Genetic and environmental influences on emotion-modulated startle reflex: a twin study

Genetics of human startle reactivity: A systematic review to acquire targets for an anxiety endophenotype

OBJECTIVES

Startle response is an objective physiological measure integral to the human defense system and a promising target for endophenotype investigations of anxiety. Given the alterations in startle reactivity observed among anxiety and related disorders, we searched for genetic variants associated with startle reactivity as they may be further involved in pathological anxiety risk.

METHODS

A systematic literature review was performed to identify genetic variants associated with startle reactivity in humans, specifically baseline and fear- or anxiety-potentiated startle.

RESULTS

The polymorphisms Val66Met (rs6265) from brain-derived neurotrophic factor (BDNF), Val158Met (rs4680) from catechol-O-methyltransferase (COMT), and the serotonin transporter-linked polymorphic region (5-HTTLPR) from the serotonin transporter gene (SLC6A4) were most commonly studied in human startle. In addition, several other genetic variants have also been identified as potential candidates that warrant further research, especially given their novelty in in the context of anxiety.

CONCLUSIONS

Similar to psychiatric genetic studies, the studies on startle reactivity primarily focus on candidate genes and are plagued by non-replication. Startle reactivity is a promising endophenotype that requires concerted efforts to collect uniformly assessed, large, well-powered samples and hypothesis-free genome-wide strategies. To further support startle as an endophenotype for anxiety, this review suggests advanced genetic strategies for startle research.

The effect of polymorphisms in startle-related genes on anxiety symptom severity

Given the limited effectiveness of treatments for pathological anxiety, there is a pressing need to identify genetic markers that can aid the precise selection of treatments and optimize treatment response. Anxiety and startle response levels demonstrate a direct relationship, and previous literature suggests that exaggerated startle reactivity may serve as an endophenotype of pathological anxiety. In addition, genetic variants related to startle reactivity may play a role in the etiology of pathological anxiety. In the current study, we selected 22 single nucleotide polymorphisms (SNPs) related to startle reactivity in the literature, and examined their association with anxiety symptom severity across psychiatric disorders (n = 508), and in a subset of patients with an anxiety disorder (n = 298). Overall, none of the SNPs pass correction for multiple independent tests. However, across psychiatric patients, rs6323 from the monoamine oxidase A (MAOA) gene and rs324981 from the neuropeptide S receptor 1 (NPSR1) gene were nominally associated with baseline anxiety symptom severity (p = 0.017, 0.023). These preliminary findings provide support for investigating startle-related genetic variants to identify biomarkers of anxiety symptom severity.

Fear-potentiated startle response as an endophenotype: Evaluating metrics and methods for genetic applications

The modulation of the startle response (SR) by threatening stimuli (fear-potentiated startle; FPS) is a proposed endophenotype for disorders of the fearful-fearlessness spectrum. FPS has failed to show evidence of heritability, raising concerns. However, metrics used to index FPS-and, importantly, other conditional phenotypes that are dependent on a baseline-may not be suitable for the approaches used in genetic epidemiology studies. Here, we evaluated multiple metrics of FPS in a population-based sample of preadolescent twins (N = 569 from 320 twin pairs, M_age = 11.4) who completed a fear-conditioning paradigm with airpuff-elicited SR on two occasions (~1 month apart). We applied univariate and multivariate biometric modeling to estimate the heritability of FPS using several proposed standardization procedures. This was extended with data simulations to evaluate biases in heritability estimates of FPS (and similar metrics) under various scenarios. Consistent with previous studies, results indicated moderate test-retest reliability (r = 0.59) and heritability of the overall SR (h² = 34%) but poor reliability and virtually no unique genetic influences on FPS when considering a raw or standardized differential score that removes baseline SR. Simulations demonstrated that the use of differential scores introduces bias in heritability estimates relative to jointly analyzing baseline SR and FPS in a multivariate model. However, strong dependency of FPS on baseline levels makes unique genetic influences virtually impossible to detect regardless of methodology. These findings indicate that FPS and other conditional phenotypes may not be well suited to serve as endophenotypes unless such codependency can be disentangled.

GLRB allelic variation associated with agoraphobic cognitions, increased startle response and fear network activation: a potential neurogenetic pathway to panic disorder

The molecular genetics of panic disorder (PD) with and without agoraphobia (AG) are still largely unknown and progress is hampered by small sample sizes. We therefore performed a genome-wide association study with a dimensional, PD/AG-related anxiety phenotype based on the Agoraphobia Cognition Questionnaire (ACQ) in a sample of 1370 healthy German volunteers of the CRC TRR58 MEGA study wave 1. A genome-wide significant association was found between ACQ and single non-coding nucleotide variants of the GLRB gene (rs78726293, P=3.3 × 10^-8; rs191260602, P=3.9 × 10^-8). We followed up on this finding in a larger dimensional ACQ sample (N=2547) and in independent samples with a dichotomous AG phenotype based on the Symptoms Checklist (SCL-90; N=3845) and a case-control sample with the categorical phenotype PD/AG (N_combined =1012) obtaining highly significant P-values also for GLRB single-nucleotide variants rs17035816 (P=3.8 × 10^-4) and rs7688285 (P=7.6 × 10^-5). GLRB gene expression was found to be modulated by rs7688285 in brain tissue, as well as cell culture. Analyses of intermediate PD/AG phenotypes demonstrated increased startle reflex and increased fear network, as well as general sensory activation by GLRB risk gene variants rs78726293, rs191260602, rs17035816 and rs7688285. Partial Glrb knockout mice demonstrated an agoraphobic phenotype. In conjunction with the clinical observation that rare coding GLRB gene mutations are associated with the neurological disorder hyperekplexia characterized by a generalized startle reaction and agoraphobic behavior, our data provide evidence that non-coding, although functional GLRB gene polymorphisms may predispose to PD by increasing startle response and agoraphobic cognitions.

