Andrew E. Weller

Association between the catechol-O-methyltransferase Val158Met polymorphism and cocaine dependence.

Dopaminergic brain systems have been documented to have a major role in drug reward, thus making genes involved in these circuits plausible candidates for susceptibility to substance use disorders. The catechol-O-methyltransferase (COMT) is involved in the degradation of catecholamines and a functional polymorphism (Val158Met) has been suggested to influence enzyme activity. In this study we hypothesize that genetic variation in the COMT gene contributes to increased risk for cocaine dependence. Cocaine-dependent individuals (n=330) and screened unaffected normal controls (n=255) were genotyped for three SNPs in the COMT gene (rs737865, rs4680 (Val158Met), rs165599). All cases and controls were of African descent. Genotype and allele frequencies differed significantly for the Val158Met polymorphism between cases (f(Met)=35%) and controls (f(Met)=27%) (p=0.004; corrected p=0.014; OR 1.44; 95% CI 1.12–1.86). Haplotype analysis showed a significant association for a two-marker haplotype rs737865–Val158Met (p=0.005). Results suggest that variation in COMT increases risk for cocaine dependence. The low enzyme activity 158Met allele or haplotypes containing this variant might have functional effects on dopamine-derived reward processes and cortical functions resulting in increased susceptibility for cocaine dependence. Additional studies are required to elucidate the role of COMT in the pathophysiology of substance use disorders.

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 between Polymorphisms in the Vesicular Monoamine Transporter 1 Gene (VMAT1/SLC18A1) on Chromosome 8p and Schizophrenia

Linkage studies have suggested a susceptibility locus for schizophrenia (SZ) exists on chromosome 8p21–22. The vesicular monoamine transporter 1 gene (VMAT1), also known as SLC18A1, maps to this SZ susceptibility locus. Vesicular monoamine transporters are involved in the presynaptic vesicular packaging of monoamine neurotransmitters, which have been postulated to play a role in the etiology of SZ. Variations in the VMAT1 gene might affect transporter function and/or expression, and might be involved in the etiology of SZ. Genotypes of 62 patients with SZ and 188 control subjects were obtained for 4 missense single nucleotide polymorphisms (Thr4Pro, Thr98Ser, Thr136Ile, Val392Leu) and 2 noncoding single nucleotide polymorphisms (rs988713, rs2279709). All cases and controls were of European descent. The frequency of the minor allele of the Thr4Pro polymorphism was significantly increased in SZ patients when compared to controls (p = 0.0140; d.f. = 1; OR = 1.69; 95% CI = 1.11–2.57). Assuming a recessive mode of inheritance, the frequency of homozygote 4Pro carriers was significantly increased in the SZ patients when compared to controls (24 vs. 8%, respectively; p = 0.0006; d.f. = 1; OR = 3.74; 95% CI = 1.703–8.21). Haplotype analysis showed nominal significance for an individual risk haplotype (p = 0.013); however, after permutation correction, the global p value did not attain a statistically significant level (p = 0.07). Results suggest that variations in the VMAT1 gene may confer susceptibility to SZ in patients of European descent. Further studies are necessary to confirm this effect, and to elucidate the role of VMAT1 in central nervous system physiology and possible involvement in the genetic origins of SZ.

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.

Genetic variants in the cocaine- and amphetamine-regulated transcript gene (CARTPT) and cocaine dependence

Dopaminergic brain systems have been implicated to play a major role in drug reward, thus making genes involved in these circuits plausible candidates for susceptibility to substance use disorders. The cocaine- and amphetamine-regulated transcript peptide (CARTPT) is involved in reward and feeding behavior and has functional characteristics of an endogenous psychostimulant. In this study we tested the hypothesis that variation in the CARTPT gene increases susceptibility to cocaine dependence in individuals of African descent. Genotypes of three HapMap tagging SNPs (rs6894758; rs11575893; rs17358300) across the CARTPT gene region were obtained in cocaine dependent individuals (n=348) and normal controls (n=256). All subjects were of African descent. There were no significant differences in allele, genotype or haplotype frequencies between cases and controls for any of the tested SNPs. Our results do not support an association of the CARTPT gene with cocaine dependence; however, additional studies using larger samples, comprehensive SNP coverage, and different populations are necessary to conclusively rule out CARTPT as a contributing factor in the etiology of cocaine dependence.

