Nuclear Localization of the Autism Candidate Gene Neurobeachin and Functional Interaction with the NOTCH1 Intracellular Domain Indicate a Role in Regulating Transcription.

BACKGROUND: Neurobeachin (NBEA) is an autism spectrum disorders (ASD) candidate gene. NBEA deficiency affects regulated secretion, receptor trafficking, synaptic architecture and protein kinase A (PKA)-mediated phosphorylation. NBEA is a large multidomain scaffolding protein. From N- to C-terminus, NBEA has a concanavalin A-like lectin domain flanked by armadillo repeats (ACA), an A-kinase anchoring protein domain that can bind to PKA, a domain of unknown function (DUF1088) and a BEACH domain, preceded by a pleckstrin homology-like domain and followed by WD40 repeats (PBW). Although most of these domains mediate protein-protein interactions, no interaction screen has yet been performed. METHODS: Yeast two-hybrid screens with the ACA and PBW domain modules of NBEA gave a list of interaction partners, which were analyzed for Gene Ontology (GO) enrichment. Neuro-2a cells were used for confocal microscopy and nuclear extraction analysis. NOTCH-mediated transcription was studied with luciferase reporter assays and qRT-PCR, combined with NBEA knockdown or overexpression. RESULTS: Both domain modules showed a GO enrichment for the nucleus. PBW almost exclusively interacted with transcription regulators, while ACA interacted with a number of PKA substrates. NBEA was partially localized in the nucleus of Neuro-2a cells, albeit much less than in the cytoplasm. A nuclear localization signal was found in the DUF1088 domain, which was shown to contribute to the nuclear localization of an EGFP-DPBW fusion protein. Yeast two-hybrid identified the Notch1 intracellular domain as a physical interactor of the PBW domain and a role for NBEA as a negative regulator in Notch-mediated transcription was demonstrated. CONCLUSION: Defining novel interaction partners of conserved NBEA domain modules identified a role for NBEA as transcriptional regulator in the nucleus. The physical interaction of NBEA with NOTCH1 is most relevant for ASD pathogenesis because NOTCH signaling is essential for neural development.

Drosophila rugose is a functional homolog of mammalian Neurobeachin and affects synaptic architecture, brain morphology, and associative learning.

Neurobeachin (Nbea) is implicated in vesicle trafficking in the regulatory secretory pathway, but details on its molecular function are currently unknown. We have used Drosophila melanogaster mutants for rugose (rg), the Drosophila homolog of Nbea, to further elucidate the function of this multidomain protein. Rg is expressed in a granular pattern reminiscent of the Golgi network in neuronal cell bodies and colocalizes with transgenic Nbea, suggesting a function in secretory regulation. In contrast to Nbea(-/-) mice, rg null mutants are viable and fertile and exhibit aberrant associative odor learning, changes in gross brain morphology, and synaptic architecture as determined at the larval neuromuscular junction. At the same time, basal synaptic transmission is essentially unaffected, suggesting that structural and functional aspects are separable. Rg phenotypes can be rescued by a Drosophila rg+ transgene, whereas a mouse Nbea transgene rescues aversive odor learning and synaptic architecture; it fails to rescue brain morphology and appetitive odor learning. This dissociation between the functional redundancy of either the mouse or the fly transgene suggests that their complex composition of numerous functional and highly conserved domains support independent functions. We propose that the detailed compendium of phenotypes exhibited by the Drosophila rg null mutant provided here will serve as a test bed for dissecting the different functional domains of BEACH (for beige and human Chediak-Higashi syndrome) proteins, such as Rugose, mouse Nbea, or Nbea orthologs in other species, such as human.

SCAMP5, NBEA and AMISYN: three candidate genes for autism involved in secretion of large dense-core vesicles.

Autism is a neurodevelopmental disorder characterized by impaired social reciprocity, impaired communication and stereotypical behaviors. Despite strong evidence for a genetic basis, few susceptibility genes have been identified. Here, we describe the positional cloning of SCAMP5, CLIC4 and PPCDC as candidate genes for autism, starting from a person with idiopathic, sporadic autism carrying a de novo chromosomal translocation. One of these genes, SCAMP5 is silenced on the derivative chromosome, and encodes a brain-enriched protein involved in membrane trafficking, similar to the previously identified candidate genes NBEA and AMISYN. Gene silencing of Nbea, Amisyn and Scamp5 in mouse beta-TC3 cells resulted in a 2-fold increase in stimulated secretion of large dense-core vesicles (LDCVs), while overexpression suppressed secretion. Moreover, ultrastructural analysis of blood platelets from the patients with haploinsufficieny of one of the three candidate genes, showed morphological abnormalities of dense-core granules, which closely resemble LDCVs. Taken together, this study shows that in three independent patients with autism three different negative regulators of LDCV secretion are affected, respectively, suggesting that in at least a subgroup of patients the regulation of neuronal vesicle trafficking may be involved in the pathogenesis of autism.

Position effect leading to haploinsufficiency in a mosaic ring chromosome 14 in a boy with autism.

We describe an individual with autism and a coloboma of the eye carrying a mosaicism for a ring chromosome consisting of an inverted duplication of proximal chromosome 14. Of interest, the ring formation was associated with silencing of the amisyn gene present in two copies on the ring chromosome and located at 300 kb from the breakpoint. This observation lends further support for a locus for autism on proximal chromosome 14. Moreover, this case suggests that position effects need to be taken into account, when analyzing genotype-phenotype correlations based on chromosomal imbalances.

Hyperphagia and early-onset obesity due to a novel homozygous missense mutation in prohormone convertase 1/3.

CONTEXT: Congenital deficiency of the neuroendocrine-specific enzyme prohormone convertase (PC) 1/3 leads to a syndrome characterized by obesity, small intestinal dysfunction, and dysregulation of glucose homeostasis in humans. To date, only two unrelated subjects with this disorder have been reported. RESEARCH DESIGN AND METHODS: We now report a third proband, a 6-yr-old boy, offspring of a consanguineous union of parents of North African origin, who was homozygous for a novel missense mutation Ser307Leu. We characterized the functional properties of the mutant PC1/3 and characterized the clinical phenotype of the patient. RESULTS: In vitro this mutation markedly impairs the catalytic activity of the convertase. However, in contrast to other previously described naturally occurring mutations, intracellular trafficking of this mutant enzyme appeared normal. The Ser307Leu mutant retained some autocatalytic activity, even though it was completely inactive on other substrates. As with the previous two patients, this child had obesity and persistent diarrhea, however, there was no history of reactive hypoglycemia. The patient showed markedly increased food intake at an ad libitum test meal, confirming that hyperphagia makes a major contribution to the obesity seen in this syndrome. CONCLUSION: This case extends the clinical and molecular spectrum of human congenital PC1/3 deficiency.