REVIEW ARTICLE Neuropsychiatric Genetics
Aggressive Behavior in Humans: Genes and Pathways Identified Through Association Studies Noelia Fernandez-Castillo1,2,3* and Bru Cormand1,2,3* 1
Departament de Genetica, Facultat de Biologia, Universitat de Barcelona, Catalonia, Spain
2
Institut de Biomedicina de la Universitat de Barcelona (IBUB), Catalonia, Spain
3
Centro de Investigacion Biomedica en Red de Enfermedades Raras (CIBERER), Spain
Manuscript Received: 26 June 2015; Manuscript Accepted: 4 January 2016
Aggressive behavior has both genetic and environmental components. Many association studies have been performed to identify genetic factors underlying aggressive behaviors in humans. In this review we summarize the previous work performed in this field, considering both candidate gene (CGAS) and genome-wide association studies (GWAS), excluding those performed in samples where the primary diagnosis is a psychiatric or neurological disorder other than an aggression-related phenotype. Subsequently, we have studied the enrichment of pathways and functions in GWAS data. The results of our searches show that most CGAS have identified associations with genes involved in dopaminergic and serotonergic neurotransmission and in hormone regulation. On the other hand, GWAS have not yet identified genome-wide significant associations, but top nominal findings are related to several signaling pathways, such as axon guidance or estrogen receptor signaling, and also to neurodevelopmental processes and synaptic plasticity. Future studies should use larger samples, homogeneous phenotypes and standardized measurements to identify genes that underlie aggressive behaviors in humans. Ó 2016 Wiley Periodicals, Inc.
Key words: aggression; genetics; association studies; GWAS; candidate genes
INTRODUCTION From an evolutionary point of view, aggressive behavior in animals is related to individual fitness since it enables surviving and mating when competing for limited resources, defending from predators and establishing hierarchies. In social animals, both low and high levels of aggression are detrimental for surviving, and aggressive behavior may be under stabilizing selection [Anholt and Mackay, 2012]. Similarly, in humans, aggressive behavior has some advantages in competition, reproduction and dominance in hierarchy, but when expressed in the wrong context it may lead to injury or death and cause serious social problems, especially when this behavior is impulsive and uncontrolled or deliberate and premeditate. Human aggressive behavior has been usually classified in proactive and reactive aggression. Proactive aggression is premeditated
Ó 2016 Wiley Periodicals, Inc.
How to Cite this Article: Fernandez-Castillo N, Cormand B. 2016. Aggressive Behavior in Humans: Genes and Pathways Identified Through Association Studies. Am J Med Genet Part B 9999:1–21.
and related to reduced emotional sensitivity, often without remorse or regret, whereas reactive aggression is related to excessive emotional sensitivity and an overreaction towards threat perception, triggered by negative emotions, anger, anxiety or bad experiences [Robinson and Wilkowski, 2010]. Aggressive behavior and the inability to control aggressive impulses seem to be influenced by both genetic and environmental factors. A number of twin and adoption studies have investigated the genetic and environmental architecture of aggressive behavior, including several meta-analyses. Taken together, they show that about 50% of the variance in aggression is explained by genetic influences and the remaining 50% is explained by environmental factors not shared by family members [Tuvblad and Baker, 2011]. Age and form of aggression (reactive, proactive, direct/physical, indirect/relational) have an influence on these figures, whereas they seem to be sex-independent, although males are more likely than Conflicts of interest: The authors declare no conflicts of interest. Grant sponsor: Centro de Investigaci on Biomedica en Red de Enfermedades Raras (CIBERER); Grant sponsor: Spanish “Ministerio de Economı´a y Competitividad”; Grant number: SAF2012-33484; Grant sponsor: AGAUR; Grant number: 2014SGR932; Grant sponsor: European Community’s Seventh Framework Programme; Grant number: 602805. Correspondence to: Noelia Fernandez Castillo and Bru Cormand, Departament de Genetica, Facultat de Biologia, Universitat de Barcelona, Avinguda Diagonal 643, edifici Prevosti, 3ª planta, 08028, Barcelona, Catalonia, Spain. E-mail:
[email protected] (N.F.-C.);
[email protected] (B.C.) Article first published online in Wiley Online Library (wileyonlinelibrary.com): 00 Month 2015 DOI 10.1002/ajmg.b.32419
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2 females to show aggressive behaviors. Regarding age, the genetic influences become increasingly more important at later stages in life (e.g., from 55% at 1–5 years to 63% at 11–18 years) (see a metaanalysis by Burt [2009]). Adoption and twin studies also show that the genetic effects on the aggressive phenotype depend upon environmental factors (GXE) such as family adversity and antisocial disadvantage, violent media exposure or alcohol use [Craig, 2007; Craig and Halton, 2009; Tuvblad and Baker, 2011; Pavlov et al., 2012]. Based on the substantial genetic component that underlies aggressive behavior, several linkage and association studies have been performed to identify genes that participate in modulating human aggressive behavior. In this review we have focused on genes involved in human aggressive behavior identified through association studies that assessed this phenotype as a dimensional trait (externalizing behavior, anger, delinquency, criminality, violence) or as a diagnostic category (conduct disorder, oppositional defiant disorder, callous unemotional, and antisocial personality), and aimed at identifying pathways that underlie this complex behavior.
