Serotonin transporter promoter variants: Analysis in Indian autistic and control population

Autism spectrum disorder in India: a scoping review

Autism is a grand challenge in global mental health to be dealt with on a priority basis. Phenotypic knowledge, biological understanding, and evidence-based intervention studies are all from western countries. We know very little about autism in the low- and middle-income countries (LMIC). Lack of infrastructure and difficulties in operationalizing research has widened the knowledge gap. We performed a comprehensive scoping review of research in Autism Spectrum Disorder in India to have an overall impression, identify gaps, and formulate evidence-based recommendations for further study. We searched PubMed, SCOPUS, and Cochrane Library to identify relevant Indian studies. A hundred and fifty-nine publications met the inclusion criteria. Most of the research contribution in autism is from few tertiary care medical centres, technological institutes, and not-for-profit organizations. We identified various themes of research like clinical profile, interventions, biomarkers, psychological, social, epidemiological, and risk factors. Evidence-based intervention studies, translation and adaptation of standard diagnostic instruments, and qualitative research on the experience of autism appeared to be state of the art. However, epidemiological studies, biomarkers identification, risk assessment studies were of low quality. There is a need for nationwide studies with representative sampling on epidemiology, biomarkers, and risk factors for a complete evaluation of the actual burden and biology of autism in India. Also, there is a need to design implementation research to evaluate the effectiveness of evidence-based interventions in routine healthcare settings. We recommend that future research should fill these gaps in understanding autism and improving its outcome in India.

Significant associations between 5-hydroxytryptaminetransporter-linked promoter region polymorphisms of the serotonin transporter (solute carrier family 6 member 4) gene and Thai patients with autism spectrum disorder

Autism spectrum disorder (ASD) is a form of pervasive developmental disorder manifested by impairment in social interactions and repetitive behaviors. Although genetic contribution is strongly suspected in autism, the specific genetic factors remain unidentified. Hyperserotoninemia has been reported in some autistic patients, and several studies have demonstrated an association between 5-hydroxytryptamine-transporter-linked promoter region (5-HTTLPR) polymorphisms and rs25531 single nucleotide polymorphism in the serotonin transporter gene (solute carrier family 6 member 4; SLC6A4) and ASD, indicating a possible involvement of the serotonin system in the etiology of ASD.To explore this situation further, a case-control association study of 5-HTTLPR and rs25531 polymorphisms on Thai ASD patients was conducted. A total of 188 ASD cases fulfilling the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) criteria (156 males and 32 females) and a total of 250 normal controls were recruited from the same ethnic backgrounds. 5-HTTLPR polymorphisms (Long, L; Short, S) and rs25531 (A/G) single nucleotide polymorphism were genotyped and compared between the patients and normal controls using chi-square statistics.The L/L genotype was more common in patients than in the controls (13.8% vs 5.2%, P = .006), and the LA haplotype was found in patients more than the controls (16.9% vs 12.2%, P = .048). When male patients were analyzed alone (156 individuals), the associations were also statistically significant with P = .017 for L/L genotype, and P = .019 for LA haplotype distribution.Our findings support previous reports suggesting an association between the 5-HTTLPR and rs25531 polymorphisms of SLC6A4 and patients with ASD.

Tackling Missing Heritability by Use of an Optimum Curve: A Systematic Review and Meta-Analysis

Missing heritability is a common problem in psychiatry that impedes precision medicine approaches to autism and other heritable complex disorders. This proof-of-concept study uses a systematic review and meta-analysis of the association between variants of the serotonin transporter promoter (5-HTTLPR) and autism to explore the hypothesis that some missing heritability can be explained using an optimum curve. A systematic literature search was performed to identify transmission disequilibrium tests on the short/long (S/L) 5-HTTLPR polymorphism in relation to autism. We analysed five American, seven European, four Asian and two American/European samples. We found no transmission preference in the joint samples and in Europe, preferential transmission of S in America and preferential transmission of L in Asia. Heritability will be underestimated or missed in genetic association studies if two alternative genetic variants are associated with the same disorder in different subsets of a sample. An optimum curve, relating a multifactorial biological variable that incorporates genes and environment to a score for a human trait, such as social competence, can explain this. We suggest that variants of functionally related genes will sometimes appear in fixed combinations at both sides of an optimum curve and propose that future association studies should account for such combinations.

