Will schizophrenia become a graveyard for molecular geneticists?

Unraveling the genetic architecture of copy number variants associated with schizophrenia and other neuropsychiatric disorders

Recent studies show that the complex genetic architecture of schizophrenia (SZ) is driven in part by polygenic components, or the cumulative effect of variants of small effect in many genes, as well as rare single-locus variants with large effect sizes. Here we discuss genetic aberrations known as copy number variants (CNVs), which fall in the latter category and are associated with a high risk for SZ and other neuropsychiatric disorders. We briefly review recurrent CNVs associated with SZ, and then highlight one CNV in particular, a recurrent 1.6-Mb deletion on chromosome 3q29, which is estimated to confer a 40-fold increased risk for SZ. Additionally, we describe the use of genetic mouse models, behavioral tools, and patient-derived induced pluripotent stem cells as a means to study CNVs in the hope of gaining mechanistic insight into their respective disorders. Taken together, the genomic data connecting CNVs with a multitude of human neuropsychiatric disease, our current technical ability to model such chromosomal anomalies in mouse, and the existence of precise behavioral measures of endophenotypes argue that the time is ripe for systematic dissection of the genetic mechanisms underlying such disease. © 2016 Wiley Periodicals, Inc.

Genetic Evaluation of Schizophrenia Using the Illumina HumanExome Chip

INTRODUCTION

Schizophrenia is a genetically heterogeneous disorder that is associated with several common and rare genetic variants. As technology involved, cost advantages of chip based genotyping was combined with information about rare variants, resulting in the Infinium HumanExome Beadchip. Using this chip, a sample of 493 patients with schizophrenia or schizoaffective disorder and 484 healthy controls was genotyped.

RESULTS

From the initial 242901 SNVs, 88306 had at least one minor allele and passed quality control. No variant reached genomewide-significant results (p<10(-8)). The SNP with the lowest p-value was rs1230345 in WISP3 (p = 3.05*10(-6)), followed by rs9311525 in CACNA2D3 (p = 1.03*10(-5)) and rs1558557 (p = 3.85*10(-05)) on chromosome 7. At the gene level, 3 genes were of interest: WISP3, on chromosome 6q21, a signally protein from the extracellular matrix. A second candidate gene is CACNA2D3, a regulator of the intracerebral calcium pathway. A third gene is TNFSF10, associated with p53 mediated apoptosis.

What have the genomics ever done for the psychoses?

BACKGROUND

Despite the substantial heritability of the psychoses and their genuine public health burden, the applicability of the genomic approach in psychiatry has been strongly questioned or prematurely dismissed.

METHODS

selective review of the recent literature on molecular genetic and genomic approaches to the psychoses including the early output from genome-wide association studies and the genomic analysis of DNA structural variation.

RESULTS

Susceptibility variants at strong candidate genes have been identified including neuregulin, dysbindin, DISC1 and neurexin 1. Rare but highly penetrant copy number variants and new mutations affecting genes involved in neurodevelopment, cell signalling and synaptic function have been described showing some overlapping genetic architecture with other developmental disorders including autism. The de-novo mutations described offer an explanation for the familial sporadic divide and the persistence of schizophrenia in the population. The functional effects of risk variants at the level of cognition and connectivity has been described and recently, ZNF804A has been identified, and the MHC re-identified as risk loci, and it has been shown that at least a third of the variation in liability is due to multiple common risk variants of small effect with a substantial shared genetic liability between schizophrenia and bipolar affective disorder.

CONCLUSIONS

The genomics have done much for the psychoses to date and more is anticipated.

Psychiatric genetics: back to the future

For the last decade or more geneticists have been predicting that advances in molecular genetics are going to revolutionize our understanding of psychiatric disorders and human behavior. However, with a few exceptions, these expectations have yet to be fulfilled. As the century draws to a close and we contemplate the prospect of the complete sequence of the human genome it seems timely to consider the state of the field and to consider carefully how it might advance, the problems to be faced and the resources required. Molecular Psychiatry (2000) 5, 22-31.

Esquizofrenia

Neste artigo revisamos e resumimos os avanços atuais sobre o mapeamento de genes relacionados à esquizofrenia. Listamos as regiões de interesse identificadas até o momento e discutimos as dúvidas pertinentes, bem como as perspectivas para o sucesso futuro.

The molecular genetics of schizophrenia: progress so far

In 1988, a report of a genetic linkage between schizophrenia and markers on chromosome 5 caused considerable excitement. Many hoped that a cause for schizophrenia had been found. Unfortunately, subsequent results failed to replicate the finding, and there was little progress in the molecular understanding of the disorder over the next five years. However, within the past two years, there have been reports of positive linkages on chromosome arms 22q and 6p that, unlike previous reports, have received support from several teams. Here, we review the evidence for these linkages, as well as findings from association studies that have not yet received as much independent confirmation.

