Monoamine oxidase genes polymorphisms and mood disorder

MAOA Variants and Genetic Susceptibility to Major Psychiatric Disorders

Monoamine oxidase A (MAOA) is a mitochondrial enzyme involved in the metabolism of several biological amines such as dopamine, norepinephrine, and serotonin, which are important neurochemicals in the pathogenesis of major psychiatric illnesses. MAOA is regarded as a functional plausible susceptibility gene for psychiatric disorders, whereas previous hypothesis-driven association studies obtained controversial results, a reflection of small sample size, genetic heterogeneity, or true negative associations. In addition, MAOA is not analyzed in most of genome-wide association studies (GWAS) on psychiatric disorders, since it is located on Chromosome Xp11.3. Therefore, the effects of MAOA variants on genetic predisposition to psychiatric disorders remain obscure. To fill this gap, we collected psychiatric phenotypic (schizophrenia, bipolar disorder, and major depressive disorder) and genetic data in up to 18,824 individuals from diverse ethnic groups. We employed classical fixed (or random) effects inverse variance weighted methods to calculate summary odds ratios (OR) and 95 % confidence intervals (CI). We identified a synonymous SNP rs1137070 showing significant associations with major depressive disorder (p = 0.00067, OR = 1.263 for T allele) and schizophrenia (p = 0.0039, OR = 1.225 for T allele) as well as a broad spectrum of psychiatric phenotype (p = 0.000066, OR = 1.218 for T allele) in both males and females. The effect size was similar between different ethnic populations and different gender groups. Collectively, we confirmed that MAOA is a risk gene for psychiatric disorders, and our results provide useful information toward a better understanding of genetic mechanism involving MAOA underlying risk of complex psychiatric disorders.

Androgen receptor and monoamine oxidase polymorphism in wild bonobos

Androgen receptor gene (AR), monoamine oxidase A gene (MAOA) and monoamine oxidase B gene (MAOB) have been found to have associations with behavioral traits, such as aggressiveness, and disorders in humans. However, the extent to which similar genetic effects might influence the behavior of wild apes is unclear. We examined the loci AR glutamine repeat (ARQ), AR glycine repeat (ARG), MAOA intron 2 dinucleotide repeat (MAin2) and MAOB intron 2 dinucleotide repeat (MBin2) in 32 wild bonobos, Pan paniscus, and compared them with those of chimpanzees, Pan troglodytes, and humans. We found that bonobos were polymorphic on the four loci examined. Both loci MAin2 and MBin2 in bonobos showed a higher diversity than in chimpanzees. Because monoamine oxidase influences aggressiveness, the differences between the polymorphisms of MAin2 and MBin2 in bonobos and chimpanzees may be associated with the differences in aggression between the two species. In order to understand the evolution of these loci and AR, MAOA and MAOB in humans and non-human primates, it would be useful to conduct future studies focusing on the potential association between aggressiveness, and other personality traits, and polymorphisms documented in bonobos.

Meta-analysis of genetic association studies on bipolar disorder

Numerous candidate gene association studies of bipolar disorder (BP) have been carried out, but the results have been inconsistent. Individual studies are typically underpowered to detect associations with genes of small effect sizes. We conducted a meta-analysis of published candidate gene studies to evaluate the cumulative evidence. We systematically searched for all published candidate gene association studies of BP. We then carried out a random-effects meta-analysis on all polymorphisms that were reported on by three or more case-control studies. The results from meta-analyses of these genes were compared with the findings from a recent mega-analysis of eleven genome-wide association studies (GWAS) in BP performed by the Psychiatric GWAS Consortium (PGC). A total of 487 articles were included in our review. Among these, 33 polymorphisms in 18 genes were reported on by three or more case-control studies and included in the random-effects meta-analysis. Polymorphisms in BDNF, DRD4, DAOA, and TPH1, were found to be nominally significant with a P-value < 0.05. However, none of the findings were significant after correction for multiple testing. Moreover, none of these polymorphisms were nominally significant in the PGC-BP GWAS. A number of plausible candidate genes have been previously associated with BP. However, the lack of robust findings in our review of these candidate genes highlights the need for more atheoretical approaches to study the genetics of BP afforded by GWAS. The results of this meta-analysis and from other on-going genomic experiments in BP are available online at Metamoodics (http://metamoodics.igm.jhmi.edu).

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.

