Genetic association of the GDNF alpha-receptor genes with schizophrenia and clozapine response

GFRA4 improves the neurogenic potential of enteric neural crest stem cells via hedgehog pathway

BACKGROUND

Hirschsprung disease (HSCR) is a congenital intestinal disease characterised by functional obstruction of the colon. Herein, we investigated the role and mechanism of the gene GFRA4 in HSCR.

METHODS

GFRA4 expression in the ganglionic and aganglionic segment tissues in patients with HSCR and healthy colon tissues were detected using qRT-PCR, western blot, and immunohistochemistry. Cell proliferation, cycle distribution, apoptosis, changes in mitochondrial membrane potential, and differentiation were assessed in mouse enteric neural crest stem cells (ENCSCs) using the CCK-8 assay, EdU staining, flow cytometry, JC-1 probe, and immunofluorescence, respectively. GSEA analysis was performed to screen the signaling pathways regulated by GFRA4.

RESULTS

GFRA4 was downregulated in aganglionic segment tissues compared to control and ganglionic segment tissues. GFRA4 overexpression promoted proliferation and differentiation, and inhibited apoptosis in ENCSCs, while GFRA4 down-regulation had the opposite result. GFRA4 activated the hedgehog pathway. GFRA4 overexpression enhanced the expression of key factors of the hedgehog pathway, including SMO, SHH, and GLI1. However, GFRA4 down-regulation reduced their expression. An antagonist of hedgehog pathway, cyclopamine, attenuated the effect of GFRA4 overexpression on proliferation, differentiation, and apoptosis of ENCSCs.

CONCLUSION

GFRA4 promotes proliferation and differentiation but inhibits apoptosis of ENCSCs via the hedgehog pathway in HSCR.

IMPACT

This study confirms that GFRA4 improves the proliferation and differentiation of ENCSCs via modulation of the hedgehog pathway. This study for the first time revealed the role and the mechanism of the action of GFRA4 in HSCR, which indicates that GFRA4 may play a role in the pathological development of HSCR. Our findings may lay the foundation for further investigation of the mechanisms underlying HSCR development and into targets of HSCR treatment.

MicroRNA-Mediated Suppression of Glial Cell Line-Derived Neurotrophic Factor Expression Is Modulated by a Schizophrenia-Associated Non-Coding Polymorphism

Plasma levels of glial cell line-derived neurotrophic factor (GDNF), a pivotal regulator of differentiation and survival of dopaminergic neurons, are reportedly decreased in schizophrenia. To explore the involvement of GDNF in the pathogenesis of the disease, a case-control association analysis was performed between five non-coding single nucleotide polymorphisms (SNP) across the GDNF gene and schizophrenia. Of them, the 'G' allele of the rs11111 SNP located in the 3' untranslated region (3'-UTR) of the gene was found to associate with schizophrenia. In silico analysis revealed that the rs11111 'G' allele might create binding sites for three microRNA (miRNA) species. To explore the significance of this polymorphism, transient co-transfection assays were performed in human embryonic kidney 293T (HEK293T) cells with a luciferase reporter construct harboring either the 'A' or 'G' allele of the 3'-UTR of GDNF in combination with the hsa-miR-1185-1-3p pre-miRNA. It was demonstrated that in the presence of the rs11111 'G' (but not the 'A') allele, hsa-miR-1185-2-3p repressed luciferase activity in a dose-dependent manner. Deletion of the miRNA binding site or its substitution with the complementary sequence abrogated the modulatory effect. Our results imply that the rs11111 'G' allele occurring more frequently in patients with schizophrenia might downregulate GDNF expression in a miRNA-dependent fashion.

Morphogenetic theory of mental and cognitive disorders: the role of neurotrophic and guidance molecules

Mental illness and cognitive disorders represent a serious problem for the modern society. Many studies indicate that mental disorders are polygenic and that impaired brain development may lay the ground for their manifestation. Neural tissue development is a complex and multistage process that involves a large number of distant and contact molecules. In this review, we have considered the key steps of brain morphogenesis, and the major molecule families involved in these process. The review provides many indications of the important contribution of the brain development process and correct functioning of certain genes to human mental health. To our knowledge, this comprehensive review is one of the first in this field. We suppose that this review may be useful to novice researchers and clinicians wishing to navigate the field.

