The utility of the non-human primate; model for studying gene by environment interactions in behavioral research

The molecular genetic architecture of human personality: beyond self-report questionnaires

Molecular genetic studies of personality began with two high impact papers in 1996 that showed provisional associations between the dopamine DRD4 exon III repeat region and Novelty Seeking/Extraversion. These first two reports were shortly followed by an investigation linking Neuroticism/Harm Avoidance with the serotonin transporter (SLC6A4) promoter region polymorphism (5-HTTLPR). In the ensuing decade, thousands of subjects have been studied for association between these genes and personality, assessed by using self-report questionnaires, with erratic success in replication of the first findings for Novelty Seeking (DRD4) and Harm Avoidance (5-HTTLPR). Small effect sizes characteristic of non-Mendelian traits, polygenic patterns of inheritance and true heterogeneity between studies confound attempts to reach a consensus regarding the role of common polymorphisms in contributing to personality domains. Nevertheless, the current state of personality genetics is far from being bleak. Several new paradigms especially functional neuroimaging or 'imaging genomics' have strengthened the connection between 5-HTTLPR and anxiety-related personality traits. The demonstrations that early environmental information can considerably strengthen and even uncover associations between genes and behavior (Caspi's seminal studies and more recently the demonstration that early environment impacts on DRD4 and Novelty Seeking) are notable and herald a new era of personality genetics. Finally, consideration of the broader phenotypic expression of common polymorphisms (e.g. the 'social brain', altruism, etc.) and the use of new experimental paradigms including neurophysiological, neuropsychological and computer games that go beyond the narrow self-report questionnaire design will enable a deeper understanding of how common genetic polymorphisms modulate human behavior. Human personality, defined by Webster as the quality or state of being a person or the complex of characteristics that distinguishes an individual, surely requires a more encompassing view towards understanding its complex molecular genetic architecture.

Stressful life events, 5-HTT genotype and risk of depression

Studies of how genetic and environmental exposures interact may be essential for understanding the aetiology of complex psychiatric disorders. In this issue of the Journal an Australian study reports evidence of such an interaction on risk of depression. We discuss findings in this field in the context of the limitations inherent in studies of gene-environment interactions.

Biological Aspects of Social Bonding and the Roots of Human Violence

The brain systems that motivate humans to form emotional bonds with others probably first evolved to mobilize the high-quality maternal care necessary for reproductive success in placental mammals. In these species, the helplessness of infants at birth and their dependence upon nutrition secreted from their mothers' bodies (milk) and parental body heat to stay warm required the evolution of a new motivational system in the brain to stimulate avid and sustained mothering behavior. Other types of social bonds that emerged subsequently in placental mammals, in particular monogamous bonds between breeding pairs, appear to have evolved from motivational brain systems that stimulate maternal behavior. This chapter focuses on aspects of the evolution and neurobiology of maternal and pair bonding and associated behavioral changes that may provide insights into the origins of human violence. The roles of the neuropeptides oxytocin and vasopressin as well as the neurotransmitter dopamine will be emphasized. Maternal and pair bonding are accompanied by increased aggressiveness toward perceived threats to the object of attachment as well as diminished fear and anxiety in stressful situations. The sustained closeness with mother required for the survival of infant mammals opened a new evolutionary niche in which aspects of the mother's care became increasingly important in regulating development in offspring. The quantity and quality of maternal care received during infancy determines adult social competence, ability to cope with stress, aggressiveness, and even preference for addictive substances. Indeed, the development of neurochemical systems within the brain that regulate mothering, aggression, and other types of social behavior, such as the oxytocin and vasopressin systems, are strongly affected by parental nurturing received during infancy. Evidence will be reviewed that the neural circuitry and neurochemistry implicated in studies of lower mammals also facilitate primate/human interpersonal bonding. It is hypothesized that neural bonding systems may also be important for the development in individuals of loyalty to the social group and its culture. Neglect and abuse during early life may cause bonding systems to develop abnormally and compromise capacity for rewarding interpersonal relationships and commitment to societal and cultural values later in life. Other means of stimulating reward pathways in the brain, such as drugs, sex, aggression, and intimidating others, could become relatively more attractive and less constrained by concern about violating trusting relationships. The ability to modify behavior based on negative experiences may be impaired. Unmet needs for social bonding and acceptance early in life might increase the emotional allure of groups (gangs, sects) with violent and authoritarian values and leadership. Social neurobiology has the potential to provide new strategies for treating and preventing violence and associated social dysfunction.

