Investigation of Notch3 as a candidate gene for bipolar disorder using brain hyperintensities as an endophenotype

Weighted Gene Coexpression Network Analysis Reveals Essential Genes and Pathways in Bipolar Disorder

Bipolar disorder (BD) is a major and highly heritable mental illness with severe psychosocial impairment, but its etiology and pathogenesis remains unclear. This study aimed to identify the essential pathways and genes involved in BD using weighted gene coexpression network analysis (WGCNA), a bioinformatic method studying the relationships between genes and phenotypes. Using two available BD gene expression datasets (GSE5388, GSE5389), we constructed a gene coexpression network and identified modules related to BD. The analyses of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathways were performed to explore functional enrichment of the candidate modules. A protein-protein interaction (PPI) network was further constructed to identify the potential hub genes. Ten coexpression modules were identified from the top 5,000 genes in 77 samples and three modules were significantly associated with BD, which were involved in several biological processes (e.g., the actin filament-based process) and pathways (e.g., MAPK signaling). Four genes (NOTCH1, POMC, NGF, and DRD2) were identified as candidate hub genes by PPI analysis and CytoHubba. Finally, we carried out validation analyses in a separate dataset, GSE12649, and verified NOTCH1 as a hub gene and the involvement of several biological processes such as actin filament-based process and axon development. Taken together, our findings revealed several candidate pathways and genes (NOTCH1) in the pathogenesis of BD and call for further investigation for their potential research values in BD diagnosis and treatment.

The Epigenetic Overlap between Obesity and Mood Disorders: A Systematic Review

(1) Background: Obesity and mood disorders are considered as the most prevalent morbidities in many countries. We suppose that epigenetic mechanisms may induce higher rates of obesity in subjects who suffer from mood disorders. In this systematic review, we focused on the potential roles of DNA methylation on mood disorders and obesity development. (2) Methods: This systematic review was conducted in accordance with the PRISMA statement and registered in Prospero. A systematic search was conducted in MEDLINE, Scopus, Web of Science, Cochrane Central database, EMBASE, and CINHAL. We also conducted a Grey literature search, such as Google Scholar. (3) Results: After deduplication, we identified 198 potentially related citations. Finally, ten unique studies met our inclusion criteria. We have found three overlap genes that show significant DNA methylation changes, both in obesity and depression. Pathway analysis interaction for TAPBP, BDNF, and SORBS2 confirmed the relation of these genes in both obesity and mood disorders. (4) Conclusions: While mechanisms linking both obesity and mood disorders to epigenetic response are still unknown, we have already known chronic inflammation induces a novel epigenetic program. As the results of gene enrichment, pathways analysis showed that TAPBP, BDNF, and SORBS2 linked together by inflammatory pathways. Hypermethylation in these genes might play a crucial rule in the co-occurrence of obesity and mood disorders.

Attenuated Notch signaling in schizophrenia and bipolar disorder

The Notch signaling pathway plays a crucial role in neurodevelopment and in adult brain homeostasis. We aimed to further investigate Notch pathway activity in bipolar disorder (BD) and schizophrenia (SCZ) by conducting a pathway analysis. We measured plasma levels of Notch ligands (DLL1 and DLK1) using enzyme immunoassays in a large sample of patients (SCZ n = 551, BD n = 246) and healthy controls (HC n = 639). We also determined Notch pathway related gene expression levels by microarray analyses from whole blood in a subsample (SCZ n = 338, BD n = 241 and HC n = 263). We found significantly elevated Notch ligand levels in plasma in both SCZ and BD compared to HC. Significant gene expression findings included increased levels of RFNG and KAT2B (p < 0.001), and decreased levels of PSEN1 and CREBBP in both patient groups (p < 0.001). RBPJ was significantly lower in SCZ vs HC (p < 0.001), and patients using lithium had higher levels of RBPJ (p < 0.001). We provide evidence of altered Notch signaling in both SCZ and BD compared to HC, and suggest that Notch signaling pathway may be disturbed in these disorders. Lithium may ameliorate aberrant Notch signaling. We propose that drugs targeting Notch pathway could be relevant in the treatment of psychotic disorders.

