Association and excess of transmission of a DRD2 haplotype in a sample of French schizophrenic patients.

Abstract

The gene which codes for dopamine receptor D2 (DRD2) is considered as one of the most relevant candidate genes in schizophrenia. Previous genetic studies focusing on this gene gave conflicting results, potentially because of the differences in methodology (linkage versus association studies), and the different loci analyzed (the DRD2 gene having many polymorphisms). We used a progressive strategy with three different approaches (case/control, haplotype relative risk and transmission disequilibrium test) and investigated two genetic polymorphisms ( TaqI B1/B2 and TaqI A1/A2, spanning the coding region of the DRD2 gene) in 50 patients with DSM-IV diagnoses of schizophrenia, in their 100 parents and in 50 healthy, matched controls. Firstly, we found a significant excess of the two alleles (B2 and A2) in the schizophrenic group compared to unaffected controls. Secondly, we found an excess of transmission from the parents to their affected children, using the haplotype relative risk design applied to the B2A2 haplotype. Finally, the transmission disequilibrium test showed evidence for linkage between B2A2 haplotype and schizophrenia. The significant excess of the B2A2 haplotype in schizophrenic patients is specifically observed in a subsample of patients with a disease onset occurring after 20 years of age. As the haplotype contains nearly the entire DRD2 gene, we found convergent evidence in our sample for a significant role of the DRD2 gene in the risk for schizophrenia. This haplotype may be more specifically involved in the disorder's onset at a later age in some patients, or, alternatively, may be implicated as a modifying factor acting on age of onset.