A preliminary association study between serotonin transporter (5-HTTLPR), receptor polymorphisms (5-HTR1A, 5-HTR2A) and depression symptom-clusters in a north Indian population suffering from Major Depressive Disorder (MDD)

INTRODUCTION

Major Depressive Disorder (MDD) is a broad heterogeneous diagnostic construct. Previous studies have shown that it can be resolved into several symptom-clusters which are proposed to be associated with single nucleotide polymorphisms (SNPs) of the serotonergic pathway (5-HTTLPR, 5HTR1A, 5-HTR2A).

METHODS AND MATERIAL

In a cross-sectional study conducted at a tertiary level mental health care set-up in north India, 80 out-patients with MDD were evaluated with Montgomery Asberg Depression Rating Scale (MADRS) and then genotyping was done. The different clinical and genetic variables were compared across the factor structures of MADRS. Also, the comparison of the genetic data of cases was done with the pre-existing database of the non-blood related healthy ethnically-matched controls.

RESULTS

There was no significant association between age, gender, other clinical variables, SNPs like 5-HTTLPR SS/SL, rs6295 CC/CG/GG, rs6311GG/GA/AA, rs6313 CC/CT/TT and different factor-structures like 'detachment' consisting of items like concentration difficulty, lassitude, inability to feel; 'psychic anxiety' consisting of suicidal thoughts and inner tension; 'mood-pessimism' consisting of symptoms like apparent sadness, reported sadness, pessimistic thoughts and 'vegetative symptoms' like decreased sleep, poor appetite. Neither there was any association between genotype of the cases compared with the controls.

CONCLUSIONS

No significant association was obtained between the four-factor structures of depression in MADRS and serotonin transporter and receptor SNPs in a study with a small sample size. This study evaluates whether depression symptom-clusters have distinct genotypic determinants and necessitates more comprehensive studies for unravelling the genetic determinants of depression.

Serotonin mediated immunoregulation and neural functions: Complicity in the aetiology of autism spectrum disorders

Serotonergic system has long been implicated in the aetiology of autism spectrum disorders (ASD), since platelet hyperserotonemia is consistently observed in a subset of autistic patients, who respond well to selective serotonin reuptake inhibitors. Apart from being a neurotransmitter, serotonin functions as a neurotrophic factor directing brain development and as an immunoregulator modulating immune responses. Serotonin transporter (SERT) regulates serotonin level in lymphoid tissues to ensure its proper functioning in innate and adaptive responses. Immunological molecules such as cytokines in turn regulate the transcription and activity of SERT. Dysregulation of serotonergic system could trigger signalling cascades that affect normal neural-immune interactions culminating in neurodevelopmental and neural connectivity defects precipitating behavioural abnormalities, or the disease phenotypes. Therefore, we suggest that a better understanding of the cross talk between serotonergic genes, immune systems and serotonergic neurotransmission will open wider avenues to develop pharmacological leads for addressing the core ASD behavioural deficits.

A study of blood serotonin and serotonin transporter promoter variant (5-HTTLPR) polymorphism in Egyptian autistic children

Background:

Autism spectrum disorder (ASD) is a complex, heterogeneous neurodevelopmental disorder with onset during early childhood. Most studies have reported an elevation in platelet serotonin in persons with autism. The serotonin (5-hydroxytryptamine; 5-HT) transporter in the brain uptakes 5-HT from extracellular spaces. It is also present in platelets, where it takes up 5-HT from plasma. Polymorphisms in serotonin transporter gene (SLC6A4) were frequently studied in many neuropsychiatric disorders.

Materials and Methods:

We have measured the plasma 5-HT levels in 20 autistic male children and 20 control male children by the enzyme-linked immunosorbent assay (ELISA) method. In addition, the SLC6A4 promoter region (5-HTTLPR) insertion/deletion (I/D) polymorphism was studied, using whole genomic DNA.

Results:

Plasma serotonin was significantly low in autistic children compared to control (P = 0.001), although correlation to severity of autism was not significant. The frequency of short (S) allele in autism cases was 10% and in the control group it was absent.

Conclusion:

Our study demonstrated an increased prevalence of 5-HTTLPR S allele in autism subjects. Significantly decreased plasma serotonin was detected in autism subjects, with no significant relationship between 5-HTTLPR genotype and plasma 5-HT being evident.

