DNA for genotyping was extracted from peripheral blood leukocytes from EDTA whole blood or blood clots, stored in the HUNT 2 Biobank, manually with the use of Puregene kit (Gentra Systems Inc., Minneapolis, MN) or with the Autopure LS (Gentra Systems Inc., Minneapolis, MN). The laboratory technicians were blinded for other HUNT data. Genotypes of the COMT Val158Met polymorphism were determined using a LightCycler Real Time PCR machine (Roche Diagnostics Scandinavia AB, Bromma, Sweeden) [19]. PCR-technique was carried out in 20 μL reagent on a LightCycler System by using 2 μL genomic DNA and LightCycler-FastStart DNA Master Hybridization Probes kit (Roche Diagnostics, Bromma, Sweeden). Details on PCR primers and hybridization probes used have been published elsewhere [17]. Based on melting-curve profiles, the genotypes of the participants were classified as Val/Val, Val/Met or Met/Met.