Bipolar II disorder as the initial presentation of CADASIL: an underdiagnosed manifestation

Mood disturbances have been documented in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). The highly varied morbidity indicates that the affective symptoms in CADASIL have not been cataloged systematically, leading to ineffective treatment, affecting the patients' quality of life, and possibly resulting in suicide. We present a case of CADASIL with bipolar II disorder as the first manifestation. A middle-aged female reported recurrent depressive episodes and appeared treatment resistant to adequate dosages and durations of antidepressants. Following a structured psychiatric interview and neuropsychological assessment, a past episode of hypomania was identified. Added treatment with sodium valproate alleviated most symptoms. Considering late-onset bipolar disorder with unexplained decline in cognition, a medical history of migraine, and a suspected family history of stroke, further cranial magnetic resonance imaging scan was performed and revealed severe leukoencephalopathy, prompting further investigation. The diagnosis was revised to CADASIL after Arg587Cys NOTCH3 mutation was confirmed. This case highlights the evolving process of affective disorder diagnosis and underlying organic etiologies. Based on the overlap of white matter hyperintensities, NOTCH3 mutation, and valproate therapy in bipolar disorder and CADASIL, bipolar II depression may be a poorly recognized manifestation of CADASIL. Well-designed clinical trials are warranted to verify the current findings.

Frontotemporal White Matter in Adolescents with, and at-Risk for, Bipolar Disorder

Frontotemporal neural systems are highly implicated in the emotional dysregulation characteristic of bipolar disorder (BD). Convergent genetic, postmortem, behavioral and neuroimaging evidence suggests abnormalities in the development of frontotemporal white matter (WM) in the pathophysiology of BD. This review discusses evidence for the involvement of abnormal WM development in BD during adolescence, with a focus on frontotemporal WM. Findings from diffusion tensor imaging (DTI) studies in adults and adolescents are reviewed to explore possible progressive WM abnormalities in the disorder. Intra- and interhemispheric frontotemporal abnormalities were reported in adults with BD. Although evidence in children and adolescents with BD to date has been limited, similar intrahemispheric and interhemispheric findings have also been reported. The findings in youths suggest that these abnormalities may represent a trait marker present early in the course of BD. Functional connectivity studies, demonstrating a relationship between WM abnormalities and frontotemporal dysfunction in BD, and DTI studies of vulnerability in first-degree relatives of individuals with BD, are discussed. Together, findings suggest the involvement of abnormal frontotemporal WM development in the pathophysiology of BD and that these abnormalities may be early trait markers of vulnerability; however, more studies are critically needed.

White matter alterations in bipolar disorder: potential for drug discovery and development

OBJECTIVES

Brain white matter (WM) alterations have recently emerged as potentially relevant in bipolar disorder. New techniques such as diffusion tensor imaging allow precise exploration of these WM microstructural alterations in bipolar disorder. Our objective was to critically review WM alterations in bipolar disorder, using neuroimaging and neuropathological studies, in the context of neural models and the potential for drug discovery and development.

METHODS

We conducted a systematic PubMed and Google Scholar search of the WM and bipolar disorder literature up to and including January 2013.

RESULTS

Findings relating to WM alterations are consistent in neuroimaging and neuropathology studies of bipolar disorder, especially in regions involved in emotional processing such as the anterior frontal lobe, corpus callosum, cingulate cortex, and in fronto-limbic connections. Some of the structural alterations are related to genetic risk factors for bipolar disorder and may underlie the dysfunctional emotional processing described in recent neurobiological models of bipolar disorder. Medication effects in bipolar disorder, from lithium and other mood stabilizers, might impact myelinating processes, particularly by inhibition of glycogen synthase kinase-3 beta.

CONCLUSIONS

Pathways leading to WM alterations in bipolar disorder represent potential targets for the development and discovery of new drugs. Myelin damage in bipolar disorder suggests that the effects of existing pro-myelinating drugs should also be evaluated to improve our understanding and treatment of this disease.

Neuroprogression in bipolar disorder

OBJECTIVE

Recent theories regarding the neuropathology of bipolar disorder suggest that both neurodevelopmental and neurodegenerative processes may play a role. While magnetic resonance imaging has provided significant insight into the structural, functional, and connectivity abnormalities associated with bipolar disorder, research assessing longitudinal changes has been more limited. However, such research is essential to elucidate the pathophysiology of the disorder. The aim of our review is to examine the extant literature for developmental and progressive structural and functional changes in individuals with and at risk for bipolar disorder.

