Sexual dimorphic effect in the genetic association of monoamine oxidase A (MAOA) markers with autism spectrum disorder
Introduction
Autism spectrum disorder (ASD) is a group of heterogeneous neurodevelopmental disorders, characterized by impairments in reciprocal social interaction, communication and restricted behavior. ASD has a high prevalence rate of 1 in 88 children with boys being more affected than girls in the ratio of 4:1 (CDC, 2012). Besides that, its heritability is greater than 90% and the data obtained from twin and sibling-based family studies suggest that the observed familial clustering is largely explained by genetics and partly by environmental factors (Bailey et al., 1995, Hallmayer et al., 1996).
Overall pattern of inheritance in families and the observation that relatives of the probands show subtle symptoms that are common to ASD suggest that the disorder might be caused by a constellation of multiple genes, each contributing small increments of risk towards the phenotype (Freitag et al., 2007). A very recent review article by Xu et al. has retrieved all the available information on the genes and its markers that are involved in ASD genetics to generate a database comprising of 2193 genes, 2806 single nucleotide polymorphisms/variable number of tandem repeats (SNPs/VNTRs), 4544 copy number variations (CNVs) and 158 linkage regions that are implicated in autism (Xu et al., 2012).
With the advent of genomic era and rapidly advancing research technologies, several evidences in support of the genetic origin of ASD has come into the limelight. Multiple genome-wide scan studies are currently available on ASD, which shows evidence of linkage in almost all the chromosomes (Yonan et al., 2003). It has been suggested that normal brain development is disrupted in individuals with autism and abnormal synaptic homeostasis is a risk factor for this condition (Bourgeron, 2009), where neurogenesis, synaptogenesis and plasticity have been shown to be abnormal. Experimental evidences from various pathophysiological, neurochemical, neuroanatomical and genetic studies propose that neurotransmitter and neurotrophic systems play major roles in its etiology (Cusco et al., 2009, Nickl-Jockschat and Michel, 2011, Pardo and Eberhart, 2007).
Serotonin is a monoamine neurotransmitter that participates in the control of behavior, mood, memory, learning and endocrine functions (Haavik et al., 2008). Its level has been shown to be elevated in the blood of one third of autistic individuals (Burgess et al., 2006, Cook and Leventhal, 1996, Croonenberghs et al., 2000). The serotonin synthesis capacity in individuals with autism is also abnormal. This raises the possibility of involvement of serotonergic system in ASD pathophysiology. Neurotransmission and metabolism of serotonin as well as serotonergic gene expression are regulated in a highly complex manner by various proteins in the pathway (Scott and Deneris, 2005). Monoamine oxidase (MAO) is the enzyme responsible for the degradation of serotonin (5-hydroxytryptamine or 5-HT) to 5-hydroxy-3-indoleacetaldehyde (5-HIAL), thereby regulating its level both in CNS and periphery. Level of 5-hydroxy-3-indoleacetacetic acid (5-HIAA) has been found to be elevated in the urine of hyperserotonemic ASD subjects (Haney et al., 1990, Mulder et al., 2010). A study by Manuck et al. (2000) has shown preliminary evidence of association between a polymorphic variant in the MAOA gene with interindividual variability in aggressiveness, impulsivity and central nervous system serotonergic responsivity (Manuck et al., 2000).
Monoamine oxidases are outer mitochondrial membrane bound FAD containing enzymes, encoded by two X-chromosome linked MAO genes (Lan et al., 1989), which exists in two isoforms MAOA and MAOB having 70% sequence identity (Bach et al., 1988). MAOA has high affinity for 5-HT and norepinephrine (NE) whereas MAOB mostly prefers β-phenylethylamine (Shih et al., 1999). According to Chen et al., increased brain serotonin and decreased 5-HIAA level have been observed in MAOA/B KO mice, which lead to extreme predisposition to reactive aggression, repetitive responses and other behavioral perturbations (Chen et al., 2004). Similar behavioral attributes along with developmental delay, intermittent hypotonia and stereotypic hand movements have been reported in a male patient with 240 kb deletion Xp11.3–p11.4 encompassing both MAOA and MAOB genes (Whibley et al., 2010). Evidences gathered from few studies have revealed a causal relationship between low catalytic activity allele MAOA-upstream VNTR (uVNTR) in the promoter region with behavioral, cognitive, neuroanatomical and neuropharmacological impairments in ASD (Cohen et al., 2003, Davis et al., 2008, Yirmiya et al., 2002). Studies on MAOA single and MAOA/B double knockout mice models further established that MAOA deficiency leads to neurochemical imbalances, which culminate in neuroanatomical abnormalities such as reduced thickness of corpus callosum, increased dendritic arborization of pyramidal neurons in the prefrontal cortex and disrupted microarchitecture of cerebellum in addition to the manifestation of numerous behavioral hallmarks of ASD (Bortolato et al., 2013).
