**3.3 Selected candidate genes**

As it is becoming apparent, a genetic predisposition to ASD may involve one or more interconnected genetic networks involving neurogenesis, neuronal migration, synaptogenesis, axon pathfinding and neuronal or glial structure regionalization [110]. Function-targeted studies, mainly by association that focus exclusively on the candidate genes, including some of the most widely studied will be reviewed in the following section (table 2).

Reelin is an extracellular matrix glycoprotein responsible for guiding the migration of several neural cell types and the establishment of neural connection. In the 1980s, it was discovered that reelin plays important roles in the positioning of neuronal cells in the inferior olivery complex, cerebral cortex and cerebellum early in embryonic development [203-205]. Further research has confirmed and further extended our knowledge about the widespread functions reelin plays in laminated regions of the brain, both embryonically and postnatally [206-208].

Given the critical functions of reelin in brain development, and knowing there are neuroanatomical abnormities in autism [209], the reelin gene (*RELN*) was a plausible candidate to investigate in ASDs. Significantly reduced levels of reelin in the human cortex, cerebellum and peripheral blood were confirmed in ASD at both the protein and mRNA levels [210-212]. Genome-wide scans also identified 7q22 as an autism critical region, where *RELN* is located [213].

CNVs were confirmed [102]. A Korean group recently reported deletion CNVs at 8p23.1 and 17p11.2 using whole-genome aCGH [103]. Using aCGH with a mean 19 kb resolution, 51 autism-specific CNV were identified in 397 unrelated ASD subjects [100]. Similarly, Qiao and colleagues performed aCGH on 100 autistic subjects and identified 9 CNVs, three of which were unique to their cohort [104]. A Spanish group recently reported the identification of 13 CNVs containing 24 different genes in their sample of 96 ASD

Single-nucleotide polymorphism (SNP) array analysis, primarily developed to determine linkage, now is also employed to determine genomic CNVs [106]. Marshall performed a genome-wide assessment via SNP array analysis. They genotyped proximately 500,000 SNPs for each sample and detected 13 loci with recurrent or overlapping CNVs in a sample of 427 ASD cases [101]. Using SNP markers, another group identified 6 CNVs within a 2.2 megabase (Mb) intergenic Chr 2 region between cadherin 10 (CDH10) and cadherin 9 (CDH9) in a combined sample set of 1,984 ASD probands of European ancestry [107]. In addition, SNP array analysis offers some special advantages in the exploration of potentially relevant gene networks. Two recent reports have provided strong evidence for the involvement of certain genes in important gene networks including neuronal cell-adhesion,

Currently available aCGH methods for identifying CNV typically assay the genome in the 40-kb to several Mb range. Methodological improvements that employ oligonucleotides are providing a high potential resolution down to approximately the 5-kb resolution level for aCGH with genome-wide detection of CNVs [106]. Thus, SNP or oligonucleotide aCGH analysis can detect a CNV as small as a few kilobases. Therefore, it is clear that the higherdensity oligonucleotide or SNP arrays offer the higher resolution for analysis of CNVs in the

As it is becoming apparent, a genetic predisposition to ASD may involve one or more interconnected genetic networks involving neurogenesis, neuronal migration, synaptogenesis, axon pathfinding and neuronal or glial structure regionalization [110]. Function-targeted studies, mainly by association that focus exclusively on the candidate genes, including some of the most widely studied will be reviewed in the following section

Reelin is an extracellular matrix glycoprotein responsible for guiding the migration of several neural cell types and the establishment of neural connection. In the 1980s, it was discovered that reelin plays important roles in the positioning of neuronal cells in the inferior olivery complex, cerebral cortex and cerebellum early in embryonic development [203-205]. Further research has confirmed and further extended our knowledge about the widespread functions reelin plays in laminated regions of the brain, both embryonically and

Given the critical functions of reelin in brain development, and knowing there are neuroanatomical abnormities in autism [209], the reelin gene (*RELN*) was a plausible candidate to investigate in ASDs. Significantly reduced levels of reelin in the human cortex, cerebellum and peripheral blood were confirmed in ASD at both the protein and mRNA levels [210-212]. Genome-wide scans also identified 7q22 as an autism critical region, where

ubiquitin degradation and GTPase/Ras signaling [108, 109].

subjects [105].

future.

