Joo Heon Shin

Distribution, recognition and regulation of non-CpG methylation in the adult mammalian brain

DNA methylation has critical roles in the nervous system and has been traditionally considered to be restricted to CpG dinucleotides in metazoan genomes. Here we show that the single base–resolution DNA methylome from adult mouse dentate neurons consists of both CpG (∼75%) and CpH (∼25%) methylation (H = A/C/T). Neuronal CpH methylation is conserved in human brains, enriched in regions of low CpG density, depleted at protein-DNA interaction sites and anticorrelated with gene expression. Functionally, both methylated CpGs (mCpGs) and mCpHs can repress transcription in vitro and are recognized by methyl-CpG binding protein 2 (MeCP2) in neurons in vivo. Unlike most CpG methylation, CpH methylation is established de novo during neuronal maturation and requires DNA methyltransferase 3A (DNMT3A) for active maintenance in postmitotic neurons. These characteristics of CpH methylation suggest that a substantially expanded proportion of the neuronal genome is under cytosine methylation regulation and provide a new foundation for understanding the role of this key epigenetic modification in the nervous system.

Intersection of diverse neuronal genomes and neuropsychiatric disease: The Brain Somatic Mosaicism Network

Single-cell diversity in the brain The cells that make up an organism may all start from one genome, but somatic mutations mean that somewhere along the line of development, an organisms individual cellular genomes diverge. McConnell et al. review the implications and causes of single-cell genomic diversity for brain function. Somatic mutations caused by mobile genetic elements or errors in DNA repair may underlie certain neuropsychiatric disorders. Science, this issue p. eaal1641 BACKGROUND Elucidating the genetic architecture of neuropsychiatric disorders remains a major scientific and medical challenge. Emerging genomic technologies now permit the analysis of somatic mosaicism in human tissues. The measured frequencies of single-nucleotide variants (SNVs), small insertion/deletion (indel) mutations, structural variants [including copy number variants (CNVs), inversions, translocations, and whole-chromosome gains or losses], and mobile genetic element insertions (MEIs) indicate that each neuron may harbor hundreds of somatic mutations. Given the long life span of neurons and their central role in neural circuits and behavior, somatic mosaicism represents a potential mechanism that may contribute to neuronal diversity and the etiology of numerous neuropsychiatric disorders. ADVANCES Somatic mutations that confer cellular proliferative or cellular survival phenotypes have been identified in patients with cortical malformations. These data have led to the hypothesis that somatic mutations may also confer phenotypes to subsets of neurons, which could increase the risk of developing certain neuropsychiatric disorders. Genomic technologies, including advances in long-read, next-generation DNA sequencing technologies, single-cell genomics, and cutting-edge bioinformatics, can now make it possible to determine the types and frequencies of somatic mutations within the human brain. However, a comprehensive understanding of the contribution of somatic mosaicism to neurotypical brain development and neuropsychiatric disease requires a coordinated, multi-institutional effort. The National Institute of Mental Health (NIMH) has formed a network of 18 investigative teams representing 15 institutions called the Brain Somatic Mosaicism Network (BSMN). Each research team will use an array of genomic technologies to exploit well-curated human tissue repositories in an effort to define the frequency and pattern of somatic mutations in neurotypical individuals and in schizophrenia, autism spectrum disorder, bipolar disorder, Tourette syndrome, and epilepsy patient populations. Collectively, these efforts are estimated to generate a community resource of more than 10,000 DNA-sequencing data sets and will enable a cross-platform integrated analysis with other NIMH initiatives, such as the PsychENCODE project and the CommonMind Consortium. OUTLOOK A fundamental open question in neurodevelopmental genetics is whether and how somatic mosaicism may contribute to neuronal diversity within the neurotypical spectrum and in diseased brains. Healthy individuals may harbor known pathogenic somatic mutations at subclinical frequencies, and the local composition of neural cell types may be altered by mutations conferring prosurvival phenotypes in subsets of neurons. By extension, the neurotypical architecture of somatic mutations may confer circuit-level differences that would not be present if every neuron had an identical genome. Given the apparent abundance of somatic mutations within neurons, an in-depth understanding of how different types of somatic mosaicism affect neural function could yield mechanistic insight into the etiology of neurodevelopmental and neuropsychiatric disorders. The BSMN will examine large collections of postmortem brain tissue from neurotypical individuals and patients with neuropsychiatric disorders. By sequencing brain DNA and single neuronal genomes directly, rather than genomic DNA derived from peripheral blood or other somatic tissues, the BSMN will test the hypothesis that brain somatic variants contribute to neuropsychiatric disease. Notably, it is also possible that some inherited germline variants confer susceptibility to disease, which is later exacerbated by somatic mutations. Confirming such a scenario could increase our understanding of the genetic risk architecture of neuropsychiatric disease and may, in part, explain discordant neuropsychiatric phenotypes between identical twins. Results from these studies may lead to the discovery of biomarkers and genetic targets to improve the treatment of neuropsychiatric disease and may offer hope for improving the lives of patients and their families. Collectively, somatic SNVs, indels, structural variants (e.g., CNVs), and MEIs (e.g., L1 retrotransposition events) shape the genomic landscape of individual neurons. The Brain Somatic Mosaicism Network aims to systematically generate pioneering data on the types and frequencies of brain somatic mutations in both neurotypical individuals and those with neuropsychiatric disease. The resulting data will be shared as a large community resource. Neuropsychiatric disorders have a complex genetic architecture. Human genetic population-based studies have identified numerous heritable sequence and structural genomic variants associated with susceptibility to neuropsychiatric disease. However, these germline variants do not fully account for disease risk. During brain development, progenitor cells undergo billions of cell divisions to generate the ~80 billion neurons in the brain. The failure to accurately repair DNA damage arising during replication, transcription, and cellular metabolism amid this dramatic cellular expansion can lead to somatic mutations. Somatic mutations that alter subsets of neuronal transcriptomes and proteomes can, in turn, affect cell proliferation and survival and lead to neurodevelopmental disorders. The long life span of individual neurons and the direct relationship between neural circuits and behavior suggest that somatic mutations in small populations of neurons can significantly affect individual neurodevelopment. The Brain Somatic Mosaicism Network has been founded to study somatic mosaicism both in neurotypical human brains and in the context of complex neuropsychiatric disorders.

