Evidence for linkage disequilibrium between serotonin transporter protein gene (SLC6A4) and obsessive compulsive disorder

Obsessive compulsive disorder (OCD) is characterized by recurrent and intrusive thoughts that are distressing (obsessions) and/or repetitive behaviors or mental acts that the person feels driven to perform (compulsions), OCD has a partly genetic basis. For treatment of OCD, potent serotonin reuptake inhibitor (SRI) drugs (clomipramine (Anafranil), fluvoxamine (Luvox), fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil)), which act on the serotonin transporter protein, are uniquely efficacious, A polymorphism in the promoter region of the gene (SLC6A4) encoding this protein,(1) was recently reported to affect protein expression and to be associated with measures of anxiety and depression(2) and with autism (using a family-controlled transmission disequilibrium test (TDT) design).(3) SLC6A4 therefore has strong a priori support for potentially influencing risk for OCD: the protein it encodes is a medication target; a polymorphism in the gene affects function; and that polymorphism has been shown to be associated with behavioral phenotypes. We used the TDT with a set of 34 European-American family trios, 30 unrelated and four drawn from an extended pedigree, to test for linkage disequilibrium between OCD and alleles at the SLC6A4 promoter polymorphic locus. Of 35 heterozygous parents, 24 transmitted the 'I' SLC6A4 allele and 11 transmitted the 's' allele (chi(TDT)(2) = 4.83; P<0.03). Considering only the 13 SRI drug nonresponders, there were 13 heterozygous parents, of whom 10 transmitted the 'I' allele and three the 's' allele (chi(TDT)(2) = 3.77; P<0.052), These data provide preliminary support for association and linkage disequilibrium between the SLC6A4 'I' allele and OCD.