METHODS

We conducted a literature review using MEDLINE and the following search terms: bipolar disorder, risk, child, adolescent, bipolar offspring, MRI, fMRI, DTI, PET, SPECT, cross-sectional, longitudinal, progressive, and developmental. Further relevant articles were identified by cross-referencing with identified manuscripts.

CONCLUSIONS

There is some evidence for developmental and progressive neurophysiological alterations in bipolar disorder, but the interpretation of correlations between neuroimaging findings and measures of illness exposure or age in cross-sectional studies must be performed with care. Prospective longitudinal studies placed in the context of normative developmental and atrophic changes in neural structures and pathways thought to be involved in bipolar disorder are needed to improve our understanding of the neurodevelopmental underpinnings and progressive changes associated with bipolar disorder.

Mapping vulnerability to bipolar disorder: a systematic review and meta-analysis of neuroimaging studies

BACKGROUND

Although early interventions in individuals with bipolar disorder may reduce the associated personal and economic burden, the neurobiologic markers of enhanced risk are unknown.

METHODS

Neuroimaging studies involving individuals at enhanced genetic risk for bipolar disorder (HR) were included in a systematic review. We then performed a region of interest (ROI) analysis and a whole-brain meta-analysis combined with a formal effect-sizes meta-analysis in a subset of studies.

RESULTS

There were 37 studies included in our systematic review. The overall sample for the systematic review included 1258 controls and 996 HR individuals. No significant differences were detected between HR individuals and controls in the selected ROIs: striatum, amygdala, hippocampus, pituitary and frontal lobe. The HR group showed increased grey matter volume compared with patients with established bipolar disorder. The HR individuals showed increased neural response in the left superior frontal gyrus, medial frontal gyrus and left insula compared with controls, independent from the functional magnetic resonance imaging task used. There were no publication biases. Sensitivity analysis confirmed the robustness of these results.

LIMITATIONS

As the included studies were cross-sectional, it remains to be determined whether the observed neurofunctional and structural alterations represent risk factors that can be clinically used in preventive interventions for prodromal bipolar disorder.

CONCLUSION

Accumulating structural and functional imaging evidence supports the existence of neurobiologic trait abnormalities in individuals at genetic risk for bipolar disorder at various scales of investigation.

White matter hyperintensities: from medical comorbidities to bipolar disorders and back

White matter hyperintensities (WMHs) are among the most replicated neuroimaging findings in studies of patients with bipolar disorders (BD). Despite the high rates of WMHs, their role and etiology in BD are not well understood. WMHs occur in multiple other conditions frequently co-morbid with BD. From the available studies it seems that WMHs are not a primary risk factor/endophenotype for BD. More likely, these lesions indicate the presence of medical co-morbidities with specific links to BD. Furthermore, the etiology of the WMHs in BD may represent different processes depending on age. In certain forms of BD, such as pediatric BD, WMHs may represent co-morbidity with developmental disorders. High frequency of migraine in BD and high prevalence of WMHs in migraine may suggest that a substantial proportion of WMHs in early adulthood to midlife BD subjects may be related to co-morbidity with migraine. Among elderly subjects with BD, or those with late-onset BD, WMHs are likely related to the presence of cardiovascular/metabolic disorders. With further research WMHs may enhance our knowledge about various pathological pathways involved in BD, help in decreasing the etiological heterogeneity of BD, and become useful as markers of severity or subtype of BD.

A meta-analysis of whole-brain diffusion tensor imaging studies in bipolar disorder

OBJECTIVES

White matter abnormalities are one of the most consistently reported findings in neuroimaging studies of bipolar disorder (BD). We conducted an anatomical likelihood estimation meta-analysis of BD whole-brain diffusion tensor imaging (DTI) studies, with the aim of identifying statistically consistent fractional anisotropy (FA) changes reflecting microstructural modifications to white matter in BD.

