Association Between GABA(A) Receptor Subunit Polymorphisms and Autism Spectrum Disorder (ASD) in an Argentinean Sample

Background: Autism Spectrum Disorders (ASDs) can be conceptualized as a genetic dysfunction disrupting brain circuits mediating social cognition and language. Some forms of ASD might be associated with a connectopathy resulting in increases/decreases in neuronal excitability. GABA has been implicated since during development is an excitatory neurotransmitter and in the adult brain GABA acts on the GABA(A) receptor (GABAR) complex and mediates synaptic inhibition.

Objectives: To study single-nucleotide polymorphisms (SNPs) in GABAR subunit genes and to evaluate their contribution to the aetiology of ASD on a sample of Argentinean ASD patients.

Methods : A hundred and fifty three ASD patients (86% males) with DSM-IV-TR evaluation and 166 controls (78% males) were studied. Twenty one SNPs located within the GABAR subunit genes were genotyped using DNA sequencing: GABRA4, GABRB3, GABRD and GABRG2. Genetic association to test whether single-locus allele frequencies, genotype frequencies and multilocus haplotype frequencies show differences between cases and controls, as well as Hardy–Weinberg equilibrium (HWE) test for each SNP and pair-wise linkage disequilibrium (LD) estimations were performed using SNPStats and UNPHASED.

Results: Both populations were in HWE (p>0.05). Significant differences in allele and genotypic frequencies (under dominant model) were observed between groups for SNP rs1912960 in GABRA4. The C allele frequency was significantly increased (OR=1.53, 95%CI: 1.10-2.12, p=0.01) while G allele was significantly decreased (OR=0.65, 95%CI: 0.47-0.90, p=0.01). In the same way, C-C/C-G genotype frequency was significantly increased (OR=1.79, 95%CI: 1.13-2.81, p=0.01) and might confer risk for ASD while G/G genotype was significantly decreased (OR=0.56, 95%CI: 0.35-0.88, p=0.01) and might confer protection. No significant association with ASD was found for the other markers studied for allelic and genotypic frequencies (p>0.05). SNPs within genes were in LD (p<0.05) and there was no LD between markers across genes studied (p>0.05). Haplotype analysis was performed using tagging SNPs within each gene. The combination between markers rs1912960 (GABRA4) and rs211037 (GABRG2) showed a significant association with autism. As a result, two haplotypes were observed: C-T risk haplotype (OR=3.1, 95%CI 1.42-6.8, p=0.001) and G-C protective haplotype (OR=0.71, 95%CI 0.51-0.99, p=0.04). No significant difference between genders was observed (p> 0.05).

Conclusions: It has been suggested that α4 subunit might be involved in neuronal hyperexcitability and rs1912960 in GABRA4 has been associated with ASD in different ethnic groups.

Also, the marker rs211037 in GABRG2 was associated with susceptibility to epilepsy and in-silico analysis indicated that this marker plays a role in the transcriptional and splicing regulation.

The frequent co-occurrence of ASD and epilepsy (at least in certain cases) might result from common developmental pathophysiological mechanisms leading to abnormalities in connectivity, imbalance in excitation/inhibition and disrupted synaptic plasticity.

Given that multiple GABAR subunits are blended in varying combinations to form a functional GABA receptor, even minor changes in levels of a particular subunit might modify the make-up of receptors within a particular cell type and alter the GABAergic signalling.

We can conclude that GABRA4 could be involved in the aetiology of autism in the Argentinean dataset both independently and in combination with GABRG2.