Genetic association study in psychiatry: analytical evaluation and a recommendation

G-protein beta 3 subunit polymorphisms and essential hypertension: a case-control association study in northern Han Chinese

OBJECTIVE

To explore the association between the three polymorphisms [ C825T, C1429T and G(-350)A] of the gene encoding the G protein beta 3 subunit (GNB3) and hypertension by performing a case-control study in the northern Han Chinese population.

METHODS

We recruited 731 hypertensive patients and 673 control subjects (the calculated power value was > 0.8). Genotyping was performed to identify C825T, C1429T and G(-350)A polymorphisms using the TaqMan assay. Comparisons of allelic and genotypic frequencies between cases and controls were made by using the chi-square test. Logistic regression analyses were performed to investigate the relationships between the three polymorphisms of GNB3 gene under different genetic models (additive, dominant and recessive models).

RESULTS

The genotype distribution and allele frequencies of C825T, C1429T and G(-350)A polymorphisms did not differ significantly between hypertensive patients and control subjects, either when the full sample was assessed, or when the sample was stratified by gender. No significant association was observed between C825T, C1429T and G(-350)A polymorphisms and the risk of essential hypertension in any genetic model. Linkage disequilibrium was only detected between C825T and C1429T polymorphisms. Haplotype analyses observed that none of the three estimated haplotypes significantly increased the risk of hypertension.

CONCLUSIONS

Our study suggested that the GNB3 gene polymorphisms [C825T, C1429T and G(-350)A] were not significantly associated with essential hypertension in northern Han Chinese population.

Hypothesizing dopaminergic genetic antecedents in schizophrenia and substance seeking behavior

The dopamine system has been implicated in both substance use disorder (SUD) and schizophrenia. A recent meta-analysis suggests that A1 allele of the DRD2 gene imposes genetic risk for SUD, especially alcoholism and has been implicated in Reward Deficiency Syndrome (RDS). We hypothesize that dopamine D2 receptor (DRD2) gene Taq1 A2 allele is associated with a subtype of non-SUD schizophrenics and as such may act as a putative protective agent against the development of addiction to alcohol or other drugs of abuse. Schizophrenics with SUD may be carriers of the DRD2 Taq1 A1 allele, and/or other RDS reward polymorphisms and have hypodopaminergic reward function. One plausible mechanism for alcohol seeking in schizophrenics with SUD, based on previous research, may be a deficiency of gamma type endorphins that has been linked to schizophrenic type psychosis. We also propose that alcohol seeking behavior in schizophrenics, may serve as a physiological self-healing process linked to the increased function of the gamma endorphins, thereby reducing abnormal dopaminergic activity at the nucleus accumbens (NAc). These hypotheses warrant further investigation and cautious interpretation. We, therefore, encourage research involving neuroimaging, genome wide association studies (GWAS), and epigenetic investigation into the relationship between neurogenetics and systems biology to unravel the role of dopamine in psychiatric illness and SUD.

Neurogenetics of dopaminergic receptor supersensitivity in activation of brain reward circuitry and relapse: proposing "deprivation-amplification relapse therapy" (DART)

BACKGROUND AND HYPOTHESIS

It is well known that after prolonged abstinence, individuals who use their drug of choice experience a powerful euphoria that often precipitates relapse. While a biological explanation for this conundrum has remained elusive, we hypothesize that this clinically observed "supersensitivity" might be tied to genetic dopaminergic polymorphisms. Another therapeutic conundrum relates to the paradoxical finding that the dopaminergic agonist bromocriptine induces stronger activation of brain reward circuitry in individuals who carry the DRD2 A1 allele compared with DRD2 A2 allele carriers. Because carriers of the A1 allele relative to the A2 allele of the DRD2 gene have significantly lower D2 receptor density, a reduced sensitivity to dopamine agonist activity would be expected in the former. Thus, it is perplexing that with low D2 density there is an increase in reward sensitivity with the dopamine D2 agonist bromocriptine. Moreover, under chronic or long-term therapy with D2 agonists, such as bromocriptine, it has been shown in vitro that there is a proliferation of D2 receptors. One explanation for this relates to the demonstration that the A1 allele of the DRD2 gene is associated with increased striatal activity of L-amino acid decarboxylase, the final step in the biosynthesis of dopamine. This appears to be a protective mechanism against low receptor density and would favor the utilization of an amino acid neurotransmitter precursor like L-tyrosine for preferential synthesis of dopamine. This seems to lead to receptor proliferation to normal levels and results in significantly better treatment compliance only in A1 carriers.

