Disease relevance of UBE3A

• Imprinting of the Angelman syndrome gene, UBE3A, is restricted to brain [1].
• Angelman syndrome is characterised by neurodevelopmental impairment (with or without epileptic seizures) associated with functional deficit of the UBE3A gene [2].
• This imprinted domain arose after a region bearing UBE3A (Angelman syndrome) fused with an unlinked region bearing SNRPN (Prader-Willi syndrome), which had duplicated from the non-imprinted SNRPB/B'. This region independently acquired several retroposed gene copies and arrays of small nucleolar RNAs from different parts of the genome [3].
• On the other hand, low-risk HPV 11 E6 does not interact with E6AP even at relatively high concentrations [4].
• High-risk human papillomavirus (HPV) can immortalize epithelial cells in culture through degradation of the retinoblastoma protein by HPV E7 and activation of hTERT transcription, the catalytic subunit of telomerase, by the heterodimer HPV E6/E6-associated protein (E6AP) [5].

Psychiatry related information on UBE3A

• MeCP2 deficiency in Rett syndrome causes epigenetic aberrations at the PWS/AS imprinting center that affects UBE3A expression [6].
• In their original configurations, SNRPN and UBE3A are expressed from both alleles, implying that acquisition of imprinting occurred after their rearrangement and required the evolution of a control locus [3].
• 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 [7].
• Evidence for likely altered regulation of UBE3A expression in maternal 15q11-q13 duplications suggests further investigation of the regulatory regions of this gene in autistic disorder [8].

High impact information on UBE3A

• Because attempts to identify any novel maternally expressed transcripts were unsuccessful and because the UBE3A gene remained within a narrowed AS critical region, we searched for mutations in UBE3A in 11 AS patients without known molecular defects (large deletion, uniparental disomy, or imprinting mutation) [9].
• Although mutations in UBE3A cause AS, indicating that maternal-specific expression of UBE3A is essential for a normal phenotype, evidence for maternal-specific expression of UBE3A has been lacking [10].
• Here we report the purification and identification of the factors necessary for the E6-E6-AP-mediated ubiquitination of p53 [11].
• Furthermore, E6-AP appears to have ubiquitin-protein ligase activity in the absence of E6 [11].
• The ubiquitination of p53 requires the E1 enzyme and a novel E2 in mammalian cells, while E3 activity is conferred by the E6-E6-AP complex [11].

Chemical compound and disease context of UBE3A

• E6 oncoproteins from HPV-16 and bovine papillomavirus type 1 (BPV-1) bind to similar leucine-rich peptides termed charged leucine motifs found on the cellular focal adhesion protein paxillin and the E3 ubiquitin ligase E6AP [12].

Biological context of UBE3A

• Point mutations were found in the gene for E6-AP ubiquitin-protein ligase (UBE3A) identifying it as the AS gene, and tissue-specific imprinting (maternal expression) was shown in the human brain and in hippocampal neurons and Purkinje cells in the mouse [13].
• Angelman syndrome (AS) is caused by chromosome 15q11-q13 deletions of maternal origin, by paternal uniparental disomy (UPD) 15, by imprinting defects, and by mutations in the UBE3A gene [14].
• In two familial cases and one sporadic case, mosaicism for UBE3A mutations was detected: in the mother of three AS sons, in the maternal grandfather of two AS first cousins, and in the mother of an AS daughter [14].
• Here we describe UBE3A coding-region mutations detected by SSCP analysis in 13 AS individuals or families [14].
• Among the other 11 unique mutations, 8 were small deletions or insertions predicted to cause frameshifts, 1 was a mutation to a stop codon, 1 was a missense mutation, and 1 was predicted to cause insertion of an isoleucine in the hect domain of the UBE3A protein, which functions in E2 binding and ubiquitin transfer [14].

Anatomical context of UBE3A

• Excess total maternal-UBE3A RNA was confirmed by Northern blot analysis of cell lines carrying 15q11- q13 duplications or triplications [15].
• These results demonstrate that: (1) UBE3A is imprinted in fibroblasts, lymphoblasts and neural-precursor cells; (2) allelic imprint status is maintained in the majority of cells upon duplication both in cis and in trans; and (3) alleles on specific types of duplications may exhibit an increase in expression levels/loss of expression constraints [15].
• Northern blot analyses revealed two UBE3A transcripts 1.4 and 2kb that were abundantly expressed in human testis [16].
• We also showed that both VCY2 and UBE3A mRNAs were expressed in ejaculated human spermatozoa, indicating that both genes localize in the germ cell compartment [16].
• Using a quantitative PCR assay, we have found that the ratio of SNURF-SNRPN/UBE3A transcript levels is increased in blood cells of AS patients with an imprinting defect, but not in AS patients with a UBE3A mutation or an unknown defect [17].