Genetics of Affective and Anxiety Disorders

The study of the genetics of complex behaviors has evolved dramatically from the days of the nature versus nurture debates that dominated much of the past century. Here we discuss advances in our understanding of the genetics of affective and anxiety disorders. In particular, we highlight our growing understanding of specific gene-environment interactions that occur during critical periods in development, setting the stage for later behavioral phenotypes. We review the recent literature in the field, focusing on recent advances in our understanding of the role of the serotonin system in establishing normal anxiety levels during development. We emphasize the importance of understanding the effect of genetic variation at the level of functional circuits and provide examples from the literature of how such an approach has been exploited to study novel genetic endpoints, including genetically based variation in response to medication, a potentially valuable phenotype that has not received much attention to date.

Inactivation of 5HT transport in mice: modeling altered 5HT homeostasis implicated in emotional dysfunction, affective disorders, and somatic syndromes

Animal models have not only become an essential tool for investigating the neurobiological function of genes that are involved in the etiopathogenesis of human behavioral and psychiatric disorders but are also fundamental in the development novel therapeutic strategies. As an example, inactivation of the serotonin (5HT) transporter (5Htt, Slc6a4) gene in mice expanded our view of adaptive 5HT uptake regulation and maintenance of 5HT homeostasis in the developing human brain and molecular processes underlying anxiety-related traits, as well as affective spectrum disorders including depression. 5Htt-deficient mice have been employed as a model complementary to direct studies of genetically complex traits and disorders, with important findings in biochemical, morphological, behavioral, and pharmacological areas. Based on growing evidence for a critical role of the 5HTT in the integration of synaptic connections in the rodent, nonhuman primate, and human brain during critical periods of development and adult life, more in-depth knowledge of the molecular mechanisms implicated in these fine-tuning processes is currently evolving. Moreover, demonstration of a joint influence of the 5HTT variation and environmental sources during early brain development advanced our understanding of the mechanism of genexgene and genexenvironment interactions in the developmental neurobiology of anxiety and depression. Lastly, imaging techniques, which become increasingly elaborate in displaying the genomic influence on brain system activation in response to environmental cues, have provided the means to bridge the gap between small effects of 5HTT variation and complex behavior, as well as psychopathological dimensions. The combination of elaborate genetic, epigenetic, imaging, and behavioral analyses will continue to generate new insight into 5HTT's role as a master control gene of emotion regulation.

Differential Functional Variability of Serotonin Transporter and Monoamine Oxidase A Genes in Macaque Species Displaying Contrasting Levels of Aggression-Related Behavior

