Human monoamine oxidase gene (MAOA): chromosome position (Xp21-p11) and DNA polymorphism

The association of genetic polymorphisms within the dopaminergic system with nicotine dependence: A narrative review

Nicotine, the main compound in cigarettes, leads to smoking addiction. Nicotine acts on the limbic dopamine reward loop in the midbrain by binding to nicotinic acetylcholine receptors, promoting the release of dopamine, and resulting in a rewarding effect or satisfaction. This satisfaction is essential for continued and compulsive tobacco use, and therefore dopamine plays a crucial role in nicotine dependence. Numerous studies have identified genetic polymorphisms of dopaminergic pathways which may influence susceptibility to nicotine addiction. Dopamine levels are greatly influenced by synthesis, storage, release, degradation, and reuptake-related genes, including genes encoding tyrosine hydroxylase, dopamine decarboxylase, dopamine transporter, dopamine receptor, dopamine 3-hydroxylase, catechol-O-methyltransferase, and monoamine oxidase. In this paper, we review research progress on the effects of polymorphisms in the above genes on downstream smoking behavior and nicotine dependence, to offer a theoretical basis for the elucidation of the genetic mechanism underlying nicotine dependence and future personalized treatment for smoking cessation.

Expanding the phenotype of Brunner syndrome from childhood to adulthood: Description of the second pediatric patient and his mother

Brunner syndrome is a recessive X-linked disorder caused by pathogenic variants in the monoamine oxidase A gene (MAOA). It is characterized by distinctive aggressive behavior, mild intellectual disability, sleep disturbances, and typical biochemical alterations deriving from the impaired monoamine metabolism. We herein describe a 5-year-old boy with developmental delay, autistic features, and myoclonic epilepsy, and his mother, who had mild intellectual disability and recurrent episodes of palpitations, headache, abdominal pain, and abdominal bloating. Whole exome sequencing allowed detection of the maternally-inherited variant c.410A>G, (p.Glu137Gly) in the MAOA gene. The subsequent biochemical studies confirmed the MAOA deficiency both in the child and his mother. Given the serotonergic symptoms associated with high serotonin levels found in the mother, treatment with a serotonin reuptake inhibitor and dietary modifications were carried out, resulting in regression of the biochemical abnormalities and partial reduction of symptoms. Our report expands the phenotypic spectrum of Brunner disease, bringing new perspectives on the behavioral and neurodevelopmental phenotype from childhood to adulthood.

Genetic and epigenetic serotonergic markers predict the ability to recognize mental states

The serotonergic (5-HT) system is related to affective and cognitive processes and explains behavioral variability in the normal and psychopathological range. For this reason, the hypothesis was put forward that genetic and epigenetic markers related to 5-HT metabolism predict individual differences in social cognitive functioning. Social cognitions are complex mental processes necessary for perceiving, interpreting and reacting to the behaviors of others. In order to test this hypothesis one of the most prominent theory of mind tasks, the reading the mind in the eye test (RMET), was administered to N = 435 participants and measures of performance were related to the functional MAO-A VNTR polymorphism (relevant for 5-HT catabolism) and to epigenetic markers in the promoter of the TPH-2 gene (relevant for 5-HT synthesis). It was postulated that genetic and epigenetic markers of high 5-HT activity are positively related to RMET performance. Results show that the MAO-A high activity allele, together with the degree of methylation at a promoter CpG site on the TPH-2 gene explain significant proportions of variance in the RMET performance even after controlling for age and sex effects. Present findings yield evidence for the importance of 5-HT for social cognition. Based on additional findings, the role of a TRP-rich diet for theory of mind functions is discussed.

The role of monoamine oxidase A in the neurobiology of aggressive, antisocial, and violent behavior: A tale of mice and men

Over the past two decades, research has revealed that genetic factors shape the propensity for aggressive, antisocial, and violent behavior. The best-documented gene implicated in aggression is MAOA (Monoamine oxidase A), which encodes the key enzyme for the degradation of serotonin and catecholamines. Congenital MAOA deficiency, as well as low-activity MAOA variants, has been associated with a higher risk for antisocial behavior (ASB) and violence, particularly in males with a history of child maltreatment. Indeed, the interplay between low MAOA genetic variants and early-life adversity is the best-documented gene × environment (G × E) interaction in the pathophysiology of aggression and ASB. Additional evidence indicates that low MAOA activity in the brain is strongly associated with a higher propensity for aggression; furthermore, MAOA inhibition may be one of the primary mechanisms whereby prenatal smoke exposure increases the risk of ASB. Complementary to these lines of evidence, mouse models of Maoa deficiency and G × E interactions exhibit striking similarities with clinical phenotypes, proving to be valuable tools to investigate the neurobiological mechanisms underlying antisocial and aggressive behavior. Here, we provide a comprehensive overview of the current state of the knowledge on the involvement of MAOA in aggression, as defined by preclinical and clinical evidence. In particular, we show how the convergence of human and animal research is proving helpful to our understanding of how MAOA influences antisocial and violent behavior and how it may assist in the development of preventative and therapeutic strategies for aggressive manifestations.

Association Analysis of Monoamine Oxidase-A Gene Promoter Polymorphism (MAOA uVNTR) for Antisocial Behavior: Absence of the Counting Number Repeats in Central Iran

Background: The MAOA gene is located on the X chromosome (Xp11.23). Several studies have established a VNTR (Variable Number Tandem Repeat) polymorphism in the upstream of the MAOA gene transcriptional initiation region named uVNTR which is correlated with the risk of antisocial behavior. Objectives: This study aimed to investigate the association between MAOA genotypes and the risk of violent behavior in a cohort of violent and age-matched non-violent individuals. Methods: In the current case-control study, MAOA uVNTR was genotyped in a cohort of 88 violent and 95 age-matched non-violent individuals. Individuals were genotyped for the MAOA uVNTR by performing PCR, gel electrophoresis, and sequencing. Furthermore, a chi-square test was performed using SPSS, and a p-value of less than 0.05 was considered statistically significant. Results: We identified three MAOA uVNTR allelic variants: They were harboring 3.5, 4.5, and 5.5 repeated sequences. Alleles with 2, 3, 4, 5, and 6 repeats were not observed in any of the two examined groups. Conclusions: We did not detect a statistically appreciable association between antisocial behavior and allele frequencies in the studied population in central Iran.