Literature Searches and Bioinformatic Analyses In this review we have focused on aggressive behaviors including aggression traits (aggressiveness, impulsive aggression, anger, externalizing behavior, violence, delinquency or criminality) or diagnostic categories (oppositional defiant disorder [ODD], conduct disorder [CD], antisocial behavior or antisocial personality disorder [ASPD], callous unemotional [CU], and psychopathy), and have discussed them separately. We have not considered studies performed in samples with other psychiatric or neurological disorders (e.g., drug use or dependence, bipolar disorder, schizophrenia, major depression). Bibliography included in this review considered searches performed by Vassos et al. [2014] and Gunter et al. [2010]. To update their searches from December 2009 until February 2015 we searched in PubMed using the terms “(aggression OR aggressivity OR aggressive OR anger OR hostility OR irritability OR violence OR convict OR crimin OR offend OR externalizing OR conduct OR antisocial OR impulsive aggression OR psychopathy OR ODD OR oppositional defiant OR callous unemotional) AND (genetics OR gene OR polymorphism OR genotype OR allele OR genome OR haplotype)”, with an output of 7,202 articles. We filtered articles written in English, performed in humans, including sample characteristics and performing genetic association studies. We selected 268 potential articles within those dates and added 263 articles from a previous review [Gunter et al., 2010] and from a meta-analysis [Vassos et al., 2014]. From these 531 articles we selected those studies that included traits related to aggression or diagnostic categories specified above, excluding studies performed in samples of individuals with other psychiatric disorders. A total of 277 articles were finally considered for this review. In order to minimize possible sources of clinical and etiological heterogeneity, the results of reports that assess traits of aggression or specific diagnostic categories are discussed separately. For bioinformatic analyses aimed at identifying functions and pathways involved in aggressive behaviors we considered data of
AMERICAN JOURNAL OF MEDICAL GENETICS PART B the six genome-wide association studies (GWAS) meeting the inclusion criteria described above. For that purpose we asked the authors of these studies to provide us with the data on associated SNPs in their GWAS studies, considering a threshold set at P ¼ 5e-05. For the study performed in hostility by Merjonen et al. [2011] we only included signals associated with anger, considering mean values across the four measures, whereas signals associated with cynicism and paranoia were discarded. For the study performed by Anney et al. [2008] we considered associations only under the additive model, and not under the other genetic models tested. For the study of Mick et al. [2014] we included associations with anger, considering both angry temperament and angry reaction measures. Finally, for the study of Viding et al. [2010] a complete list of p-values from the discovery sample was not available. However, as the study included a replication sample, we considered the top 30 replicated signals reported in the paper. A total of 559 signals were selected across the six GWAS [Anney et al., 2008; Viding et al., 2010; Merjonen et al., 2011; Mick et al., 2011, 2014; Tielbeek et al., 2012]. We used the PLINK software to retrieve information of the nearby genes for each of those SNPs, considering a window of 100 Kb distal and proximal from each SNP. When more than one gene was present in the window, we chose only the one closest to the SNP. A total of 156 genes were finally included, two of them, A2BP1 and NFKB1, being identified in two different studies (see Fig. 1). To identify over-representation of pathways and functions among the genes associated with aggressive behavior we used WebGESTALT (http://bioinfo.vanderbilt.edu/webgestalt) and the Ingenuity Pathway Analysis (IPA) 8.8 software (Ingenuity Systems, Redwood city, CA, USA). Pathway enrichment was performed using KEGG pathways and Common pathways analyses of WebGESTALT. Function over-representation was performed using the Gene Ontology (GO) analysis of WebGESTALT and Diseases and Bio Functions analysis of IPA. Finally, gene network analyses were performed with IPA.
Genetic Association Studies of Aggressive Behavior Association studies performed to identify genetic variants involved in the susceptibility to aggression have followed two different approaches: candidate gene association studies (CGAS) or genome-wide association studies (GWAS). Most of the studies examining aggressive behavior traits and phenotypes have focused on candidate genes, based on previous evidence or biological plausibility. The majority of investigated genes encode proteins involved in dopamine or serotonin neurotransmission as well as hormone enzymes or receptors. In contrast, only a few GWAS scans have been performed so far, which assess different phenotypes related to aggressive behaviors. The genome-wide hypothesis-free approach has allowed identification of several genes that had not been considered in CGAS and highlight novel pathways and functions potentially relevant to these behaviors. The most studied variants in CGAS have often shown conflicting association results in the different reports, so meta-analytical approaches have been used to try to clarify their contribution to aggression. The most comprehensive meta-analysis investigated 31 genes from 185 studies and did not identify any significant
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FIG. 1. Selection of variants and genes associated with aggressive behavior from GWAS studies. The criteria for the selection of association signals and the number of genes associated within a window of 100 kb from each SNP are indicated. A total of 156 genes showing a nominal association with aggressive behavior were selected, two of them in two independent studies.
association [Vassos et al., 2014]. However, there was a remarkable heterogeneity in the phenotypes assessed, as the study included samples with psychiatric or neurological diseases (such as substance use disorders, schizophrenia, Alzheimer or intellectual disability) that may drive the aggression behaviors, biasing the results. In this review we have not considered studies performed in samples with other psychiatric or neurological disorders.