Treatment-Resistant Schizophrenia, Clozapine Resistance, Genetic Associations, and Implications for Precision Psychiatry: A Scoping Review

Treatment-resistant schizophrenia (TRS) is often associated with severe burden of disease, poor quality of life and functional impairment. Clozapine is the gold standard for the treatment of TRS, although it is also known to cause significant side effects in some patients. In view of the burgeoning interest in the role of genetic factors in precision psychiatry, we conducted a scoping review to narratively summarize the current genetic factors associated with TRS, clozapine resistance and side effects to clozapine treatment. We searched PubMed from inception to December 2022 and included 104 relevant studies in this review. Extant evidence comprised associations between TRS and clozapine resistance with genetic factors related to mainly dopaminergic and serotoninergic neurotransmitter systems, specifically, TRS and rs4680, rs4818 within COMT, and rs1799978 within DRD2; clozapine resistance and DRD3 polymorphisms, CYP1A2 polymorphisms; weight gain with LEP and SNAP-25 genes; and agranulocytosis risk with HLA-related polymorphisms. Future studies, including replication in larger multi-site samples, are still needed to elucidate putative risk genes and the interactions between different genes and their correlations with relevant clinical factors such as psychopathology, psychosocial functioning, cognition and progressive changes with treatment over time in TRS and clozapine resistance.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Human iPSC-Derived Neuronal Cells From CTBP1-Mutated Patients Reveal Altered Expression of Neurodevelopmental Gene Networks

A recurrent de novo mutation in the transcriptional corepressor CTBP1 is associated with neurodevelopmental disabilities in children (Beck et al., 2016, 2019; Sommerville et al., 2017). All reported patients harbor a single recurrent de novo heterozygous missense mutation (p.R342W) within the cofactor recruitment domain of CtBP1. To investigate the transcriptional activity of the pathogenic CTBP1 mutant allele in physiologically relevant human cell models, we generated induced pluripotent stem cells (iPSC) from the dermal fibroblasts derived from patients and normal donors. The transcriptional profiles of the iPSC-derived “early” neurons were determined by RNA-sequencing. Comparison of the RNA-seq data of the neurons from patients and normal donors revealed down regulation of gene networks involved in neurodevelopment, synaptic adhesion and anti-viral (interferon) response. Consistent with the altered gene expression patterns, the patient-derived neurons exhibited morphological and electrophysiological abnormalities, and susceptibility to viral infection. Taken together, our studies using iPSC-derived neuron models provide novel insights into the pathological activities of the CTBP1 p.R342W allele.

Impact of Auditory Integration Therapy (AIT) on the Plasma Levels of Human Glial Cell Line-Derived Neurotrophic Factor (GDNF) in Autism Spectrum Disorder

Neurotrophic factors, including the glial cell line-derived neurotrophic factor (GDNF), are of importance for synaptic plasticity regulation, intended as the synapses' ability to strengthen or weaken their responses to differences in neuronal activity. Such plasticity is essential for sensory processing, which has been shown to be impaired in autism spectrum disorder (ASD). This study is the first to investigate the impact of auditory integration therapy (AIT) of sensory processing abnormalities in autism on plasma GDNF levels. Fifteen ASD children, aged between 5 and 12 years, were enrolled and underwent the present research study. AIT was performed throughout 10 days with a 30-min session twice a day. Before and after AIT, Childhood Autism Rating Scale (CARS), Social Responsiveness Scale (SRS), and Short Sensory Profile (SSP) scores were calculated, and plasma GDNF levels were assayed by an EIA test. A substantial decline in autistic behavior was observed after AIT in the scaling parameters used. Median plasma GDNF level was 52.142 pg/ml before AIT. This level greatly increased immediately after AIT to 242.05 pg/ml (P < 0.001). The levels were depressed to 154.00 pg/ml and 125.594 pg/ml 1 month and 3 months later, respectively, but they were still significantly higher compared with the levels before the treatment (P = 0.001, P = 0.01, respectively). There was an improvement in the measures of autism severity as an effect of AIT which induced the up-regulation of GDNF in plasma. Further research, on a large scale, is needed to evaluate if the cognitive improvement of ASD children after AIT is related or not connected to the up-regulation of GDNF.

Altered serum levels of glial cell line-derived neurotrophic factor in male chronic schizophrenia patients with tardive dyskinesia

OBJECTIVES

Many research indicate that the tardive dyskinesia (TD) is generally linked with long-term antipsychotic therapy for schizophrenia. Glial cell line-derived neurotrophic factor (GDNF) is a critical role in the protection of catecholaminergic, dopaminergic, and cholinergic neurons. Thus, we examined the serum GDNF levels in schizophrenia patients with TD (WTD) and without TD (NTD) and compared with healthy controls (HC), respectively.

