The diacylglycerol kinase eta gene and bipolar disorder: a replication study in a Sardinian sample

Using Dictyostelium to Advance Our Understanding of the Role of Medium Chain Fatty Acids in Health and Disease

Ketogenic diets have been utilized for many years to improve health, and as a dietary approach for the treatment of a range of diseases, where the mechanism of these low carbohydrate and high fat diets is widely considered to be through the production of metabolic products of fat breakdown, called ketones. One of these diets, the medium chain triglyceride ketogenic diet, involves high fat dietary intake in the form of medium chain fatty acids (MCFAs), decanoic and octanoic acid, and is commonly used in endurance and high intensity exercises but has also demonstrated beneficial effects in the treatment of numerous pathologies including drug resistant epilepsy, cancer, and diabetes. Recent advances, using Dictyostelium discoideum as a model, have controversially proposed several direct molecular mechanisms for decanoic acid in this diet, independent of ketone generation. Studies in this model have identified that decanoic acid reduces phosphoinositide turnover, diacylglycerol kinase (DGK) activity, and also inhibits the mechanistic target of rapamycin complex 1 (mTORC1). These discoveries could potentially impact the treatment of a range of disorders including epilepsy, cancer and bipolar disorder. In this review, we summarize the newly proposed mechanisms for decanoic acid, identified using D. discoideum, and highlight potential roles in health and disease treatment.

Membrane shape as determinant of protein properties

Membrane lipids play a role in the modulation of a variety of biological processes. This is often achieved through fine-tuned changes in membrane physical and chemical properties. While some membrane physical properties (e.g., curvature, lipid domains, fluidity) have received increased scientific attention over the years, only recently has membrane shape emerged as an active modulator of protein properties. Biological membranes are mostly found organized into a lipid bilayer arrangement, in which the spontaneous shape is an intrinsically flat, planar morphology (in relation to the size of proteins). However, it is known that many cells and organelles have non-planar morphologies. In addition, perturbations in membrane morphology occur in a variety of biological processes. Recent studies have shown that membrane shape can modulate a variety of biological processes by determining protein properties. While membrane shape generation modulates proteins via changes in membrane mechanical properties, membrane shape recognition regulates proteins by providing the optimal surface for interaction. Hence, membranes have evolved an elegant mechanism to couple mesoscopic perturbations to molecular properties and vice-versa. In this review, the regulation of the enzymatic properties of two isoforms of mammalian diacylglycerol kinase, which play important roles in cellular signal transductions, will be used to exemplify the recent advancements in the field of membrane shape recognition, as well as future challenges and perspectives.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α–κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK–PA–PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG–DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

Diacylglycerol kinase η regulates C2C12 myoblast proliferation through the mTOR signaling pathway

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to produce phosphatidic acid (PA). The η isozyme of DGK is abundantly expressed in C2C12 myoblasts. However, the role of DGKη in skeletal muscle cells remains unknown. In the present study, we showed that DGKη was downregulated at an early stage of myogenic differentiation. The knockdown of DGKη by siRNAs significantly inhibited C2C12 myoblast proliferation but did not inhibit differentiation. Moreover, the suppression of DGKη expression decreased the expression levels of mammalian target of rapamycin (mTOR), which is a key regulator of cell proliferation, and fatty acid synthase (FASN), which catalyzes the de novo synthesis of fatty acids for cell proliferation and is transcriptionally regulated via mTOR signaling. Furthermore, the knockdown of mTOR or raptor, which is a component of mTOR complex 1 (mTORC1), decreased the amount of FASN. These results indicate that DGKη regulates myoblast proliferation through the mTOR (mTORC1)-FASN pathway. Interestingly, the knockdown of mTOR reduced the expression levels of DGKη, implying mutual regulation between DGKη and mTOR. In DGKη-knockdown myoblasts, C30-C36-PA species, mTOR activators, were decreased, suggesting that the modulation of mTOR activity through these PA species also plays an important role in myoblast proliferation.