SLC6A4 markers modulate platelet 5-HT level and specific behaviors of autism: a study from an Indian population

Presence of platelet hyperserotonemia and effective amelioration of behavioral dysfunctions by selective serotonin reuptake inhibitors (SSRI) in autism spectrum disorders (ASD) indicate that irregularities in serotonin (5-HT) reuptake and its homeostasis could be the basis of behavioral impairments in ASD patients. SLC6A4, the gene encoding serotonin transporter (SERT) is considered as a potential susceptibility gene for ASD, since it is a quantitative trait locus for blood 5-HT levels. Three functional polymorphisms, 5-HTTLPR, STin2 and 3'UTR-SNP of SLC6A4 are extensively studied for possible association with the disorder, with inconclusive outcome. In the present study, we investigated association of these polymorphisms with platelet 5-HT content and symptoms severity as revealed by childhood autism rating scale in ASD children from an Indian population. Higher 5-HT level observed in ASD was highly significant in children with heterozygous and homozygous genotypes comprising of minor alleles of the markers. Quantitative transmission disequilibrium test demonstrated significant genetic effect of STin2 allele as well as STin2/3'UTR-SNP and 5-HTTLPR/3'UTR-SNP haplotypes on 5-HT levels, but no direct association with overall CARS score and ASD phenotype. Significant genetic effect of the markers on specific behavioral phenotypes was observed for various sub-phenotypes of CARS in quantitative trait analysis. Even though the 5-HT level was not associated with severity of behavioral CARS score, a significant negative relationship was observed for 5-HT levels and level and consistency of intellectual response and general impression in ASD children. Population-based study revealed higher distribution of the haplotype 10/G of STin2/3'UTR-SNP in male controls, suggesting protective effect of this haplotype in male cases. Overall results of the study suggest that SLC6A4 markers have specific genetic effect on individual ASD behavioral attributes, might be through the modulation of 5-HT content.

A randomized double-blind placebo-controlled clinical trial of adjuvant buspirone for irritability in autism

BACKGROUND

The brain serotonin level is decreased in individuals with autism. Buspirone is a 5-HT(1A) receptor agonist with antiaggressive effects increasing prosocial behaviors.

METHODS

We conducted an 8-week randomized double-blind placebo-controlled clinical trial. Participants included 40 outpatient children and adolescents with autism. The patients took buspirone plus risperidone or risperidone plus placebo during 8 weeks. The patients were assessed at baseline, week 4, and week 8 using the Aberrant Behavior Checklist-Community Rating Scale.

RESULTS

Eighteen patients in the placebo group and 16 patients in the buspirone group completed this trial. The mean dose of buspirone was 6.7 (SD 2.7) mg/day. Irritability subscale score significantly decreased during this trial in both groups (buspirone group: declined from 25.7 [SD 5.7] to 16.3 [SD 8.5]; placebo group: declined from 24.7 [SD 7.6] to 18.2 [SD 7.7]). The Cohen d effect size was .45. Thirteen (81.2%) of 16 patients in the buspirone group and 7 (38.9%) of 18 patients in the placebo group showed a ≥30% decline in irritability score. The relative risk for treatment was 2.1. There were no serious adverse effects. The most common adverse effects in the buspirone group were increased appetite, drowsiness, and fatigue.

CONCLUSION

This clinical trial supports that low dose buspirone plus risperidone is more effective than risperidone plus placebo for treating irritability in individuals with autism.

Genes associated with autism spectrum disorder

Autism spectrum disorder (ASD) is a heterogeneous grouping of neurodevelopmental disorders characterized by impairment in social interaction, verbal communication and repetitive/stereotypic behaviors. Much evidence suggests that ASD is multifactorial with a strong genetic basis, but the underlying mechanisms are far from clear. Recent advances in genetic technologies are beginning to shed light on possible etiologies of ASD. This review discusses current evidence for several widely studied candidate ASD genes, as well as various rare genes that supports their relationship to the etiology of ASD. The majority of the data are based on molecular, cytogenetic, linkage and association studies of autistic subjects, but newer methods, including whole-exome sequencing, are also beginning to make significant contributions to our understanding of autism.

Serotonin transporter gene polymorphism and psychiatric disorders: is there a link?