Candidate Gene Association Studies (CGAS) Since the identification in 1993 of a truncating mutation in the Xlinked MAOA gene responsible for a severe aggressive and violent behavior in the male individuals of a Dutch family [Brunner et al., 1993], and the confirmation that Maoa knock-out mice show aggressive behaviors [Cases et al., 1995], dopamine and serotonin neurotransmission have been the main focus of aggression studies. On the other hand, gender-specific genes and genes related to hormonal function (e.g., hormone metabolizers or receptors) have also received attention, since aggressive behavior is more frequent in males. Genetic factors involved in aggressive behavior are likely to have a small effect size, and may contribute to the phenotype in some cases in combination with specific environmental factors. For this reason, several association studies assess gene by environmental interactions (G E). Dopamine and serotonin neurotransmission. The four most studied genes related to dopamine and serotonin encode enzymes
involved in the metabolism of these neurotransmitters (MAOA and COMT), the serotonin transporter (5HTT) and a dopamine receptor (DRD4), but other genes have also been investigated (Tables I and II). We will first discuss studies performed on genes coding for enzymes involved in the metabolism of both dopamine and serotonin, then we will focus on serotonergic genes, and finally on dopaminergic genes. The MAOA gene encodes monoamine oxidase A, an enzyme responsible for the catabolism of amines (dopamine, serotonin and noradrenalin). This gene is located on chromosome X and most of the published studies have been performed in male samples or have identified associations in males. Results obtained in females are difficult to interpret because of the lack of information on chromosome X inactivation. An upstream variable number of tandem repeats polymorphism (uVNTR) located in the promoter region and with a repeat unit length of 30 bp, has been the most studied genetic variant in aggressive behaviors. Low activity alleles of this VNTR (2 and 3 repeats), which determine reduced transcriptional levels, have been associated with aggressive behavior, increased traits of aggression or high aggression scores, aggressiveness and impulsivity, reactive aggressiveness, violent behavior, delinquency, use of weapons, stabbing and shooting [Manuck et al., 2000, 2002; Eisenberger et al., 2007; Reif et al., 2007; Guo et al., 2008; Beaver et al., 2010a, 2014; Gallardo-Pujol et al., 2013; Kuepper et al., 2013; Tiihonen et al., 2015]. Also, studies considering GxE interactions showed that the low activity variant in the VNTR interacts with
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TABLE I. Candidate Genes Investigated in Aggressive Traits Gene symbol ABCG1
Gene name
Associated phenotype
References (positive and negative associations)
Aggression and anger
Gietl et al. [2007]
Anger Externalizing behavior Aggression
Richter et al. [2011] Logue et al. [2013] Damberg et al. [2000]
AR
ATP-binding cassette, sub-family G (WHITE), member 1 A kinase (PRKA) anchor protein 5 ankyrin 3, node of Ranvier (ankyrin G) transcription factor AP-2 beta (activating enhancer binding protein 2 beta) androgen receptor
Violent criminal behavior, aggression
AVP AVPR1A AVPR1B BDNF
arginine vasopressin arginine vasopressin receptor 1A arginine vasopressin receptor 1B brain-derived neurotrophic factor
CDH13 CHRM2
cadherin 13 cholinergic receptor, muscarinic 2
Aggression Aggression and anger Aggression Aggressive behavior, aggression and impulsivity Violent behavior Externalizing behavior
COMT
catechol-O-methyltransferase
Aggression, externalizing and anger
CRHR1 CYP17
Aggressive behavior — Externalizing behavior
Miodovnik et al. [2012]
—
Miodovnik et al. [2012]
Anger
Reuter et al. [2009]
DRD3 DRD4
dopamine receptor D3 dopamine receptor D4
Aggressive hostility, impulsivity and neuroticism Aggressive behavior and violent delinquency Impulsivity in violent offenders Aggression, aggressive behavior, externalizing behavior and delinquency
Hess et al. [2009]
DRD2
corticotropin releasing hormone receptor 1 cytochrome P450, family 17, subfamily A, polypeptide 1 cytochrome P450, family 19, subfamily A, polypeptide 1 cytochrome P450, family 1, subfamily B, polypeptide 1 protein phosphatase 1, regulatory (inhibitor) subunit 1B dopamine beta-hydroxylase (dopamine beta-monooxygenase) dopamine receptor D2
Cheng et al. [2006]; Jonsson et al. [2001]; Rajender et al. [2008] Malik et al. [2014] Malik et al. [2014]; Moons et al. [2014] Luppino et al. [2014]; Zai et al. [2012b] Kretschmer et al. [2014]; Musci et al. [2014]; Perroud et al. [2010] Tiihonen et al. [2015] Dick et al. [2008]; Dick et al. [2011]; Latendresse et al. [2011] Albaugh et al. [2010]; Calati et al. [2011]; Flory et al. [2007]; Hirata et al. [2013]; Kang et al. [2008]; Kulikova et al. [2008]; Perroud et al. [2010]; Rujescu et al. [2003]; Shehzad et al. [2012] Chen et al. [2014] Miodovnik et al. [2012]
ESR1 FKBP5 GABRA2
estrogen receptor 1 FK506 binding protein 5 gamma-aminobutyric acid (GABA) A receptor, alpha 2 5-hydroxytryptamine (serotonin) receptor 1A, G protein-coupled 5-hydroxytryptamine (serotonin) receptor 1B, G protein-coupled 5-hydroxytryptamine (serotonin) receptor 2A, G protein-coupled
Anger Aggressive and violent behavior Externalizing behavior
5-hydroxytryptamine (serotonin) receptor 2C, G protein-coupled monoamine oxidase A
—
AKAP5 ANK3 AP2B
CYP19 CYP1B1 DARPP32 DBH
HTR1A HTR1B HTR2A
HTR2C MAOA
— Aggressive behavior, anger and hostility Aggression, anger, hostility and criminality
Aggression, anger, externalizing behavior, impulsivity, hostility, use of weapons, delinquent, violent and criminal behaviors.