Reprogramming fatty acyl specificity of lipid kinases via C1 domain engineering

C1 domains are lipid-binding modules that regulate membrane activation of kinases, nucleotide exchange factors and other C1-containing proteins to trigger signal transduction. Despite annotation of typical C1 domains as diacylglycerol (DAG) and phorbol ester sensors, the function of atypical counterparts remains ill-defined. Here, we assign a key role for atypical C1 domains in mediating DAG fatty acyl specificity of diacylglycerol kinases (DGKs) in live cells. Activity-based proteomics mapped C1 probe binding as a principal differentiator of type 1 DGK active sites that combined with global metabolomics revealed a role for C1s in lipid substrate recognition. Protein engineering by C1 domain swapping demonstrated that exchange of typical and atypical C1s is functionally tolerated and can directly program DAG fatty acyl specificity of type 1 DGKs. Collectively, we describe a protein engineering strategy for studying metabolic specificity of lipid kinases to assign a role for atypical C1 domains in cell metabolism.

Roles of DGKs in neurons: Postsynaptic functions?

Diacylglycerol kinases (DGKs) contribute to an important part of intracellular signaling because, in addition to reducing diacylglycerol levels, they generate phosphatidic acid (PtdOH) Recent research has led to the discovery of ten mammalian DGK isoforms, all of which are found in the mammalian brain. Many of these isoforms have studied functions within the brain, while others lack such understanding in regards to neuronal roles, regulation, and structural dynamics. However, while previously a neuronal function for DGKθ was unknown, it was recently found that DGKθ is required for the regulation of synaptic vesicle endocytosis and work is currently being conducted to elucidate the mechanism behind this regulation. Here we will review some of the roles of all mammalian DGKs and hypothesize additional roles. We will address the topic of redundancy among the ten DGK isoforms and discuss the possibility that DGKθ, among other DGKs, may have unstudied postsynaptic functions. We also hypothesize that in addition to DGKθ's presynaptic endocytic role, DGKθ might also regulate the endocytosis of AMPA receptors and other postsynaptic membrane proteins.

The Role of Pharmacogenomics in Bipolar Disorder: Moving Towards Precision Medicine

Bipolar disorder (BD) is a common and disabling psychiatric condition with a severe socioeconomic impact. BD is treated with mood stabilizers, among which lithium represents the first-line treatment. Lithium alone or in combination is effective in 60% of chronically treated patients, but response remains heterogenous and a large number of patients require a change in therapy after several weeks or months. Many studies have so far tried to identify molecular and genetic markers that could help us to predict response to mood stabilizers or the risk for adverse drug reactions. Pharmacogenetic studies in BD have been for the most part focused on lithium, but the complexity and variability of the response phenotype, together with the unclear mechanism of action of lithium, limited the power of these studies to identify robust biomarkers. Recent pharmacogenomic studies on lithium response have provided promising findings, suggesting that the integration of genome-wide investigations with deep phenotyping, in silico analyses and machine learning could lead us closer to personalized treatments for BD. Nevertheless, to date none of the genes suggested by pharmacogenetic studies on mood stabilizers have been included in any of the genetic tests approved by the Food and Drug Administration (FDA) for drug efficacy. On the other hand, genetic information has been included in drug labels to test for the safety of carbamazepine and valproate. In this review, we will outline available studies investigating the pharmacogenetics and pharmacogenomics of lithium and other mood stabilizers, with a specific focus on the limitations of these studies and potential strategies to overcome them. We will also discuss FDA-approved pharmacogenetic tests for treatments commonly used in the management of BD.

Diacylglycerol kinase (DGKA) regulates the effect of the epilepsy and bipolar disorder treatment valproic acid in Dictyostelium discoideum