Though still in infancy, the field of psychiatric genetics holds great potential to contribute to the development of new diagnostic and therapeutic options to treat these disorders. Among a large number of existing neurotransmitter systems, the serotonin system dysfunction has been implicated in many psychiatric disorders and therapeutic efficacy of many drugs is also thought to be based on modulation of serotonin. Serotonin transporter gene polymorphism is one of the most extensively studied polymorphisms in psychiatric behavioral genetics. In this article, we review the status of evidence for association between the serotonin gene polymorphism and some common mental disorders like affective disorders, post-traumatic stress disorder, obsessive-compulsive disorder, suicide, autism, and other anxiety and personality disorders. Going beyond traditional association studies, gene-environment interaction, currently gaining momentum, is also discussed in the review. While the existing information of psychiatric genetics is inadequate for putting into practice genetic testing in the diagnostic work-up of the psychiatric patient, if consistent in future research attempts, such results can be of great help to improve the clinical care of a vast majority of patients suffering from such disorders.

MAOA, DBH, and SLC6A4 variants in CHARGE: a case-control study of autism spectrum disorders

Genetic factors are established to contribute to the development of autism. We examined three loci, serotonin transporter (SLC6A4), dopamine β-hydroxylase (DBH), and the variable number of tandem repeat promoter of the monoamine oxidase A (MAOA) for association with autism in participants from the Childhood Autism Risks from Genetics and the Environment (CHARGE ) Study, the first large-scale population-based case-control investigation of both environmental and genetic contributions to autism risk. Among male children enrolled in the CHARGE study we tested associations between each of the three polymorphisms and autism (AU) (n = 119), or a combined group of autism and other autism spectrum disorders (AU+ASD, which includes an additional n = 53) as compared with typically developing controls (TD, n = 137). The case-control association analysis showed neither SLC6A4 nor DBH to be statistically significantly associated with AU or ASD. However, the male children carrying 4 tandem repeats in the promoter region of the MAOA gene showed a two-fold higher risk of AU (or AU+ASD) than those carrying allele 3, adjusted for confounders (OR = 2.02, 95% CI = 1.12, 3.65, P = 0.02 for AU vs. TD, and OR = 2.05, 95% CI = 1.19, 3.53, P = 0.01 for ASD vs. TD). In addition, children of mothers homozygous for the 4 tandem repeat allele showed at least a three-fold higher risk of AU (or AU+ASD) than those with mothers homozygous for allele 3 (OR = 3.07, 95% CI = 1.19, 7.91, P = 0.02 for AU vs. TD, and OR = 3.26, 95% CI = 1.35, 7.89, P = 0.009 for AU+ASD vs. TD). These results suggest a potential role of the functional MAOA promoter alleles in the male child, the mother, or both in ASD.

Lack of association of the serotonin transporter polymorphism with the sudden infant death syndrome in the San Diego Dataset

Dysfunction of medullary serotonin (5-HT)-mediated respiratory and autonomic function is postulated to underlie the pathogenesis of the majority of sudden infant death syndrome (SIDS) cases. Several studies have reported an increased frequency of the LL genotype and L allele of the 5-HT transporter (5-HTT) gene promoter polymorphism (5-HTTLPR), which is associated with increased transcriptional activity and 5-HT transport in vitro, in SIDS cases compared with controls. These findings raise the possibility that this polymorphism contributes to or exacerbates existing medullary 5-HT dysfunction in SIDS. In this study, we tested the hypothesis that the frequency of LL genotype and L allele are higher in 179 SIDS cases compared with 139 controls of multiple ethnicities in the San Diego SIDS Dataset. We observed no significant association of genotype or allele with SIDS cases either in the total cohort or on stratification for ethnicity. These observations do not support previous findings that the L allele and/or LL genotype of the 5-HTTLPR are associated with SIDS.

Serotonin transporter promoter variant: Analysis in Indian IBS patients and control population

BACKGROUND

Studies of serotonin reuptake transporter (SERT-P) polymorphism and irritable bowel syndrome (IBS) have shown diverse results among different populations, which might be due to racial and ethnic difference.

AIM

This study was to investigate the potential association between the SERT-P polymorphism and clinical subtypes of IBS patients in the Indian population.

METHOD

This prospective case-control study included 151 IBS patients. Ninety-two patients were diarrhea-predominant IBS, 44 were constipation-predominant IBS (C-IBS), 15 were alternating diarrhea and constipation IBS, and 100 were healthy controls. SERT gene polymorphism was studied by polymerase chain reaction.

