Association study of dopamine D2, D4 receptor gene, GABAA receptor beta subunit gene, serotonin transporter gene polymorphism with children of alcoholics in Korea: a preliminary study

Evaluation of the Relationship Between Dopamine Receptor D2 Gene TaqIA1 Polymorphism and Alcohol Dependence Risk

Several studies are published, that investigated dopamine receptor 2 (DRD2) gene TaqIA polymorphism as a risk factor for alcohol dependence (AD) with positive and negative associations. To derive a more precise estimation of the relationship, a meta-analysis of case-control studies that examined the association between DRD2 gene Taq1A polymorphism and alcohol dependence was performed. Eligible articles were identified through a search of databases including PubMed, Science Direct, Springer link, and Google Scholar. The association between the DRD2 TaqIA polymorphism and AD susceptibility was conducted using odds ratios (ORs) and 95% confidence intervals (95% CIs) as association measures. A total of 69 studies with 9125 cases and 9123 healthy controls were included in the current meta-analysis. Results of the present analysis showed significant association between DRD2 TaqIA polymorphism and AD risk using five genetic modes (allele contrast model-OR 1.22, 95% CI 1.13-1.32, p < 0.0001; homozygote model-OR 1.35, 95%CI 1.18-1.55; p ≤ 0.0001; dominant model-OR 1.29; 95% CI 1.20-1.39; p < 0.0001; recessive model-OR 1.21; 95% CI 1.08-1.36; p = 0.0006). There was no significant association found in subgroup analysis, TaqIA polymorphism was not significantly associated with AD risk in the Asian population under all genetic models, but in the Caucasian population, TaqIA polymorphism was significantly associated with AD risk. Overall, results support the hypothesis that DRD2 Taq1A polymorphism plays a role in alcohol dependence.

Dopaminergic dysfunction: Role for genetic & epigenetic testing in the new psychiatry

Reward Deficiency Syndrome (RDS), particularly linked to addictive disorders, costs billions of dollars globally and has resulted in over one million deaths in the United States (US). Illicit substance use has been steadily rising and in 2021 approximately 21.9% (61.2 million) of individuals living in the US aged 12 or older had used illicit drugs in the past year. However, only 1.5% (4.1 million) of these individuals had received any substance use treatment. This increase in use and failure to adequately treat or provide treatment to these individuals resulted in 106,699 overdose deaths in 2021 and increased in 2022. This article presents an alternative non-pharmaceutical treatment approach tied to gene-guided therapy, the subject of many decades of research. The cornerstone of this paradigm shift is the brain reward circuitry, brain stem physiology, and neurotransmitter deficits due to the effects of genetic and epigenetic insults on the interrelated cascade of neurotransmission and the net release of dopamine at the Ventral Tegmental Area -Nucleus Accumbens (VTA-NAc) reward site. The Genetic Addiction Risk Severity (GARS) test and pro-dopamine regulator nutraceutical KB220 were combined to induce "dopamine homeostasis" across the brain reward circuitry. This article aims to encourage four future actionable items: 1) the neurophysiologically accurate designation of, for example, "Hyperdopameism /Hyperdopameism" to replace the blaming nomenclature like alcoholism; 2) encouraging continued research into the nature of dysfunctional brainstem neurotransmitters across the brain reward circuitry; 3) early identification of people at risk for all RDS behaviors as a brain check (cognitive testing); 4) induction of dopamine homeostasis using "precision behavioral management" along with the coupling of GARS and precision Kb220 variants; 5) utilization of promising potential treatments include neuromodulating modalities such as Transmagnetic stimulation (TMS) and Deep Brain Stimulation(DBS), which target different areas of the neural circuitry involved in addiction and even neuroimmune agents like N-acetyl-cysteine.

High Mobility Group Box 1/Toll-like Receptor 4 Signaling Increases GABRB3 Expression in Alcohol Exposure

INTRODUCTION

Prefrontal cortex (PFC) and striatal neurotransmitter homeostasis is affected by alcohol dependence. In this study, the microarray dataset from the Gene Expression Omnibus (GEO) database were downloaded. The prefrontal and striatum data were cross-analyzed to reveal the co-effects of alcohol dependence on the two brain regions of mice.

METHODS

The GSE123114 microarray profile was downloaded from the GEO database, and differentially expressed genes (DEGs) between the two groups were acquired by GEO2R. KEGG analyses were performed to identify the pivotal pathways of these DEGs. Key differential gene expressions and their mechanism associated with alcohol exposure were investigated by an intraperitoneal alcohol model.

