Am J Pharmacogenomics 2005; 5 (3): 149-160

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 1. Issue in Psychiatric Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 1.1 Pleiotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 1.2 Epigenetic Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 1.3 Polygeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 1.4 Ethics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 2. Genetic Studies in the Field of Psychiatry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 2.1 Brain Dopaminergic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 2.1.1 Cathechol-O-Methyl Transferase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 2.1.2 Dopamine Transporter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 2.1.3 Dopamine D1 receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 2.1.4 Dopamine D2 Receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 2.1.5 Dopamine D2-like receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 2.2 Brain-Derived Neurotrophic Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 2.3 Reelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 2.4 Serotoninergic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 2.5 Glutaminergic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 2.6 Other Genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 3. Gene-Gene and Gene-Environment Interactions: an Integrative Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 4. Clinical Implications of Genetic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 No specific gene has been identified for any major psychiatric disorder, including schizophrenia, in spite of Abstract strong evidence supporting a genetic basis for these complex and devastating disorders. There are several likely reasons for this failure, ranging from poor study design with low statistical power to genetic mechanisms such as polygenic inheritance, epigenetic interactions, and pleiotropy. Most study designs currently in use are inadequate to uncover these mechanisms. However, to date, genetic studies have provided some valuable insight into the causes and potential therapies for psychiatric disorders. There is a growing body of evidence suggesting that the understanding of the genetic etiology of psychiatric illnesses, including schizophrenia, will be more successful with integrative approaches considering both genetic and epigenetic factors. For example, several genes including those encoding dopamine receptors (DRD2, DRD3, 150 Abdolmaleky et al. and DRD4), serotonin receptor 2A (HTR2A) and catechol-O-methyltransferase (COMT) have been implicated in the etiology of schizophrenia and related disorders through meta-analyses and large, multicenter studies. There is also growing evidence for the role of DRD1, NMDA receptor genes (GRIN1, GRIN2A, GRIN2B), brain-derived neurotrophic factor (BDNF), and dopamine transporter (SLC6A3) in both schizophrenia and bipolar disorder. Recent studies have indicated that epigenetic modification of reelin (RELN), BDNF, and the DRD2 promoters confer susceptibility to clinical psychiatric conditions. Pharmacologic therapy of psychiatric disorders will likely be more effective once the molecular pathogenesis is known. For example, the hypoactive alleles of DRD2 and the hyperactive alleles of COMT, which degrade the dopamine in the synaptic cleft, are associated with schizophrenia. It is likely that insufficient dopaminergic transmission in the frontal lobe plays a role in the development of negative symptoms associated with this disorder. Antipsychotic therapies with a partial dopamine D2 receptor agonist effect may be a plausible alternative to current therapies, and would be effective in symptom reduction in psychotic individuals. It is also possible that therapies employing dopamine D1/D2 receptor agonists or COMT inhibitors will be beneficial for patients with negative symptoms in schizophrenia and bipolar disorder. The complex etiology of schizophrenia, and other psychiatric disorders, warrants the consideration of both genetic and epigenetic systems and the careful design of experiments to illumine the genetic mechanisms conferring liability for these disorders and the benefit of existing and new therapies. 