Attention deficit hyperactivity disorder (ADHD) is a developmental disorder which is characterized by inattention, impulsiveness and hyperactivity.¹ The prevalence of ADHD was reported as 5.3% in childhood and estimated as 1-4% in adulthood.² The high prevalence of ADHD made it necessary to understand the etiology of this disorder and also to further refine the diagnosis and treatment of it.³

The etiology of ADHD is not completely understood. ADHD is a complex disorder in which multiple genes exert moderate effects. The twin studies find little evidence of shared environmental influences on familiarity, although the role of the environment may still be pivotal, acting through mechanisms of gene-environment interactions. Based on twin and adoption studies, genetic factors can play an important role in the etiology of ADHD.⁴ Many candidate genes were reported in the literature associated with ADHD. Significant associations between ADHD and DAT-1, DRD4, DRD5, 5HTT, HTR1B and SNAP-25 (The synaptosomal-associated protein, 25 kDa) were identified.⁵

Recently, there is increasing evidence to suggest that polymorphisms within the synaptosomal-associated protein, 25 kDa (SNAP-25) may play a role in susceptibility to ADHD. Human SNAP-25 gene is located on chromosome 20p11.2. SNAP-25, a pre-synaptic plasma membrane protein, an integral part of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein complex. This complex is composed of SNAP-25, syntaxin 1A and synaptobrevin. SNAP-25 plays an important role in synaptic vesicle membrane docking and fusion, which is involved in the regulation of neurotransmitter release by Ca⁺²-mediated exocytosis.⁶ Thus, at least it is possible that any variation in SNAP-25 might interfere in the susceptibility of ADHD by influencing the release of dopamine as well as other neurotransmitters.⁷

Wilson⁸ reported that hyperkinesis was ameliorated by low doses of the psychostimulant d-amphetamine and could be rescued genetically by a transgene encoding SNAP-25, located within the mutation coloboma (Cm) deletion. He also emphasized that despite the ubiquitous role of SNAP-25 in synaptic transmission and uniformly decreased expression in the mutants; coloboma mice had shown marked deficits in Ca⁺² dependent dopamine release selectively in dorsal but not ventral striatum. He suggested that haploinsufficiency of SNAP-25 revealed a specific vulnerability of the nigrostriatal pathway which regulated motor activity and might provide a model for impaired striatal input into executive functions encoded by the prefrontal cortex associated with ADHD.

Initially, Barr et al.⁹ identified two single-nucleotide polymorphisms (SNPs) (Mnll, Ddel) located four bases apart in the 3' UTR of SNAP-25. Since then, the association of these two SNPs and ADHD has been investigated. Some of the studies reported a significant association with ADHD and Ddel, Mnll or both.¹⁰

To the best of our knowledge, there is no genetic study about the association of genetic polymorphisms of SNAP-25 gene with adult ADHD in the English literature. In this study, we aimed to investigate whether Ddel and Mnll polymorphisms of SNAP-25 gene were associated with ADHD in an adult Turkish sample.

A total of 139 patients with adult ADHD and 73 healthy controls were included into the study. Mean age of the study group was 27.24±9.62 and mean age of the control group was 26.64±6.71. There was no statistically significant difference between the study and the control groups in terms of age (p>0.05). The study group consisted of 38 women (27.3%) and 101 men (72.7%) and the control group consisted of 23 women (31.5%) and 50 men (68.5%). There was also no statistically significant difference between the study and the control groups in terms of gender (p>0.05). Of the 139 patients with adult ADHD, 44 (31.7%) were diagnosed as predominantly inattentive type, 33 (23.7%) were diagnosed as predominantly hyperactive-impulsive, and the rest of them (44.6%) were diagnosed as combined type ADHD.

SNAP-25 gene Ddel polymorphism was analyzed in both of the study and the control groups. T allele was the most common allele in both groups (study: 84.3%, control: 89.0%). C allele distribution in both groups was as follows; study: 15.7%, control: 11.0%. The most common genotypes were T/T (study: 71.2%, control: 80.8%). In the study group, 35 (25.2%) of them had T/C genotype and 2 (2.5%) of them had C/C genotype. In the control group, 12 (16.5%) of them had T/C genotype and 5 (3.6%) of them had C/C genotype. There was no statistically significant difference between the study and the control groups regarding Ddel polymorphism (p>0.05).

T allele was determined in 96.4% of the ADHD group and in 97.3% of the control group, and C allele was determined in 28.8% of the ADHD group and in 19.2% of the control group and the difference between the groups was not found statistically significant (p>0.05) (Table 1).

