Phenotypes from cell-free DNA
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Cell-free DNA (cfDNA) has the potential to enable non-invasive detection of disease states and progression. Beyond its sequence, cfDNA also represents the nucleosomal landscape of cell(s)-of-origin and captures the dynamics of the epigenome. In this review, we highlight the emergence of cfDNA epigenomic methods that assess disease beyond the scope of mutant tumour genotyping. Detection of tumour mutations is the gold standard for sequencing methods in clinical oncology. However, limitations inherent to mutation targeting in cfDNA, and the possibilities of uncovering molecular mechanisms underlying disease, have made epigenomics of cfDNA an exciting alternative. We discuss the epigenomic information revealed by cfDNA, and how epigenomic methods exploit cfDNA to detect and characterize cancer. Future applications of cfDNA epigenomic methods to act complementarily and orthogonally to current clinical practices has the potential to transform cancer management and improve cancer patient outcomes.Keywords:
Epigenomics
Epigenome
Cell-free fetal DNA
Abstract: Epigenomes are comprised, in part, of all genome-wide chromatin modifications, including DNA methylation and histone modifications. Unlike the genome, epigenomes are dynamic during development and differentiation to establish and maintain cell type–specific gene expression states that underlie cellular identity and function. Chromatin modifications are particularly labile, providing a mechanism for organisms to respond and adapt to environmental cues. Results from studies in animal models clearly demonstrate that epigenomic variability leads to phenotypic variability, including susceptibility to disease that is not recognized at the DNA sequence level. Thus, capturing epigenomic information is invaluable for comprehensively understanding development, differentiation, and disease. Herein, we provide a brief overview of epigenetic processes, how they are relevant to human health, and review studies using technologies that enable epigenome mapping. We conclude by describing feasible applications of epigenome mapping, focusing on epigenome-wide association studies (eGWAS), which have the potential to revolutionize current studies of human diseases and will likely promote the discovery of novel diagnostic, preventative, and treatment strategies.
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Epigenetics is a gene regulation mechanism that does not depend on genomic DNA sequences but depends on chemical modification of genomic DNA and histone proteins around which DNA is wrapped. The failure of epigenetic mechanisms is known to cause various congenital disorders. It is also known that the failures of epigenetic mechanisms causes various acquired disorders since epigenetic modifications of the genome (i.e., "epigenome") are more vulnerable to environmental stress, such as malnutrition, environmental chemicals, and mental stress, than the "genome," especially during the early period of life. However, the epigenome has a reversible property since it is based on removable residues on genomic DNA. Thus, environmentally induced epigenomic alterations can be potentially restored. In fact, some medicines, especially for psychiatric diseases, are known to restore an altered epigenome, resulting in the correction of gene expression. Several lines of evidence suggest that environmentally induced epigenomic alterations are not erased completely during gametogenesis, but are transmitted to subsequent generations with disease phenotypes. In accordance with these understandings, I would like to propose the development of epigenomic-based preemptive medicine that consists of the early detection of the developmental origins of diseases using epigenomic signatures and the early intervention that take advantages of the use of epigenomic reversibility.
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It is tempting to assume that a gradual accumulation of damage 'causes' an organism to age, but other biological processes present during the lifespan, whether 'programmed' or 'hijacked', could control the type and speed of aging. Theories of aging have classically focused on changes at the genomic level; however, individuals with similar genetic backgrounds can age very differently. Epigenetic modifications include DNA methylation, histone modifications and ncRNA. Environmental cues may be 'remembered' during lifespan through changes to the epigenome that affect the rate of aging. Changes to the epigenomic landscape are now known to associate with aging, but so far causal links to longevity are only beginning to be revealed. Nevertheless, it is becoming apparent that there is significant reciprocal regulation occurring between the epigenomic levels. Future work utilizing new technologies and techniques should build a clearer picture of the link between epigenomic changes and aging.