RESULTS

A total of 13 overlapping DEGs from the PFC and striatal datasets of the GSE123114 microarray profile were identified, and they were significantly enriched in the morphine addiction pathway. The transcript levels and protein expression of Gabrb3 were consistent with the microarray data both in the PFC and striatum. The transcript levels of HMGB1, TLR4, TNFα and IL-1β were upregulated in the PFC and striatum of mice in the alcohol group. The HMGB1 inhibitor decreased Gabrb3 transcript and protein levels as well as TNFα and IL-1β transcript levels both in the PFC and striatum in the intraperitoneal alcohol model mice.

DISCUSSION

Through the reanalysis of GSE123114 microarray profile, we found that Gabrb3 is a key gene associated with alcohol exposure. In further experiments, our findings suggest that alcohol exposure modulates Gabrb3 expression through the HMGB1/TLR4 pathway. Moreover, inflammation-associated factors, such as IL-1β and TNFα, may be related to the HMGB1/TLR4-mediated regulation of GABRB3 expression in alcohol exposure.

Associations Among Internet Addiction, Genetic Polymorphisms, Family Functioning, and Psychopathological Risk: Cross-Sectional Exploratory Study

BACKGROUND

International research has emphasized that youths are at higher risk for the onset of internet addiction (IA), but studies investigating biological, psychological, and social factors associated with this condition are limited.

OBJECTIVE

This study aims to investigate the possible association between IA and genetic polymorphisms in monoamine oxidase A (MAO-A), serotonin-transporter (5-HTTPR), dopamine receptor (DRD4), and dopamine transporter (DAT1) genes by considering the role played by the perception of young adults in their family functioning and their depression, anxiety, and avoidant personality problems.

METHODS

In a sample of 104 male and female young adults aged between 19 and 23 years (mean age 21.87, SD 2.29 years) recruited from universities in the central southern part of Italy, we addressed the presence of IA using the Young criteria of the IA test. Moreover, the perception of young adults of their family functioning and their psychopathological symptoms were assessed through the Family Assessment Device (FAD) and the Adult Self-Report, respectively.

RESULTS

We found no significant association between IA and any genetic polymorphisms, neither among males or females. Young adults with IA reported significantly higher scores in the subscale of FAD affective responsiveness (AR; P=.01) and in depressive problems (P=.02), anxiety problems (P=.009), and avoidant personality problems (P=.003) than those in the control group. Results of mediation analyses showed a mediation role played by depressive symptoms (B=0.99; 95% CI 0.22 to 1.97) and avoidant personality problems (B=1.09; 95% CI 0.32 to 2.05) of young adults on the relationship between the FAD, AR, and IA. Finally, this relationship was moderated by the genotype of the 5-HTTLPR (P<.001), DAT1 (P<.001), and MAO-A (P<.001) genes in young adults.

CONCLUSIONS

This exploratory study supports the recent evidence on the mutual relationship among biological, individual, and social risk factors associated with IA in young adulthood. Our findings may have important clinical implications for the development of prevention and treatment programs.

In Search of Reward Deficiency Syndrome (RDS)-free Controls: The "Holy Grail" in Genetic Addiction Risk Testing

BACKGROUND

The search for an accurate, gene-based test to identify heritable risk factors for Reward Deficiency Syndrome (RDS) was conducted based on hundreds of published studies about the role of dopamine in addictive behaviors, including risk for drug dependence and compulsive/impulsive behavior disorders. The term RDS was first coined by Blum's group in 1995 to identify a group of behaviors with a common neurobiological mechanism associated with a polymorphic allelic propensity for hypodopaminergia.

OBJECTIVES

To outline the process used to select risk alleles of reward genes for the Genetic Addiction Risk Score (GARS) test. Consequently, to address the limitations caused by inconsistent results that occur in many case-control behavioral association studies. These limitations are perhaps due to the failure of investigators to adequately screen controls for drug and alcohol use disorder, and any of the many RDS behaviors, including nicotine dependence, obesity, pathological gambling, and internet gaming addiction.

METHODS

Review of the literature related to the function of risk alleles of reward genes associated with hypodopaminergia relevant case-control association studies for the selection of alleles to be measured by the Genetic Addiction Risk Score (GARS) test.