Modulation of defensive reactivity by GLRB allelic variation: converging evidence from an intermediate phenotype approach

Representing a phylogenetically old and very basic mechanism of inhibitory neurotransmission, glycine receptors have been implicated in the modulation of behavioral components underlying defensive responding toward threat. As one of the first findings being confirmed by genome-wide association studies for the phenotype of panic disorder and agoraphobia, allelic variation in a gene coding for the glycine receptor beta subunit (GLRB) has recently been associated with increased neural fear network activation and enhanced acoustic startle reflexes. On the basis of two independent healthy control samples, we here aimed to further explore the functional significance of the GLRB genotype (rs7688285) by employing an intermediate phenotype approach. We focused on the phenotype of defensive system reactivity across the levels of brain function, structure, and physiology. Converging evidence across both samples was found for increased neurofunctional activation in the (anterior) insular cortex in GLRB risk allele carriers and altered fear conditioning as a function of genotype. The robustness of GLRB effects is demonstrated by consistent findings across different experimental fear conditioning paradigms and recording sites. Altogether, findings provide translational evidence for glycine neurotransmission as a modulator of the brain's evolutionary old dynamic defensive system and provide further support for a strong, biologically plausible candidate intermediate phenotype of defensive reactivity. As such, glycine-dependent neurotransmission may open up new avenues for mechanistic research on the etiopathogenesis of fear and anxiety disorders.

The genetics of anxiety-related negative valence system traits

NIMH's Research Domain Criteria (RDoC) domain of negative valence systems (NVS) captures constructs of negative affect such as fear and distress traditionally subsumed under the various internalizing disorders. Through its aims to capture dimensional measures that cut across diagnostic categories and are linked to underlying neurobiological systems, a large number of phenotypic constructs have been proposed as potential research targets. Since "genes" represent a central "unit of analysis" in the RDoC matrix, it is important for studies going forward to apply what is known about the genetics of these phenotypes as well as fill in the gaps of existing knowledge. This article reviews the extant genetic epidemiological data (twin studies, heritability) and molecular genetic association findings for a broad range of putative NVS phenotypic measures. We find that scant genetic epidemiological data is available for experimentally derived measures such as attentional bias, peripheral physiology, or brain-based measures of threat response. The molecular genetic basis of NVS phenotypes is in its infancy, since most studies have focused on a small number of candidate genes selected for putative association to anxiety disorders (ADs). Thus, more research is required to provide a firm understanding of the genetic aspects of anxiety-related NVS constructs. © 2016 Wiley Periodicals, Inc.

Endophenotype best practices

This review examines the current state of electrophysiological endophenotype research and recommends best practices that are based on knowledge gleaned from the last decade of molecular genetic research with complex traits. Endophenotype research is being oversold for its potential to help discover psychopathology relevant genes using the types of small samples feasible for electrophysiological research. This is largely because the genetic architecture of endophenotypes appears to be very much like that of behavioral traits and disorders: they are complex, influenced by many variants (e.g., tens of thousands) within many genes, each contributing a very small effect. Out of over 40 electrophysiological endophenotypes covered by our review, only resting heart, a measure that has received scant advocacy as an endophenotype, emerges as an electrophysiological variable with verified associations with molecular genetic variants. To move the field forward, investigations designed to discover novel variants associated with endophenotypes will need extremely large samples best obtained by forming consortia and sharing data obtained from genome wide arrays. In addition, endophenotype research can benefit from successful molecular genetic studies of psychopathology by examining the degree to which these verified psychopathology-relevant variants are also associated with an endophenotype, and by using knowledge about the functional significance of these variants to generate new endophenotypes. Even without molecular genetic associations, endophenotypes still have value in studying the development of disorders in unaffected individuals at high genetic risk, constructing animal models, and gaining insight into neural mechanisms that are relevant to clinical disorder.

Winter is coming: Seasonality and the acoustic startle reflex

Circannual rhythms and seasonality have long been in the interest of research. In humans, seasonal changes in mood have been extensively investigated since a substantial part of the population experiences worsening of mood during winter. Questions remain regarding accompanying physiological phenomena. We report seasonal effects on the acoustic startle response in a cross-sectional (n=124) and a longitudinal sample (n=23). Startle magnitudes were larger in winter (sample 1: p=0.026; sample 2: p=0.010) compared to summer months. Although the findings need to be replicated they may have implications regarding the timing of startle experiments.

Reactivity to uncertain threat as a familial vulnerability factor for alcohol use disorder

BACKGROUND

When sober, problematic drinkers display exaggerated reactivity to threats that are uncertain (U-threat). Since this aversive affective state can be alleviated via acute alcohol intoxication, it has been posited that individuals who exhibit heightened reactivity to U-threat at baseline are motivated to use alcohol as a means of avoidance-based coping, setting the stage for excessive drinking. To date, however, no study has attempted to characterize the dispositional nature of exaggerated reactivity to U-threat and test whether it is a vulnerability factor or exclusively a disease marker of problematic alcohol use.