Functional allelic variation in the transcriptional control region of the serotonin transporter and monoamine oxidase A genes has been associated with anxiety- and aggression-related behavior in humans and, more recently, in nonhuman primates. Here, we have genotyped these polymorphic regions in seven species of the genus Macaca. Macaques exhibit exceptional inter-species variation in aggression-related social behavior as illustrated by recent studies showing overlapping patterns of aggression-based social organization grades and macaque phylogeny. We cloned and sequenced two new alleles of the serotonin transporter gene-linked polymorphic region in Barbary and Tibetan macaques. In addition, we observed that species displaying tolerant societies, with relaxed dominance and high levels of conciliatory tendency, were monomorphic for both the serotonin transporter gene and, with the exception of Tonkean macaques, the monoamine oxidase A gene. In contrast, those species known to exhibit intolerant, hierarchical and nepotistic societies were polymorphic at one or more of these loci. Rhesus (M. mulatta), the most intolerant and hierarchical species of macaques, showed the greatest degree of allelic variation in both genes. Additional investigation of a polymorphic repeat in exon III of the dopamine receptor D4 as well as a repeat/single nucleotide polymorphism in the 3' untranslated region of the dopamine transporter which have both been implicated in the modulation of complex behavior failed to reveal a relationship between allelic variability and social organization grade. Taken together, these findings suggest that genetic variation of serotonergic neurotransmission may play an important role in determining inter-species differences in aggression related behavior in macaques.

Leveraging human genomic information to identify nonhuman primate sequences for expression array development

Background

Nonhuman primates (NHPs) are essential for biomedical research due to their similarities to humans. The utility of NHPs will be greatly increased by the application of genomics-based approaches such as gene expression profiling. Sequence information from the 3' end of genes is the key resource needed to create oligonucleotide expression arrays.

Results

We have developed the algorithms and procedures necessary to quickly acquire sequence information from the 3' end of nonhuman primate orthologs of human genes. To accomplish this, we identified terminal exons of over 15,000 human genes by aligning mRNA sequences with genomic sequence. We found the mean length of complete last exons to be approximately 1,400 bp, significantly longer than previous estimates. We designed primers to amplify genomic DNA, which included at least 300 bp of the terminal exon. We cloned and sequenced the PCR products representing over 5,500 Macaca mulatta (rhesus monkey) orthologs of human genes. This sequence information has been used to select probes for rhesus gene expression profiling. We have also tested 10 sets of primers with genomic DNA from Macaca fascicularis (Cynomolgus monkey), Papio hamadryas (Baboon), and Chlorocebus aethiops (African green monkey, vervet). The results indicate that the primers developed for this study will be useful for acquiring sequence from the 3' end of genes for other nonhuman primate species.

Conclusion

This study demonstrates that human genomic DNA sequence can be leveraged to obtain sequence from the 3' end of NHP orthologs and that this sequence can then be used to generate NHP oligonucleotide microarrays. Affymetrix and Agilent used sequences obtained with this approach in the design of their rhesus macaque oligonucleotide microarrays.

Serotonin and aggressive behavior in rodents and nonhuman primates: predispositions and plasticity

This review analyzes psychosocial and genetic determinants of aggressive behavior in rodents and nonhuman primates and the role of the serotonin (5-HT) system on aggressive behaviors in order to trace possible evolutionary common origins between psychopathological and adaptive forms of aggression. Studies in primates suggest that deficit in serotonin activity, as indicated by the levels of the cerebrospinal fluid (CSF) serotonin major metabolite 5-hydroxyindoleacetic acid (5-HIAA) correlates with impulsive and aggressive behavior. It is possible that CSF 5-HIAA reflects the prevailing serotonergic tone and may be related to an aggressive trait. Superimposed on this tone are phasic serotonin changes that may be related to the inhibition of aggressive acts. Genetic factors determine aggressive behaviors as demonstrated by classic selection and strain comparison studies. Manipulations of genes targeting 5-HT receptors, transporters and enzymes can influence aggression. Some of these genes related to the serotonin transporter (5-HTT) and the monoamine oxidase A (MAO-A) show a polymorphism that may predispose, under specific environmental conditions, certain individuals to display pathological forms of aggression.

Alcohol dependence and gene x environment interaction in emotion regulation: Is serotonin the link?