METHODS

We performed online searches of the PUBMED and EMBASE databases in January 2011. Studies were considered for inclusion if they used diffusion tensor MRI, compared a group of subjects with BD with healthy controls and involved whole-brain white matter analysis of FA. The analyses were conducted in Talairach space, using the activation likelihood estimation technique. We carried out a meta-analysis restricted to studies reporting a lower FA in patients with BD than in healthy controls.

RESULTS

Ten studies were included. We identified two significant clusters of decreased FA on the right side of the brain. The first was located in the right white matter, close to the parahippocampal gyrus. Four of the ten studies included contributed to this cluster. The second cluster was located close to the right anterior and subgenual cingulate cortex. These two clusters of decreased FA in BD are crossed by several white matter tracts.

CONCLUSIONS

These two clusters of altered FA may underlie the abnormal emotional processing and altered functional limbic connectivity in BD. Explorations based on DTI-based tractography are required to identify the tracts involved in the pathophysiology of BD.

Hippocampal morphology in lithium and non-lithium-treated bipolar I disorder patients, non-bipolar co-twins, and control twins

BACKGROUND

Bipolar I disorder is a highly heritable psychiatric illness with undetermined predisposing genetic and environmental risk factors. We examined familial contributions to hippocampal morphology in bipolar disorder, using a population-based twin cohort design.

METHODS

We acquired high-resolution brain MRI scans from 18 adult patients with bipolar I disorder [BPI; mean age 45.6 ± 8.69 (SD); 10 lithium-treated], 14 non-bipolar co-twins, and 32 demographically matched healthy comparison twins. We used three-dimensional radial distance mapping techniques to visualize hippocampal shape differences between groups.

RESULTS

Lithium-treated BPI patients had significantly larger global hippocampal volume compared to both healthy controls (9%) and non-bipolar co-twins (12%), and trend-level larger volumes relative to non-lithium-treated BPI patients (8%). In contrast, hippocampal volumes in non-lithium-treated BPI patients did not differ from those of non-bipolar co-twins and control twins. 3D surface maps revealed thicker hippocampi in lithium-treated BPI probands compared with control twins across the entire anterior-to-posterior extent of the cornu ammonis (CA1 and 2) regions, and the anterior part of the subiculum. Unexpectedly, co-twins also showed significantly thicker hippocampi compared with control twins in regions that partially overlapped those showing effects in the lithium treated BPI probands.

CONCLUSIONS

These findings suggest that regionally thickened hippocampi in bipolar I disorder may be partly due to familial factors and partly due to lithium-induced neurotrophy, neurogenesis, or neuroprotection. Unlike schizophrenia, hippocampal alterations in co-twins of bipolar I disorder probands are likely to manifest as subtle volume excess rather than deficit, perhaps indicating protective rather than risk effects.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL): a familial cause of depression and headache

A 31 year-old man with a history of a depressive episode presented with acute severe 'thunderclap' headache. Magnetic resonance imaging (MRI) revealed abnormalities typical of cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL), which was subsequently confirmed by genetic analysis. The psychiatric features of this genetic cause of depression and headache are discussed.

Genetic risk for white matter abnormalities in bipolar disorder

White matter deficits have been demonstrated in people with bipolar disorder, schizophrenia and their unaffected relatives. These deficits are supported by evidence from post-mortem studies, including microarray investigations which have repeatedly implicated abnormal myelin-associated gene expression. Furthermore, several risk-associated genes have now been identified that encode for proteins which have effects on white matter integrity. These genes include neuregulin-1 (NRG1) polymorphisms of which have been associated with risk to bipolar disorder. NRG1 has been shown to have effects on axonal migration, myelination and oligodendrocyte function. We and others have also shown that 5' risk-associated genetic variants in NRG1 are associated with reductions in both white matter density and integrity in regions associated with prefrontal connectivity. These findings are discussed in the context of the current literature, along with possible future research directions.

