Although no single SNP showed significant association after correction for genome-wide testing in either of our populations, some noteworthy associations were observed with BD candidate genes, as well as with genes known to be expressed in human brain. Of particular interest are SLITRK2 and NTRK2. SLITRK2 is a member of a family of six genes which are widely expressed in neural tissue (22), producing proteins which are membrane bound. SLITRK2 regulates neurite outgrowth in vitro. Thus, SLITRK2 is a logical bipolar risk gene. Another member of this gene family, SLITRK1, has been reported to be associated with Tourette’s syndrome (23). NTRK2 (also known as TrkB) is a tyrosine kinase receptor which binds brain derived neurotrophic factor (BDNF) and possibly other neurotrophins, i.e., (for review see 24). NTRK2 is a high priority bipolar candidate gene for several reasons. There is abundant evidence from animal models of depression that hippocampal neurogenesis is decreased during the behavioral syndrome, that antidepressants increase neurogenesis, and that BDNF has antidepressant-like properties in these animal models (25). BDNF expression is increased in animals by treatment with antidepressants
