John P. Dahl

Confirmation of Association Between the Val66Met Polymorphism in the Brain-Derived Neurotrophic Factor (BDNF) Gene and Bipolar I Disorder

Recent studies have indicated that the brain‐derived neurotrophic factor (BDNF) gene is involved in the etiology of bipolar disorder (BPD). Two family‐based association studies showed that the Val allele of the functional polymorphism Val66Met in the BDNF gene is associated with BPD; however, others could not confirm the results. Here we performed a replication study in an independent sample and tested the hypothesis that the Val66 allele in the BDNF gene confers susceptibility to bipolar I disorder (BPI). Six hundred twenty‐one patients with BPI and 998 control subjects were genotyped for the Val66Met polymorphism. All cases and controls were of European descent. All BPI patients had a positive family history of affective disorder. The frequency of the Val allele was significantly increased in BPI patient when compared to controls (χ2 = 4.8; df = 1; P = 0.028; two‐sided; OR = 1.22; 95% CI: 1.02–1.47). Results confirm previous findings and suggest that the Val allele increases risk for BPI in patients of European descent. Further studies are necessary to elucidate the involvement of the BDNF gene in the pathophysiology of BPD.

Interaction between variation in the D2 dopamine receptor (DRD2) and the neuronal calcium sensor-1 (FREQ) genes in predicting response to nicotine replacement therapy for tobacco dependence

We have previously demonstrated that a functional dopamine D2 receptor promoter variant (DRD2 −141 Ins/Del) predicts response to nicotine replacement therapy (NRT). The present study extends this finding in the same population of 363 NRT-treated subjects, by examining variation in the gene encoding the neuronal calcium sensor-1 protein (FREQ), which functions to regulate D2 receptor desensitization. The results indicate a statistically significant interaction effect of DRD2−141 and FREQ genotypes on abstinence at the end of the NRT treatment phase; 62% of the smokers with at least one copy of the DRD2 −141 Del allele and two copies of the FREQ rs1054879 A allele were abstinent from smoking, compared to 29–38% abstinence rates for other smokers in the trial. This result suggests that the interaction between variation in the DRD2 and FREQ genes, which both encode components of the D2 dopamine receptor signal transduction pathway, impacts the efficacy of NRT.

Variations in the vesicular monoamine transporter 1 gene (VMAT1/SLC18A1) are associated with bipolar I disorder

The vesicular monoamine transporter 1 gene (VMAT1/SLC18A1) maps to the shared bipolar disorder (BPD)/schizophrenia (SZ) susceptibility locus on chromosome 8p21. Vesicular monoamine transporters are involved in transport of monoamine neurotransmitters which have been postulated to play a relevant role in the etiology of BPD and/or SZ. Variations in the VMAT1 gene might affect transporter function and/or expression and might be involved in the etiology of BPD and/or SZ. Genotypes of 585 patients with BPD type I and 563 control subjects were obtained for three missense single nucleotide polymorphisms (SNPs) (Thr4Pro, Thr98Ser, Thr136Ile) and four non-coding SNPs (rs988713, rs2279709, rs3735835, rs1497020). All cases and controls were of European descent. Allele frequencies differed significantly for the potential functional polymorphism Thr136Ser between BPD patients and controls (p=0.003; df=1; OR=1.34; 95% CI: 1.11–1.62). Polymorphisms in the promoter region (rs988713: p=0.005, df=1; OR=1.31; 95% CI: 1.09–1.59) and intron 8 (rs2279709: p=0.039, df=1; OR=0.84; 95% CI: 0.71–0.99) were also associated with disease. Expression analysis confirmed that VMAT1 is expressed in human brain at the mRNA and protein level. Results suggest that variations in the VMAT1 gene may confer susceptibility to BPD in patients of European descent. Additional studies are necessary to confirm this effect and to elucidate the role of VMAT1 in central nervous system physiology.