RESULTS

The prevalence of the DRD2 A1 allele in unscreened controls (33.3%), compared to "Super-Controls" [highly screened RDS controls (3.3%) in proband and family] is used to exemplify a possible solution.

CONCLUSION

Unlike one gene-one disease (OGOD), RDS is polygenetic, and very complex. In addition, any RDS-related behaviors must be eliminated from the control group in order to obtain the best possible statistical analysis instead of comparing the phenotype with disease-ridden controls.

Explicating the role of empathic processes in substance use disorders: A conceptual framework and research agenda

ISSUES

Elucidating the role of empathic processes in developmental pathways to substance use disorders could have important implications for prevention.

APPROACH

We searched the biomedical and social sciences literature to determine what is known about empathy and psychopathological manifestations of severe lack of empathy in the initiation, development and maintenance of psychoactive substance use. Thirty-seven empirical studies were identified and formally reviewed.

KEY FINDINGS

Adults with alcohol and stimulant use disorders exhibited detectable impairments in both cognitive and affective empathy, measured behaviourally, neuroanatomically and by self-report, relative to controls. There were no developmental studies specifically designed to test the role of empathy in substance use pathways, but several studies that included measures of empathy suggest that empathy may be protective. Studies on severe empathic deficits were mixed regarding a unique role of empathy in substance use trajectories, independent of interpersonal style, impulsivity and social deviance. Implications and Conclusions. In the context of findings and methodological limitations of this review, we recommend more rigorous examination of empathy across the spectrum of substance use behaviour. Future work should utilise the following: (i) prospective assessment of empathic capacity in substance abusers during and following treatment; (ii) large, developmentally based prospective designs beginning prior to substance initiation incorporating multiple measures of empathy; (iii) assessment of the moderating role of gender, race and ethnicity; and (iv) prospective study of empathy in children at elevated risk for substance use disorders. [Massey SH, Newmark RL, Wakschlag LS. Explicating the role of empathic processes in substance use disorders: A conceptual framework and research agenda.

A Shared Molecular and Genetic Basis for Food and Drug Addiction: Overcoming Hypodopaminergic Trait/State by Incorporating Dopamine Agonistic Therapy in Psychiatry

This article focuses on the shared molecular and neurogenetics of food and drug addiction tied to the understanding of reward deficiency syndrome. Reward deficiency syndrome describes a hypodopaminergic trait/state that provides a rationale for commonality in approaches for treating long-term reduced dopamine function across the reward brain regions. The identification of the role of DNA polymorphic associations with reward circuitry has resulted in new understanding of all addictive behaviors.

Association between urine phthalate levels and poor attentional performance in children with attention-deficit hyperactivity disorder with evidence of dopamine gene-phthalate interaction

Although there is some evidence supporting the existence of an association between prenatal maternal or postnatal child’s urine phthalate metabolite concentrations and poor attentional performances, the interaction between urine phthalate metabolite levels and genetic variation for neuropsychological deficit of attention-deficit hyperactivity disorder (ADHD) has not been examined. The aim of this study was to determine whether phthalate metabolites in urine are associated with poor neuropsychological performance in children with ADHD, and whether such association is affected by genotype-phthalate interaction. A cross-sectional examination of urine phthalate metabolite concentrations and the continuous performance test (CPT) were performed in 179 Korean children with ADHD recruited from department of psychiatry of university hospital. Correlations between urine phthalate metabolite concentrations and the CPT scores were investigated, and the interaction of phthalate metabolite levels with the selected polymorphisms at major candidate genes for ADHD, namely dopamine receptor D4 (DRD4), dopamine transporter, α-2A-adrenergic receptor, and norepinephrine transporter genes. For the subjects with the DRD4 4/4 genotype, there were significant associations of the urine phthalate metabolite concentrations with the number of omission errors, the number of commission errors, and the response time variability scores on the CPT. However, for the subjects without the DRD4 4/4 genotype, no significant associations were found. The results of this study suggest a possible association between phthalate metabolite concentrations and poor attentional performances of ADHD as well as a genetic influence on this association. Further prospective and epigenetic studies are needed to investigate causality and pathophysiological mechanisms.