METHOD

The current investigation utilized a family study design to address these gaps by examining whether (1) reactivity to U-threat is associated with risk for problematic alcohol use, defined by family history of alcohol use disorder (AUD) and (2) reactivity to U-threat is correlated amongst adult biological siblings. A total of 157 families, and 458 individuals, participated in the study and two biological siblings completed a threat-of-shock task designed to probe reactivity to U-threat and predictable threat (P-threat). Startle potentiation was collected as an index of aversive responding.

RESULTS

Within biological siblings, startle potentiation to U-threat [intraclass correlation (ICC) = 0.35] and P-threat (ICC = 0.63) was significantly correlated. In addition, independent of an individuals' own AUD status, startle potentiation to U-threat, but not P-threat, was positively associated with risk for AUD (i.e. AUD family history).

CONCLUSION

This suggests that heightened reactivity to U-threat may be a familial vulnerability factor for problematic drinking and a novel prevention target for AUD.

Heritability of startle reactivity and affect modified startle

Startle reflex and affect-modified startle reflex are used as indicators of defensive reactivity and emotional processing, respectively. The present study investigated the heritability of both the startle blink reflex and affect modification of this reflex in a community sample of 772 twins ages 14-15years old. Subjects were shown affective picture slides falling in three valence categories: negative, positive and neutral; crossed with two arousal categories: high arousal and low arousal. Some of these slides were accompanied with a loud startling noise. Results suggested sex differences in mean levels of startle reflex as well as in proportions of variance explained by genetic and environmental factors. Females had higher mean startle blink amplitudes for each valence-arousal slide category, indicating greater baseline defensive reactivity compared to males. Startle blink reflex in males was significantly heritable (49%), whereas in females, variance was explained primarily by shared environmental factors (53%) and non-shared environmental factors (41%). Heritability of affect modified startle (AMS) was found to be negligible in both males and females. These results suggest sex differences in the etiology of startle reactivity, while questioning the utility of the startle paradigm for understanding the genetic basis of emotional processing.

Psychophysiology of threat response, paradigm shifts in psychiatry, and RDoC: Implications for genetic investigation of psychopathology

Two articles in this issue seek to further the goal of NIMH's Research Domain Criteria (RDoC) initiative to develop new approaches for investigating mental disorders, using fundamental dimensions that cut across traditional disorder categories and align more closely with mechanisms that underlie psychopathology. One article, by Lang, McTeague, and Bradley, describes the construction of such a disorder cross-crossing dimension of fear response. The other, by Kozak and Cuthbert, expounds upon the basis and conceptualization of research targets under RDoC. Possible implications of these developments for psychiatric genetics research are discussed.

Startle amplitude during unpleasant pictures is greater in veterans with a history of multiple-suicide attempts and predicts a future suicide attempt

Recent studies demonstrate that veterans exhibit higher suicide risk compared with the general U.S.

POPULATION

A prior suicide attempt is a well-documented predictor of suicide death. Despite increased attention to clinical risk factors of suicide and efforts to develop psychosocial interventions to reduce suicide risk, the underlying biological factors that confer this risk are not well understood. This study examined affect-modulated startle (AMS) during a series of intermixed unpleasant, neutral, and pleasant pictures in a sample of 108 demographically-matched veterans at low (passive ideators: n = 26) and high risk (active ideators: n = 29; single attempters: n = 28; and multiple attempters: n = 25) for suicide based on the Columbia Suicide Severity Rating Scale. An exploratory aim involved a longitudinal component in a subset of the high-risk sample that went on to participate in a randomized 6-month clinical trial. We investigated whether baseline AMS predicts a subsequent suicide attempt at 12-month follow-up. Compared with the other three groups, multiple attempters showed greater startle potentiation during unpleasant pictures and deficient overall startle habituation from early to later trials. The groups did not differ in startle during neutral or pleasant pictures, or self-reported picture valence. Greater startle during unpleasant pictures was associated with greater emotion dysregulation as measured by the Difficulties in Emotion Regulation Scale and a future suicide attempt assessed prospectively at 12-month follow-up. These findings suggest that startle potentiation during unpleasant pictures in multiple-suicide attempters is a promising psychophysiological biomarker of suicide risk and underscore the clinical importance of targeting emotion dysregulation in the treatment of patients at-risk for suicide.

Comparing the validity of informant and self-reports of personality using laboratory indices of emotional responding as criterion variables

Personality traits relate to risk for psychopathology and can inform predictions about treatment outcome. In an effort to obtain a comprehensive index of personality, informant reports of personality are sometimes obtained in addition to self-reports of personality. However, there is limited research comparing the validity of self- and informant reports of personality, particularly among those with internalizing psychopathology. This is important given that informants may provide an additional (and perhaps different) perspective on individuals' personality. The present study therefore compared how both reports of positive affectivity (PA) and negative affectivity (NA) relate to psychophysiological and subjective measures of emotional responding to positive and negative stimuli. Given that our sample (n = 117) included individuals with no history of psychopathology, as well as individuals with major depressive disorder (MDD) and/or panic disorder (PD), we were also able to explore whether these internalizing diagnoses moderated the association between personality reports and measures of emotional responding. Informant-reported PA predicted physiological responses to positive stimuli (but not negative). Informant-reported NA predicted physiological responses to negative stimuli (but not positive). Self-reported personality did not predict physiological responding, but did predict subjectively measured emotional responding (NA for negative responding, PA for positive responding). Diagnoses of internalizing psychopathology (PD or MDD) did not moderate these associations. Results suggest self- and informant reports of personality may each provide valid indices of an individual's emotional response tendencies, but predict different aspects of those tendencies.