Genetic variation in mu-opioid-receptor-interacting proteins and smoking cessation in a nicotine replacement therapy trial

Extending a previous finding of an association between functional genetic variation in the mu-opioid receptor gene and response to nicotine replacement therapy, we explored the role of genetic variants in two genes encoding mu-opioid-receptor-interacting proteins, namely ARRB2 and HINT1. Participants were 374 smokers treated for nicotine dependence with either transdermal nicotine or nicotine nasal spray for 8 weeks in an open-label randomized trial. In a logistic regression model controlling for OPRM1 genotype, treatment type, and other covariates, we found no significant main effect of ARRB2 genotype on abstinence at either end of treatment or 6-month follow-up. Participants with the HINT1 TT genotype had significantly higher abstinence rates at 6-month follow-up, but this may not be a pharmacogenetic effect, given that the participants were drug free during this time. Haplotype analysis did not reveal any significant associations for either gene. We found an interaction of ARRB2 and OPRM1 genotype on abstinence at 6 months that approached significance; however, interpretation of this finding is limited by the small number of participants with the minor alleles for both genes. Although these data do not provide support for the role of genetic variation in these mu-opioid-receptor-interacting proteins and smoking cessation, further exploration of opioid pathway genes in larger prospective pharmacogenetic trials may be warranted.

Confirmation of the association between a polymorphism in the promoter region of the prodynorphin gene and cocaine dependence.

The endogenous opioid system has been shown to have a role in the biological processes involved in addiction to numerous drugs of abuse including cocaine. It has recently been reported that the variable nucleotide tandem repeat (VNTR) polymorphism in the 5′ promoter region of the prodynorphin gene, which encodes the precursor for three endogenous opioid peptides, is associated with the cocaine dependent phenotype. In order to confirm this finding, we genotyped the prodynorphin promoter polymorphism in cocaine dependent (n = 167) and control (n = 88) individuals of African descent. The results from this experiment indicate a statistically significant (χ2 = 5.64, OR = 1.59, P = 0.018) association between the prodynorphin promoter VNTR polymorphism and the cocaine dependent phenotype. In contrast to previous work showing increased risk conferred by one or two copies of the prodynorphin VNTR, the genotyping results from this study indicate that persons with three or four copies of this polymorphism are more likely to become cocaine dependent. This disparity suggests that the prodynorphin promoter VNTR may not be the functional polymorphism associating with the cocaine dependent phenotype. It is possible that different alleles of the prodynorphin promoter VNTR in the independent populations used for this and the previous study may be in linkage disequilibrium with a yet to be identified functional polymorphism in this gene.

Association of a polymorphism in the Homer1 gene with cocaine dependence in an African American population.

Objective While twin and adoption studies have demonstrated that up to 70% of the risk for becoming addicted to cocaine is due to genetic factors, identifying specific genes involved in the development or progression of cocaine dependence has been difficult. The purpose of this study is to determine whether single-nucleotide polymorphisms in the Homer1 and Homer2 genes associate with the cocaine-dependent phenotype in an African American population. Methods This study utilized a case–control design in which the genotype and allele frequencies for four single-nucleotide polymorphisms in the Homer1 gene and three single-nucleotide polymorphisms in the Homer2 gene were compared between African American individuals with a diagnosis of cocaine dependence (n=170) and African American individuals with no history of substance abuse (n=90). Results The data indicate that one single-nucleotide polymorphism, rs6871510, located in intron 1 of the Homer1 gene significantly (P=0.029) associates with cocaine dependence at the genotype level, and trends toward a significant association at the allele frequency level (χ2=2.62, df=1, P=0.106, OR=1.71). None of the single-nucleotide polymorphisms analyzed in the Homer2 gene associates with cocaine dependence. Conclusions The results of this study suggest that a polymorphism in the Homer1 gene, rs6871510, is a potential risk factor for the development of cocaine dependence in an African American population, whereas polymorphisms in the Homer2 gene are not.