Moderator Effects of Life Stress on the Association between MAOA-uVNTR, Depression, and Burnout

BACKGROUND

The serotonergic and noradrenergic systems have a strong impact on several affective disorders and are key targets for psychopharmacological therapy. With respect to pathogenesis, there is a growing body of evidence showing an influence of a promoter repeat polymorphism (MAOA-uVNTR) altering the expression rate of monoamine oxidase A. However, only a few studies investigate its influence on depression with only 2 of them considering the moderating effects of life stress. For burnout, there are no studies so far investigating the genetic basis.

OBJECTIVES

The aim of the present study was to replicate an interaction effect of MAOA-uVNTR and life stress on depression, and extend these possible findings to the burnout syndrome. Especially, the latter one might help in understanding the underlying mechanisms of burnout and its association to depression.

METHOD

A total of n = 1,541 participants (n = 1,099 healthy controls, n = 442 inpatients with affective disorders) provided genetic samples and filled in self-report questionnaires measuring depression, burnout, and the extent of experienced stressful life events (SLEs).

RESULTS

A life stress x MAOA-uVNTR interaction on depression and burnout was observed in women suggesting that carriers of the high expressing allele (MAO-H) with many SLEs had the highest scores in both burnout and depression. In men, there was only a weak effect of MAOA-uVNTR on depression.

CONCLUSIONS

The results suggest a more pronounced reactivity to adverse environmental factors in carriers of the MAO-H allele. Especially the effect of life stress and MAOA-uVNTR on burnout should be independently replicated in the future as this is the first study showing this association.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

Integrated analysis of whole-exome sequencing and transcriptome profiling in males with autism spectrum disorders

BACKGROUND

Autism spectrum disorders (ASD) are a group of neurodevelopmental disorders with high heritability. Recent findings support a highly heterogeneous and complex genetic etiology including rare de novo and inherited mutations or chromosomal rearrangements as well as double or multiple hits.

METHODS

We performed whole-exome sequencing (WES) and blood cell transcriptome by RNAseq in a subset of male patients with idiopathic ASD (n = 36) in order to identify causative genes, transcriptomic alterations, and susceptibility variants.

RESULTS

We detected likely monogenic causes in seven cases: five de novo (SCN2A, MED13L, KCNV1, CUL3, and PTEN) and two inherited X-linked variants (MAOA and CDKL5). Transcriptomic analyses allowed the identification of intronic causative mutations missed by the usual filtering of WES and revealed functional consequences of some rare mutations. These included aberrant transcripts (PTEN, POLR3C), deregulated expression in 1.7% of mutated genes (that is, SEMA6B, MECP2, ANK3, CREBBP), allele-specific expression (FUS, MTOR, TAF1C), and non-sense-mediated decay (RIT1, ALG9). The analysis of rare inherited variants showed enrichment in relevant pathways such as the PI3K-Akt signaling and the axon guidance.

CONCLUSIONS

Integrative analysis of WES and blood RNAseq data has proven to be an efficient strategy to identify likely monogenic forms of ASD (19% in our cohort), as well as additional rare inherited mutations that can contribute to ASD risk in a multifactorial manner. Blood transcriptomic data, besides validating 88% of expressed variants, allowed the identification of missed intronic mutations and revealed functional correlations of genetic variants, including changes in splicing, expression levels, and allelic expression.

Family-based association study between monoamine oxidase A (MAOA) gene promoter VNTR polymorphism and Tourette's syndrome in Chinese Han population

To clarify the association of monoamine oxidase A- variable number of tandem repeat (MAOA-pVNTR) with susceptibility to Tourette's syndrome (TS) in Chinese Han population we discuss the genetic contribution of MAOA-VNTR in 141 TS patients including all their parents in Chinese Han population using transmission disequilibrium test (TDT) design. Our results revealed that no significant association was found in the MAOA gene promoter VNTR polymorphism and TS in Chinese Han population (TDT = 1.515, df = 1, p > 0.05). The negative result may be mainly due to the small sample size, but we don't deny the role of gene coding serotonergic or monoaminergic structures in the etiology of TS.

Gene-sex interactions in schizophrenia: focus on dopamine neurotransmission

Schizophrenia is a severe mental disorder, with a highly complex and heterogenous clinical presentation. Our current perspectives posit that the pathogenic mechanisms of this illness lie in complex arrays of gene × environment interactions. Furthermore, several findings indicate that males have a higher susceptibility for schizophrenia, with earlier age of onset and overall poorer clinical prognosis. Based on these premises, several authors have recently begun exploring the possibility that the greater schizophrenia vulnerability in males may reflect specific gene × sex (G×S) interactions. Our knowledge on such G×S interactions in schizophrenia is still rudimentary; nevertheless, the bulk of preclinical evidence suggests that the molecular mechanisms for such interactions are likely contributed by the neurobiological effects of sex steroids on dopamine (DA) neurotransmission. Accordingly, several recent studies suggest a gender-specific association of certain DAergic genes with schizophrenia. These G×S interactions have been particularly documented for catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO), the main enzymes catalyzing DA metabolism. In the present review, we will outline the current evidence on the interactions of DA-related genes and sex-related factors, and discuss the potential molecular substrates that may mediate their cooperative actions in schizophrenia pathogenesis.

The current state of play on the molecular genetics of depression

BACKGROUND

It has been well established that both genes and non-shared environment contribute substantially to the underlying aetiology of major depressive disorder (MDD). A comprehensive overview of genetic research in MDD is presented. Method Papers were retrieved from PubMed up to December 2011, using many keywords including: depression, major depressive disorder, genetics, rare variants, gene-environment, whole genome, epigenetics, and specific candidate genes and variants. These were combined in a variety of permutations.

RESULTS

Linkage studies have yielded some promising chromosomal regions in MDD. However, there is a continued lack of consistency in association studies, in both candidate gene and genome-wide association studies (GWAS). Numerous factors may account for variable results including the use of different diagnostic approaches, small samples in early studies, population stratification, epigenetic phenomena, copy number variation (CNV), rare variation, and phenotypic and allelic heterogeneity. The conflicting results are also probably, in part, a consequence of environmental factors not being considered or controlled for.