Association study of polymorphisms in the autosomal mitochondrial complex I subunit gene, NADH dehydrogenase (ubiquinone) flavoprotein 2, and bipolar disorder.

The mitochondrial dysfunction hypothesis of bipolar disorder (BPD) was proposed based on a maternal inheritance pattern in family-based studies (McMahon et al., 1995). For the BPD linkage signal around human chromosome 18p11.2, Gershon et al. (1996) found a parent-of-origin effect only when maternal and paternal inheritance was considered. An autosomally encoded mitochondrial protein imported into mitochondria to perform important functions might explain this finding. NDUFV2, encoding the 24 kDa subunit of mitochondrial complex I, is one such gene found within the aforementioned BPD linkage region. Single nucleotide polymorphisms (SNPs) in NDUFV2 showed nominally significant associations with BPD in various ethnic populations (Washizuka et al., 2003; Xu et al., 2008; Zhang et al., 2009). To replicate or extend these findings, we studied three SNPs in NDUFV2 (Chr18:9,102,675-9,134,336; build GRCh37:Feb2009:hg19): −3188C>T (rs2377961, Chr18:9,099,554) and −602G>A (rs1156044, Chr18:9,102,140) in the promoter, and +86C>T (rs906807, Chr18:9,117,867), a missense (A29V) SNP in exon 2, using the National Institute of Mental Health (NIMH) Caucasian control (no psychiatric or chronic neurological disease history) and BPDI (ascertained by DSM-IV criteria) populations. Informed consent was obtained from all individuals according to Institutional Review Board (IRB) requirements. 741 control and 569 BPDI individuals were genotyped using the Taqman® Genotyping Assay system (Applied Biosystems, Inc., Foster City, CA). Individuals with “undetermined” genotypes at any locus, representing 1.5% of control and 6.0% of BPDI samples, were removed from further analysis. Thus, 730 control and 535 bipolar individuals were included in our data analyses. All SNPs were in Hardy-Weinberg equilibrium (rs2377961, BPDI p=0.140, Controls p=0.281; rs1156044, BPDI p=0.494, Controls p=0.794; rs906807, BPDI p=0.408, Controls p=0.889). rs2377961 was in strong LD with rs1156044 (D’=0.843, r2=0.288) and rs906807 (D’=0.832, r2=0.325), and rs1156044 was in strongest LD with rs906807 (D’=0.914, r2=0.721). There were no statistically significant associations between genotypes and BPDI at rs2377961 (X2=3.873; p=0.144), rs906807 (X2=4.711; p=0.095) or rs1156044 (X2=5.798; p=0.055). Neither allele at rs2377961 (X2=0.572; p=0.449) showed association with BPDI; however, the “A” allele at rs1156044 (X2=5.362; p=0.021) and “C” allele at rs906807 (X2=4.173; p=0.041) were nominally associated with BPDI. Notably, association of the “A” allele at rs1156044 agrees with a study of a smaller Caucasian population (Xu et al., 2008), but is opposite the trend observed for BPDII in a Japanese population (Washizuka et al., 2003). While our findings may bolster those of Xu et al. (2008), statistical significances of the allelic associations in the present study were not upheld after Bonferroni correction (α=0.0167 for 3 tests). Washizuka et al. (2003) found association of a “CTGT” promoter haplotype in NDUFV2 with BPD, where “G” is rs1156044. Haplotype (rs1156044-rs906807) analysis of our data revealed no statistically significant association of the “GT” (X2=2.765; p=0.096) or “AT” (X2=1.259; p=0.262) haplotypes, but the “AC” haplotype showed statistical significance (X2=7.033; p=0.008) after Bonferroni correction (α=0.0125 for 4 haplotype comparisons). However, after 10,000 permutations of our data, no associations remained statistically significant (lowest permutation adjusted p-value=0.064 for the “AC” haplotype). Thus, we cautiously conclude that the “AC” haplotype in NDUFV2 may be associated with BPDI in this Caucasian population.

