A DRD4/BDNF gene-gene interaction associated with maximum BMI in women with bulimia nervosa

The BDNF Val66Met polymorphism serves as a potential marker of body weight in patients with psychiatric disorders

Brain-derived neurotrophic factor (BDNF) is a predominant neurotrophic factor in the brain, indispensable for neuronal growth, synaptic development, neuronal repair, and hippocampal neuroplasticity. Among its genetic variants, the BDNF Val66Met polymorphism is widespread in the population and has been associated with the onset and aggravation of diverse pathologies, including metabolic conditions like obesity and diabetes, cardiovascular ailments, cancer, and an array of psychiatric disorders. Psychiatric disorders constitute a broad category of mental health issues that influence mood, cognition, and behavior. Despite advances in research and treatment, challenges persist that hinder our understanding and effective intervention of these multifaceted conditions. Achieving and maintaining stable body weight is pivotal for overall health and well-being, and the relationship between psychiatric conditions and body weight is notably intricate and reciprocal. Both weight gain and loss have been linked to varying mental health challenges, making the disentanglement of this relationship critical for crafting holistic treatment strategies. The BDNF Val66Met polymorphism's connection to weight fluctuation in psychiatric patients has garnered attention. This review investigated the effects and underlying mechanisms by which the BDNF Val66Met polymorphism moderates body weight among individuals with psychiatric disorders. It posits the polymorphism as a potential biomarker, offering prospects for improved monitoring and therapeutic approaches for mental illnesses.

Humanized dopamine D4.7 receptor male mice display risk-taking behavior and deficits of social recognition and working memory in light/dark-dependent manner

The dopamine D4 receptor 7-repeat allele (D4.7 R) has been linked with psychiatric disorders such as attention-deficit-hyperactivity disorder, autism, and schizophrenia. However, the highly diverse study populations and often contradictory findings make it difficult to draw reliable conclusions. The D4.7 R has the potential to explain individual differences in behavior. However, there is still a great deal of ambiguity surrounding whether it is causally connected to the etiology of psychiatric disorders. Therefore, humanized D4.7 R mice, with the long third intracellular domain of the human D4.7 R, may provide a valuable tool to examine the relationship between the D4.7 R variant and specific behavioral phenotypes. We report that D4.7 R male mice carrying the humanized D4.7 R variant exhibit distinct behavioral features that are dependent on the light-dark cycle. The behavioral phenotype was characterized by a working memory deficit, delayed decision execution in the light phase, decreased stress and anxiety, and increased risk behavior in the dark phase. Further, D4.7 R mice displayed impaired social recognition memory in both the light and dark phases. These findings provide insight into the potential causal relationship between the human D4.7 R variant and specific behaviors and encourage further consideration of dopamine D4 receptor (DRD4) ligands as novel treatments for psychiatric disorders in which D4.7 R has been implicated.

Genetic polymorphisms in the serotonin, dopamine and opioid pathways influence social attention in rhesus macaques (Macaca mulatta)

Behaviour has a significant heritable component; however, unpicking the variants of interest in the neural circuits and molecular pathways that underpin these has proven difficult. Here, we present a comprehensive analysis of the relationship between known and new candidate genes from identified pathways and key behaviours for survival in 109 adult rhesus macaques (Macaca mulatta). Eight genes involved in emotion were analysed for variation at a total of nine loci. Genetic data were then correlated with cognitive and observational measures of behaviour associated with wellbeing and survival using MCMC-based Bayesian GLMM in R, to account for relatedness within the macaque population. For four loci the variants genotyped were length polymorphisms (SLC6A4 5-hydroxytryptamine transporter length-polymorphic repeat (5-HTTLPR), SLC6A4 STin polymorphism, Tryptophan 5-hydroxylase 2 (TPH2) and Monoamine oxidase A (MAOA)) whilst for the other five (5-hydroxytryptamine receptor 2A (HTR2A), Dopamine Receptor D4 (DRD4), Oxytocin receptor (OXTR), Arginine vasopressin receptor 1A (AVPR1a), Opioid receptor mu(μ) 1 (OPRM1)) SNPs were analysed. STin genotype, DRD4 haplotype and OXTR haplotype were significantly associated with the cognitive and observational measures of behaviour associated with wellbeing and survival. Genotype for 5-HTTLPR, STin and AVPR1a, and haplotype for HTR2A, DRD4 and OXTR were significantly associated with the duration of behaviours including fear and anxiety. Understanding the biological underpinnings of individual variation in negative emotion (e.g., fear and anxiety), together with their impact on social behaviour (e.g., social attention including vigilance for threat) has application for managing primate populations in the wild and captivity, as well as potential translational application for understanding of the genetic basis of emotions in humans.

Behavioural effects of APH199, a selective dopamine D4 receptor agonist, in animal models

RATIONALE

The dopamine D4 receptors (DRD4) play a key role in numerous brain functions and are involved in the pathogenesis of various psychiatric disorders. DRD4 ligands have been shown to moderate anxiety, reward and depression-like behaviours, and cognitive impairments. Despite a series of promising but ambiguous findings, the therapeutic advantages of DRD4 stimulation remain elusive.

OBJECTIVES

The investigation focused on the behavioural effects of the recently developed DRD4 agonist, APH199, to evaluate its impact on anxiety, anhedonia, behavioural despair, establishment and retrieval of alcohol reinforcement, and amphetamine (AMPH)-induced symptoms.

METHODS

Male C57BL/6 J mice and Sprague-Dawley rats were examined in five independent experiments. We assessed APH199 (0.1-5 mg/kg, i.p.) effects on a broad range of behavioural parameters in the open field (OF) test, conditioned place preference test (CPP), elevated plus maze (EPM), light-dark box (LDB), novelty suppressed feeding (NSF), forced swim test (FST), sucrose preference test (SPT), AMPH-induced hyperlocomotion test (AIH), and prepulse inhibition (PPI) of the acoustic startle response in AMPH-sensitized rats.

RESULTS

APH199 caused mild and sporadic anxiolytic and antidepressant effects in EPM and FST, but no remarkable impact on behaviour in other tests in mice. However, we found a significant increase in AMPH-induced hyperactivity, suggesting an exaggeration of the psychotic-like responses in the AMPH-sensitized rats.

CONCLUSIONS

Our data challenged the hypothesis of the therapeutic benefits of DRD4 agonists, pointing out a possible aggravation of psychosis. We suggest a need for further preclinical studies to ensure the safety of antipsychotics with DRD4 stimulating properties.

Metabolic homeostasis via BDNF and its receptors

Metabolic disorders result from dysregulation of central nervous system and peripheral metabolic energy homeostatic pathways. To maintain normal energy balance, neural circuits must integrate feedforward and feedback signals from the internal metabolic environment to orchestrate proper food intake and energy expenditure. These signals include conserved meal and adipocyte cues such as glucose and leptin, respectively, in addition to more novel players including brain-derived neurotrophic factor (BDNF). In particular, BDNF's two receptors, tropomyosin related kinase B (TrkB) and p75 neurotrophin receptor (p75NTR), are increasingly appreciated to be involved in whole body energy homeostasis. At times, these two receptors even seem to functionally oppose one another's actions, providing the framework for a potential neurotrophin mediated energy regulatory axis, which we explore further here.