(table 2).

postnatally [206-208].

*RELN* is located [213].

**3.3 Selected candidate genes** 


Table 2. Selected candidate genes

*i. Reelin gene (RELN)* 

Additionally, case-control and family-based studies provided further evidence supporting the association of *RELN* and ASD. Persico identified a *RELN*–related polymorphic GGC repeat located immediately 5' of the ATG initiator codon in Italian and American subjects [120]. Using the similar methods and 126 multiplex ASD families, Zhang *et al* examined the polymorphic CGG-repeat of *RELN* [118]. Family-based association tests showed that larger *RELN* alleles (≥11 repeats) were transmitted more often than expected to autistic children. Independant studies regarding the CGG-repeat of *RELN* have also supported its contribution to the genetic risk of autism [112, 113, 115]. Others have also reported significant differences in the transmission of the reelin alleles of exon 22 and intron 59 SNPs to autistic subjects [114]. However, results have not been uniformly positive. Krebs *et al* performed a transmission disequilibrium test (TDT) analysis of the CGG-repeat polymorphism in 167 Caucasian families and found no evidence of linkage or association [119]. Similarly, another two groups failed to find a significant association of *RELN* CGG repeat polymorphisms with liability to autism [116, 117].

The association between *RELN* and ASD were also found in other ethnic groups besides Caucasian populations. Recently, a significant genetic association between the *RELN* SNP2 (located in intron 59) and ASD was reported in a Chinese Han population, and the combination of *RELN* SNP1/SNP2/SNP3/SNP4, all in strong linkage disequilibrium, were reported to have a significant association with ASD [111].

Autoimmune Disorder and Autism 527

respectively. They encode a family of cell-adhesion molecules, the neuroligans, essential for

The earliest report regarding the potential association of *NLGN* genes and ASD came from the study of multiple Swedish families [163]. The authors screened for *NLGN3* mutations in 36 affected sib-pairs and 122 trios with ASD. They found one *de nov*o mutation in *NLGN4* in one family. This mutation creates a stop codon leading to premature termination of the protein. In another family, a C to T transition in *NLGN3* was identified that changed a highly conserved arginine residue into cysteine (R451C) within the esterase domain. It was inherited from the mother. Following this report, several other groups studied this gene but found little support for common mutations of the gene. Limited support came from a Portuguese group, who found missense changes in *NLGN4* as well as the protein-truncating mutations in ASD [162]. A Finnish group conducted a molecular genetic analysis of *NLGN1, NLGN3, NLGN4*, and *NLNG4Y*. Their results suggested neuroligin mutations most probably represent rare causes of autism and concluded that it was unlikely that the allelic variants in these genes would be major risk factors for autism [166]. Others have also failed to obtain

Other reports about mutations of *NLGN3* or *NLGN4* have identified splice variants in both genes [161]. Three groups recently reported one missense variant and two single substitutions in independent autistic samples, indicating that a defect of synaptogenesis

Oxytocin is a nine-amino-acid peptide synthesized in the hypothalamus. Apart from regulating lactation and uterine contraction, oxytocin acts as a neuromodulator in the central nervous system [219, 220]. Both animal experiments and clinical research have confirmed the role oxytocin plays in social and repetitive behaviors [221]. Therefore the oxytocin system might be potentially involved in the pathogenesis of ASD, and the human oxytocin receptor gene (*OXTR*) has been regarded as a most promising candidate gene to

Indeed, research pertaining to the potential association between *OXTR* and autism has come to positive conclusions. Using family-based and population-based association tests, SNPs and haplotypes in the *OXTR* have been reported to confer risk for ASD in different ethnic groups [170, 172-174]. They have also been associated with IQ and adaptive behavior scale scores [172]. Furthermore, a recent study identified significant increases in the DNA methylation status of OXTR in peripheral blood cells and temporal cortex, as well as decreased expression of *OXTR* mRNA in the temporal cortex of autism cases, suggesting

