Investigation of dopamine receptor (DRD4) and dopamine transporter (DAT) polymorphisms for genetic linkage or association to panic disorder

Biological markers for anxiety disorders, OCD and PTSD - a consensus statement. Part I: Neuroimaging and genetics

OBJECTIVES

Biomarkers are defined as anatomical, biochemical or physiological traits that are specific to certain disorders or syndromes. The objective of this paper is to summarise the current knowledge of biomarkers for anxiety disorders, obsessive-compulsive disorder (OCD) and post-traumatic stress disorder (PTSD).

METHODS

Findings in biomarker research were reviewed by a task force of international experts in the field, consisting of members of the World Federation of Societies for Biological Psychiatry Task Force on Biological Markers and of the European College of Neuropsychopharmacology Anxiety Disorders Research Network.

RESULTS

The present article (Part I) summarises findings on potential biomarkers in neuroimaging studies, including structural brain morphology, functional magnetic resonance imaging and techniques for measuring metabolic changes, including positron emission tomography and others. Furthermore, this review reports on the clinical and molecular genetic findings of family, twin, linkage, association and genome-wide association studies. Part II of the review focuses on neurochemistry, neurophysiology and neurocognition.

CONCLUSIONS

Although at present, none of the putative biomarkers is sufficient and specific as a diagnostic tool, an abundance of high-quality research has accumulated that will improve our understanding of the neurobiological causes of anxiety disorders, OCD and PTSD.

The DRD4 gene and severity of tics and comorbid symptoms: main effects and interactions with delivery complications

In this study, we investigated the role of the dopamine receptor D4 (DRD4) 48-base pairs (bp) variable number of tandem repeats (VNTR) and perinatal adversities regarding severity of tics and comorbid symptoms in children with tic disorders. We genotyped 110 children with tics with regard to the 48-bp VNTR and assessed presence of prenatal smoking exposure, and pregnancy and delivery complications by parent questionnaires. We examined associations between 2, 3, 4, and 7 repeat (R) alleles and severity of tics and comorbid obsessive-compulsive, depressive, anxious, and autistic symptoms. Through linear regressions, we investigated whether perinatal adversities and the 2R, 3R, 4R, and 7R alleles would interact with severity ratings of tics or comorbid symptoms as outcome. Presence of a 2R allele was related to more severe obsessive-compulsive symptoms, and presence of a 3R allele to increased severity of autistic features. Pregnancy complications were associated with decreased obsessive-compulsive symptom severity, and prenatal smoking exposure to more severe depressive and autistic symptoms. In children without a 3R allele delivery complications were associated with more severe tics, but in children with a 3R variant an inverse relation between delivery complications and tic severity was found. Moreover, the relation between delivery complications and internalizing symptom severity appeared to be most pronounced in children with a 2R allele. In conclusion, this study provides evidence for a role of the 48-bp VNTR in the etiology of tic and associated disorders, and for interactions with delivery complications regarding severity of tics and co-occurring internalizing symptoms.

Candidate genes for panic disorder: insight from human and mouse genetic studies

Panic disorder is a major cause of medical attention with substantial social and health service cost. Based on pharmacological studies, research on its etiopathogenesis has been focused on the possible dysfunction of specific neurotransmitter systems. However, recent work has related the genes involved in development, synaptic plasticity and synaptic remodeling to anxiety disorders. This implies that learning processes and changes in perception, interpretation and behavioral responses to environmental stimuli are essential for development of complex anxiety responses secondary to the building of specific brain neural circuits and to adult plasticity. The focus of this review is on progress achieved in identifying genes that confer increased risk for panic disorder through genetic epidemiology and the use of genetically modified mouse models. The integration of human and animal studies targeting behavioral, systems-level, cellular and molecular levels will most probably help identify new molecules with potential impact on the pathogenetic aspects of the disease.