Underlying Susceptibility to Eating Disorders and Drug Abuse: Genetic and Pharmacological Aspects of Dopamine D4 Receptors

The dopamine D4 receptor (DRD4) has a predominant expression in the prefrontal cortex (PFC), brain area strictly involved in the modulation of reward processes related to both food and drug consumption. Additionally, the human DRD4 gene is characterized by a variable number of tandem repeats (VNTR) in the exon 3 and, among the polymorphic variants, the 7-repeat (7R) allele appears as a contributing factor in the neurobiological mechanisms underlying drug abuse, aberrant eating behaviors and related comorbidities. The 7R variant encodes for a receptor with a blunted intracellular response to dopamine, and carriers of this polymorphism might be more tempted to enhance dopamine levels in the brain, through the overconsumption of drugs of abuse or palatable food, considering their reinforcing properties. Moreover, the presence of this polymorphism seems to increase the susceptibility of individuals to engage maladaptive eating patterns in response to negative environmental stimuli. This review is focused on the role of DRD4 and DRD4 genetic polymorphism in these neuropsychiatric disorders in both clinical and preclinical studies. However, further research is needed to better clarify the complex DRD4 role, by using validated preclinical models and novel compounds more selective for DRD4.

Predicted DRD4 prefrontal gene expression moderates snack intake and stress perception in response to the environment in adolescents

Body weight is substantially determined by eating behaviors, which are themselves driven by biological factors interacting with the environment. Previous studies in young children suggest that genetic influences on dopamine function may confer differential susceptibility to the environment in such a way that increases behavioral obesity risk in a lower socioeconomic status (SES) environment but decreases it in a higher SES environment. We aimed to test if this pattern of effect could also be observed in adolescence, another critical period for development in brain and behavior, using a novel measure of predicted expression of the dopamine receptor 4 (DRD4) gene in prefrontal cortex. In a sample of 76 adolescents (37 boys and 39 girls from Baltimore, Maryland/US, aged 14-18y), we estimated individual levels of DRD4 gene expression (PredDRD4) in prefrontal cortex from individual genomic data using PrediXcan, and tested interactions with a composite SES score derived from their annual household income, maternal education, food insecurity, perceived resource availability, and receipt of public assistance. Primary outcomes were snack intake during a multi-item ad libitum meal test, and food-related impulsivity assessed using a food-adapted go/no-go task. A linear regression model adjusted for sex, BMI z-score, and genetic ethnicity demonstrated a PredDRD4 by composite SES score interaction for snack intake (p = 0.009), such that adolescents who had lower PredDRD4 levels exhibited greater snack intake in the lower SES group, but lesser snack intake in the higher SES group. Exploratory analysis revealed a similar pattern for scores on the Perceived Stress Scale (p = 0.001) such that the low PredDRD4 group reported higher stress in the lower SES group, but less stress in the higher SES group, suggesting that PredDRD4 may act in part by affecting perceptions of the environment. These results are consistent with a differential susceptibility model in which genes influencing environmental responsiveness interact with environments varying in obesogenicity to confer behavioral obesity risk in a less favorable environment, but behavioral obesity protection in a favorable one.

Role of eating disorders-related polymorphisms in obesity pathophysiology

Human biological system provides innumerable neuroendocrine inputs for food intake control, with effects on appetite's modulation and the satiety signs. Its regulation is very complex, engaging several molecular interactions with many tissues, hormones, and neural circuits. Thus, signaling molecules that control food intake are critical for normal energy homeostasis and a deregulation of these pathways can lead to eating disorders and obesity. In line of this, genetic factors have a significantly influence of the regulation of neural circuits controlling the appetite and satiety pathways, as well as the regulation of brain reward systems. Single Nucleotide Polymorphisms (SNPs) in genes related to hypothalamic appetite and satiety mechanisms, further in multiple neurotransmitter systems may contribute to the development of major Eating Disorders (EDs) related to obesity, among them Binge Eating Disorder (BED) and Bulimia Nervosa (BN), which are discussed in this review.

Gene and environment interaction: Is the differential susceptibility hypothesis relevant for obesity?

The differential susceptibility model states that a given genetic variant is associated with an increased risk of pathology in negative environments but greater than average resilience in enriched ones. While this theory was first implemented in psychiatric-genetic research, it may also help us to unravel the complex ways that genes and environments interact to influence feeding behavior and obesity. We reviewed evidence on gene vs. environment interactions that influence obesity development, aiming to support the applicability of the differential susceptibility model for this condition, and propose that various environmental "layers" relevant for human development should be considered when bearing the differential susceptibility model in mind. Mother-child relationship, socioeconomic status and individual's response are important modifiers of BMI and food intake when interacting with gene variants, "for better and for worse". While only a few studies to date have investigated obesity outcomes using this approach, we propose that the differential susceptibility hypothesis is in fact highly applicable to the study of genetic and environmental influences on feeding behavior and obesity risk.

Influence of SNP*SNP interaction on BMI in European American adolescents: findings from the National Longitudinal Study of Adolescent Health

BACKGROUND

Adolescent obesity is predictive of future weight gain, obesity and adult onset severe obesity (body mass index [BMI] ≥40 kg m(-2) ). Despite successful efforts to identify Single Nucleotide Polymorphisms (SNPs) influencing BMI, <5% of the 40-80% heritability of the phenotype has been explained. Identification of gene-gene (G-G) interactions between known variants can help explain this hidden heritability as well as identify potential biological mechanisms affecting weight gain during this critical developmental period.

OBJECTIVE

We have recently shown distinct genetic effects on BMI across the life course, and thus it is important to examine the evidence for epistasis in adolescence.

METHODS

In adolescent participants of European descent from wave II of the National Longitudinal Study of Adolescent Health (Add Health, n = 5072, ages 12-21, 52.5% female), we tested 34 established BMI-related SNPs for G-G interaction effects on BMI z-score. We used mixed-effects regression, assuming multiplicative interaction models adjusting for age, sex and geographic region, with random effects for family and school.

RESULTS

For 28 G-G interactions that were nominally significant (P < 0.05), we attempted to replicate our results in an adolescent sample from the Childhood European American Cohort from Philadelphia. In the replication study, one interaction (PRKD1-FTO) was significant after correction for multiple testing.

CONCLUSIONS

Our results are suggestive of epistatic effects on BMI during adolescence and point to potentially interactive effects between genes in biological pathways important in obesity.

NT-3 protein levels are enhanced in the hippocampus of PRG1-deficient mice but remain unchanged in PRG1/LPA2 double mutants

The plasticity-related gene 1 (PRG1) modulates bioactive lipids at the postsynaptic density and is a novel player in neuronal plasticity and regulation of glutamatergic transmission at principal neurons. PRG1, a neuronal molecule, is highly expressed during development and regeneration processes at the postsynaptic density, modulates synaptic lysophosphatidic acid (LPA) levels and is related to epilepsy and brain injury. In the present study, we analyzed the interaction between the synaptic molecules PRG1 and LPA2R with other plasticity-related molecules the neurotrophins. The protein levels of NGF, BDNF and NT-3 were measured using ELISA in hippocampal tissue of homozygous (PRG(-/-)) and heterozygous (PRG(+/-)) PRG1 deficient mice and compared to their wild type (PRG(+/+)/WT) littermates. In the hippocampus, protein levels of NT-3 were significantly increased in PRG(-/-) mice (compared to WT-litters) while protein levels of NGF and BDNF were not affected. Since PRG1 deficiency leads to increased neuronal excitability and higher hippocampal network activity, which may well influence neurotrophin levels, we further assessed PRG1 deficient mice on an LPA2-receptor (LPA2R) deficient background, reported to normalize hippocampal over-excitability in PRG1(-/-) mice. However, on an LPA2R deficient background, protein levels of NT-3 in PRG1(-/-) mice (PRG1(-/-)/LPA2R(-/-)) were not significantly different when compared to WT animals. Since PRG1 deficient mice showed over-excitability in glutamatergic neurons. This was normalized by additional LPA2R deletion, and we conclude the increased NT3-levels were directly or indirectly attributable to increased hippocampal network activity, possibly exerting a protective effect against over-excitability.