METHODS

Totally 75 males with schizophrenia were recruited into this study. All were measured by the Diagnostic and Statistical Manual of Mental Disorders, fifth edition, the Positive and Negative Syndrome Scale, and the Abnormal Involuntary Movement Scale (AIMS). The patient group was divided into two subgroups: WTD (n = 32) and NTD (n = 43) according to the AIMS score. Fifty-three healthy controls matching in age and gender were also enlisted from the region. GDNF levels were examined with sandwich enzyme-linked immunosorbent assay.

RESULTS

Analysis of variance indicated significant differences between the three groups (P = 0.012); GDNF levels in the WTD group were significantly different from those in the NTD (P = 0.030) and HC (P = 0.003) groups.

CONCLUSION

Decreased GDNF levels in TD patients indicated that alterations in neurotrophic factors may be involved in the pathophysiology of TD, but the exact mechanisms need further investigation.

Biological Predictors of Clozapine Response: A Systematic Review

Background: Clozapine is the recommended antipsychotic for treatment-resistant schizophrenia (TRS) but there is significant variability between patients in the degree to which clozapine will improve symptoms. The biological basis of this variability is unknown. Although clozapine has efficacy in TRS, it can elicit adverse effects and initiation is often delayed. Identification of predictive biomarkers of clozapine response may aid initiation of clozapine treatment, as well as understanding of its mechanism of action. In this article we systematically review prospective or genetic studies of biological predictors of response to clozapine. Methods: We searched the PubMed database until 20th January 2018 for studies investigating "clozapine" AND ("response" OR "outcome") AND "schizophrenia." Inclusion required that studies examined a biological variable in relation to symptomatic response to clozapine. For all studies except genetic-studies, inclusion required that biological variables were measured before clozapine initiation. Results: Ninety-eight studies met the eligibility criteria and were included in the review, including neuroimaging, blood-based, cerebrospinal fluid (CSF)-based, and genetic predictors. The majority (70) are genetic studies, collectively investigating 379 different gene variants, however only three genetic variants (DRD3 Ser9Gly, HTR2A His452Tyr, and C825T GNB3) have independently replicated significant findings. Of the non-genetic variables, the most consistent predictors of a good response to clozapine are higher prefrontal cortical structural integrity and activity, and a lower ratio of the dopamine and serotonin metabolites, homovanillic acid (HVA): 5-hydroxyindoleacetic acid (5-HIAA) in CSF. Conclusions: Recommendations include that future studies should ensure adequate clozapine trial length and clozapine plasma concentrations, and may include multivariate models to increase predictive accuracy.

Association between serum levels of glutamate and neurotrophic factors and response to clozapine treatment

Clozapine is the only available therapy for about 30% of schizophrenia patients otherwise refractory to antipsychotics. Unfortunately, the mechanism of action of the drug is still unknown and there are no biomarkers that can predict a positive response to clozapine. We aimed to examine serum neurotrophins and glutamate levels as putative biomarkers for clozapine response based on the hypothesized mode-of-action of the compound. Blood samples of 89 chronic schizophrenia patients maintained on clozapine were analyzed in a cross-sectional design. Serum brain derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), neurotrophic growth factor (NGF), glial derived neurotrophic factor (GDNF) and glutamate were determined. Differences between responders and non-responders to clozapine and correlation between clinical and biological measures were analyzed. Our sample consisted of 54 (61%) responders and 35 (39%) non-responders. Responders had higher mean BDNF levels than non-responders (2066±814 vs. 1668±820pg/ml, p<0.05. respectively) and higher serum glutamate levels (1.61±2.2 vs. 0.66±0.9pg/ml, respectively, p<0.05). Furthermore, there was a significant correlation between serum glutamate levels and positive symptoms among the clozapine-responder group (rho=0.47, p<0.005). High serum levels of BDNF and glutamate were associated with response to clozapine, while glutamate levels correlated with the psychosis severity in clozapine responders only. Large-scale, prospective longitudinal studies are needed to support these findings and the assumption that serum glutamate and BDNF can discriminate between clozapine responders and non-responders.