BDNF val(66)met genotype shows distinct associations with the acoustic startle reflex and the cortisol stress response in young adults and children

Brain Derived Neurotrophic Factor (BDNF) is a crucial regulator of neuronal development, organization and function and the val(66)met polymorphism in the BDNF gene has been associated with several (endo-) phenotypes of cognitive and affective processing. The BDNF met allele is considered a risk factor for anxiety and fear related phenotypes although findings are not entirely consistent. Here, the impact of BDNF val(66)met on two parameters of anxiety and stress was investigated in a series of studies. Acoustic startle responses were assessed in three adult samples (N1=117, N2=104, N3=116) as well as a children sample (N4=123). Cortisol increase in response to the Trier Social Stress Test (TSST) was measured in one adult sample (N3) and in the children sample (N4). The BDNF met allele was associated with enhanced cortisol responses in young adults (p=0.039) and children (p=0.013). On the contrary, BDNF met allele carriers showed a reduced acoustic startle response which reached significance in most samples (N1: p=0.004; N2: p=0.045; N3: n.s., N4: p=0.043) pointing to differential effects of BDNF val(66)met on distinct endophenotypes of anxiety and stress-related responses. However, small effect sizes suggest substantial additional genetic as well as environmental contributors.

Genetic influences on the neural and physiological bases of acute threat: A research domain criteria (RDoC) perspective

The NIMH Research Domain Criteria (RDoC) initiative aims to describe key dimensional constructs underlying mental function across multiple units of analysis-from genes to observable behaviors-in order to better understand psychopathology. The acute threat ("fear") construct of the RDoC Negative Valence System has been studied extensively from a translational perspective, and is highly pertinent to numerous psychiatric conditions, including anxiety and trauma-related disorders. We examined genetic contributions to the construct of acute threat at two units of analysis within the RDoC framework: (1) neural circuits and (2) physiology. Specifically, we focused on genetic influences on activation patterns of frontolimbic neural circuitry and on startle, skin conductance, and heart rate responses. Research on the heritability of activation in threat-related frontolimbic neural circuitry is lacking, but physiological indicators of acute threat have been found to be moderately heritable (35-50%). Genetic studies of the neural circuitry and physiology of acute threat have almost exclusively relied on the candidate gene method and, as in the broader psychiatric genetics literature, most findings have failed to replicate. The most robust support has been demonstrated for associations between variation in the serotonin transporter (SLC6A4) and catechol-O-methyltransferase (COMT) genes with threat-related neural activation and physiological responses. However, unbiased genome-wide approaches using very large samples are needed for gene discovery, and these can be accomplished with collaborative consortium-based research efforts, such as those of the Psychiatric Genomics Consortium (PGC) and Enhancing Neuro Imaging Genetics through Meta-Analysis (ENIGMA) Consortium.

Psychopathology research in the RDoC era: Unanswered questions and the importance of the psychophysiological unit of analysis

The NIMH Research Domain Criteria (RDoC) initiative seeks to re-conceptualize psychopathology by identifying transdiagnostic constructs that reflect core mechanisms of psychopathology. Although the RDoC framework has been discussed in many prior papers, there are several methodological and conceptual points that have yet to be fully specified. For example, little discussion exists on the importance of distinguishing each construct's nomological network and linking it to risk for psychopathology. It has also been unclear the extent to which RDoC constructs (within and across systems) should relate to one another and how these associations may differ as a function of developmental period. These are important questions as we enter the RDoC era and psychophysiological measures represent an exciting tool to address these issues. In this paper, we discuss the currently un- (or under-)specified aspects of the RDoC initiative and highlight the advantages of the psychophysiological 'unit of analysis.' We also briefly review existing psychophysiological studies, within the positive and negative valence systems, that exemplify the RDoC approach and make recommendations for how future studies can help the field progress in this mission.

Biomarkers of intergenerational risk for depression: a review of mechanisms in longitudinal high-risk (LHR) studies

BACKGROUND

Longitudinal research is critical for understanding the biological mechanisms underlying the development of depression. Researchers recruit high-risk cohorts to understand how risk is transmitted from one generation to the next. Biological measurements have been incorporated into these longitudinal high-risk (LHR) studies in order to illuminate mechanistic pathways.

METHODS

To frame our review, we first present heritability estimates along the gene-by-environment continuum as a foundation. We then offer a Biomarkers of Intergenerational Risk for Depression (BIRD) model to describe the multiple hits individuals at risk receive and to allow for greater focus on the interactive effects of markers. BIRD allows for the known multifinality of pathways towards depression and considers the context (i.e., environment) in which these mechanisms emerge. Next, we review the extant LHR cohort studies that have assessed central nervous system (electroencephalography (EEG), neuroimaging), endocrine (hypothalamic-pituitary-adrenal axis (HPA)/cortisol), autonomic (startle, heart rate), genetic, sleep, and birth characteristics.