The human *MET* gene encodes a transmembrane receptor tyrosine kinase of the hepatocyte growth factor/scatter factor (HGF/SF) [222]. Though primarily identified as an oncogene, MET plays crucial roles in neuronal development [222-224]. Moreover, impaired MET signaling causes abnormal interneuron migration and neural growth in the cortex, as well as decreased proliferation of granule cells, which matches many of the features found in

Campbell and colleagues have done a series of studies regarding the association between MET signaling and autism. They first reported the genetic association of a common C allele

that epigenetic dysregulation may be involved in the pathogenesis of ASD [171].

positive results, casting doubt on the earlier conclusion [164, 165, 167-169].

the formation of functional neural synapses [163, 169].

may predispose to autism [158-160]. *v. Human oxytocin receptor gene (OXTR)* 

study.

*vi. MET* 

autistic brains [223, 225].

#### *ii. Human serotonin transporter gene (SLC6A4)*

The human serotonin transporter, encoded by *SLC6A4,* localizes to chromosome 17q11.1-q12 and consists of 15 exons [214]. *SLC6A4* was considered as a candidate gene for autism primarily based on the elevated blood serotonin levels found in a number of autistic probands, as well as the efficacy of potent serotonin transporter inhibitors in reducing rituals and routines [215, 216]. Using the TDT, positive associations of a 5-HTTLPR polymorphism found in the promoter region of the *SLC6A4* gene with autism have been identified by 4 family-based studies and 2 case-control studies [121, 123, 125-127]. Other groups have performed both family-based and case-control analysis and found significant associations of the *SLC6A4* polymorphism with autism [122, 124]. In contrast to these positive reports, 9 family-based studies failed to find evidence for associations of the *SLC6A4* polymorphism with autism [130, 132-134, 136-140], as well as a case-control study [128]. An Indian group performed a series of studies but found no persuasive evidence of the association of the *SLC6A4* polymorphisms with autism [129, 135, 217]. In addition, a systematic review and meta-analysis failed to find a significant overall association of the serotonin polymorphisms examined and autism [131].

#### *iii. Gamma-aminobutyric acid receptor gene (GABR)*

Gamma-aminobutyric acid (GABA) is the chief inhibitory neurotransmitter in the brain, acting by binding to a GABA receptor. The receptor is a multimeric transmembrane receptor that consists of five subunits arranged around a central pore. The GABA receptor subunits are homologous, but are both structurally and functionally diverse [144]. Three of the GABA receptor subunit genes (*GABRB3, GABRA5* and *GABRG3*) are localized to chromosome 15q11-q13, one of the most complex regions in the genome involved with genome instability, gene expression, imprinting and recombination [156].

The region 15q11-q13 was originally associated with ASD based on several studies which reported a common duplication of this region in ASD subjects [147, 148, 152, 154]. A chromosome-engineered mouse model for human 15q11-13 duplication was developed with autistic features [141, 143, 153]. Cook *et al* examined markers across this region for linkage disequilibrium in 140 families with ASD, detecting significant linkage disequilibrium between *GABRB3* and ASD [218]. This finding was confirmed by others as well [145, 146, 151]. Also, two SNPs located within the *GABRG3* gene were associated with ASD using the Pedigree Disequilibrium Test (PDT) [144]. An independent study demonstrated nominally signicant associations between six markers across the *GABRB3* and *GABRA5* genes [142]. Moreover, using ordered-subset analysis (OSA) another group provided evidence of increased linkage at the *GABRB3* locus [149]. Other research has also identified significant association and gene-gene interactions of GABA receptor subunit genes in autism [150].

Nonetheless, conflicting evidence has also been reported. Other groups have reported limited or no association between GABA receptor polymorphisms and autism [155, 156]. Similarly, another group conducted a full genome search for autism susceptibility loci including seven microsatellite markers from 15q11-q13, and found no significant evidence of association or linkage [157]. Thus the linkage results are at best inconclusive.