High-activity variants of the uMAOA polymorphism increase the risk for depression in a large primary care sample

Studies on the association between the functional uMAOA polymorphism and depression have yielded non-conclusive results up till now. One thousand two hundred twenty eight consecutive Spanish primary care attendees, participating in the PREDICT study, agreed to take part in this genetic PREDICT-Gene study. We explored the association between depression and either high-activity uMAOA alleles or genotypes. Depression was diagnosed using the Composite International Diagnostic Interview (CIDI) to establish three different depressive outcomes (ICD-10 Depressive Episode (DE), ICD-10 Severe Depressive Episode (SDE) and DSM-IV Major Depression (MD)). uMAOA genetic variation was determined by PCR amplification and subsequent electrophoresis. Crude and adjusted (gender and/or age) odds ratios, with 95% confidence intervals, were calculated for the associations between allele or genotype frequencies and all three depressive outcomes. We found associations between all three depressive phenotypes and either high-activity alleles or high-activity genotypes in both sexes. The associations were statistically significant for females but not for males. Testing the same associations on the entire sample (males and females) also yielded significant associations between depression and either high-activity alleles or high-activity genotype distribution that were independent of age and/or gender (ICD-10 DE: OR = 1.98; 95% CI: 1.42-1.77; P = 0.00002; ICD-10-SDE: OR = 2.05; 95% CI: 1.38-3.05; P = 0.0002; DSM-IV MD: OR = 1.91; 95% CI: (1.26-2.91); P = 0.0014). Our results provide fairly consistent evidence that high-activity variants of the MAOA promoter polymorphism confer a modestly higher risk for depression.

Meta-analyses of genetic studies on major depressive disorder

The genetic basis of major depressive disorder (MDD) has been investigated extensively, but the identification of MDD genes has been hampered by conflicting results from underpowered studies. We review all MDD case-control genetic association studies published before June 2007 and perform meta-analyses for polymorphisms that had been investigated in at least three studies. The study selection and data extraction were performed in duplicate by two independent investigators. The 183 papers that met our criteria studied 393 polymorphisms in 102 genes. Twenty-two polymorphisms (6%) were investigated in at least three studies. Seven polymorphisms had been evaluated in previous meta-analyses, 5 of these had new data available. Hence, we performed meta-analyses for 20 polymorphisms in 18 genes. Pooled odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Statistically significant associations were found for the APOE varepsilon2 (OR, 0.51), GNB3 825T (OR, 1.38), MTHFR 677T (OR, 1.20), SLC6A4 44 bp Ins/Del S (OR, 1.11) alleles and the SLC6A3 40 bpVNTR 9/10 genotype (OR, 2.06). To date, there is statistically significant evidence for six MDD susceptibility genes (APOE, DRD4, GNB3, MTHFR, SLC6A3 and SLC6A4).

Association analysis of monoamine oxidase A gene and bipolar affective disorder in Han Chinese

Background

Monoamine oxidase A (MAOA) is a mitochondrial enzyme involved in degrading several different biological amines, including serotonin. Although several pieces of evidence suggested that MAOA is important in the etiology of bipolar affective disorder (BPD), associations for markers of the MAOA gene with BPD were not conclusive and the association has not been investigated in Taiwanese population. This study was designed to illustrate the role of MAOA in the etiology of BPD in Han Chinese.

Methods

Two markers, a dinucleotide polymorphism in exon 2 and a functional uVNTR on the promoter of the MAOA gene, were used to study the genetic association in 108 unrelated patients with BPD and 103 healthy controls. Allelic distributions of two polymorphisms were analyzed and, caused the MAOA located at X chromosome, haplotype association was performed using haplotype unambiguously assigned in male participants.

Results

While no difference in allelic distributions of two MAOA polymorphisms was found, the risk haplotype 114S was associated with BPD in male patients (P = 0.03). The significance, however, was not found in female patients with 114S haplotype.

Conclusion

Results from this study suggest that MAOA may have a gender-specific and small effect on the etiology of BPD in Taiwan. Due to the limited sample size, results from this study need to be confirmed in replicates.

Recent findings on the genetic basis of bipolar disorder

What began as a search for a specific gene for bipolar disorder has now become a search for multiple susceptibility genes as it has be-come clear that the genetic basis of bipolar disorder probably involves multiple genes interacting with each other and with environmental components in as-yet mysterious ways. This article reviews the most recent findings and the emerging picture in the genetics of bipolar disorder.

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.