Alcohol dependence is characterized by frequent, compulsive and uncontrolled consumption of alcohol associated with behavior of maladaption and destruction. It is an etiologically and clinically heterogeneous syndrome, moderately to highly heritable, and caused by interaction of genes and environment. Alcohol dependence is related to other psychiatric diseases by common neurobiological pathways, including those that modulate reward, behavioral control as well as anxiety and stress response. Alcohol induces adaptive changes in brain function providing the basis for tolerance, craving, withdrawal, and emotional disturbance. The differentiation of psychobiological traits of addictive behavior reflecting neurobiological processes is therefore of particular importance for the dissection of the complex genetic susceptibility to alcohol dependence. A central serotonin (5-HT) deficit is thought to be involved in the pathogenesis of alcohol dependence by modulating motivational behavior, neuroadaptive processes, and resulting emotional disturbance. 5-HT-related impulsive, aggressive, and suicidal behavior has been linked to a primordial personality that is susceptible to alcohol dependence. Although variations in many of the genes that encode receptors, enzymes, and transporters of the 5-HT system have been tested as risk factors for alcohol dependence, genetic analyses of 5-HT signaling in alcohol dependence have mainly been focused on the 5-HT transporter (5-HTT) gene. Due to its central role in the fine-tuning serotonergic neurotransmission, a regulatory variant of the 5-HTT, which is associated with anxiety related traits, is not only a key player in the neurobiological mechanism of gene x environment interaction in the etiology of depression, but also contributes to the risk to develop alcohol dependence with antisocial behavior and suicidality. Evidence for a modulatory effect of allelic variation of 5-HTT function on limbic circuit responses to emotional stimuli suggests that genotype-endophenotype correlations may be accessible to molecular functional imaging of the brain. These new developments have broad implications for our understanding how genetic vulnerability to alcohol dependence is manifested in the brain's response to emotional stimuli.

Maternal care and development of stress responses in baboons

The ability to mount a successful response to threats is critical for an organism's survival. A key element of the stress response is its nonspecificity toward the stress source, with similar endocrine and behavioral changes expected under a variety of stressors. In this project we utilized an experimental design that accounts for multiple sources of variation to further understand the nature of stress responsivity and its relationship to the early rearing environment. A sample of baboons (n=73) was observed during the early phase of life in their social group, and then tested as juveniles in a challenging situation. Maternal cortisol levels were measured during the peripartum period. The challenging situation (individuals were isolated for a few minutes in a single cage) was designed to be a moderate source of psychological stress. Patterns in individual differences during the stress test were "mapped" by means of multidimensional scaling (MDS). After the observation was made, the subject was sedated and a blood sample was taken to measure cortisol levels. Our results indicate that when juvenile baboons are confronted with a source of psychological stress, they show a multidimensional behavioral response, probably mediated by the activation and synergic interaction among different neurohormonal systems that, ultimately, act on the hypothalamus-pituitary-adrenal (HPA) axis. Different components of the multidimensional, or nonspecific, behavioral response are associated with the quality and quantity of interactions with their mothers during early life. Juveniles whose mothers displayed higher levels of positive interaction were characterized by vigilant but less active reactions to the stress test, whereas juveniles of mothers that displayed high levels of stress-related behaviors had higher cortisol and locomotion levels.

Association between 5-HTTLPR genotypes and persisting patterns of anxiety and alcohol use: results from a 10-year longitudinal study of adolescent mental health

The serotonin transporter gene (5-HTT) encodes a transmembrane protein that plays an important role in regulating serotonergic neurotransmission and related aspects of mood and behaviour. The short allele of a 44 bp insertion/deletion polymorphism (S-allele) within the promoter region of the 5-HTT gene (5-HTTLPR) confers lower transcriptional activity relative to the long allele (L-allele) and may act to modify the risk of serotonin-mediated outcomes such as anxiety and substance use behaviours. The purpose of this study was to determine whether (or not) 5-HTTLPR genotypes moderate known associations between attachment style and adolescent anxiety and alcohol use outcomes. Participants were drawn from an eight-wave study of the mental and behavioural health of a cohort of young Australians followed from 14 to 24 years of age (Victorian Adolescent Health Cohort Study, 1992 - present). No association was observed within low-risk attachment settings. However, within risk settings for heightened anxiety (ie, insecurely attached young people), the odds of persisting ruminative anxiety (worry) decreased with each additional copy of the S-allele (approximately 30% per allele: OR 0.77, 95% CI 0.62-0.97, P=0.029). Within risk settings for binge drinking (ie, securely attached young people), the odds of reporting persisting high-dose alcohol consumption (bingeing) decreased with each additional copy of the S-allele (approximately 35% per allele: OR 0.74, 95% CI 0.64-0.86, P<0.001). Our data suggest that the S-allele is likely to be important in psychosocial development, particularly in those settings that increase risk of anxiety and alcohol use problems.