SNAP-25 gene Mnll polymorphism was analyzed in both of the study and the control groups. While 58.3% of them were having T allele and 41.7% of them were having G allele in the study group, 77.4% of them had T allele and 22.6% of them had G allele in the control group. The difference between the groups was found statistically significant (p<0.001). Among the ADHD patients, 50 (36.0%) of them had T/T, 62 (44.6%) of them had T/G and 27 (19.4%) of them had G/G genotype. In the control group, 47 (64.4%) of them had T/T, 19 (26.0%) of them had T/G and 7 (9.6%) of them had G/G genotype. The difference between the groups was found statistically significant (p<0.001).

T allele was determined in 80.6% of ADHD group and in 90.4% of the control group and there was no significant difference between the groups. When the groups were compared with respect to the G allele, ADHD group and the control group were found to have significantly different (p<0.001) rates of G allele which were 64.0% and 35.6% respectively (Table 2).

When ADHD patients were evaluated in terms of scale scores, the subjects having G/G genotype of the SNAP-25 gene Mnll polymorphism had higher Wender-Utah scores and higher scores in the 1st and 3rd parts of adult ADD/ADHD Scale (p<0.05) (Table 3).

The relation between ADHD and SNAP-25 gene was investigated in many studies among children. Feng et al.¹⁴ examined 12 SNPs in two independent samples of ADHD families and found significant over-transmission of rs66039806-C, rs362549-A, rs362987-A and rs362998-C alleles in families. Mill et al.¹⁵ have reported a strong relation between rs363006 and ADHD. Renner et al.¹⁶ and Zhang et al.⁴ have reported an association between rs362549 and ADHD similar to Feng et al.¹⁴ The studies investigating the relation between ADHD and MnlI and DdeI polymorphisms of SNAP-25 gene reported different results. Kim et al.¹⁷ examined 61 SNPs in 229 families and found a statistically significant association with only a single SNP (rs3787283), which was in strong linkage disequilibrium with MnlI and DdeI polymorphisms (p=0.002). Although, they also emphasized that combined analysis of the pooled data of the whole SNPs was modestly significant for MnlI (p=0.048) and rs6077690 (p=0.031). Choi et al.¹⁸ have detected a significant association between ADHD and MnlI polymorphism. In the present study, we determined a significant association between ADHD and MnlI polymorphism, which is similar to the findings of Kim et al.¹⁷ and Choi et al.¹⁸

While Brophy et al.¹⁹ have reported no significant relation between ADHD and MnlI polymorphism; they have reported significant relation between ADHD and DdeI polymorphism. Kustanovich et al.³ reported an increased transmission of the Ddel polymorphism of SNAP-25 gene. On the other hand, Barr et al.⁹ have reported significant relation between ADHD and MnlI and DdeI polymorphisms of SNAP-25 gene. Faraone and Mick²⁰ have performed a meta-analysis, consisted of four family-based studies of SNAP25 examining two SNPs (Ddel and Mnll), and they have shown significant evidence for an association with ADHD and Mnll (OR=1.19, 95% CI: 1.03-1.38); similar to our results.

Recently, a relation between ADHD and MnlI and DdeI polymorphisms of SNAP-25 gene is mentioned in the pharmacogenetic studies. Öner et al.²¹ have reported that hemodynamic changes in the brain induced by methylphenidate might be related with SNAP-25 gene DdeI T/T and MnlI T/T genotypes in a fNIRS (Functional Near-Infrared Spectroscopy) study performed with 15 ADHD adults and 16 ADHD children. They suggested that their results were in line with Barr et al.⁹ findings; combination of DdeI and MnlI polymorphisms of SNAP-25 gene might be important in ADHD. In a study investigating the response of methylphenidate in adult ADHD and DdeI and MnlI polymorphisms of SNAP-25 gene, Ünal et al.²² reported that considerable increase was determined in N-acetylaspartate (NAA) levels after methylphenidate at anterior cingulate cortex in the samples having SNAP-25 Ddel polymorphism T/T genotype and Mnll polymorphism G/G genotype. In the present study, an association between ADHD and MnlI polymorphism G allele was determined. Also, the subjects having MnlI polymorphism G/G genotype had higher Wender-Utah scores and higher scores in the adult ADD/ADHD Scale. Therefore, these findings may suggest much more impairment of neuronal integrity and functionality. When it is considered that NAA is a marker of neuronal integrity and functionality, our results is consistent with the findings of Ünal et al.,²² indicating considerable increase in NAA levels after methylphenidate in the samples having Mnll polymorphism G/G genotype. Aforementioned studies performed among adult ADHD patients involving SNAP-25 gene are pharmacogenetic studies, but not association studies. Our study was the first report on the association of SNAP-25 gene with adult ADHD.

Limitations of the study are the number of the patient and the control groups not to be much enough, impossibility of excluding familial genetic load completely, interaction of the genes with each other besides their relation with the disorder.

In conclusion, it is supposed that the supplied evidence supporting the association of SNAP-25 gen polymorphisms and ADHD may be important in the genetic etiology of ADHD.