RESULTS

Results to date, in conjunction with the proposed model, point towards several pathways of discovery in understanding mechanisms, providing clear direction for future research examining potential endophenotypes.

LIMITATIONS

Our review is based on relatively narrow inclusion and exclusion criteria. As such, many interesting studies were excluded, but this weakness is offset by strengths such as the increased reliability of findings.

CONCLUSIONS

Blanket prevention programs are inefficient and plagued by low effect sizes due to low rates of actual conversion to disorder. The inclusion of biomarkers of risk may lead to enhanced program efficiency by targeting those at greatest risk.

Heritability and molecular genetic basis of acoustic startle eye blink and affectively modulated startle response: a genome-wide association study

Acoustic startle responses have been studied extensively in relation to individual differences and psychopathology. We examined three indices of the blink response in a picture-viewing paradigm-overall startle magnitude across all picture types, and aversive and pleasant modulation scores-in 3,323 twins and parents. Biometric models and molecular genetic analyses showed that half the variance in overall startle was due to additive genetic effects. No single nucleotide polymorphism was genome-wide significant, but GRIK3 produced a significant effect when examined as part of a candidate gene set. In contrast, emotion modulation scores showed little evidence of heritability in either biometric or molecular genetic analyses. However, in a genome-wide scan, PARP14 produced a significant effect for aversive modulation. We conclude that, although overall startle retains potential as an endophenotype, emotion-modulated startle does not.

Empathy as a "risky strength": a multilevel examination of empathy and risk for internalizing disorders

Learning to respond to others' distress with well-regulated empathy is an important developmental task linked to positive health outcomes and moral achievements. However, this important interpersonal skill set may also confer risk for depression and anxiety when present at extreme levels and in combination with certain individual characteristics or within particular contexts. The purpose of this review is to describe an empirically grounded theoretical rationale for the hypothesis that empathic tendencies can be "risky strengths." We propose a model in which typical development of affective and cognitive empathy can be influenced by complex interplay among intraindividual and interindividual moderators that increase risk for empathic personal distress and excessive interpersonal guilt. These intermediate states in turn precipitate internalizing problems that map onto empirically derived fear/arousal and anhedonia/misery subfactors of internalizing disorders. The intraindividual moderators include a genetically influenced propensity toward physiological hyperarousal, which is proposed to interact with genetic propensity to empathic sensitivity to contribute to neurobiological processes that underlie personal distress responses to others' pain or unhappiness. This empathic personal distress then increases risk for internalizing problems, particularly fear/arousal symptoms. In a similar fashion, interactions between genetic propensities toward negative thinking processes and empathic sensitivity are hypothesized to contribute to excess interpersonal guilt in response to others' distress. This interpersonal guilt then increases the risk for internalizing problems, especially anhedonia/misery symptoms. Interindividual moderators, such as maladaptive parenting or chronic exposure to parents' negative affect, further interact with these genetic liabilities to amplify risk for personal distress and interpersonal guilt as well as for consequent internalizing problems. Age-related increases in the heritability of depression, anxiety, and empathy-related constructs are consistent with developmental shifts toward greater influence of intraindividual moderators throughout childhood and adolescence, with interindividual moderators exerting their greatest influence during early childhood. Efforts to modulate neurobiological and behavioral expressions of genetic dysregulation liabilities and to promote adaptive empathic skills must thus begin early in development.

The impact of sex and menstrual cycle on the acoustic startle response

Sex differences in fear and anxiety have been widely reported although results are not entirely consistent depending on measures used. Also, a possible influence of the menstrual cycle is often not taken into account, and effect sizes are not always discussed. In a sample of healthy young adults (n=111 women without hormonal contraceptives and n=107 men) the acoustic startle response (ASR) and emotional ASR modulation were analysed. We found no significant effect of sex on ASR (p=.269) but a significant effect of menstrual cycle (p=.027, η(2)=0.105). Compared to men, women showed increased ASR during the late luteal phase probably reflecting elevated negative emotionality, and during ovulation which, however, might be due to increased auditory sensitivity and changes in general CNS arousal. Neither sex nor menstrual cycle affected startle modulation. Thus, at least in young adults, menstrual cycle but not sex per se appears to contribute significantly to ASR variance.

Transcription factor 4 (TCF4) and schizophrenia: integrating the animal and the human perspective