CONCLUSIONS

Each research group has to identify what issues their sample may best address. We suggest that, where possible, more emphasis should be placed on the environment in molecular behavioural genetics to identify individuals at environmental high risk in addition to genetic high risk. Sequencing should be used to identify rare and alternative variation that may act as a risk factor, and a systems biology approach including gene-gene interactions and pathway analyses would be advantageous. GWAS may require even larger samples with reliably defined (sub)phenotypes.

The role of the serotonergic system in suicidal behavior

Serotonin is a widely investigated neurotransmitter in several psychopathologies, including suicidal behavior (SB); however, its role extends to several physiological functions involving the nervous system, as well as the gastrointestinal and cardiovascular systems. This review summarizes recent research into ten serotonergic genes related to SB. These genes - TPH1, TPH2, SLC6A4, SLC18A2, HTR1A, HTR1B, HTR2A, DDC, MAOA, and MAOB - encode proteins that are vital to serotonergic function: tryptophan hydroxylase; the serotonin transporter 5-HTT; the vesicular transporter VMAT2; the HTR1A, HTR1B, and HTR2A receptors; the L-amino acid decarboxylase; and the monoamine oxidases. This review employed a systematic search strategy and a narrative research methodology to disseminate the current literature investigating the link between SB and serotonin.

Association between a functional polymorphism in the MAOA gene and sudden infant death syndrome

OBJECTIVES

Abnormalities in the serotonergic as well as the noradrenergic neuronal systems are believed to contribute to sudden infant death syndrome (SIDS). The X-chromosomal monoamine oxidase A (MAOA) gene is of importance for both systems and up to now no systematic study on a functional polymorphism in this gene has been performed in a sufficiently large group.

METHODS

We investigated a functional MAOA promoter length polymorphism in 156 white SIDS cases and 260 gender- and age-matched control subjects by using capillary electrophoresis and fluorescence dye labeled primers.

RESULTS

The pooled low-expressing alleles *2 and *3 were more frequent in the 99 male SIDS cases than in 161 male control subjects (44.4% vs 25.5%). However, there were no differences in female cases. The frequency of low expression alleles varied significantly with the age at death and were significantly more frequent in children who died between an age of 46 and 154 days than at an older age (54.9% vs 22.6%).

CONCLUSIONS

Our results indicate a relationship between SIDS and the MAOA genotype in boys via influencing serotonergic and noradrenergic neurons in the brainstem. This locus is the first X-chromosomal locus associated with SIDS. Our results support the theory that abnormalities in the brainstem contribute to a subset of SIDS, at least in boys. Moreover, we argue that not only the serotonergic system but also other neuronal systems, among those the noradrenergic one, are involved.

Association of the MAOA promoter uVNTR polymorphism with suicide attempts in patients with major depressive disorder

Background

The MAOA uVNTR polymorphism has been documented to affect the MAOA gene at the transcriptional level and is associated with aggressive impulsive behaviors, depression associated with suicide (depressed suicide), and major depressive disorder (MDD). We hypothesized that the uVNTR polymorphism confers vulnerability to MDD, suicide or both. The aim of this study was to explore the association between the MAOA uVNTR and depressed suicide, using multiple controls.

Methods

Four different groups were included: 432 community controls, 385 patients with MDD who had not attempted suicide, 96 community subjects without mental disorders who had attempted suicide, and 109 patients with MDD who had attempted suicide. The MAOA uVNTR polymorphism was genotyped by a PCR technique. The symptom profiles and personal characteristics in each group were also compared.

Results

The MAOA 4R allele was more frequent in males with MDD than in male community controls (χ² = 4.182, p = 0.041). Logistic regression analysis showed that, among the depressed subjects, those younger in age, more neurotic or who smoked had an increased risk of suicide (β = -0.04, p = 0.002; β = 0.15, p = 0.017; β = 0.79, p = 0.031, respectively). Moreover, among those who had attempted suicide, those younger in age, with more paternal overprotection, and more somatic symptoms were more likely to be in the MDD group than in the community group (β = -0.11, p < 0.001; β = 0.15, p = 0.026; β = 1.11, p < 0.001). Structural equation modeling (SEM) showed that nongenetic factors, such as age, paternal overprotection, and somatic symptoms, were associated with MDD, whereas depressed suicide were associated with severity of depression, personality traits, age, marital status, and inversely associated with anxiety symptoms. However, depression did not affect suicidal behavior in the community group.

Conclusion

The MAOA 4R allele is associated with enhanced vulnerability to suicide in depressed males, but not in community subjects. The MAOA 4R allele affects vulnerability to suicide through the mediating factor of depressive symptoms. Further large-scale studies are needed to verify the psychopathology of the relationships among MAOA uVNTR polymorphism, symptom profiles, and suicidal behavior.

Deviant peer affiliation and antisocial behavior: interaction with Monoamine Oxidase A (MAOA) genotype

Although genetic and environmental factors are separately implicated in the development of antisocial behavior (ASB), interactive models have emerged relatively recently, particularly those incorporating molecular genetic data. Using a large sample of male Caucasian adolescents and young adults from the National Longitudinal Study of Adolescent Health (Add Health), the association of deviant peer affiliation, the 30-base pair variable number tandem repeat polymorphism in promoter region of the monoamine oxidase-A (MAOA) gene, and their interaction, with antisocial behavior (ASB) was investigated. Weighted analyses accounting for over-sampling and clustering within schools as well as controlling for age and wave suggested that deviant peer affiliation and MAOA genotype were each significantly associated with levels of overt ASB across a 6-year period. Only deviant peer affiliation was significantly related to covert ASB, however. Additionally, there was evidence suggestive of a gene-environment interaction (G × E) where the influence of deviant peer affiliation on overt ASB was significantly stronger among individuals with the high-activity MAOA genotype than the low-activity genotype. MAOA was not significantly associated with deviant peer affiliation, thus strengthening the inference of G × E rather than gene-environment correlation (rGE). Different forms of gene-environment interplay and implications for future research on ASB are discussed.