Genetic Addiction Risk Score (GARS): molecular neurogenetic evidence for predisposition to Reward Deficiency Syndrome (RDS)

We have published extensively on the neurogenetics of brain reward systems with reference to the genes related to dopaminergic function in particular. In 1996, we coined “Reward Deficiency Syndrome” (RDS), to portray behaviors found to have gene-based association with hypodopaminergic function. RDS as a useful concept has been embraced in many subsequent studies, to increase our understanding of Substance Use Disorder (SUD), addictions, and other obsessive, compulsive, and impulsive behaviors. Interestingly, albeit others, in one published study, we were able to describe lifetime RDS behaviors in a recovering addict (17 years sober) blindly by assessing resultant Genetic Addiction Risk Score (GARS™) data only. We hypothesize that genetic testing at an early age may be an effective preventive strategy to reduce or eliminate pathological substance and behavioral seeking activity. Here, we consider a select number of genes, their polymorphisms, and associated risks for RDS whereby, utilizing GWAS, there is evidence for convergence to reward candidate genes. The evidence presented serves as a plausible brain-print providing relevant genetic information that will reinforce targeted therapies, to improve recovery and prevent relapse on an individualized basis. The primary driver of RDS is a hypodopaminergic trait (genes) as well as epigenetic states (methylation and deacetylation on chromatin structure). We now have entered a new era in addiction medicine that embraces the neuroscience of addiction and RDS as a pathological condition in brain reward circuitry that calls for appropriate evidence-based therapy and early genetic diagnosis and that requires further intensive investigation.

Multi-cultural association of the serotonin transporter gene (SLC6A4) with substance use disorder

A number of studies have reported associations between the serotonin transporter gene (SLC6A4) and alcohol, heroin, cocaine, or methamphetamine abuse. Other studies have yielded contrary results. There are a number of reasons for non-replication, including inadequate statistical power, population stratification, and poor phenotype definition. This study was to test the association using a meta-analytic approach across a variety of racial and ethnic populations. Using the genotype data of 55 studies (7999 cases, 8264 controls, and 676 families or parent-offspring trios) published in the past 15 years, we have conducted comprehensive meta-analyses to examine the associations of the 5-HTTLPR and STin2 polymorphisms with substance use disorder. The meta-analyses support the associations of 5-HTTLPR with alcohol, heroin, cocaine, and methamphetamine dependence and abuse (eg, the smallest P-values were 0.0058 with odds ratio (OR)=0.54 (0.35, 0.84); 0.0024 with OR=0.77 (0.66, 0.91); 0.018 with OR=1.38 (1.06, 1.81); and 0.028 with OR=0.46 (0.23, 0.92) for alcohol, heroin, cocaine, and methamphetamine dependence/abuse, respectively). When all the phenotypes are combined, the P-value was 0.0006 with OR=0.86 (0.78, 0.94) in the combined European, Asian, and Mexican populations and P-value was 0.0028 with OR=1.41 (1.13, 1.78) in the African populations. Evidence of significant associations was also identified in other subgroup analyses regarding differently combined substance and populations. The effect sizes of 5-HTTLPR were comparable among the European, Asian, and Mexican populations, however, the risk allele was more frequent in Asians than in Europeans and Mexicans. The opposite directions of risk allele in African population might be driven by the opposite directions of risk allele in cocaine dependence. This meta-analysis supports that the association of the SLC6A4 gene with substance use disorder varies depending on substances with different risk allele frequencies in the multi-cultural populations. Further studies using larger sample size are warranted.

Dopamine Genetics and Function in Food and Substance Abuse

Having entered the genomics era with confidence in the future of medicine, including psychiatry, identifying the role of DNA and polymorphic associations with brain reward circuitry has led to a new understanding of all addictive behaviors. It is noteworthy that this strategy may provide treatment for the millions who are the victims of "Reward Deficiency Syndrome" (RDS) a genetic disorder of brain reward circuitry. This article will focus on drugs and food being mutuality addictive, and the role of dopamine genetics and function in addictions, including the interaction of the dopamine transporter, and sodium food. We will briefly review our concept that concerns the genetic antecedents of multiple-addictions (RDS). Studies have also shown that evaluating a panel of established reward genes and polymorphisms enables the stratification of genetic risk to RDS. The panel is called the "Genetic Addiction Risk Score (GARS)", and is a tool for the diagnosis of a genetic predisposition for RDS. The use of this test, as pointed out by others, would benefit the medical community by identifying at risk individuals at a very early age. We encourage, in depth work in both animal and human models of addiction. We encourage further exploration of the neurogenetic correlates of the commonalities between food and drug addiction and endorse forward thinking hypotheses like "The Salted Food Addiction Hypothesis".