Association analysis between polymorphisms in the myo-inositol monophosphatase 2 (IMPA2) gene and bipolar disorder

Linkage studies in bipolar disorder (BPD) suggest that a susceptibility locus exists on chromosome 18p11. The myo-inositol monophosphatase 2 gene (IMPA2) maps to this genomic region. Myo-inositol monophosphatase dephosphorylates inositol monophosphate, regenerating free inositol. Lithium, a common treatment for BPD, has been shown to inhibit IMPA2 activity and decrease levels of inositol. It is hypothesized that lithium conveys its therapeutic effect for BPD patients partially through inositol regulation. Hence, dysfunction of inositol caused by IMPA2 irregularity may contribute to the pathophysiology of BPD. In this study, we hypothesize that genetic variations in the IMPA2 gene contributes to increased susceptibility to BPD. We tested this hypothesis by genotyping 9 SNPs (rs1787984; rs585247; rs3974759; rs650727; rs589247; rs669838; rs636173; rs3786285; rs613993) in BPD patients (n=556) and controls (n=735). Genotype 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. Single marker analysis revealed several associations between IMPA2 variations and BPD, which were subsequently rendered non-significant after correction for multiple testing. Although our study did not show strong support for an association between the tested IMPA2 polymorphisms and susceptibility to BPD, additional larger studies are necessary to comprehensively investigate a role of the IMPA2 gene in the pathophysiology of BPD.

Association analysis of the pituitary adenylate cyclase-activating polypeptide (PACAP/ADCYAP1) gene in bipolar disorder.

Background Linkage studies in bipolar disorder (BPD) suggest that a susceptibility locus exists on chromosome 18p11. The pituitary adenylate cyclase-activating polypeptide/adenylate cyclase-activating polypeptide 1 (pituitary) (PACAP/ADCYAP1) gene maps to this region. PACAP is a neuropeptide involved in neurotransmission in both the peripheral nervous system and central nervous system and is required for catecholamine secretion. Animal models of PACAP mutations show remarkable behavioral defects, including hyperactivity and increased exploratory behavior. Objective In this study we tested the hypothesis that genetic variations in the human PACAP gene contribute to BPD. Methods Genotyping of seven single nucleotide polymorphisms (rs1893154; rs2846811; rs8192595; rs2856966; rs928978; rs2231187; rs1610037) was performed in BPD patients (n=570) and healthy controls (n=710). Genotypes and allele frequencies were compared between groups using χ2 contingency analysis. Linkage disequilibrium between markers was calculated and estimated haplotype frequencies were compared between groups. Main results There were no significant differences between groups on the allele, genotype or haplotype level for any of the tested single nucleotide polymorphisms. Conclusion Our results provide no evidence of an association of the PACAP gene with BPD in this group of patients and controls. Additional studies are necessary to elucidate the BPD susceptibility locus on chromosome 18p.

Association between polymorphisms in the metallophosphoesterase (MPPE1) gene and bipolar disorder.

Genetic linkage studies in bipolar disorder (BPD) suggest that a susceptibility locus exists on chromosome 18p11. The metallophosphoesterase (MPPE1) gene maps to this region. Dysregulation of protein phosphorylation and subsequent abnormal cellular signaling has been postulated to be involved in neuropsychiatric disorders thus making MPPE1 a plausible biological candidate gene for BPD. In this study, we hypothesized that genetic variation in the MPPE1 gene contributes to BPD. We tested this hypothesis by genotyping four SNPs (rs871044; rs3974590; rs593713; rs602201) in BPD patients (n = 570) and healthy controls (n = 725). 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. Single marker analysis revealed an association of rs3974590 with BPD (P = 0.009; permutation corrected P = 0.046). Haplotype analysis did not show any significant association with disease after permutation correction. Our results provide evidence of an association between a polymorphism in the MPPE1 gene and BPD. Additional studies are necessary to confirm and elucidate the role of MPPE1 as a susceptibility gene for BPD on chromosome 18p.