In collaboration with 10 academic centers across the United States, we have recruited a large sample of families in which at least 2 siblings suffer from bipolar disorder or related mood disorders. This is the largest sample ever to participate in a genetic study of bipolar disorder. All research participants have undergone a diagnostic interview and provided a blood sample for DNA analysis. Genetic linkage studies have been performed using molecular markers evenly spaced across all chromosomes. These studies suggested several chromosomal regions may contain genes that contribute to bipolar disorder in these families. Ongoing work is aimed at identifying the actual genes involved. Using the latest genotyping chip technology and DNA pooling, we conducted the first genome-wide association study of bipolar disorder. The results implicated several genes, each of small effect, suggesting that bipolar disorder is a polygenic disease. Meta-analysis of independent case-control studies of bipolar disorder supported association with several distinct genes that play a role in zinc metabolism (SLC39A3), cell adhesion (JAM3), and maintenance of normal neuronal functioning (ANK3). We have also found evidence that 2 independent regions of the ANK3 gene are risk factors for bipolar disorder. We are also exploring variables that might be used to estimate an individual's response to lithium, one of the most effective current treatments for bipolar disorder. We have organized a large international collaboration, known as the Consortium on Lithium Genetics (ConLiGen), which aims to characterize lithium response in a large group of patients using reliable instruments, then perform a genome-wide association study. So far we have collected data and DNA from over 1000 cases. During the past year, we have shown that models based on large numbers of markers can distinguish between cases and controls in independent datasets with high significance, but only modest predictive value. These studies further suggest that common alleles predisposing to bipolar disorder also predispose to major depression and schizophrenia, but not to neurological diseases such as Parkinson Disease. We have also shown that DGKH, a gene implicated in the first genome-wide association study of bipolar disorder that we published in 2007, but inconsistently replicated in subsequent studies, shows differential expression in post-mortem brain tissue taken from patients diagnosed with bipolar disorder. This suggests that DGKH may indeed play a role in the disorder, even though genetic assocaition methods may sometimes fail to detect it. Finally, we have completed the initial ConLiGen collection and will begin genotyping in the fall of 2010. In collaboration with other investigators at the NIMH Intramural Program, we have also investigated neuroimaging measures that have been associated with bipolar disorder and may reveal biological aspects of the disease. We have carried out candidate gene and genome-wide association studies of muscarinic-2 receptor and serotonin-transporter binding potential, as measured by positron emission tomography (PET), and amygdala activation, measured by functional magnetic resonance imaging (fMRI). These studies have implicated additional genes that were not detected in our studies based solely on the clinical diagnosis, suggesting other biological pathways that may be involved in mood disorders. In the past year, we have shown how genetic variation in CHRM2 modulates muscarinic-2 receptor binding in vivo and accounts for reduced binding in bipolar disorder. We have also demonstrated that a Bcl-2 polymorphism influences gray matter volume in healthy volunteers, suggesting a mechanism whereby Bcl-2 may play a role in mood disorder susceptibility. Ongoing work is aimed at further developing multi-locus models that may better capture the multi-genic nature of this complex genetic disorder. In an effort to identify genetic variants that may have a much larger impact on individual risk, we are beginning large scale sequencing studies in selected populations, such as the Amish in Holmes County, Ohio.
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