#### *iv. Neuroligin genes (NLGN)*

The marked difference in sex ratio for ASD justifies the exploration of genes on the sex chromosome, among which the neuroligin genes (*NLGN*) are perhaps the most widely studied. Five *NLGN* have been identified in the human genome, which are localized at 3q26(*NLGN1*), 17p13 (*NLGN2*), Xq13 (*NLGN3*), Xp22.3 (*NLGN4*), and Yq11.2 (*NLGN4Y*)

The human serotonin transporter, encoded by *SLC6A4,* localizes to chromosome 17q11.1-q12 and consists of 15 exons [214]. *SLC6A4* was considered as a candidate gene for autism primarily based on the elevated blood serotonin levels found in a number of autistic probands, as well as the efficacy of potent serotonin transporter inhibitors in reducing rituals and routines [215, 216]. Using the TDT, positive associations of a 5-HTTLPR polymorphism found in the promoter region of the *SLC6A4* gene with autism have been identified by 4 family-based studies and 2 case-control studies [121, 123, 125-127]. Other groups have performed both family-based and case-control analysis and found significant associations of the *SLC6A4* polymorphism with autism [122, 124]. In contrast to these positive reports, 9 family-based studies failed to find evidence for associations of the *SLC6A4* polymorphism with autism [130, 132-134, 136-140], as well as a case-control study [128]. An Indian group performed a series of studies but found no persuasive evidence of the association of the *SLC6A4* polymorphisms with autism [129, 135, 217]. In addition, a systematic review and meta-analysis failed to find a significant overall association of the

Gamma-aminobutyric acid (GABA) is the chief inhibitory neurotransmitter in the brain, acting by binding to a GABA receptor. The receptor is a multimeric transmembrane receptor that consists of five subunits arranged around a central pore. The GABA receptor subunits are homologous, but are both structurally and functionally diverse [144]. Three of the GABA receptor subunit genes (*GABRB3, GABRA5* and *GABRG3*) are localized to chromosome 15q11-q13, one of the most complex regions in the genome involved with genome

The region 15q11-q13 was originally associated with ASD based on several studies which reported a common duplication of this region in ASD subjects [147, 148, 152, 154]. A chromosome-engineered mouse model for human 15q11-13 duplication was developed with autistic features [141, 143, 153]. Cook *et al* examined markers across this region for linkage disequilibrium in 140 families with ASD, detecting significant linkage disequilibrium between *GABRB3* and ASD [218]. This finding was confirmed by others as well [145, 146, 151]. Also, two SNPs located within the *GABRG3* gene were associated with ASD using the Pedigree Disequilibrium Test (PDT) [144]. An independent study demonstrated nominally signicant associations between six markers across the *GABRB3* and *GABRA5* genes [142]. Moreover, using ordered-subset analysis (OSA) another group provided evidence of increased linkage at the *GABRB3* locus [149]. Other research has also identified significant association and gene-gene interactions of GABA receptor subunit genes in autism [150]. Nonetheless, conflicting evidence has also been reported. Other groups have reported limited or no association between GABA receptor polymorphisms and autism [155, 156]. Similarly, another group conducted a full genome search for autism susceptibility loci including seven microsatellite markers from 15q11-q13, and found no significant evidence of

*ii. Human serotonin transporter gene (SLC6A4)* 

serotonin polymorphisms examined and autism [131].

instability, gene expression, imprinting and recombination [156].

association or linkage [157]. Thus the linkage results are at best inconclusive.

The marked difference in sex ratio for ASD justifies the exploration of genes on the sex chromosome, among which the neuroligin genes (*NLGN*) are perhaps the most widely studied. Five *NLGN* have been identified in the human genome, which are localized at 3q26(*NLGN1*), 17p13 (*NLGN2*), Xq13 (*NLGN3*), Xp22.3 (*NLGN4*), and Yq11.2 (*NLGN4Y*)

*iii. Gamma-aminobutyric acid receptor gene (GABR)* 

*iv. Neuroligin genes (NLGN)* 

respectively. They encode a family of cell-adhesion molecules, the neuroligans, essential for the formation of functional neural synapses [163, 169].