Mesolimbic dopamine and its neuromodulators in obesity and binge eating

Obesity has reached epidemic prevalence, and much research has focused on homeostatic and nonhomeostatic mechanisms underlying overconsumption of food. Mesocorticolimbic circuitry, including dopamine neurons of the ventral tegmental area (VTA), is a key substrate for nonhomeostatic feeding. The goal of the present review is to compare changes in mesolimbic dopamine function in human obesity with diet-induced obesity in rodents. Additionally, we will review the literature to determine if dopamine signaling is altered with binge eating disorder in humans or binge eating modeled in rodents. Finally, we assess modulation of dopamine neurons by neuropeptides and peripheral peptidergic signals that occur with obesity or binge eating. We find that while decreased dopamine concentration is observed with obesity, there is inconsistency outside the human literature on the relationship between striatal D2 receptor expression and obesity. Finally, few studies have explored how orexigenic or anorexigenic peptides modulate dopamine neuronal activity or striatal dopamine in obese models. However, ghrelin modulation of dopamine neurons may be an important factor for driving binge feeding in rodents.

A draft network of ligand-receptor-mediated multicellular signalling in human

Cell-to-cell communication across multiple cell types and tissues strictly governs proper functioning of metazoans and extensively relies on interactions between secreted ligands and cell-surface receptors. Herein, we present the first large-scale map of cell-to-cell communication between 144 human primary cell types. We reveal that most cells express tens to hundreds of ligands and receptors to create a highly connected signalling network through multiple ligand-receptor paths. We also observe extensive autocrine signalling with approximately two-thirds of partners possibly interacting on the same cell type. We find that plasma membrane and secreted proteins have the highest cell-type specificity, they are evolutionarily younger than intracellular proteins, and that most receptors had evolved before their ligands. We provide an online tool to interactively query and visualize our networks and demonstrate how this tool can reveal novel cell-to-cell interactions with the prediction that mast cells signal to monoblastic lineages via the CSF1-CSF1R interacting pair.

Genetics and Epigenetics of Eating Disorders

Eating disorders (EDs) are serious psychiatric conditions influenced by biological, psychological, and sociocultural factors. A better understanding of the genetics of these complex traits and the development of more sophisticated molecular biology tools have advanced our understanding of the etiology of EDs. The aim of this review is to critically evaluate the literature on the genetic research conducted on three major EDs: anorexia nervosa (AN), bulimia nervosa (BN), and binge eating disorder (BED). We will first review the diagnostic criteria, clinical features, prevalence, and prognosis of AN, BN, and BED, followed by a review of family, twin, and adoption studies. We then review the history of genetic studies of EDs covering linkage analysis, candidate gene association studies, genome-wide association studies, and the study of rare variants in EDs. Our review also incorporates a translational perspective by covering animal models of ED-related phenotypes. Finally, we review the nascent field of epigenetics of EDs and a look forward to future directions for ED genetic research.

Pharmacological manipulations in animal models of anorexia and binge eating in relation to humans

Eating disorders, such as anorexia nervosa (AN), bulimia nervosa (BN) and binge eating disorders (BED), are described as abnormal eating habits that usually involve insufficient or excessive food intake. Animal models have been developed that provide insight into certain aspects of eating disorders. Several drugs have been found efficacious in these animal models and some of them have eventually proven useful in the treatment of eating disorders. This review will cover the role of monoaminergic neurotransmitters in eating disorders and their pharmacological manipulations in animal models and humans. Dopamine, 5-HT (serotonin) and noradrenaline in hypothalamic and striatal regions regulate food intake by affecting hunger and satiety and by affecting rewarding and motivational aspects of feeding. Reduced neurotransmission by dopamine, 5-HT and noradrenaline and compensatory changes, at least in dopamine D2 and 5-HT(2C/2A) receptors, have been related to the pathophysiology of AN in humans and animal models. Also, in disorders and animal models of BN and BED, monoaminergic neurotransmission is down-regulated but receptor level changes are different from those seen in AN. A hypofunctional dopamine system or overactive α2-adrenoceptors may contribute to an attenuated response to (palatable) food and result in hedonic binge eating. Evidence for the efficacy of monoaminergic treatments for AN is limited, while more support exists for the treatment of BN or BED with monoaminergic drugs.

BDNF genetic variability modulates psychopathological symptoms in patients with eating disorders

The brain-derived neurotrophic factor (BDNF) gene may influence eating behavior, body weight and cognitive impairments. We aimed to investigate whether BDNF genetic variability may affect anthropometric and psychological parameters in patients with anorexia or bulimia nervosa (AN, BN) and/or modulate the risk for the disorder. A total of 169 unrelated female patients and 312 healthy controls were genotyped for two common BDNF single-nucleotide polymorphisms (SNPs), Val66Met and C-270T, and several selected tag-SNPs. Associated personality characteristics and psychopathological symptoms were assessed by the EDI-2 and SCL-90R inventories, respectively. No single SNP or haplotype played a relevant role in the risk for AN or BN. The rs16917237 TT genotype was significantly associated with increased weight (74.63 ± 16.58 vs. 57.93 ± 13.02) and body mass index (28.94 ± 6.22 vs. 22.23 ± 4.77) in the BN group after correcting for multiple testing. Haplotype analyses using a sliding window approach with three adjacent SNPs produced four loci of interest. Locus 3 (rs10835210/rs16917237/C-270T) showed a broad impact on the measured psychopathological symptoms. Haplotypes CGC and CGT in this locus correlated with scores in all three scales of the SCL-90R inventory, both in AN and BN patients. In contrast, the results of the EDI-2 inventory were largely unaffected. These preliminary results suggest that variability in the BDNF gene locus may contribute to anthropometric characteristics and also psychopathological symptoms that are common but not exclusive of ED patients.

The role of leptin, melanocortin, and neurotrophin system genes on body weight in anorexia nervosa and bulimia nervosa

OBJECTIVE

Although low weight is a key factor contributing to the high mortality in anorexia nervosa (AN), it is unclear how AN patients sustain low weight compared with bulimia nervosa (BN) patients with similar psychopathology. Studies of genes involved in appetite and weight regulation in eating disorders have yielded variable findings, in part due to small sample size and clinical heterogeneity. This study: (1) assessed the role of leptin, melanocortin, and neurotrophin genetic variants in conferring risk for AN and BN; and (2) explored the involvement of these genes in body mass index (BMI) variations within AN and BN.

METHOD

Our sample consisted of 745 individuals with AN without a history of BN, 245 individuals with BN without a history of AN, and 321 controls. We genotyped 20 markers with known or putative function among genes selected from leptin, melanocortin, and neurotrophin systems.