1. Issue in Psychiatric Genetics common gene. Dopamine receptor genes (DRD2, for example) and serotonin receptor and transporter genes (e.g. HTR2A and Epidemiologic data and familial studies strongly suggest a SLC6A4) may be considered pleiotropic genes, since they have genetic basis for major psychiatric disorders. During the past 2 various manifestations in different organs and developmental peridecades, more than 1000 linkage and associations studies have ods. For example, DRD2 hypoactivity may result in attentionbeen conducted in a worldwide search to find the genes underlying deficit hyperactivity disorder (ADHD) during childhood, drug psychiatric disorders. Almost all chromosomes harbor genes reabuse in adolescence, and post-traumatic stress disorder (PTSD) ported to be linked to psychiatric disorders and at least 150 genes and depressive disorders in adulthood.[3] It is also likely that this have been implicated in the pathogenesis of schizophrenia and gene has a role in schizophrenia pathogenesis.[4] Also, since esbipolar disorder. However, no specific gene has been definitively trogen up-regulates the DRD2 gene,[5] family members who have a identified as having a major or even moderate role in these dysfunctional gene may have varying phenotypes depending upon devastating disorders. Part of this failure could be due to the gender. This can confound family genetic studies examining specomplexity of mental disorders; the following issues may also cific genetic associations. For example, the COMT Val158 allele contribute to the difficulty of gene discovery for these diseases. (the overactive allele of the Val158Met polymorphism) is associ1.1 Pleiotropy ated with schizophrenia in males but not females.[6] Since estrogen down-regulates COMT gene expression,[7] the effects of this risk Pleiotropy is one issue that is often overlooked and may be very allele for schizophrenia may be attenuated by gender and ageimportant in the field of psychiatric genetics. Pleiotropy occurs related estrogen down regulation. The same scenario is likely true when one gene has several functions in different tissues at the for the DRD2 Taq1A1 (hypoactive) allele of the Taq1A restriction same time or during different developmental periods.[1] Thus, if a fragment length polymorphism (RFLP), which is related to drug pleiotropic gene is dysfunctional there may be several abnormal abuse in men, but not in women.[3] In this case, the estrogen that manifestations in a specific period or in different life stages. It is up-regulates the DRD2 gene expression[5] may compensate for well known that genes can be activated during critical periods in hypoactivity. reaction to environmental or hormonal stimulations through the Unfortunately, most genetic studies have focused primarily on removal of methylation of their promoters.[2] Accordingly, the binary phenotype definitions and have not considered sub-types or activated dysfunctional pleiotropic gene may have various maniquantitative measures of psychiatric disorders, leading to underesfestations at the critical period and at other times in which it timation of the gene-disease associations and false negative becomes activated. linkage results. Wilcox et al.[8] constructed a set of quantitative Psychiatric disorders with a high prevalence of comorbid psychiatric or somatic disorders are more likely to be influenced by a traits for schizophrenia. Using the derived trait, they were able to  2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) Genetics and Epigenetics in Major Psychiatric Disorders 151 replicate prior findings and propose new loci that were not linked facilitate the development of an integrated model for the susceptito the usual binary DSM (Diagnostic and Statistical Manual of bility to schizophrenia and other psychiatric disorders. Mental Disorders) trait. Obsessive compulsive disorder (OCD), eating disorders, impulse control disorders, and some paraphilias 1.3 Polygeneity may belong to a specific category named ‘OCD-related disorders’. The common genetic liability for these comorbid conditions has The polygenic nature of the pathogenesis of psychiatric disorbeen suggested in several studies.[9,10] Thus, more elaborate study ders is another reason genetic studies tend to be inconclusive and designs with careful attention to phenotype definitions and close not replicable. A conclusion that can be drawn from the history of collaboration between geneticists and clinicians are needed to genetic research in psychiatry indicates that, as is the case with identify the effects of specific genes in psychiatric disorders other complex diseases, psychiatric disorders are likely to be generally, and schizophrenia specifically. multi-factorial and polygenic in origin.