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Chromatin epimutations in transgenerational epigenetic inheritance

ABSTRACT: Epigenetic inheritance is the transmission of altered gene expression states across dividing cells without DNA sequence variation. Induced epigenetic changes (epimutations) have been extensively studied, but importantly, epimutations also occur spontaneously in a manner reminiscent of DNA sequence change. Small RNAs are established as a source of spontaneous epimutations which transmit gene expression changes over multiple generations in C.elegans. These changes are generally short lived, but rarely long-lived changes occur. Given that chromatin state is a critical regulator of gene expression, we explored whether chromatin states also undergo spontaneous change and whether this could be a potential alternative mechanism for transgenerational epigenetic inheritance. We compared the chromatin profiles and gene expression profiles at matched time points from a twenty generation bottlenecked lineage of C.elegans. We found that heritable changes in gene expression arose at a higher rate than the estimate for DNA sequence variation, indicating that these were true epigenetic events. In addition to recapitulating the known role for small RNAs, we provide evidence for chromatin mediated epimutations as an alternative mechanism driving epigenetic inheritance. Chromatin epimutations were heritable, enriched in the germline and synchronised with inherited gene expression changes. As with small RNA mediated epimutations, they were generally short lived with a tendency to resolve to baseline after 2-3 generations. A subset of chromatin epimutations however were long lasting, and these were uniquely enriched in piRNA clusters. Genes with long lived epimutations were implicated in multiple components of xenobiotic response pathways. This points to a mechanism for epigenetic ‘anticipation’ of environmental stressors. Adaptive traits driven by epimutations may be selected for under adverse conditions and lost on return to normal conditions, enabling soft adaptation in the absence of genetic sequence change.

ORGANISM(S): Caenorhabditis elegans

PROVIDER: GSE211846 | GEO | 2023/01/08

REPOSITORIES: GEO

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Project description:The prevailing theory for the molecular basis of evolution involves genetic mutations that ultimately generate the heritable phenotypic variation on which natural selection acts. However, epigenetic transgenerational inheritance of phenotypic variation may also play an important role in evolutionary change. A growing number of studies have demonstrated the presence of epigenetic inheritance in a variety of different organisms that can persist for hundreds of generations. The possibility that epigenetic changes could accumulate over macroevolutionary time has been considered, but not yet seldom been tested empirically. The current study was designed to compare epigenetic changes among several closely related species of Darwin’s finches, a well-known example of adaptive radiation. Erythrocyte DNA was obtained from five species of sympatric Darwin's finches that vary in phylogenetic relatedness. Genome wide alterations in genetic mutations using copy number variation (CNV) were compared to epigenetic alterations associated with differential DNA methylation regions (epimutations). Epimutations were more common than genetic CNV mutations among the five species; furthermore, the number of epimutations increased monotonically with phylogenetic distance. Interestingly, the number of genetic CNV mutations did not consistently increase with phylogenetic distance. The number, chromosomal locations, regional clustering, and lack of overlap of epimutations and genetic mutations suggests that epigenetic changes are distinct and that they correlate with the evolutionary history of Darwin’s finches. The potential functional significance of the epimutations was explored by comparing their locations on the genome to the location of evolutionarily important genes and cellular pathways in birds. Specific epimutations were associated with genes related to the bone morphogenic protein (BMP), toll receptor, and melanogenesis signaling pathways. Species- specific epimutations were significantly over-represented in these pathways. Since environmental factors are known to rapidly alter heritable changes in the epigenome, it is possible that epigenetic changes have played a contributing role in the molecular basis of the evolution of Darwin's finches.

Project description:Phenotypic differences caused by epimutations rather than changes in DNA sequence have been described in various organisms. Epimutations are potentially adaptive if inherited across generations and might even respond to environmental challenges. Yet experimental evidence is scarce1. Examples of heritable phenotypic changes are paramutation in plants and RNA-induced epigenetic silencing (RNAe) in nematodes, in which small RNAs trigger the formation of silent epialleles that are stable across generations2-7. Consequently, RNA-directed epimutagenesis has been associated with persistent gene repression. Here, using the fission yeast Schizosaccharomyces pombe, we demonstrate that RNA-induced epimutations are stably transmitted to subsequent generations also after reactivation of the silenced gene. This enables descendants to remember and reinstate preceding silencing episodes that had occurred in their ancestors. We show that temporary expression of hairpin-derived small interfering RNAs (siRNAs) triggers RNAe that is stably propagated for at least 18 generations upon inactivation of the polymerase-associated factor 1 complex (Paf1C). In the presence of Paf1C, the silent state is lost, but resumes if Paf1C is again impaired, even in the absence of the original trigger and in later generations. Thus, in yeast RNAe installs a molecular imprint that can be inherited through meiosis with no discernible phenotypic consequences. We show that trimethylated histone H3 lysine 9 (H3K9me3) is a major constituent of this imprint, and that stable propagation thereof is coupled to RNA interference (RNAi). Our findings reveal a distinct form of epigenetic memory in which fission yeast cells acquire heritable, transcriptionally active epialleles that confer gene silencing and manifest a phenotype only in particular, recurring situations. This demonstrates that epigenetic inheritance is adaptive and could potentially constitute a strategy to cope with adverse environmental conditions. If conserved in other eukaryotes, this phenomenon will impact our comprehension of natural variation and epigenetics in evolution.

Project description:Environmental compounds including fungicides, plastics, pesticides, dioxin and hydrocarbons can promote the epigenetic transgenerational inheritance of adult-onset disease in future generation progeny following ancestral exposure during the critical period of fetal gonadal sex determination. This study examined the actions of the pesticide methoxychlor to promote the epigenetic transgenerational inheritance of adult-onset disease and associated differential DNA methylation regions (i.e. epimutations) in sperm. Gestating F0 generation female rats were transiently exposed to methoxychlor during fetal gonadal development (gestation days 8 to 14) and then adult-onset disease was evaluated in adult F1 and F3 (great-grand offspring) generation progeny for control (vehicle exposed) and methoxychlor lineage offspring. There were increases in the incidence of kidney disease, ovary disease, and obesity in the methoxychlor lineage animals. In females and males the incidence of disease increased in both the F1 and the F3 generations and the incidence of multiple disease increased in the F3 generation. There was increased disease incidence in F4 generation reverse outcross (female) offspring indicating disease transmission was primarily transmitted through the female germline. Analysis of the F3 generation sperm epigenome of the methoxychlor lineage males identified differentially DNA methylated regions (DMR) termed epimutations in a genome-wide gene promoters analysis. These epimutations were found to be methoxychlor exposure specific in comparison with other exposure specific sperm epimutation signatures. Observations indicate that the pesticide methoxychlor has the potential to promote the epigenetic transgenerational inheritance of disease and the sperm epimutations appear to provide exposure specific epigenetic biomarkers for transgenerational disease and ancestral environmental exposures. Methylated sperm DNA was isolated from rats ancestrally exposed to methoxychlor. Three independent samples from each treatment group were obtained. Differential DNA methylation between treatment groups was determined using Nimblegen microarrays. Treated samples were paired with control samples and hybridized together on arrays, resulting in three arrays for the treatment.

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Chromatin epimutations in transgenerational epigenetic inheritance

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