PROPOSAL AND CONCLUSION

We propose that low D2 receptor density and polymorphisms of the D2 gene are associated with risk for relapse of substance abuse, including alcohol dependence, heroin craving, cocaine dependence, methamphetamine abuse, nicotine sensitization, and glucose craving. With this in mind, we suggest a putative physiological mechanism that may help to explain the enhanced sensitivity following intense acute dopaminergic D2 receptor activation: "denervation supersensitivity." Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D2 receptor proliferation occurs (20%-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. Thus, the administration of dopamine D2 agonists would target D2 sensitization and attenuate relapse, especially in D2 receptor A1 allele carriers. This hypothesized mechanism is supported by clinical trials utilizing amino acid neurotransmitter precursors, enkephalinase, and catechol-O-methyltransferase (COMT) enzyme inhibition, which have resulted in attenuated relapse rates in reward deficiency syndrome (RDS) probands. If future translational research reveals that dopamine agonist therapy reduces relapse in RDS, it would support the proposed concept, which we term "deprivation-amplification relapse therapy" (DART). This term couples the mechanism for relapse, which is "deprivation-amplification," especially in DRD2 A1 allele carriers with natural D2 agonist therapy utilizing amino acid precursors and COMT and enkepalinase inhibition therapy.

Correlation of angiotensin-converting enzyme 2 gene polymorphisms with stage 2 hypertension in Han Chinese

Experimental evidence indicates that angiotensin-converting enzyme 2 (ACE2), a homologue of human ACE, might negatively regulate the activated renin-angiotensin-aldosterone system (RAAS) and might function as a protective regulator in the pathogenesis of hypertension. However, association studies regarding ACE2 are sparse in the literature, with negative results in the majority of cases. Here we conducted an association study between 2 intronic polymorphisms (A1075G and G8790A) of the ACE2 gene and stage 2 hypertension in Han Chinese. We genotyped the 2 polymorphisms in 1494 subjects (808 stage 2 hypertensives and 686 normotensives) recruited from the Fangshan district (Beijing). Data were analyzed using chi(2) test, 1-way analysis of variance, and logistic regression where appropriate. The frequency of A1075G allele distribution in males differed significantly (P < 0.0001), whereas the genotype and allele distributions of G8790A polymorphism were similar, between stage 2 hypertensives and normotensives. Systolic blood pressure (SBP) differed significantly in females across both genotypes: SBP was significantly lower in subjects with the 1075AA and 8790GG genotypes, higher in the 1075GG (+13.65 mm Hg versus AA) and 8790AA (+13.36 mm Hg versus GG) genotypes, and intermediate in the 1075AG (+5.76 mm Hg versus AA) and 8790GA (+5.65 mm Hg versus GG) genotypes. Our data suggest that the polymorphism (A1075G) might be a risk factor-at least a marker-for stage 2 hypertension in males and that the 2 studied polymorphisms might be the indicators of systolic hypertension in females.

Social adjustment could be associated with the serotonin transporter gene in remitted patients with mood disorders and healthy subjects

We investigated the serotonin transporter gene (5-HTTLPR) in the functioning of euthymic patients with mood disorders (n=285) and healthy controls (n=94). The 5-HTTLPR genotype showed marginal associations with working, familial and social adjustment, with marked differences between controls and patients. Because of the exploratory nature of our analyses, all results should be viewed with caution. However, we observed how the s/s variant, commonly associated with detrimental effects, could be beneficial in specific areas of social adjustment.

