A long-range restriction map across 3 Mb of the chromosome 11 breakpoint region of a translocation linked to schizophrenia: localization of the breakpoint and the search for neighbouring genes

Shifting towards a model of mGluR5 dysregulation in schizophrenia: Consequences for future schizophrenia treatment

Metabotropic glutamate receptor subtype 5 (mGluR5), encoded by the GRM5 gene, represents a compelling novel drug target for the treatment of schizophrenia. mGluR5 is a postsynaptic G-protein coupled glutamate receptor strongly linked with several critical cellular processes that are reported to be disrupted in schizophrenia. Accordingly, mGluR5 positive allosteric modulators show encouraging therapeutic potential in preclinical schizophrenia models, particularly for the treatment of cognitive dysfunctions against which currently available therapeutics are largely ineffective. More work is required to support the progression of mGluR5-targeting drugs into the clinic for schizophrenia treatment, although some obstacles may be overcome by comprehensively understanding how mGluR5 itself is involved in the neurobiology of the disorder. Several processes that are necessary for the regulation of mGluR5 activity have been identified, but not examined, in the context of schizophrenia. These processes include protein-protein interactions, dimerisation, subcellular trafficking, the impact of genetic variability or mutations on protein function, as well as epigenetic, post-transcriptional and post-translational processes. It is essential to understand these aspects of mGluR5 to determine whether they are affected in schizophrenia pathology, and to assess the consequences of mGluR5 dysfunction for the future use of mGluR5-based drugs. Here, we summarise the known processes that regulate mGluR5 and those that have already been studied in schizophrenia, and discuss the consequences of this dysregulation for current mGluR5 pharmacological strategies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

The DISC locus in psychiatric illness

The DISC locus is located at the breakpoint of a balanced t(1;11) chromosomal translocation in a large and unique Scottish family. This translocation segregates in a highly statistically significant manner with a broad diagnosis of psychiatric illness, including schizophrenia, bipolar disorder and major depression, as well as with a narrow diagnosis of schizophrenia alone. Two novel genes were identified at this locus and due to the high prevalence of schizophrenia in this family, they were named Disrupted-in-Schizophrenia-1 (DISC1) and Disrupted-in-Schizophrenia-2 (DISC2). DISC1 encodes a novel multifunctional scaffold protein, whereas DISC2 is a putative noncoding RNA gene antisense to DISC1. A number of independent genetic linkage and association studies in diverse populations support the original linkage findings in the Scottish family and genetic evidence now implicates the DISC locus in susceptibility to schizophrenia, schizoaffective disorder, bipolar disorder and major depression as well as various cognitive traits. Despite this, with the exception of the t(1;11) translocation, robust evidence for a functional variant(s) is still lacking and genetic heterogeneity is likely. Of the two genes identified at this locus, DISC1 has been prioritized as the most probable candidate susceptibility gene for psychiatric illness, as its protein sequence is directly disrupted by the translocation. Much research has been undertaken in recent years to elucidate the biological functions of the DISC1 protein and to further our understanding of how it contributes to the pathogenesis of schizophrenia. These data are the main subject of this review; however, the potential involvement of DISC2 in the pathogenesis of psychiatric illness is also discussed. A detailed picture of DISC1 function is now emerging, which encompasses roles in neurodevelopment, cytoskeletal function and cAMP signalling, and several DISC1 interactors have also been defined as independent genetic susceptibility factors for psychiatric illness. DISC1 is a hub protein in a multidimensional risk pathway for major mental illness, and studies of this pathway are opening up opportunities for a better understanding of causality and possible mechanisms of intervention.

Schizophrenia: a common disease caused by multiple rare alleles

Schizophrenia is widely held to stem from the combined effects of multiple common polymorphisms, each with a small impact on disease risk. We suggest an alternative view: that schizophrenia is highly heterogeneous genetically and that many predisposing mutations are highly penetrant and individually rare, even specific to single cases or families. This "common disease--rare alleles" hypothesis is supported by recent findings in human genomics and by allelic and locus heterogeneity for other complex traits. We review the implications of this model for gene discovery research in schizophrenia.

Invited commentary: taking the search for causes of schizophrenia to a different level

In recent years, epidemiologists have established major variations in the incidence of schizophrenia and have begun to investigate the causes of these variations. The report by Pedersen and Mortensen (Am J Epidemiol 2006;163:971-8) in this issue of the Journal examines the contribution of family-level factors to the urban-rural difference in the incidence of schizophrenia. Their results suggest that familial life in urban environments confers some effect that persists after families move to rural settings. Taking these findings together with those of previous studies, it appears that factors operating at the level of the social context, the family, and the individual may all contribute to the urban-rural difference in schizophrenia incidence. This work exemplifies an integrative, multilevel approach to epidemiologic research that employs principles central to eco-epidemiology and other, similar frameworks.