[12] Polygenic disorders result from the interaction of multiple common responsible genes. It is usually true that co-inheritance of a few of the genes is 1.2 Epigenetic Interaction necessary to reach the disease threshold and observe the psychiatric phenotype. For example, if we assume the frequency of a risk Epigenetic interaction is another overlooked issue in the psyallele of each of several genes contributing to the liability of a chiatric genetic studies. Epigenetics refers to modifications in gene disorder is approximately 20%, at least three risk alleles are expression that are controlled by heritable but potentially reversnecessary to produce the observed population prevalence of schizible changes in DNA methylation and/or chromatin structure.[11] In ophrenia or bipolar disorder (0.2 × 0.2 × 0.2 = 0.008). Therefore, fact, a portion of the variability in disease phenotypes that cannot the assumption of a single deleterious gene, made for most associbe explained based on traditional genetic data may arise from ation studies (and determining the effect size based upon that epigenetic interactions. For example, even monozygotic twins, assumption), is not accurate, especially with a limited sample size. who share 100% of their genes, can display very different psychiIn these situations, a much larger sample and a homogeneous atric phenotypes.[12] To understand this variability, the interaction population is needed to detect the polygenic effects conferring of the epigenetic risk factors and susceptibility genes should be disease liability. To overcome these limitations, multicenter studinvestigated more thoroughly. ies and meta-analyses of the existing association studies are valuaAlteration in the pattern of DNA methylation is an efficient ble to determine population heterogeneity and to obtain a sample mechanism for genes to adapt functionality in response to a large enough to provide statistical power to detect several genes of variable environment. This can compensate for the effects of a small effect. Fortunately, there are several meta-analyses as well malfunctional polymorphism. DNA methylation at CpG islands is as multicenter studies in the field of psychiatric genetics in the mediated by the addition of a methyl group to cytosine, in a literature and in progress. Therefore, in this review we focus on reaction catalyzed by one of the several DNA methyltransferases meta-analyses and multicenter studies. in an interactive manner.[13] S-adenosyl methionine is the major provider of methyl groups for methylation, while folic acid and 1.4 Ethics vitamin B12/cyanocobalamin are also necessary for recruitment of the demethylated S-adenosyl methionine. Environmental insults could also cause altered DNA methylation patterns, leading to In spite of above mentioned dilemmas, as well as a long delay corresponding changes in gene expression levels. These insults due to the abuse of psychiatric genetics during World War II, could be more problematic in individuals with a genetic susceptigenetic studies have provided valuable insight into the causes and bility to a specific disease, including psychiatric disorders. For potential remedies of psychiatric disorders. These disorders are example, if there is already a weak/susceptible allele and the level devastating and have substantial public health implications both in of illness is not at the threshold, epigenetic insults such as promotterms of personal suffering and public fiscal burden. Most major er DNA methylation could worsen the background hypoor hyperpsychiatric disorders have an early age of onset. This may warrant activity, allowing the disease phenotype to be expressed (for a focus on the identification of genetic liabilities for the purpose of review see Abdolmaleky et al.[14]). Therefore, a comprehensive early intervention and prevention. However, the ethical issues view of the genetic liability for psychiatric disorders will involve associated with the genetic liabilities for disease in general and the investigation of brain DNA methylation and gene expression psychiatric disorders specifically have not been adequately adanalyses as well as functional genetic polymorphisms. This will dressed, making this a difficult proposition.  2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) 152 Abdolmaleky et al. 2. Genetic Studies in the Field of Psychiatry phrenia patients indicated that COMT gene expression was reduced in superficial cortical layers, but increased in deep layers.[25] Another study analyzing COMT expression in schizophrenia and 2.1 Brain Dopaminergic System mood disorders indicated no difference in the levels of transcripts Dopaminergic neurons are those that release dopamine, which between cases and controls.