RESULT

A genotypic association was observed between SS genotype of SERT-P polymorphism and C-IBS (P<0.05). When the L/S and L/L genotypes were combined into one group, the frequency of the S/S genotype was significantly higher than that of the non-S/S genotype between C-IBS and the control group (P<0.05). There was no significant difference in the SERT-P genotype and allele frequency between c-ibs, alternating diarrhea and constipation IBS, all types of IBS cases, and controls.

CONCLUSIONS

A significant association was observed between the SS genotype of SERT-P polymorphism and C-IBS in the Indian population.

Autism and serotonin transporter gene polymorphisms: a systematic review and meta-analysis

The serotonin transporter gene (5-HTT) plays a crucial role in serotonergic neurotransmission and has been found to be associated, with varying degrees of significance, with many diseases, including autism. Prior association studies of autism have yielded conflicting results regarding the association between two common 5-HTT polymorphisms, the promoter insertion/deletion (5-HTTLPR) and the intron 2 VNTR (STin2 VNTR). We conducted a systematic review and meta-analysis to test the following hypotheses: (i) there is an association between autism and either or both of the 5-HTTLPR and STin2 VNTR polymorphisms, and (ii) the S allele of 5-HTTLPR and/or the STin2.12 allele of the VNTR are the specific risk alleles for autism. All published family-based and population based studies were examined to determine the overall strength of association between 5-HTT polymorphisms and autism. After exclusion of studies with overlapping samples and studies whose data did not allow for calculation of an odds ratio, 16 studies were included for final analyses, all but two of which used a family-based design. The meta-analysis failed to find a significant overall association between either of the 5-HTT polymorphisms examined and autism. Further, no allelic transmission distortion was found when studies of simplex (11 studies) and multiplex (3 studies) family samples were analyzed separately. However, there was significant heterogeneity by ethnicity; family based studies of US mixed population samples showed preferential transmission of the S allele of 5-HTTLPR (S allele:L allele = 247:183), while there was no allelic distortion among the family-based studies of European and Asian samples.

Polymorphisms of the serotonin transporter gene (5-HTTLPR, A/G SNP in 5-HTTLPR, and STin2 VNTR) and their relation to personality traits in healthy individuals from Russia

OBJECTIVE

Numerous studies have reported association of the serotonin transporter gene (5-HTT) polymorphisms and neuroticism and traits characterizing sociability and activity. This study aimed to define a single genotype effect of three polymorphic markers in the 5-HTT gene (5-HTTLPR, A/G SNP in 5-HTTLPR and STin2 VNTR) and to check possible association of the 5-HTT haplotypes and personality traits [assessed with Eysenck Personality Inventory (EPI) and Temperament and Character Inventory (TCI) questionnaires] in 301 healthy young individuals.

METHODS

To investigate single genotype and haplotype effects of all polymorphic markers, multivariate analysis of variance and haplotype trend regression analyses were conducted correspondingly.

RESULTS

Individuals with STin2.10 allele scored significantly lower on Neuroticism (EPI) (P=0.007) and Harm Avoidance (P=0.005) in the overall sample. The same pattern of association was reported in women: carriers of STin2.10 allele scored lower on Harm Avoidance (TCI) (P=0.008). Haplotype trend regression analyses revealed that carriers of S12 haplotype had lower sociability-related traits such as Extraversion (EPI) and Novelty Seeking (TCI), whereas Harm Avoidance (TCI) (anxiety-related trait) was higher. Opposite association was observed for S10 haplotype: Extraversion (EPI) score was higher, whereas Harm Avoidance (TCI) score was lower in carriers of this haplotype.

CONCLUSION

As single polymorphism effect of STin2 was observed in relation to anxiety-related traits, opposite S10 and S12 haplotype effects on Neuroticism and Harm Avoidance could be explained by the larger impact of STin2 polymorphism. Controversially, we consider that the variance in sociability-related traits is related to specific haplotypes of 5-HTT gene.