Valproic acid (VPA) provides a common treatment for both epilepsy and bipolar disorder; however, common cellular mechanisms relating to both disorders have yet to be proposed. Here, we explore the possibility of a diacylglycerol kinase (DGK) playing a role in regulating the effect of VPA relating to the treatment of both disorders, using the biomedical model Dictyostelium discoideum. DGK enzymes provide the first step in the phosphoinositide recycling pathway, implicated in seizure activity. They also regulate levels of diacylglycerol (DAG), thereby regulating the protein kinase C (PKC) activity that is linked to bipolar disorder-related signalling. Here, we show that ablation of the single Dictyostelium dgkA gene results in reduced sensitivity to the acute effects of VPA on cell behaviour. Loss of dgkA also provides reduced sensitivity to VPA in extended exposure during development. To differentiate a potential role for this DGKA-dependent mechanism in epilepsy and bipolar disorder treatment, we further show that the dgkA null mutant is resistant to the developmental effects of a range of structurally distinct branched medium-chain fatty acids with seizure control activity and to the bipolar disorder treatment lithium. Finally, we show that VPA, lithium and novel epilepsy treatments function through DAG regulation, and the presence of DGKA is necessary for compound-specific increases in DAG levels following treatment. Thus, these experiments suggest that, in Dictyostelium, loss of DGKA attenuates a common cellular effect of VPA relating to both epilepsy and bipolar disorder treatments, and that a range of new compounds with this effect should be investigated as alternative therapeutic agents.

This article has an associated First Person interview with the first author of the paper.

Editor's choice: Here, using a tractable model system, Dictyostelium discoideum, we show that diacylglycerol kinase activity might contribute to the cellular mechanism of action of the epilepsy and bipolar disorder treatment, valproic acid.

Expressional profile of the diacylglycerol kinase eta gene DGKH

Bipolar disorder (BPD) is a genetically complex mental disorder, which is characterized by recurrent depressive and manic episodes, occurring with a typical cyclical course. In a recent study, we were able to identify a risk haplotype for BPD, as well as for unipolar depression and adult attention-deficit/hyperactivity disorder (ADHD), within the DGKH gene. DGKH codes for the eta (η) isoform of diacylglycerol kinase, which is involved in the phosphoinositol pathway. In the present study, we determined the expressional profile of Dgkh using quantitative real-time PCR (qPCR), in situ hybridization and immunohistological staining in the human and in the mouse brain. Expression studies showed that two different Dgkh transcripts exhibited distinct occurrence in a variety of murine tissues and also differed in their expression levels. The proteins encoded by those transcripts differ in functional protein domains suggesting distinct biochemical and cell biological properties and functions. qPCR analyses revealed an increase in Dgkh expression during mouse brain development indicating a possible role of this kinase in late developmental stages. Immunostainings revealed strong Dgkh expression in neurons of the hippocampus and the cerebellum of the murine brain, whereas highest expression levels of DGKH in the human brain were found in the striatum. Taken together, our studies revealed expressional changes during mouse brain development and occurrence of Dgkη in neurons of regions that have been linked to BPD as well as ADHD in humans providing evidence for the implication of DGKH in those disorders.

Pharmacogenomic aspects of bipolar disorder: An update

The hopes for readily implementable precision medicine are high. For many complex disorders, such as bipolar disorder, these hopes critically hinge on tangible successes in pharmacogenetics of treatment response or susceptibility to adverse events. In this article, we review the current state of pharmacogenomics of bipolar disorder including latest results from candidate genes and genome-wide association studies. The majority of studies focus on response to lithium treatment. Although a host of genes has been studied, hardly any replicated findings have emerged so far. Very small samples sizes and heterogeneous phenotype definition may be considered the major impediments to success in this field. Drawing from current experiences and successes in studies on diagnostic psychiatric phenotypes, we suggest several approaches for our way forward.

Diacylglycerol Kinases as Emerging Potential Drug Targets for a Variety of Diseases: An Update

Ten mammalian diacylglycerol kinase (DGK) isozymes (α–κ) have been identified to date. Our previous review noted that several DGK isozymes can serve as potential drug targets for cancer, epilepsy, autoimmunity, cardiac hypertrophy, hypertension and type II diabetes (Sakane et al., 2008). Since then, recent genome-wide association studies have implied several new possible relationships between DGK isozymes and diseases. For example, DGKθ and DGKκ have been suggested to be associated with susceptibility to Parkinson's disease and hypospadias, respectively. In addition, the DGKη gene has been repeatedly identified as a bipolar disorder (BPD) susceptibility gene. Intriguingly, we found that DGKη-knockout mice showed lithium (BPD remedy)-sensitive mania-like behaviors, suggesting that DGKη is one of key enzymes of the etiology of BPD. Because DGKs are potential drug targets for a wide variety of diseases, the development of DGK isozyme-specific inhibitors/activators has been eagerly awaited. Recently, we have identified DGKα-selective inhibitors. Because DGKα has both pro-tumoral and anti-immunogenic properties, the DGKα-selective inhibitors would simultaneously have anti-tumoral and pro-immunogenic (anti-tumor immunogenic) effects. Although the ten DGK isozymes are highly similar to each other, our current results have encouraged us to identify and develop specific inhibitors/activators against every DGK isozyme that can be effective regulators and drugs against a wide variety of physiological events and diseases.