Serotonin gene polymorphisms and bipolar I disorder: focus on the serotonin transporter

The pathogenesis of bipolar disorder may involve, at least in part, aberrations in serotonergic neurotransmission. Hence, serotonergic genes are attractive targets for association studies of bipolar disorder. We have reviewed the literature in this field. It is difficult to synthesize results as only one polymorphism per gene was typically investigated in relatively small samples. Nevertheless, suggestive associations are available for the 5HT2A receptor and the serotonin transporter genes. With the availability of extensive polymorphism data and high throughput genotyping techniques, comprehensive evaluation of these genes using adequately powered samples is warranted. We also report on our investigations of the serotonin transporter, SLC6A4 (17q11.1-q12). An insertion/deletion polymorphism (5HTTLPR) in the promoter region of this gene has been investigated intensively. However, the results have been inconsistent. We reasoned that other polymorphism/s may contribute to the associations and the inconsistencies may be due to variations in linkage disequilibrium (LD) patterns between samples. Therefore, we conducted LD analyses, as well as association and linkage using 12 polymorphisms, including 5HTTLPR. We evaluated two samples. The first sample consisted of 135 US Caucasian nuclear families having a proband with bipolar I disorder (BDI, DSM IV criteria) and available parents. For case-control analyses, the patients from these families were compared with cord blood samples from local Caucasian live births (n = 182). Our second, independent sample was recruited through the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD, 545 cases, 548 controls). No significant associations were detected at the individual polymorphism or haplotype level using the case-control or family-based analyses. Our analyses do not support association between SLC6A4 and BDI families. Further studies using sub-groups of BDI are worthwhile.

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.

An association between a functional polymorphism in the monoamine oxidase a gene promoter, impulsive traits and early abuse experiences

Monoamine oxidase A (MAOA) activity is altered in mood disorders and lower activity associated with aggressive behavior. The gene has a functional polymorphism with a variable number tandem repeat (VNTR) in the upstream regulatory region (MAOA-uVNTR). In this study, we examined possible associations between the MAOA-uVNTR polymorphism and mood disorders, suicidal behavior, aggression/impulsivity, and effects of reported childhood abuse. In total, 663 unrelated subjects with a psychiatric disorder and 104 healthy volunteers were genotyped for the 30 base pair functional VNTR. A novel repeat variation was identified. No statistically significant associations were found between this functional MAOA-uVNTR polymorphism and mood disorders or suicide attempts. However, the lower expression allele was associated with a history of abuse before 15 years of age in male subjects and with higher impulsivity in males but not females. Our results suggest that the lower expression of the MAOA-uVNTR polymorphism is related to a history of early abuse and may sensitize males, but not females, to the effects of early abuse experiences on impulsive traits in adulthood. The polymorphism may be a marker for impulsivity that in turn may contribute to the risk for abuse. This trait could then be further aggravated by abuse.

Review of bipolar molecular linkage and association studies

This paper reviews the history of molecular genetic linkage and linkage disequilibrium (LD) or association studies of bipolar disorder (BPD). The topic is introduced with a brief discussion of various genetic concepts, including linkage and linkage disequilibrium. It is emphasized that criteria for declaring linkage must include independent confirmation by multiple groups of investigators. Given that the inherited susceptibility for BPD is most likely explained by multiple genes of small effect, simulations indicate that universal confirmation of valid linkages cannot be expected due to sampling variation and genetic heterogeneity. With this background, several valid linkages of BPD to genomic regions are reviewed, including some which may be shared with schizophrenia. These results suggest that nosology must be changed to reflect the genetic origins of the multiple disorders which are collectively described by the term, BPD. The history of BPD LD studies is reviewed, using monoamine oxidase as as an example. Some suggestions of improving these BPD LD are offered.

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 genetic and family studies in affective disorders: state of the art

The identification of genes responsible for mood disorders will contribute to significant advances in the awareness of diagnosis (diagnostic process and early recognition), pathophysiology, epidemiology and treatment issues. During the past two decades, the search for genes for mood disorders has mainly contributed to better understand and confirm the genetic complexities inherent to these disorders. The large amount of results available and the difficulty to digest them corroborate this observation. The major contribution of these findings should be integrated in the context of the world-wide efforts to identify the thousands of genes of the human genome. Some of these genes may be identified within the next decade. Several consistent hypotheses are currently being tested and will, hopefully, speed up the process of narrowing the important regions when the complete genome map will be available. The most promising chromosomal regions have been localized on chromosomes 4, 5, 11, 12, 18, 21 and X. A number of candidate genes have also been investigated, some of these are directly linked to neurobiological hypotheses of the aetiology of affective disorders. In parallel, specific hypotheses have been implicated, such as anticipation and dynamic mutations. Further research should concentrate on these hypotheses and confirm positive findings through interdisciplinary and multicenter projects.