Role of the serotonin transporter gene and family function in adolescent alcohol consumption

BACKGROUND

That the extent to which a particular individual will engage in problematic behaviors such as delinquency, violence, or drug abuse is determined by the way psychosocial, situational, and hereditary factors interact is widely accepted. However, only recently have researchers begun to investigate the interactions between specific genotypes and psychosocial factors in relation to behavior. The purpose of the present study was to investigate possible interactions between a polymorphism in the promoter region of the serotonin transporter (5-HTT) gene and family relations on adolescent alcohol consumption.

METHODS

A cross-sectional study with a randomized sample from a total population of 16- and 19-year-old adolescents from a Swedish county was conducted. Eighty-one male and 119 female adolescents, who volunteered to participate after having answered a questionnaire, were randomly selected from quartiles of volunteers representing various degrees of psychosocial risk behavior.

RESULTS

5-HTT genotype (p=0.029) and family relations (p=0.022) predicted alcohol consumption independently as well as through an interaction with one another (p=0.05). The model explained 11% of the variance in alcohol consumption. In a binary logistic model, we found that adolescents with the LS variant of the 5-HTT gene and with family relations being "neutral" or "bad" had a 12- to 14-fold increased risk for high intoxication frequency.

CONCLUSIONS

In sum, our results show that a functional polymorphism of the 5-HTT genotype, family relations, and interactions between these variables predict adolescent alcohol consumption in a randomized sample of adolescents.

Biological sensitivity to context: II. Empirical explorations of an evolutionary–developmental theory

In two studies comprising 249 children and their families, the authors utilized secondary, exploratory data analyses to examine Boyce and Ellis' (this issue) evolutionary-developmental theory of biological sensitivity to context. The theory proposes that individual differences in stress reactivity constitute variation in susceptibility to environmental influence, both positive and negative, and that early childhood exposures to either highly protective or acutely stressful environments result in heightened reactivity. In Study 1, 127 3- to 5-year old children were concurrently assessed on levels of support/adversity in home and preschool environments and on cardiovascular reactivity to laboratory challenges. In Study 2, 122 children were prospectively assessed on familial stress in both infancy and preschool and on autonomic and adrenocortical reactivity to laboratory challenges at age 7. In both studies, a disproportionate number of children in supportive, low stress environments displayed high autonomic reactivity. Conversely, in Study 2, a relatively high proportion of children in very stressful environments showed evidence of heightened sympathetic and adrenocortical reactivity. Consistent with the evolutionary-developmental theory, the exploratory analyses also generated the testable hypothesis that relations between levels of childhood support/adversity and the magnitude of stress reactivity are curvilinear, with children from moderately stressful environments displaying the lowest reactivity levels in both studies.

