Vidar M. Steen

Genome-wide association studies establish that human intelligence is highly heritable and polygenic.

General intelligence is an important human quantitative trait that accounts for much of the variation in diverse cognitive abilities. Individual differences in intelligence are strongly associated with many important life outcomes, including educational and occupational attainments, income, health and lifespan. Data from twin and family studies are consistent with a high heritability of intelligence, but this inference has been controversial. We conducted a genome-wide analysis of 3511 unrelated adults with data on 549 692 single nucleotide polymorphisms (SNPs) and detailed phenotypes on cognitive traits. We estimate that 40% of the variation in crystallized-type intelligence and 51% of the variation in fluid-type intelligence between individuals is accounted for by linkage disequilibrium between genotyped common SNP markers and unknown causal variants. These estimates provide lower bounds for the narrow-sense heritability of the traits. We partitioned genetic variation on individual chromosomes and found that, on average, longer chromosomes explain more variation. Finally, using just SNP data we predicted ∼1% of the variance of crystallized and fluid cognitive phenotypes in an independent sample (P=0.009 and 0.028, respectively). Our results unequivocally confirm that a substantial proportion of individual differences in human intelligence is due to genetic variation, and are consistent with many genes of small effects underlying the additive genetic influences on intelligence.

Nomenclature for human CYP2D6 alleles.

To standardize CYP2D6 allele nomenclature, and to conform with international human gene nomenclature guidelines, an alternative to the current arbitrary system is described. Based on recommendations for human genome nomenclature, we propose that alleles be designated by CYP2D6 followed by an asterisk and a combination of roman letters and arabic numerals distinct for each allele with the number specifying the key mutation and, where appropriate, a letter specifying additional mutations. Criteria for classification as a separate allele and protein nomenclature are also presented.

Detection of the poor metabolizer-associated CYP2D6(D) gene deletion allele by long-PCR technology

The cytochrome P450 enzyme debrisoquine 4-hydroxylase metabolizes many different classes of commonly used drugs, such as antidepressants and neuroleptics. Deficient hydroxylation of debrisoquine, known as the poor metabolizer (PM) phenotype, affects 5-10% of Caucasians and may lead to adverse reactions upon administration of drugs in standard doses. This autosomal recessive metabolic deficiency is caused by the possession of two PM-associated mutations in the human CYP2D6 gene locus coding for the enzyme. These mutations include at least four different single base mutations and two different large gene deletion alleles. The single base mutations can be rapidly detected by PCR methods. In contrast, the large gene deletions have so far only been directly identified by RFLP analysis. By the use of sequence data previously published by others, we report here an alignment of different CYP2D alleles to focus on the presence of almost completely identical sequences immediately downstream of both CYP2D7 and CYP2D6 which may seriously complicate and interfere with PCR-based detection of the gene deletion. Based on this analysis, we have developed a rapid assay which, for the first time, detects the 13kb (also called 11.5 kb) Xba I gene deletion allele by the use of long-PCR technology. The primers were designed to amplify a 3.5 kb PCR product in the presence of this D6(D) allele. We have evaluated the method on 23 different DNA samples heterozygous (n = 22) or homozygous (n = 1) for the 13 kb gene deletion allele (previously typed by RFLP analyses). All samples were correctly identified by the assay. The PCR method did not detect the rare 11 kb Xba I gene deletion allele (n = 5), and there was no false positive amplification from deletion negative DNA samples (n = 47). This sensitive and specific PCR-based assay for detection of the D6(D) allele will improve the scientific and clinical use of CYP2D6 genotyping.

Pharmacogenetics of Tardive Dyskinesia: Combined Analysis of 780 Patients Supports Association with Dopamine D3 Receptor Gene Ser9Gly Polymorphism