Overexpression of diacylglycerol kinase η enhances Gαq-coupled G protein-coupled receptor signaling

Multiple genome-wide association studies have linked diacylglycerol kinase η (DGKη) to bipolar disorder (BPD). Moreover, DGKη expression is increased in tissue from patients with BPD. How increased levels of this lipid kinase might affect cellular functions is currently unclear. Here, we overexpressed mouse DGKη in human embryonic kidney 293 cells to examine substrate specificity and signaling downstream of endogenous G protein-coupled receptors (GPCRs). We found that DGKη can phosphorylate diacylglycerol (DAG) with different acyl side chains (8:0, 12:0, 18:1). In addition, overexpression of DGKη enhanced calcium mobilization after stimulating muscarinic receptors with carbachol and after stimulating purinergic receptors with ATP. This effect required DGKη catalytic activity, as assessed using a kinase-dead (G389D) mutant and multiple truncation constructs. DGKη was localized throughout the cytosol and did not translocate to the plasma membrane after stimulation with carbachol. Since protein kinase C (PKC) can be activated by DAG and promotes receptor desensitization, we also examined functional interactions between PKC and DGKη. We found that acute activation of PKC with phorbol 12-myristate 13-acetate shortened carbachol-evoked calcium responses and occluded the effect of overexpressed DGKη. Moreover, inhibition of PKC activity with bisindolylmaleimide I (BIM I) produced the same enhancing effect on carbachol-evoked calcium mobilization as overexpressed DGKη, and overexpression of DGKη produced no additional effect on calcium mobilization in the presence of BIM I. Taken together, our data suggest that DGKη enhances GPCR signaling by reducing PKC activation.

Molecular actions and clinical pharmacogenetics of lithium therapy

Mood disorders, including bipolar disorder and depression, are relatively common human diseases for which pharmacological treatment options are often not optimal. Among existing pharmacological agents and mood stabilizers used for the treatment of mood disorders, lithium has a unique clinical profile. Lithium has efficacy in the treatment of bipolar disorder generally, and in particular mania, while also being useful in the adjunct treatment of refractory depression. In addition to antimanic and adjunct antidepressant efficacy, lithium is also proven effective in the reduction of suicide and suicidal behaviors. However, only a subset of patients manifests beneficial responses to lithium therapy and the underlying genetic factors of response are not exactly known. Here we discuss preclinical research suggesting mechanisms likely to underlie lithium's therapeutic actions including direct targets inositol monophosphatase and glycogen synthase kinase-3 (GSK-3) among others, as well as indirect actions including modulation of neurotrophic and neurotransmitter systems and circadian function. We follow with a discussion of current knowledge related to the pharmacogenetic underpinnings of effective lithium therapy in patients within this context. Progress in elucidation of genetic factors that may be involved in human response to lithium pharmacology has been slow, and there is still limited conclusive evidence for the role of a particular genetic factor. However, the development of new approaches such as genome-wide association studies (GWAS), and increased use of genetic testing and improved identification of mood disorder patients sub-groups will lead to improved elucidation of relevant genetic factors in the future.