Behavioral outcomes of monoamine oxidase deficiency: preclinical and clinical evidence

Monoamine oxidase (MAO) isoenzymes A and B are mitochondrial-bound proteins, catalyzing the oxidative deamination of monoamine neurotransmitters as well as xenobiotic amines. Although they derive from a common ancestral progenitor gene, are located at X-chromosome and display 70% structural identity, their substrate preference, regional distribution, and physiological role are divergent. In fact, while MAO-A has high affinity for serotonin and norepinephrine, MAO-B primarily serves the catabolism of 2-phenylethylamine (PEA) and contributes to the degradation of other trace amines and dopamine. Convergent lines of preclinical and clinical evidence indicate that variations in MAO enzymatic activity--due to either genetic or environmental factors--can exert a profound influence on behavioral regulation and play a role in the pathophysiology of a large spectrum of mental and neurodegenerative disorders, ranging from antisocial personality disorder to Parkinson's disease. Over the past few years, numerous advances have been made in our understanding of the phenotypical variations associated with genetic polymorphisms and mutations of the genes encoding for both isoenzymes. In particular, novel findings on the phenotypes of MAO-deficient mice are highlighting novel potential implications of both isoenzymes in a broad spectrum of mental disorders, ranging from autism and anxiety to impulse-control disorders and ADHD. These studies will lay the foundation for future research on the neurobiological and neurochemical bases of these pathological conditions, as well as the role of gene × environment interactions in the vulnerability to several mental disorders.

Molecular characteristics of a single and novel form of carp (Cyprinus carpio) monoamine oxidase

Two mammalian monoamine oxidases (MAO), MAO-A and MAO-B, are similar in primary structures but have unique substrate/inhibitor selectivities. Carp (Cyprinus carpio) contains a MAO enzyme (C-MAO) with properties different from MAO-A and MAO-B. To determine the molecular characteristics of C-MAO and its phylogenetic relationship with other fish and mammalian MAOs, the primary structure of C-MAO was estimated. The putative C-MAO cDNA encodes 526 amino acids with 59.001 Da, and the deduced amino acid sequence showed as much as 68.9% homology with some mammalian MAO-A proteins, 69.8% homology with some mammalian MAO-B proteins, and as much as 92.4% homology with some fish MAOs. Comparison of two regions in the polypeptide sequence of C-MAO determining possible substrate/inhibitor preferences of MAO-A and MAO-B showed both 79.5% homologies.

Association of MAOA gene functional promoter polymorphism with CSF dopamine turnover and atypical depression

OBJECTIVE

Monoamine oxidase-A (MAO-A) is a key mitochondrial enzyme that metabolizes biogenic amine neurotransmitters such as dopamine and serotonin. Individuals with atypical depression (AD) are particularly responsive to treatment with MAO inhibitors (MAOIs). Biomarker tests are essential for prompt diagnosis of AD, and to identify those with an altered brain neurotransmitter metabolism who may selectively respond to MAOI therapy.

METHODS

In a sample of 118 Scandinavian patients with treatment-resistant depression who are naive to MAOI therapy, we investigated the associations between a common MAOA functional promoter polymorphism (MAOA-uVNTR), cerebrospinal fluid (CSF) neurotransmitter metabolites, and AD susceptibility. The metabolites for dopamine (homovanillic acid, HVA), serotonin (5-hydroxyindoleacetic acid) and noradrenaline (3-methoxy-4-hydroxyphenylglycol) were measured in the CSF.

RESULTS

AD was associated with the female sex and a higher HVA in CSF (P=0.008). The carriers of the MAOA-uVNTR short allele were significantly overrepresented among women with AD (P=0.005; odds ratio=4.76; 95% confidence interval=1.5-13.1; statistical power=80.0%). Moreover, the MAOA-uVNTR genotype significantly influenced the HVA concentration (P=0.01) and showed a strong trend in relation to 5-hydroxyindoleacetic acid concentration (P=0.057) in women. The mediational statistical analyses showed the CSF-HVA concentration as a key driver of the relationship between MAOA-uVNTR genotype and AD.

CONCLUSION

The association of the MAOA-uVNTR with both susceptibility to AD and dopamine metabolite (HVA) concentration lends further biological plausibility for high MAO-A enzyme activity as a mechanistic factor for genetic predisposition to AD through altered dopamine turnover. Our observations provide new evidence on the in-vivo functional significance of the MAOA-uVNTR short allele as a high activity variant.

An association study of monoamine oxidase A (MAOA) gene polymorphism in methamphetamine psychosis

Methamphetamine continues to be the most widely abused drug in Japan. Chronic methamphetamine users show psychiatric signs, including methamphetamine psychosis. Monoamine oxidase A (MAOA) is one of the major enzymes responsible for the degradation of neurotransmitters. Abnormalities in MAO levels have been related to a wide range of psychiatric disorders. We examined whether or not the MAOA-u variable-number tandem repeat (VNTR) has a functional polymorphism in methamphetamine psychosis and whether or not such a polymorphism is related to the prolongation of psychosis. As expected, there was a significant difference in the MAOA-u VNTR between males with persistent versus transient methamphetamine psychosis (p=0.018, odds ratio (OR)=2.76, 95% CI: 1.18-6.46). Our results suggest that the high-activity allele class of MAOA-u VNTR in males may be involved in susceptibility to a persistent course of methamphetamine psychosis. We found no differences among females. The sample size of females with methamphetamine psychosis was too small to have significant analysis.

Direct analysis of candidate genes in impulsive behaviours

Antisocial behaviour is both heterogeneous and the product of interacting genetic and environmental factors acting at different levels of causation. Heritability studies show that individual differences in predisposition to antisocial behaviour are transmitted vertically in families by genetic mechanisms. Owing to aetiological heterogeneity and complexity, study of a variety of other behavioural phenotypes may shed more light on the antecedents of antisocial behaviour than direct studies on antisocial behaviour. Identification of genetic vulnerability factors would clarify mechanisms of vulnerability and the role of the environment. Direct gene analysis and genetic linkage analysis have identified structural variants in genes involved in neurotransmitter function, and some progress has been made towards relating these genetic variants to antisocial personality and other behaviours. Thyroid hormone receptor variants can cause attention deficit/hyperactivity disorder, and a monoamine oxidase A variant leads to aggressive behaviour in one family. Direct gene analyses have revealed non-conservative amino acid substitutions and structural variants (generally rare) at DRD2, DRD3 and DRD4 dopamine receptors and 5-HT1A, 5-HT2A, 5-HT2C and 5-HT7 serotonin receptors. The stage is set to identify the phenotypic significance of these as well as genetic variants at other loci which may be relevant as candidate genes for antisocial behaviour and related behavioural differences.

