Rebecca S. Devon

A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2

Amyotrophic lateral sclerosis 2 (ALS2) is an autosomal recessive form of juvenile ALS and has been mapped to human chromosome 2q33. Here we report the identification of two independent deletion mutations linked to ALS2 in the coding exons of the new gene ALS2. These deletion mutations result in frameshifts that generate premature stop codons. ALS2 is expressed in various tissues and cells, including neurons throughout the brain and spinal cord, and encodes a protein containing multiple domains that have homology to RanGEF as well as RhoGEF. Deletion mutations are predicted to cause a loss of protein function, providing strong evidence that ALS2 is the causative gene underlying this form of ALS.

Genomic structure and localisation within a linkage hotspot of Disrupted In Schizophrenia 1 , a gene disrupted by a translocation segregating with schizophrenia

Two overlapping and antiparallel genes on chromosome 1, Disrupted In Schizophrenia 1 and 2 (DISC1 and DISC2), are disrupted by a (1;11)(q42.1;q14.3) translocation which segregates with schizophrenia through at least four generations of a large Scottish family. Consequently, these genes are worthy of further investigation as candidate genes potentially involved in the aetiology of major psychiatric illness. We have constructed a contiguous clone map of PACs and cosmids extending across at least 400 kb of the chromosome 1 translocation breakpoint region and this has provided the basis for examination of the genomic structure of DISC1. The gene consists of thirteen exons, estimated to extend across at least 300 kb of DNA. The antisense gene DISC2 overlaps with exon 9. Exon 11 contains an alternative splice site that removes 66 nucleotides from the open reading frame. The final intron of DISC1 belongs to the rare AT-AC class of introns. We have also mapped marker DIS251 in close proximity to DISC1, localising the gene within a critical region identified by several independent studies. Information regarding the structure of the DISC1 gene will facilitate assessment of its involvement in the aetiology of major mental illness in psychotic individuals unrelated to carriers of the translocation.

Identification of polymorphisms within Disrupted in Schizophrenia 1 and Disrupted in Schizophrenia 2, and an investigation of their association with schizophrenia and bipolar affective disorder

We have undertaken a search for polymorphic sequence variation within Disrupted in Schizophrenia 1 and Disrupted in Schizophrenia 2 (DISC1 and DISC2), which are both novel genes that span a translocation breakpoint strongly associated with schizophrenia and related psychoses in a large Scottish family. A scan of the coding sequence, intron/exon boundaries, and part of the 5′ and 3′ untranslated regions of DISC1, plus 2.7 kb at the 3′ end of DISC2, has revealed a novel microsatellite and 15 novel single nucleotide polymorphisms (SNPs). We have tracked the inheritance of four of the SNPs through multiply affected families, and carried out case–control association studies using the microsatellite and four common SNPs on populations of patients with schizophrenia or bipolar affective disorder versus normal control subjects. Neither co‐segregation with disease status nor significant association was detected; however, we could not detect linkage disequilibrium between all these markers in the control population, arguing that an even greater density of informative markers is required to test rigorously for association in this genomic region.

A genome scan and follow-up study identify a bipolar disorder susceptibility locus on chromosome 1q42

In this study, we report a genome scan for psychiatric disease susceptibility loci in 13 Scottish families. We follow up one of the linkage peaks on chromosome 1q in a substantially larger sample of 22 families affected by schizophrenia (SCZ) or bipolar affective disorder (BPAD). To minimise the effect of genetic heterogeneity, we collected mainly large extended families (average family size >18). The families collected were Scottish, carried no chromosomal abnormalities and were unrelated to the large family previously reported as segregating a balanced (1 : 11) translocation with major psychiatric disease. In the genome scan, we found linkage peaks with logarithm of odds (LOD) scores >1.5 on chromosomes 1q (BPAD), 3p (SCZ), 8p (SCZ), 8q (BPAD), 9q (BPAD) and 19q (SCZ). In the follow-up sample, we obtained most evidence for linkage to 1q42 in bipolar families, with a maximum (parametric) LOD of 2.63 at D1S103. Multipoint variance components linkage gave a maximum LOD of 2.77 (overall maximum LOD 2.47 after correction for multiple tests), 12 cM from the previously identified SCZ susceptibility locus DISC1. Interestingly, there was negligible evidence for linkage to 1q42 in the SCZ families. These results, together with results from a number of other recent studies, stress the importance of the 1q42 region in susceptibility to both BPAD and SCZ.

An ALS2 gene mutation causes hereditary spastic paraplegia in a Pakistani kindred

Hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis (ALS) are genetically heterogeneous progressive neurodegenerative disorders distinguished by differential motor neuron involvement. In ALS, both upper and lower motor neurons are affected, whereas in HSP only upper motor neuron function is affected, resulting in a less severe disease. Recently, juvenile forms of ALS (ALS2) and primary lateral sclerosis (PLS) were shown to be caused by mutations in the ALS2 gene, which encodes a putative GTPase regulator. The ALS2 gene has 34 exons with at least two splice variants. The long variant (6,394 nucleotides) is expressed in various tissues with highest expression in the brain. The protein, alsin, shows similarity to three domains (RCC1, Pleckstrin-DB1, VPS9) and to membrane occupation and recognition nexus repeat motifs, which are characteristic of various guanine exchange factors (Ran, Rho, and Rab, respectively). We report the identification of a novel ALS2 mutation in a large consanguineous Pakistani kindred with infantileonset autosomal recessive complicated HSP. The proband initially presented with gait disturbance and hyperreflexia at 18 months. Currently, at age 12 years, she is anarthric and confined to a wheelchair. Family history indicates that the disease slowly progresses to tetraplegia and death by the fourth decade of life, with relatively preserved intellect. The clinical picture is similar to that recently reported by Eymard-Pierre and colleagues. Because our family is clinically related to previously reported ALS2 families and the fact that an HSP locus (SPG13) initially overlapped with the ALS2 locus, the proband was analyzed for mutations in the ALS2 gene using the denaturing high performance liquid chromatography (DHPLC)–WAVE system (Transgenomics, Mountain View, CA). Sequencing of a DHPLC variant in exon 32 showed a 1bp deletion (4844delT; Fig, a). Cosegregation of the mutation and the disease in the family was confirmed, and the mutation was absent in 155 control individuals. The deletion occurs at the beginning of the VPS9 domain and adds 43 unique residues to the truncated protein (see Fig, b). Our results show that absence of a functional VPS9 domain of alsin is sufficient to cause neurodegeneration. The yeast VPS9 protein and its mammalian homolog RABEX-5 are guanine nucleotide exchange factors for specific proteins thought to be involved in vacuolar endocytic transport. Disruption of intracellular trafficking has long been suggested to cause selective degeneration of the long axons of the pyramidal tract in HSP, but identification of additional mutations, functional studies, and animal models are necessary to further understand the pathogenesis resulting from ALS2 mutations.

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin‐coated vesicles that plays a role in clathrin‐mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1−/−). HIP1−/− mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1‐containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose‐dependent defect in clathrin‐mediated internalization of GluR1‐containing AMPA receptors was observed in neurons from HIP1−/− mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin‐mediated endocytosis.

The first nonsense mutation in alsin results in a homogeneous phenotype of infantile-onset ascending spastic paralysis with bulbar involvement in two siblings.

Eight mutations in the ALS2 gene have been described as causing autosomal‐recessive juvenile‐onset forms of the motor neuron diseases amyotrophic lateral sclerosis, primary lateral sclerosis and hereditary spastic paraplegia. All mutations are small deletions that are predicted to result in a frameshift and premature truncation of the alsin protein. Here we describe a ninth ALS2 mutation, in two siblings affected by infantile‐onset ascending spastic paraplegia with bulbar involvement. This mutation is predicted to result in the substitution of an amino acid by a stop codon, and thus is the first nonsense mutation detected in this gene. It is probable that full‐length alsin is required for the proper development and/or functioning of upper motor neurons.

Evolutionary constraints on the Disrupted in Schizophrenia locus.

The Disrupted in Schizophrenia (DISC) locus on human chromosome 1q42 has been strongly implicated by genetic studies as a susceptibility locus for major mental illnesses. In humans the locus is transcriptionally complex, with multiple alternate splicing events, antisense transcription, and intergenic splicing all evident. We have compared the genomic sequence and transcription maps of this locus between human, mouse, pufferfish (Fugu rubripes), and, in part, zebrafish (Danio rerio). The order and orientation of EGLN1, TSNAX, and DISC1 genes are conserved between mammals and F. rubripes. Intergenic splicing and short intergenic transcripts are not found to be conserved features. DISC2, a putative noncoding transcript partially antisense to DISC1, is not conserved in mouse or F. rubripes. Alternate splice forms of the protein-coding DISC1 gene are conserved even though the genomic structure is not. The amino acid sequence of DISC1 is diverging rapidly, although a putative nuclear localization signal and discrete blocks of coiled coil are specifically conserved features.

The genomic organisation of the metabotropic glutamate receptor subtype 5 gene, and its association with schizophrenia

The G-protein coupled metabotropic glutamate receptors (GRMs/mGluRs) have been implicated in the aetiology of schizophrenia as they modulate the NMDA response and that of other neurotransmitters including dopamine and GABA.1–3 Electrophysiological studies in GRM subtype 5 knockout mice reveal, in one study, a sensorimotor gating deficit characteristic of schizophrenia4 and in another, a key rôle for this gene in the modulation of hippocampal NMDA-dependent synaptic plasticity.5 In humans, GRM5 levels are increased in certain pyramidal cell neurons in schizophrenics vscontrols.6 Finally, GRM5 has been mapped to 11q14, neighbouring a translocation that segregates with schizophrenia and related psychoses in a large Scottish family, F23 (MLOD score 6.0).7,8 We determined the intron/exon structure of GRM5 and identified a novel intragenic microsatellite. A case-control association study identified a significant difference in allele frequency distribution between schizophrenics and controls (P = 0.04). This is suggestive of involvement of the GRM5 gene in schizophrenia in this population.

Chromosome Workshop: chromosomes 11, 14, and 15.

This report describes linkage data presented at the Workshop on Chromosomes 11, 14, and 15 at the Sixth World Congress of Psychiatric Genetics in Bonn, Germany, together with relevant linkage data submitted to the chair and co-chair, and it is presented in the context of the previous literature concerning these chromosomes. We have attempted to collate current linkage data to provide a guide to potentially interesting findings on chromosomes 11, 14, and 15 for the phenotypes of bipolar disorder, schizophrenia, alcoholism, autism, and spelling and reading disability. We discuss methodological limitations and provide chromosome ideograms and tables summarizing findings to date. The most promising region currently appears to be 15q13-q15 in the region of the alpha 7 nicotinic receptor for the phenotype of schizophrenia (and, perhaps, more generally for functional psychosis). Additionally, 15q11-q13 in the region of GABRB3 holds interest as a potential site of a susceptibility gene for autism. Two regions on chromosome 11, 11p15 in the region of tyrosine hydroxylase gene and 11q22-q23 in the region of DRD2, continue to retain some interest for functional psychosis.