Manic-depression genes and the new millennium: poised for discovery

Manic-depressive illness is a common psychiatric disorder with complex etiology that likely involves multiple genes and non-genetic influences. The uncertain path to gene discovery has spurred considerable debate over genetic findings and gene-finding strategies. In this article, I review the main findings, with a focus on: (1) putative linked loci on chromosomes 1q31-32, 4p16, 6pter-p24, 10p14, 10q21-26, 12q23-24, 13q31-32, 18p11, 18q21-23, 21q22, 22q11-13, and Xq24-28; and (2) association studies with candidate genes, dynamic mutations, mitochondrial mutations, and chromosomal aberrations. Although no gene has been identified, promising findings are emerging. I then discuss the challenges and opportunities ahead, with special emphasis on gene-finding methods-in particular, questions pertaining to phenotype definition, linkage and association mapping, gene markers, sampling, study population, multigene systems, lessons from other disorders, animal models, and bioinformatics. The progress to date, together with rapid advances in genomics, analytical and computational methods, and bioinformatics, holds promise for new insights into the genetics of manic-depression, in the new millennium.

Serotonin transporter gene (5-HTTLPR) and major psychoses

Serotoninergic neurotransmitter systems have been implicated in the pathogenesis of major psychoses. A functional polymorphism (5-HTTLPR) in the upstream regulatory region of the gene (SLC6A4) has been associated with a number of psychiatric disturbances, but conflicting replication followed. The aim of this study was to investigate the possibility that the 5-HTTLPR might be associated with major psychoses. One thousand, eight hundred and twenty inpatients (789 bipolars, 667 major depressives, 66 delusionals, 261 schizophrenics, 37 psychotics not otherwise specified-NOS) and 457 control subjects were included in this study. A subsample of 1235 patients (523 bipolars, 359 major depressives, 259 schizophrenics, 66 delusionals, 28 psychotic NOS) were evaluated for lifetime psychotic symptomatology using the Operational Criteria for Psychotic illness (OPCRIT) checklist. The subjects were also typed for 5-HTTLPR variants using PCR techniques. 5-HTTLPR allele frequencies were not significantly different between controls and bipolars, major depressives, schizophrenics, delusionals and psychotic NOS; genotype analysis also did not show any association. The analysis of symptomatology did not show significant differences. Consideration of possible stratification factors such as sex and age of onset did not significantly influence results. 5-HTTLPR variants are not therefore a liability factor for major psychoses or for major psychoses symptomatology.

Association between a polymorphism in the pseudoautosomal X-linked gene SYBL1 and bipolar affective disorder

In the past decade, several chromosomal regions have been analyzed for linkage with bipolar affective disorder (BPAD). There have been conflicting results regarding the involvement of X-chromosomal regions in harboring susceptibility genes for BPAD. Recently, a new candidate gene (SYBL1) for BPAD has been described on Xq28. SYBL1, which maps to the Xq pseudoautosomal region (PAR), encodes a member of the synaptobrevin family of proteins involved in synaptic vesicle docking, exocytosis, and membrane transport. A subsequent case-control association study, including 110 US-American patients with BPAD and 119 unrelated controls, investigated a potential etiological role of a novel polymorphism (G-->C transversion) in a regulatory region of the SYBL1 gene. In this analysis, the C allele showed a statistical trend to be more frequent in males with BPAD than in respective controls (P=0.06). This finding prompted us to verify whether a similar effect was also present in a larger German sample of 164 unrelated patients with BPAD (148 patients with BP I disorder, 16 patients with BP II disorder) and 267 controls. We observed a significantly increased frequency of genotypes homozygous for the C allele in females with BPAD in comparison with controls (P=0.017). Thus, our data strengthen the role of the SYBL1 gene as a candidate gene for BPAD.