The earliest report regarding the potential association of *NLGN* genes and ASD came from the study of multiple Swedish families [163]. The authors screened for *NLGN3* mutations in 36 affected sib-pairs and 122 trios with ASD. They found one *de nov*o mutation in *NLGN4* in one family. This mutation creates a stop codon leading to premature termination of the protein. In another family, a C to T transition in *NLGN3* was identified that changed a highly conserved arginine residue into cysteine (R451C) within the esterase domain. It was inherited from the mother. Following this report, several other groups studied this gene but found little support for common mutations of the gene. Limited support came from a Portuguese group, who found missense changes in *NLGN4* as well as the protein-truncating mutations in ASD [162]. A Finnish group conducted a molecular genetic analysis of *NLGN1, NLGN3, NLGN4*, and *NLNG4Y*. Their results suggested neuroligin mutations most probably represent rare causes of autism and concluded that it was unlikely that the allelic variants in these genes would be major risk factors for autism [166]. Others have also failed to obtain positive results, casting doubt on the earlier conclusion [164, 165, 167-169].

Other reports about mutations of *NLGN3* or *NLGN4* have identified splice variants in both genes [161]. Three groups recently reported one missense variant and two single substitutions in independent autistic samples, indicating that a defect of synaptogenesis may predispose to autism [158-160].

#### *v. Human oxytocin receptor gene (OXTR)*

Oxytocin is a nine-amino-acid peptide synthesized in the hypothalamus. Apart from regulating lactation and uterine contraction, oxytocin acts as a neuromodulator in the central nervous system [219, 220]. Both animal experiments and clinical research have confirmed the role oxytocin plays in social and repetitive behaviors [221]. Therefore the oxytocin system might be potentially involved in the pathogenesis of ASD, and the human oxytocin receptor gene (*OXTR*) has been regarded as a most promising candidate gene to study.

Indeed, research pertaining to the potential association between *OXTR* and autism has come to positive conclusions. Using family-based and population-based association tests, SNPs and haplotypes in the *OXTR* have been reported to confer risk for ASD in different ethnic groups [170, 172-174]. They have also been associated with IQ and adaptive behavior scale scores [172]. Furthermore, a recent study identified significant increases in the DNA methylation status of OXTR in peripheral blood cells and temporal cortex, as well as decreased expression of *OXTR* mRNA in the temporal cortex of autism cases, suggesting that epigenetic dysregulation may be involved in the pathogenesis of ASD [171].

#### *vi. MET*

The human *MET* gene encodes a transmembrane receptor tyrosine kinase of the hepatocyte growth factor/scatter factor (HGF/SF) [222]. Though primarily identified as an oncogene, MET plays crucial roles in neuronal development [222-224]. Moreover, impaired MET signaling causes abnormal interneuron migration and neural growth in the cortex, as well as decreased proliferation of granule cells, which matches many of the features found in autistic brains [223, 225].

Campbell and colleagues have done a series of studies regarding the association between MET signaling and autism. They first reported the genetic association of a common C allele

Autoimmune Disorder and Autism 529

*TPH1* and *TPH2* encode rate-limiting enzymes that control serotonin biosynthesis. TPH1 is primarily expressed peripherally, while TPH2 is found exclusively in brain tissue. However, despite evidence for the potential involvement of the serotonin system in the etiology of autism, only one of three reports to date conservatively has supported the notion that *TPH2*

The association between prenatal insults and the pathogenesis of autism has been reported recent decades. Early in 2005, Beversdorf *et al*. conducted surveys regarding incidence and timing of prenatal stressors. They found a higher incidence of prenatal stressors in autism at 21-32 weeks gestation, which peaks at 25-28 weeks. Their finding supported the hypothesis of prenatal stressors as a potential contributor to autism, and the timing was consistent with the embryological age suggested by neuroanatomical findings seen in the cerebellum in autism [231]. More specifically, Meyer *et al* demonstrate that the effects of maternal immune challenge between middle and late gestation periods in mice are dissociable in terms of several neuropsychiatric disorders including autism [232]. However, this conclusion was challenged by another group of scientists. Ploeger *et al*. proposed pleiotropic effects during a very early and specic stage of embryonic development, namely early organogenesis (day 20 to day 40 after fertilization) in order to explain the effect of uterine disturbances to the development of autism [233]. They provided ample evidence from literature for the association between autism and many different kinds of physical anomalies such as limb deformities, craniofacial malformations, brain pathology, and anomalies in other organs, which agrees with the hypothesis that pleiotropic effects are involved in the development of