Impaired Latent Inhibition in GDNF-Deficient Mice Exposed to Chronic Stress

Increased reactivity to stress is maladaptive and linked to abnormal behaviors and psychopathology. Chronic unpredictable stress (CUS) alters catecholaminergic neurotransmission and remodels neuronal circuits involved in learning, attention and decision making. Glial-derived neurotrophic factor (GDNF) is essential for the physiology and survival of dopaminergic neurons in substantia nigra and of noradrenergic neurons in the locus coeruleus. Up-regulation of GDNF expression during stress is linked to resilience; on the other hand, the inability to up-regulate GDNF in response to stress, as a result of either genetic or epigenetic modifications, induces behavioral alterations. For example, GDNF-deficient mice exposed to chronic stress exhibit alterations of executive function, such as increased temporal discounting. Here we investigated the effects of CUS on latent inhibition (LI), a measure of selective attention and learning, in GDNF-heterozygous (HET) mice and their wild-type (WT) littermate controls. No differences in LI were found between GDNF HET and WT mice under baseline experimental conditions. However, following CUS, GDNF-deficient mice failed to express LI. Moreover, stressed GDNF-HET mice, but not their WT controls, showed decreased neuronal activation (number of c-Fos positive neurons) in the nucleus accumbens shell and increased activation in the nucleus accumbens core, both key regions in the expression of LI. Our results add LI to the list of behaviors affected by chronic stress and support a role for GDNF deficits in stress-induced pathological behaviors relevant to schizophrenia and other psychiatric disorders.

Cognitive impairment in first-episode drug-naïve patients with schizophrenia: Relationships with serum concentrations of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor

OBJECTIVES

Evidence suggests that brain-derived neurotrophic factor (BDNF) and glial cell line -derived neurotrophic factor (GDNF) are important in the regulation of synaptic plasticity, which plays a key role in the cognitive processes in psychiatric disorders. Our work aimed at exploring the associations between serum BDNF and GDNF levels and cognitive functions in first-episode drug-naïve (FEDN) patients with schizophrenia.

METHODS

The BDNF and GDNF levels of 58 FEDN patients and 55 age- and sex-matched healthy controls were measured and test subjects were examined using several neurocognitive tests including the verbal fluency test (VFT), the trail making test (TMT), the digit span test (DST), and the Stroop test.

RESULTS

Patients performed significantly worse than controls in nearly all neurocognitive performances except the forward subscale part of the DST. BDNF levels were inversely correlated to TMT-part B scores and positively correlated to VFT-action in the FEDN group. GDNF levels showed a positive correlation with VFT-action scores and a negative correlation with TMT-part B scores of these patients.

CONCLUSION

Current data suggests that cognitive dysfunction widely exists in the early stages of schizophrenia. BDNF and GDNF may be jointly contributed to the pathological mechanisms involved in cognitive impairment in FEDN patients with schizophrenia.

Association Between RET (rs1800860) and GFRA1 (rs45568534, rs8192663, rs181595401, rs7090693, and rs2694770) Variants and Kidney Size in Healthy Newborns

BACKGROUND

Abnormal congenital nephron number has been implicated in the pathogenesis of hypertension and renal disease. The RET receptor complex propagates signals essential for nephrogenesis and the RET c.1296G>A polymorphism, leading to aberrant splicing of exon 7, is associated with reduced kidney volume, a surrogate for nephron endowment. The glial cell-derived neurotrophic factor (GDNF) family receptor alpha 1 (GFRA1) is a component of the RET receptor complex, and three alternatively spliced GFRA1 transcripts (with or without exon 5) have been identified. In rats, exclusion of exon 5 results in stronger GDNF binding affinity and RET activation. The aims of this study were to investigate further the relationship between RET c.1296G>A and kidney volume, and also to investigate the association between the GFRA1 polymorphisms near and within the alternatively spliced exon 5, as well as the functional 5'-UTR c.-193C>G with kidney volume.

MATERIALS AND METHODS

The study included 188 healthy full-term newborns. Genotyping of the RET (NM_020975.4:c.1296G>A, rs1800860) and GFRA1 (NM_005264.5:c.-193C>G, rs45568534; c.419-87A>G, rs8192663; c.429G>A, rs181595401; c.433+127A>G, rs7090693; c.433+245A>G, rs2694770) polymorphisms was performed using polymerase chain reaction-restriction fragment length polymorphism, minisequencing, or sequencing. Total kidney volume (TKV) was determined by ultrasound and normalized to body surface area (TKV/BSA). Both marker-by-marker and haplotype-based methods were used to test for associations between polymorphisms and TKV/BSA.

RESULTS

TKV/BSA in RET c.1296A allele carriers was significantly lower compared with GG homozygotes (103 ± 23 vs. 110 ± 19 mL/m², p = 0.034). c.429G>A was invariant in our sample. There was no association between any of the GFRA1 polymorphisms and renal volume.

CONCLUSIONS

RET c.1296A may be a common susceptibility allele for nephron underdosing-related diseases. The 5'-UTR and intronic variants near exon 5 of GFRA1 are not associated with nephron endowment.