Variability among individuals in their therapeutic response to psychotropic drugs and in susceptibility to adverse effects is considerable. Pharmacogenetics addresses the contribution of genetic factors to this variability. An important focus of interest in pharmacogenetics has been on candidate genes that play a role in susceptibility to the antipsychotic drug-induced adverse effect, tardive dyskinesia (TD). Four published studies have reported an association between a serine (ser) to glycine (gly) polymorphism in exon 1 of the dopamine D3 receptor gene (DRD3) and TD; three failed to replicate this finding and one found an insignificant trend. We examined the association in a pooled sample of 780 patients (317 with TD and 463 without TD) drawn from 6 research centers, who were divided into 8 groups based on their population origin. The analysis employed stepwise logistic regression so as to allow confounding effects of group, age, and gender to be taken into account. TD was significantly associated with DRD3 gly allele carrier status (x2=4.46, df 1, p = .04) and with DRD3 genotype (x2=6.62, df 2, p = .04) over and above the effect of group. Similar positive effects were observed when controlling for age and gender (x2=5.02, df 1, p = .02 for gly allele carrier status; x2 = 7.51, df 2, p = .002 for genotype). Examining abnormal involuntary movement scores as a continuous variable, we found that patients homozygous for the gly allele had significantly higher scores than ser-gly heterozygotes (p = .006) or ser-ser homozygotes (p < .0001). We also performed a meta-analysis that included, besides the groups in the combined analysis, three other published studies on DRD3 and TD. The Mantel-Haenszel pooled odds ratio for DRD3 gly allele carrier status increasing susceptibility to TD was 1.33 (95% CI 1.04–1.70, p = .02); the cumulative pooled estimate showed an odds ratio of 1.52 (95% CI 1.08–1.68, p < .0001). These findings support a small but significant contribution of the DRD3 ser9gly polymorphism to TD susceptibility that is demonstrable over and above population effects and the effect of age and gender on the phenotype.

Dopamine D3-receptor gene variant and susceptibility to tardive dyskinesia in schizophrenic patients.

Schizophrenia is a serious psychiatric illness with a life-time risk of approximately one percent. Many of the patients, but not all, benefit from treatment with anti-psychotic drugs known to block dopamine D2-like receptors. The use of conventional neuroleptics is, however, hampered by the risk of extrapyramidal side-effects. Tardive dyskinesia (TD) is usually regarded as the most serious of these drug-induced movement disorders due to its high prevalence and potentially irreversible nature. In this study, we have investigated the genetic variation of the dopamine D3 receptor gene (DRD3) as a putative risk factor for TD in schizophrenic patients receiving long-term anti-psychotic drug therapy. We found a high frequency (22–24%) of homozygosity for the Ser9Gly variant (allele 2) of the DRD3 gene among subjects with TD in both a cross-sectional and a longitudinal evaluation, as compared with the relative under-representation (4–6%) of this genotype in patients with no or fluctuating TD. This result indicates that autosomal inheritance of two polymorphic Ser9Gly alleles (2-2 genotype), but not homozygosity for the wild-type allele (1-1 genotype), is a susceptibility factor for the development of TD, an observation which may improve the understanding of the pathophysiological mechanisms of TD and influence the design and choice of future anti-psychotic drugs. The correlation between a serious motor side-effect and a genetic marker could lead to selection bias in the sampling of schizophrenic patients for genetic studies, and may therefore explain the apparent association reported between susceptibility for schizophrenia per se and homozygosity for the DRD3 gene.

Ultrarapid metabolizers of debrisoquine: Characterization and PCR-based detection of alleles with duplication of the CYP2D6 gene

Up to 7% of Caucasians may demonstrate ultrarapid metabolism of debrisoquine due to inheritance of alleles with duplicated functional CYP2D6 genes. Here we describe the genomic organization of the duplicated CYP2D6 genes in the 42 kb XbaI allele. We postulate that this duplication originates from a homologous, unequal cross‐over event which involved two 29 kb XbaI wild‐type alleles, and had break points within a 2.8 kb direct repeat (CYP‐REP) flanking the CYP2D6 gene. Moreover, we have designed two different PCR assays for detection of alleles with duplicated CYP2D6 genes. Both assays correctly identified 29 out of 29 subjects positive for the 42 kb XbaI allele. No false negative or false positive reactions were observed.

Gene variants associated with schizophrenia in a Norwegian genome-wide study are replicated in a large European cohort

We have performed a genome-wide association study (GWAS) of schizophrenia in a Norwegian discovery sample of 201 cases and 305 controls (TOP study) with a focused replication analysis in a larger European sample of 2663 cases and 13,780 control subjects (SGENE-plus study). Firstly, the discovery sample was genotyped with Affymetrix Genome-Wide Human SNP Array 6.0 and 572,888 markers were tested for schizophrenia association. No SNPs in the discovery sample attained genome-wide significance (P<8.7 x 10(-8)). Secondly, based on the GWAS data, we selected 1000 markers with the lowest P values in the discovery TOP sample, and tested these (or HapMap-based surrogates) for association in the replication sample. Sixteen loci were associated with schizophrenia (nominal P value<0.05 and concurring OR) in the replication sample. As a next step, we performed a combined analysis of the findings from these two studies, and the strongest evidence for association with schizophrenia was provided for markers rs7045881 on 9p21, rs433598 on 16p12 and rs10761482 on 10q21. The markers are located in PLAA, ACSM1 and ANK3, respectively. PLAA has not previously been described as a susceptibility gene, but 9p21 is implied as a schizophrenia linkage region. ACSM1 has been identified as a susceptibility gene in a previous schizophrenia GWAS study. The association of ANK3 with schizophrenia is intriguing in light of recent associations of ANK3 with bipolar disorder, thereby supporting the hypothesis of an overlap in genetic susceptibility between these psychopathological entities.