A human-specific AS3MT isoform and BORCS7 are molecular risk factors in the 10q24.32 schizophrenia-associated locus

Genome-wide association studies (GWASs) have reported many single nucleotide polymorphisms (SNPs) associated with psychiatric disorders, but knowledge is lacking regarding molecular mechanisms. Here we show that risk alleles spanning multiple genes across the 10q24.32 schizophrenia-related locus are associated in the human brain selectively with an increase in the expression of both BLOC-1 related complex subunit 7 (BORCS7) and a previously uncharacterized, human-specific arsenite methyltransferase (AS3MT) isoform (AS3MTd2d3), which lacks arsenite methyltransferase activity and is more abundant in individuals with schizophrenia than in controls. Conditional-expression analysis suggests that BORCS7 and AS3MTd2d3 signals are largely independent. GWAS risk SNPs across this region are linked with a variable number tandem repeat (VNTR) polymorphism in the first exon of AS3MT that is associated with the expression of AS3MTd2d3 in samples from both Caucasians and African Americans. The VNTR genotype predicts promoter activity in luciferase assays, as well as DNA methylation within the AS3MT gene. Both AS3MTd2d3 and BORCS7 are expressed in adult human neurons and astrocytes, and they are upregulated during human stem cell differentiation toward neuronal fates. Our results provide a molecular explanation for the prominent 10q24.32 locus association, including a novel and evolutionarily recent protein that is involved in early brain development and confers risk for psychiatric illness.

Dynamic regulation of RNA editing in human brain development and disease.

RNA editing is increasingly recognized as a molecular mechanism regulating RNA activity and recoding proteins. Here we surveyed the global landscape of RNA editing in human brain tissues and identified three unique patterns of A-to-I RNA editing rates during cortical development: stable high, stable low and increasing. RNA secondary structure and the temporal expression of adenosine deaminase acting on RNA (ADAR) contribute to cis- and trans-regulatory mechanisms of these RNA editing patterns, respectively. Interestingly, the increasing pattern was associated with neuronal maturation, correlated with mRNA abundance and potentially influenced miRNA binding energy. Gene ontology analyses implicated the increasing pattern in vesicle or organelle membrane-related genes and glutamate signaling pathways. We also found that the increasing pattern was selectively perturbed in spinal cord injury and glioblastoma. Our findings reveal global and dynamic aspects of RNA editing in brain, providing new insight into epitranscriptional regulation of sequence diversity.