Update on the use of MR for assessment and diagnosis of psychiatric diseases

The lack of quantitative objective measures of psychiatric diseases such as anxiety and depression is one reason that the causative factors of psychiatric diseases remain obscure. The fact that human behavior is complex and cannot be easily tested in laboratories or reproduced in animal models further complicates our understanding of psychiatric diseases. During the past 3 decades, several magnetic resonance (MR)-based tools such as MR morphometry, diffusion-tensor imaging, functional MR imaging, and MR spectroscopy have yielded findings that provide tangible evidence of the neurobiologic manifestations of psychiatric diseases. In this article, we summarize major MR findings of schizophrenia, bipolar disorder, anxiety disorders, and attention deficit-hyperactivity disorder as examples to illustrate the promise that MR techniques hold for not only revealing the neurobiological underpinnings of psychiatric disorders but also enhancing our understanding of healthy human behavior. However, many radiologists remain skeptical about the diagnostic value of MR in psychiatric disease. Many inconsistent, noncomparable reports in the literature contribute to this skepticism. The aims of this article are to (a) illustrate the most reported MR findings of major psychiatric disorders such as schizophrenia, mood disorders, anxiety disorders, and attention deficit-hyperactivity disorder; (b) inform radiologists of the potential roles of MR imaging in psychiatric imaging research; and (c) discuss several confounding factors in the design and interpretation of MR imaging findings in psychiatry.

Valproate activates the Notch3/c-FLIP signaling cascade: a strategy to attenuate white matter hyperintensities in bipolar disorder in late life?

OBJECTIVES

Increased prevalence of deep white matter hyperintensities (DWMHs) has been consistently observed in patients with geriatric depression and bipolar disorder. DMWHs are associated with chronicity, disability, and poor quality of life. They are thought to be ischemic in their etiology and may be related to the underlying pathophysiology of mood disorders in the elderly. Notably, these lesions strikingly resemble radiological findings related to the cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephelopathy (CADASIL) syndrome. CADASIL arises from mutations in Notch3, resulting in impaired signaling via cellular Fas-associated death domain-like interleukin-1-beta-converting enzyme-inhibitory protein (c-FLIP) through an extracellular signal-regulated kinase (ERK)-dependent pathway. These signaling abnormalities have been postulated to underlie the progressive degeneration of vascular smooth muscle cells (VSMC). This study investigates the possibility that the anticonvulsant valproate (VPA), which robustly activates the ERK mitogen-activated protein kinase (MAPK) cascade, may exert cytoprotective effects on VSMC through the Notch3/c-FLIP pathway.

METHODS

Human VSMC were treated with therapeutic concentrations of VPA subchronically. c-FLIP was knocked down via small interfering ribonucleic acid transfection. Cell survival, apoptosis, and protein levels were measured.

RESULTS

VPA increased c-FLIP levels dose- and time-dependently and promoted VSMC survival in response to Fas ligand-induced apoptosis in VSMC. The anti-apoptotic effect of VPA was abolished by c-FLIP knockdown. VPA also produced similar in vivo effects in rat brain.

CONCLUSIONS

These results raise the intriguing possibility that VPA may be a novel therapeutic agent for the treatment of CADASIL and related disorders. They also suggest that VPA might decrease the liability of patients with late-life mood disorders to develop DWMHs.

Hyperintense MRI lesions in bipolar disorder: A meta-analysis and review

BACKGROUND

Cortical and subcortical hyperintensities in magnetic resonance imaging (MRI) scans are thought to represent areas of ischemic damage to brain tissue. Researchers have focused on the possible role these lesions may have in psychiatric disorders, including bipolar disorder. In 1997, the proposed 'vascular mania' diagnosis suggested utilizing not only the presence of strokes, but also confluent hyperintensities in its diagnostic criteria. This study was conducted to use meta-analytic techniques to investigate the association of hyperintensities and bipolar illness and to evaluate the current state of the literature.

METHODS

Using the PubMed and MEDLINE databases, we conducted a systematic literature search of studies investigating hyperintensities in subjects with bipolar disorder and controls or other psychiatric illnesses. We identified 44 publications from which 35 studies were included for review and 27 were selected for meta-analysis. Summary statistics of the prevalence were estimated through odds-ratios and confidence interval. Heterogeneity of the results across studies was tested using Q-statistics.

