Imke Puls

Molecular Mechanisms of Schizophrenia

Schizophrenia is a complex disorder, where family, twin and adoption studies have been demonstrating a high heritability of the disease and that this disease is not simply defined by several major genes but rather evolves from addition or potentiation of a specific cluster of genes, which subsequently determines the genetic vulnerability of an individual. Linkage and association studies suggest that a genetic vulnerablility, is not forcefully leading to the disease since triggering factors and environmental influences, i.e. birth complications, drug abuse, urban background or time of birth have been identified. This has lead to the assumption that schizophrenia is not only a genetically defined static disorder but a dynamic process leading to dysregulation of multiple pathways. There are several different hypothesis based on several facets of the disease, some of them due to the relatively well-known mechanisms of therapeutic agents. The most widely considered neurodevelopmental hypothesis of schizophrenia integrates environmental influences and causative genes. The dopamine hypothesis of schizophrenia is based on the fact that all common treatments involve antidopaminergic mechanisms and genes such as DRD2, DRD3, DARPP-32, BDNF or COMT are closely related to dopaminergic system functioning. The glutamatergic hypothesis of schizophrenia lead recently to a first successful mGlu2/3 receptor agonistic drug and is underpinned by significant findings in genes regulating the glutamatergic system (SLC1A6, SLC1A2 GRIN1, GRIN2A, GRIA1, NRG1, ErbB4, DTNBP1, DAAO, G72/30, GRM3). Correspondingly, GABA has been proposed to modulate the pathophysiology of the disease which is represented by the involvement of genes like GABRA1, GABRP, GABRA6 and Reelin. Moreover, several genes implicating immune, signaling and networking deficits have been reported to be involved in the disease, i.e. DISC1, RGS4, PRODH, DGCR6, ZDHHC8, DGCR2, Akt, CREB, IL-1B, IL-1RN, IL-10, IL-1B. However, molecular findings suggest that a complex interplay between receptors, kinases, proteins and hormones is involved in schizophrenia. In a unifying hypothesis, different cascades merge into another that ultimately lead to the development of symptoms adherent to schizophrenic disorders.

A common polymorphism in the cannabinoid receptor 1 (CNR1) gene is associated with antipsychotic-induced weight gain in schizophrenia.

Antipsychotic-induced weight gain has emerged as a serious complication in the treatment of patients with atypical antipsychotic drugs. The cannabinoid receptor 1 (CNR1) is expressed centrally in the hypothalamic region and associated with appetite and satiety, as well as peripherally. An antagonist of CNR1 (rimonabant) has been effective in causing weight loss in obese patients indicating that CNR1 might be important in antipsychotic-induced weight gain. Twenty tag SNPs were analyzed in 183 patients who underwent treatment (with either clozapine, olanzapine, haloperidol, or risperidone) for chronic schizophrenia were evaluated for antipsychotic-induced weight gain for up to 14 weeks. The polymorphism rs806378 was nominally associated with weight gain in patients of European ancestry treated with clozapine or olanzapine. ‘T’ allele carriers (CT+TT) gained more weight (5.96%), than the CC carriers (2.76%, p=0.008, FDR q-value=0.12). This translated into approximately 2.2 kg more weight gain in patients carrying the T allele than the patients homozygous for the CC genotype (CC vs CT+TT, 2.21±4.51 vs 4.33±3.89 kg; p=0.022). This was reflected in the allelic analysis (C vs T allele, 3.84 vs 5.83%, p=0.035). We conducted electrophoretic mobility shift assays which showed that the presence of the T allele created a binding site for arylhydrocarbon receptor translocator (ARNT), a member of the basic helix–loop–helix/Per–Arnt–Sim protein family. In this study, we provide evidence that the CNR1 gene may be associated with antipsychotic-induced weight gain in chronic schizophrenia patients. However, these observations were made in a relatively small patient population; therefore these results need to be replicated in larger sample sets.

Serotonergic dysfunction: brain imaging and behavioral correlates

Identification of gene-environment and gene-gene interactions has become increasingly important in understanding psychiatric disorders. Dysfunction of central serotonergic neurotransmission has been implicated in alcoholism, depression, and anxiety. We review the literature on nonhuman primates that assesses the interaction between the genetic constitution of the regulatory region of the serotonin transporter (5-HTT) and environmental factors. Prospective studies in nonhuman primates that underwent social stress found a reduction of the serotonin turnover rate among carriers of one or two short alleles in a functional polymorphism of the 5-HTT promoter. In these primates, brain imaging studies showed a relative increase in the availability of raphe serotonin transporters. A low serotonin turnover rate and a high availability of serotonin transporters were associated with reduced response to excessive alcohol intake, anxiety, and impulsive aggression. Animal experiments point to a relationship between serotonergic dysfunction, negative mood states, and excessive alcohol intake, which may in part be mediated by reduced alcohol-induced sedation.