Meta-analysis of COMT val158met in panic disorder: ethnic heterogeneity and gender specificity

There is strong evidence for a genetic contribution to the pathogenesis of panic disorder, with the functional catechol-O-methyltransferase (COMT) val158met polymorphism having been suggested as a potential susceptibility factor. In the present study, a meta-analysis of six available case-control studies (557 patients with panic disorder and 763 healthy controls in total) on the role of the COMT val158met polymorphism in panic disorder was conducted in an attempt to reconcile previous conflicting results and to facilitate evaluation of the role of COMT gene variation in panic disorder. Overall, no significant association, but strong between-study heterogeneity, was discerned. Analysis of studies pooled by ancestry yielded a significant association of the COMT 158val allele with panic disorder in Caucasian samples and, conversely, a trend towards association of the COMT 158met allele with the disorder in Asian samples. Interestingly, stratification for gender as well as ethnicity revealed that association of the 158val allele in Caucasians and, reciprocally, the 158met allele in Asian samples was restricted to females. The present meta-analysis provides tentative support for the COMT val158met polymorphism as a possible risk factor for panic disorder, with differential effects in Caucasian and Asian populations, and suggests a female-specific effect. However, given the relatively small number of case-control studies presently available, several more association studies, preferably including a larger number of family-based studies, are warranted for conclusive evaluation of the COMT val158met polymorphism as a vulnerability factor in panic disorder.

Genetic epidemiology of anxiety disorders

This chapter reviews the genetic epidemiology of the major subtypes of anxiety disorders including panic disorder, phobic disorders, generalized anxiety disorder, and obsessive-compulsive disorder. Controlled family studies reveal that all of these anxiety subtypes are familial, and twin studies suggest that the familial aggregation is attributable in part to genetic factors. Panic disorder and, its spectrum have the strongest magnitude of familial clustering and genetic underpinnings. Studies of offspring of parents with anxiety disorders an increased risk of mood and anxiety disorders, but there is far less specificity of the manifestations of anxiety in children and young adolescents. Although there has been a plethora of studies designed to identify genes underlying these conditions, to date, no specific genetic loci have been identified and replicated in independent samples.

The genetics of panic disorder: state of the art

Panic disorder (PD) is a highly prevalent, debilitating disorder. The heritability of the disease has been estimated by twin studies to be between 30 and 60%. The vulnerability for PD overlaps with an increased risk of bipolar disorder in some families. Classical genetic methods such as linkage analysis and association studies have not yet identified genetic risk factors beyond doubt. However, two independent studies confirm linkage of a specific syndrome characterized by PD, bladder problems, severe headaches, mitral valve prolapse and thyroid dysfunction to genetic markers on chromosome 13q. Association studies, although showing divergent results, give some support to a causative role for the genes encoding for monoamine oxidase A (MAO-A), cholecystokinin (CCK) and catechol-O-methyltransferase (COMT). Finally, a somatic duplication of a 19-Mb region on chromosome 15 has been associated with PD, but this intriguing finding awaits confirmation.

Cocaine, reward, movement and monoamine transporters

Recent evidence enriches our understanding of the molecular sites of action of cocaine reward and locomotor stimulation. Dopamine transporter blockade by cocaine appears a sufficient explanation for cocaine-induced locomotion. Variation in DAT appears to cause differences in locomotion without drug stimulation. However, previously-held views that DAT blockade was the sole site for cocaine reward have been replaced by a richer picture of multitransporter involvement with the rewarding and aversive actions of cocaine. These new insights, derived from studies of knockout mice with simultaneous deletions and/or blockade of multiple transporters, provide a novel model for the rewarding action of this heavily-abused substance and implicate multiple monoamine systems in cocaine's hedonic activities.

Evidence for a susceptibility locus for panic disorder near the catechol-O-methyltransferase gene on chromosome 22

BACKGROUND

A well-characterized single nucleotide polymorphism (472G/A-Val/Met-SNP8) in the coding sequence of the catechol-O-methyltransferase (COMT) gene leads to a three- to fourfold difference in enzymatic activity and clinical and animal studies suggest a role in anxiety states like panic disorder.

METHODS

Subjects from 70 panic disorder pedigrees, and 83 "triads", were genotyped at seven single nucleotide polymorphisms (SNPs), polymorphic microsatellites in the first intron of COMT and approximately 339kb upstream of COMT (D22S944) and analyzed for genetic association and linkage.