Pharmacogenetics of clozapine response and induced weight gain: A comprehensive review and meta-analysis

Clozapine (CLZ) is the prototype atypical antipsychotic and it has many advantages over other antipsychotic drugs. Several data suggest that both CLZ response and induced weight gain are strongly determined by genetic variability. However, results remain mainly inconclusive. We aim to review the literature data about pharmacogenetics studies on CLZ efficacy, focusing on pharmacodynamic genes. Further, we performed meta-analyses on response when at least three studies for each polymorphism were available. Sensitivity analyses were conducted on Caucasian population when feasible. Electronic literature search was performed to identify pertinent studies published until May 2014 using PubMed, ISI Web of Knowledge and PsycINFO databases. For meta-analyses, data were entered and analyzed through RevMan version 5.2 using a random-effect model. Our literature search yielded 9266 articles on CLZ; among these, we identified 59 pertinent pharmacogenetic studies. Genotype data were retrieved for 14 polymorphisms in 9 genes. Among these, we had available data from at least three independent samples for 8 SNPs in 6 genes to perform meta-analyses: DRD2 rs1799732, DRD3 rs6280, HTR2A rs6313, rs6311, rs6314, HTR2C rs6318, HTR3A rs1062613, TNFa rs1800629. Although literature review provided conflicting results, in meta-analyses three genetic variants within serotonin genes resulted associated to CLZ response: rs6313 and rs6314 within HTR2A gene and rs1062613 within HT3A gene. On the other hand, no clear finding emerged for CLZ-induced weight gain. Our results suggest a possible serotonergic modulation of CLZ clinical response.

DNA methylation in peripheral tissue of schizophrenia and bipolar disorder: a systematic review

BACKGROUND

Increasing evidence suggests the involvement of epigenetic processes in the development of schizophrenia and bipolar disorder, and recent reviews have focused on findings in post-mortem brain tissue. A systematic review was conducted to synthesise and evaluate the quality of available evidence for epigenetic modifications (specifically DNA methylation) in peripheral blood and saliva samples of schizophrenia and bipolar disorder patients in comparison to healthy controls.

METHODS

Original research articles using humans were identified using electronic databases. There were 33 included studies for which data were extracted and graded in duplicate on 22 items of the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement, to assess methodological precision and quality of reporting.

RESULTS

There were 15 genome-wide and 18 exclusive candidate gene loci investigations for DNA methylation studies. A number of common genes were identified as differentially methylated in schizophrenia/bipolar disorder, which were related to reelin, brain-derived neurotrophic factor, dopamine (including the catechol-O-methyltransferase gene), serotonin and glutamate, despite inconsistent findings of hyper-, hypo-, or lack of methylation at these and other loci. The mean STROBE score of 59% suggested moderate quality of available evidence; however, wide methodological variability contributed to a lack of consistency in the way methylation levels were quantified, such that meta-analysis of the results was not possible.

CONCLUSIONS

Moderate quality of available evidence shows some convergence of differential methylation at some common genetic loci in schizophrenia and bipolar disorder, despite wide variation in methodology and reporting across studies. Improvement in the clarity of reporting clinical and other potential confounds would be useful in future studies of epigenetic processes in the context of exposure to environmental and other risk factors.

Pharmacogenetics of second-generation antipsychotics

This review considers pharmacogenetics of the so called 'second-generation' antipsychotics. Findings for polymorphisms replicating in more than one study are emphasized and compared and contrasted with larger-scale candidate gene studies and genome-wide association study analyses. Variants in three types of genes are discussed: pharmacokinetic genes associated with drug metabolism and disposition, pharmacodynamic genes encoding drug targets, and pharmacotypic genes impacting disease presentation and subtype. Among pharmacokinetic markers, CYP2D6 metabolizer phenotype has clear clinical significance, as it impacts dosing considerations for aripiprazole, iloperidone and risperidone, and variants of the ABCB1 gene hold promise as biomarkers for dosing for olanzapine and clozapine. Among pharmacodynamic variants, the TaqIA1 allele of the DRD2 gene, the DRD3 (Ser9Gly) polymorphism, and the HTR2C -759C/T polymorphism have emerged as potential biomarkers for response and/or side effects. However, large-scale candidate gene studies and genome-wide association studies indicate that pharmacotypic genes may ultimately prove to be the richest source of biomarkers for response and side effect profiles for second-generation antipsychotics.

Use of candidate gene markers to guide antipsychotic dosage adjustment

OBJECTIVE

To improve antipsychotic treatment in schizophrenia patients, many studies have investigated genetic polymorphisms associated with antipsychotic metabolizing enzymes and receptors. While these studies have typically focused on drug response, few have investigated genetic influences on antipsychotic dosage. This study set out to analyze the association between 134 SNPs in 38 candidate genes and antipsychotic dosage in schizophrenia patients.