Schizophrenia is a genetically complex disease considered to have a neurodevelopmental pathogenesis and defined by a broad spectrum of positive and negative symptoms as well as cognitive deficits. Recently, large genome-wide association studies have identified common alleles slightly increasing the risk for schizophrenia. Among the few schizophrenia-risk genes that have been consistently replicated is the basic Helix-Loop-Helix (bHLH) transcription factor 4 (TCF4). Haploinsufficiency of the TCF4 (formatting follows IUPAC nomenclature: TCF4 protein/protein function, Tcf4 rodent gene cDNA mRNA, TCF4 human gene cDNA mRNA) gene causes the Pitt-Hopkins syndrome-a neurodevelopmental disease characterized by severe mental retardation. Accordingly, Tcf4 null-mutant mice display developmental brain defects. TCF4-associated risk alleles are located in putative coding and non-coding regions of the gene. Hence, subtle changes at the level of gene expression might be relevant for the etiopathology of schizophrenia. Behavioural phenotypes obtained with a mouse model of slightly increased gene dosage and electrophysiological investigations with human risk-allele carriers revealed an overlapping spectrum of schizophrenia-relevant endophenotypes. Most prominently, early information processing and higher cognitive functions appear to be associated with TCF4 risk genotypes. Moreover, a recent human study unravelled gene × environment interactions between TCF4 risk alleles and smoking behaviour that were specifically associated with disrupted early information processing. Taken together, TCF4 is considered as an integrator ('hub') of several bHLH networks controlling critical steps of various developmental, and, possibly, plasticity-related transcriptional programs in the CNS and changes of TCF4 expression also appear to affect brain networks important for information processing. Consequently, these findings support the neurodevelopmental hypothesis of schizophrenia and provide a basis for identifying the underlying molecular mechanisms.

Genetic psychophysiology: advances, problems, and future directions

This paper presents an overview of historical advances and the current state of genetic psychophysiology, a rapidly developing interdisciplinary research linking genetics, brain, and human behavior, discusses methodological problems, and outlines future directions of research. The main goals of genetic psychophysiology are to elucidate the neural pathways and mechanisms mediating genetic influences on cognition and emotion, identify intermediate brain-based phenotypes for psychopathology, and provide a functional characterization of genes being discovered by large association studies of behavioral phenotypes. Since the initiation of this neurogenetic approach to human individual differences in the 1970s, numerous twin and family studies have provided strong evidence for heritability of diverse aspects of brain function including resting-state brain oscillations, functional connectivity, and event-related neural activity in a variety of cognitive and emotion processing tasks, as well as peripheral psychophysiological responses. These data indicate large differences in the presence and strength of genetic influences across measures and domains, permitting the selection of heritable characteristics for gene finding studies. More recently, candidate gene association studies began to implicate specific genetic variants in different aspects of neurocognition. However, great caution is needed in pursuing this line of research due to its demonstrated proneness to generate false-positive findings. Recent developments in methods for physiological signal analysis, hemodynamic imaging, and genomic technologies offer new exciting opportunities for the investigation of the interplay between genetic and environmental factors in the development of individual differences in behavior, both normal and abnormal.

Asians demonstrate reduced sensitivity to unpredictable threat: a preliminary startle investigation using genetic ancestry in a multiethnic sample

Research has indicated that individuals of Asian descent, relative to other racial groups, demonstrate reduced emotional responding and lower prevalence rates of several anxiety disorders. It is unclear though whether these group differences extend to biomarkers of anxiety disorders and whether genetic differences play a role. This study compared self-identified Caucasian, Latino, and Asian persons (total N = 174) on startle response during a baseline period and while anticipating unpredictable threat-a putative biomarker for certain anxiety disorders--as well as predictable threat. In addition, the association between genetic ancestry and startle response was examined within each racial group to determine potential genetic influences on responding. For the baseline period, Asian participants exhibited a smaller startle response relative to Caucasian and Latino participants, who did not differ. Within each racial group, genetic ancestry was associated with baseline startle. Furthermore, genetic ancestry mediated racial group differences in baseline startle. For the threat conditions, a Race × Condition interaction indicated that Asian participants exhibited reduced startle potentiation to unpredictable, but not predicable, threat relative to Caucasian and Latino participants, who did not differ. However, genetic ancestry was not associated with threat-potentiated startle in any racial group. This study adds to the growing literature on racial differences in emotional responding and provides preliminary evidence suggesting that genetic ancestry may play an important role. Moreover, reduced sensitivity to unpredictable threat may reflect a mechanism for why individuals of Asian descent are at less risk for particular anxiety disorders relative to other racial groups.

Influence of catechol-O-methyltransferase Val158Met polymorphism on startle response in the presence of high estradiol levels

INTRODUCTION

both human and animal studies have shown a somewhat complex COMT-by-sex interaction effect on brain function and dysfunction. A functional variation in the gene coding for the catechol-O-methyltransferase (COMT) enzyme, which metabolizes dopamine and noradrenaline, has been related to executive and emotional functions, and to sex dimorphism.

AIM

to investigate if COMT Val158Met genotype influences startle response in pregnant women, given their physiologically elevated estradiol levels.

METHODS

seventy-three pregnant women were assessed in gestational week 38 for acoustic startle response, measured by electromyography of the blink reflex, during control condition, positive and negative anticipation stimuli, and pleasant and unpleasant image stimuli. A blood sample was taken for measurement of estradiol levels and genetic analysis.

RESULTS

the results indicated a COMT Val158Met effect on startle response across all conditions (main effect of genotype, F(2,70=3.58), p=0.033), where Val/Val women displayed higher startle magnitudes than Val/Met carriers (Cohen's d=0.71). No significant difference by genotype was found in affective modulation. The findings also suggested an estrogen dose-dependent effect of COMT Val158Met on startle reflex. Among women with higher pregnancy-induced estradiol levels, Val/Val carriers had markedly higher startle response across conditions than heterozygotes (Cohen's d=1.36; F(4,21=11.07); p=0.003), while this effect was not present in women with estradiol levels under the median concentration.

