Candidate gene based association studies have also identified susceptibility genes that may increase the risk for both schizophrenia and bipolar disorder. These genes include DISC1, DTNBP1, COMT, NRG1, and DAOA (G72/G30) (for review see Craddock et al., 2005, 2006; Owen et al., 2007). The strongest support for a transcategorical overlap has been reported for G72/G30, a gene in the 13q candidate region (Chumakov et al., 2002; Schumacher et al., 2004; Jamra et al., 2006). For some of the loci and genes described above, detailed analysis has shown that the association is due to subgroups of patients who suffer from symptoms that are common to both disorders. An association of G72/G30 with schizophrenia and bipolar disorder, for example, was found to be mediated mainly by bipolar patients with psychotic symptoms (Schulze et al., 2005).