METHODS

For our analysis, 300 patients with a diagnosis of either schizophrenia or schizoaffective disorder were recruited between the ages of 18 and 75. A cross-sectional assessment was used, in which data were collected from each participant through an interview and self-report questionnaire. Antipsychotic dose was standardized according to the chlorpromazine equivalents, defined daily dose and relative to the maximum dose specified in the product monograph. Participants were genotyped using a Customized Illumina Chip comprising 134 SNPs, and all markers were screened for nominal significance.

RESULTS

The analysis showed a nominally significant association with the GFRA1 gene.

CONCLUSION

The common variants investigated in this study had no major influence on the antipsychotic dosage prescribed in study participants. It remains, though, that this strategy may prove valuable clinically and warrants further investigation.

Pharmacogenetics in schizophrenia: a review of clozapine studies

OBJECTIVES

Clozapine is quite effective to treat schizophrenia, but its use is complicated by several factors. Although many patients respond to antipsychotic therapy, about 50% of them exhibit inadequate response, and ineffective medication trials may entail weeks of unremitted illness, potential adverse drug reactions, and treatment nonadherence. This review of the literature sought to describe the main pharmacogenetic studies of clozapine and the genes that potentially influence response to treatment with this medication in schizophrenics.

METHODS

We searched the PubMed database for studies published in English in the last 20 years using keywords related to the topic.

RESULTS AND CONCLUSIONS

Our search yielded 145 studies that met the search and selection criteria. Of these, 21 review articles were excluded. The 124 studies included for analysis showed controversial results. Therefore, efforts to identify key gene mechanisms that will be useful in predicting clozapine response and side effects have not been fully successful. Further studies with new analysis approaches and larger sample sizes are still required.

Pharmacogenetics of antipsychotics

OBJECTIVE

During the past decades, increasing efforts have been invested in studies to unravel the influence of genetic factors on antipsychotic (AP) dosage, treatment response, and occurrence of adverse effects. These studies aimed to improve clinical care by predicting outcome of treatment with APs and thus allowing for individualized treatment strategies. We highlight most important findings obtained through both candidate gene and genome-wide association studies, including pharmacokinetic and pharmacodynamic factors.

METHODS

We reviewed studies on pharmacogenetics of AP response and adverse effects published on PubMed until early 2012. Owing to the high number of published studies, we focused our review on findings that have been replicated in independent studies or are supported by meta-analyses.

RESULTS

Most robust findings were reported for associations between polymorphisms of the cytochrome P450 system, the dopamine and the serotonin transmitter systems, and dosage, treatment response, and adverse effects, such as AP-induced weight gain or tardive dyskinesia. These associations were either detected for specific medications or for classes of APs.

CONCLUSION

First promising and robust results show that pharmacogenetics bear promise for a widespread use in future clinical practice. This will likely be achieved by developing algorithms that will include many genetic variants. However, further investigation is warranted to replicate and validate previous findings, as well as to identify new genetic variants involved in AP response and for replication of existing findings.

DNA methylation signatures of peripheral leukocytes in schizophrenia

Schizophrenia (SCZ) is a complex psychiatric disease with a lifetime morbidity rate of 0.5-1.0 %. To date, aberrant DNA methylation in SCZ has been reported in several studies. However, no comprehensive studies using medication-free subjects with SCZ have been conducted. In addition, most of these studies have been limited to the analysis of the CpG sites in CpG islands (CGIs) in the gene promoter regions, so little is known about the DNA methylation signatures across the whole genome in SCZ. Genome-wide DNA methylation profiling (485,764 CpG sites) of peripheral leukocytes was conducted in the first set of samples (24 medication-free patients with SCZ and 23 non-psychiatric controls) using Infinium HumanMethylation450 Beadchips. Second, a monozygotic twin study was performed using three pairs of monozygotic twins that were discordant for SCZ. Finally, the data from these two independent cohorts were compared. A total of 234 differentially methylated CpG sites that were common between these two cohorts were identified. Of the 234 CpG sites, 153 sites (65.4 %) were located in the CGIs and in the regions flanking CGIs (CGI: 40.6 %; CGI shore: 13.3 %; CGI shelf: 11.5 %). Of the 95 differently methylated CpG sites in the CGIs, most of them were located in the promoter regions (promoter: 75.8 %; gene body: 14.7 %; 3'-UTR: 2.1 %). Aberrant DNA methylation in SCZ was identified at numerous loci across the whole genome in peripheral leukocytes using two independent sets of samples. These findings support the notion that altered DNA methylation could be involved in the pathophysiology of SCZ.

