Association of polymorphic variants in serotonin re-uptake transporter gene with Crohn's disease: a retrospective case-control study

Abstract

Aim: To analyze the distribution of SLC6A4 gene polymorphisms in Crohn's disease (CD) patients and their association with the disease.

Methods: We evaluated the presence/absence of promoter (5-HTTLPR, rs25531) and intron 2 (STin2 VNTR) polymorphic variants of SLC6A4 gene in a retrospective case-control study including 192 CD patients and 157 healthy controls (HC). Genotyping was performed by polymerase chain reaction. The association of polymorphisms with CD and its clinical subtypes was analyzed using χ2 and Fisher exact test, binary logistic regression, and haplotype analysis.

Results: CD patients and healthy controls had similar sex (88 [45.8%] vs 84 [53.5%] women, respectively; P=0.154) and age (41.3±12.8 years vs 41.7±8.8 years, respectively, P=0.091) distribution. Significant differences were observed in the STin2 genotype and allele distribution between CD patients and healthy controls (P=0.003 and P=0.002, respectively) and between the corresponding female subgroups (P=0.004 and P=0.007, respectively), with a significant negative association of biallelic ss (STin2.9 and Stin2.10) STin2 genotype with CD (P=0.013, age- and sex-adjusted odds ratio [OR] 0.5, 95% confidence interval [CI] 0.29-0.86; women: P=0.006, age-adjusted OR 0.32, 95% CI 0.14-0.72) and a significantly higher S-STin2.12 (5-HTTLPR/rs25531: S-STin2: STin2.12) haplotype distribution in CD patients (P=0.004, OR 1.62, 95% CI 1.16-2.26). There was no significant association between 5-HTTLRP and rs25531 genotype or allele frequencies and CD and between any SLC6A4 polymorphic loci with clinical CD subtypes.

Conclusion: STin2 VNTR polymorphism of SLC6A4 gene may contribute to CD pathogenesis.

Figure 1

Location of SLC6A4 gene polymorphic regions and their genotype combinations examined in this study. SLC6A4 gene is affected by the insertion/deletion variation of the promoter region 5-HTTLPR, resulting in two alleles, long L and small S. This promoter region also functionally couples with the common rs25531 (A/G) single nucleotide polymorphism (SNP), resulting in alleles L_A and L_G. L_A allele is associated with the higher transcriptional activity, whereas the L_G allele exhibits lower serotonin uptake and the transcriptional activity equivalent to S allele of the 5-HTTLPR polymorphic region. Transcriptional activity of SLC6A4 gene is further modulated by the enhancer activity of STin2 polymorphic region, 17–18 bp long variable number of tandem repeats (VNTR), found in intron 2. The STin2 allelic variants were identified as 10-repeat and 12-repeat alleles that have been identified in all ethnicities, and the less common 9-repeat allele found only in individuals of European or African descent. Thus, promoter polymorphic region may be presented in biallelic, triallelic, and clustered biallelic genotype model while STin2 VNTR region may be presented in triallelic and biallelic genotype forms. Rare allele forms (eg, XL or STin2.7) not encountered in this study, are not presented.

Figure 2

Linkage disequilibrium (LD) in SLC6A4 gene with D' and r2 values.