RESULTS

Meta-analysis identified an odds ratio of 2.5 (95% CI 1.9, 3.3) for hyperintensities in bipolar subjects compared to controls; however, there was significant heterogeneity among the studies (Q-statistics = 32; p = 0.04). This finding was most prominent for adolescents and children where the odds ratio was 5.7 (95% CI 2.3, 13.7). Deep white matter hyperintensities (odd ratio 3.2; 95% CI 2.2, 4.5) and subcortical grey matter hyperintensities (odds ratio 2.7; 95% CI 1.3, 2.9) were more strongly associated with bipolar subjects. There were no differences between bipolar subjects and controls for perivascular hyperintensities (odds ratio 1.3; 95% CI 0.8, 1.9). Though hyperintensities were numerically greater in bipolar subjects, meta-analysis did not demonstrate any significant differences between bipolar subjects and unipolar depression subjects (OR 1.6; 95% CI 0.9, 2.7) nor subjects with schizophrenia (OR 1.5; 95% CI 0.9, 2.7).

CONCLUSIONS

This meta-analysis continues to support the association of bipolar disorder and hyperintensities, especially in the deep white matter and subcortical grey matter. It also highlights the increased incidence in children and adolescence with bipolar disorder. However, hyperintensities are not specific to bipolar disorder, but appear at similar rates in unipolar depression and schizophrenia. Thus, the role of hyperintensities in the pathogenesis, pathophysiology, and treatment of bipolar disorder remains unclear. Further studies are required that are large enough to decrease the heterogeneity of the samples and MRI techniques, assess size and location of hyperintensities, and the impact on treatment response. Coordination with newer imaging techniques, such as diffusion tensor imaging (DTI) may be especially helpful in understanding the pathology of these lesions.

Relationship of cerebrospinal fluid glucose metabolites to MRI deep white matter hyperintensities and treatment resistance in bipolar disorder patients

OBJECTIVES

Both diabetes mellitus and magnetic resonance image (MRI) deep white matter hyperintensities (WMHs) are more common in bipolar disorder (BD) patients than in matched controls. Deep-as opposed to periventricular--WMHs and diabetes are associated with treatment resistance and poorer outcome. This study investigated whether brain glucose metabolism by the polyol pathway--a pathway linked to nervous tissue disease in diabetes--is related to deep WMH volume and treatment resistance in BD patients.

METHODS

Volumes of fluid-attenuated inversion recovery WMHs were quantified and correlated with cerebrospinal fluid (CSF) concentrations of glucose metabolites in 20 nondiabetic patients with BD and nondiabetic comparison subjects with schizophrenia (n = 15) or transient neurologic symptoms (neurologic controls, n = 15).

RESULTS

BD patients, but not schizophrenic patients, had significantly greater volumes of deep but not periventricular WMHs compared to neurologic controls. BD subjects also had significantly greater CSF concentrations of sorbitol and fructose (the polyol pathway metabolites of glucose) compared to controls. Significant positive correlations between CSF metabolites and WMH volumes were found only in the BD group and were between deep WMH volumes and CSF sorbitol (rho = 0.487, p = 0.029) and fructose (rho = 0.474, p = 0.035). An index of treatment resistance correlated significantly with deep WMH volume (rho = 0.578, p = 0.008), sorbitol (rho = 0.542, p = 0.013), and fructose (rho = 0.692, p = 0.001) in BD subjects but not in other subjects.

CONCLUSIONS

This is the first reported evidence of relationships between abnormal brain glucose metabolism and both deep WMHs and treatment resistance in a group of BD patients. Further studies are necessary to determine the significance of these findings to BD pathophysiology.

Neuropsychiatric manifestations in CADASIL

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an inherited small-artery disease of mid-adulthood caused by mutations of the NOTCH3 gene. The disease is responsible for widespread white-matter lesions associated with lacunar infarctions in various subcortical areas. The disease is responsible for migraine with aura and ischemic strokes, and is associated with various degrees of cognitive impairment and with mood disturbances. CADASIL is considered as a unique model to investigate what is known as "subcortical ischemic vascular dementia." Recent data suggest that the number of lacunar infarctions and severity of cerebral atrophy are the main magnetic resonance imaging markers associated with cognitive and motor disabilities in this disorder. Mood disturbances are reported in 10% to 20% of patients, most often in association with cognitive alterations. Their exact origin remains unknown; the presence of ischemic lesions within the basal ganglia or the frontal white matter may promote the occurrence of these symptoms. Further studies are needed to better understand the relationships between cerebral lesions and both cognitive and psychiatric symptoms in this small-vessel disease of the brain.