Male Reproductive Timing in Rhesus Macaques Is Influenced by the 5HTTLPR Promoter Polymorphism of the Serotonin Transporter Gene1

The 5HTTLPR polymorphism in the promoter region of the human serotonin transporter (SLC6A4) gene is known to be associated with various stress-related psychological and psychiatric phenomena. We observed that a similar diallelic polymorphism in the orthologous gene of rhesus macaques (Macaca mulatta) was related to the reproductive life history of 580 males residing in the free-ranging colony of Cayo Santiago, Puerto Rico, between 1985 and 1998. At first glance, the polymorphism appeared to be selectively neutral because no difference in total reproductive output was noted between males of different 5HTTLPR genotypes. However, whereas heterozygotes were significantly more reproductive than homozygotes at intermediate age (10-13 yr), the opposite held true before and after this period (n = 682 offspring; randomization P = 0.014). This association, which explains approximately 7% of the observed variation in sire age, most likely reflects different natal dispersal patterns and represents the first reported instance of a genetic influence on reproductive timing in mammals.

Monoamine oxidase A gene promoter variation and rearing experience influences aggressive behavior in rhesus monkeys

BACKGROUND

Allelic variation of the monoamine oxidase A (MAOA) gene has been implicated in conduct disorder and antisocial, aggressive behavior in humans when associated with early adverse experiences. We tested the hypothesis that a repeat polymorphism in the rhesus macaque MAOA gene promoter region influences aggressive behavior in male subjects.

METHODS

Forty-five unrelated male monkeys raised with or without their mothers were tested for competitive and social group aggression. Functional activity of the MAOA gene promoter polymorphism was determined and genotypes scored for assessing genetic and environmental influences on aggression.

RESULTS

Transcription of the MAOA gene in rhesus monkeys is modulated by an orthologous polymorphism (rhMAOA-LPR) in its upstream regulatory region. High- and low-activity alleles of the rhMAOA-LPR show a genotype x environment interaction effect on aggressive behavior, such that mother-reared male monkeys with the low-activity-associated allele had higher aggression scores.

CONCLUSIONS

These results suggest that the behavioral expression of allelic variation in MAOA activity is sensitive to social experiences early in development and that its functional outcome might depend on social context.

Genetic alterations of the murine serotonergic gene pathway: the neurodevelopmental basis of anxiety

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

Social supports and serotonin transporter gene moderate depression in maltreated children

In this study, measures of the quality and availability of social supports were found to moderate risk for depression associated with a history of maltreatment and the presence of the short (s) allele of the serotonin transporter gene promoter polymorphism (5-HTTLPR). The present investigation (i) replicates research in adults showing that 5-HTTLPR variation moderates the development of depression after stress, (ii) extends the finding to children, and (iii) demonstrates the ability of social supports to further moderate risk for depression. Maltreated children with the s/s genotype and no positive supports had the highest depression ratings, scores that were twice as high as the non-maltreated comparison children with the same genotype. However, the presence of positive supports reduced risk associated with maltreatment and the s/s genotype, such that maltreated children with this profile had only minimal increases in their depression scores. These findings are consistent with emerging preclinical and clinical data suggesting that the negative sequelae associated with early stress are not inevitable. Risk for negative outcomes may be modified by both genetic and environmental factors, with the quality and availability of social supports among the most important environmental factors in promoting resiliency in maltreated children, even in the presence of a genotype expected to confer vulnerability for psychiatric disorder.

Trauma, Genes, and the Neurobiology of Personality Disorders

A model for personality dysfunction posits an interaction between inherited susceptibility and environmental factors such as childhood trauma. Core biological vulnerabilities in personality include dimensions of affective instability, impulsive aggression, and cognition/perceptual domains. For the dimension of impulsive aggression, often seen in borderline personality disorder (BPD), the underlying neurobiology involves deficits in central serotonin function and alterations in specific brain regions in the cingulate and the medial and orbital prefrontal cortex. The role of trauma in the development of personality disorder and especially for BPD remains unclear. Although recent studies suggest that BPD is not a trauma-spectrum disorder and that it is biologically distinct from posttraumatic stress disorder, high rates of childhood abuse and neglect do exist for individuals with personality dysfunction. Personality symptom clusters seem to be unrelated to specific abuses, but they may relate to more enduring aspects of interpersonal and family environments in childhood. Whereas twin and family studies indicate a partially heritable basis for impulsive aggression, studies of serotonin-related genes to date suggest only modest contributions to behavior. Gene-environment interactions involving childhood maltreatment are demonstrated in recent studies on antisocial behaviors and aggressive rhesus monkeys and highlight the need for further research in this important area.