Chen et al. [2005]; Guo et al. [2007]; Zai et al. [2012a] Retz et al. [2003] Boutwell and Beaver [2008];Buchmann et al. [2014]; Dmitrieva et al. [2011]; Farbiash et al. [2014]; Hohmann et al. [2009]; Kang et al. [2008]; Marsman et al. [2013]; Nobile et al. [2007]; Schlomer et al. [2015]; Zai et al. [2012a] Vermeersch et al. [2013] Bevilacqua et al. [2012] Dick et al. [2009] Keltikangas-Jarvinen et al. [2008]; Serretti et al. [2007] Conner et al. [2010]; Hakulinen et al. [2013]; Zouk et al. [2007] Banlaki et al. [2015]; Berggard et al. [2003]; Dijkstra et al. [2013]; Giegling et al. [2006]; Keltikangas-Jarvinen et al. [2008]; Mik et al. [2007] Serretti et al. [2007] Antypa et al. [2013]; Armstrong et al. [2014]; Beaver and Holtfreter [2009]; Beaver et al. [2010a]; Beaver et al. [2010b]; Beaver et al. [2014]; Edwards et al. [2010]; Eisenberger et al. [2007]; Frazzetto et al. [2007]; Gallardo-Pujol et al. [2013];Gorodetsky et al. [2014];
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Gene symbol
Gene name
Associated phenotype
NOS1
nitric oxide synthase 1 (neuronal)
Impulsive aggressivity and aggression
NOS3 OXTR SLC6A4 (5HTT)
nitric oxide synthase 3 (endothelial cell) oxytocin receptor solute carrier family 6 (neurotransmitter transporter), member 4 (serotonin transporter)
Aggressive behavior Aggression Aggression, anger, impulsive aggressivity, hostility, neuroticism, externalizing behavior, violence, delinquency and criminality
SLC6A3 (DAT1)
SLIT2 TBX19 TH TPH1
solute carrier family 6 (neurotransmitter transporter), member 3 (dopamine transporter) slit homolog 2 (Drosophila) T-box 19 tyrosine hydroxylase tryptophan hydroxylase 1
Externalizing behavior, pathological violence, serious delinquency and criminal conduct Anger Angry hostility Angry hostility and neuroticism Aggression, aggressive behavior, anger and violence
TPH2
tryptophan hydroxylase 2
Anger
adverse environmental factors (e.g., maltreatment, childhood trauma, social exclusion) contributing to the occurrence of aggressive behaviors such as aggression, violent behavior, criminal and delinquent behavior and anger proneness [Frazzetto et al., 2007; Reif et al., 2007; Weder et al., 2009; Beaver et al., 2010b; Edwards et al., 2010; Gallardo-Pujol et al., 2013; Pickles et al., 2013; Armstrong et al., 2014; Gorodetsky et al., 2014]. These findings are consistent, since only a few studies have failed to replicate the results or have identified high activity alleles associated with aggressive behaviors, some of them performed only in females [Huizinga et al., 2006; Sjoberg et al., 2007; Yang et al., 2007; Beaver and Holtfreter, 2009; van der Vegt et al., 2009; Perroud et al., 2010; Verhoeven et al., 2012]. Studies evaluating the MAOA uVNTR in ODD and CD only identified associations with CD and conduct problems in the presence of adverse childhood environments [Foley et al., 2004; Haberstick et al., 2005; Young et al., 2006;
References (positive and negative associations) Guo et al. [2008]; Huizinga et al. [2006]; Kuepper et al. [2013]; Manuck et al. [2000, 2002]; Perroud et al. [2010]; Pickles et al. [2013]; Pingault et al. [2013]; Reif et al. [2007]; Sjoberg et al. [2007]; Tiihonen et al. [2015]; van der Vegt et al. [2009]; Verhoeven et al. [2012]; Weder et al. [2009]; Yang et al. [2007] Reif et al. [2009]; Retz et al. [2010]; Rujescu et al. [2008] Rujescu et al. [2008] Malik et al. [2012, 2014] Aslund et al. [2013]; Baca-Garcia et al. [2004]; Beitchman et al. [2006]; Cadoret et al. [2003]; Conway et al. [2012]; Davidge et al. [2004]; Gerra et al. [2005]; Gonda et al. [2009]; Greenberg et al. [2000]; Gyurak et al. [2013]; Haberstick et al. [2006]; Hohmann et al. [2009]; Liao et al. [2004]; Lopez-Castroman et al. [2014]; Nobile et al. [2007]; Perroud et al. [2010]; Reif et al. [2007]; Retz et al. [2004]; Sysoeva et al. [2009]; Terracciano et al. [2009]; Verona et al. [2006]; Yang et al. [2010]; Zalsman et al. [2001]; Zimmermann et al. [2009] Beaver et al. [2008]; Chen et al. [2005]; Guo et al. [2007]; Young et al. [2002]; Zai et al. [2012a] Sokolowski et al. [2010] Wasserman et al. [2007] (Persson et al. [2000] Evans et al. [2000]; Hennig et al. [2005]; Manuck et al. [1999]; Reuter and Hennig [2005]; Perroud et al. [2010]; Rotondo et al. [1999]; Rujescu et al. [2002]; Yang et al. [2010] Mann et al. [2008]; Yang et al. [2010]; Yoon et al. [2012]