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Abstract The role of the epigenome in phenotypic plasticity is unclear presently. Here we used a multiomics approach (RNA-seq, ChIP-seq, ATAC-seq and Hi-C) to explore the nature of the epigenome in developing honeybee (Apis mellifera) workers and queens. Our data showed that the distinct queen and worker epigenomic landscapes form during the developmental process. Differences in gene expression between workers and queens precede other epigenomic modifications, but the epigenomic differences between workers and queens become more extensive and more layered during development. Genes known to be important for caste differentiation were more likely to be multiply differentially regulated by more than one epigenomic system than other differentially expressed genes. This indicates a multidimensional regulation of expression of key genes, presumably to canalise differences in gene expression. Our data indicate that the epigenome interacts with diverging developmental trajectories rather than controlling them since different epigenomic landscapes form in concert with different developmental outcomes.
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The epigenome undergoes significant remodeling during tissue and organ development, which coincides with a period of exquisite sensitivity to environmental exposures. In the case of endocrine-disrupting compounds (EDCs), exposures can reprogram the epigenome of developing tissues to increase susceptibility to diseases later in life, a process termed “developmental reprogramming.” Both DNA methylation and histone modifications have been shown to be vulnerable to disruption by EDC exposures, and several mechanisms have been identified by which EDCs can reprogram the epigenome. These include altered methyl donor availability, loss of imprinting control, changes in dioxygenase activity, altered expression of noncoding RNAs, and activation of cell signaling pathways that can phosphorylate, and alter the activity of, histone methyltransferases. This altered epigenomic programming can persist across the life course, and in some instances generations, to alter gene expression in ways that correlate with increased disease susceptibility. Together, these studies on developmental reprogramming of the epigenome by EDCs are providing new insights into epigenomic plasticity that is vulnerable to disruption by environmental exposures.
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Epigenetic modifications, DNA methylation and histone modifications, mark genes to be used and not to be used, and are stably inherited in somatic cells. Epigenome is their genome-wide compilation, and undergoes dynamic changes in development, differentiation, and reprogramming. Epigenomic changes are causally involved in cancer development and progression by inducing silencing of tumor-suppressor genes and genomic instability. Aberrant DNA methylation can accumulate in a large fraction of cells even in tissues without clonal lesions, which indicates that epigenomic changes can potentially affect functions of a tissue. Multiple reports show that epigenomic changes are present in acquired neurological, metabolic, and immunological disorders, and more research in the field is urgently necessary.
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An epigenome is a chemical modification pattern of genomic DNA that determines the on/off status of genes, and its abnormalities are known to cause congenital diseases. Recent studies have shown that epigenomic patterns are altered by various environmental factors, indicating that epigenomic abnormalities also cause acquired mental and neurological diseases. An epigenomic pattern that reflects past environmental insults is an epigenomic signature. Therefore, it will be used as a marker for preemptive medicine in that it is not based on the population but is instead based on individuals.
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It is undeniably one of the greatest findings in biology that (with some very minor exceptions) every cell in the body possesses the whole genetic information needed to generate a complete individual. Today, this concept has been so thoroughly assimilated that we struggle to still see how surprising this finding actually was: all cellular phenotypes naturally occurring in one person are generated from genetic uniformity, and thus are per definition epigenetic. Transcriptional mechanisms are clearly critical for developing and protecting cell identities, because a mis-expression of few or even single genes can efficiently induce inappropriate cellular programmes. However, how transcriptional activities are molecularly controlled and which of the many known epigenomic features have causal roles remains unclear. Today, clarification of this issue is more pressing than ever because profiling efforts and epigenome-wide association studies (EWAS) continuously provide comprehensive datasets depicting epigenomic differences between tissues and disease states. In this commentary, we propagate the idea of a widespread follow-up use of epigenome editing technology in EWAS studies. This would enable them to address the questions of which features, where in the genome, and which circumstances are essential to shape development and trigger disease states.
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