RESULTS

There were no significant differences in allele frequencies among individuals with AN, BN, and controls. AGRP rs13338499 polymorphism was associated with lowest illness-related BMI in those with AN (p = 0.0013), and NTRK2 rs1042571 was associated with highest BMI in those with BN (p = 0.0018).

DISCUSSION

To our knowledge, this is the first study to address the issue of clinical heterogeneity in eating disorder genetic research and to explore the role of known or putatively functional markers in genes regulating appetite and weight in individuals with AN and BN. If replicated, our results may serve as an important first step toward gaining a better understanding of weight regulation in eating disorders.

The association between dopamine receptor (DRD4) gene polymorphisms and second language learning style and behavioral variability in undergraduate students in Turkey

The dopamine D4 receptor gene (DRD4) encodes a receptor for dopamine, a chemical messenger used in the brain. One variant of the DRD4 gene, the 7R allele, is believed to be associated with attention deficit hyperactivity disorder (ADHD). The aim of this study was to investigate the relationships between repeat polymorphisms in dopamine DRD4 and second language learning styles such as visual (seeing), tactile (touching), auditory (hearing), kinesthetic (moving) and group/individual learning styles, as well as the relationships among DRD4 gene polymorphisms and ADHD in undergraduate students. A total of 227 students between the ages of 17-21 years were evaluated using the Wender Utah rating scale and DSM-IV diagnostic criteria for ADHD. Additionally, Reid's perceptual learning style questionnaire for second language learning style was applied. In addition, these students were evaluated for social distress factors using the list of Threatening Events (TLE); having had no TLE, having had just one TLE or having had two or more TLEs within the previous 6 months before the interview. For DRD4 gene polymorphisms, DNA was extracted from whole blood using the standard phenol/chloroform method and genotyped using polymerase chain reaction. Second language learners with the DRD4.7+ repeats showed kinaesthetic and auditory learning styles, while students with DRD4.7-repeats showed visual, tactile and group learning, and also preferred the more visual learning styles [Formula: see text]. We also demonstrated that the DRD4 polymorphism significantly affected the risk effect conferred by an increasing level of exposure to TLE.

Association analyses for dopamine receptor gene polymorphisms and weight status in a longitudinal analysis in obese children before and after lifestyle intervention

Background

Dopamine receptors are involved in midbrain reward circuit activation. Polymorphisms in two dopamine receptor genes, DRD2 and DRD4, have been associated with altered perception of food reward and weight gain. The objective of this study was to determine whether the same risk alleles were associated with overweight/obesity and with lower reduction of overweight after a 1-year lifestyle intervention.

Methods

In a longitudinal study the association of polymorphisms in DRD2 (rs18000497, risk allele: T, formerly A1 allele at the TaqI A1 polymorphism) and DRD4 (variable number of tandem repeats (VNTR); 48 bp repeat in exon III; risk alleles: 7 repeats or longer: 7R+) was tested on weight loss success following a 1-year lifestyle childhood obesity intervention (OBELDICKS). An additional exploratory cross-sectional case-control study was performed to compare the same DRD polymorphisms in these overweight/obese children and adolescents versus lean adult controls. Subjects were 423 obese and 28 overweight children participating in lifestyle intervention (203 males), age median 12.0 (interquartile range 10.0–13.7) years, body mass index - standard deviation score (BMI-SDS) 2.4 ± 0.5; 583 lean adults (232 males); age median 25.3 (interquartile range 22.5–26.8) years, BMI 19.1 ± 1.9 kg/m². BMI, BMI-SDS and skinfold thickness measures were assessed at baseline and after 1 year; genotyping was performed for DRD2 risk variant rs1800497 and DRD4 exon III VNTR.

Results

The DRD2 genotype had a nominal effect on success in the weight loss intervention. The weakest BMI-SDS reduction was in children homozygous for two rs1800497 T-alleles (n = 11) compared to the combined group with zero (n = 308) or one (n = 132) rs1800497 T-allele (-0.08 ± 0.36 vs. -0.28 ± 0.34; p < 0.05). There was no association between the DRD4 VNTR alleles and genotypes and success in the weight loss intervention. No associations of the risk alleles of the DRD2 and DRD4 polymorphisms and obesity were observed in the cross-sectional part of the study.

Conclusions

We did not find association between polymorphisms in DRD2 and DRD4 genes and weight status. However, obese carriers of two DRD2 rs1800497 T-alleles may be at risk for weak responses to lifestyle interventions aimed at weight reduction.

Trial registration

Obesity intervention program “Obeldicks” is registered at clinicaltrials.gov (NCT00435734).

Association between the seven-repeat allele of the dopamine-4 receptor gene (DRD4) and spontaneous food intake in pre-school children

BACKGROUND

Studies in adults show associations between the hypofunctional seven-repeat allele (7R) of the dopamine-4 receptor gene (DRD4), increased eating behaviour and/or obesity, particularly in females. We examined whether 7R is associated with total caloric intake and/or food choices in pre-schoolers.

METHODS

150 four-year-old children taking part in a birth cohort study in Canada were administered a snack test meal in a laboratory setting. Mothers also filled out a food frequency questionnaire to address childrens' habitual food consumption. Total caloric and individual macronutrient intakes during the snack meal and specific types of foods as reported in the food diaries were compared across 7R allele carriers vs. non-carriers, using current BMI as a co-variate.

RESULTS

We found significant sex by genotype interactions for fat and protein intake during the snack test. Post hoc testing revealed that in girls, but not boys, 7R carriers ate more fat and protein than did non-carriers. Based on the food diaries, across both sexes, 7R carriers consumed more portions of ice cream and less vegetables, eggs, nuts and whole bread, suggesting a less healthy pattern of habitual food consumption.

CONCLUSION

The 7R allele of DRD4 influences macronutrient intakes and specific food choices as early as four years of age. The specific pattern of results further suggests that prior associations between the 7R allele and adult overeating/obesity may originate in food choices observable in the preschool years. Longitudinal follow-up of these children will help establish the relevance of these findings for obesity risk and prevention.

The genetics of eating disorders

Over the past decade, considerable advances have been made in understanding genetic influences on eating pathology. Eating disorders aggregate in families, and twin studies reveal that additive genetic factors account for approximately 40% to 60% of liability to anorexia nervosa (AN), bulimia nervosa (BN), and binge eating disorder (BED). Molecular genetics studies have been undertaken to identify alterations in deoxyribonucleic acid sequence and/or gene expression that may be involved in the pathogenesis of disordered eating behaviors, symptoms, and related disorders and to uncover potential genetic variants that may contribute to variability of treatment response. This article provides an in-depth review of the scientific literature on the genetics of AN, BN, and BED including extant studies, emerging hypotheses, future directions, and clinical implications.

The role of D4 receptor gene exon III polymorphisms in shaping human altruism and prosocial behavior

Human beings are an extraordinarily altruistic species often willing to help strangers at a considerable cost (sometimes life itself) to themselves. But as Darwin noted "… he who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature." Hence, this is the paradox of altruism. Twin studies have shown that altruism and other prosocial behavior show considerable heritability and more recently a number of candidate genes have been identified with this phenotype. Among these first provisional findings are genes encoding elements of dopaminergic transmission. In this article we will review the evidence for the involvement of one of these, the dopamine D4 receptor (DRD4) gene, in shaping human prosocial behavior and consider the methodologies employed in measuring this trait, specific molecular genetic findings and finally, evidence from several Gene × Environment (G × E) studies that imply differential susceptibility of this gene to environmental influences.

