GeNome ‐ Wide aSSociatioN StUdieS iN SchizoPhreNia , aNd PoteNtial etioloGical aNd FUNctioNal imPlicatioNS oF their reSUltS

Background: despite the fact that the genetic basis of schizophrenia has been intensively studied for more than two decades, our contemporary knowledge in this field is rather fractional, and a substantial part of it is still missing. The aim of this review article is to sum up the data coming from genome ‐wide association genetic studies in schizophrenia, and indicate prospective directions of further scientific endeavour. Methods: We searched the National Human Genome Research institute’s catalog of genome ‐wide association studies for schizophrenia to identify all papers related to this topic. In consequence, we looked up the possible relevancy of these findings for etiology and pathogenesis of schizophrenia using the computer gene and pubMed databases. Results: eighteen genome ‐wide association studies in schizophrenia have been published till now, referring to fifty ‐seven genes supposedly involved into schizophrenia’s etiopathogenesis. Most of these genes are related to neurodevelopment, neuroendocrinology, and immunology. conclusions: it is reasonable to predict that complex studies of sufficiently large samples, involving detection of copy number variants and assessment of endopheno‐ types, will produce definitive discoveries of genetic risk factors for schizophrenia in the future.

Review aRticle introduction schizophrenia (sZ) is a chronic disabling disease of the brain.sZ affects 0.5-1 percent of the adult population worldwide.it is commonly manifested by auditory hallucinations, paranoid or bizarre delusions, and disorganized speech and thinking.Schizophrenia results in a significant social or occupational dysfunction.nine to thirteen percent of patients with schizophrenia eventually commit suicide (9). the causes of sZ consist of genetic and environmental factors (27).heritability of schizophrenia is usually mentioned in the range 0.4-0.7 (32).even though we already have certain empirical data about the genetic basis of schizophrenia that implicate specific DNA loci (17), our recent knowledge on the genetics of schizophrenia is still nascent.
a genome -wide association study (gwas), also known as a whole genome association study (wgas), is an examination of frequencies of single nucleotide polymorphisms in most of the genes of different individuals with vs without a certain disease in order to see how much the polymorphisms in genes vary among the affected (cases) as against the unaffected subjects (controls).the individuals are tested for single dna mutations (single -nucleotide polymorphisms, snps), but gwass are also able to detect copy number variants (cnvs; dna deletions or duplications).
acta Medica (hradec Králové) 2012; 55: [3][4][5][6][7][8][9][10][11] a common practice is that the original gwas is subsequently replicated by the same authors in an independent sample.gwass incorporate the power to detect small effects with the advantage of the positional genetics design, which requires no specific knowledge of pathogenesis.The first genome -wide association study of age -related macular degeneration appeared in 2005 (19).since then, more than one thousand gwas articles have been published in the national human genome Research institute (nhgRi) gwas catalog (30), as accessed on the 23rd of september, 2011.the nhgRi is one of the 27 national institutes of health established pursuant to federal legislation in the U.s. GWASs are usually useful in finding the molecular pathways of the disease (47).genome -wide association studies recently represent the most comprehensive procedure to discover a genetic background of complex diseases.the aim of this article is to sum up the recent gwas findings in SZ patients, and imply their possible significance in schizophrenia etiopathogenesis.

methods
we searched the nhgRi gwas catalog (30) for schizophrenia on 25th september 2011.afterwards, we attempted to ascertain the potential etiopathogenetic role of the genes found to be associated with sZ using computer gene databases (28,46).lastly, we discussed a prospective meaning of these findings, their limitations, and the direction for a further research.

