Neural basis of genetic vulnerability to bipolar disorder

Abnormalities of reward processing, decision-making and emotion processing are core features of bipolar I disorder (BD). These processes are closely linked with fronto-striatal and midbrain circuitry. I sought to test whether dysfunctions of these pathways were present in BD and whether they related to genetic vulnerability to illness or resilience. I recruited twenty-five BD I patients each with their unaffected sibling, and compared them to 24 healthy age- and gender-matched controls. In chapter 1, I provide a research background and literature review. Chapter 2 describes the neuropsychological assessments which demonstrated trait-related deficits in working memory with slower processing speed representing an endophenotype. Chapter 3 describes the implicit/ explicit facial emotion processing task performed during event-related functional MRI (erfMRI). Pairwise comparisons demonstrated implicit processing was associated with increases in lingual gyrus and insula activations and explicit processing elicited reduced fusiform activations in patients compared with controls. Increased posterior cingulate activations and reductions in putamen and cerebellar activity were found in siblings compared to controls, and reductions in parietal activations were noted in siblings compared to their ill relatives. These findings suggest over-activations in regions involved in facial expression recognition and attentional shifting (lingual and insula respectively) and deactivations in a region important for the perception and recognition of faces (fusiform) represent correlates of disease expression. Additionally regional deactivations associated with category learning and attentional processing (parietal, putamen and cerebellar) and increased activations in a region involved in emotional salience (posterior cingulate) may represent adaptive responses associated with resilience. Chapter 4 describes an instrumental reward-learning task performed during erfMRI. Data were analysed at whole brain level and using a priori region of interest analyses in ventral striatum/midbrain and prefrontal cortex (PFC). Results included increased ventral striatum activation in association with the difference between observed and expected rewarding outcomes (the prediction error (PE)) in patients compared to controls. Decreased prefrontal activations were seen in the patient and sibling groups compared to controls in association with the learning of the value of the conditioned stimulus. These findings suggest that i) PE associated circuitry (striatal) overactivation, and ii) prefrontal deactivations underlie the genetic vulnerability to BD

Preliminary investigation of miRNA expression in individuals at high familial risk of bipolar disorder

AbstractBipolar disorder (BD) is a highly heritable psychiatric disorder characterised by recurrent episodes of mania and depression. Many studies have reported altered gene expression in BD, some of which may be attributable to the dysregulated expression of miRNAs. Studies carried out to date have largely studied medicated patients, so it is possible that observed changes in miRNA expression might be a consequence of clinical illness or of its treatment. We sought to establish whether altered miRNA expression might play a causative role in the development of BD by studying young, unmedicated relatives of individuals with BD, who are at a higher genetic risk of developing BD themselves (high-risk individuals). The expression of 20 miRNAs previously implicated in either BD or schizophrenia was measured by qRT-PCR in whole-blood samples from 34 high-risk and 46 control individuals. Three miRNAs, miR-15b, miR-132 and miR-652 were up-regulated in the high-risk individuals, consistent with previous reports of increased expression of these miRNAs in patients with schizophrenia. Our findings suggest that the altered expression of these miRNAs might represent a mechanism of genetic susceptibility for BD. Moreover, our observation of altered miRNA expression in the blood prior to the onset of illness provides hope that one day blood-based tests may aid in the risk-stratification and treatment of BD

DNA methylation in a Scottish family multiply affected by bipolar disorder and major depressive disorder

Background: Bipolar disorder (BD) is a severe, familial psychiatric condition. Progress in understanding the aetiology of BD has been hampered by substantial phenotypic and genetic heterogeneity. We sought to mitigate these confounders by studying a multi-generational family multiply affected by BD and major depressive disorder (MDD), who carry an illness-linked haplotype on chromosome 4p. Within a family, aetiological heterogeneity is likely to be reduced, thus conferring greater power to detect illness-related changes. As accumulating evidence suggests that altered DNA methylation confers risk for BD and MDD, we compared genome-wide methylation between (i) affected carriers of the linked haplotype (ALH) and married-in controls (MIs), (ii) well unaffected haplotype carriers (ULH) and MI, (iii) ALH and ULH and (iv) all haplotype carriers (LH) and MI.Results: Nominally significant differences in DNA methylation were observed in all comparisons, with differences withstanding correction for multiple testing when the ALH or LH group was compared to the MIs. In both comparisons, we observed increased methylation at a locus in FANCI, which was accompanied by increased FANCI expression in the ALH group. FANCI is part of the Fanconi anaemia complementation (FANC) gene family, which are mutated in Fanconi anaemia and participate in DNA repair. Interestingly, several FANC genes have been implicated in psychiatric disorders. Regional analyses of methylation differences identified loci implicated in psychiatric illness by genome-wide association studies, including CACNB2 and the major histocompatibility complex. Gene ontology analysis revealed enrichment for methylation differences in neurologically relevant genes.Conclusions: Our results highlight altered DNA methylation as a potential mechanism by which the linked haplotype might confer risk for mood disorders. Differences in the phenotypic outcome of haplotype carriers might, in part, arise from additional changes in DNA methylation that converge on neurologically important pathways. Further work is required to investigate the underlying mechanisms and functional consequences of the observed differences in methylation

Genetic influences on schizophrenia and subcortical brain volumes:Large-scale proof of concept

Schizophrenia is a devastating psychiatric illness with high heritability. Brain structure and function differ, on average, between people with schizophrenia and healthy individuals. As common genetic associations are emerging for both schizophrenia and brain imaging phenotypes, we can now use genome-wide data to investigate genetic overlap. Here we integrated results from common variant studies of schizophrenia (33,636 cases, 43,008 controls) and volumes of several (mainly subcortical) brain structures (11,840 subjects). We did not find evidence of genetic overlap between schizophrenia risk and subcortical volume measures either at the level of common variant genetic architecture or for single genetic markers. These results provide a proof of concept (albeit based on a limited set of structural brain measures) and define a roadmap for future studies investigating the genetic covariance between structural or functional brain phenotypes and risk for psychiatric disorders