Genetic control of lithium sensitivity and regulation of inositol biosynthetic genes.

Lithium (Li(+)) is a common treatment for bipolar mood disorder, a major psychiatric illness with a lifetime prevalence of more than 1%. Risk of bipolar disorder is heavily influenced by genetic predisposition, but is a complex genetic trait and, to date, genetic studies have provided little insight into its molecular origins. An alternative approach is to investigate the genetics of Li(+) sensitivity. Using the social amoeba Dictyostelium, we previously identified prolyl oligopeptidase (PO) as a modulator of Li(+) sensitivity. In a link to the clinic, PO enzyme activity is altered in bipolar disorder patients. Further studies demonstrated that PO is a negative regulator of inositol(1,4,5)trisphosphate (IP(3)) synthesis, a Li(+) sensitive intracellular signal. However, it was unclear how PO could influence either Li(+) sensitivity or risk of bipolar disorder. Here we show that in both Dictyostelium and cultured human cells PO acts via Multiple Inositol Polyphosphate Phosphatase (Mipp1) to control gene expression. This reveals a novel, gene regulatory network that modulates inositol metabolism and Li(+) sensitivity. Among its targets is the inositol monophosphatase gene IMPA2, which has also been associated with risk of bipolar disorder in some family studies, and our observations offer a cellular signalling pathway in which PO activity and IMPA2 gene expression converge.

Pharmacogenetics: defining the genetic basis of drug action and inositol trisphosphate analysis.

Medicinal drugs do not always have clearly understood mechanisms of action, especially as regards psychiatric treatment. Identification of genes involved in drug resistance is an important step toward elucidating the genetic basis of disease and the molecular mechanism of drug action. However, this approach is impractical in higher animals, as ablation and screening of every gene in an animal is not currently possible. Dictyostelium has proven a good model system for molecular pharmacological research as a result of its genetic tractability, ease of gene knockout, and creation of isogenic lines. In this system, we have identified genes that confer resistance to bipolar disorder drugs. This work has implicated inositol (1,4,5) trisphosphate (InsP3) signaling as a common mechanism of action for these drugs in patients.

An inverse PCR technique to rapidly isolate the flanking DNA of dictyostelium insertion mutants.

Restriction enzyme mediated integration is a widely used and effective method for insertional mutagenesis in Dictyostelium discoideum. In this method, plasmid rescue is used to clone the genomic deoxyribonucleic acid (DNA) sequences that flank the insertion site. For this to be effective, it is necessary to first find a convenient restriction enzyme site within the genomic DNA. This is a time-consuming process that requires Southern blot analysis of the mutant DNA. In addition, plasmid rescue requires transformation into highly competent Escherichia coli. Problems can arise owing to unstable genomic sequences, damage to the plasmid DNA and exogenous plasmid contamination. We have established a simple and rapid polymerase chain reaction-based technique that works for all mutants and circumvents the need for Southern blot analysis and plasmid rescue.