No allele variation of the MAOA gene promoter in male Chinese subjects with attention deficit hyperactivity disorder

The aim of the current study was to examine any difference in the repeat sequence of the monoamine oxidase A (MAOA) gene promoter between males with attention deficit hyperactivity disorder (ADHD) and randomly selected subjects in a community. The role of the MAOA gene promoter in ADHD cases was also investigated. The total of 244 participants included 57 male ADHD subjects as the case group and 187 males, also in southern Taiwan, as the community controls. There was no significantly different distribution in the repeat sequence of the MAOA gene promoter (chi2 = 3.895, d.f. = 3, p = 0.273), and no significantly different distribution of 'long-form' and 'short-form' alleles between the ADHD group and the male community group was noted (chi2 = 2.484, d.f. = 1, p = 0.115). Some aspects of clinical response are mentioned in the discussion of this study which are worth exploring further in the future.

Childhood maltreatment, subsequent antisocial behavior, and the role of monoamine oxidase A genotype

BACKGROUND

A functional promoter polymorphism in monoamine oxidase A (MAOA) has been implicated as a moderating factor in the relationship between childhood maltreatment and later adolescent and adult antisocial behavior. Despite wide interest in this hypothesis, results remain mixed from the few attempts at replication.

METHODS

Regression-based analyses were conducted to test for a genotype-environment interaction using self-reported physical abuse and MAOA genotype to predict later antisocial behavior and arrests for violence by participants in the National Youth Survey Family Study. We also examined the interaction using a measure of violent victimization. The analysis sample included 277 Caucasian male respondents, aged 11-15 in 1976, who provided buccal swab DNA samples and who were successfully genotyped for the variable number tandem repeat (VNTR) in the MAOA promoter using polymerase chain reaction.

RESULTS

Maltreatment by a parent during adolescence was a risk factor for adolescent and adult antisocial and violence related behavioral problems. Tests for the main effect of MAOA and a MAOA-maltreatment interaction were nonsignificant. Similar results were obtained using the measure of adolescent violent victimization.

CONCLUSIONS

Findings from this general population sample could not confirm the hypothesis that MAOA moderates the relationship between adolescent maltreatment and adolescent or adult antisocial behavior.

A functional polymorphism in the promoter region of the monoamine oxidase A gene is associated with the cigarette smoking quantity in alcohol-dependent heavy smokers

Tobacco smoking represents a leading cause of morbidity and mortality with a strong dose-response relation between the amount of smoking and the risks of tobacco-related diseases and death. The quantity that is smoked is determined predominantly by genetic factors. The present study examined whether there is an association between the quantity of cigarettes smoked and length variation of a functional 30-bp repeat polymorphism in the promoter region of the monoamine oxidase A (MAO-A) gene. The number of 30-bp repeats, which is associated with enzyme activity was assessed in 121 Caucasian men suffering from both alcohol and tobacco dependence. Analysis revealed that the highly active long allele (4 repeat) is associated with a significantly greater amount of cigarette smoking in comparison with the less active short allele (3 repeat). In a logistic regression model (dichotomized), smoking quantity was significantly predicted by MAO-A genotype while no other variable (age, height, body weight, frequency of smoking, quantity and frequency of alcohol consumption) met the significance level. Since tobacco smoke is a potent inhibitor of MAO-A, this result could be regarded as a genotype-related dosage effect. Taken together, in alcohol-dependent heavily smoking men there is evidence for a MAO-A gene-associated effect on the quantity that is smoked as reflected by the daily number of cigarettes consumed.

Science and society: applications of behavioural genetics: outpacing the science?

Human behavioural genetics is an established research discipline of the genomic age, and applications for behavioural genetic information are most likely to emerge in areas such as criminal justice, education, employment and insurance. However, behavioural genetic research into personality traits and antisocial behaviour poses several risks; for example, tentative or preliminary research findings might be misused in legal and commercial settings. Scientific caution, public and media education, expert consultation and confidentiality protection are essential for the responsible use of behavioural genetics.

Association analysis between a functional polymorphism in the monoamine oxidase A gene promoter and severe mood disorders

Monoamine oxidase A (MAOA) has been suggested to be involved in human behaviour and physiology due to its key role in the metabolism of several different biological amines including the neurotransmitters serotonin, norepinephrin and dopamine.Recently, a 30 bp repeat in the MAOA gene promoter (uMAOA) has been demonstrated to be polymorphic and to affect transcriptional activity. In the context of an association case-control study design, we analysed the uMAOA polymorphism in 389 unrelated patients affected by severe mood disorders (88 bipolar subjects and 301 major depressive individuals) and in 156 controls. No association was found between the uMAOA locus and bipolar disorder or major depression. However, an increase of high-activity uMAOA alleles was found in major depression female patients presenting a seasonal pattern (chi2=3.013, P=0.05) or psychotic symptoms in their episodes (chi2=2.679, P=0.07). In female bipolar disorder patients, long alleles were associated with longest times of admission (F=4.604, P=0.037). A trend for association with seasonal pattern was also defined in this group (data not corrected for multiple testing). Our results suggest that MAOA gene variation may modulate the expression of some clinical aspects of severe mood disorders, especially in females, and support the existence of a genetic and aetiologic heterogeneity underlying the diagnoses of bipolar disorder and major depression.

