes is a challenge because the affected chromosomal segment can contain many genes. The identification of genes that are relevant to these disorders often requires the analysis of individuals that carry rare, small deletions, translocations or single-gene mutations. Research into the chromosome 22 deletion (del22q11) syndrome, which encompasses DiGeorge and velocardiofacial syndrome, has taken a different path in recent years, using mouse models to circumvent the paucity of informative human material. These mouse models have provided new insights into the pathogenesis of del22q11 syndrome and have established strategies for research into chromosomal-deletion and -duplication syndromes.

Molecular mechanisms in neurologic disorders

Although many pediatric neurologic disorders, such as epilepsy and mental retardation, are the result of a combination of genetic and environmental factors, many others are the result of mutations of single genes. Most of these single gene traits are inherited in autosomal dominant, autosomal recessive, or X-linked fashion. The diversity of mutations that are responsible for these diseases produces variability in phenotypic expression. However, there are other important features of many neurologic disorders that cannot be explained by standard models of mendelian inheritance. This review focuses on recently described mechanisms, such as genomic imprinting, germline mosaicism, mitochondrial inheritance, and triplet repeat expansion. The diagnostic evaluation, prognostic significance, and recurrence risk for specific neurogenetic disorders is correlated with these underlying disease mechanisms.

Imprinted genes and mental dysfunction

There is a rapidly accumulating body of evidence from family, adoption and twin studies suggestive of a genetic component to many common mental disorders. In some cases, the transmission of abnormalities has been shown to be dependent upon the sex of the parent from whom they are inherited. Such 'parent-of-origin effects' may be explained by a number of genetic mechanisms, one of which is 'genomic imprinting'. In imprinted genes one allele is silenced according to its parental origin. This in turn means that imprinted traits are passed down the maternal or paternal line, in contrast to the more frequent Mendelian mode of inheritance that is indifferent to the parental origin of the allele. In the present review, we survey the evidence for the influence of imprinted genes on a number of mental disorders, ranging from explicit imprinted conditions, where in some cases abnormalities have been mapped to particular gene candidates, to examples where the evidence for parent-of-origin effects is less strong. We also consider, briefly, the wider implications of imprinted effects on mental dysfunction, in particular with respect to evolutionary pressures on mammalian brain development and function.

The ubiquitin-proteasome pathway and proteasome inhibitors

The ubiquitin-proteasome pathway has emerged as a central player in the regulation of several diverse cellular processes. Here, we describe the important components of this complex biochemical machinery as well as several important cellular substrates targeted by this pathway and examples of human diseases resulting from defects in various components of the ubiquitin-proteasome pathway. In addition, this review covers the chemistry of synthetic and natural proteasome inhibitors, emphasizing their mode of actions toward the 20S proteasome. Given the importance of proteasome-mediated protein degradation in various intracellular processes, inhibitors of this pathway will continue to serve as both molecular probes of major cellular networks as well as potential therapeutic agents for various human diseases.

Prader-Willi and Angelman syndromes: sister imprinted disorders

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are clinically distinct complex disorders mapped to chromosome 15q11-q13. They both have characteristic neurologic, developmental, and behavioral phenotypes plus other structural and functional abnormalities. However, the cognitive and neurologic impairment is more severe in AS, including seizures and ataxia. The behavioral and endocrine disorders are more severe in PWS, including obsessive-compulsive symptoms and hypothalamic insufficiency. Both disorders can result from microdeletion, uniparental disomy, or an imprinting center defect in 15q11-q13, although the abnormality is on the paternally derived chromosome 15 for PWS and the maternally derived 15 for AS because of genomic imprinting. Although the same gene may control imprinting for both disorders, the gene(s) causing their phenotypes differ. AS results from underexpression of a single gene, UBE3A, which codes for E6-AP, a protein that functions to transfer small ubiquitin molecules to certain target proteins, to enable their degradation. The genes responsible for PWS are not determined, although several maternally imprinted genes in 15q11-q13 are known. The most likely candidate is SNRPN, which codes for a small nuclear ribonucleoprotein, a ribosome-associated protein that controls gene splicing and thus synthesis of critical proteins in the brain. Animal models exist for both disorders. The genetic relationship between PWS and AS makes them unique and potentially highly instructive disorders that contribute substantially to the population burden of cognitive impairment.