Genome-wide analysis of polymorphisms associated with cytokine responses in smallpox vaccine recipients

The role that genetics play in response to infection or disease is becoming increasingly clear as we learn more about immunogenetics and host-pathogen interactions. Here we report a genome-wide analysis of the effects of host genetic variation on cytokine responses to vaccinia virus stimulation in smallpox vaccine recipients. Our data show that vaccinia stimulation of immune individuals results in secretion of inflammatory and Th1 cytokines. We identified multiple SNPs significantly associated with variations in cytokine secretion. These SNPs are found in genes with known immune function, as well as in genes encoding for proteins involved in signal transduction, cytoskeleton, membrane channels and ion transport, as well as others with no previously identified connection to immune responses. The large number of significant SNP associations implies that cytokine secretion in response to vaccinia virus is a complex process controlled by multiple genes and gene families. Follow-up studies to replicate these findings and then pursue mechanistic studies will provide a greater understanding of how genetic variation influences vaccine responses.

Pharmacogenetics of response to antipsychotics in patients with schizophrenia

This review presents the findings of pharmacogenetic studies exploring the influence of gene variants on antipsychotic treatment response, in terms of both symptom improvement and adverse effects, in patients with schizophrenia. Despite numerous studies in the field, replicating findings across different cohorts that include subjects of different ethnic groups has been challenging. It is clear that non-genetic factors have an important contribution to antipsychotic treatment response. Differing clinical, demographic and environmental characteristics of the cohorts studied have added substantial complexity to the interpretation of the positive and negative findings of many studies. Pharmacogenomic genome-wide investigations are beginning to yield interesting data although they have failed to replicate the most robust findings of candidate gene studies, and are limited by the sample size, especially given the need for studying homogeneous cohorts. Most of the studies conducted on cohorts treated with single antipsychotics have investigated clozapine, olanzapine or risperidone response. These studies have provided some of the most replicated associations with treatment efficacy. Serotonergic system gene variants are significantly associated with the efficacy of clozapine and risperidone, but may have less influence on the efficacy of olanzapine. Dopamine D3 receptor polymorphisms have been more strongly associated with the efficacy of clozapine and olanzapine, and D2 genetic variants with the efficacy of risperidone. Serotonin influences the control of feeding behaviour and has been hypothesized to have a role in the development of antipsychotic-induced weight gain. Numerous studies have linked the serotonin receptor 2C (5-HT2C) -759-C/T polymorphism with weight gain. The leptin gene variant, -2548-G/A, has also been associated with weight gain in several studies. Pharmacogenetic studies support the role of cytochrome P450 enzymes and dopamine receptor variants in the development of antipsychotic-induced movement disorders, with a contribution of serotonergic receptors and other gene variants implicated in the mechanism of action of antipsychotics. Clozapine-induced agranulocytosis has been associated with polymorphisms in the major histocompatibility complex gene (HLA).

Lack of association of NALCN genetic variants with schizophrenia

NALCN (sodium leak channel, non-selective) is located on chromosome 13q (suggested linkage region for schizophrenia). We analyzed 21 polymorphisms in 464 schizophrenia subjects, 220 controls subjects and 119 small nuclear families. We observed nominal association with rs9518320 and rs9518331, suggesting that NALCN is not related to schizophrenia risk.

Are serotonin 3A and 3B receptor genes associated with suicidal behavior in schizophrenia subjects?

Suicide is a major contributor to the morbidity and mortality of schizophrenia, accounting for approximately 10% of deaths in these patients. Genetic factors have been reported to modulate the risk for suicide, although the precise mechanism and magnitude of the genetic contribution are unknown. Further, suicide attempters present abnormalities in the serotonergic system. We evaluated whether genetic variants in the serotonin receptors HTR3A (rs897692, rs1150226, rs1176724, rs2276302, rs3737457, rs897687 and rs1176713) and HTR3B (rs3758987, rs10502180, rs11606194, rs17116121, rs1176744, rs17116138, rs2276307, rs3782025 and rs1176761) were susceptibility components for suicidal behavior in 154 Caucasians schizophrenia subjects (20.1% of suicide attempters). In a second step, we compared haplotype and gene-gene interaction approaches because both genes are located in the chromosome 11q23 approximately 28Kbp apart. We did not observe allelic or genotypic associations. Six haplotypes were nominally significant associated with suicide. Gene-gene interaction using Helix Tree software showed two nominally significant interactions reproduced by haplotype association. Likewise, haplotypes composed by the markers included in the best multidimensional reduction three-locus model were nominally significant. Our results suggest that HTR3A and HTR3B polymorphisms may not play a major role in the susceptibility for suicidal behavior in schizophrenia subjects. Moreover, gene-gene interaction and haplotype association may have consistent results for genes located in the same chromosome.