Epistatic interactions implicating dopaminergic genes in bulimia nervosa (BN): relationships to eating- and personality-related psychopathology

We explored the influence of interactions between polymorphisms acting upon postsynaptic receptors (DRD2 TaqA1 rs1800497 and DRD4 7R) and dopamine regulators (COMT rs4680 and DAT1) on the expression of eating symptoms and personality traits in women with bulimia-spectrum eating disorders. We had 269 bulimic women provide blood for genetic assays, and measured eating-disorder symptoms and psychopathological traits using structured interviews and self-report questionnaires. We observed two epistatic interactions on symptom indices: interactions (in predicted directions) of DRD2 by DAT were seen on Body Mass Index (p=.023), and of DRD4 by COMT on self-harming behaviors (p=.014)--with genetic effects that would correspond to reduced dopamine transmission coinciding with more-pathological scores. Our findings suggest that genes acting in the dopamine system interact to influence both eating-related and personality psychopathology, with the result that lower levels of dopamine neuro-transmission correspond to increased psychopathology and body mass in women with bulimia-spectrum disorders. We discuss the implications of our observations.

Reward circuitry dysfunction in psychiatric and neurodevelopmental disorders and genetic syndromes: animal models and clinical findings

This review summarizes evidence of dysregulated reward circuitry function in a range of neurodevelopmental and psychiatric disorders and genetic syndromes. First, the contribution of identifying a core mechanistic process across disparate disorders to disease classification is discussed, followed by a review of the neurobiology of reward circuitry. We next consider preclinical animal models and clinical evidence of reward-pathway dysfunction in a range of disorders, including psychiatric disorders (i.e., substance-use disorders, affective disorders, eating disorders, and obsessive compulsive disorders), neurodevelopmental disorders (i.e., schizophrenia, attention-deficit/hyperactivity disorder, autism spectrum disorders, Tourette's syndrome, conduct disorder/oppositional defiant disorder), and genetic syndromes (i.e., Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, Angelman syndrome, and Rett syndrome). We also provide brief overviews of effective psychopharmacologic agents that have an effect on the dopamine system in these disorders. This review concludes with methodological considerations for future research designed to more clearly probe reward-circuitry dysfunction, with the ultimate goal of improved intervention strategies.

Possible association of the DRD4 gene with a history of attention-deficit/hyperactivity disorder in women with bulimia nervosa

OBJECTIVE

Up to one-third of patients with bulimia nervosa (BN) report a history of attention-deficit/hyperactivity disorder (ADHD) symptoms, and the dopaminergic genes, especially the dopamine receptor D4 (DRD4), have been associated independently with both binge eating and ADHD.

METHOD

The purpose of this study was to (1) compare the frequency of DRD4 exon III VNTR variants in 157 BN probands and equal number of healthy controls; (2) assess transmission of DRD4 alleles in 68 families of patients with BN; and (3) explore the possible role of DRD4 gene in childhood ADHD history in a subgroup of 81 women with BN.

RESULTS

Our results showed that the BN probands and controls did not differ in DRD4 allele frequency. We also did not find evidence for preferential transmission of any DRD4 variant to the affected offspring with BN. However, the presence of either the 2-repeat or 7-repeat allele was associated with a history of childhood ADHD in BN (p = .038, odds ratio = 3.2).

DISCUSSION

Our findings suggest that although DRD4 may not be associated with the diagnosis of BN, its variants are associated with a history of childhood ADHD in BN probands. This may have relevance for the understanding, prevention, and treatment of BN that evolves in the context of childhood ADHD.

The influence of dopamine receptor d4 polymorphism on resting EEG in healthy young females

The polymorphism of variable number of tandem repeat (VNTR) in dopamine receptor D4 (DRD4) gene exon III has been linked to various neuro-psychiatric conditions with disinhibition/impulsivity as one of the core features. This study examined the modulatory effects of long-allele variant of DRD4 VNTR on the regional neural activity as well as inter-regional neural interactions in a young female population. Blood sample and resting state eyes-closed EEG signals were collected in 233 healthy females, stratified into two groups by polymerase chain reaction: long-allele carriers (>4- repeat) and non-carriers (<=4-repeat/<=4-repeat). The values of mean power of 18 electrodes and mutual information of 38 channel pairs across theta, alpha, and beta frequencies were analyzed. Our connectivity analysis was based on information theory, which combined Morlet wavelet transform and mutual information calculation. Between-group differences of regional power and connectivity strength were quantified by independent t-test, while between-group differences in global trends were examined by non-parametric analyses. We noticed that DRD4 VNTR long-allele was associated with decreased global connectivity strength (from non-parametric analysis), especially over bi-frontal, biparietal and right fronto-parietal and right fronto-temporal connections (from independent t-tests). The between-group differences in regional power were not robust. Our findings fit with the networks of response inhibition, providing evidence bridging DRD4 long-allele and disinhibition/impulsivity in neuropsychiatric disorders. We suggest future DRD4 studies of imaging genetics incorporate connectivity analysis to unveil its impact on cerebral network.

Pharmacogenetics of antipsychotic-induced weight gain: review and clinical implications

Second-generation antipsychotics (SGAs), such as risperidone, clozapine and olanzapine, are the most common drug treatments for schizophrenia. SGAs presented an advantage over first-generation antipsychotics (FGAs), particularly regarding avoidance of extrapyramidal symptoms. However, most SGAs, and to a lesser degree FGAs, are linked to substantial weight gain. This substantial weight gain is a leading factor in patient non-compliance and poses significant risk of diabetes, lipid abnormalities (that is, metabolic syndrome) and cardiovascular events including sudden death. The purpose of this article is to review the advances made in the field of pharmacogenetics of antipsychotic-induced weight gain (AIWG). We included all published association studies in AIWG from December 2006 to date using the Medline and ISI web of knowledge databases. There has been considerable progress reaffirming previous findings and discovery of novel genetic factors. The HTR2C and leptin genes are among the most promising, and new evidence suggests that the DRD2, TNF, SNAP-25 and MC4R genes are also prominent risk factors. Further promising findings have been reported in novel susceptibility genes, such as CNR1, MDR1, ADRA1A and INSIG2. More research is required before genetically informed, personalized medicine can be applied to antipsychotic treatment; nevertheless, inroads have been made towards assessing genetic liability and plausible clinical application.

Dopamine D4 receptor gene DRD4 and its association with psychiatric disorders

Dopamine receptors control neural signals that modulates behavior. Dopamine plays an important role in normal attention; that is the reason for studying the genes of the dopaminergic system, mainly in connection with disorders of attention. DRD4 influences the postsynaptic action of dopamine and is implicated in many neurological processes, exhibits polymorphism and is one of the most studied genes in connection with psychiatric disorders. Associations were found with ADHD (attention deficit hyperactivity disorder), substance dependences, several specific personality traits, and reaction to stress. These findings have implications for pharmacogenetics. This article reviews the principle published associations of DRD4 variants with psychiatric disorders.