results
eighteen relevant published gwas studies were found by a selecting program.their key description and results are displayed in tables 1a and 1b.
the potential etiologic and functional role of the genes associated with schizophrenia according to the presented gwass is shown in tables 2a, 2b, and 2c (28,46).note: in the tables 2a, 2b, and 2c, the genes related to neurodevelopment and neuroplasticity are written in bold.names of the genes in relation to immunology are in italics and the genes related to neuroendocrinology are underlined.the association studies for each gene can be found at http://www .schizophreniaforum.org (schizophrenia Research forum).
for most genes listed in tables 2a, 2b, and 2c, no results were found in the pubMed database (29) on the 4th of october, 2011 using the name of the gene and schizophrenia as key words.nevertheless, a possible role of certain genes in schizophrenia etiology and pathophysiology was already mentioned.this is recapitulated in the following text.
according to Berretta (6), emerging evidence points to the involvement of the brain extracellular matrix (ecM) in the pathophysiology of schizophrenia.solid evidence supports the involvement of reelin, an ecM glycoprotein related to corticogenesis, synaptic functions and glutamate nMda receptor regulation.Reelin is expressed prevalently in gaBaergic neurons, which secrete it into the ecM.Marked changes of reelin expression in sZ have typically been reported in association with gaBa -related abnormalities.ecM anomalies may contribute to disrupted connectivity and neuronal migration, synaptic abnormalities and altered gaBaergic, glutamatergic and dopaminergic neurotransmission in schizophrenia.
pRodh (proline dehydrogenase) gene, encoding proline oxidase (poX), has been associated with schizophrenia through linkage, association, and the 22q11 deletion syndrome (16).the pRodh gene polymorphisms are related to structure, function, and connectivity of striatum and prefrontal cortex, which was found in a family -based sample.this circuitry is implicated in the pathophysiology of schizophrenia. the schizophrenia risk haplotype was associated with decreased striatal volume and increased striatal -frontal functional connectivity.according to li et al. (24), pRodh gene was associated with executive function in schizophrenic families (n = 167) in china.
wang et al. (45) summed up the knowledge on the role of the notch4 locus (neurogenic locus notch homolog protein 4) in sZ. the notch4 gene was found to be associated with schizophrenia among the British population in 2000.the results from independent studies are inconsistent.allelic heterogeneity, heterogeneity of clinical diagnosis, ethnic vari-ance of researched population, and linkage disequilibrium structures may be the reasons for a poor replication.a part of the studies suggested that the notch4 gene could play a role in a subgroup of the disease, such as early -onset schizophrenia and negative symptoms.a single study revealed a strong association with the frontal lobe cognitive performance.according to wang et al. (45), the notch4 gene may be associated with schizophrenia, but we do not still know how the gene contributes to the etiology of this illness.
Roussos et al. (38) revealed that the anK3 (ankyrin 3) rs9804190 c allele was associated with lower anK3 mRna expression levels, higher risk for schizophrenia, and poorer working memory and executive function performance in the case -control analysis in sZ patients (n = 272) versus healthy controls (n = 513).lennertz et al. (23) analysed the impact of the tcf4 (transcription factor 4) variant rs9960767 on early information processing and cognitive functions in schizophrenic patients (N = 401).TCF4 influenced verbal memory in the Rey auditory verbal learning test.tcf4 did not impact on various other cognitive functions in the domains of attention and executive functions.the sZ risk allele c of tcf4 rs9960767 reduced sensorimotor gating as measured by prepulse inhibition in electrophysiological examination.this indicates that TCF4 influences key mechanisms of information processing, which may contribute to the pathogenesis of schizophrenia. the role of the sZ risk allele c of the tcf4 rs9960767 polymorphism in disrupting sensorimotor gating in schizophrenia spectrum and healthy volunteers was also confirmed by Quednow et al. (36).
Based on a systematic review of literature, crespi et al. (11) concluded that the hla -dRB1*13 alleles are associated with a higher risk of schizophrenia compared to the hla -dRB1*04 alleles.the risk of individual dRB1 (dR beta 1) variants for rheumatoid arthritis is quite the opposite.These findings from genetics and epidemiology imply that a subset of schizophrenia cases may be underlain by genetically based neuroimmune alterations.analyses of the causes of risk and protective effects from dRB1 variants may provide new approaches to therapy.
according to Krug et al. (20), neurogranin is the main postsynaptic protein regulating the availability of calmodulin -ca ++ in neurons.neurogranin is expressed exclusively in the brain, particularly in dendritic spines and has been implicated in spatial learning and hippocampal plasticity.Krug et al. having used cognitive tests and functional magnetic resonance imaging in a large sample of healthy subjects demonstrated that rs12807809 of the neurogranin gene is associated with differential neural functioning in the anterior and posterior cingulate.these areas are involved in episodic memory processes and have been implicated in the pathophysiology of sZ in structural and functional imaging as well as post mortem studies.hashimoto et al. (13) mentioned that functional magnetic resonance imaging studies in healthy subjects demonstrated the association of the ZNF804A (zinc finger protein 804A) variants with neural activation during a memory task. in their own research of 113 patients with schizophrenia and 184 healthy subjects, hashimoto et al. investigated the potential relationship between the Znf804a rs1344706 polymorphism and memory function.patients with sZ exhibited poorer performance on verbal memory, visual memory, attention/concentration and delayed recall as compared to healthy control subjects (p < 0.001).patients with the highrisk ZNF804A T/T genotype scored significantly lower on visual memory than the g carriers did (p = 0.018).this data suggests that the Znf804a gene rs1344706 polymorphism may be related to memory dysfunction in schizophrenia.
