A cost-efficient way to increase power in a genetic association study is to pool controls from different sources. The genotyping effort can then be directed to large case series. The Nordic Control database, NordicDB, has been set up as a unique resource in the Nordic area and the data are available for authorized users through the web portal (http://www.nordicdb.org). The current version of NordicDB pools together high-density genome-wide SNP information from ~5000 controls originating from Finnish, Swedish and Danish studies and shows country-specific allele frequencies for SNP markers. The genetic homogeneity of the samples was investigated using multidimensional scaling (MDS) analysis and pairwise allele frequency differences between the studies. The plot of the first two MDS components showed excellent resemblance to the geographical placement of the samples, with a clear NW–SE gradient. We advise researchers to assess the impact of population structure when incorporating NordicDB controls in association studies. This harmonized Nordic database presents a unique genome-wide resource for future genetic association studies in the Nordic countries.The first thing that stands out to me in the MDS plot from the NordicDB website is the substantial overlap between the Danish sample and the CEU HapMap sample (Utah whites):
Top axes of genetic variation in the Nordic Control Database (4620 samples) contrasted with the CEU population (108 samples) HapMap and a Finnish reference population (81 samples). The MDS analysis was performed on approximately 45K SNPs that were common between the genotyping platforms. The samples are represented with the color of their country of origin: Finland (red), Sweden (green) and Denmark (yellow).