5-HTT genotype effect on prefrontal–amygdala coupling differs between major depression and controls

RationaleIn major depression, prefrontal regulation of limbic brain areas may be a key mechanism that is impaired during the processing of affective information. This prefrontal–limbic interaction has been shown to be modulated by serotonin (5-HTT) genotype, indicating a higher risk for major depressive disorder (MDD) with increasing number of 5-HTT low-expression alleles.ObjectiveFunctional magnetic resonance imaging was used to assess neural response to uncued unpleasant affective pictures in 21 unmedicated patients with MDD compared to 21 matched healthy controls, taking into account genetic influences of the 5-HTT (SCL6A4) high- and low-expression genotype.ResultsHealthy controls displayed greater prefrontal activation (BA10) to uncued negative pictures compared to patients with MDD. While in healthy controls prefrontal (BA10) activation and BA10–amygdala coupling increased with the number of 5-HTT low-expression risk alleles, this effect was abolished, and even reversed, in patients with MDD. In MDD, connectivity decreased with severity of depressive symptoms (HAMD total score).ConclusionThese findings suggest that increased medial prefrontal (BA10) activation and BA10–amygdala connectivity may counteract the risk for MDD in healthy carriers of 5-HTT low-expression alleles, while this protective factor might be lost in patients who actually suffer from MDD. Prefrontal–limbic regulation in risk populations could be a target of early interventions and should be the focus of further research.

Catechol-O-methyltransferase val158met genotype influences neural processing of reward anticipation

Reward processing depends critically on dopaminergic neurotransmission in the ventral striatum. The common polymorphism val(158)met of catechol-O-methyltransferase (COMT) accounts for significant interindividual variations in dopamine (DA) degradation, although the direct effect of COMT on striatal DA might be limited. Using fMRI we assessed the influence of COMT val(158)met genotype on brain activations elicited by the anticipation of monetary gains and losses in forty-four healthy volunteers. We found that the met(158) allele, which is presumably linked to higher synaptic DA levels, was associated with higher responses in ventral striatum to loss incentives. There was a linear relationship between the number of met(158) alleles and ventral striatal activity. Furthermore, we observed a similar gene-dose effect in the anterior temporal cortex, a region that has been linked to the coupling of sensory information with emotional contents. Temporal cortex also showed enhanced connectivity to the ventral striatum during the processing of incentive stimuli. Increased ventral striatal reactivity to loss incentives related to the met(158) allele might contribute to the observed association of the met(158) allele to higher loss aversion behaviour. Current evidence and our results are compatible with an interpretation that construes this effect of COMT genotype on striatal reactivity as a result of a cortico-striatal interaction.

Pharmacogenetic insights to monoaminergic dysfunction in alcohol dependence

RationaleAlcohol dependence is characterized by the development of tolerance, withdrawal symptoms, and craving for alcohol. Chronic alcohol consumption causes neuroadaptive changes in the central dopaminergic and serotonergic system, which are partially reversible after detoxification. The severity and time-course of recovery of these neuroadaptive changes may depend on the genetic constitution of monoamine transporters and receptors and contribute to the relapse risk of alcoholics.ObjectivesTo assess the interaction between the genetic constitution and the in vivo availability of dopamine and serotonin transporters and receptors, chronic alcohol intake, alcohol craving and withdrawal.MethodsReview of brain imaging studies that assess the genotype and availability of dopamine and serotonin transporters in detoxified alcoholics and healthy control subjects.ResultsChronic alcohol intake induced neuroadaptive reductions in striatal dopamine transporter (DAT) availability, which were reversible during early abstinence. A polymorphism of the DAT gene (SLC6A3) was associated with the in vivo transporter availability and with the severity of alcohol withdrawal. Neurotoxic reductions in 5-HTT protein expression were limited to homozygous carriers of the long allele in the 5-HTT gene (SCL6A4) regulatory region and correlated with negative mood states.ConclusionGenetic constitution interacts with the in vivo availability of central dopamine and serotonin transporters during alcohol detoxification and may affect the severity of alcohol withdrawal and clinical depression.