CONCLUSIONS

the observed effect of COMT Val158Met by estradiol on overall startle response is likely to be due to a variable noradrenergic transmission depending on COMT activity in a possible interaction with estradiol.

Affect-modulated startle: interactive influence of catechol-O-methyltransferase Val158Met genotype and childhood trauma

The etiology of emotion-related disorders such as anxiety or affective disorders is considered to be complex with an interaction of biological and environmental factors. Particular evidence has accumulated for alterations in the dopaminergic and noradrenergic system--partly conferred by catechol-O-methyltransferase (COMT) gene variation--for the adenosinergic system as well as for early life trauma to constitute risk factors for those conditions. Applying a multi-level approach, in a sample of 95 healthy adults, we investigated effects of the functional COMT Val158Met polymorphism, caffeine as an adenosine A2A receptor antagonist (300 mg in a placebo-controlled intervention design) and childhood maltreatment (CTQ) as well as their interaction on the affect-modulated startle response as a neurobiologically founded defensive reflex potentially related to fear- and distress-related disorders. COMT val/val genotype significantly increased startle magnitude in response to unpleasant stimuli, while met/met homozygotes showed a blunted startle response to aversive pictures. Furthermore, significant gene-environment interaction of COMT Val158Met genotype with CTQ was discerned with more maltreatment being associated with higher startle potentiation in val/val subjects but not in met carriers. No main effect of or interaction effects with caffeine were observed. Results indicate a main as well as a GxE effect of the COMT Val158Met variant and childhood maltreatment on the affect-modulated startle reflex, supporting a complex pathogenetic model of the affect-modulated startle reflex as a basic neurobiological defensive reflex potentially related to anxiety and affective disorders.

Variation in genes involved in dopamine clearance influence the startle response in older adults

The dopamine transporter (DAT) and the enzyme catechol-O-methyltransferase (COMT) both terminate synaptic dopamine action. Here, we investigated the influence of two polymorphisms in the respective genes: DAT1 (SLC6A3) VNTR and COMT val(158)met (rs4680). Startle magnitudes to intense noise bursts as measured with the eye blink response were recorded during the presentation of pictures of three valence conditions (unpleasant, pleasant and neutral) and during baseline without additional pictorial stimulation in a sample of healthy older adults (N = 94). There was a significant Bonferroni corrected main effect of COMT genotype on the overall startle responses, with met/met homozygotes showing the highest and participants with the val/val genotype showing the lowest startle response, while participants with the val/met genotype displayed intermediate reactions. There was also a DAT1 VNTR main effect, which, after Bonferroni correction, still showed a tendency toward significance with carriers of at least one 9-repeat (R) allele showing smaller overall startle responses compared to 10R/10R homozygotes. Thus, older adult carriers of COMT variants, which result in lower enzyme activity and therefore probably enhanced dopamine signaling, showed stronger startle activity. Although the functional significance of DAT1 VNTR is less defined, our results point to a potential influence of SLC6A3 on startle magnitude.

Low baseline startle and deficient affective startle modulation in remitted bipolar disorder patients and their unaffected siblings

We examined whether startle abnormalities are present in bipolar disorder (BD) patients and their unaffected siblings. Twenty-one remitted patients with BD, 19 unaffected siblings, and 42 controls were presented with 18 pleasant, 18 unpleasant, and 18 neutral pictures. Acoustic probes (104 dB) were presented during 12 of 18 pictures in each affective category at 300, 3000, and 4500 ms after picture onset, so that there were 4 pictures per valence per probe onset type. Baseline startle was assessed during blank screens and was found reduced in patients and sibling groups. We found startle inhibition with the 300 probes and a linear increase in amplitude with valence with the late probes in controls; these effects were absent in patients and their siblings. Low startle and blunted startle reactivity may represent trait deficits in remitted BD patients and their relatives, possibly associated with attentional deficits and adaptive down-regulation of emotion.

Influence of functional tryptophan hydroxylase 2 gene variation and sex on the startle response in children, young adults, and older adults

Serotonin, a key regulator of emotional behavior, is synthesized by tryptophan hydroxylase (TPH). Allelic variation of TPH2 gene expression influences serotonin synthesis in the brain and therefore may modulate emotional processing. Here, we investigated the influence of the -703 G/T polymorphism in the regulatory promoter region of the TPH2 gene on the startle response in three different age samples: children (N=110), young adults (N=209), and older adults (N=95). Startle magnitudes to intense noise bursts were recorded during baseline and while participants viewed unpleasant, pleasant or neutral pictures. There was a significant TPH2xsex interaction effect in young adults with male T allele carriers showing stronger overall startle responses compared to male G/G homozygotes while in young women this effect appeared to be reversed. The difference between TPH2 genotype groups also reached significance in the female subsample when including menstrual cycle phase. In contrast, there was no effect of TPH2 or a TPH2xsex interaction effect in children or in older adults.