The endophenotype concept in psychiatric genetics

The idea that some phenotypes bear a closer relationship to the biological processes that give rise to psychiatric illness than diagnostic categories has attracted considerable interest. Much effort has been devoted to finding such endophenotypes, partly because it is believed that the genetic basis of endophenotypes will be easier to analyse than that of psychiatric disease. This belief depends in part on the assumption that the effect sizes of genetic loci contributing to endophenotypes are larger than those contributing to disease susceptibility, hence increasing the chance that genetic linkage and association tests will detect them. We examine this assumption by applying meta-analytical techniques to genetic association studies of endophenotypes. We find that the genetic effect sizes of the loci examined to date are no larger than those reported for other phenotypes. A review of the genetic architecture of traits in model organisms also provides no support for the view that the effect sizes of loci contributing to phenotypes closer to the biological basis of disease is any larger than those contributing to disease itself. While endophenotype measures may afford greater reliability, it should not be assumed that they will also demonstrate simpler genetic architecture.

Psychiatric endophenotypes and the development of valid animal models

Endophenotypes are quantifiable components in the genes-to-behaviors pathways, distinct from psychiatric symptoms, which make genetic and biological studies of etiologies for disease categories more manageable. The endophenotype concept has emerged as a strategic tool in neuropsychiatric research. This emergence is due to many factors, including the modest reproducibility of results from studies directed toward etiologies and appreciation for the complex relationships between genes and behavior. Disease heterogeneity is often guaranteed, rather than simplified, through the current diagnostic system; inherent benefits of endophenotypes include more specific disease concepts and process definitions. Endophenotypes can be neurophysiological, biochemical, endocrine, neuroanatomical, cognitive or neuropsychological. Heritability and stability (state independence) represent key components of any useful endophenotype. Importantly, they characterize an approach that reduces the complexity of symptoms and multifaceted behaviors, resulting in units of analysis that are more amenable to being modeled in animals. We discuss the benefits of more direct interpretation of clinical endophenotypes by basic behavioral scientists. With the advent of important findings regarding the genes that predispose to psychiatric illness, we are at an important crossroads where, without anthropomorphizing, animal models may provide homologous components of psychiatric illness, rather than simply equating to similar (loosely analogized) behaviors, validators of the efficacy of current medications or models of symptoms. We conclude that there exists a need for increased collaboration between clinicians and basic scientists, the result of which should be to improve diagnosis, classification and treatment on one end and to increase the construct relevance of model organisms on the other.

A guide to the genetics of psychiatric disease

The road to scientific discovery begins with an awareness of what is unknown. Research in science can in some ways be like putting together the pieces of a puzzle without having the benefit of the box-top picture of the completed puzzle. The "picture" in science is an understanding of how nature works in a particular instance, and it takes many separate pieces of the "puzzle" to put this understanding together. These pieces are always of different kinds of data, often obtained using different approaches and techniques. The challenge of the researcher is to picture or hypothesize each of the missing pieces before actually having them in hand, so they can be sought and tested in the laboratory. This "picturing" is actually having a clear idea of what you don't know: having a clear image of the "shape" of the missing piece. This is easy when the puzzle surrounding the missing piece is already in hand, but more difficult with less of it constrained by what is already known. In putting paper puzzles together, the shape of the pieces is not the only limitation that needs to be satisfied. There is also the picture to satisfy, that is, the picture usually has to make sense. In science these constraints can be manifold, and usually the quality of the research is judged by the number of ways a piece of data integrates into and brings together the rest of the puzzle. The multidimensionality of scientific questions makes it virtually essential that as many different pieces of the puzzle as possible be obtained. The more that is not known about the puzzle, the more pieces you need. Thus it is with the genetics of psychiatric diseases. In this guide, we will explore as many of the domains of the genetic puzzle as we are aware of. We will learn a bit of the language of each and how they fit into the puzzle with at least one anecdote to serve as an example. Mapping unknown territory is always a process, but we hope this guide will increase the reader's awareness of what is unknown.

T2 Hyperintensities in Bipolar Patients and their Healthy Siblings

BACKGROUND

This study investigates the frequency of hyperintensities, which are common in bipolar disorder, in sufferers' siblings who are free of bipolar disorder and to ascertain whether these lesions reflect any familial characteristics. It offers an assessment of the relationship between these lesions and clinical characteristics.