Chromosomal fragility, structural rearrangements and mobile element activity may reflect dynamic epigenetic mechanisms of importance in neurobehavioural genetics

Advances in human genome analyses have not yet allowed identification of specific genetic mechanisms underlying the expression of human neurobehavioural disorders. There is an increasing awareness that several genes may contribute to behavioural phenotypes and these genes appear to interact in as yet undetermined ways. It has been suggested that the problem needs elucidation from an epigenetic, gene expression perspective. Cytogenetic instability manifesting as chromosomal fragile sites, translocations, duplications, deletions and inversions, when co-occurring with neurobehavioural disorders, may offer a doorway to the investigation of such chromatin level, regulatory region, epigenetic processes. Due to earlier indications of non-specificity of chromosomal aberrations, poor phenotype:genotype correlations and a shift to analysing candidate coding regions on high resolution map level, the only utility of chromosomal breakpoints came to be seen as harbouring possible candidate genes of interest when segregating together with particular neurobehavioural disorders. More recent findings of the expression of highly specific subsets of fragile sites in association with Tourette and Rett syndromes need to be extended to other neurobehavioural disorders to ascertain whether observed patterns can be considered representative of 'chromatin endophenotypes' correlating with discrete sets of neurobehavioural symptoms. Environmental/epigenetic factors could affect the chromatin characteristics of the genome arising through DNA strand breakage, mobile element activity and retroinsertion, establishing new architectural features of regulatory control networks very rapidly in comparison to coding region evolution rates. Microarray-based techniques for the genome-wide mapping of in vivo protein-DNA interactions offer increasingly comprehensive views of genetic and epigenetic regulatory networks. It may be informative to include functionally significant chromatin structural variation analyses when considering candidate genes for neurobehavioural disorders.

Linkage studies of schizophrenia

The study of schizophrenia genetics has revealed much about the disease but none of the essential secrets of its etiology, so far, for numerous reasons. First, schizophrenia is a complex trait, influenced by both genes and environment. Second, it appears to be a highly heterogeneous disease, with locus and allelic heterogeneity both between and within families likely. Third, since it is common, it is likely that the genetic liability variants are common, and so are found with relatively high frequency in the general population. Fourth, linkage methods, which deliver rapid coverage of the genome, have great power to identify single genes causing Mendelian disorders but are poorly suited to the genetic architecture of complex traits. Although association methods are undeniably more powerful in such situations, affordable technologies to deliver the much higher density whole genome coverage required are not yet available and candidate gene studies of schizophrenia have not produced robust and replicable results. In spite of these limitations, there are now sufficient data to support several conclusions. Numerous regions of the human genome give consistent, though by no means unanimous, support for linkage. The precise nature of these signals is not yet understood, and power to position the effects is poor, but metanalyses show the co-occurrence is unlikely to be due to chance. Combined approaches utilizing linkage for rapid genome coverage and association for fine-scale follow-up have identified several promising candidate genes. Although the definition of replication in a complex trait is itself complex, a number of these candidates have been supported by numerous studies. These converging lines of evidence suggest that the genetics of schizophrenia, long considered a most intractable problem, are at last beginning to be unraveled.

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 schizophrenia and in another, a key rôle for this gene in the modulation of hippocampal NMDA-dependent synaptic plasticity. In humans, GRM5 levels are increased in certain pyramidal cell neurons in schizophrenics vs controls.(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). 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.

Identification of Genes from a Schizophrenia-Linked Translocation Breakpoint Region

The translocation t(1:11)(q42.1,q14.3) has previously been found to be linked with schizophrenia. Genes present at the chromosome 1 breakpoint have been investigated in some detail but little was known about genes in the chromosome 11 breakpoint region. Here we report a BAC clone contig encompassing 2.51 Mb around the chromosome 11 breakpoint, which was constructed computationally using draft genomic sequence data and existing mapping data for the region. The contig includes 26 clones and has led to the identification and relative ordering of 10 candidate genes in the region, including 2 novel transcripts. It constitutes a resource for polymorphic marker discovery and association studies to validate or reject candidate genes. Four candidate genes appear to be particularly promising based upon their proximity to the breakpoint and their likely functional roles. Three of these are involved in glutamatergic neurotransmission (the glutamate receptor GRM5, NAALADase II, and a close homolog), perturbation of which is one of the most widely held theories on the underlying biochemistry of schizophrenia. The 4th gene, tyrosinase, has been previously linked to schizophrenia through the cosegregation of oculocutaneous albinism with psychosis in several pedigrees.

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.