Association study of polymorphisms in leptin and leptin receptor genes with antipsychotic-induced body weight gain

BACKGROUND Antipsychotic-induced weight gain (AIWG) is a serious side-effect of antipsychotic medication leading to metabolic syndrome and increased cardiovascular morbidity. Unfortunately, there are still no valid predictors to assess an individuals risk to gain weight. Previous studies have indicated an impact of genetic variation in the genes encoding leptin, LEP, and leptin receptor, LEPR, on AIWG, but results have not been conclusive. Thus, we investigated polymorphisms in both genes for an association with AIWG. METHODS A total of 181 schizophrenic and schizoaffective patients treated with various antipsychotics were included. In a small subset of patients, leptin plasma levels were additionally obtained. Five polymorphisms in LEP and LEPR (LEP: rs7799039 (-2548G/A polymorphism), rs10954173, rs3828942; LEPR: rs1327120, rs1137101 (Q223R polymorphism) were genotyped using TaqMan assays. Statistical association with % weight change from baseline weight was performed using ANCOVA with baseline weight as covariate. RESULTS ANCOVA showed a non-significant trend for genotype association of the rs7799039 marker (p=.068). No significant association of the other LEP and LEPR SNPs with AIWG was detected. However, we found a significant association between a haplotype of LEP rs7799039G-rs10954173G-rs3828942G (p=.035) and AIWG. The rs7799039 G-allele (p=.042) and G-allele of rs3828942 (p=.032) were associated with higher weight gain. CONCLUSION Our study supports the hypothesis of an impact of LEP gene variation on AIWG. Limitations of our study include heterogeneous samples, short treatment duration and multiple comparisons. Our findings were compared to previous studies in detail in order to provide the readers with a more conclusive picture. However, further studies are warranted including more gene variants and interaction analyses with other genes of the leptin-melanocortin pathway.

Neurotransmitter systems in alcohol dependence.

An interplay of different neurotransmitter systems has been implicated in the development and maintenance of alcohol dependence. Here we focus on neuroadaptations in reward-related neurotransmitter systems and their impact on central processing of alcohol-associated and reward-indicating stimuli. We discuss genotype effects on cue-induced neuronal activation and present new computational methods based on machine learning to deal with complex genotype-phenotype interactions, e.g. between brain atrophy and genes associated with glutamatergic and dopaminergic neurotransmission.

Association of the Nicotinic Receptor α7 Subunit Gene (CHRNA7) with Schizophrenia and Visual Backward Masking.

The nicotinic system is involved in the pathophysiology of schizophrenia. However, very little is known about its genetic basis and how it relates to clinical symptoms and potentially pharmacological intervention. Here, we investigated five single nucleotide polymorphisms (SNPs) [rs3826029] [rs2337506] [rs982574] [rs904952] [rs2337980] of the cholinergic nicotinic receptor gene, alpha 7 subunit (CHRNA7) and their association to schizophrenia. We found an association with rs904952 (p = 0.009) in a German sample of 224 schizophrenic patients and 224 healthy control subjects. The same trend was shown in an independent Georgian sample of 50 schizophrenic patients, 57 first order unaffected relatives, and 51 healthy controls. In addition, visual backward masking (VBM), a sensitive test for early visual information processing, was assessed in the Georgian sample. In line with prior studies, VBM performance deficits were much more pronounced in schizophrenic patients and their unaffected relatives compared to healthy controls (schizophrenic patients: 156 ms; unaffected relatives: 60 ms; healthy controls: 33 ms). VBM was strongly correlated with SNP rs904952 (H[2] = 7.3, p = 0.026). Our results further support the notion that changes in the nicotinic system are involved in schizophrenia and open the avenue for pharmacological intervention.

Smoking, but not malnutrition, influences promoter-specific DNA methylation of the proopiomelanocortin gene in patients with and without anorexia nervosa.

Objective: Our pilot study evaluates the impact of environmental factors, such as nutrition and smoking status, on epigenetic patterns in a disease-associated gene. Method: We measured the effects of malnutrition and cigarette smoking on proopiomelanocortin (POMC) promoter-specific DNA methylation in female patients with and without anorexia nervosa (AN). POMC and its derived peptides (alpha melanocyte stimulating hormone and adrenocorticotropic hormone) are implicated in stress and feeding response. Promoter-specific DNA methylation of the POMC gene was determined in peripheral blood mononuclear cells of 54 healthy female control subjects, 40 underweight patients with AN, and 21 weight-restored patients with AN using bisulfite sequencing. Malnutrition was characterized by plasma leptin. Results: POMC promoter-specific DNA methylation was not affected by diagnosis or nutritional status but significantly negatively associated with cigarette smoking. Conclusions: Although malnutrition may be expected to reduce DNA methylation through its effects on one-carbon metabolism, our negative results are in line with several in vitro and clinical studies that did not show a direct relation between gene-specific DNA methylation and folate levels. In contrast, smoking has been repeatedly reported to alter DNA methylation of specific genes and should be controlled for in future epigenetic studies.