BrainSeq: Neurogenomics to Drive Novel Target Discovery for Neuropsychiatric Disorders

We outline an ambitious project to characterize the genetic and epigenetic regulation of multiple facets of transcription in distinct brain regions across the human lifespan in samples of major neuropsychiatric disorders and controls. Initially focused on schizophrenia and mood disorders, the goal of this consortium is to elucidate the underlying molecular mechanisms of genetic associations with the goal of identifying novel therapeutic targets. The consortium currently consists of seven pharmaceutical companies and a not-for-profit medical research institution working as a precompetitive team to generate and analyze publicly available archival brain genomic data related to neuropsychiatric illness.

Differential Effects of Common Variants in SCN2A on General Cognitive Ability, Brain Physiology, and messenger RNA Expression in Schizophrenia Cases and Control Individuals

IMPORTANCE One approach to understanding the genetic complexity of schizophrenia is to study associated behavioral and biological phenotypes that may be more directly linked to genetic variation. OBJECTIVE To identify single-nucleotide polymorphisms associated with general cognitive ability (g) in people with schizophrenia and control individuals. DESIGN, SETTING, AND PARTICIPANTS Genomewide association study, followed by analyses in unaffected siblings and independent schizophrenia samples, functional magnetic resonance imaging studies of brain physiology in vivo, and RNA sequencing in postmortem brain samples. The discovery cohort and unaffected siblings were participants in the National Institute of Mental Health Clinical Brain Disorders Branch schizophrenia genetics studies. Additional schizophrenia cohorts were from psychiatric treatment settings in the United States, Japan, and Germany. The discovery cohort comprised 339 with schizophrenia and 363 community control participants. Follow-up analyses studied 147 unaffected siblings of the schizophrenia cases and independent schizophrenia samples including a total of an additional 668 participants. Imaging analyses included 87 schizophrenia cases and 397 control individuals. Brain tissue samples were available for 64 cases and 61 control individuals. MAIN OUTCOMES AND MEASURES We studied genomewide association with g, by group, in the discovery cohort. We used selected genotypes to test specific associations in unaffected siblings and independent schizophrenia samples. Imaging analyses focused on activation in the prefrontal cortex during working memory. Brain tissue studies yielded messenger RNA expression levels for RefSeq transcripts. RESULTS The schizophrenia discovery cohort showed genomewide-significant association of g with polymorphisms in sodium channel gene SCN2A, accounting for 10.4% of g variance (rs10174400, P = 9.27 × 10(-10)). Control individuals showed a trend for g/genotype association with reversed allelic directionality. The genotype-by-group interaction was also genomewide significant (P = 1.75 × 10(-9)). Siblings showed a genotype association with g parallel to the schizophrenia group and the same interaction pattern. Parallel, but weaker, associations with cognition were found in independent schizophrenia samples. Imaging analyses showed a similar pattern of genotype associations by group and genotype-by-group interaction. Sequencing of RNA in brain revealed reduced expression in 2 of 3 SCN2A alternative transcripts in the patient group, with genotype-by-group interaction, that again paralleled the cognition effects. CONCLUSIONS AND RELEVANCE The findings implicate SCN2A and sodium channel biology in cognitive impairment in schizophrenia cases and unaffected relatives and may facilitate development of cognition-enhancing treatments.

Genome-wide antagonism between 5-hydroxymethylcytosine and DNA methylation in the adult mouse brain

Mounting evidence points to critical roles for DNA modifications, including 5-methylcytosine (5mC) and its oxidized forms, in the development, plasticity and disorders of the mammalian nervous system. The novel DNA base 5- hydroxymethylcytosine (5hmC) is known to be capable of initiating passive or active DNA demethylation, but whether and how extensively 5hmC functions in shaping the post-mitotic neuronal DNA methylome is unclear. Here we report the genome-wide distribution of 5hmC in dentate granule neurons from adult mouse hippocampus in vivo. 5hmC in the neuronal genome is highly enriched in gene bodies, especially in exons, and correlates with gene expression. Direct genome-wide comparison of 5hmC distribution between embryonic stem cells and neurons reveals extensive differences, reflecting the functional disparity between these two cell types. Importantly, integrative analysis of 5hmC, overall DNA methylation and gene expression profiles of dentate granule neurons in vivo reveals the genome-wide antagonism between these two states of cytosine modifications, supporting a role for 5hmC in shaping the neuronal DNA methylome by promoting active DNA demethylation.