COMT Val158Met variant and functional haplotypes associated with childhood ADHD history in women with bulimia nervosa

Up to one third of patients with bulimia nervosa (BN) report a history of ADHD symptoms, and both disorders may also be associated with dopaminergic abnormalities. COMT gene, coding for an enzyme responsible for the degradation of dopamine, may play a part in the etiology of ADHD and BN. This study aimed to (1) examine if certain variants of the COMT genetic markers (rs6269, rs4633, rs4818 and rs4680) are more common in BN versus controls; (2) assess transmission of COMT alleles in BN families; and (3) explore the role of COMT genotypes and haplotypes in bulimic women with childhood ADHD history. 72 BN probands and unaffected relatives were genotyped for COMT rs4680 (Val158Met) and three adjacent markers. The remaining 165 probands were matched with nonpsychiatric controls. We also investigated if COMT variants and haplotypes were associated with childhood ADHD history in a subgroup of 86 BN probands who completed the Wender Utah Rating Scale (WURS). Our results showed that cases and controls did not differ in COMT allele and haplotype frequencies. In contrast, specific alleles of all four COMT markers and the medium-activity haplotype were preferentially transmitted to the offspring with BN. COMT Val158 allele was overrepresented and the medium-activity haplotype was underrepresented in BN with childhood ADHD history (p=0.010). These findings suggest a possible role for COMT variants and related haplotypes in BN and its subphenotypes. If replicated, these preliminary findings may have implications for the prevention and treatment of BN that emerges in the context of childhood ADHD.

The dark side of food addiction

In drug addiction, the transition from casual drug use to dependence has been linked to a shift away from positive reinforcement and toward negative reinforcement. That is, drugs ultimately are relied on to prevent or relieve negative states that otherwise result from abstinence (e.g., withdrawal) or from adverse environmental circumstances (e.g., stress). Recent work has suggested that this "dark side" shift also is a key in the development of food addiction. Initially, palatable food consumption has both positively reinforcing, pleasurable effects and negatively reinforcing, "comforting" effects that can acutely normalize organism responses to stress. Repeated, intermittent intake of palatable food may instead amplify brain stress circuitry and downregulate brain reward pathways such that continued intake becomes obligatory to prevent negative emotional states via negative reinforcement. Stress, anxiety and depressed mood have shown high comorbidity with and the potential to trigger bouts of addiction-like eating behavior in humans. Animal models indicate that repeated, intermittent access to palatable foods can lead to emotional and somatic signs of withdrawal when the food is no longer available, tolerance and dampening of brain reward circuitry, compulsive seeking of palatable food despite potentially aversive consequences, and relapse to palatable food-seeking in response to anxiogenic-like stimuli. The neurocircuitry identified to date in the "dark" side of food addiction qualitatively resembles that associated with drug and alcohol dependence. The present review summarizes Bart Hoebel's groundbreaking conceptual and empirical contributions to understanding the role of the "dark side" in food addiction along with related work of those that have followed him.

Central dopaminergic circuitry controlling food intake and reward: implications for the regulation of obesity

Prevalence of obesity in the general population has increased in the past 15 years from 15% to 35%. With increasing obesity, the coincident medical and social consequences are becoming more alarming. Control over food intake is crucial for the maintenance of body weight and represents an important target for the treatment of obesity. Central nervous system mechanisms responsible for control of food intake have evolved to sense the nutrient and energy levels in the organism and to coordinate appropriate responses to adjust energy intake and expenditure. This homeostatic system is crucial for maintenance of stable body weight over long periods of time of uneven energy availability. However, not only the caloric and nutritional value of food but also hedonic and emotional aspects of feeding affect food intake. In modern society, the increased availability of highly palatable and rewarding (fat, sweet) food can significantly affect homeostatic balance, resulting in dysregulated food intake. This review will focus on the role of hypothalamic and mesolimbic/mesocortical dopaminergic (DA) circuitry in coding homeostatic and hedonic signals for the regulation of food intake and maintenance of caloric balance. The interaction of dopamine with peripheral and central indices of nutritional status (e.g., leptin, ghrelin, neuropeptide Y), and the susceptibility of the dopamine system to prenatal insults will be discussed. Additionally, the importance of alterations in dopamine signaling that occur coincidently with obesity will be addressed.

Dopamine-based reward circuitry responsivity, genetics, and overeating

Data suggest that low levels of dopamine D2 receptors and attenuated responsivity of dopamine-target regions to food intake is associated with increased eating and elevated weight. There is also growing (although mixed) evidence that genotypes that appear to lead to reduced dopamine signaling (e.g., DRD2, DRD4, and DAT) and certain appetite-related hormones and peptides (e.g., ghrelin, orexin A, leptin) moderate the relation between dopamine signaling, overeating, and obesity. This chapter reviews findings from studies that have investigated the relation between dopamine functioning and food intake and how certain genotypes and appetite-related hormones and peptides affect this relation.

Role of the neurotrophin network in eating disorders' subphenotypes: body mass index and age at onset of the disease

Eating disorders (ED) are severe psychiatric diseases that most likely result from, and are sustained by socio-cultural, psychological and biological factors. We explored whether members of the neurotrophin family are disease-modifying factors of quantitative traits, potentially contributing to the outcome or prognosis of the disease. We studied lifetime minimum and maximum body mass index (minBMI and maxBMI) and age at onset of the disease in a sample of 991 ED patients from France, Germany, Italy and Spain and analysed 183 genetic variants located in 10 candidate genes encoding different neurotrophins and their receptors. We used a hierarchical model approach to include prior genetic knowledge of the specific and found that variants in CNTF, in its receptor CNTFR, and in NTRK2 were significantly associated with a lower age at onset of the ED. In addition, one variant in NTRK1 was associated with a higher minBMI. The results suggest that for these two subphenotypes, CNTF, CNTFR, NTRK1 and NTRK2 might act as disease-modifying factors and add preliminary evidence to the global hypothesis that EDs are the result of complex interactions and reciprocal controls between the immune, endocrine and central nervous systems.

Systematic analysis of dopamine receptor genes (DRD1-DRD5) in antipsychotic-induced weight gain

Antipsychotic-induced weight gain has emerged as a serious complication in the treatment of patients with most antipsychotics. We have conducted the first in-depth examination of dopamine receptor genes in antipsychotic-induced weight gain. A total of 206 patients (139 of European descent and 56 African Americans) who underwent treatment for chronic schizophrenia or schizoaffective disorder were evaluated after on average over 6 weeks of treatment. Thirty-six tag single nucleotide polymorphisms (SNPs) and one variable-number tandem repeat, spanning the five dopamine receptor genes (DRD1-DRD5) were analyzed. In the total sample, we found a nominally significant association between the DRD2 rs1079598 marker and weight change using a cutoff of 7% gain (P=0.03). When stratifying the sample according to ethnicity and antipsychotics with highest risk for weight gain, we found significant associations in three DRD2 SNPs: rs6277 (C957T), rs1079598 and rs1800497 (TaqIA). The other genes were primarily negative. We provide evidence that dopamine receptor DRD2 gene variants might be associated with antipsychotic-induced weight gain in chronic schizophrenia patients.