Drugs are the most important prenatal factors affecting embryo and fetal development. Cumulating data support the relationships between maternal medication and fetogeneous diseases including autism. The obnoxious drug thalidomide turned out not only to relate to fetal abnormality but also to autism. Stromland group retrospectively investigated 100 Swedish thalidomide embryopathy cases and found possible association of thalidomide embryopathy with autism [234]. Another example of drug relating to autism is valproate. Williams *et al* reported six cases whose clinical phenotype was compatible with both fetal valproate syndrome (FVS) and autism. Although the sample size is small, the authors claimed the association between this known teratogen and autism had both clinical and research implications [235]. Similarly, Rasalam group provided another line of evidence that prenatal exposure to sodium valproate is a risk factor for the development of an ASD [236]. Another prenatal factor is intrauterine inflammation. Kannan *et al* conducted an animal study to demonstrate intrauterine inflammation results in alterations in cortical serotonin and disruption of serotonin-regulated thalamocortical development in the newborn brain therefore resulting in impairment of the somatosensory system, such as autism [237]. More persuasive evidence comes from Girard's report. According to their results, end of gestation exposure of pregnant rats to systemic microbial product such as lipopolysacharide (LPS) is an independent risk factor for neurodevelopmental diseases such as cerebral palsy, mental deficiency, and autism. And coadministration of IL-1 receptor antagonist with LPS

plays a role in autism susceptibility [197, 201, 202].

alleviated the detrimental effects caused by LPS [238].

**4. Environmental factors** 

**4.1 Prenatal factors** 

autism.

in the promoter region of *MET*, which results in significant decrease in *MET* promoter activity and altered binding of specific transcription factor complexes [179]. Then they found significantly decreased MET protein levels and increased mRNA expression for proteins involved in regulating MET signaling activity [226]. Furthermore, they screened the exons and 5' promoter regions for variants in the five genes encoding the proteins that regulate *MET* expression, finding that genetic susceptibility impacting multiple components of the MET signaling pathway contributes to ASD risk [178]. Most recently, they found that the *MET* C allele inuences two of the behavioral domains of the autism triad [175]. Other groups have also provided supportive evidence that *MET* gene variations may play a role in autism susceptibility [176, 177].

#### *vii. SLC25A12*

*SLC25A12* locates in the chromosome 2q31 region, encoding the mitochondrial aspartate/glutamate carrier (AGC1), a key protein involved in mitochondrial function and ATP synthesis. Since the physiological function of neurons greatly depends on energy supply, any alteration in mitochondrial function or ATP synthesis could lead to corresponding changes in neurons [227]. Recently mitochondrial hyperproliferation and partial respiratory chain block were found in two autistic patients, suggesting *SLC25A12* could be a promising candidate gene [228].

Following this report, several studies for genetic variants of the gene were performed. Three different ethnic groups reported linkage and association between ASD and two SNPs (i.e. rs2056202 and rs2292813) in *SLC25A12* [180, 182, 183], while another three independent groups failed to reveal significant association [184-186]. Another group associated one SNP (rs2056202) with ASD but not the other [181]. Thus, the findings so far are inconclusive.

#### *viii. Other candidate genes*

The glutamate receptor 6 gene (*GRIK2* or *GluR6*) is located at chromosome 6q21. Given that glutamate is the principal excitatory neurotransmitter in the brain and it is involved in cognitive functions such as memory and learning, *GRIK2* was proposed as a gene candidate for ASD [229]. Unfortunately, the limited reports have very different results. Genetic studies in a Caucasian population, Chinese Han and Korean trios provided positive evidence, but using different SNPs [187-189]. Another report failed to find any association of *GRIK2* with autism in an Indian population [189].

Contactin associated protein-2 (*CNTNAP2*) belongs to the neurexin family, within which several members have been identified as being related to autism [230]. A recent research report identified a homozygous mutation of *CNTNAP2* in Amish children with pervasive developmental disorders, seizures, and language regression [196]. Five other studies have supported this finding that *CNTNAP2* may be a genetic susceptibility factor in autism [191- 195]. Another group found that *CNTNAP2* provided a strong male affection bias in ASD [193].