[26] Recently, we found a highly signifis involved in several brain activities including attention, execuicant level of hypomethylation of the MB-COMT promoter in tive memory, desire, hedonic activities, natural rewards, and biopostmortem brains of patients with schizophrenia and bipolar logical activities such as cell signaling.[12,15] Most of these effects disorder compared with control subjects, particularly in the left are mediated by dopamine D1and D2-like receptors that act on frontal lobe. In addition, the degree of DNA methylation was other cell-signaling pathways. As we will discuss later, D1and inversely correlated with transcript quantities as determined by D2-like receptor polymorphisms are involved in psycho-pathogenquantitative real-time PCR, using primers that exclusively amplify esis. The functional consequences of polymorphisms in many the MB-COMT isoform (Abdolmaleky HM et al., unpublished genes involved in dopamine production are not well known. data). The primers of the previous studies were not specific for the However, the recognized polymorphisms in the genes encoding MB-COMT isoform, which is the predominant form involved in the catabolizing enzyme cathechol-O-methyl transferase (COMT), the degradation of synaptic dopamine in the human brain. and the dopamine transporter (SLC6A3), involved with reuptake of These results suggest that hyperactivity of the COMT gene may dopamine from the synaptic cleft, may both have roles in psychiatconfer liability for schizophrenia and mood disorders through ric disorders. rapid degradation of dopamine in the synaptic cleft, leading to a frontal lobe hypodopaminergic state and the reported frontal hy2.1.1 Cathechol-O-Methyl Transferase poactivity in schizophrenic patients.[27] The known schizophreniaThe COMT gene is located in the 22q11.21 chromosomal band. associated problems in attention, desire, hedonic and social activiSeveral linkage studies have linked this region to schizophrenia ty, cognitive processes, and working memory could be related to and bipolar disorder.[16,17] COMT encodes two isoforms of the dopamine dysfunction of the frontal lobe. Therefore, new antipCOMT enzyme, a membrane-bound form (MB-COMT) and a sychotic drugs with partial agonistic effects on the D2 receptor, or soluble form (S-COMT), which result from transcription from dopamine auto receptors agonist drugs,[28] may be more useful in different promoters. The MB-COMT form is involved in improving attention, cognitive processing, and negative symptoms dopamine catabolism in the human brain.[18] COMT is involved in in patients with schizophrenia. attention, executive cognition, and working memory performance.[19] COMT overactivity is related to social withdrawal[12] and 2.1.2 Dopamine Transporter neuroticism.[20] The dopamine transporter (DAT) is involved in dopamine The COMT transcript encoding MB-COMT has a functional reuptake from the synaptic cleft, while COMT has catabolic effect polymorphism at codon 158 (Val158Met). Homozygosity for the over dopamine. Several reports indicate that dysfunctional COMT Val158 allele leads to a 3to 4-fold increase in enzymatic polymorphisms in the gene encoding DAT (SLC6A3) are related to activity, compared with homozygosity for COMT Met158.[21] Gebipolar disorder (e.g. Greenwood et al.[29]); however, there are netic studies regarding the association of this polymorphism with many negative reports as well (e.g. Georgieva et al.[30]). A recent schizophrenia have been inconclusive and somewhat contradictostudy reported a highly significant (p = 0.0003) association of an ry; however, a recent study with a very large sample (720 patients SLC6A3 promoter polymorphism in schizophrenic patients.[31] and 2970 controls) reported the association of Val158 homozygosConcurrent dysfunction of the SLC6A3 gene may exaggerate the ity (the over-active allele) with schizophrenia in men.[6] Since effects of the above-mentioned COMT over-activity. Thus, invesestrogen down-regulates the COMT production,[7] this sexual diftigating the interaction of SLC6A3 and COMT functional ference might explain the gender difference in the findings. Furpolymorphisms will likely improve our understanding of genethermore, recently a family-based meta-analysis confirmed the gene interactions in psycho-pathogenesis and provide an integraassociation of the Val158 allele with schizophrenia.[22] Another tive model of the genetic liability for psychiatric disorders. meta-analysis reported evidence for the association of Met158 2.1.3 Dopamine D1 receptor allele with OCD.[23] However, a large, multicenter study reported the association of the Val158 allele with early onset major depresThe dopamine D1 receptor (DRD1) is involved in the modulasive disorder.