Rai1 haploinsufficiency causes reduced Bdnf expression resulting in hyperphagia, obesity and altered fat distribution in mice and humans with no evidence of metabolic syndrome

Smith-Magenis syndrome (SMS) is a genetic disorder caused by haploinsufficiency of the retinoic acid induced 1 (RAI1) gene. In addition to intellectual disabilities, behavioral abnormalities and sleep disturbances, a majority of children with SMS also have significant early-onset obesity. To study the role of RAI1 in obesity, we investigated the growth and obesity phenotype in a mouse model haploinsufficient for Rai1. Data show that Rai1(+/-) mice are hyperphagic, have an impaired satiety response and have altered abdominal and subcutaneous fat distribution, with Rai1(+/-) female mice having a higher proportion of abdominal fat when compared with wild-type female mice. Expression analyses revealed that Bdnf (brain-derived neurotrophic factor), a gene previously associated with hyperphagia and obesity, is downregulated in the Rai1(+/-) mouse hypothalamus, and reporter studies show that RAI1 directly regulates the expression of BDNF. Even though the Rai1(+/-) mice are significantly obese, serum analyses do not reveal any evidence of metabolic syndrome. Supporting these findings, a caregiver survey revealed that even though a high incidence of abdominal obesity is observed in females with SMS, they did not exhibit a higher incidence of indicators of metabolic syndrome above the general population. We conclude that Rai1 haploinsufficiency represents a single-gene model of obesity with hyperphagia, abnormal fat distribution and altered hypothalamic gene expression associated with satiety, food intake, behavior and obesity. Linking RAI1 and BDNF provides a more thorough understanding of the role of Rai1 in growth and obesity and insight into the complex pathogenicity of obesity, behavior and sex-specific differences in adiposity.

Interaction effects between COMT and BDNF polymorphisms on boredom susceptibility of sensation seeking traits

Sensation seeking is a temperament associated with willingness to take risks to obtain arousal. We investigated the relationship between the polymorphisms of the COMT Val158Met and BDNF Val66Met and sensation seeking traits. The Sensation Seeking Scale (SSS) and genotyping were performed in 277 Korean healthy volunteers (165 males, 112 females). Multivariate analysis of covariance was used to test the association between the COMT and BDNF functional polymorphisms and dimensions of sensation seeking, namely, disinhibition, boredom susceptibility, experience seeking and thrill/adventure seeking. No main and interaction effects of the COMT and BDNF on SSS were observed for total subjects. However, in females, a significant gene-gene interaction effect on the boredom susceptibility of SSS was shown. Among females with COMT Met present genotype (Val/Met+Met/Met), subjects with BDNF Met absent genotype (Val/Val) had significantly higher boredom susceptibility than subjects with BDNF Met. The effects of BDNF Val66Met polymorphism on boredom susceptibility of sensation seeking traits could be modulated by COMT Val158Met polymorphism in female.

BDNF Val66Met polymorphism, energy intake and BMI: a follow-up study in schoolchildren at risk of eating disorders

BACKGROUND

Eating disorders (ED) have a multifactorial aetiology in which genetics play an important role. Several studies have found an association between the Val66Met (G196A) polymorphism of the Brain-Derived Neurotrophic Factor (BDNF) and Eating disorders.The aim of this study was to determine the association of the Val66Met (G196A) polymorphism of the BDNF gene and its effect on eating disorders (ED), energy intake and BMI in schoolchildren.

METHODS

Two-year cohort study (preadolescence to adolescence). From an initial sample of 1336 Caucasian children (mean age = 11.37 years), a group at risk of ED (n = 141) and a control group (n = 117) were selected using the Children's Eating Attitudes Test. Two years later, they were re-classified into an at-risk group (n = 41) and a control group (n = 159) using the Eating Attitudes Test. The diagnosis of the individuals at risk of ED was confirmed by means of the Diagnostic Interview for Children and Adolescents. BMI, energy intake and the Val66Met (G196A) polymorphism of the BDNF gene were analysed in the at-risk and control groups.

RESULTS

The frequency of genotypes of the Val66Met (G196A) polymorphism of the BDNF gene is 28.6% (95% CI: 22.4-34.9) in the heterozygous form (Val/Met) and 5% (95% CI: 2.4-9) in the homozygous form (Met/Met). We detected no association between Val66Met genotypes and the severity of ED. Over time, the carriers of the Met66 allele with a persistent risk of ED significantly restricted energy intake (507 Kcal/day; p = 0.033).

CONCLUSION

We have not found an association between Val66Met (G196A) polymorphism of the BDNF and ED in schoolchildren from general population. The relationship found between this polymorphism and energy intake restriction in adolescents with a persistent risk of ED should be replicated in a larger sample.

Altered striatal response to reward in bulimia nervosa after recovery

OBJECTIVE

It is possible that disturbances of systems modulating reward may contribute to a vulnerability to develop an eating disorder.

METHOD

This hypothesis was tested by assessing functional magnetic resonance brain imaging response to a monetary reward task known to activate the anterior ventral striatum (AVS), a region implicated in motivational aspects toward stimuli. To avoid the confounding effects of malnutrition, 10 women who had recovered from bulimia nervosa (BN) were compared with 10 healthy comparison women (CW).

RESULTS

For the AVS, CW distinguished positive and negative feedback, whereas recovered BN women had similar responses to both conditions. In addition, these groups had similar patterns of findings for the dorsal caudate.

DISCUSSION

We have previously shown that individuals recovered from anorexia nervosa (AN) also had altered striatal responses and difficulties in differentiating positive and negative feedback. Thus BN and AN individuals may share a difficulty in discriminating the emotional significance of a stimulus.

Dopaminergic dysregulation in mice selectively bred for excessive exercise or obesity

Dysregulation of the dopamine system is linked to various aberrant behaviors, including addiction, compulsive exercise, and hyperphagia leading to obesity. The goal of the present experiments was to determine how dopamine contributes to the expression of opposing phenotypes, excessive exercise and obesity. We hypothesized that similar alterations in dopamine and dopamine-related gene expression may underly obesity and excessive exercise, as competing traits for central reward pathways. Moreover, we hypothesized that selective breeding for high levels of exercise or obesity may have influenced genetic variation controlling these pathways, manifesting as opposing complex traits. Dopamine, dopamine-related peptide concentrations, and gene expression were evaluated in dorsal striatum (DS) and nucleus accumbens (NA) of mice from lines selectively bred for high rates of wheel running (HR) or obesity (M16), and the non-selected ICR strain from which these lines were derived. HPLC analysis showed significantly greater neurotransmitter concentrations in DS and NA of HR mice compared to M16 and ICR. Microarray analysis showed significant gene expression differences between HR and M16 compared to ICR in both brain areas, with changes revealed throughout the dopamine pathway including D1 and D2 receptors, associated G-proteins (e.g., Golf), and adenylate cyclase (e.g., Adcy5). The results suggest that similar modifications within the dopamine system may contribute to the expression of opposite phenotypes in mice, demonstrating that alterations within central reward pathways can contribute to both obesity and excessive exercise.

Reward circuitry responsivity to food predicts future increases in body mass: moderating effects of DRD2 and DRD4

OBJECTIVE

To determine whether responsivity of reward circuitry to food predicts future increases in body mass and whether polymorphisms in DRD2 and DRD4 moderate these relations.

DESIGN

The functional magnetic resonance imaging (fMRI) paradigm investigated blood oxygen level dependent activation in response to imagined intake of palatable foods, unpalatable foods, and glasses of water shown in pictures. DNA was extracted from saliva samples using standard salting-out and solvent precipitation methods.

PARTICIPANTS

Forty-four adolescent female high school students ranging from lean to obese.

MAIN OUTCOME

Future increases in body mass index (BMI).