Pharmacogenomics of bipolar disorder

Bipolar disorder (BD) is a lifelong severe psychiatric condition with high morbidity, disability and excess mortality. The longitudinal clinical trajectory of BD is significantly modified by pharmacological treatment(s), both in acute and in long-term stages. However, a large proportion of BD patients have inadequate response to pharmacological treatments. Pharmacogenomic research may lead to the identification of molecular predictors of treatment response. When integrated with clinical information, pharmacogenomic findings may be used in the future to determine the probability of response/nonresponse to treatment on an individual basis. Here we present a selective review of pharmacogenomic findings in BD. In light of the evidence suggesting a genetic effect of lithium reponse in BD, we focused particularly on the pharmacogenomic literature relevant to this trait. The article contributes a detailed overview of the current status of pharmacogenomics in BD and offers a perspective on the challenges that can hinder its transition to personalized healthcare.

Assessment of Response to Lithium Maintenance Treatment in Bipolar Disorder: A Consortium on Lithium Genetics (ConLiGen) Report

OBJECTIVE

The assessment of response to lithium maintenance treatment in bipolar disorder (BD) is complicated by variable length of treatment, unpredictable clinical course, and often inconsistent compliance. Prospective and retrospective methods of assessment of lithium response have been proposed in the literature. In this study we report the key phenotypic measures of the "Retrospective Criteria of Long-Term Treatment Response in Research Subjects with Bipolar Disorder" scale currently used in the Consortium on Lithium Genetics (ConLiGen) study.

MATERIALS AND METHODS

Twenty-nine ConLiGen sites took part in a two-stage case-vignette rating procedure to examine inter-rater agreement [Kappa (κ)] and reliability [intra-class correlation coefficient (ICC)] of lithium response. Annotated first-round vignettes and rating guidelines were circulated to expert research clinicians for training purposes between the two stages. Further, we analyzed the distributional properties of the treatment response scores available for 1,308 patients using mixture modeling.

RESULTS

Substantial and moderate agreement was shown across sites in the first and second sets of vignettes (κ = 0.66 and κ = 0.54, respectively), without significant improvement from training. However, definition of response using the A score as a quantitative trait and selecting cases with B criteria of 4 or less showed an improvement between the two stages (ICC1 = 0.71 and ICC2 = 0.75, respectively). Mixture modeling of score distribution indicated three subpopulations (full responders, partial responders, non responders).

CONCLUSIONS

We identified two definitions of lithium response, one dichotomous and the other continuous, with moderate to substantial inter-rater agreement and reliability. Accurate phenotypic measurement of lithium response is crucial for the ongoing ConLiGen pharmacogenomic study.

Effect of variation in diacylglycerol kinase η (DGKH) gene on brain function in a cohort at familial risk of bipolar disorder

Several lines of evidence indicate that the diacylglycerol kinase eta (DGKH) gene is implicated in the etiology of bipolar disorder (BD). However, the functional neural mechanisms of DGKH's risk association remain unknown. Therefore, we examined the effects of three haplotype-tagging risk variants in DGKH (single nucleotide polymorphisms rs9315885, rs1012053, and rs1170191) on brain activation using a verbal fluency functional magnetic resonance imaging task. The subject groups consisted of young individuals at high familial risk of BD (n=81) and a comparison group of healthy controls (n=75). Individuals were grouped based on risk haplotypes described in previous studies. There was a significant risk haplotype*group interaction in the left medial frontal gyrus (BA10, involving anterior cingulate BA32), left precuneus, and right parahippocampal gyrus. All regions demonstrated greater activation during the baseline condition than sentence completion. Individuals at high familial risk for BD homozygous for the DGKH risk haplotype demonstrated relatively greater activation (poor suppression) of these regions during the task vs the low-risk haplotype subjects. The reverse pattern was seen for the control subjects. These findings suggest that there are differential effects of the DGKH gene in healthy controls vs the bipolar high-risk group, which manifests as a failure to disengage default-mode regions in those at familial risk carrying the risk haplotype.

Database of genetic studies of bipolar disorder

This study describes the construction and preliminary analysis of a database of summary level genetic findings for bipolar disorder from the literature. The database is available for noncommercial use at http://bioprogramming.bsd.uchicago.edu/BDStudies/. This may be the first complete collection of published gene-specific linkage and association findings on bipolar disorder, including genome-wide association studies. Both the positive and negative findings have been incorporated so that the statistical and contextual significance of each finding may be compared semi-quantitatively and qualitatively across studies of mixed technologies. The database is appropriate for searching a literature populated by mainly underpowered studies, and if 'hits' are viewed as tentative knowledge for future hypothesis generation. It can serve as the basis for a mega-analysis of candidate genes. Herein, we discuss the most robust and best replicated gene findings to date in a contextual manner.

