Allele-Skewed DNA Modification in the Brain: Relevance to a Schizophrenia GWAS.

Numerous recent studies have suggested that phenotypic effects of DNA sequence variants can be mediated or modulated by their epigenetic marks, such as allele-skewed DNA modification (ASM). Using Affymetrix SNP microarrays, we performed a comprehensive search of ASM effects in human post-mortem brain and sperm samples (total n = 256) from individuals with major psychosis and control individuals. Depending on the phenotypic category of the brain samples, 1.4%-7.5% of interrogated SNPs exhibited ASM effects. Next, we investigated ASM in the context of genetic studies of schizophrenia and detected that brain ASM SNPs were significantly overrepresented among sub-threshold SNPs from a schizophrenia genome-wide association study (GWAS). Brain ASM SNPs showed a much stronger enrichment in a schizophrenia GWAS than in 17 large GWASs of non-psychiatric diseases and traits, arguing that ASM effects are at least partially tissue specific. Studies of germline and control brain ASM SNPs supported a causal association between ASM and schizophrenia. Finally, significantly higher proportions of ASM SNPs than of non-ASM SNPs were detected at loci exhibiting epigenetic signatures of enhancers and promoters, and they were overrepresented within transcription factor binding regions and DNase I hypersensitive sites. All of these findings collectively indicate that ASM SNPs should be prioritized in follow-up GWASs.

Epigenetic approaches to psychiatric disorders.

Psychiatric diseases place a tremendous burden on affected individuals, their caregivers, and the health care system. Although evidence exists for a strong inherited component to many of these conditions, dedicated efforts to identify DNA sequence-based causes have not been exceptionally productive, and very few pharmacologic treatment options are clinically available. Many features of psychiatric diseases are consistent with an epigenetic dysregulation, such as discordance of monozygotic twins, late age of onset, parent-of-origin and sex effects, and fluctuating disease course. In recent years, experimental technologies have significantly advanced, permitting in-depth studies of the epigenome and its role in maintenance of normal genomic functions, as well as disease etiopathogenesis. Here, we present an epigenetic explanation for many characteristics of psychiatric disease, review the current literature on the epigenetic mechanisms involved in major psychosis, Alzheimer's disease, and autism spectrum disorders, and describe some future directions in the field of psychiatric epigenomics.

Lactase nonpersistence is directed by DNA-variation-dependent epigenetic aging.

The inability to digest lactose, due to lactase nonpersistence, is a common trait in adult mammals, except in certain human populations that exhibit lactase persistence. It is not known how the lactase gene is dramatically downregulated with age in most individuals but remains active in some individuals. We performed a comprehensive epigenetic study of human and mouse small intestines, by using chromosome-wide DNA-modification profiling and targeted bisulfite sequencing. Epigenetically controlled regulatory elements accounted for the differences in lactase mRNA levels among individuals, intestinal cell types and species. We confirmed the importance of these regulatory elements in modulating lactase mRNA levels by using CRISPR-Cas9-induced deletions. Genetic factors contribute to epigenetic changes occurring with age at the regulatory elements, because lactase-persistence and lactase-nonpersistence DNA haplotypes demonstrated markedly different epigenetic aging. Thus, genetic factors enable a gradual accumulation of epigenetic changes with age, thereby influencing phenotypic outcome.

DNA unmethylome profiling by covalent capture of CpG sites.

Dynamic patterns of cytosine-5 methylation and successive hydroxylation are part of epigenetic regulation in eukaryotes, including humans, which contributes to normal phenotypic variation and disease risk. Here we present an approach for the mapping of unmodified regions of the genome, which we call the unmethylome. Our technique is based on DNA methyltransferase-directed transfer of activated groups and covalent biotin tagging of unmodified CpG sites followed by affinity enrichment and interrogation on tiling microarrays or next generation sequencing. Control experiments and pilot studies of human genomic DNA from cultured cells and tissues demonstrate that, along with providing a unique cross-section through the chemical landscape of the epigenome, the methyltransferase-directed transfer of activated groups-based approach offers high precision and robustness as compared with existing affinity-based techniques.

5-hmC in the brain is abundant in synaptic genes and shows differences at the exon-intron boundary.

The 5-methylcytosine (5-mC) derivative 5-hydroxymethylcytosine (5-hmC) is abundant in the brain for unknown reasons. Here we characterize the genomic distribution of 5-hmC and 5-mC in human and mouse tissues. We assayed 5-hmC by using glucosylation coupled with restriction-enzyme digestion and microarray analysis. We detected 5-hmC enrichment in genes with synapse-related functions in both human and mouse brain. We also identified substantial tissue-specific differential distributions of these DNA modifications at the exon-intron boundary in human and mouse. This boundary change was mainly due to 5-hmC in the brain but due to 5-mC in non-neural contexts. This pattern was replicated in multiple independent data sets and with single-molecule sequencing. Moreover, in human frontal cortex, constitutive exons contained higher levels of 5-hmC relative to alternatively spliced exons. Our study suggests a new role for 5-hmC in RNA splicing and synaptic function in the brain.

DNA methylation profiles in monozygotic and dizygotic twins.

Twin studies have provided the basis for genetic and epidemiological studies in human complex traits. As epigenetic factors can contribute to phenotypic outcomes, we conducted a DNA methylation analysis in white blood cells (WBC), buccal epithelial cells and gut biopsies of 114 monozygotic (MZ) twins as well as WBC and buccal epithelial cells of 80 dizygotic (DZ) twins using 12K CpG island microarrays. Here we provide the first annotation of epigenetic metastability of approximately 6,000 unique genomic regions in MZ twins. An intraclass correlation (ICC)-based comparison of matched MZ and DZ twins showed significantly higher epigenetic difference in buccal cells of DZ co-twins (P = 1.2 x 10(-294)). Although such higher epigenetic discordance in DZ twins can result from DNA sequence differences, our in silico SNP analyses and animal studies favor the hypothesis that it is due to epigenomic differences in the zygotes, suggesting that molecular mechanisms of heritability may not be limited to DNA sequence differences.

Epigenetics and complex disease: from etiology to new therapeutics.

Epigenetics is a new development in complex non-Mendelian disease, which may not only uncover etiologic and pathogenic mechanisms but may also provide the basis for the development of medications that would target the primary epigenetic causes of such diseases. Such epigenetic drugs would be novel, potentially possessing substantially higher therapeutic potential and a much lower rate of adverse effects in comparison to current symptomatic treatments. A collection of epigenetic drugs already exist at various stages of development and, although their effectiveness has yet to be maximized, they show great promise in the treatment of cancer, psychiatric disorders, and other complex diseases. Here we present a review of the epigenetic theory of complex disease and an evaluation of current epigenetic therapies, as well as predictions of the future directions in this expanding field.