A study on ASD by Tassone et al. (2011) suggested a potential role of functional MAOA promoter alleles in the male children, their mothers or both (Tassone et al., 2011). Previously it was shown by Cohen et al. (2003) that MAOA-uVNTR alleles may act as a genetic modifier for autism severity in males (Cohen et al., 2003). In 2009, a study conducted by Yoo et al. in a Korean population also threw light on the hypothesis that MAOA is a candidate gene for ASD and has shown statistically significant association of five markers in 151 families and 193 unrelated controls (Yoo et al., 2009). The findings by Cohen et al. (2011) suggest that maternal genotypes have a significant effect on the association of MAOA and other genes with behavior in male offspring (Cohen et al., 2011). All the above findings indicate that concomitant behavioral disturbances in affected children with MAOA genotypes in ASD warrant consideration of MAOA as a candidate gene for the disorder. Although this is considered as a gene of interest in regard to autism, no conclusive data is available to confirm this. Therefore, in this present study an effort has been made to examine whether this gene poses any risk for ASD through population based approach. Here we have genotyped 421 subjects including 194 ASD patients and 227 controls from West Bengal in India for 30 bp-uVNTR at 5′untranslated region (5′UTR) and seven SNPs (rs5906883, rs1465107 & rs1465108 in intron1, rs5905809 & rs5906957 in intron2, rs6323 in exon8 and rs1137070 in exon14) of MAOA gene.
Section snippets
Selection of subjects
A total of 421 study subjects were recruited for this study, the details of which are presented in Table 1. The autistic cohort included 194 individuals of West Bengal origin that met the DSM-IV-TR criteria for autistic disorder, Asperger's syndrome and PDD-NOS. Assessments of the cases were carried out using Childhood Autism Rating Scale (CARS) (Schopler et al., 1980). The diagnosis and assessment were carried out by an experienced psychiatrist and a clinical psychologist. Cases with gross
Description of MAOA markers
A diagrammatic representation of MAOA gene is provided in Fig. 1. Relative positions of the eight markers (30 bp-uVNTR, rs5906883, rs5905809, rs5906957, rs1465107, rs1465108, rs6323 and rs1137070) are marked in the diagrammatic gene structure. As has been mentioned in the Materials and methods section, the markers rs5905809 and rs5906957 (C–G & G–C) as well as rs1465107 and rs1465108 (G–G & A–A) are in complete LD in all the samples. Therefore, subsequent genetic analyses were performed only for
Discussion
ASD is a neurodevelopmental disorder with behavioral manifestations. Increasing evidences have suggested perturbation of serotonergic system in autism pathology. The disorder is known to exhibit gender differences with higher affection status in males, which probably suggests involvement of X-chromosomal genes. Growing evidences from neurochemical, epidemiological and genetic studies support MAOA as one of the putative autism susceptibility genes for this disorder. Since MAOA is responsible for
Conclusion
In conclusion, the present study highlights the importance of MAOA gene in ASD etiology and it shows sexual dimorphic effect in genetic association. The low activity allele of rs6323 poses increased risk in males, whereas no effect has been detected in females. Random association of various alleles of the markers also showed differential effect in males and females. The low activity alleles supposedly increase the 5-HT level in the brain. During the critical stages of brain development, such a
Acknowledgments
The present study is supported by the Department of Biotechnology (DBT), Govt. of India grant to KPM, KM and UR vide BT/PR14637MED/30/561/2010 dated 28/11/2011 and theCouncil of Scientific and Industrial Research (CSIR), Govt. of India (Junior Research Fellowship to DV, award no. Admn 9/840(0009) EMR—I/2011 dated 25/03/2011). BC and AK have been working for this project as Junior Research Fellows. TB was a summer project student who worked in this project.
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2016, Progress in Neuro-Psychopharmacology and Biological PsychiatryCitation Excerpt :As MAO enzymes modulate behavioral phenotype by regulating the monoamine levels in the brain and peripheral system, MAOB gene has been chosen in the present study as a potential candidate for ASD susceptibility. Moreover, male-specific effect of MAOA in the Indian ASD population (Verma et al., 2014) and its homology with MAOB indicates a risk association for MAOB in ASD. X-chromosomal localization of MAOB also increases this possibility as it is a male predominant disorder.
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2015, Neuroscience and Biobehavioral ReviewsCitation Excerpt :This finding was supported by one case–control study (Yoo et al., 2009), and contradicted by another study through family and case–control approach (Yirmiya et al., 2002). Recently Verma et al. (2014) reported the sexual dimorphic effect in the association of MAO-A polymorphisms with ASD and the results are suggestive of the sex ratio differences in the incidence of this disorder. The only study examining association of MAO-B with autism using small sample size reported that G allele of rs1799836, which showed higher distribution in autistic patients was associated with reduced plasma MAO-B activity in male cases (Salem et al., 2013).
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These authors contributed equally to this work.