The use of adolescent nonhuman primates to model human alcohol intake: neurobiological, genetic, and psychological variables

Traits characteristic of type I and type II alcoholism are thought to relate to dysregulated central nervous system serotonin functioning. In this review, we discuss variables associated with high adolescent alcohol consumption and other risk-taking behaviors in a nonhuman primate model. Adolescent primates with low CSF concentrations of the serotonin metabolite 5-HIAA are more impulsive and exhibit increased levels of alcohol consumption. Both genetic and environmental factors contribute to alcohol-seeking behavior in adolescent macaques. Sequence variation within serotonin system genes, for example, a repeat polymorphism in the transcriptional control region of the monoamine oxidase gene (MAOA-LPR), increases the propensity for adolescent males to consume alcohol. Environmental factors, such as early life stress in the form of peer-rearing or early age of exposure to alcohol, are also associated with increased alcohol consumption. Peer-reared females, especially those exposed to alcohol during early adolescence, exhibit increased rates of alcohol consumption compared to those exposed to alcohol later in development. When genetic variables are also considered, there is an interaction between the low activity serotonin transporter gene promoter s allele (rh5-HTTLPR) and rearing condition on alcohol preference in females but not males, suggesting that the interactions between genes and the environment may be sexually dichotomous. By learning more about the interactions between genes, early experience, and alcohol intake in the adolescent nonhuman primate, we may be able to identify factors that contribute to the susceptibility, pathogenesis, and progression of impulse control disorders, such as alcoholism.

Sexual dichotomy of an interaction between early adversity and the serotonin transporter gene promoter variant in rhesus macaques

A polymorphism in the human serotonin transporter gene promoter (5-HTTLPR) is associated with anxiety and increased risk for developing depression in the face of adversity. Here, we report that among infant rhesus macaques, an orthologous polymorphism (rh5-HTTLPR) interacts with adversity in the form of peer rearing to influence adrenocorticotropic hormone (ACTH) response to stress and, further, that this interaction is sexually dichotomous. ACTH responses to separation are higher in l/s than in l/l males. In females, however, it is only among those with a history of adversity that the s allele is associated with increased ACTH responses to stress. Of interest, peer-reared animals, in particular females carrying the s allele, also exhibit lower cortisol responses to stress, a pattern that has been recognized in association with certain stress-related neuropsychiatric disorders. By extension, our findings suggest the intriguing possibility that human females carrying the 5-HTTLPR s allele could be more vulnerable to the effects of early adversity. This interactive effect may underlie the increased incidence of certain stress-related disorders in women.

Creation of non-human primate neurogenetic disease models by gene targeting and nuclear transfer

Genetically modified rhesus macaques are necessary because mouse models are not suitable for a number of important neurogenetic disorders; for example, Kallmann's syndrome, Lesch-Nyhan's disease and Ataxia-Telangiectasia. Mouse models may not be suitable because there may be no mouse ortholog of the human gene of interest, as is the case for Kallmann's syndrome, or because mutant mice do not exhibit the same phenotype observed in humans, as is the the case for Lesch-Nyhan's disease and Ataxia-Telangiectasia. Non-human primate models of neurogenetic diseases are expected to more closely resemble human diseases than existing mouse models. Genetically modified rhesus macaques can be created by modifying the genome of a somatic cell and then transferring the nucleus from this cell to an enucleated oocyte. Random integration of a transgene is sufficient to create models of gain-of-function genetic diseases. Stable expression of green fluorescent protein has been achieved in rhesus macaque fibroblasts. However, gene targeting is necessary to create models of loss-of-function genetic diseases. Several technical challenges must be overcome before null mutant non-human primates can be produced. In our experience, fetal fibroblasts frequently become senescent before selection procedures can be completed. We have overcome this problem by transfecting somatic cells with human telomerase reverse transcriptase. This enzyme extends the telomeres, and lifespan, of somatic cells. Long and accurate polymerase chain reaction can be used to obtain sufficient regions of homology of isogenic rhesus genomic DNA for targeting constructs. This should improve gene targeting efficiency. Gene targeting experiments are currently underway. Null mutant rhesus macaques will likely result in breakthrough advances in the understanding of neurogenetic disease and prove invaluable for preclinical trials of new therapies.