Prom-Wormley et al., 2009; Qian et al., 2009; Derringer et al., 2010; Wakschlag et al., 2010; Fergusson et al., 2011; McGrath et al., 2012; Kieling et al., 2013]. The low activity alleles have also been found associated with ASPD and antisocial behavior [Nilsson et al., 2006; Williams et al., 2009; Beach et al., 2010; Reti et al., 2011], and also in the presence of adverse environments (e.g., sexual assault, abuse, maltreatment) in GxE studies [Caspi et al., 2002; Derringer et al., 2010; Cicchetti et al., 2012; Fergusson et al., 2012]. However, some studies did not replicate those results in antisocial behavior [Widom and Brzustowicz, 2006; Prichard et al., 2007; Haberstick et al., 2014]. Psychopathy seems to be also associated with MAOA low activity alleles, and in some cases in the presence of children maltreatment [Kim-Cohen et al., 2006; Fowler et al., 2009; Sadeh et al., 2013]. Recent meta-analyses have evaluated the contribution of the MAOA uVNTR to aggressive behavior with positive results. Ficks and Waldman [2014] identified association between the low
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TABLE II. Candidate Genes Investigated in CD, ODD, CU, Antisocial Behavior and Psychopathy Gene symbol AR AVPR1A BDNF
androgen receptor arginine vasopressin receptor 1A brain-derived neurotrophic factor
Antisocial behavior — ODD, CU and psychopathy
COMT
catechol-O-methyltransferase
CD
DBH
—
DRD2
dopamine beta-hydroxylase (dopamine beta-monooxygenase) dopamine receptor D2
—
DRD4
dopamine receptor D4
CD, ODD and CU
ESR1
estrogen receptor 1
HTR1B
5-hydroxytryptamine (serotonin) receptor 1B, G protein-coupled 5-hydroxytryptamine (serotonin) receptor 2A, G protein-coupled monoamine oxidase A
Antisocial behavior, neuroticism and psychoticism CD
HTR2A MAOA
NR4A2 OXTR
Gene name
nuclear receptor subfamily 4, group A, member 2 oxytocin receptor
Associated phenotype
CU and antisocial behavior CD and ODD with adverse childhood environment, Antisocial behavior, ASPD, conduct problems and psychopathy
Antisocial behavior CD and CU
SLC6A4 (5HTT)
solute carrier family 6 (neurotransmitter transporter), member 4 (serotonin transporter)
CD, conduct problems, antisocial behavior and psychopathy
SLC6A3 (DAT1)
solute carrier family 6 (neurotransmitter transporter), member 3 (dopamine transporter) synaptosomal-associated protein, 25kDa transcription factor AP-2 beta (activating enhancer binding protein 2 beta) tryptophan hydroxylase 1 tryptophan hydroxylase 2
ODD and conduct problems
SNAP25 TFAP2B TPH1 TPH2
References (positive and negative associations) Prichard et al. [2007] Prichard et al. [2007] Kourmouli et al. [2013]; Willoughby et al. [2013] Caspi et al. [2008]; DeYoung et al. [2010]; Fowler et al. [2009]; Qian et al. [2009] Prichard et al. [2007] Beaver et al. [2007]; Prichard et al. [2007] Beaver et al. [2007]; Kirley et al. [2004]; Martel et al. [2011]; Nikitopoulos et al. [2014]; Zohsel et al. [2014] Prichard et al. [2007]; Westberg et al. [2003] Jensen et al. [2009]; Moul et al. [2013]; Wang et al. [2012] (Burt and Mikolajewski [2008]; Moul et al. [2013] Beach et al. [2010]; Byrd and Manuck, 2014]; Caspi et al. [2002]; Cicchetti et al. [2012]; Derringer et al. [2010]; Fergusson et al. [2012]; Fergusson et al. [2011]; Foley et al. [2004]; Fowler et al. [2009]; Haberstick et al. [2005]; Haberstick et al. [2014]; Kieling et al. [2013]; Kim-Cohen et al. [2006]; McGrath et al. [2012]; Nilsson et al. [2006]; Philibert et al. [2011]; Prichard et al. [2007]; Prom-Wormley et al. [2009]; Qian et al. [2009]; Reti et al. [2011]; Sadeh et al. [2013]; Wakschlag et al. [2010]; Williams et al. [2009]; Young et al. [2006]; Widom and Brzustowicz [2006] Prichard et al. [2007] Beitchman et al. [2012]; Dadds et al. [2014]; Malik et al. [2012]; Prichard et al. [2007]; Sakai et al. [2012]; Smearman et al. [2015] Brody et al. [2011]; Cicchetti et al. [2012]; Ficks and Waldman [2014]; Fowler et al. [2009]; Garcia et al. [2010]; Malmberg et al. [2008]; Sadeh et al. [2013]; Sakai et al. [2006, 2007, 2010] Burt and Mikolajewski [2008]; Jorm et al. [2001]; Lee et al. [2007]
Antisocial personality disorder Antisocial behavior
Basoglu et al. [2011] Prichard et al. [2007]
— —
Cicchetti et al. [2012] Burt and Mikolajewski [2008]
CD, conduct disorder; ODD, oppositional defiant disorder; CU, callous-unemotional; ASPD, antisocial personality disorder.
