In the control samples, all polymorphisms were in the Hardy–Weinberg equilibrium. The results of the polymorphic loci genotyping in cases and control samples are shown in Table 2.
The strongest association was found for VNTR-polymorphism of the SLC6A3 gene. Given the data suggesting the effect of length of this locus on gene expression and high activity of the 10R allele compared with the 9R allele [23–26], we classified cases and controls as carriers of the long (≥10) and short (<10) alleles. The short allele (S) was more common in cases than it was in the control group (p=0.009). SS and LS genotype frequencies were higher and LL genotype frequency was lower in cases than in controls (p=0.019).
Another gene of the dopaminergic system associated with suicide attempts was the COMT gene. In the group of cases, the G-allele (Val) frequency of COMT rs4680 was significantly higher than with the control group (p = 0.028). However, the genotype distributions of COMT rs4680 were not found to be significantly different between cases and controls, AA genotype frequency was decreased and GA and GG frequencies were increased in the group of cases compared with the control group (17%, 53%, 30%, respectively, in cases and 29.4%, 47.9%, 22.7%, respectively, in controls; p = 0.064).
We also found a statistically significant association between the T-allele of BDNF rs6264 and suicidal behavior. In the group of patients, the T-allele was more common than with controls (p = 0.029). The difference in genotype distribution was not statistically significant. However, CC genotype tends to be less common in the group of cases, while frequencies of CT and TT genotypes are higher in cases than with the control group (61%, 32%, 7%, respectively, in cases and 70.7%, 26.6%, 2.7%, respectively, in controls; p = 0.076).
We analysed two loci of the SLC6A4 gene: rs25531 and VNTR 44 bp polymorphism. We found that the rs25531 polymorphism of SLC6A4 showed some statistically significant associations. The A-allele frequency of rs25531 was higher in cases than it was in controls (p=0.047). VNTR-polymorphism of SLC6A4 was not found to be associated with suicidal behavior.
Because of the complexity of SLC6A4 organization, we additionally analysed the distribution frequencies of its allelic variants according to their functional characteristics. To verify whether alleles with low or high expression activity are associated with suicidal behavior, we grouped alleles and genotypes according to their expression levels: high (LA) and low (S and LG) alleles with high and low expression activity, respectively, and high/high (LA/LA), high/low (LA/SA, LA/LG) and low/low (SA/SA, SA/LG, LG/LG) genotypes.
We found that the distribution of SLC6A4 gene genotypes and alleles showed that alleles with high expression tend to be increased in patients with suicidal behavior compared with the control group, but this association was not statistically significant (p = 0.098).
The alleles of the dopamine receptor DRD4 exon 3 were grouped into long (≥7) and short (<7) allelic variants, associations between this locus and suicide were not found. No significant associations between suicidal behavior and DRD2 rs1800497, DRD4 In/Del, 5HTR1A rs6295, 5HTR2A rs6311 and 5HTR1B rs6296 alleles and genotypes were discovered. All results of genotyping of cases and controls are shown in Additional files 1 and 2, respectively.
To determine the effect of studied polymorphisms on the patient’s clinical characteristics we conducted the one-way ANOVA. This analysis showed the lack of association between loci which were significant in the Chi-squared test and severity of the symptoms of depression, situational anxiety, and personal anxiety in patients with suicidal behavior. However, we observed a significant effect of SLC6A4 VNTR 44 bp on the situational anxiety and HTR1B loci on the personal anxiety (p = 0,0358 and p = 0,0132, respectively ). The most significant results of one-way ANOVA are shown in Table 3 and results for all loci are shown in Additional file 3.