A large-scale meta-analysis of the association between the ANKK1/DRD2 Taq1A polymorphism and alcohol dependence

Alcohol dependence (AD) is a common neuropsychiatric disorder with high heritability. A number of studies have analyzed the association between the Taq1A polymorphism (located in the gene cluster ANKK1/DRD2) and AD. In the present study, we conducted a large-scale meta-analysis to confirm the association between the Taq1A polymorphism and the risk for AD in over 18,000 subjects included in 61 case-control studies that were published up to August 2012. Our meta-analysis demonstrated both allelic and genotypic association between the Taq1A polymorphism and AD susceptibility [allelic: P(Z) = 1.1 × 10(-5), OR = 1.19; genotypic: P(Z) = 3.2 × 10(-5), OR = 1.24]. The association remained significant after adjustment for publication bias using the trim and fill method. Sensitivity analysis showed that the effect size of the Taq1A polymorphism on AD risk was moderate and not influenced by any individual study. The pooled odds ratio from published studies decreased with the year of publication, but stabilized after the year 2001. Subgroup analysis indicated that publication bias could be influenced by racial ancestry. In summary, this large-scale meta-analysis confirmed the association between the Taq1A polymorphism and AD. Future studies are required to investigate the functional significance of the ANKK1/DRD2 Taq1A polymorphism in AD.

Best friends and alcohol use in adolescence: the role of the dopamine D4 receptor gene

The influence of friends and peers is theoretically one of the most consistent and important factors explaining adolescent alcohol use. However, not all adolescents are equally likely to be influenced by their friends' drinking behaviors. Genetic factors may underlie these inter-individual differences in susceptibility to the drinking behavior of friends. Because the long allele (≥ 7 repeats) of the dopamine D4 receptor (DRD4) gene has been associated with susceptibility to alcohol and alcohol-related cues, we tested whether associations between best friend's and adolescent's alcohol use differed for DRD4 genotypes. A Dutch nationwide sample of 308 adolescents (age 13 at baseline) participated in a prospective, community-based study with five annual waves. A cross-lagged path analysis was carried out in Mplus to examine bi-directional relations between friends' and adolescents' weekly alcohol use (number of drinks). A multi-group approach was applied to test for moderation effects of a 48-base pair variable number of tandem repeats polymorphism in exon 3 of the DRD4 gene. Additionally, with latent growth curve models, it was examined whether the interaction between friends' drinking and DRD4 genotype predicted the development of adolescents' alcohol use. Results showed that both cross-sectionally and longitudinally higher levels of friends' alcohol use resulted in higher levels of adolescents' alcohol consumption over time (and vice versa). No significant moderation of DRD4 genotype was found: Associations between adolescents' and friends' drinking did not differ for adolescent carriers of the DRD4 long allele, when compared with adolescents without the DRD4 long allele. Because this is the first study to examine DRD4 × friends' drinking effects prospectively, replication is essential. Future longitudinal studies, possibly with observational or diary designs, are needed to increase our understanding of the interplay between genetic and environmental risk factors for adolescent alcohol use.

Genetic variations in the dopaminergic system and alcohol use: a system-level analysis

Alcohol use is highly heritable and has been associated with many gene variants, including those related to dopamine (DA). However, single gene association studies have shown inconsistent and small effects. Using a system-level approach, the current study aimed to estimate the overall effect of genetic variations in the DA system on alcohol use among male drinkers. One hundred seventy-six male college students who reported to have ever drunk alcohol were enrolled. Alcohol use was measured using the Alcohol Use Disorders Identification Test. Ninety-eight representative polymorphisms in all major DA neurotransmitter genes were genotyped. Using analysis of variance, we identified six single-nucleotide polymorphisms (SNP)s that made statistically significant contributions to alcohol use. Next, main effects and interactions of these SNPs were assessed using multiple regression. The final model accounted for approximately 20% of the variance for alcohol use. Finally, permutation analyses ascertained the probability of obtaining these findings by chance to be low, p ranging from 0.024 to 0.048. These results confirmed that DA-related gene variants made strong contributions to reported alcohol use and suggest that multiple regression can be a promising way to explore the genetic basis for multi-gene-determined human behaviors.

Confirmation of prior evidence of genetic susceptibility to alcoholism in a genome-wide association study of comorbid alcoholism and bipolar disorder

OBJECTIVES

Alcoholism and affective disorders are both strongly comorbid and heritable. We have investigated the genetic comorbidity between bipolar affective disorder and alcoholism.