Glyoxalase 1 is a cytosolic, ubiquitously expressed, zinc metalloenzyme enzyme involved in scavenging toxic α-oxoaldehydes formed during cellular metabolic reactions. Proteomics analysis found glyoxalase 1 increased in autism brains, and subsequent sequencing of its gene (*GLO1*) identified that homozygosity for a polymorphism of the gene, A419 *GLO1,* resulted in decreased enzyme activity and association with autism [198], although this conclusion was not confirmed by other studies [199, 200]. In addition, one group found a protective effect of the A419 allele of *GLO1* [197].

in the promoter region of *MET*, which results in significant decrease in *MET* promoter activity and altered binding of specific transcription factor complexes [179]. Then they found significantly decreased MET protein levels and increased mRNA expression for proteins involved in regulating MET signaling activity [226]. Furthermore, they screened the exons and 5' promoter regions for variants in the five genes encoding the proteins that regulate *MET* expression, finding that genetic susceptibility impacting multiple components of the MET signaling pathway contributes to ASD risk [178]. Most recently, they found that the *MET* C allele inuences two of the behavioral domains of the autism triad [175]. Other groups have also provided supportive evidence that *MET* gene variations may play a role in

*SLC25A12* locates in the chromosome 2q31 region, encoding the mitochondrial aspartate/glutamate carrier (AGC1), a key protein involved in mitochondrial function and ATP synthesis. Since the physiological function of neurons greatly depends on energy supply, any alteration in mitochondrial function or ATP synthesis could lead to corresponding changes in neurons [227]. Recently mitochondrial hyperproliferation and partial respiratory chain block were found in two autistic patients, suggesting *SLC25A12*

Following this report, several studies for genetic variants of the gene were performed. Three different ethnic groups reported linkage and association between ASD and two SNPs (i.e. rs2056202 and rs2292813) in *SLC25A12* [180, 182, 183], while another three independent groups failed to reveal significant association [184-186]. Another group associated one SNP (rs2056202) with ASD but not the other [181]. Thus, the findings so far are inconclusive.

The glutamate receptor 6 gene (*GRIK2* or *GluR6*) is located at chromosome 6q21. Given that glutamate is the principal excitatory neurotransmitter in the brain and it is involved in cognitive functions such as memory and learning, *GRIK2* was proposed as a gene candidate for ASD [229]. Unfortunately, the limited reports have very different results. Genetic studies in a Caucasian population, Chinese Han and Korean trios provided positive evidence, but using different SNPs [187-189]. Another report failed to find any association of *GRIK2* with

Contactin associated protein-2 (*CNTNAP2*) belongs to the neurexin family, within which several members have been identified as being related to autism [230]. A recent research report identified a homozygous mutation of *CNTNAP2* in Amish children with pervasive developmental disorders, seizures, and language regression [196]. Five other studies have supported this finding that *CNTNAP2* may be a genetic susceptibility factor in autism [191- 195]. Another group found that *CNTNAP2* provided a strong male affection bias in ASD

Glyoxalase 1 is a cytosolic, ubiquitously expressed, zinc metalloenzyme enzyme involved in scavenging toxic α-oxoaldehydes formed during cellular metabolic reactions. Proteomics analysis found glyoxalase 1 increased in autism brains, and subsequent sequencing of its gene (*GLO1*) identified that homozygosity for a polymorphism of the gene, A419 *GLO1,* resulted in decreased enzyme activity and association with autism [198], although this conclusion was not confirmed by other studies [199, 200]. In addition, one group found a

autism susceptibility [176, 177].

*viii. Other candidate genes* 

[193].

autism in an Indian population [189].

protective effect of the A419 allele of *GLO1* [197].

could be a promising candidate gene [228].

*vii. SLC25A12* 

*TPH1* and *TPH2* encode rate-limiting enzymes that control serotonin biosynthesis. TPH1 is primarily expressed peripherally, while TPH2 is found exclusively in brain tissue. However, despite evidence for the potential involvement of the serotonin system in the etiology of autism, only one of three reports to date conservatively has supported the notion that *TPH2* plays a role in autism susceptibility [197, 201, 202].