Monoamine Oxidase A and Tryptophan Hydroxylase Gene Polymorphisms

Most of the candidate gene studies in bipolar disorder have focused on the major neurotransmitter systems that are influenced by drugs used in the treatment of this disorder. The monoamine oxidase A (MAOA) and the tryptophan hydroxylase (TPH1, TPH2) genes are two of the candidates that have been tested in a series of association studies using unrelated or family-based controls. This review summarizes the existing association studies regarding these genes. Most of these studies were based on the unrelated case-control design with samples of 50 to 600 subjects. Regarding MAOA, three meta-analyses with partially overlapping samples supported a modest effect of this gene in bipolar disorder in female Caucasians. However, as several studies could not replicate these findings, more work is necessary to demonstrate unequivocally the involvement of MAOA in bipolar disorder and establish the biological mechanism underlying the genetic association. With respect to TPH1 and TPH2, the majority of studies did not provide evidence for an association between these genes and bipolar disorder. The genes are more likely to be related to suicidal behavior than to bipolar disorder.

Association between a promoter variant in the monoamine oxidase A gene and schizophrenia

Monoaminergic transmission has been implicated in the pathophysiology of schizophrenia. We investigated a putative functional promoter polymorphism in the monoamine oxidase A (MAOA) gene in schizophrenic patients (n=133) and control subjects (n=377). In men, there was an association between the less efficiently transcribed alleles and schizophrenia (chi(2)=4.01, df=1, p<0.05). In women, no significant differences were found. The present results support the involvement of the MAOA gene in men with schizophrenia in the investigated Swedish population but should be interpreted with caution until replicated.

Monoamine oxidase polymorphisms and smoking behaviour in Japanese

Although nicotine dependence is one of the primary reasons why smokers cannot quit smoking, nicotine cannot explain all of the psychopharmacological effects of tobacco smoke. Accumulating evidence points to potent inhibition of monoamine oxidase (MAO) which metabolizes neurotransmitters relating to additive behaviour. We have therefore investigated the association between smoking behaviour and MAO ( variable number of tandem repeat in the promoter region and A644G) polymorphisms. The genotypes were examined in 504 Japanese outpatients (217 men and 287 women) who visited Aichi Cancer Centre Hospital. The age-adjusted odds ratios (aORs) were estimated by a logistic model. Among males, we did not find a significant association of the smoking habit with either of the polymorphisms. The median Fargastrom test for nicotine dependence (FTND) score among male current smokers was significantly higher with than without the 4-repeat allele (5.8 and 4.7, respectively). The aOR of FTND 6 versus FTND 6 was 2.72 (95% confidence interval 1.13-6.50) for males with the 4-repeat allele. Among females, the aOR of being current smokers compared to never smokers was 0.49 (0.26-0.93) for individuals with the 4-repeat allele. Our results indicate that the polymorphisms of influence the smoking habit for female, as well as the nicotine dependence and smoking initiation for male smokers. These findings among male smokers support the view that MAO affects a smokers' requirement for nicotine and may explain why some people are predisposed to tobacco addiction and why some individuals find it difficult to stop smoking.

No evidence of an association between a functional monoamine oxidase a gene polymorphism and completed suicides

Monoamine oxidase A (MAOA) has been implicated in the control of aggression and/or impulsivity in humans and been involved in suicide. This gene has a functional polymorphism in which there is a variable number tandem repeat (VNTR) in the upstream region (MAOA-uVNTR). We hypothesized that MAOA dysfunction due to this polymorphism was associated with suicide genetically through the disinhibition of aggression and/or impulsivity. We performed an association study between completed suicides and the MAOA-uVNTR polymorphism. No significant difference in genotype distribution or allele frequencies was found between completed suicides and comparison groups either in males or females. These results show no evidence of an association between the MAOA-uVNTR polymorphism and completed suicides and suggest that MAOA is not involved in the susceptibility to suicide.

Clinical and molecular genetics of ADHD and Tourette syndrome. Two related polygenic disorders

ADHD is a polygenic disorder due to the additive effect of genes affecting dopamine, norepinephrine, serotonin, GABA, and other neurotransmitters. Some of the specific loci involved are dopamine genes--DRD2, DRD4, DRD5, and the dopamine transporter; norepinephrine (NE) and epinephrine (EPI) genes--dopamine beta-hydroxylase, ADRA2A, ADRA2C, PNMT, norepinephrine transporter, MAOA, COMT; serotonin genes--TDO2, HTR1A, HTR1DA, serotonin transporter; GABA genes--GABRB3; androgen receptor and other genes. This model is consistent with all of the present knowledge about ADHD including (a) the increased frequency of ADHD in the relatives of ADHD probands, (b) the presence of a wide spectrum of comorbid behaviors (depression, anxiety, learning, conduct, oppositional-defiant, conduct and substance abuse disorders) in ADHD probands and their relatives on both parental sides, (c) the close relationship to Tourette syndrome (TS), (d) the failure to find the genes for TS using linkage analysis, (e) the brain imaging studies showing hypometabolism of the frontal lobes, (f) the relationship between dopamine D2 receptor density and regional blood flow, (g) the correlation between tics and dopamine D2 receptor density in TS, (h) the motor hyperactivity of dopamine transporter and dopamine D3 receptor gene knockout mice, (i) the LeMoal and Shaywitz dopamine deficiency animal models of ADHD, (j) the NE models of ADHD, (k) the failure to explain ADHD on the basis of any single neurotransmitter defect, (l) the response of ADHD to dopamine and alpha 2-adrenergic agonists, (m) the small percentage of the variance of specific behaviors accounted for by each gene, and numerous other aspects of ADHD. The implications of the polygenic model for the understanding, diagnosis and treatment of ADHD and TS, as well as other psychiatric disorders, are reviewed.

Association analysis of the functional monoamine oxidase A gene promoter polymorphism in psychiatric disorders

Functional characterization studies revealed that transcriptional activity of the human monoamine oxidase A (MAOA) gene is modulated by a polymorphic repetitive sequence located approximately 1.2 kb upstream of the ATG codon. To investigate the possible influence of the allelic variants of the MAOA gene-linked polymorphic region (MAOA-LPR) on the genetic predisposition to psychiatric disorders, we have performed a case-control association study. 174 patients with affective disorders and 258 patients with schizophrenia according to DSM-IV, as well as 229 population controls were tested. Statistical analysis showed no significant differences in allele or genotype frequencies between control and patient groups. Our results suggest that there is no association between MAOA-LPR genotype and susceptibility to recurrent major depression, bipolar disorder, and schizophrenia in our population.