METHODS

The study group consisted of 12 patients with bipolar disorder, their siblings who had no history of mental disorder, and a matched control group. All three groups were assessed using SCID-I (Structured Clinical Interview for DSM-IV Axis I Disorders) for diagnosis, and MRI (magnetic resonance imaging) for signal hyperintensity.

RESULTS

No periventricular white matter lesions were encountered in any groups. It was found that in the patients and siblings, the location was in the right cerebral hemisphere in the deep white matter, whereas it was in the left cerebral hemisphere for the control group. Only in the patient group were lesions detected in subcortical white matter in the right cerebral hemisphere. The lesions were localized in the fronto-parietal area. Considering the brain as a whole, more hyperintensities were detected in the patient group compared to the other two groups. The onset of the disorder with a manic episode was increasing probably in the presence of hyperintensities.

CONCLUSIONS

Despite being alike in terms of age, more hyperintensities were detected in the patient group than in the siblings and control groups. Contrary to the control group, however, hyperintensities were localized in the right cerebral hemisphere in both the patients and the siblings groups. Hyperintensities seen in the left cerebral hemisphere yield a nonspecific impression. Siblings who have hyperintensities in the right cerebral hemisphere should be followed up and investigated with regard to bipolar disorder.

Neuroanatomical abnormalities as risk factors for bipolar disorder

OBJECTIVE

Neuroimaging studies show structural brain abnormalities in bipolar patients. Some of the abnormalities may represent biological risk factors conveying vulnerability for the disease. This paper aims to identify neuroanatomical risk factors for bipolar disorder (BD).

METHODS

We reviewed magnetic resonance imaging (MRI) findings in populations in which the effects of the disease or treatment are minimal or where the chances of finding genetically coded risk factors shared within the families are increased. Such populations include unaffected relatives of bipolar patients, first-episode patients, children or adolescents with BD and patients with familial BD.

RESULTS

MEDLINE search revealed 30 relevant scientific papers. Abnormalities in the volume of the striatum, left hemispheric white matter, thalamus and anterior cingulate as well as quantitative MRI signal hyperintensities were identified already in unaffected relatives of bipolar patients. Subjects in the early stages of the disease showed volume changes of the ventricles, white matter, caudate, putamen, amygdala, hippocampus and the subgenual prefrontal cortex. Reduction in the subgenual prefrontal cortex volume was replicated in three of four studies in patients with familial BD.

CONCLUSIONS

Possible candidates for neuroanatomical risk factors for BD are volumetric abnormalities of the subgenual prefrontal cortex, striatum, white matter, and probably also the hippocampus and amygdala. Qualitative finding of white matter hyperintensities was already utilized as an endophenotype.

The phenotypes of bipolar disorder: relevance for genetic investigations

The search for susceptibility genes for bipolar disorder (BD) depends on appropriate definitions of the phenotype. In this paper, we review data on diagnosis and clinical features of BD that could be used in genetic studies to better characterize patients or to define homogeneous subgroups. Clinical symptoms, long-term course, comorbid conditions, and response to prophylactic treatment may define groups associated with more or less specific loci. One such group is characterized by symptoms of psychosis and linkage to 13q and 22q. A second group includes mainly bipolar II patients with comorbid panic disorder, rapid mood switching, and evidence of chromosome 18 linkage. A third group comprises typical BD with an episodic course and favourable response to lithium prophylaxis. Reproducibility of cognitive deficits across studies raises the possibility of using cognitive profiles as endophenotypes of BD, with deficits in verbal explicit memory and executive function commonly reported. Brain imaging provides a more ambiguous data set consistent with heterogeneity of the illness.

The neurobiology of bipolar disorder

The neurobiology of bipolar disorder is reviewed. Bipolar disorder is associated with alterations in central nervous system (CNS) function from the level of large-scale brain circuits to intracellular signal transduction mechanisms. Because of the broad spectrum over which these abnormalities appear, the causative effects are most likely present in the lowest common denominator of all of these systems. Current evidence points to subtle alterations in signal transduction that reverberate downstream both intra- and extracellularly to produce the symptoms of bipolar disorder.