The last few years have seen concrete demonstrations of “pleiotropy”, i.e. the sharing of genetic factors, between human complex traits. For example, a systematic analysis of the NHGRI catalog of published GWAS (http://www.genome.gov/gwastudies/) showed that 16.9% of the reported genes and 4.6% of the reported SNPs are associated with multiple traits [16]. Through a “pleiotropic enrichment” method, Andreassen et al showed that it is possible to improve the power to detect schizophrenia-associated genetic variants by using the pleiotropy between schizophrenia (SCZ) and cardiovascular-disease [17]. A more recent study identified four significant loci (p-value ) to have pleiotropic effects by analyzing GWAS data of 33,332 cases and 27,888 controls for five psychiatric disorders [18]. Further analysis suggested a very significant genetic correlation between schizophrenia and bipolar disorder ( s.e.) [12]. Pleiotropy has also been demonstrated among several other types of traits, for example, metabolic syndrome traits [11] and cancers [19].