RESULTS

Linkage analysis showed elevated LOD scores for 472G/A (SNP 8), silent exon 3 substitution (186C/T-SNP 5), and the marker D22S944 (2.88, 2.62, and 2.93, respectively), using a variety of diagnostic and genetic models. Association tests were not significant for the SNPs, but were highly significant for D22S944 (p =.0001-.0003). One three-marker haplotype formed from the above three polymorphisms was significantly associated with panic disorder (p =.0001), as was the "global" p value for this combination (p =.005). In addition, numerous haplotypes with combinations of D22S944 and COMT SNPs were found to be significantly associated with panic disorder.

CONCLUSIONS

Our findings provide strong evidence for a susceptibility locus for panic disorder either within the COMT gene or in a nearby region of chromosome 22.

Idiopathic environmental intolerance: increased prevalence of panic disorder-associated cholecystokinin B receptor allele 7

BACKGROUND

A growing body of evidence suggests that idiopathic environmental intolerance (IEI) is a psychophysiologic disorder with prominent features of anxiety/panic and somatization, although proponents of a toxicogenic explanation claim, despite a lack of convincing evidence, that symptoms arise from exposure to otherwise nonnoxious environmental agents. Patient behaviour is characterized by strenuous avoidance of perceived triggers to the point of severe impairment of normal social and vocational functioning. IEI proponents claim that previous studies showing a high prevalence of psychopathology in patients with IEI and studies showing panic responses to known panicogenic challenges merely reflect the anxiety-producing result of living with IEI.

OBJECTIVE

We explored whether IEI and panic disorder, personality traits, or both shared an underlying neurogenetic basis that would predate the anxiety of IEI symptomatology. The DNA of patients with IEI was examined for the presence of known panic disorder-associated cholecystokinin B (CCK-B) receptor alleles and for personality trait-associated dopamine D4 receptor polymorphisms.

METHODS

Eleven patients with typical IEI symptoms were recruited and were individually matched to normal control subjects from an existing bank for age, sex, and ethnic background. Genomic DNA was extracted from peripheral blood samples. CCK-B and dopamine D4 receptor polymorphisms were examined by using standard PCR-based techniques.

RESULTS

There was a significantly higher prevalence of the panic disorder-associated CCK-B receptor allele 7 in subjects with IEI (9/22 [40.9%]) compared with control subjects (2/22 [9.1%], P =.037). There was no difference in personality trait-associated polymorphisms of the gene encoding dopamine D4 receptor between patients and control subjects.

CONCLUSIONS

These findings provide preliminary evidence that IEI and panic disorder share a common neurogenetic basis, which would predate the anxiety-producing effects of IEI symptoms. Further studies with larger samples are warranted, but these results support previous studies that suggest that panic disorder may account for much of the symptomatology in at least some cases of IEI and provide a basis for rational treatment strategies.

The genetics of panic disorder

Of the anxiety disorders, panic disorder (PD) has been the most extensively studied from a genetic standpoint. Results of family studies have consistently demonstrated that PD runs in families, and twin studies indicate that genes contribute to this familiality. However, phenotypic and genetic complexity has made finding the specific genes involved in PD a challenge. There is still uncertainty about how best to define the phenotype for genetic studies and whether it is the clinical phenotype of PD or more latent psychologic and biologic traits that are inherited. To date, molecular genetic studies have suggested some chromosomal regions and genes that may contribute to risk, but none of these have been established. We review the genetic epidemiology of PD as well as recent molecular genetic studies of the disorder, and conclude with a discussion of promising strategies that attempt to uncover specific genetic loci involved in the etiology of PD.

Electroretinogram B-wave amplitude in panic disorder

Abnormal light-related behaviors have been described for patients with panic disorder (PD). The present study was undertaken to investigate the retinal light response in PD using electroretinography (ERG). The authors conducted b-wave ERG measurements with a bright light (after dark adaptation) in 28 patients with PD and 28 control subjects. There were no significant differences in the mean b-wave amplitude between the two groups, but the retinal response to light in PD patients was generally lower than in healthy subjects. A large interindividual variability was found; also noted was a significant difference in the mean b-wave amplitude between the right and left eyes in the control group. The data indicate subtle variation of retinal photosensitivity in a subgroup of patients with PD. Because dopaminergic retinal activity affects b-ERG amplitude, the authors hypothesize that the dopaminergic system is involved in the response to light in PD patients.