Influence of serotonin transporter gene polymorphisms on clozapine response in Brazilian schizophrenics

Schizophrenia is a severe mental illness affecting around 1% of adults with a high degree of morbidity and mortality. Affected individuals are at increased risk for unemployment, handicap, obesity, diabetes mellitus, hearth attack and suicide. Antipsychotic drugs are the best treatment for this disease, but about 20% of patients display drug resistance, or refractoriness, and may receive a special neuroleptic named Clozapine. Despite its superiority from other neuroleptics, only 30-60% of drug-resistant patients are responsive to clozapine. Clozapine's action results from interactions between dopaminergic and serotonergic neurotransmitter systems and since clozapine appears to exert its effect strongly through the serotonergic systems, alterations in serotonin synaptic levels may influence antipsychotic response. The serotonin transporter (5-HTT) is responsible for pre-synaptic re-uptake of serotonin, making this transporter a logical candidate gene for prediction of clozapine response and to increase understanding about mechanisms of refractoriness. Therefore, we investigated the influence of two polymorphisms in the 5-HTT gene (HTTLPR/rs25531 and VNTR Stin2) in clozapine response in a sample of 116 schizophrenic individuals of European descent from South-Brazil. Significant differences between responders and non-responders to clozapine were observed for the HTTLPR/rs25531 polymorphism. Nonresponders to clozapine showed a higher frequency of S'-allele (P = 0.01) and also were more likely to be S'/S' homozygous or S'/L' heterozygous than those who did respond (P = 0.04). After controlling for confounding variables, logistic regression analyses confirmed this association (OR = 3.15; 95% CI: 1.13-8.80). The observed association suggests that increased availability of extracellular serotonin concentrations at all synapses may reduce clozapine effect.