Our present study has clearly shown, on the basis of analysis of genome-wide SNP genotypes that most Japanese individuals fall into two main clusters: the Hondo cluster and the Ryukyu cluster. Our results also show that local regions in Honshu Island (the largest island of Japan) are still genetically differentiated, even though human migration within Japan has become rather frequent in the past 100 years or so. Our finding that the individuals from Tohoku were less related to Han-Chinese individuals than were the individuals from Kinki and Kyushu suggests that the individuals in Tohoku were less affected by immigrants from the Asian continent than were the individuals in Kinki. The immigrants who came to Japan from the Asian continent through the Korean Peninsula may have entered Japan from northern Kyushu, the Japan Sea side of Kinki or Chugoku.
American Journal of Human Genetics doi: doi:10.1016/j.ajhg.2008.08.019
Japanese Population Structure, Based on SNP Genotypes from 7003 Individuals Compared to Other Ethnic Groups: Effects on Population-Based Association Studies
Yumi Yamaguchi-Kabata et al.
Because population stratification can cause spurious associations in case-control studies, understanding the population structure is important. Here, we examined Japanese population structure by “Eigenanalysis,” using the genotypes for 140,387 SNPs in 7003 Japanese individuals, along with 60 European, 60 African, and 90 East-Asian individuals, in the HapMap project. Most Japanese individuals fell into two main clusters, Hondo and Ryukyu; the Hondo cluster includes most of the individuals from the main islands in Japan, and the Ryukyu cluster includes most of the individuals from Okinawa. The SNPs with the greatest frequency differences between the Hondo and Ryukyu clusters were found in the HLA region in chromosome 6. The nonsynonymous SNPs with the greatest frequency differences between the Hondo and Ryukyu clusters were the Val/Ala polymorphism (rs3827760) in the EDAR gene, associated with hair thickness, and the Gly/Ala polymorphism (rs17822931) in the ABCC11 gene, associated with ear-wax type. Genetic differentiation was observed, even among different regions in Honshu Island, the largest island of Japan. Simulation studies showed that the inclusion of different proportions of individuals from different regions of Japan in case and control groups can lead to an inflated rate of false-positive results when the sample sizes are large.