Genetic analysis of reelin gene (RELN) SNPs: no association with autism spectrum disorder in the Indian population

Involvement of reelin with Autism spectrum disorder (ASD) has been implicated through several biochemical as well as genetic studies. Reelin is an extracellular signaling protein, which plays a significant role in cytoarchitectonic pattern formation of different brain areas during development. Reelin gene (RELN) is located on chromosome 7q22; an important autism critical region identified through several genome-wide scans. A number of genetic studies have been carried out to investigate the association of reelin with autism. Recently we reported possible paternal effect in the transmission of CGG repeat alleles of RELN in the susceptibility towards autism. Further analysis on other polymorphisms is warranted to validate the status of RELN as a candidate for autism. Therefore in the present study, we have investigated six more SNPs (rs727531, rs2072403, rs2072402, rs362691, rs362719, rs736707) in 102 patients, 182 parents and 101 healthy controls. We have followed DSM-IV criteria and the screening for autism was carried out using CARS. Genomic DNA isolated from blood was used for PCR and subsequent RFLP analysis. Finally, case-control and family-based association studies were carried out to examine the genetic association of these SNP markers with ASD in the Indian population. But, we failed to detect either preferential parental transmission of any alleles of the markers to affected offspring or any biased allelic or genotypic distribution between the cases and controls. Thus the present study suggests that these SNPs of RELN are unlikely to be associated with ASD in the Indian population.

The loss of methyl-CpG binding protein 1 leads to autism-like behavioral deficits

Methyl-CpG binding proteins (MBDs) are central components of DNA methylation-mediated epigenetic gene regulation. Alterations of epigenetic pathways are known to be associated with several neurodevelopmental disorders, particularly autism. Our previous studies showed that the loss of Mbd1 led to reduced hippocampal neurogenesis and impaired learning in mice. However, whether MBD1 regulates the autism-related cognitive functions remains unknown. Here we show that Mbd1 mutant (Mbd1(-/-)) mice exhibit several core deficits frequently associated with autism, including reduced social interaction, learning deficits, anxiety, defective sensory motor gating, depression and abnormal brain serotonin activity. Furthermore, we find that Mbd1 can directly regulate the expression of Htr2c, one of the serotonin receptors, by binding to its promoter, and the loss of Mbd1 led to elevated expression of Htr2c. Our results, therefore, demonstrate the importance of epigenetic regulation in mammalian brain development and cognitive functions. Understanding how the loss of Mbd1 could lead to autism-like behavioral phenotypes would reveal much-needed information about the molecular pathogenesis of autism.

Lack of association between down syndrome and polymorphisms in dopamine receptor D4 and serotonin transporter genes

Down Syndrome (DS) patients suffer from cognitive dysfunction, depression, hyperactivity, irritability etc. Dopamine (DA) and serotonin (5HT) are known to control cognitive and behavioral attributes. An increased number of the DA receptor 4 (DRD4) is detected in brain regions primarily involved in cognition. Impairments in executive function have also been reported with depletion in 5HT. A variable number of tandem repeat (VNTR) in the exon 3 of DRD4 and an insertion/deletion polymorphism in the promoter region of 5HT transporter (5HTTLPR) have been found to be associated with different neurobehavioral disorders; however, association of these polymorphisms with DS has never been explored. The present family-based analysis on DS revealed significant over-transmission of a DRD4 VNTR allele which encodes for D4 receptor with average activity. No association was noticed for the 5HTTLPR. We may conclude that these genetic polymorphisms are not contributing to the neuromotor and cognitive dysfunctions observed in DS.

The 5-HTTLPR polymorphism in South African healthy populations: a global comparison

The serotonin transporter promoter length polymorphism (5-hydroxytryptamine transporter length polymorphism, 5-HTTLPR) in serotonin transporter gene has been implicated in numerous psychiatric disorders. Having a high affinity for the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT), serotonin transporter controls the duration, availability and signaling capacity of 5-HT in the synapse. Association studies have focused extensively on this polymorphic region as the frequencies of long- and short-alleles of this gene differ greatly amongst populations and association studies have either reported conflicting results or nothing significant at all. In this study, the genotype and allele frequencies of 5-HTTLPR polymorphism were determined in the healthy South African (SA) individuals belonging to diverse ethnic backgrounds. Cheek cell samples were collected from the three major ethnic groups namely: Caucasians, Africans and coloreds/Mixed population. The DNA was extracted and genotyped for the 5-HTTLPR. Genotypes were compared amongst the three major ethnic groups from SA as well as to that of other studies around the world. This is the first study to report significant differences in the 5-HTTLPR genotype and allelic frequencies among various ethnic groups in SA. Future studies will target larger population groups and the estimation of frequency of these alleles in individuals with autism.