EH, Ledbetter DH. The human aminophospholipid-transporting ATPase gene ATP10C maps adjacent to UBE3A and exhibits similar imprinted expression

Maternal duplications of the imprinted 15q11-13 domain result in an estimated 1%-2% of autism-spectrum disorders, and linkage to autism has been identified within 15q12-13. UBE3A, the Angelman syndrome gene, has, to date, been the only maternally expressed, imprinted gene identified within this region, but mutations have not been found in autistic patients. Here we describe the characterization of ATP10C, a new human imprinted gene, which encodes a putative protein homologous to the mouse aminophospholipid-transporting ATPase Atp10c. ATP10C maps within 200 kb distal to UBE3A and, like UBE3A, also demonstrates imprinted, preferential maternal expression in human brain. The location and imprinted expression of ATP10C thus make it a candidate for chromosome 15-associated autism and suggest that it may contribute to the Angelman syndrome phenotype.

The ubiquitin protein catabolic disorders

The ubiquitin-proteasome system of intracellular proteolysis is essential for cell viability. We propose the concept that neurodegenerative diseases such as Alzheimer's and Parkinson's, as well as other conditions including some types of cancer, collectively represent a raft of 'ubiquitin protein catabolic disorders' in which altered function of the ubiquitin-proteasome system can cause or directly contribute to disease pathogenesis. Genetic abnormalities within the ubiquitin pathway, either in ubiquitin-ligase (E3) enzymes or in deubiquitinating enzymes, cause disease because of problems associated with substrate recognition or supply of free ubiquitin, respectively. In some cases, mutations in protein substrates of the ubiquitin-proteasome system may directly contribute to disease progression because of inefficient substrate recognition. Mutations in transcripts for the ubiquitin protein itself (as a result of 'molecular misreading') also affect ubiquitin-dependent proteolysis with catastrophic consequences. This has been shown in Alzheimer's disease and could apply to other age-associated neurodegenerative conditions. Within the nervous system, accumulation of unwanted proteins as a result of defective ubiquitin-dependent proteolysis may contribute to aggregation events, which underlie the pathogenesis of several major human neurodegenerative diseases.

Targeting protein ubiquitination for drug discovery. What is in the drug discovery toolbox?

Protein ubiquitination regulates the half-lives of many proteins by targeting them for degradation. Ubiquitination is a specific process associated with several highly regulated biological outcomes including cell cycle progression, differentiation, antigen presentation, retrovirus assembly, apoptosis, signal transduction, transcriptional activation, biological clocks, receptor downregulation and endocytosis. Newly discovered families of ubiquitination and deubiquitination enzymes participate in these processes. These enzymes could provide new families of drug targets and new ways of intervention in many human diseases; however, much work is required to validate this approach. This review will discuss what is in the drug discovery toolbox to assist in the validation of ubiquitination enzymes as therapeutic targets.

The role of genomic imprinting in human developmental disorders: lessons from Prader-Willi syndrome

Normal human development involves a delicate interplay of gene expression in specific tissues at narrow windows of time. Temporally and spatially regulated gene expression is controlled both by gene-specific factors and chromatin-specific factors. Genomic imprinting is the expression of specific genes primarily from only one allele at particular times during development, and is one mechanism implicated in the intricate control of gene expression. Two human genetic disorders, Prader-Willi syndrome (PWS, MIM 176270) and Angelman syndrome (AS, MIM 105830), result from rearrangements of chromosome 15q11-q13, an imprinted region of the human genome. Despite their rarity, disorders such as PWS and AS can give focused insight into the role of genomic imprinting and imprinted genes in human development.

Epilepsy genes: the link between molecular dysfunction and pathophysiology

Our understanding of the genetic basis of epilepsy is progressing at a rapid pace. Gene mutations causing several of the inherited epilepsies have been mapped, and several more are likely to be added in coming years. In this review, we summarize the available information on the genetic basis of human epilepsies and epilepsy syndromes, emphasizing how genetic defects may correlate with the pathophysiological mechanisms of brain hyperexcitability. Mutations leading to epilepsy have been identified in genes encoding voltage- and ligand-gated ion channels (benign familial neonatal convulsions, autosomal dominant nocturnal frontal lobe epilepsy, generalized epilepsy with febrile seizures "plus"), neurotransmitter receptors (Angelman syndrome), the molecular cascade of cellular energy production (myoclonic epilepsy with ragged red fibers), and proteins without a known role in neuronal excitability (Unverricht-Lundborg disease). Gene defects can lead to epilepsy by altering multiple and diverse aspects of neuronal function.