[24] tion of synaptic plasticity in the prefrontal cortex[32] and Gene expression analysis applying in situ hybridization histostriatum.[33,34] DRD1 also regulates expression of several genes chemistry (FISH) in the dorsolateral prefrontal cortex of schizo(e.g. brain-derived neurotropic factor gene [BDNF]) through cyc 2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) Genetics and Epigenetics in Major Psychiatric Disorders 153 lic adenosine monophosphate (cAMP) and cAMP response elethat DRD4 exon 3 variants are associated with delusional sympment modulator (CREM) by acting on the cAMP response eletomatology in major psychoses.[49] A polymorphism in DRD4 was ments (CREs).[35] also reported to be associated with ADHD in several studies, including two meta-analyses.[50,51] The DRD1 gene has several polymorphisms that are not currently known to be associated with schizophrenia or mood disor2.2 Brain-Derived Neurotrophic Factor ders. However, haplotype analysis indicates a significant association between DRD1 and bipolar disorder.[36] Moreover, patients The brain-derived neurotrophic factor (BDNF) is a member of receiving the D1/D2 receptor agonist drug pramipexole have the neurotrophin family involved in neuronal development and shown improvement in the depressive phase of bipolar disorcell survival.[52] Human studies indicate that BDNF has a role in der.[37,38] affective disorders.[53] Family-based association studies have indi2.1.4 Dopamine D2 Receptor cated that the BDNF gene is a risk locus for bipolar disorder[54,55] and the Val66 allele of the BDNF Val66Met polymorphism is The dopamine D2 receptor (DRD2) has been a central subject associated with OCD[56] and neuroticism.[57] Studies with animal of study in major psychiatric disorders for several decades. Many models of depression indicated that maternal deprivation is correstudies have shown an up-regulation of DRD2 gene expression in lated with reduced BDNF expression. This finding suggests a role schizophrenia.[39] However, there are several studies that have for BDNF in the biology of mood disorders,[58] schizophrenia,[59,60] reported its down-regulation in schizophrenia, drug dependency, and anxiety disorders. depressive disorders, and PTSD.[3] The DRD2 gene is located on the long arm of chromosome 11, a region that has been linked to The antidepressant activity of BDNF was also shown in animal schizophrenia and mood disorders.[40] Although genetic associamodel studies.[61] Most of the antidepressant drugs act through tion studies on the role of the DRD2 in schizophrenia showed increasing monoamines in the synaptic cleft and production of the contradicting results, a recent meta-analysis reported a highly BDNF protein.[53,62] Also, olanzapine, an atypical antipsychotic significant association of the DRD2 Cys311Ser polymorphism drug, inhibits stress-induced BDNF reduction in animal studies.[63] with schizophrenia.[41] This meta-analysis suggests that this DRD2 New reports indicate that olanzapine, which induces the expreshypoactive allele (Cys311) is, at least in part, involved in schizosion of the BDNF gene, may be more appropriate for psychotic phrenia pathogenesis. patients because BDNF is hypoexpressed in patients with schizophrenia.[64] There is also some evidence indicating that DRD2 promoter DNA methylation could be under epigenetic modulation. The The production of BDNF is correlated with the degree of DRD2 gene has several potential CpG dinucleotides which could methylation of its gene’s promoter secondary to neuronal activaserve as methylation targets.[42] tion through dopamine D1-like receptors. D1-like receptors, through a cascade of events (mediated by cAMP/protein kinase A Recently, an investigation of the patterns of epigenetic DNA [PKA] and CREM), activate a CRE site on the BDNF promoter, modifications in the 5′-regulatory region of the DRD2 gene of and concurrently the DNA becomes de-methylated resulting in twins with schizophrenia showed that the affected twin from a increased gene expression. Neuronal inactivation leads to BDNF discordant pair was epigenetically ‘closer’ to the affected twin of promoter methylation.[65] COMT overactivity may reduce another pair than to his unaffected monozygotic co-twin.[43] Furdopamine and the availability of other monoamines in the synaptic thermore, other experiments indicate that antipsychotic drugs cleft leading to BDNF under-stimulation and methylation of its modulate DRD2 expression through chromatin modification.