Abstract
Bipolar disorder is a serious psychiatric condition characterised by alternating episodes of major depression with mania or hypomania. The condition is relatively common, affecting 2-3% of the general population, and is generally treated with mood stabilising drugs such as lithium, valproate and carbamazepine. While considerable progress has been made over the last twenty years, both the pathology of bipolar disorder and the mechanisms by which mood-stabilising drugs relieve the symptoms of the disorder are not as yet fully understood. The high heritability of the condition, as evidenced in twin and adoption studies, suggests a genetic aetiology. The time scale of effective clinical treatment of bipolar disorder suggests that mood stabilising drugs may cause long term neural changes mediated by differences in gene regulation. Identifying genes that are differentially regulated by these medications may therefore provide insight into factors contributing to bipolar disorder and the mechanism of action of mood stabilisers.
Transcript quantitation was used to measure the expression of 20 genes across three human neural cell lines, SK-N-SH, SH-SY5Y and SK-N-DZ. Each cell line had been cultured in therapeutic concentrations of two mood stabilising drugs, valproate and carbamazepine. The genes measured were selected from a panel of genes previously established to be significantly up or downregulated after lithium treatment by a previously performed serial analysis of gene expression screen, or alternatively selected from the literature as promising candidates because of significant expression changes after lithium treatment or involvement in a lithium-affected signal transduction pathway. The brain derived neurotrophic factor (BDNF) gene was found to be consistently upregulated across all six cell line/treatment combinations while the other genes showed inconsistent regulation.
Lymphoblast cell lines were used as a model in which to attempt to replicate the changes in expression of four of the 20 initial genes – brain derived neurotrophic factor (BDNF), midkine (MDK), ras-related protein (RAB7A) and 14-3-3γ (YWHAG). Ten lymphoblast cell lines were used, five from individuals with bipolar disorder and five from unaffected control individuals. These cell lines were treated with a mood stabiliser, either lithium, valproate or carbamazepine, and differences in gene expression between treated cells and controls were measured. No consistent changes in regulation were found in any of the four genes analysed, and no significant differences were found between the control group and the bipolar group.
Single nucleotide polymorphisms from across the genes BDNF, RAB7A and YWHAG were examined for evidence of association with bipolar disorder. The polymorphisms were genotyped in the individuals of the South Island Bipolar Study (SIBS), a resource containing DNA and diagnostic information from individuals with bipolar disorder and their first and second-degree relatives, all sourced from the South Island of New Zealand. The genotypes were analysed for association with bipolar disorder using family based association testing. No polymorphisms of YWHAG showed association with bipolar disorder in the SIBS cohort. Four single nucleotide polymorphisms of BDNF were found to be associated with bipolar disorder after correction for multiple testing. Two of these associations had previously been reported in the literature, while the other two are novel. A previously unreported association with the rs13081864 SNP of RAB7A was also found. In addition, a haplotype of RAB7A consisting of five polymorphisms was found to be associated with bipolar disorder at p=0.00009.
These findings suggest BDNF and RAB7A may be involved in the pathophysiology of bipolar disorder. This study provides the first evidence that the gene RAB7A may be a contributing factor to the disorder.