Behavioral genetics of affective and anxiety disorders

As shown by clinical genetic studies, affective and anxiety disorders are complex genetic disorders with genetic and environmental factors interactively determining their respective pathomechanism. Advances in molecular genetic techniques including linkage studies, association studies, and genome-wide association studies allow for the detailed dissection of the genetic influence on the development of these disorders. Besides the molecular genetic investigation of categorical entities according to standardized diagnostic criteria, intermediate phenotypes comprising neurobiological or neuropsychological traits (e.g., neuronal correlates of emotional processing) that are linked to the disease of interest and that are heritable, have been proposed to be closer to the underlying genotype than the overall disease phenotype. These intermediate phenotypes are dimensional and more precisely defined than the categorical disease phenotype, and therefore have attracted much interest in the genetic investigation of affective and anxiety disorders. Given the complex genetic nature of affective and anxiety disorders with an interaction of multiple risk genes and environmental influences, the interplay of genetic factors with environmental factors is investigated by means of gene-environment interaction (GxE) studies. Pharmacogenetic studies aid in the dissection of the genetically influenced heterogeneity of psychotropic drug response and may contribute to the development of a more individualized treatment of affective and anxiety disorders. Finally, there is some evidence for genetic factors potentially shared between affective and anxiety disorders pointing to a possible overlapping phenotype between anxiety disorders and depression.

Cross-disorder analysis of bipolar risk genes: further evidence of DGKH as a risk gene for bipolar disorder, but also unipolar depression and adult ADHD

Recently, several genome-wide association studies (GWAS) on bipolar disorder (BPD) suggested novel risk genes. However, only few of them were followed up and further, the specificity of these genes is even more elusive. To address these issues, we genotyped SNPs in ANK3, CACNA1C, CMTM8, DGKH, EGFR, and NPAS3, which were significantly associated with BPD in previous GWAS, in a sample of 380 BPD patients. Replicated SNPs were then followed up in patients suffering from unipolar depression (UPD; n=387) or adult attention-deficit/hyperactivity disorder (aADHD; n=535). While we could not confirm an association of ANK3, CACNA1C, and EGFR with BPD, 10 SNPs in DGKH, CMTM8, and NPAS3 were nominally associated with disease, with two DGKH markers surviving correction for multiple testing. When these were followed up in UPD and aADHD, seven DGKH SNPs were also associated with UPD, while one SNP each in NPAS3 and CMTM8 and four in DGKH were linked to aADHD. Furthermore, a DGKH haplotype consisting of rs994856/rs9525580/rs9525584 GAT was associated with all disorders tested, while the complementary AGC haplotype was protective. The corresponding haploblock spans a 27-kb region covering exons coding for amino acids 65-243, and thus might include functional variants yet to be identified. We demonstrate an association of DGKH with BPD, UPD, and aADHD by applying a two-stage design. These disorders share the feature of mood instability, so that this phenotype might be associated with genetic variation in DGKH.

Regulation and roles of neuronal diacylglycerol kinases: a lipid perspective

Diacylglycerol kinases (DGKs) are a class of enzymes that catalyze the ATP-dependent conversion of diacylglycerol (DAG) to phosphatidic acid (PtdOH), resulting in the coordinate regulation of these two lipid second messengers. This regulation is particularly important in the nervous system where it is now well-established that DAG and PtdOH serve very important roles in modulating a variety of neurological functions. There are currently 10 identified mammalian DGKs, organized into five classes or "Types" based upon similarities in their primary sequences. A number of studies have identified eight of these isoforms in various regions of the mammalian central nervous system (CNS): DGK-α, DGK-β, DGK-γ, DGK-η, DGK-ζ, DGK-ι, DGK-ϵ, and DGK-θ. Further studies have provided compelling evidence supporting roles for these enzymes in neuronal spine density, myelination, synaptic activity, neuronal plasticity, epileptogenesis and neurotransmitter release. The physiological regulation of these enzymes is less clear. Like all interfacial enzymes, DGKs metabolize their hydrophobic substrate (DAG) at a membrane-aqueous interface. Therefore, these enzymes can be regulated by alterations in their subcellular localization, enzymatic activity, and/or membrane association. In this review, we summarize what is currently understood about the localization and regulation of the neuronal DGKs in the mammalian CNS.