[44] promoter. An investigational focus on this issue could provide new Although this effect was transient, it implies that DRD2 could be treatment modalities, such as COMT inhibitor drugs, which under epigenetic influences. This may increase the probability of through an increase of dopamine in the synaptic cleft may stimuschizophrenia, especially in individuals with the hypoactive allele. late BDNF expression reducing the risk of depressive or psychotic 2.1.5 Dopamine D2-like receptors disorders. The dopamine D3 and D4 receptors (DRD3 and DRD4; the D2-like receptors) have also been the focus of genetic studies. 2.3 Reelin According to two meta-analyses, the DRD3 Ser9Gly variant confers susceptibility to schizophrenia.[45,46] In another meta-analysis, Reelin is an extra-cellular matrix protein, mainly produced by the promoter polymorphism of DRD4 was associated with schizogamma-aminobutyric acid (GABA)-ergic inter-neurons involved phrenia.[47] However, exon 1 and exon 3 DRD4 polymorphisms in neuronal migration, axonal branching, and synaptogenesis were not associated with schizophrenia.[48] One report indicates throughout brain development and later life.[66,67] These activities  2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) 154 Abdolmaleky et al. are mediated through binding to lipoprotein receptors and activathese proteins for growth and long-lasting structural changes.[80] tion of a tyrosine kinase cascade leading to DAB1 phosphorylThere are several types of serotonin receptors in the brain. ation. Reelin also activates a second messenger cascade, influencThe serotonin type 2 receptor (HTR2A), which is the main ing gene expression that leads to long lasting structural target of antidepressants and atypical antipsychotic drugs, has changes.[68,69] The reelin gene (RELN) is located on chromosome been the focus of many studies in the field of psychiatric genetics. 7, having a long CpG-rich promoter.[70] Reelin promoter in vitro A meta-analysis of whole-genome linkage scans confirmed a methylation reduces the gene expression, which could be recovlinkage between schizophrenia and markers on the long arm of ered by treatment with 5-aza-deoxycytidine, confirming that its chromosome 13.[81] The HTR2A gene is located in this region,[82] expression is regulated by promoter methylation.[71] and has many known polymorphisms in the population. The 102T/ L-methionine (a known methyl donor) decreases RELN mRNA C single nucleotide polymorphism has been the subject of most levels in animal studies and is associated with an increase in the studies. In this polymorphism, the base in the nucleotide at posinumber of methylated cytosines in the CpG islands of the RELN tion 102 may be thymine (T) or cytosine (C), with possible promoter region.[72] In schizophrenia and bipolar disorder, RELN genotypes TT, TC, or CC. However, this mutation does not result mRNA is severely reduced in postmortem brain studies.[66,67,73,74] in an amino acid change. There is a significant association beL-methionine can exacerbate symptoms in most schizophrenic and tween TT genotype and platelet HTR2A density.[83] In human bipolar patients, suggesting that hypo-activity of RELN may be postmortem studies, the expression of HTR2A receptors in the due to hyper-methylation of the gene’s promoter. Interestingly, temporal cortex is about 20% less for the CC compared with the valproate, which is useful in schizophrenia and bipolar disorder, is TT genotypes. This difference is greater for schizophrenic patients known as a demethylating agent through inhibition of histone dethan healthy controls with the same alleles. Moreover, HTR2A acetylase[71,72,75] and can prevent this methionine-induced hypermRNA level is inversely correlated with the duration of neurolepmethylation in animal studies.[72] tic-free intervals indicating that neuroleptics may increase HTR2A A recent study reported an over-expression of DNA methyl expression.[84] In electro-physiologic studies, schizophrenic patransferase (DNMT)-1 (an enzyme that primarily acts to maintain tients with the TT and CC genotypes show a differential response DNA methylation) in the GABAergic interneurons of patients to treatment with clozapine.[85] Following treatment, patients howith schizophrenia using the RNA in situ hybridization techmozygous for the 102C allele had higher N100 amplitudes than nique.[76] patients with other genotypes.