Associations of UBE3A with chemical compounds

• Binding of GABA(A) receptor channel ([(35)S]t-butylbicyclophosphorothionate) and benzodiazepine ([(3)H]Ro 15-4513) site ligands was reduced in selected brain regions of the beta3-deficient mice as compared to controls, while the UBE3A-deficient mice failed to show reduced GABA(A) receptors [18].
• In addition, both in vitro and in vivo experiments indicate that E6AP self-ubiquitination results primarily from an intramolecular transfer of ubiquitin from the active-site cysteine to one or more lysine residues; however, intermolecular transfer can also occur in the context of an E6-mediated E6AP multimer [19].
• BE6 and 16E6 both bind the product of the UBE3A gene called E6AP on a charged leucine peptide, LQELL [20].
• The cysteine residue of E6-AP responsible for ubiquitin thioester formation was mapped to a region that is highly conserved among several proteins of unknown function, suggesting that these proteins share the ability to form thioesters with ubiquitin [21].
• Mutation of the E6-AP ubiquitin ligase reduces nuclear inclusion frequency while accelerating polyglutamine-induced pathology in SCA1 mice [22].

Physical interactions of UBE3A

• The crystal structure of the E6AP hect domain bound to the UbcH7 ubiquitin-conjugating enzyme (E2) reveals the determinants of E2-E3 specificity and provides insights into the transfer of ubiquitin from the E2 to the E3 [23].
• The region of E6AP involved in complex formation with UbcH8 and UbcH7 was mapped to its Hect domain [24].
• These results indicate that 16E6 may have multiple modes of interaction with E6AP and that assembly of p53 containing complexes for targeted degradation by E6AP may occur in more than one way [20].
• The E6 protein binds to a cellular protein of 100 kDa termed E6-AP [25].
• Identification of a novel telomerase repressor that interacts with the human papillomavirus type-16 E6/E6-AP complex [26].

Enzymatic interactions of UBE3A

• The dimeric complex then binds p53 and E6AP catalyzes multi-ubiquitination and degradation of p53 [27].
• Mcm7 is ubiquitinated in vivo in both an E6-AP-dependent and -independent manner [28].

Regulatory relationships of UBE3A

• Additionally, we show that UBCH7 and E6-associated protein (E6-AP) synergistically enhance PR transactivation [29].
• Our findings are compatible with the assumption that imprinted UBE3A expression is regulated through the SNURF-SNRPN sense- UBE3A antisense transcript [17].
• E6AP and calmodulin reciprocally regulate estrogen receptor stability [30].
• These data indicate that the anti-growth-suppressive properties of E6-AP in HPV-positive cells depend on its ability to induce p53 degradation [31].
• Importantly, the purified E6AP enhanced the ubiquitination and degradation of E6TP1 in the presence of E6 in vitro [32].

Other interactions of UBE3A

• Rett syndrome, an X-linked dominant disorder caused by MECP2 mutations, and Angelman syndrome, an imprinted disorder caused by maternal 15q11-q13 or UBE3A deficiency, have phenotypic and genetic overlap with autism [33].
• At least one other gene, the E6-associated protein ubiquitin-protein ligase (UBE3A) gene, has been implicated in AS, so the relative contribution of the GABRB3 gene alone or in combination with other genes remains to be established [34].
• Expression of NDNL2, just distal to the duplicated region, was not markedly altered but paralleled changes in UBE3A expression [15].
• UBE3A encodes a ubiquitin-protein ligase and shows brain-specific imprinting [14].
• However, two genes previously identified as maternally expressed (UBE3A and ATP10C) showed a significant increase in expression in UPD cell lines compared with control and PWS deletion subjects [35].

Analytical, diagnostic and therapeutic context of UBE3A

• Molecular analyses included determination of DNA methylation within the 15q11-13 region by Southern blotting and microsattelite analysis within the 15q11-13 region by PCR and the UBE3A gene was also studied by mutational screening [36].
• To clarify the potential role of GABA(A) beta3 subunit-containing inhibitory receptors in these disorders, we applied ligand autoradiography on brain sections from mice with inactivated GABRB3 or maternal UBE3A genes [18].
• Kinetic analysis of the interactions of human papillomavirus E6 oncoproteins with the ubiquitin ligase E6AP using surface plasmon resonance [4].
• Prenatal diagnosis and carrier detection for a point mutation in UBE3A causing Angelman syndrome [37].
• Angelman syndrome: AS phenotype correlated with specific EEG pattern may result in a high detection rate of mutations in the UBE3A gene [38].

References