Glutamate receptor 6 gene (GluR6 or GRIK2) polymorphisms in the Indian population: a genetic association study on autism spectrum disorder

Autism is a neurodevelopmental disorder with early manifestation. It is a multifactorial disorder and several susceptible chromosomal regions for autism are identified through genome scan studies. The gene coding for glutamate receptor 6 (GluR6 or GRIK2) has been suggested as a candidate gene for autism based on its localization in the autism specific region on chromosome 6q21 and the involvement of receptor protein in cognitive functions like learning and memory. Despite its importance, so far no studies have been carried out on possible involvement of GluR6 with autism in the Indian population. Therefore in the present study, we have performed genetic analysis of three markers of GluR6 (SNP1: rs2227281, SNP2: rs2227283, SNP3: rs2235076) for possible association with autism through population, and family-based (TDT and HHRR) approaches. DSM-IV criteria and CARS/ADI-R have been utilized for diagnosis. Genotyping analysis for the SNPs has been carried out in 101 probands with autism spectrum disorder, 180 parents and 152 controls from different regions of India. Since the minor allele frequency of SNP3 was too low, the association studies have been carried out only for SNP1 and SNP2. Even though two earlier studies have shown association of these markers with autism, the present case-control and TDT, as well as HHRR analyses have not demonstrated any biased transmission of alleles or haplotypes to the affected offspring. Thus our results suggest that these markers of GluR6 are unlikely to be associated with autism in the Indian population.

Family-based association study of 5-HTTLPR and the 5-HT2A receptor gene polymorphisms with autism spectrum disorder in Korean trios

The potential role of the serotoninergic system in the development of autistic disorder has long been suggested based on the observation of hyperserotoninemia in autistic subjects and the results of drug treatment studies. Promoter region polymorphisms (5-HTTLPR) of the serotonin transporter gene (SLC6A4) and the 5-HT2A receptor gene (HTR2A) have been studied as potential candidate genes in autism spectrum disorder (ASD). The objective of this family-based linkage/association study is to evaluate the relationship between ASD and 5-HTTLPR as well as that between some SNPs of HTR2A and ASD in Korean trios by using the transmission disequilibrium test (TDT). Genotyping was performed for 5-HTTLPR and two single nucleotide polymorphisms (SNPs) (-1438G/A and 102T/C) of HTR2A. The TDT, linkage disequilibrium (LD) analysis and haplotype analysis were performed. This study comprised 126 complete trios of ASD patients and both parents. With regard to the transmission of 5-HTTLPR, the long allelic variant was preferentially transmitted in the ASD subjects. Based on the TDT results, there was no significant difference in the transmission of the two SNPs of HTR2A. However, in the results of the haplotype analysis, the AT haplotype demonstrated significant evidence of association with autism. The global chi(2) test for haplotype transmission revealed a significant association between HTR2A and autism. Although we identified a significant association between ASD and 5-HTTLPR as well as between ASD and HTR2A, it cannot exclude the chance finding because of the low level of statistical significance and relatively small power. We believe that further studies are required to examine the relationship between serotonin-related genes and the behavioral phenotypes of ASD in the Korean population.

A review of gene linkage, association and expression studies in autism and an assessment of convergent evidence

Autism is a neurodevelopmental disorder with high heritability and a likely complex genetic architecture. Much genetic evidence has accumulated in the last 20 years but no gene has been unequivocally identified as containing risk variants for autism. In this article we review the past and present literature on neuro-pathological, genetic linkage, genetic association, and gene expression studies in this disorder. We sought convergent evidence to support particular genes or chromosomal regions that might be likely to contain risk DNA variants. The convergent evidence from these studies supports the current hypotheses that there are multiple genetic loci predisposing to autism, and that genes involved in neurodevelopment are especially important for future genetic studies. Convergent evidence suggests the chromosome regions 7q21.2-q36.2, 16p12.1-p13.3, 6q14.3-q23.2, 2q24.1-q33.1, 17q11.1-q21.2, 1q21-q44 and 3q21.3-q29, are likely to contain risk genes for autism. Taken together with results from neuro-pathological studies, genes involved in brain development located at the above regions should be prioritized for future genetic research.