Rearing condition and rh5-HTTLPR interact to influence limbic-hypothalamic-pituitary-adrenal axis response to stress in infant macaques

BACKGROUND

In humans and macaques, a promoter polymorphism that decreases transcription of the serotonin transporter gene is associated with anxiety. Serotonin transporter gene disruption in rodents produces anxious animals with exaggerated limbic-hypothalamic-pituitary-adrenal (LHPA) responses to stress. We wanted to determine whether serotonin transporter gene promoter variation (rh-5HTTLPR) and rearing condition would interact to influence endocrine responses to stress in infant rhesus macaques.

METHODS

Animals were reared with their mothers (MR, n = 141) or in peer-only groups (PR, n = 67). At 6 months of age, adrenocorticotropic hormone (ACTH) and cortisol levels were determined at baseline and during separation stress. Serotonin transporter genotype (l/l and l/s) was determined with polymerase chain reaction followed by gel electrophoresis.

RESULTS

Cortisol levels increased during separation, and there was a main effect of rearing condition, with decreased cortisol levels among PR macaques. Animals with l/s rh5-HTTLPR genotypes had higher ACTH levels than did l/l animals. Adrenocorticotropic hormone levels increased during separation, and there was a separation x rearing x rh5-HTTLPR interaction, such that PR-l/s animals had higher ACTH levels during separation than did other animals studied.

CONCLUSIONS

These data demonstrate that serotonin transporter gene variation affects LHPA axis activity and that the influence of rh5-HTTLPR on hormonal responses during stress is modulated by early experience.

Abnormal anxiety-related behavior in serotonin transporter null mutant mice: the influence of genetic background

Serotonin transporter (5-HTT) null mutant mice provide a model system to study the role genetic variation in the 5-HTT plays in the regulation of emotion. Anxiety-like behaviors were assessed in 5-HTT null mutants with the mutation placed on either a B6 congenic or a 129S6 congenic background. Replicating previous findings, B6 congenic 5-HTT null mutants exhibited increased anxiety-like behavior and reduced exploratory locomotion on the light <--> dark exploration and elevated plus-maze tests. In contrast, 129S6 congenic 5-HTT null mutant mice showed no phenotypic abnormalities on either test. 5-HTT null mutants on the 129S6 background showed reduced 5-HT(1A) receptor binding (as measured by quantitative autoradiography) and reduced 5-HT(1A) receptor function (as measured by 8-OH-DPAT-induced hypothermia). These data confirm that the 5-HTT null mutation produced alterations in brain 5-HT function in mice on the 129S6 background, thereby discounting the possibility that the absence of an abnormal anxiety-like phenotype in these mice was due to a suppression of the mutation by 129 modifier genes. Anxiety-like behaviors in the light <--> dark exploration and elevated plus-maze tests were significantly higher in 129S6 congenic +/+ mice as compared to B6 congenic +/+ mice. This suggests that high baseline anxiety-like behavior in the 129S6 strain might have precluded detection of the anxiety-like effects of the 5-HTT null mutation on this background. Present findings provide further evidence linking genetic variation in the 5-HTT to abnormalities in mood and anxiety. Furthermore, these data highlight the utility of conducting behavioral phenotyping of mutant mice on multiple genetic backgrounds.