Stathmin, a gene regulating neural plasticity, affects fear and anxiety processing in humans

The identification of biological mechanisms underlying emotional behavior is crucial for our understanding of the pathogenesis of mental disorders. Besides genes modulating neural transmission and influencing amygdala reactivity and anxiety-related temperamental traits a different plasticity regulating genes affect interindividual differences in emotional regulation. Recently it has been demonstrated that stathmin, a regulator of microtubule formation which affects long-term potentiation (LTP), controls learned and innate fear responses in rodents, but its role in human emotion regulation is unknown. We hypothesized that in humans the gene coding for stathmin (STMN1), which is highly expressed in the lateral nucleus of the amygdala and associated thalamic and cortical structures, influences behavioral responses to fear and anxiety stimuli by way of two common single nucleotide polymorphisms (rs182455, SNP1; rs213641, SNP2). These polymorphisms are located within or close to the putative transcriptional control region. We used the acoustic startle paradigm and a standardized laboratory protocol for the induction of fear and psychosocial stress in 106 healthy volunteers to investigate the impact of stathmin gene variation on two fear- and anxiety-controlling effector-systems of the amygdala. We found that STMN1 genotype interacting with individuals' gender significantly impacts fear and anxiety responses as measured with the startle and cortisol stress response. We therefore conclude that STMN1 genotype has functional relevance for the acquisition and expression of basic fear and anxiety responses also in humans.

Serotonin transporter gene variation and stressful life events impact processing of fear and anxiety

Genetic variation of the serotonin transporter (5-HTT) has been associated with fear- and anxiety-related behaviours. The amygdala is considered crucial in emotional modulation and stronger amygdala reactivity in response to fearful stimuli has been found in carriers of the short (S) allele of the 5-HTT gene in imaging studies. Additionally, reactivity of amygdala-innervated effectory systems is also of particular interest. We recently reported the impact of a functional polymorphism in the transcriptional control region of the serotonin transporter gene (5-HTTLPR) on the acoustic startle reflex. Here, we attempted to replicate and extend these findings. Startle magnitudes to intense noise bursts as measured with the eyeblink response were recorded in 106 healthy volunteers during baseline without additional stimulation and while they viewed pictures of three valence conditions: unpleasant, pleasant and neutral. Subjects were genotyped for the tri-allelic functional polymorphism 5-HTTLPR. In replication of our previous findings we found that carriers of the low-expressing S or LG alleles exhibited stronger overall startle responses across conditions than LA/LA homozygotes, while there were no differences in emotional startle modulation between the two genetic groups. In addition, we found that the recent experience of stressful life events resulted in overall higher startle responses and less startle habituation across blocks. The results replicate and emphasize the role of 5-HTTLPR and stress on the overall startle response as a possible genetically driven endophenotype for anxiety-related behaviour.

Recurrence of major depressive disorder is predicted by inhibited startle magnitude while recovered

BACKGROUND

Major depressive disorder, for some, follows a chronic relapsing course. However, no reliable marker has been established that allows the identification of this sub-group of patients. Preliminary findings suggest that baseline startle magnitude may be such a marker. This study evaluated whether differences in baseline startle magnitude during remission could prospectively predict depressive symptomatology and recurrence.

METHODS

A group of previously depressed individuals (n=25), who were in full remission at the time of testing, had their startle reflex measured via surface EMG electrodes on the left orbicularis oculi muscle. These people were then followed-up 2 years later and their depressive symptomatology during the intervening period was assessed using the psychiatric ratings scale of the Longitudinal Interval Follow-up Evaluation (LIFE; [Keller, M.B., Lavori, P.W., Friedman, B., Nielsen, E., Endicott, J., McDonald-Scott, P., et al. (1987). The Longitudinal Interval Follow-up Evaluation. A comprehensive method for assessing outcome in prospective longitudinal studies. Arch. Gen. Psychiatry, 44(6), 540-548]) and incidents of recurrence assessed using the Structured Clinical Interview for DSM-IV (SCID-I/P; [First, M.B., Spitzer, R.L., Gibbon, M., & Williams, J.B.W. (2001). Structured clinical interview for axis 1 DSM-IV disorders. New York: New York State Psychiatric Institute]).

RESULTS

It was found that a relatively attenuated startle response at initial assessment was strongly predictive of both depressive symptomatology and those who would experience relapse.

LIMITATIONS

This study has a relatively small sample size that limits the degree to which a thorough co-variant analysis can be conducted and also makes the analysis of gender-based difference impracticable. Additionally, as no healthy control group is included, we report a relative rather than absolute attenuation of startle to be indicative of symptom severity and recurrence proclivity.

CONCLUSIONS

These preliminary findings suggest that an attenuated startle response may have utility as an endophenotypic marker of risk for recurrence of Major Depression and residual sub-syndromal symptomatology. Such a marker may facilitate the early identification and treatment of those most at risk of recurrent Major Depression.

Differences in startle reflex and prepulse inhibition in European-Americans and African-Americans

The acoustic startle reflex and its modulation by a prepulse are psychophysiological phenomena that are commonly studied to evaluate various aspects of information processing. Recent reports in human populations suggest that subjects from disparate racial backgrounds may have significant differences in the startle response. To determine if this pattern could be observed in our subject population and whether it extended to prepulse inhibition (PPI), we evaluated baseline startle parameters and PPI in 53 African-Americans (AA) and 38 European-Americans (EA). In AA compared to EA, mean startle magnitude and probability of blink response were lower, with no difference in habituation. PPI was greater in AA than EA when groups were matched on baseline startle magnitude. These findings support the idea of racial differences in startle response. Implications for study design are highlighted, and possible environmental and genetic influences are considered.