qSVA framework for RNA quality correction in differential expression analysis

Significance Many studies use measurements of gene expression in human postmortem and ex vivo tissues like brain and blood to characterize genomic correlates of illness. However, molecular analyses of these tissues can be susceptible to a wide range of confounders that may be difficult to measure and remove. In this article, we describe an analysis framework for identifying and removing previously uncharacterized quality biases in measurements of RNA. Our paper critically highlights the shortcomings of standard RNA quality correction approaches, such as statistically adjusting for RNA integrity numbers. We show that the our framework removes residual confounding by RNA quality and greatly improves replication of significant differentially expressed genes across independent datasets by more than threefold compared with previous approaches. RNA sequencing (RNA-seq) is a powerful approach for measuring gene expression levels in cells and tissues, but it relies on high-quality RNA. We demonstrate here that statistical adjustment using existing quality measures largely fails to remove the effects of RNA degradation when RNA quality associates with the outcome of interest. Using RNA-seq data from molecular degradation experiments of human primary tissues, we introduce a method—quality surrogate variable analysis (qSVA)—as a framework for estimating and removing the confounding effect of RNA quality in differential expression analysis. We show that this approach results in greatly improved replication rates (>3×) across two large independent postmortem human brain studies of schizophrenia and also removes potential RNA quality biases in earlier published work that compared expression levels of different brain regions and other diagnostic groups. Our approach can therefore improve the interpretation of differential expression analysis of transcriptomic data from human tissue.

GAD2 alternative transcripts in the human prefrontal cortex, and in schizophrenia and affective disorders

Genetic variation and early adverse environmental events work together to increase risk for schizophrenia. γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in adult mammalian brain, plays a major role in normal brain development, and has been strongly implicated in the pathobiology of schizophrenia. GABA synthesis is controlled by two glutamic acid decarboxylase (GAD) genes, GAD1 and GAD2, both of which produce a number of alternative transcripts. Genetic variants in the GAD1 gene are associated with increased risk for schizophrenia, and reduced expression of its major transcript in the human dorsolateral prefrontal cortex (DLPFC). No consistent changes in GAD2 expression have been found in brains from patients with schizophrenia. In this work, with the use of RNA sequencing and PCR technologies, we confirmed and tracked the expression of an alternative truncated transcript of GAD2 (ENST00000428517) in human control DLPFC homogenates across lifespan besides the well-known full length transcript of GAD2. In addition, using quantitative RT-PCR, expression of GAD2 full length and truncated transcripts were measured in the DLPFC of patients with schizophrenia, bipolar disorder and major depression. The expression of GAD2 full length transcript is decreased in the DLPFC of schizophrenia and bipolar disorder patients, while GAD2 truncated transcript is increased in bipolar disorder patients but decreased in schizophrenia patients. Moreover, the patients with schizophrenia with completed suicide or positive nicotine exposure showed significantly higher expression of GAD2 full length transcript. Alternative transcripts of GAD2 may be important in the growth and development of GABA-synthesizing neurons as well as abnormal GABA signaling in the DLPFC of patients with schizophrenia and affective disorders.

RNA-Seq-based toxicogenomic assessment of fresh frozen and formalin-fixed tissues yields similar mechanistic insights

Formalin‐fixed, paraffin‐embedded (FFPE) pathology specimens represent a potentially vast resource for transcriptomic‐based biomarker discovery. We present here a comparison of results from a whole transcriptome RNA‐Seq analysis of RNA extracted from fresh frozen and FFPE livers. The samples were derived from rats exposed to aflatoxin B1 (AFB1) and a corresponding set of control animals. Principal components analysis indicated that samples were separated in the two groups representing presence or absence of chemical exposure, both in fresh frozen and FFPE sample types. Sixty‐five percent of the differentially expressed transcripts (AFB1 vs. controls) in fresh frozen samples were also differentially expressed in FFPE samples (overlap significance: P < 0.0001). Genomic signature and gene set analysis of AFB1 differentially expressed transcript lists indicated highly similar results between fresh frozen and FFPE at the level of chemogenomic signatures (i.e., single chemical/dose/duration elicited transcriptomic signatures), mechanistic and pathology signatures, biological processes, canonical pathways and transcription factor networks. Overall, our results suggest that similar hypotheses about the biological mechanism of toxicity would be formulated from fresh frozen and FFPE samples. These results indicate that phenotypically anchored archival specimens represent a potentially informative resource for signature‐based biomarker discovery and mechanistic characterization of toxicity. Copyright