Lack of linkage disequilibrium between serotonin transporter protein gene (SLC6A4) and bipolar disorder

The serotonin transporter (5HTT) gene appears to be of particular interest as 5HTT is the selective site of action of selective serotonin reuptake inhibitors (SSRIs) that successfully treat bipolar depression (BP). The 5HTT gene is located on chromosome 17q11.1-q12 and has a 44 bp deletion/insertion functional polymorphism in the promoter region (SLC6A4). Results from association studies on SLC6A4 and BP disorder are conflicting. The aim of the present study was to investigate for the presence of linkage disequilibrium between SLC6A4 and BP disorder. One hundred thirty-three Bipolar I or Bipolar II probands with their living parents were recruited. Diagnoses were assessed by the structured interview for the Diagnostic and Statistical Manual of Mental Disorders, fourth edition [DSM-IV, American Psychiatric Association, 1994] (SCID-I). Genotyping was performed with standard procedures and data were analyzed using the Transmission Disequilibrium Test [TDT, Spielman et al., 1993: Am J Hum Genet 52: 506-516]. One hundred two triads were informative for the analysis. Each of the two alleles of the SLC6A4 was transmitted at the same rate to bipolar probands (chi(2) = 0.692, df = 1, P = NS). Thus, it appears unlikely that the SLC6A4 plays a fundamental role in the pathogenesis of BP disorder. However, further studies focusing on the role of the 5HTT gene in predicting the response to SSRIs in BP patients might be worthwhile.

Searching high and low: a review of the genetics of bipolar disorder

OBJECTIVES

To review the methodologies and findings in the genetics of bipolar disorder (BPD), and to suggest future directions for research.

METHODS

Reports of family, twin, adoption, linkage, association, cytogenetic, and animal model studies, and segregation analyses in English, were identified from multiple MEDLINE searches. Hand searches were carried out in bibliographies from review articles.

RESULTS

Family, twin, and adoption studies have provided strong evidence for a genetic etiology in BPD. Early reports of linkage of BPD to DNA markers at several chromosomal sites have not proven robust, perhaps because of the complex nature of BPD inheritance. However, linkage findings in the 1990s, on chromosomes 18, 21q, 12q, and 4p, have provided leads that are being pursued through both genetic and physical mapping. No gene has yet been definitively implicated in BPD.

CONCLUSIONS

Strategies for increasing the power to detect BPD genes include: (1) dividing the phenotype into genetically meaningful subtypes to decrease heterogeneity: and (2) ascertaining a very large family sample--a multicenter study now in progress will collect 700 bipolar I sibling pairs. BPD may result from several genes acting in concert so that new multilocus statistical methods could enhance the capacity to detect loci involved. Family-based association studies using a very large number of newly identified single nucleotide polymorphisms (SNPs) may allow for more efficient screening of the genome. As the Human Genome Project approaches its goal of isolating all genes by 2003, the data generated is likely to speed identification of candidate BPD genes.

Association analysis between mood disorder and monoamine oxidase gene

To ascertain whether mood disorders, including bipolar and unipolar, are genetically associated with the monoamine oxidase A (MAOA) or monoamine oxidase B (MAOB) gene in the Chinese population, 132 cases of mood disorder and 88 normal controls were genotyped for the MAOA(CA)n, MAOB(GT)n, and MAOB(TG)n loci by the method of amplification fragment length polymorphism. Among 132 cases with mood disorder, eight alleles (size: 112-126 bp) of locus MAOA(CA)n, 12 alleles (size: 168-198 bp) of locus MAOB(GT)n, and nine alleles (size: 195-213 bp) of locus MAOB(TG)n were observed. Comparison of the allele frequency of the three loci showed no difference between mood disorder cases and normal controls on average. When each group was stratified into several subgroups, significant differences were found. On the MAOA(CA)n locus, the frequency of 116 bp allele was higher in the female bipolar disorder cases (0.2581) compared with that in the female unipolar disorder patients (0.1154) (Z=2.15, p<0. 05). On the MAOB(GT)n locus, the frequency of 180 bp allele was higher in bipolar disorder patients (0.1579) than that in normal controls (0.0678) (Z=2.05, p<0.05). The frequency of this allele was even higher in female bipolar disorder patients (0.1719) than that in female normal controls (0.0541). On the MAOB(TG)n locus, the frequency of 205 bp allele was higher in female bipolar disorder patients (0.6406) than that in female normal controls (0.4375) (Z=2. 17, p<0.05). For the unipolar disorder patients, the frequency of this allele was higher in female cases (0.5222) than that in male cases (0.1818) (Z=3.49, p<0.05). As for association studies, significant association between bipolar disorder and MAOB gene was detected. For the 180 bp allele of MAOB(GT)n, the relative risk (RR) of biploar versus normal control was 2.58 (p<0.05), and the RR of female bipolar disorder versus female normal control was 3.63 (p<0. 05). For the 205 bp allele of MAOB(TG)n, the RR of female bipolar disorder versus female normal control was 2.29 (p<0.05). Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:12-14, 2000.