FERNANDEZ-CASTILLO AND CORMAND activity alleles of this polymorphism and antisocial behavior (P ¼ 1.37e-06). Also, Byrd and Manuck [2014] found low activity alleles of MAOA uVNTR associated with antisocial behavior in the presence of childhood maltreatment (P ¼ 8e-07). Additionally, several SNPs in the MAOA gene have been associated with physical aggression and anger, and another VNTR with a 10-bp repeat unit and also located in the promoter region was associated with ASPD in the presence of childhood abuse [Philibert et al., 2011; Antypa et al., 2013; Pingault et al., 2013]. In contrast, no association with aggression has been detected for the MAOB gene, encoding monoamine oxidase B, with more restrictive substrates than monoamine oxidase A [Antypa et al., 2013]. The COMT gene encodes catechol-o-methyltransferase, involved in the metabolism of dopamine, epinephrine and norepinephrine. The Val/Val genotype of the common variation p. Val158Met (rs4680G>A) has been associated with aggression, externalizing behavior and anger in several studies, in one of them influenced by sexual abuse [Kulikova et al., 2008; Perroud et al., 2010; Shehzad et al., 2012]. However, other studies failed to replicate these results or found the Met allele associated with anger and aggressive behavior [Rujescu et al., 2003; Flory et al., 2007; Kang et al., 2008; Albaugh et al., 2010]. The Val allele and the Val/ Val genotype have been associated with CD and ODD [Caspi et al., 2008; DeYoung et al., 2010; Qian et al., 2009], but not with total psychopathy scores, only with emotional dysfunction [Fowler et al., 2009]. Other polymorphisms in the COMT gene have also been associated with aggression and anger reactions [Calati et al., 2011; Hirata et al., 2013], and an epistatic interaction of COMT, TPH, and HTR2A genes was found with ASPD [Cuartas Arias et al., 2011]. Other less studied genes encoding enzymes involved in the synthesis or degradation of serotonin and dopamine are TPH1, TPH2, DBH, and TH. No consistent results have been observed for the TPH1 gene [Manuck et al., 1999; Rotondo et al., 1999; Evans et al., 2000; Rujescu et al., 2002; Hennig et al., 2005; Reuter and Hennig, 2005; Perroud et al., 2010; Yang et al., 2010; Cicchetti et al., 2012] nor for TPH2 [Burt and Mikolajewski, 2008; Mann et al., 2008; Perroud et al., 2010; Yang et al., 2010; Yoon et al., 2012]. One study identified association of a functional SNP (rs1611115) in the promoter region of the DBH gene with aggressive hostility and impulsivity, whereas another one investigated a dinucleotide repeat (TG) also in the promoter region of this gene but did not detect association with antisocial behavior [Hess et al., 2009; Prichard et al., 2007]. Another repeat (TCAT) located in the first intron of the TH gene was found associated with angry hostility [Persson et al., 2000]. The serotonin transporter is encoded by the SLC6A4 gene, also known as 5HTT. A functional polymorphism in the promoter region, called 5-HTTLPR (Serotonin-Transporter-Linked Polymorphic Region) has been associated with aggressive behavior. It consists of a degenerate repeat with more than 10 variants, although researchers usually report only two, the short (S) and the long (L) alleles. The short variant (S), driving lower transcription levels, and the SS genotype, have been found associated with aggressive behavior, aggression, anger, hostility, impulsive aggressivity, neuroticism, violent behavior and criminality [Greenberg et al., 2000; Liao et al., 2004; Retz et al., 2004; Gerra et al., 2005;
7 Haberstick et al., 2006; Verona et al., 2006; Reif et al., 2007; Gonda et al., 2009; Sysoeva et al., 2009; Zimmermann et al., 2009; Conway et al., 2012; Gyurak et al., 2013; Lopez-Castroman et al., 2014]. However, other studies report divergent conclusions, associating the long (L) variant with some of these phenotypes [Zalsman et al., 2001; Cadoret et al., 2003; Beitchman et al., 2006; Nobile et al., 2007; Aslund et al., 2013] or not finding significant associations with 5-HTTLPR at all [Baca-Garcia et al., 2004; Davidge et al., 2004; Terracciano et al., 2009; Perroud et al., 2010; Yang et al., 2010]. The S variant was also found associated with conduct problems and CD [Sakai et al., 2006, 2010; Malmberg et al., 2008; Brody et al., 2011], although another study failed to detect significant results [Sakai et al., 2007]. Studies evaluating antisocial behavior also identified association with the S allele [Garcia et al., 2010; Cicchetti et al., 2012], and in a recent meta-analysis the short allele of 5-HTTLPR was significantly associated with antisocial behavior [Ficks and Waldman, 2014]. In contrast, in the case of psychopathy the associated allele is the longer variant [Fowler et al., 2009; Sadeh et al., 2013]. Also, a 12-bp VNTR in intron 2 of the 5HTT gene has been found associated with aggression and antisocial behavior [Davidge et al., 2004; Garcia et al., 2010]. Regarding serotonin receptors, several SNPs in the HTR2A gene associate with aggression, criminality, hostility and anger [Berggard et al., 2003; Giegling et al., 2006; Mik et al., 2007; KeltikangasJarvinen et al., 2008; Dijkstra et al., 2013; Banlaki et al., 2015] and also with CU and antisocial behavior [Burt and Mikolajewski, 2008; Moul et al., 2013]. Variants and haplotypes in the HTR1B gene have been associated with aggressive behavior, anger and hostility [Zouk et al., 2007; Conner et al., 2010; Hakulinen et al., 2013] and also with CD and CU, but not with ASPD [Jensen et al., 2009; Wang et al., 2012; Moul et al., 2013]. However, Perroud et al. did not observe significant associations between HTR2A and HTR1B variants and anger [Perroud et al., 2010]. In contrast with those genes, no associations were identified for HTR1A and HTR2C [Serretti et al., 2007; Keltikangas-Jarvinen et al., 2008]. The dopamine transporter is encoded by the SLC6A3 gene (or DAT1). A 40bp VNTR located in the 3’UTR region has been associated with criminal conduct, pathological violence and violent delinquency, the 10-repeat