Tryptophan hydroxylase gene associated with paroxetine antidepressant activity

The possible association of the A218C tryptophan hydroxylase (TPH) gene variant with the antidepressant activity of paroxetine was investigated in a sample of 121 inpatients affected by a major depressive episode and treated with paroxetine 20-40 mg with either placebo or pindolol in a double blind design for 4 weeks. The severity of depressive symptoms was weekly assessed with the Hamilton Rating Scale for Depression. TPH allelic variants were determined in each subject using a PCR-based technique. TPH*A/A and TPH*A/C variants were associated with a poorer response to paroxetine treatment when compared to TPH*C/C (P=0.005); this difference was not present in the pindolol augmented group. Other variables, such as sex, diagnosis, presence of psychotic features, severity of depressive symptomatology at baseline and paroxetine plasma level, were not associated with the outcome. TPH gene variants are therefore a possible modulator of paroxetine antidepressant activity.

The search for complex disease genes: fault by linkage or fault by association?

With the human DNA sequence nearing completion, the search for complex disease genes is gaining momentum, as is the debate over gene-finding strategies. This overview contrasts two pivotal methods: linkage analysis and association mapping. Linkage analysis has been used successfully to identify the genes underlying rare mendelian disorders. It has also played a role in attempts to map genes for common non-mendelian (also known as 'complex' or 'multifactorial') diseases such as psychiatric disorders. However, despite extensive efforts progress has been slow, marred by inconsistent or ambiguous results. Uncertainties about the utility of the linkage approach for complex genetic traits has spurred interest in association studies with candidate genes, as an alternate strategy. Recently, with the advent of new molecular tools, in particular high-density, single-nucleotide polymorphisms (SNPs) maps, it has been argued that, while linkage analysis may retain some role, genome-wide association studies with SNPs offer a superior strategy for unraveling genetic complexity. In this paper I review these issues, stressing the pros and cons of the various strategies. I propose that: (1) the uncertainties in association studies may have been underestimated; (2) neither method is sufficient or optimal; and (3) a joint linkage and association approach, together with genomic, statistical and computational advances, may have greater promise for understanding the genetic underpinnings of complex disorders in the new millennium.

Serotonergic gene expression and depression: implications for developing novel antidepressants

The development and configuration of several neural networks is dependent on the actions of serotonin (5-HT) acting through multiple hetero- and autoreceptor subtypes. During early brain development 5-HT modulates morphogenetic activities, such as neural differentiation, axon outgrowth, and synaptic modeling. In the adult brain, midbrain raphe serotonergic neurons project to a variety of brain regions and modulate a wide range of physiological functions. Several lines of evidence indicate that genetically determined variability in serotonergic gene expression, as it has been documented for the 5-HT transporter, influences temperamental traits and may lead to psychopathological conditions with increased anxiety, depression, and aggression. Investigation of the regulation of serotonergic gene transcription and its impact on neuronal development, synaptic plasticity, and neurogenesis spur interest to identify serotonergic gene-related molecular factors underlying disease states and to develop more effective antidepressant treatment strategies. Gene targeting strategies have increasingly been integrated into investigations of brain function and along with the fading dogma of a limited capacity of neurons for regeneration and reproducibility, it is realized that gene transfer techniques using efficient viral vectors in conjunction with neuron-selective transcriptional control systems may also be applicable to complex disorders of the brain. Given the fact that the 5-HT system continues to be an important target for drug development and production, novel strategies aiming toward the modification of 5-HT function at the level of gene expression are likely to be exploited by enterprises participating actively in the introduction of alternative therapeutic approaches.

Genotypic relative risks under ordered restriction

In mapping diseases of complex aetiology, conventional linkage approaches narrow the location of the disease susceptibility locus to quite a large region so that candidate gene association studies are then necessary to further isolate these genes. However, even in the simplest scenario where the candidate locus is bi-allelic, two statistical tests with various correcting factors have been proposed: a chi-square 1 df test (counting chromosomes) which may be slightly conservative and a 2 df chi-square test (counting genotypes) which may lack power because of the extra degree of freedom. This paper introduces a better and more powerful alternative which turns out to be a compromise between the two existing statistical tests. The asymptotic distribution of this test statistic is determined and the efficacy of the 3 tests are compared under different genetic models by simulation.