[86] This may be an important biWe recently reported increased DNA methylation of the CpG omarker for the potential efficacy of treatment with this agent. islands localized to the RELN promoter regulatory regions, includThe HTR2A 102T/C polymorphism is located in exon 1 with ing a CRE and several SP1 transcription factor binding sites and possible promoter activity that may regulate gene expression. The concurrent RELN hypo-expression, in postmortem brain samples C in position 102 is followed by G, making this mutation is a from patients who had been diagnosed with schizophrenia.[77] candidate for methylation.[87] C-methylation can prevent gene Thus, the benefits of sodium valproate in schizophrenia and expression;[2,88,89] in some locations even single nucleotide methbipolar disorder may be due to its inhibition of the histone deacetyylation can change gene expression.[90] Therefore, the 102C allele lases which results in the removal of the methyl groups from the could be under epigenetic modification influencing gene expresmethylated cytosine of the DNA.[72,75] Further attention to this sion. Moreover, new data indicates that silent mutations may class of drugs may provide additional insight. Interestingly, butyinfluence mRNA stability and gene expression level,[91] supportrate, a milk product known to inhibit histone deacetylases,[78] or ing the idea that the HTR2A 102C allele could reduce HTR2A tea polyphenol (–)-epigallocatechin-3-gallate, which inhibits expression. DNMT, could also influence the methylation status of some There have been numerous association studies of the HTR2A genes.[79] 102T/C polymorphism with aspects of schizophrenia, including a formal diagnosis,[82] earlier onset with poorer outcome,[92] drug 2.4 Serotoninergic System response,[85] and susceptibility to tardive dyskinesia.[93,94] The association between the C allele and schizophrenia was confirmed by a meta-analysis.[82] However, there is an ongoing debate beSecondary to environmental stimulation, serotonin (5-HT) acticause negative findings persist in the literature. vates genes to produce several proteins involved in synapse formation, a process mediated by cAMP and CRE-binding protein We recently performed a meta-analysis of case-control and (CREB). Those synapses already activated by serotonin can utilize family-based association studies.[95] In the 6 years between the  2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) Genetics and Epigenetics in Major Psychiatric Disorders 155 original work and our analysis, the number of available studies receptor has several polymorphisms in the coding and non-coding doubled (to 35). regions that, in most instances, do not change the protein structure.[100] Some of these silent polymorphisms are not associated Our results were similar to the previous meta-analysis[82] (odds with schizophrenia,[101,102] although other polymorphisms have ratio [OR] = 1.18). We found a significant association between the been associated with the disorder.[103,104] C allele of the HTR2A 102T/C polymorphism and schizophrenia (OR = 1.1). The OR was slightly higher for the European sample The promoter region of the GRIN2A gene exhibits a variable (OR = 1.2). We also found significant allelic heterogeneity belength GT repeat polymorphism, and a longer GT repeat has been tween European and East Asian populations. In the East Asian correlated with reduced gene expression may be a risk factor for population, the 102C allele was not associated with schizophrenia. schizophrenia and bipolar disorder.[105,106] There is also some This heterogeneity, along with the documented differences in evidence that the G alleles of GRIN1 1001G/C and 6608G/C allele frequency between the two ethnic groups, suggests that data polymorphisms may have a role in bipolar disorder.[107] Overall, from European and East Asian samples should not be pooled when genetic studies provide evidence for the involvement of the evaluating the evidence for involvement of this gene in schizoNMDA receptor in the pathogenesis of major psychiatric disorphrenia risk. ders. These findings suggest that research focusing on the genes involved in glutaminergic transmission may be successful in deciWe did not find significant evidence for an association of the phering the genetic liability for major psychiatric illnesses, includ102C allele with schizophrenia in five family-based association ing schizophrenia. However, the complexity of glutamine-related studies. However, the pooled OR from 473 parent-offspring trios genes has made this task difficult. was 1.3. Thus, the pattern of results was similar to those from casecontrol studies. This evidence may help us estimate the magnitude Evidence indicates that atypical antipsychotic drugs such as of this association in distinct population subgroups. Polymorphic clozapine, through increasing activity of the NMDA receptor, may imprinting may influence the magnitude of the effect in genetic facilitate glutamatergic transmission.