according to Bennett (5), gray matter loss in the cortex is extensive in schizophrenia, especially in the prefrontaltemporal -network (ptn).several molecules such as neuregulin -1 (nRg1) and its erbB4 receptor are encoded by candidate susceptibility genes for schizophrenia. it is suggested that one pathway involves nRg1/erbB4 determining the efficacy of N -methyl -D -aspartate receptors (NMDARs) found on dendritic spines at synapses in the ptn.another pathway involves nRg1/erbB determining the proliferation and differentiation of oligodendrocytes in the white matter as well as their capacity for myelination.in schizophrenia, a causal chain is established between dysfunctional products of susceptibility genes, the decrease of dendritic spines and synaptic terminals, and the loss of gray matter.similarly, pitcher et al. (34) propose that nRg1 -erbB4 signalling participates in cognitive dysfunction in sZ by the aberrant suppression of src -mediated enhancement of synaptic nMdaRs function.
the possible involvement of the ptgs2 (prostaglandinendoperoxide synthase 2) gene and inflammatory mechanisms in etiopathogenesis of schizophrenia may be the explanation for the antipsychotic effect of the antiinflammatory COX -2 inhibitors (26).discussion according to duan et al. (12), the data proceeding from gwas studies provide evidence for: 1. a number of chromosomal regions with common polymorphisms showing genome -wide association with schizophrenia, but only presenting small odds ratios.2. polygenic inheritance.3. involvement of rare (< 1%) and large (> 100 kb) copy number variants (cnvs) that have fairly large effect sizes on disease risk.4. a genetic overlap of schizophrenia with autism and bipolar disorder.
several candidate genes involved in neurodevelopment have been suggested in schizophrenia by the gwas studies (efhd1, eMl5, Reln, anK3, tcf4, nRg1, lnX2 etc.).therefore, the genetic basis of schizophrenia could involve different factors more or less specifically required for neuroplasticity, including the synapse maturation, as well as neurogenesis.
the presence of cytokine receptor abnormalities in schizophrenia may help to explain prior epidemiologic data relating the risk for this illness to altered rates of autoimmune disorders, prenatal infection and familial leucemia (22).
we can sum up, that the results of gwas studies implicate the significance of neurodevelopmental, neuroplastic, neuroprotective, neuroendocrinological, and immunological factors in etiology and pathogenesis of schizophrenia.
although the new data from gwass in schizophrenia are promising, they still do not meet our initial expectations (8).an obvious question is why the results of genome -wide association studies in schizophrenia do not correspond with the findings of earlier association studies aimed only at several genes or polymorhpisms?these previously discovered genes were reviewed by schwab et al. (39).the authors covered the Regulator of g -protein signalling 4 (Rgs4), d -amino acid oxidase activator (g72; daoa), neuregulin 1 (nRg1), dystrobrevin binding protein 1 (dysbindin), phosphatidylinositol -5 -phosphate -4 -kinase type ii -alpha (pip4K2a), v -akt murine thymoma viral oncogene homologue 1 (protein kinase B) (aKt1), several dopamine receptors (dRd1, dRd2, dRd3, dRd4), and disrupted in schizophrenia (disc1) genes.this might indicate that effect sizes previously reported in schizophrenia non -gwas genetics might have been overestimated.
another problem in schizophrenia genetics is that rare variants with large effect have a very low frequency in the general population and therefore will not be detected by the population -based gwas strategy.this may be overcome by studying families and ethnically homogenous populations (2).
the reason why the results of individual gwass in schizophrenia do not overlap may lie in the fact that they do not cover the same polymorphisms.the dissimilar results may also be explained by clinical or ethnic differences among the studied populations.
among clinical factors, variability of the phenotype is a major limitation in genetic research of schizophrenia (7).genetic problems include locus heterogeneity and the complex genetic architecture of the phenotype.some genes may be disease -causing, whilst others only disease -modifying in each individual.endophenotypes instead of the complex nosological entity of schizophrenia may be more appropriate for a genetic research because of their good detectability, and a relative simplicity (3).
the psychiatric gwas consortium systematically conducts gwas meta -analyses.past experiences suggest that for some disorders as many as 20,000 to 30,000 case subjects and similar number of comparison subjects are required to obtain highly robust findings (35).This may be the arrangement in future sZ genetic studies.
according to lee et al. (21), there is a pressing need to better integrate the multiple research platforms including biology computational models, genomics, epigenetics, cross disorder phenotyping studies, transcriptomics, proteomics, metabolomics, neuroimaging and clinical correlations in the studies on psychoses, including schizophrenia.conclusions the up to now results of gwas studies point out that the genetic background of schizophrenia is mostly related to neurodevelopmental, neuroendocrinological, and immuno-logical factors.it is reasonable to predict that elaborate studies of sufficiently large samples will produce definitive discoveries of genetic risk factors for schizophrenia in the future.Interleukin 3 receptor, alpha (low affinity) a subunit of a cytokine receptor note: in the tables 2a, 2b and 2c, the genes related to neurodevelopment and neuroplasticity are written in bold.names of the genes in relation to immunology are in italics and the genes related to neuroendocrinology are underlined.the association studies for each gene can be found at http://www.schizophreniaforum.org (schizophrenia Research forum).

tab. 1a:
the survey of genome -wide association studies in schizophrenia