allele (10R) being the risk factor [Chen et al., 2005; Guo et al., 2007; Beaver et al., 2008], although 9R was the risk allele for externalizing behavior [Young et al., 2002]. However, another study did not succeed in identifying association with aggression [Zai et al., 2012a]. This VNTR polymorphism was also associated with ODD and antisocial behavior, but not with CD [Jorm et al., 2001; Lee et al., 2007; Burt and Mikolajewski, 2008]. Another dopaminergic gene, DRD4, encoding the dopamine receptor D4, has been associated with several aggression phenotypes. The 7-repeat allele of a 48bp VNTR in exon 3 of the gene is associated with aggression, aggressive behavior, delinquency, externalizing behavior [Nobile et al., 2007; Boutwell and Beaver, 2008; Hohmann et al., 2009; Dmitrieva et al., 2011; Buchmann et al., 2014; Farbiash et al., 2014; Schlomer et al., 2015] and with ODD and CD [Kirley et al., 2004; Nikitopoulos et al., 2014; Zohsel et al., 2014]. Others found association between the 4-repeat allele and externalizing behavior and anger [Kang et al., 2008; Marsman et al., 2013], whereas Zai et al. [2012a] did not observe a significant association between this VNTR and aggression. Another polymor-
8 phism in the promoter region (a 120-bp repeat) was associated with ODD in the presence of inconsistent parenting [Martel et al., 2011]. The gene for the dopamine receptor 2 (DRD2) has also been associated with violent delinquency and aggressive behavior [Chen et al., 2005; Guo et al., 2007; Zai et al., 2012a], but not with antisocial behavior [Prichard et al., 2007]. A study performed by Beaver et al. did not detect independent associations between the DRD2 or DRD4 genes and CD or antisocial behavior, but identified an interaction between both genes that predicted both phenotypes [Beaver et al., 2007]. Also, an epistatic interaction of the S allele of the 5-HTTLPR variation with the 7R allele of the DRD4 VNTR associated with aggressive and delinquent behavior [Hohmann et al., 2009]. Another study identified an association between DRD3 and impulsivity in violent offenders [Retz et al., 2003]. Other genes related to dopamine, involved in signaling or in the regulation of transcription of dopaminergic genes, have also been associated with aggressive behaviors: DARPP32 was associated with anger and amygdala volume [Reuter et al., 2009], AP2B with aggression [Damberg et al., 2000], and NR4A2 and TFAP2B with antisocial behavior in women [Prichard et al., 2007]. Hormone regulation. Since aggressive behaviors are more frequently found in males than in females, genes involved in hormone regulation responsible for sex differences and genderrelated behaviors have been usual suspects in the study of the genetics of aggression. The most studied ones are those involved in steroid hormone regulation (androgen and estrogen receptors) and neurohypophysial hormones involved in sexual reproduction, and gender and social behaviors (vasopressin and its receptors, and oxytocin receptor) (Tables I and II). The androgen receptor is encoded by the AR gene. The shorter alleles of a trinucleotide repeat in exon 1 were found to be associated with violent criminal behavior and aggression in men [Jonsson et al., 2001; Cheng et al., 2006; Rajender et al., 2008]. This polymorphism was also found associated with antisocial behavior [Prichard et al., 2007]. Regarding the ESR1 gene, encoding the estrogen receptor 1, two SNPs and a microsatellite (a TG repeat in the 50 region) have been associated with anger, neuroticism, indirect aggression and antisocial behavior [Westberg et al., 2003; Prichard et al., 2007; Vermeersch et al., 2013]. Variants in genes coding for vasopressin and its receptors (AVP, AVPR1A and AVPR1B) have been reported to be associated with aggression. Two SNPs in the AVP and AVPR1A genes were associated with pervasive aggression only in males, whereas a microsatellite (RS1) in the promoter region of AVPR1A was associated with aggression and anger [Malik et al., 2014; Moons et al., 2014]. An association identified between reactive aggression and the SNP rs35369693 in the AVPR1B gene was replicated by other authors in the same phenotype [Zai et al., 2012b; Luppino et al., 2014]. Two microsatellites repeats in the AVPR1A genes were studied, but no association was found with antisocial behavior [Prichard et al., 2007]. SNPs in the oxitocin receptor gene (OXTR) have been associated with aggression [Malik et al., 2012, 2014], and also with conduct problems and CD [Sakai et al., 2012; Smearman et al., 2015], as well as CU [Beitchman et al., 2012; Dadds et al., 2014]. However, two studies did not identify associations between variants in this gene and antisocial behavior or CU [Prichard et al., 2007; Malik et al., 2012]. A SNP and a haplotype in the gene
AMERICAN JOURNAL OF MEDICAL GENETICS PART B encoding the corticotropin releasing hormone receptor 1 (CRHR1) have been associated with aggressive behavior [Chen et al., 2014]. A study investigated functional polymorphisms in the genes CYP17, CYP19, and CYP1B1, which control the major enzymatic steps in sex steroid synthesis and metabolism, and identified association between externalizing problems in boys and a functional SNP in the CYP19 gene which affects levels of estradiol and testosterone (rs10046 in the 30 UTR) [Miodovnik et al., 2012]. Several SNPs in the ABCG1 gene, encoding a transporter involved in cholesterol and sterol homeostasis, showed association with aggression and anger [Gietl et al., 2007]. The FKBP5 gene, coding for a chaperone involved in glucocorticoid response, was associated with aggressive and violent behavior in individuals exposed to childhood trauma [Bevilacqua et al., 2012]. Other genes. Other candidate genes have been investigated in aggressive behaviors (Tables I and II). Thus, genes involved in neurotransmission, such as those related to acetylcholine (Ach), nitric oxide (NO) and GABA, have also been found associated with aggression. The CHRM2 gene, encoding the cholinergic muscarinic receptor 2, was associated