Genetic studies of substance abuse

Genetic studies of substance abuse indicate that variation in the risk for the disorder in the population is contributed by differences in both individual genotypes and environment. Recent developments in genetics raise the possibility of disentangling the complex system of genotype-environment interaction that determines the development of the individual behavioral phenotype. This paper reviews the concepts, methods and results pertaining to genetic investigation of substance abuse.

Linkage of mood disorders with D2, D3 and TH genes: a multicenter study

BACKGROUND

It has been suggested that the dopaminergic system is involved in the pathophysiology of mood disorders. We conducted a multicenter study of families with mood disorders, to investigate a possible linkage with genes coding for dopamine receptor D2, dopamine receptor D3 and tyrosine hydroxylase (TH).

METHODS

Twenty three mood disorder pedigrees collected within the framework of the European Collaborative Project on Affective Disorders were analyzed with parametric and non-parametric linkage methods. Various potential phenotypes were considered, from a narrow (only bipolar as affected) to a broad (bipolar+major depressive+schizoaffective disorders) definition of affection status.

RESULTS

Parametric analyses excluded linkage for all the candidate genes, even though small positive LOD (Limit of Detection) scores were observed for TH in three families. Non-parametric analyses yielded negative results for all markers.

CONCLUSION

The D2 and D3 dopamine receptors were, therefore, not a major liability factor for mood disorders in our sample, whereas TH may play a role in a subgroup of patients.

Dopamine receptor D2 Ser/Cys311 variant associated with disorganized symptomatology of schizophrenia

The dopamine D2 receptor gene has been proposed as a genetic risk factor for schizophrenia (Arinami et al., 1994). However, a number of replications failed to confirm the initial report. The finding of a stronger association considering schizophrenics with the absence of negative symptoms (Arinami et al., 1996) suggested that the influence of DRD2 variants should be analyzed more at the level of symptoms rather than syndromes. One hundred and four inpatients affected by schizophrenia (n = 99) and delusional disorder (n = 5) (DSM IV) were assessed at admission by the Operational Criteria for Psychotic Illness (OPCRIT) and were typed for DRD2 variants using polymerase chain reaction (PCR) techniques. Subjects with the S311C variant presented a higher score on the 'Disorganization' factor (P = 0.012). Consideration of possible stratification effects such as sex and age of onset did not reveal any deviation from the whole sample. In conclusion, our preliminary report suggests that the DRD2 S311C variant may be a liability factor for disorganized symptoms among schizophrenics or for a subtype of schizophrenia characterized by highly disorganized symptomatology.

G protein beta3 subunit variant and essential hypertension in Japanese

Enhanced G protein activation has been implicated to underlie the increased sodium-proton transport in blood cells, an impaired characteristic observed in 30% to 50% of patients with essential hypertension. Recently, significant association between a C825T polymorphism of the gene encoding the G protein beta3 subunit and hypertension was demonstrated in a white population, together with the finding that the T825 variant might be related to alternative splicing through unidentified mechanisms. We therefore investigated the disease relevance of this candidate gene by conducting an association study in a relatively large Japanese population. Participants comprised 718 hypertensive case subjects (without diabetes mellitus), 515 normotensive control subjects, and 191 hypertensive subjects with borderline or established diabetes mellitus; all individuals were recruited at a single institution. Genotype distribution of the C825T polymorphism was compared between hypertensive subjects, with or without diabetes, and the control group with chi2 statistics. No significant association was observed in the present study. Results were still not significant when the case group was subdivided according to more stringent classification criteria. Allele frequencies of T825 proved to be almost concordant among the 3 study groups and higher in Japanese (49.0% to 49.6%) compared with a reported prevalence of 25% to 31% in whites. Our data suggest that the T825 variant of the G protein beta3 subunit gene is unlikely to constitute major susceptibility for essential hypertension in the Japanese population studied. However, further investigation is required to answer the question of whether the lack of association reflects ethnic differences in the nature of genetic susceptibility loci.