Analysis of variations in the tryptophan hydroxylase-2 (TPH2) gene in cocaine dependence

While the exact physiological mechanisms underlying cocaine dependence remain unclear, a growing body of evidence indicates a role for the serotonergic neurotransmitter system in the pathology of this substance use disorder. The focus of the present study is to determine whether genetic variation in the tryptophan hydroxylase‐2 (TPH2) gene, which encodes the enzyme responsible for synthesis of the majority of the serotonin contained in neurons of the central nervous system, contributes to the pathophysiology of cocaine dependence. To examine this hypothesis, we used a case–control study design in which the genotype and allele distributions for six single nucleotide polymorphisms (SNPs) in the TPH2 gene were compared between cocaine‐dependent (n = 299) and control individuals (n = 208) of African descent. The results indicate that none of the SNPs in the TPH2 gene examined in this study associate with the cocaine‐dependent phenotype. This work suggests that variations in the TPH2 gene are not a risk factor for the development of cocaine dependence, but these findings require confirmation in larger, independent samples of cocaine‐dependent and control subjects.

No association between common variations in the human alpha 2 subunit gene (ATP1A2) of the sodium-potassium-transporting ATPase and idiopathic generalized epilepsy

Quantitative trait loci studies in inbred mice have identified a locus on chromosome 1 (Szs1) of fundamental importance to seizure susceptibility. High-ranking candidate genes in this susceptibility region include KCNJ9, KCNJ10 and ATP1A2. We performed a systematic mutation scan of the coding region of the human ATP1A2 gene and performed a case-control association study with seven common markers. Genotypes were assessed in 152 idiopathic generalized epilepsy (IGE) patients of German ancestry and 111 healthy German controls for all seven polymorphisms. No significant differences were found in genotype or allele frequencies for any of the variations between the IGE patients and controls. No haplotypes were associated with IGE when compared to controls. Linkage disequilibrium was demonstrated throughout the gene. Results suggest that the polymorphisms we studied in the ATP1A2 gene do not represent major susceptibility factors for common forms of IGE.

Lack of association between variations in the brain-derived neurotrophic factor (BDNF) gene and temporal lobe epilepsy

Recently, an association between the C240T polymorphism in the brain-derived neurotrophic factor (BDNF) gene and partial epilepsy was demonstrated in a Japanese population. In this study we attempted to replicate the initial finding in a patient-control population of European ancestry and in addition tested whether the functional Val66Met polymorphism is associated with temporal lobe epilepsy (TLE). Genotypes of 151 TLE patients and 189 controls did not differ significantly for either of the variations. Results suggest that neither of the studied polymorphisms are strong susceptibility factors for TLE in this sample of individuals of European ancestry.

No association between common variations in the neuronal nicotinic acetylcholine receptor alpha2 subunit gene (CHRNA2) and bipolar I disorder

The neuronal nicotinic acetylcholine receptor alpha2 subunit gene (CHRNA2) maps to the bipolar susceptibility locus on chromosome 8p21-22. Given the biological role of the neuronal nicotinic acetylcholine receptors and the substantial comorbidity of nicotine dependence in psychiatric disorders, the CHRNA2 gene is a plausible candidate gene for bipolar disorder (BPD). We tested the hypothesis that variations in the CHRNA2 gene confer susceptibility to bipolar I disorder in a case-control association study. Genotypes of one amino acid substitution polymorphism (Ala125Thr) and five non-coding variations across the CHRNA2 gene were obtained from 345 unrelated bipolar I patients and 273 control samples. Genotypes and allele frequencies were compared between groups using chi-square contingency analysis. Linkage disequilibrium (LD) between markers was calculated, and estimated haplotype frequencies were compared between groups. We observed no statitistically significant difference in genotype and allele frequencies for all six variations between bipolar patients and controls, but we did demonstrate strong LD throughout the gene. Haplotype analysis showed that no combinations of alleles were associated with illness. Our results suggest that common variations in the CHRNA2 gene are unlikely to confer susceptibility to BPD in this sample. Further studies are required to elucidate the susceptibility locus for BPD on chromosome 8p21-22.