with externalizing behavior [Dick et al., 2008, 2011; Latendresse et al., 2011]. Also, variants in the gene cluster of cholinergic nicotinic receptors CHRNA5/ CHRNA3/CHRNB4 were found associated with the same phenotype [Stephens et al., 2012]. Regarding the NO neurotransmitter, polymorphic variants in the NOS1 gene, encoding the nitric oxide synthase 1, have been associated with aggressive behavior, aggression and impulsivity in offenders [Rujescu et al., 2008; Reif et al., 2009; Retz et al., 2010], and a NOS3 haplotype displayed association with increased aggression [Rujescu et al., 2008]. The GABA receptor encoded by GABRA2 was found associated with high externalizing behavior [Dick et al., 2009]. A common variant in the promoter region of the adrenergic alpha 2A receptor gene (ADRA2A) was found associated with aggression, irritability and hostility [Comings et al., 2000]. Also, genes involved in neuroadaptations have been assessed. Variants in BDNF (brain-derived neurotrophic factor) have been associated with aggressive behavior and aggression and impulsivity, but not with anger [Perroud et al., 2010; Kretschmer et al., 2014; Musci et al., 2014]. A BDNF SNP (rs6265, p.Val66Met) was also associated with ODD and CU, and psychopathology, but the identity of the risk allele is not consistent across studies [Kourmouli et al., 2013; Willoughby et al., 2013]. Sokolowski et al. investigated 14 genes involved in axonal guidance and identified an association of anger with SLIT2 [Sokolowski et al., 2010], which has been shown to determine dopaminergic and serotonergic circuits in the forebrain [Bagri et al., 2002; Lin et al., 2005]. A SNP in the gene encoding the A-kinase-anchoring protein 5 (AKAP5), a post-synaptic multi-adaptor molecule, was found associated with aggressive behavior and anger [Richter et al., 2011]. The ankyrin 3 gene (ANK3), which is involved in neuronal activity, showed association with externalizing behavior [Logue et al., 2013]. The synaptosomal-associated protein 25 gene (SNAP25), related to neurotransmitter release, was found associated with antisocial personality disorder [Basoglu et al., 2011]. CDH13, encoding cadherin 13, a neuronal membrane adhesion protein, showed association with extremely violent behavior [Tiihonen et al., 2015].
FERNANDEZ-CASTILLO AND CORMAND Finally, another study identified association between angry and hostility and the gene TBX19, encoding the hypothalamicpituitary-adrenocortical axis regulatory factor [Wasserman et al., 2007].
Genome-Wide Association Studies Only a few association studies of aggression have been performed using hypothesis-free approaches through GWAS. Although none of them identified genome-wide significant findings, we will discuss a number of suggestive associations. These studies have allowed the identification of genes involved in aggressive behavior that had not been considered in any previous GCAS. Interestingly, several of them are involved in synaptic plasticity. Mick et al. performed two GWAS on aggressive traits. The first one assessed the Child Behavior Checklist Dysregulation Profile (CBCL-DP), including three different clinical subscales (aggressive behavior, anxiety/depression, attention problems), on ADHD affected family trios [Mick et al., 2011]. CBCL-DL elevated scores increase susceptibility to aggressive behavior and psychopathology. Increased aggressive behavior scores were found nominally associated with several genes, including LRRC7, STIP1 and TMEM132D. These three genes are involved in neuronal excitability, astrocyte differentiation and anxiety-related behaviors, respectively. The second GWAS reported investigated proneness to anger by assessing measures of angry temperament and angry reaction [Mick et al., 2014]. A nominal association was found between anger and the FYN gene, involved in calcium influx and release in the postsynaptic density and also in long-term potentiation. The long-term potentiation pathway could play a role in aggressive behavior both in children and in adults, since FYN, LRRC7 and STIP1, identified in this GWAS, as well as other nominally associated genes in the previous one, such as BDNF, NTRK2 and CAMK2A, are mediators in this pathway [Mick et al., 2011, 2014]. In a third GWAS, Merjonen et al. [2011] assessed hostility in adolescents and in adult males, considering three dimensions: anger, cynicism and paranoia. They identified several SNPs showing nominal associations with anger, several of them located in the PURG and SHISA6 genes. However, little is known about the actual function of the encoded proteins [Merjonen et al., 2011]. Anney et al. [2008] performed a family-based genome-wide study considering three measures of conduct problems. They identified nine genes nominally associated with conduct problems considering the dominant, recessive or additive models: A2BP1, c12orf28, FLJ39061, KIRREL3, LOC729257, PAWR, PKD1L2, PKD1L3 and RGL1. A2BP1 and KIRREL3 encode proteins involved in neuron development and synaptic plasticity, respectively, and PAWR participates in the regulation of dopamine receptor D2 signaling. However, little is known about the function of the other genes in the brain. Another GWAS studied the interaction between genes and environmental risk factors (GxE) [Sonuga-Barke et al., 2008], and found nominal associations between CD and mother’s warmth interacting with several variants in five genes: RIT1, ADH1C, SLC6A1, A2BP1, and MFHAS1. The SLC6A1 gene codes for a GABA transporter, and the proteins encoded by RIT1 and A2BP1 are involved in neuronal development and regeneration. Interestingly, A2BP1 was also associated with CD in the GWAS discussed above [Anney et al., 2008].
9 A meta-analysis of ADHD GWAS data showed that polygenic risk for ADHD was higher in ADHD with CD, and that it was mainly associated with aggression [Hamshere et al., 2013]. In another GWAS, Tielbeek et al. [2012] assessed antisocial behavior in adults using a self-report questionnaire. The top signal from that GWAS implicated DYRK1A (with 30 SNPs showing pvalues