A critical review of genetic studies of schizophrenia. II. Molecular genetic studies

Recent molecular genetic studies of schizophrenia have, until now, been unable to demonstrate any specific major gene for schizophrenia. On the contrary, linkage and association studies have yielded almost exclusively negative or contradictory results. Such studies have involved certain candidate genes, such as the genes for dopamine receptors and other brain neurotransmitters. Some of these candidate genes have now actually been excluded as specific aetiological factors in schizophrenia. Similarly, studies searching for a major gene for susceptibility to schizophrenia involving the whole human genome or large parts of chromosomes have not yielded unambiguously positive results. However, the most recent empirical evidence suggests that many polygenes, acting together, could constitute a risk factor for schizophrenia. It is thus most probable that genetic susceptibility to schizophrenic psychoses is polygenic, and that their effects are dependent on interaction with physical and psychosocial environmental factors.

Schizophrenia, bacterial toxins and the genetics of redundancy

The hypothesis proposed is that episodes of bacterial toxaemia occurring early in life cause subtle brain damage which predisposes to schizophrenia. The damage is most likely to occur in individuals who have one or more mutations in a large set of genes controlling aspects of the immune response to infection. These genes, numbering several hundreds or even thousands, control a vital but highly redundant system. Schizophrenia, by reducing fertility, causes selective pressure against the mutations which would otherwise accumulate in the genome and degrade our ability to fight infection. This hypothesis is consistent with the key features of the disease which include a strong genetic component, neuroanatomical abnormalities, a seasonal influence, lack of association with HLA subtypes and the observation that the frequency of schizophrenia is similar in widely different populations. If bacterial toxins also have a role in precipitating acute psychosis, then the age incidence of schizophrenia can be explained.

No association between schizophrenia and polymorphisms within the genes for debrisoquine 4-hydroxylase (CYP2D6) and the dopamine transporter (DAT)

It has been suggested that the cytochrome P450 mono-oxygenase, debrisoquine 4-hydroxylase, is involved in the catabolism and processing of neurotransmitters subsequent to their reuptake into target cells. It is also thought to be related to the dopamine transporter that acts to take released dopamine back up into presynaptic terminals. The present study used the association approach to test the hypothesis that mutations in the genes for debrisoquine 4-hydroxylase (CYP2D6) and the dopamine transporter (DAT) confer susceptibility to schizophrenia. There were no differences in allele or genotype frequencies between patients and controls in the mutations causing the poor metaboliser phenotype in CYP2D6. In addition there was no association found between schizophrenia and a 48bp repeat within the 3' untranslated region of DAT.

Schizophrenia epigenesis: past, present, and future

Sociopolitical and ideological attacks on the various genetically oriented programs of research into the causes of schizophrenia, even when well-motivated to prevent genetic discrimination in all its forms, endangers the enterprise. Although there have been no replicated successes in finding genes linked or associated to markers for schizophrenia yet, the solid foundation for the fact that genetic factors are importantly involved in the etiology of schizophrenia derives from replicated studies using the strategies of genetic epidemiology--families, twins, adoptees. The models to emulate in schizophrenia research should follow the paths of researchers on coronary artery disease, diabetes, and Alzheimer's disease. All are complex diseases with multifactorial and multigenic components. Even when the necessary genotype is present, it may not be expressed as clinical disease at the phenotypic level.

The molecular genetics of schizophrenia

There is strong evidence for a genetic component in schizophrenia but its precise nature remains unclear. Positional cloning and studies of potential candidate genes offer prospects for progress. The diagnosis of schizophrenia can now be made reliably but questions remain over the most valid phenotypic definition. To deal with this and uncertainties regarding mode of transmission a 'polydiagnostic' approach is advisable. A wealth of new DNA markers has enhanced the potential for linkage studies which have so far focused on large multiply-affected families. Multi-centre collaborative studies that are currently under way are likely to identify genes of major effect but other strategies are required if it turns out that most cases result from the combined effect of multiple genes.

Absence of linkage between schizophrenia and the dopamine D4 receptor gene

The dopamine D4 receptor gene and the closely placed tyrosine hydroxylase (TH) receptor gene are important candidate genes for schizophrenia; both are located on the short arm of chromosome 11. Multipoint linkage analyses excluded linkage of schizophrenia/schizoaffective disorder to both candidate genes in a sample of 15 multiplex and systematically recruited families. This result was not dependent on the definition of the affection status and on the specification of the mode of transmission (insofar as it is monogenic) of the disease. There was no evidence for a subgroup of families being linked. This result does not preclude the possibility that the D4 receptor gene or the TH gene has only a nonmajor effect on the genetic etiology of schizophrenia or that families in other populations are linked.