Influence of neurexin 1 (NRXN1) polymorphisms in clozapine response

Schizophrenia patients show alterations in the synaptic connectivity. However, it remains unknown whether antipsychotic response may be altered depending on the synaptic connectivity. We tested this association using polymorphisms in the neurexin 1 gene (NRXN1). Large NRXN1 deletions have also been associated with schizophrenia. We genotyped four variants in 140 schizophrenia patients assessed prospectively for clozapine response after 6 months. We observed a trend toward association of clozapine response with the rs12467557 (permuted p = 0.051). These results should be read with caution before independent replication.

Cocaine and amphetamine regulated transcript (CART) gene in the comorbidity of schizophrenia with alcohol use disorders and nicotine dependence

Studies have shown a genetic susceptibility to develop schizophrenia, alcohol use disorders and nicotine dependence. Brain areas related to reward and reinforcement show high expression of the cocaine and amphetamine regulated transcript (CART). Nicotine and alcohol are also able to modulate CART expression in the hypothalamic areas. In this study, we evaluated whether CART variants would influence the predisposition of schizophrenia subjects to alcohol use disorders and nicotine dependence. Clinical and genetic data were obtained from 190 unrelated Caucasian schizophrenia subjects collected at the Centre for Addiction and Mental Health. We found no association of CART variants with alcohol use disorders or nicotine dependence. We found a trend for allelic association of rs11575893 with the heaviness of smoking behaviour (p=0.057). Our results indicate that genetic variants in the CART gene may not play a major role in the vulnerability of schizophrenia subjects to concurrent alcohol use disorders and nicotine dependence. Additional association studies in independent samples can evaluate whether CART gene is playing a role in the schizophrenia comorbidity with alcohol use disorders and nicotine dependence.

Schizophrenia severity and clozapine treatment outcome association with oxytocinergic genes

Antipsychotic drugs are the best means available for symptomatically treating individuals suffering from schizophrenia; however, there is a significant variability in clinical response to these psychotropic medications. Previous findings connect oxytocin (OXT) with schizophrenia and antipsychotic action. Therefore, we evaluated if OXT and OXT receptor (OXTR) genes might play a role in the symptom severity and clozapine treatment response in schizophrenia subjects. The rs2740204 variant in the OXT gene was significantly associated with treatment response (after 1000 permutations p=0.042) and nominally associated with negative symptoms in our sample. Furthermore, variants in the OXTR were nominally associated with severity of overall symptoms accessed using the Brief Psychiatric Rating Scale (rs237885, rs237887) as well as on the improvement of the positive symptoms (rs11706648, rs4686301, rs237899). Additional association studies in independent samples will be able evaluate whether OXT and OXTR genes are truly playing a role in the clozapine treatment outcome.

Glial cell line-derived neurotrophic factor receptor alpha 2 (GFRA2) gene is associated with tardive dyskinesia

INTRODUCTION

Tardive dyskinesia (TD) has a pharmacogenetic component in which the interaction of antipsychotic exposure with individual genetic variation mediates risk. The glial cell line-derived neurotrophic factor (GDNF) signalling pathway has been associated with neuroprotective effects in central dopaminergic neurons and spinal motor neurons. Clinical trials have also investigated whether GDNF may ameliorate Parkinson's disease symptoms.

METHODS

We tested whether variants in the GDNF receptor alpha 2 (GFRA2) gene could play a role in TD susceptibility evaluating 16 variants in 172 Caucasian schizophrenia subjects.

RESULTS

We observed one significant allelic association (rs4739285, permuted p = 0.042) and two genotypic associations: rs4739285 under additive inheritance model and rs4739217 under dominant inheritance model (permuted p = 0.044). Moreover, carriers of the major alleles for both rs6587002 and rs4739217 presented significantly higher risk for TD (OR = 2.04, permuted p = 0.014), while subjects with the minor allele for rs4739217 and the major allele for rs6988470 were less likely to have TD (OR = 0.21, permuted p = 0.0007).

DISCUSSION

Haplotype results indicate that the minor allele of the rs4739217 is a risk factor for TD (permuted allelic p = 0.074). Age was also a risk factor for TD in our sample (p = 0.0001). Taken together, our findings suggest that GFRA2 genetic variants and age may play a role in TD susceptibility, but further work is required to confirm these findings.