METHODS

A genome-wide allelic association study of 506 patients from the University College London bipolar disorder case-control sample and 510 ancestrally matched supernormal controls. One hundred forty-three of the bipolar patients fulfilled the Research Diagnostic Criteria diagnosis of alcoholism. A total of 372 193 single nucleotide polymorphisms (SNPs) were genotyped. Genes previously shown to be associated with alcoholism and addiction phenotypes were then tested for association in the bipolar alcoholic sample using gene-wise permutation tests of all SNPs genotyped within a 50-kb region flanking each gene.

RESULTS

Several central nervous system genes showed significant (P<0.05) gene-wise evidence of association with bipolar alcoholism. The genes implicated, which replicated genes previously shown to be associated with alcoholism were: cadherin 11, collagen type 11 α2, neuromedin U receptor 2, exportin7, and semaphorin-associated protein 5A. The SNPs most strongly implicated in bipolar alcoholism, but, which did not meet conventional genome-wide significance criteria were the insulin-like growth factor-binding protein 7, carboxypeptidase O, cerebellin 2, and the cadherin 12 genes.

CONCLUSION

We have confirmed the role of some genes previously shown to be associated with alcoholism in the comorbid bipolar alcoholism subgroup. In this subgroup, bipolar disorder may lower the threshold for the phenotypic expression of these alcoholism susceptibility genes. We also show that some genes may independently increase susceptibility to affective disorder and alcoholism.

Molecular basis and current treatment for alcoholic liver disease

Alcohol use disorders and alcohol dependency affect millions of individuals worldwide. The impact of these facts lies in the elevated social and economic costs. Alcoholic liver disease is caused by acute and chronic exposure to ethanol which promotes oxidative stress and inflammatory response. Chronic consumption of ethanol implies liver steatosis, which is the first morphological change in the liver, followed by liver fibrosis and cirrhosis. This review comprises a broad approach of alcohol use disorders, and a more specific assessment of the pathophysiologic molecular basis, and genetics, as well as clinical presentation and current modalities of treatment for alcoholic liver disease.

Association between Dopamine D4 Receptor Gene Polymorphism and Scores on a Continuous Performance Test in Korean Children with Attention Deficit Hyperactivity Disorder

OBJECTIVE

The aim of this study was to evaluate the association between a variable number of tandem repeats polymorphism at the dopamine D4 receptor gene (DRD4) and the performance of children with attention deficit hyperactivity disorder (ADHD) in a continuous performance test (CPT).

METHODS

This study included 72 ADHD children (mean age=9.39+/-2.05 years) who were recruited from one child psychiatric clinic. The omission errors, commission errors, reaction time and reaction standardization in the CPT were computed. The number of 48-base pairs tandem repeats in the exon III of DRD4 was analyzed in a blind manner.

RESULTS

The homozygosity of the 4-repeat allele at DRD4 was significantly associated with fewer commission errors (t=2.364, df=28.685, p=0.025) and standard deviation of reaction time (t=2.351, df=24.648, p=0.027) even after adjusting for age. The results of analyses of CPT measured values among three groups showed that the group with higher frequency of the 4-repeat allele showed a lower mean score of commission errors (F=4.268, df=2, p=0.018).

CONCLUSION

These results suggest a protective role of 4-repeat allele of the DRD4 polymorphisms on commission errors in the CPT in children with ADHD.

The DRD4 exon 3 VNTR polymorphism and addiction-related phenotypes: a review

In addition to the large literatures on associations of the DRD4 VNTR polymorphism with ADHD and personality traits, there is an emerging literature linking this variant to addiction and addiction-related phenotypes. When only diagnosis-based studies are considered, an inconsistent picture emerges raising doubts as to the relevance of this polymorphism to addiction. However the use of multiple levels of analysis in examining the importance of this polymorphism has raised the possibility of an urge-related "intermediate phenotype" that puts one at risk for developing addiction but may not be found in all persons with an addiction diagnosis. From cellular assays through neuroimaging and behavioral phenotypes, these studies highlight the power of the "intermediate phenotype" approach and suggest a possible explanation of the mixed findings when diagnosis is used as the phenotype. Strengths and weaknesses of alternative DRD4 VNTR genotype grouping strategies are discussed. In sum, converging evidence across multiple methodologies supports the possibility of a robust relationship between the DRD4 exon 3 VNTR polymorphism and urge for addictive substances.