Molecular characterization of monoamine oxidases A and B

Monoamine oxidase A and B (MAO A and B) are the major neurotransmitter-degrading enzymes in the central nervous system and in peripheral tissues. MAO A and B cDNAs from human, rat, and bovine species have been cloned and their deduced amino acid sequences compared. Comparison of A and B forms of the enzyme shows approximately 70% sequence identity, whereas comparison of the A or B forms across species reveals a higher sequence identity of 87%. Within these sequences, several functional regions have been identified that contain crucial amino acid residues participating in flavin adenine dinucleotide (FAD) or substrate binding. These include a dinucleotide-binding site, a second FAD-binding site, a fingerprint site, the FAD covalent-binding site, an active site, and the membrane-anchoring site. The specific residues that play a role in FAD or substrate binding were identified by comparing sequences in wild-type and variants of MAO with those in soluble flavoproteins of known structures. The genes that encode MAO A and B are closely aligned on the X chromosome (Xp11.23), and have identical exon-intron organization. Immunocytochemical localization studies of MAO A and B in primate brain showed distribution in distinct neurons with diverse physiological functions. A defective MAO A gene has been reported to associate with abnormal aggressive behavior. A deleterious role played by MAO B is the activation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a proneurotoxin that can cause a parkinsonian syndrome in mammals. Deprenyl, an inhibitor of MAO B, has been used for the treatment of early-stage Parkinson's disease and provides protection of neurons from age-related decay.

A promoter polymorphism in the monoamine oxidase A gene and its relationships to monoamine metabolite concentrations in CSF of healthy volunteers

Concentrations of monoamine metabolites (MM) in lumbar cerebrospinal fluid (CSF) have been used extensively as indirect estimates of monoamine turnover in the brain. We investigated possible relationships between a putative functional promoter polymorphism in the monoamine oxidase A (MAOA) gene and CSF concentrations of homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), and 3-methoxy-4-hydroxyphenylglycol (MHPG) in healthy volunteers (n=88). Among women (n=37), those carrying at least one copy of the alleles associated with more efficient transcription displayed higher concentrations of HVA (p=0.01) and 5-HIAA (p=0.01). In men (n=51), however, there was a tendency in the opposite direction. The results suggest that MAOA genotypes may participate differentially in the regulation of dopamine and serotonin turnover rates under presumed steady state in the central nervous system. The results should be interpreted with caution until replicated because of the limited sample size.

Association analysis between a regulatory-promoter polymorphism of the human monoamine oxidase A gene and idiopathic generalized epilepsy

Monoaminergic neurotransmission plays an important role in the regulation of neuronal network excitability and seizure activity. Therapeutic inhibition of the mitochondrial enzyme monoamine oxidase A (MAO-A), which is involved in the degradation and inactivation of monoaminergic neurotransmitters, has been shown to confer a potent anticonvulsant effect. These and other findings suggest a possible role of the X-linked MAO-A gene in epileptogenesis. Therefore, our study was designed to test for an association between a novel MAO-A gene promoter polymorphism and common subtypes of idiopathic generalized epilepsy (IGE). The length of a 30-bp repetitive sequence approximately 1.2 kb upstream of the ATG initiation codon was assessed in 126 patients with juvenile myoclonic epilepsy (JME), 122 patients with idiopathic absence epilepsy (IAE), and 248 healthy controls of German descent. Both sexes were analyzed separately. Although we observed a trend towards a lower number of heterozygotes carrying the 3 and 4 copy alleles in female IAE patients (chi2 = 3.813, df = 1, P = 0.053), allele frequencies did not deviate significantly between patients and controls. Thus, our results do not provide evidence for a contribution of the functional MAO-A gene promoter polymorphism to the pathogenesis of common IGE subtypes.

Excess of high activity monoamine oxidase A gene promoter alleles in female patients with panic disorder

A genetic contribution to the pathogenesis of panic disorder has been demonstrated by clinical genetic studies. Molecular genetic studies have focused on candidate genes suggested by the molecular mechanisms implied in the action of drugs utilized for therapy or in challenge tests. One class of drugs effective in the treatment of panic disorder is represented by monoamine oxidase A inhibitors. Therefore, the monoamine oxidase A gene on chromosome X is a prime candidate gene. In the present study we investigated a novel repeat polymorphism in the promoter of the monoamine oxidase A gene for association with panic disorder in two independent samples (German sample, n = 80; Italian sample, n = 129). Two alleles (3 and 4 repeats) were most common and constituted >97% of the observed alleles. Functional characterization in a luciferase assay demonstrated that the longer alleles (3a, 4 and 5) were more active than allele 3. Among females of both the German and the Italian samples of panic disorder patients (combined, n = 209) the longer alleles (3a, 4 and 5) were significantly more frequent than among females of the corresponding control samples (combined, n = 190, chi2 = 10.27, df = 1, P = 0.001). Together with the observation that inhibition of monoamine oxidase A is clinically effective in the treatment of panic disorder these findings suggest that increased monoamine oxidase A activity is a risk factor for panic disorder in female patients.

Association study between schizophrenia and monoamine oxidase A and B DNA polymorphisms

Monoamine oxidases (MAO) A and B, which are encoded by two distinct genes located on the human X chromosome, are both involved in the oxidative metabolism of dopamine. Decreased levels of platelet MAO-B activity has been reported in patients with schizophrenia and genetic variation in MAO activity had been proposed as a significant factor in the etiology of this disease. We carried out an association study using two intragenic polymorphisms within the MAO-A and MAO-B genes in 110 schizophrenic patients and 87 control subjects. For each polymorphic marker, no significant difference in allelic frequencies was observed between patients and controls. Nevertheless, a trend toward an association between allele 1 of the MAO-B gene and paranoid schizophrenia was found. Our results do not support the hypothesis that inherited variants of MAO genes might play a major role in a genetic predisposition to schizophrenia. Since several previous reports found a low MAO-B platelet activity in patients with paranoid schizophrenia, the identification of polymorphisms related to enzyme activity would be useful.

