The International Warfarin Pharmacogenetics Consortium (IWPC), which involves investigators from eight institutions including Stanford University, has identified a SNP that is associated with warfarin dose requirement in the African American population. As described in Lancet, rs12777823, located in the CYP2C cluster, shows an effect in African Americans that is independent from those of CYP2C9*2 and CYP2C9*3. In this first investigation of warfarin dose requirements in African Americans, this new association reached genome-wide significance in a GWAS and was also found to be significant in a replication cohort. Individuals heterozygous for the rs12777823 A allele need a dose reduction of 6·92 mg/week and those homozygous for this allele need reduction in dose of 9·34 mg/week.
The data used in these studies are available on the PharmGKB.