Nominal association between a polymorphism in DGKH and bipolar disorder detected in a meta-analysis of East Asian case-control samples

AIM

Recent genome-wide association studies (GWAS) of bipolar disorder (BD) have detected new candidate genes, including DGKH, DFNB31 and SORCS2. However, the results of these GWAS were not necessarily consistent, indicating the importance of replication studies. In this study, we tested the genetic association of DGKH, DFNB31 and SORCS2 with BD.

METHODS

We genotyped 18 single-nucleotide polymorphisms (SNP) in DGKH, DFNB31 and SORCS2 using Japanese samples (366 cases and 370 controls). We also performed a meta-analysis of four SNP in DGKH, using the previously published allele frequency data of Han-Chinese case-control samples (1139 cases and 1138 controls).

RESULTS

IN the association analysis using Japanese samples, a SNP in SORCS2 (rs10937823) showed nominal genotypic association. However, we could not find any association in an additional analysis of tag SNP around rs10937823. In the meta-analysis of SNP in DGKH, rs9315897, which was not significantly associated with BD in the previous Chinese study, showed nominal association.

CONCLUSION

Although the association was not strong, the result of this study would support the association between DGKH and BD.

Response to treatment in bipolar disorder

PURPOSE OF REVIEW

Bipolar disorder is a complex psychiatric condition that has been shown to carry a great degree of genetic loading. This review addresses current research in the genetics of treatment response in bipolar disorder, with a focus on findings that have shaped our understanding of the changing direction of this field in light of recent technological advancements.

RECENT FINDINGS

The recent publications in bipolar disorder treatment response have helped consolidate or improve upon knowledge of susceptibility loci and genes in the field. There seems to be an increasing trend toward functionally assessing the role played by putative candidate genes and molecular factors modulating expression in bipolar disorder, as well as a movement toward more global, pathway and genome-wide-oriented research.

SUMMARY

Genetic and molecular research to date in bipolar disorder treatment response has not completely answered all the lingering questions in the field, but has contributed to the development of a more patient-based understanding of treatment. In order to apply these findings at a clinical level, more comprehensive treatment response studies are imperative, combining recent advances in high-throughput genomics with functional molecular research.

Pharmacogenomics of mood stabilizers in the treatment of bipolar disorder

Bipolar disorder (BD) is a chronic and often severe psychiatric illness characterized by manic and depressive episodes. Among the most effective treatments, mood stabilizers represent the keystone in acute mania, depression, and maintenance treatment of BD. However, treatment response is a highly heterogeneous trait, thus emphasizing the need for a structured informational framework of phenotypic and genetic predictors. In this paper, we present the current state of pharmacogenomic research on long-term treatment in BD, specifically focusing on mood stabilizers. While the results provided so far support the key role of genetic factors in modulating the response phenotype, strong evidence for genetic predictors is still lacking. In order to facilitate implementation of pharmacogenomics into clinical settings (i.e., the creation of personalized therapy), further research efforts are needed.

Lithium: a key to the genetics of bipolar disorder

Since the 1950s, lithium salts have been the main line of treatment for bipolar disorder (BD), both as a prophylactic and as an episodic treatment agent. Like many psychiatric conditions, BD is genetically and phenotypically heterogeneous, but evidence suggests that individuals who respond well to lithium treatment have more homogeneous clinical and molecular profiles. Response to lithium seems to cluster in families and can be used as a predictor for recurrence of BD symptoms. While molecular studies have provided important information about possible genes involved in BD predisposition or in lithium response, neither the mechanism of action of this drug nor the genetic profile of bipolar disorder is, as yet, completely understood.