[108] As there is a close interstudies. Bunzel et al.,[96] showed that approximately 20% of indiaction with the dopaminergic system,[109] any malfunction of the viduals have mono-allelic expression of HTR2A in the brain, glutamatergic pathway affects dopaminergic pathways as well. confounding the magnitude of association of a specific allele. Clozapine increases dopamine release in the frontal lobe thereby improving cognition in schizophrenic patients.[110] The prevalence The HTR2A receptor has modulatory effects on dopaminergic of smoking in schizophrenic and bipolar patients is significantly neurons.[97] Part of its role in the pathogenesis of schizophrenia higher than in the general population.[111] One extant hypothesis is may be through actions on the dopaminergic system. The that nicotine (as a dopamine agonist) is abused to overcome dopaminergic system is under the influence of several other neurocognitive and depressive symptoms. As clozapine affects the transmitters. Thus, estimating the effect of the HTR2A 102C allele NMDA receptor, concurrent use of nicotine with classical antipwould be confounded by the influences of these other genes and sychotic drugs also has some positive effects on the NMDA environmental factors. receptor activity.[112] In light of these effects, physicians and researchers should consider more appropriate drugs to overcome 2.5 Glutaminergic System these symptoms and avoid self-medication by patients using otherwise harmful drugs. N-methyl-D-aspartate (NMDA) receptor antagonists produce a schizophrenia-like syndrome. NMDA receptors include an ionotropic NMDA-1 (GRIN1) subunit and one of four GRIN2 subunits 2.6 Other Genes (A, B, C, or D). The NMDA receptor 2B (GRIN2B) subunit is one of the brain-specific proteins in the postsynaptic density at glutaIn a small number of studies, other genes were also reported to matergic synapses. The carboxyl-terminal domain of the GRIN2B be linked to schizophrenia, including those encoding nitric oxide subunit is involved in the intracellular signal transduction. The synthase 1 (NOS1),[113] the apolipoprotein L genes APOL1, NMDA receptor participates in the regulation of dopamine, norAPOL2, and APOL4,[114] neuregulin 1 (NRG1),[115] tumor protein epinephrine (noradrenaline), acetylcholine, and GABA.[98] Sup53 (TP53),[116] the DAOA (D-amino acid oxidase activator; G72), portive evidence for involvement of NMDA in schizophrenia is D-amino-acid oxidase (DAO), proline dehydroxylase 2 provided from pharmacological studies and from animal models (PRODH2), dystrobrevin binding protein 1 (DTNBP1), neuronal showing schizophrenia-like symptoms (decreased social activity nicotinic cholinergic receptor alpha polypeptide 7 (CHRNA7), and and increased stereotypy) in mice that have a deficiency in expresthe protein kinase oncogene AKT1, as reviewed comprehensively sion of NMDA receptors GRIN1 and GRIN2.[99] The NMDA elsewhere.[117] The function of most of these genes is not well  2005 Adis Data Information BV. All rights reserved. Am J Pharmacogenomics 2005; 5 (3) 156 Abdolmaleky et al. understood in the brain, and the therapeutic application of these findings is unclear at present. Among these genes, there is most supporting evidence for the involvement of NRG1 (located in the short arm of chromosome 8) in the pathogenesis of schizophrenia in a minority of patients. The NRG1 risk haplotype is present in 15.4% of patients with schizophrenia, compared with 7.5% of the general population.[115] This gene is implicated in glutamate receptor expression and therefore justifies the current extensive investigation in its role in psychiatric disorders worldwide. However, NRG1 is a very complex gene with several isoforms. More than 1000 SNPs have been identified to date.[115] However, no specific functional polymorphism has been linked to schizophrenia. This is probably due to the complexity of this gene. 3. Gene-Gene and Gene-Environment Interactions: an Integrative Model Although several lines of evidence indicate that COMT, RELN, BDNF, HTR2A, GRIN1, GRIN2A, GRIN2B, SLC6A3, DRD1, and DRD2-like receptor genes are involved in major psychiatric disorders, genetic polymorphism studies have shown only weak association. New evidence strongly suggests a potential role for DNA Coding region Regulatory region Gene expression · Chemical and physical stress · Experience and habits · Culture · Nutrition Behavioral change Translation to biochemical language