Norepinephrine Transporter Gene (SLC6A2) Is Quantitatively Associated with Behavioral and Cognitive Functions in Autism Spectrum Disorder

Background:  Autism spectrum disorder (ASD) is a highly heritable condition, which is related to complex genetic mechanisms involving multiple neurotransmitter systems. The high level of anxiety and stress sensitivity in ASD suggest the possible involvement of the norepinephrine system, but the role of this system has been little explored. An association with the norepinephrine transporter gene (SLC6A2) was mainly reported in attention deficit hyperactivity disorder, but any genetic association studies have not been reported in ASD.

Objectives:  The objective of this study is evaluating relations SLC6A2 gene and behavioral and cognitive phenotypes of ASD in Korean population. In the family-based association study of ASD, significant association of SNPs in SLC6A2 gene and ASD might be expected and the specific genotype of SLC6A2 might be related to certain phenotypes of ASD, especially social anxiety, vigilance to specific sensory stimuli, stereotyped and repetitive behaviors, and executive dysfunction as well as attention problems.

Methods:  All subjects were initially screened for pervasive developmental disorders by two child psychiatrists based on DSM-IV-R criteria, using Korean versions of ADOS and ADI-R. Behavioral and cognitive characteristics were assessed by Social responsiveness scale, Asperger syndrome diagnostic scale (ASDS), Aberrant behavior checklist, Leiter’s international performance test, and executive functions tests. Any subjects with clinically significant neurological diseases, serious medical conditions or known chromosomal anomalies were excluded. The probands and siblings were ascertained by same protocol. Ten SNPs in SLC6A2 were selected from common tagSNP s (Minor allele frequency = 0.1, r² = 0.8) for Asian populations at the international HapMap project homepage. Genotyping was performed using genomic DNA isolated from blood samples, and SNPs were genotyped using the GoldenGate^TM Assay (Illumina, San Diego, CA, USA). We checked for Mendelian inheritance errors by using the Merlin-1.1.2 software to assess data quality and detect genotypic errors; the genotypes of families with Mendelian inheritance error were reset to 0 at the time of coding. We performed the family-based candidate gene analysis using using transmission disequilibrium test (TDT), DFAM and QFAM tests (PLINK v1.07) for SNPs and behavioral domains of ADOS, ADI-R, rating scales measuring behaviors of ASD and executive functions tests.

Results:  Total 811 individuals (205 probands, 155 siblings, 410 parents and 41 other relatives) were recruited for analyses (mean age of probands = 95.02±51.63, 14.0% female). (1) One SNP (rs41153) is significantly associated with ASD in trio-only analysis by TDT with recessive mode (p<.05). (2) There was difference in the severity of Restricted, repetitive, & stereotyped patterns of behavior measured by ADI-R according to genotypes of 3 SNPs of SLC6A2 gene (rs4783899, rs1814270, rs187714).(3) There are significant association between behavioral and SNPs of SLC6A2 gene by QFAM test, including language/behavior problems in ASDS and  total performance IQ, word stroop test, response time variability in continuous performance test and color sequence errors in Children’s color trail making test 2 (p<.05).

Conclusions:  This family-based association study suggests that SLC6A2 might be involved in pathogenesis of ASD, especially in specific phenotype traits, especially on cognitive functions including performance IQ and attention.