Functional Genomics of Bipolar Disorder

National Institute Of Mental Health

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Abstract

NCT00001174 Despite strong evidence of heritability and growing discovery of genetic markers for major mental illness, little is known about how gene expression in the brain differs across psychiatric diagnoses, or how inherited genetic risk factors shape these differences. We have studied expression of genes and gene transcripts in postmortem subgenual anterior cingulate cortex (sgACC), a key component of limbic circuits linked to mental illness. Deep sequencing was carried out in RNA obtained postmortem from 200 donors diagnosed with bipolar disorder, schizophrenia, major depression, or no psychiatric disorder. Case-control comparisons detected modest expression differences that were similar across disorders, although transcript-level differences were more pronounced. The 250 rare transcripts that were differentially expressed were enriched for genes involved in synapse formation, cell junctions, and heterotrimeric G-protein complexes. Relative abundances of alternatively spliced transcripts were associated with common genetic variants that accounted for disproportionate fractions of diagnosis-specific heritability. Inherited genetic risk factors shape the brain transcriptome and contribute to diagnostic differences between broad classes of mental illness. We also seek to model the impact of disease-related genes in cells derived from induced pluripotent stem cell (iPSC) lines. This project aims to explore ways in which we can use iPSC technology to study the biological impact of genes and genetic mutations that we identify in our other ongoing studies. Working with the National Heart, Lung and Blood Institute (NHLBI) stem cell core we have so far successfully reprogrammed fibroblasts into iPSCs from over 72 study participants. We are developing a large iPSC-based resource and associated work-flows that constitute a living catalog of psychiatric risk alleles. iPSC-derived cells are studied with high-resolution microscopic imaging, electrophysiology, and gene expression methods. These data could reveal differences between control and patient-derived cells and the impact of known and novel therapeutic agents. In collaboration with scientists at the New York Stem Cell Foundation Research Institute, we are also exploring ways to measure the functional impact of genetic mutations at the cellular level and to use genome editing tools such as CRISPR-Cas9 to rescue cellular phenotypes and establish a causal role for specific genetic mutations. A previous proof-of-concept study demonstrated the value of iPSC-based assays for translating even common, low-risk alleles identified by GWAS into novel genetic, neurobiological, and pharmacological insights. Currently we are working on methods to map regulatory chromatin contacts in neural progenitor cells. This will enable the identification of developmental stage and treatment specific influences on chromatin structure and gene expression. Our ongoing CNV studies examine the impact of known pathogenic CNVs at the level of iPSC-derived neural cells and in post-mortem brain. In the past year, we carried out extensive morphological and transcriptomic characterization of neural cells carrying a duplication CNV on chromosome 16p11.2 which has previously been associated with BD and other psychiatric disorders. Transcriptomic analyses indicate that some genes in the duplicated region show increased expression in neurons, but that many other genes are also dysregulated in CNV carriers. Overexpressed genes are enriched for several pathways, including neuronal growth and proliferation, MAP kinase signaling, and cell migration. Comparisons between carriers and sex-matched non-carriers revealed major differences in cellular differentiation and growth. Carrier neurons also showed fewer post-synaptic structures. While non-carrier NPCs easily differentiated into astrocytes using standard protocols, carrier NPCs did not differentiate into mature astrocytes. Astrocytes are known to play a critical role in the development and maintenance of healthy neurons and synapses. We have found that exogenous astrocytes from mouse rescue most of the neurodevelopmental and synaptic deficits in 16p11.2 duplication carriers. These results demonstrate that developmental deficits in astrocytes among carriers play a causal role in the syndrome. We are now screening a variety of medications to identify drugs that can help compensate for reduced astrocyte function. Multigenic disorders such as BD pose special challenges for experimental studies, since a single causative mutation is usually not identifiable. Thus we are also studying rare, single-gene disorders whose symptoms overlap with those seen in common mental illnesses. Smith-Magenis syndrome (SMS) is a neurodevelopmental disorder characterized by behavioral abnormalities and disruptions in circadian rhythm. Cells from people living with SMS obtained in collaboration with Ann Smith (NHGRI) have been reprogrammed into iPSCs and differentiated into neurons and other brain cells. We are using these cells to explore gene-expression, molecular, and morphological traits associated with SMS mutations in cultured brain cells. Causal connections with candidate cellular phenotypes will be established using gene editing techniques such as CRISPR. The results suggest that SMS mutations cause increases in proliferation and neurite outgrowth especially among excitatory neurons, consistent with proposed excitation/inhibition imbalance models of pathogenesis. Findings from this study may have relevance to other neuropsychiatric disorders such as depression, autism, and BD. In the coming year, we will continue studies in neural cells derived from people with BD, carriers of pathogenic CNVs, and rare damaging mutations within GWAS or CNV loci that run together with BD and related conditions in families. If successful, these projects will help unpack the biology behind GWAS results, identify high-risk alleles, and shed new light on how risk alleles act within neural cells to generate biological changes in the brain. The findings may also identify new targets that lead to better methods of diagnosis and treatment for neuropsychiatric disorders.