For each participant genomic DNA was extracted from buffy-coats prepared from EDTA blood samples (9 mL) using the method of Miller [54]. Genotyping was performed using the Affymetrix Genome-Wide Human SNP Array 6.0 (http://www.affymetrix.com), as described by the Affymetrix user manual. Genotypes were called using the Affymetrix Birdseed-V2 calling algorithm and quality control was performed using GenABEL (http://mga.bionet.nsc.ru/nlru/GenABEL/). Individuals with a call rate below 97% or a too high autosomal heterozygosity (False Discovery Rate <1%) were excluded. After applying standard quality criteria (minor allele frequency >1%, genotype call rate >98% and P-value of deviation from Hardy-Weinberg equilibrium >10−4), 675,350 out of 900,392 SNPs remained for analysis.