Identification of genes co‐upregulated with Arc during BDNF‐induced long‐term potentiation in adult rat dentate gyrus in vivo

Brain‐derived neurotrophic factor (BDNF) is a critical regulator of transcription‐dependent adaptive neuronal responses, such as long‐term potentiation (LTP). Brief infusion of BDNF into the dentate gyrus of adult anesthetized rats triggers stable LTP at medial perforant path‐granule synapses that is transcription‐dependent and requires induction of the immediate early gene Arc. Rather than acting alone, Arc is likely to be part of a larger BDNF‐induced transcriptional program. Here, we used cDNA microarray expression profiling to search for genes co‐upregulated with Arc 3 h after BDNF‐LTP induction. Of nine cDNAs encoding for known genes and up‐regulated more than four‐fold, we selected five genes, Narp, neuritin, ADP‐ribosylation factor‐like protein‐4 (ARL4L), TGF‐β‐induced immediate early gene‐1 (TIEG1) and CARP, for further validation. Real‐time PCR confirmed robust up‐regulation of these genes in an independent set of BDNF‐LTP experiments, whereas infusion of the control protein cytochrome C had no effect. In situ hybridization histochemistry further revealed up‐regulation of all five genes in somata of post‐synaptic granule cells following both BDNF‐LTP and high‐frequency stimulation‐induced LTP. While Arc synthesis is critical for local actin polymerization and stable LTP formation, several of the co‐upregulated genes have known functions in excitatory synaptogenesis, axon guidance and glutamate receptor clustering. These results provide novel insight into gene expression responses underlying BDNF‐induced synaptic consolidation in the adult brain in vivo.

Mutations in ABHD12 Cause the Neurodegenerative Disease PHARC: An Inborn Error of Endocannabinoid Metabolism

Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) is a neurodegenerative disease marked by early-onset cataract and hearing loss, retinitis pigmentosa, and involvement of both the central and peripheral nervous systems, including demyelinating sensorimotor polyneuropathy and cerebellar ataxia. Previously, we mapped this Refsum-like disorder to a 16 Mb region on chromosome 20. Here we report that mutations in the ABHD12 gene cause PHARC disease and we describe the clinical manifestations in a total of 19 patients from four different countries. The ABHD12 enzyme was recently shown to hydrolyze 2-arachidonoyl glycerol (2-AG), the main endocannabinoid lipid transmitter that acts on cannabinoid receptors CB1 and CB2. Our data therefore represent an example of an inherited disorder related to endocannabinoid metabolism. The endocannabinoid system is involved in a wide range of physiological processes including neurotransmission, mood, appetite, pain appreciation, addiction behavior, and inflammation, and several potential drugs targeting these pathways are in development for clinical applications. Our findings show that ABHD12 performs essential functions in both the central and peripheral nervous systems and the eye. Any future drug-mediated interference with this enzyme should consider the potential risk of long-term adverse effects.

Antipsychotic drugs activate SREBP-regulated expression of lipid biosynthetic genes in cultured human glioma cells: a novel mechanism of action?

Several studies have reported on structural abnormalities, decreased myelination and oligodendrocyte dysfunction in post-mortem brains from schizophrenic patients. Glia-derived cholesterol is essential for both myelination and synaptogenesis in the CNS. Lipogenesis and myelin synthesis are thus interesting etiological candidate targets in schizophrenia. Using a microarray approach, we here demonstrate that the antipsychotic drugs clozapine and haloperidol upregulate several genes involved in cholesterol and fatty acid biosynthesis in cultured human glioma cells, including HMGCR (3-hydroxy-3-methylglutaryl-coenzyme A reductase), HMGCS1 (3-hydroxy-3-methylglutaryl-coenzyme A synthase-1), FASN (fatty acid synthase) and SCD (stearoyl-CoA desaturase). The changes in gene expression were followed by enhanced HMGCR-enzyme activity and elevated cellular levels of cholesterol and triglycerides. The upregulated genes are all known to be controlled by the sterol regulatory element-binding protein (SREBP) transcription factors. We show that clozapine and haloperidol both activate the SREBP system. The antipsychotic-induced SREBP-mediated increase in glial cell lipogenesis could represent a novel mechanism of action, and may also be relevant for the metabolic side effects of antipsychotics.