Mutational analysis of the human MAOA gene

The monoamine oxidases (MAO-A and MAO-B) are the enzymes primarily responsible for the degradation of amine neurotransmitters, such as dopamine, norepinephrine, and serotonin. Wide variations in activity of these isozymes have been reported in control humans. The MAOA and MAOB genes are located next to each other in the p11.3-11.4 region of the human X chromosome. Our recent documentation of an MAO-A-deficiency state, apparently associated with impulsive aggressive behavior in males, has focused attention of genetic variations in the MAOA gene. In the present study variations in the coding sequence of the MAOA gene were evaluated by RT-PCR, SSCP, and sequencing a mRNA or genomic DNA in 40 control males with > 100-fold variations of MAO-A activity, as measured in cultured skin fibroblasts. Remarkable conservation of the coding sequence was found with only 5 polymorphisms observed. All but one of these were in the third codon position and thus did not alter the deduced amino acid sequence. The one amino acid alteration observed, lys --> arg, was neutral and should not affect the structure of the protein. This study demonstrates high conservation of coding sequence in the human MAOA gene in control males, and provides primer sets which can be used to search genomic DNA for mutations in this gene in males with neuropsychiatric conditions.

Evidence for a genetic association between alleles of monoamine oxidase A gene and bipolar affective disorder

We present evidence of a genetic association between bipolar disorder and alleles at 3 monoamine oxidase A (MAOA) markers, but not with alleles of a monoamine oxidase B (MAOB) polymorphism. The 3 MAOA markers, including one associated with low MAOA activity, show strong allelic association with each other but surprisingly not with MAOB. Our results are significant only for females, though the number of males in our sample is too small to draw any definite conclusions. Our data is consistent with recent reports of reduced MAOA activity in patients with abnormal behavioral phenotypes. The strength of the association is weak, but significant, which suggests that alleles at the MAOA locus contribute to susceptibility to bipolar disorder rather than being a major determinant.

Identifying alcoholism vulnerability alleles

Identification of vulnerability alleles is one starting point for elucidating the web of interactions leading to alcoholism so that treatment and prevention can be improved. Heritability studies indicate that vulnerability alleles exist. Two molecular approaches for identifying them, direct analysis of candidate genes and genetic linkage, are highlighted in this review. Methodological problems that have been partially addressed and limitations for the applicability of the genetic findings are discussed.

The promoter of the human monoamine oxidase A gene

Monoamine oxidase (MAO) A (EC 1.4.3.4) oxidizes norepinephrine and serotonin and is expressed in a cell type-specific manner. Evidence that MAO A deficient males in a large Dutch kindred suffer from mild mental retardation and occasional episodes of impulsive-aggressive behavior makes it important to understand how the human MAO A promoter is regulated. Workers in multiple laboratories have isolated and characterized protein-coding sequences of the human MAO A gene and the DNA region where mRNA synthesis is initiated. After summarizing our published findings concerning where transcription of the human MAO A gene is initiated, I summarize representative results of transient expression assays aimed at assessing whether some potential gene regulatory agents affect the expression of luciferase from MAO A promoter reporter constructs when transfected into a mouse L cell line which expresses MAO A. These studies revealed no specific regulatory effects of serum, dexamethasone or a stable cyclic-AMP analogue on the human MAO A promoter introduced.

Pharmacological and clinical implications of MAO-B inhibitors

Although the involvement of monoamine oxidase B (MAO-B) in physiological function is not yet well understood, its inhibitors have been shown to be quite useful in the treatment of various neuropsychiatric disorders. Platelet MAO-B activity has been found to be reduced in several psychiatric disorders, related to substance abuse and associated with different personalities. 1-Deprenyl (selegiline), an archetypical MAO-B inhibitor, alone does not seem to exert an antidepressive effect, however, it may become useful when administered in combination with amine neurotransmitter precursors. MAO-B inhibitors are useful adjunct drugs to 1-DOPA in the symptomatic treatment of Parkinson's disease. Interestingly, 1-deprenyl alone can slow down the progress of otherwise disabled syndromes of Parkinson's disease. It has been proposed that 1-deprenyl may play a role in neuroprotection and neurorescue. MAO-B inhibitors can selectively and dramatically increase the level of beta-phenylethylamine, which has been shown to potentiate dopamine and noradrenaline function in the central nervous system. Several new types of highly selective, reversible and irreversible MAO-B inhibitors have recently been developed. The mechanism(s) of neuroprotective and rescue actions of 1-deprenyl and other MAO-B inhibitors will help to shed some light on our understanding of the neurodegenerative process.

Hereditary variations in monoamine oxidase as a risk factor for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder caused by loss of dopaminergic neurons in the brainstem. Recent studies suggest that several genes may have a role in determining individual susceptibility to this disease, and the degradative enzyme monoamine oxidase (MAO) has been implicated in the disease process. Wide differences in activity levels for both forms of this enzyme (MAO-A and MAO-B) exist in the human population, and levels of both are genetically determined. Here we have compared the frequency of haplotypes at the MAOA and MAOB loci on the X chromosome in 91 male patients with PD and 129 male controls. Alleles were marked using two restriction fragment length polymorphisms (RFLPs), a (GT)n repeat in the MAOA locus, and a (GT)n repeat in the MAOB locus. One particular haplotype marked by the RFLP's at MAOA was three times more frequent in patients with PD as compared with controls, and the overall distribution of these alleles was significantly different (p = 0.03) between these two groups. Another MAOA haplotype was about threefold more common in controls than in patients with PD (p = 0.005). No associations were observed between individual MAOB alleles and the disease state, but the frequency distribution for all alleles was significantly different in the two populations (p = 0.046). These findings support the idea that the MAO genes may be among the hereditary factors that influence susceptibility of individuals to PD.

Identification of human monoamine oxidase (MAO) A and B gene promoters

The promoter of human monoamine oxidase (MAO) A and B genes have been identified. The core promoter region of MAO A is comprised of two 90 bp repeats each of which contains two Sp1 elements and lacks a TATA box. The MAO B core promoter region contains two sets of overlapping Sp1 sites which flank a CACCC element all upstream of a TATA box. The different organization of the MAO A and B promoters may underlie their different cell and tissue specific expression.