Pseudoautosomal region in schizophrenia: linkage analysis of seven loci by sib-pair and lod-score methods

In a previous study, we reported a nonrandom segregation between schizophrenia and the pseudoautosomal locus DXYS14 in a sample of 33 sibships. That study has been extended by the addition of 16 new sibships from 16 different families. Data from six other loci of the pseudoautosomal region and of the immediately adjacent part of the X specific region have also been analyzed. Two methods of linkage analysis were used: the affected sibling pair (ASP) method and the lod-score method. Lod-score analyses were performed on the basis of three different models--A, B, and C--all shown to be consistent with the epidemiological data on schizophrenia. No clear evidence for linkage was obtained with any of these models. However, whatever the genetic model and the disease classification, maximum lod scores were positive with most of the markers, with the highest scores generally being obtained for the DXYS14 locus. When the ASP method was used, the earlier finding of nonrandom segregation between schizophrenia and the DXYS14 locus was still supported in this larger data set, at an increased level of statistical significance. Findings of ASP analyses were not significant for the other loci. Thus, findings obtained from analyses using the ASP method, but not the lod-score method, were consistent with the pseudoautosomal hypothesis for schizophrenia.

Sequential strategy to identify a susceptibility gene for schizophrenia: report of potential linkage on chromosome 22q12-q13.1: Part 1

To identify genes responsible for the susceptibility for schizophrenia, and to test the hypothesis that schizophrenia is etiologically heterogeneous, we have studied 39 multiplex families from a systematic sample of schizophrenic patients. Using a complex autosomal dominant model, which considers only those with a diagnosis of schizophrenia or schizoaffective disorder as affected, a random search of the genome for detection of linkage was undertaken. Pairwise linkage analyses suggest a potential linkage (LRH = 34.7 or maximum lod score = 1.54) for one region (22q12-q13.1). Reanalyses, varying parameters in the dominant model, maximized the LRH at 660.7 (maximum lod score 2.82). This finding is of sufficient interest to warrant further investigation through collaborative studies.

Bipolar affective puerperal psychosis associated with consanguinity

Most clinical and genetic evidence suggests that puerperal psychosis is closely related to bipolar affective disorder. During a linkage study of bipolar disorder we ascertained a British family in which puerperal psychosis was associated with consanguinity in three sisters. All three subjects had lifetime RDC diagnoses of bipolar I or manic disorder. An inbred brother also had bipolar I disorder. The only female member of the sibship to escape puerperal psychosis was outbred. These findings are consistent with several genetic models for bipolar disorder in this family. The most interesting possibility is a single major susceptibility locus of recessive effect. Under this assumption, the family could be used for homozygosity mapping to help localise the putative recessive locus. If other inbred families can be found in which the same putative recessive locus is operating, the power to localise the gene by homozygosity mapping would be greatly increased.

No association between schizophrenia and homozygosity at the D3 dopamine receptor gene

The D3 dopamine receptor gene is an important candidate gene for schizophrenia, since (because of its almost exclusive expression in the limbic system) it combines the dopamine receptor hypothesis with the limbic system hypothesis of schizophrenia. A BalI restriction fragment length polymorphism of the D3 dopamine receptor gene has been typed in 107 schizophrenic patients and 98 normal controls from Sichuan (China). With regard to alleles or genotypes, no significant differences were obtained between controls from Europe and China, between patients and controls, and between patient subgroups and controls. These results indicate a lack of association between schizophrenia and the D3 dopamine receptor gene in our sample. Our findings are at variance with reports of a significant excess of homozygosity at the D3 dopamine receptor gene in schizophrenic patients from Wales (United Kingdom) and Alsace (France). In conclusion, further studies will be needed with larger samples of patients from Wales and Alsace as well as with samples of different racial groups to prove or disprove the initial positive association between schizophrenia and genotypes of the D3 dopamine receptor gene.

Association between schizophrenia and homozygosity at the dopamine D3 receptor gene

Disturbances in dopamine neurotransmission have been postulated to underlie schizophrenia. We report data from two independent studies of a BalI polymorphism in the dopamine D3 receptor gene in patients with schizophrenia. In both studies, more patients than controls were homozygous (p = 0.005, p = 0.008). When pooled data were analysed, this difference was highly significant (p = 0.0001) with a relative risk of schizophrenia in homozygotes of 2.61 (95% confidence intervals 1.60-4.26).