Self-esteem in remitted patients with mood disorders is not associated with the dopamine receptor D4 and the serotonin transporter genes

Disturbances of the dopaminergic and serotoninergic neurotransmitter systems have been implicated in the pathogenesis of depressive symptoms. Associations have been reported between markers of the two neurotransmitter systems and the presence of illness or severity of depressive episodes, but no attention has been focused on the periods of remission. The present report focuses on a possible association of self-esteem in remitted mood disorder patients with the functional polymorphism located in the upstream regulatory region of the serotonin transporter gene (5-HTTLPR) and the dopamine receptor D4 (DRD4). Inpatients (N=162) affected by bipolar (n=103) and unipolar (n=59) disorder (DSM III-R) were assessed by the Self-Esteem Scale (SES, Rosenberg, 1965) and were typed for DRD4 and 5-HTTLPR (n=58 subjects) variants at the third exon using polymerase chain reaction (PCR) techniques. Neither DRD4 nor 5-HTTLPR variants were associated with SES scores, and consideration of possible stratification effects such as sex and psychiatric diagnosis did not reveal any association either. The serotonin transporter and dopamine receptor D4 genes do not, therefore, influence self-esteem in remitted mood disorder subjects.

Lack of association between the alpha-adducin locus and essential hypertension in the Japanese population

Significant linkage and association of alpha-adducin, a cytoskeleton protein involved in transmembrane ion transport, with essential hypertension were recently shown in Caucasian populations, especially in relation to salt sensitivity. The present study investigated the relevance of this candidate gene to hypertension in a well-characterized Japanese population. A total number of 507 individuals were selected from clinic outpatients. Hypertensive subjects were defined on the basis of the individual's blood pressure readings before starting medications; the criteria included systolic blood pressure > or = 160 mm Hg and/or diastolic blood pressure > or = 95 mm Hg. Patients with diabetes mellitus, renal failure, and secondary forms of hypertension had been excluded. Control subjects had blood pressure values < 130/85 mm Hg. The allele frequency of a genetic variant at amino acid residue 460 of alpha-adducin (460Trp) was compared between cases and control subjects with chi(2) statistics; in addition, the association was tested with blood pressure as a continuous variable. No significant association was found in either of the statistics tested. The 460Trp variant appeared to be relatively common in the Japanese (54% to 60%) compared with a reported prevalence of 13% to 23% in Caucasians. The present study brought up an important issue concerning the pathophysiological role of alpha-adducin in non-Caucasian populations, given the likely ethnic variation in the nature of genetic susceptibility loci. The 460Trp variant of the alpha-adducin gene is unlikely to have a major effect on susceptibility to hypertension in the Japanese population studied, although the present study does not exclude the involvement of alpha-adducin in the pathogenesis of hypertension.

An insertion/deletion polymorphism in the angiotensin converting enzyme gene is associated with both brain substance P contents and affective disorders

Because of a potent action of angiotensin converting enzyme (ACE) to degrade substance P (SP) and an association of the insertion/deletion (I/D) polymorphism of the ACE gene with ACE activity, an association between the SP level and the ACE I/D polymorphism were examined using 20 human postmortem brain samples. The results showed a significant association between the polymorphism and SP levels in the basal ganglia and substantia nigra, where both ACE and SP concentrate, and a higher SP level in the subjects with the DD genotype than in those with the II genotype, with an intermediate level in heterozygotes. Associations of the polymorphism with schizophrenia and affective disorders were also investigated in 292 unrelated Japanese schizophrenics, 65 patients with affective disorders, and 579 controls. The D allele was significantly more frequent in the patients with affective disorders than in the controls (p < .02), and the DD genotype was significantly more frequent in the patients with affective disorders than in the controls (p < .002). There is no significant difference in the frequencies of the allele and the genotype between the controls and schizophrenics. These results suggest that the ACE I/D polymorphism is one of the genetic factors for an interindividual variability of brain SP levels, and that the ACE polymorphism may contribute to the susceptibility to affective disorders.