RESULTS

Weaker activation of the frontal operculum, lateral orbitofrontal cortex, and striatum in response to imagined intake of palatable foods, versus imagined intake of unpalatable foods or water, predicted future increases in body mass for those with the DRD2 TaqIA A1 allele or the DRD4-7R allele. Data also suggest that for those lacking these alleles, greater responsivity of these food reward regions predicted future increases in body mass.

DISCUSSION

This novel prospective fMRI study indicates that responsivity of reward circuitry to food increases risk for future weight gain, but that genes that impact dopamine signaling capacity moderate the predictive effects, suggesting two qualitatively distinct pathways to unhealthy weight gain based on genetic risk.

Eating disorders: the current status of molecular genetic research

Anorexia nervosa (AN) and bulimia nervosa (BN) are complex disorders characterized by disordered eating behavior where the patient's attitude towards weight and shape, as well as their perception of body shape, are disturbed. Formal genetic studies on twins and families suggested a substantial genetic influence for AN and BN. Candidate gene studies have initially focused on the serotonergic and other central neurotransmitter systems and on genes involved in body weight regulation. Hardly any of the positive findings achieved in these studies were unequivocally confirmed or substantiated in meta-analyses. This might be due to too small sample sizes and thus low power and/or the genes underlying eating disorders have not yet been analyzed. However, some studies that also used subphenotypes (e.g., restricting type of AN) led to more specific results; however, confirmation is as yet mostly lacking. Systematic genome-wide linkage scans based on families with at least two individuals with an eating disorder (AN or BN) revealed initial linkage regions on chromosomes 1, 3 and 4 (AN) and 10p (BN). Analyses on candidate genes in the chromosome 1 linkage region led to the (as yet unconfirmed) identification of certain variants associated with AN. Genome-wide association studies are under way and will presumably help to identify genes and pathways involved in these eating disorders. The elucidation of the molecular mechanisms underlying eating disorders might improve therapeutic approaches.

Genetic findings in anorexia and bulimia nervosa

Anorexia nervosa (AN) and bulimia nervosa (BN) are complex disorders associated with disordered eating behavior. Heritability estimates derived from twin and family studies are high, so that substantial genetic influences on the etiology can be assumed for both. As the monoaminergic neurotransmitter systems are involved in eating disorders (EDs), candidate gene studies have centered on related genes; additionally, genes relevant for body weight regulation have been considered as candidates. Unfortunately, this approach has yielded very few positive results; confirmed associations or findings substantiated in meta-analyses are scant. None of these associations can be considered unequivocally validated. Systematic genome-wide approaches have been performed to identify genes with no a priori evidence for their relevance in EDs. Family-based scans revealed linkage peaks in single chromosomal regions for AN and BN. Analyses of candidate genes in one of these regions led to the identification of genetic variants associated with AN. Currently, an international consortium is conducting a genome-wide association study for AN, which will hopefully lead to the identification of the first genome-wide significant markers.

Genetic variation in the hypothalamic pathways and its role on obesity

Over recent decades, the prevalence of obesity has increased dramatically worldwide. Although this epidemic is mainly attributable to modern (western) lifestyle, multiple twin and adoption studies indicate the significant role of genes in the individual's predisposition to becoming obese. As the hypothalamus plays a central role in controlling body weight, its regulatory circuits may represent a crucial system in the pathogenesis of the disorder. Genetic variations in genes in the hypothalamic pathways may therefore contribute to the susceptibility for obesity in humans and animals. We summarize current knowledge on the physiological role of the hypothalamus in body-weight regulation and review genetic studies on the hypothalamic candidate genes in relation to obesity. Together, data from functional and genetic studies as well as the new, common, obesity loci identified in genome-wide association scans support an important role for the hypothalamic genes in predisposing to obesity. However, findings are still inconclusive for many candidate genes. To improve our understanding of the genetic architecture of common obesity, we suggest that specific obesity phenotypes should be considered and different analytical approaches used. Such studies should consider multiple genes from the same physiological pathways, together with environmental risk factors.

Relation of obesity to consummatory and anticipatory food reward

This report reviews findings from studies that have investigated whether abnormalities in reward from food intake and anticipated food intake increase risk for obesity. Self-report and behavioral data suggest that obese relative to lean individuals show elevated anticipatory and consummatory food reward. Brain imaging studies suggest that obese relative to lean individuals show greater activation of the gustatory cortex (insula/frontal operculum) and oral somatosensory regions (parietal operculum and Rolandic operculum) in response to anticipated intake and consumption of palatable foods. Yet, data also suggest that obese relative to lean individuals show less activation in the dorsal striatum in response to consumption of palatable foods and reduced striatal D2 dopamine receptor density. Emerging prospective data also suggest that abnormal activation in these brain regions increases risk for future weight gain and that genotypes associated with lowered dopamine signaling amplify these predictive effects. Results imply that individuals who show greater activation in the gustatory cortex and somatosensory regions in response to anticipation and consumption of food, but who show weaker activation in the striatum during food intake, may be at risk for overeating, particularly those at genetic risk for lowered dopamine receptor signaling.

The biology of binge eating

OBJECTIVE

To examine the literature on binge eating to gain a better understanding of its biological foundations and their role in eating disorders.

METHOD

Literature review and synthesis.

RESULTS

Research using animal models has revealed several factors that contribute to the development and maintenance of binge eating. These factors, including stress, food restriction, the presence of palatable foods, and environmental conditioning, parallel many of the precursory circumstances leading to binge eating in individuals with bulimia nervosa and binge eating disorder.

DISCUSSION

The animal literature has opened a new avenue to aid in the understanding of the neurobiological basis of binge eating. Future endeavors examining the genetic and environmental correlates of binge eating behavior will further contribute to the understanding of the biological foundations of binge eating and assist with establishing diagnostic criteria and the development of novel treatments for eating disorders marked by binge eating.

Genetic susceptibility to eating disorders: associated polymorphisms and pharmacogenetic suggestions

Anorexia nervosa (AN), bulimia nervosa (BN) and binge-eating disorder (BED) are characterized by abnormal eating behaviors often resulting in dramatic physical consequences for the patients. The etiology of eating disorders (EDs) is currently unknown; however, a strong genetic contribution is likely to be involved. To date, the majority of genetic studies have focused on candidate genes, and polymorphic variants of genes coding for substances likely to be involved in the etiopathogenesis of EDs have been assessed for association with AN, BN, BED and/or ED-related phenotypic traits. Results have been generally inconsistent and cannot be considered conclusive because of several methodological flaws and differences, such as small sample sizes, ethnic heterogeneity of studied populations, lack of statistical correction for multiple testing, adoption of different diagnostic criteria and population stratification. Although, at present, no convincing evidence for associations of candidate genes with EDs has been provided, the 5-HT2A receptor gene and the BDNF gene seem to be promising candidates for genetic influences on AN, since polymorphic variants of these genes have been found quite consistently, although not specifically, linked to AN restricting subtype in large sample studies. Moreover, pharmacogenetic investigations have suggested a possible role of some gene polymorphisms in predicting the response to treatment with selective serotonin reuptake inhibitors in BN, but results are still preliminary. The heterogeneity of ED phenotypes is believed to represent the most relevant variable responsible for contradictory and not conclusive results. Future studies should focus on more homogeneous subgroups, either relying on specific ED traits or identifying endophenotypes. This will be useful also for prevention and treatment of EDs.