Serotonin transporter promoter gain-of-function genotypes are linked to obsessive-compulsive disorder

Abstract

A functional serotonin transporter promoter polymorphism, HTTLPR, alters the risk of disease as well as brain morphometry and function. Here, we show that HTTLPR is functionally triallelic. The L(G) allele, which is the L allele with a common G substitution, creates a functional AP2 transcription-factor binding site. Expression assays in 62 lymphoblastoid cell lines representing the six genotypes and in transfected raphe-derived cells showed codominant allele action and low, nearly equivalent expression for the S and L(G) alleles, accounting for more variation in HTT expression than previously recognized. The gain-of-function L(A)L(A) genotype was approximately twice as common in 169 whites with obsessive-compulsive disorder (OCD) than in 253 ethnically matched controls. We performed a replication study in 175 trios consisting of probands with OCD and their parents. The L(A) allele was twofold overtransmitted to the patients with OCD. The HTTLPR L(A)L(A) genotype exerts a moderate (1.8-fold) effect on risk of OCD, which crystallizes the evidence that the HTT gene has a role in OCD.

Figure 1

A, Schematic of HTTLPR alleles. An A→G substitution (boxed letter) is located at nucleotide 6 within the first of two extra 22-bp repeats that characterize the L allele, located 1,629 nt 5′ of exon 1 of HTT. This position corresponds to nucleotide 168 of the reference sequence (NCBI accession number AF126506). B, The L_G allele creates an AP2-DNA binding site. The autoradiograph shows the presence of a specific AP2 DNA-protein complex in nuclear extracts from JEG-3 cells. The gel-shift assay used ³³P-labeled L_G double-stranded oligonucleotide probe (G). Addition of unlabeled competitor DNA (G) or an AP2 consensus sequence (AP2) resulted in disappearance of the specific band. Neither the L_A competitor DNA (A) nor the SP1 competitor DNA (SP1) displaced L_G-specific binding. C, A specific DNA-protein complex was not seen with the L_A double-stranded oligonucleotide probe. D, “Supershift” gel-shift assay. A combination of 5 μg anti-AP2 antibody and 5 μg recombinant AP2 further retarded the migration of the L_G-specific DNA-protein complex, as shown by the arrow indicating the supershift band. No supershift was seen with antibody to the NF1 transcription factor. ³³P-labeled AP2 probe served as a reference.

Figure 2

A, Sequence and organization of HTTLPR repeats. Sequences from −1797 to −1268 relative to the HTT transcription start site are shown. This segment, encompassing the HTTLPR region, was inserted into the pGL4.10 expression plasmid. PCR primers used for genomic amplification are indicated in underlined lower-case letters. AP2 sites are boxed. Capital letters indicate the 43-bp insertion containing the A→G SNP, which is in brackets. Previously, this region was considered to be a 44-bp insertion/deletion, because of an additional C in the reference sequence (NCBI accession number AF116506). This region of sequence disparity is shown in bold letters (five Cs). Our sequencing results (106 samples) and those of Nakamura et al. and Kraft et al. confirm the sequence presented. A rare substitution within this region (C→T; frequency 0.0048 in present study and 0.007 in study by Nakamura et al.) is underlined. B, HTTLPR L_A and L_G alleles detected by direct sequencing. Representative electropherograms show the position of the A→G SNP (arrows).

Figure 3

Effect of HTTLPR S, L_A, and L_G alleles on HTT-specific mRNA expression in lymphoblastoid cell lines. Cell lines with genotypes SS (n=12), SL_A (n=15), SL_G (n=7), L_AL_A (n=12), L_AL_G (n=9), and L_GL_G (n=7) were identified. Mean HTT mRNA expression values were measured by real-time PCR in triplicate cultures and were normalized to 18S rRNA levels, as described in the “Material and Methods” section.

Figure 4

Relative HTT mRNA levels in 62 human lymphoblastoid cell lines with HTTLPR genotypes SS (n=12), SL (n=22), and LL (n=28), before and after treatment with 100 μM forskolin.

Figure 5

Codominance of HTT mRNA expression. The codominance of HTT mRNA expression normalized to 18S mRNA was observed in 62 lymphoblastoid cell lines representing the six HTTLPR genotypes. Expression values represent the means of 7–15 cell lines per genotype. Observed expression is plotted versus the expression expected on the basis of the codominant model and of computed allele-expression values: 0.50 for S, 0.57 for L_A, and 1.16 for L_G. For observed vs. expected, r=0.84 and P=.038.

Figure 6

Relative HTT mRNA levels in lymphoblastoid cell lines with various intron 2 VNTR genotypes.

Figure 7

Reduction of basal HTTLPR transcriptional activity by the L_G allele relative to the L_A allele. A, Effect of three HTTLPR alleles on reporter transcription. RN46A cells were transfected with HTTLPR-luciferase plasmids (pGL4.10). The effect of each allele on basal transcription was measured as relative luciferase level (ratio of firefly luciferase to Renilla luciferase) and was normalized to basal S allele expression. There was no statistically significant difference between S and L_G. However, L_A was significantly different from both S and L_G (P<.01). Transfected cells carrying each of the three different reporter plasmids and treated with 50 μM forskolin showed an approximately twofold increase in relative luciferase level, compared with the basal condition. B, Effect of an L_G decoy on expression of three HTTLPR reporters. The double-stranded L_G-specific decoy DNA (Oligo-G) containing the AP2-binding sequence was transfected into RN46A cells carrying the S, L_G, or L_A allele in HTTLPR-pGL4.10 reporter plasmids. An L_A-specific decoy DNA (Oligo-A) was used for the control. Twenty-four hours after transfection, luciferase activity was measured and normalized for transfection efficiency. Results are presented relative to expression determined for Oligo-A–treated cells carrying the S allele reporter plasmid. Treatment with Oligo-G decoy DNA equalized L_G and L_A allele reporter expression, whereas the control decoy DNA had little effect (compare basal expression in panel A with expression of Oligo-A–treated cells in panel B).