Transgenerational epigenetic inheritance in plants

Interest in transgenerational epigenetic inheritance has intensified with the boosting of knowledge on epigenetic mechanisms regulating gene expression during development and in response to internal and external signals such as biotic and abiotic stresses. Starting with an historical background of scantily documented anecdotes and their consequences, we recapitulate the information gathered during the last 60 years on naturally occurring and induced epialleles and paramutations in plants. We present the major players of epigenetic regulation and their importance in controlling stress responses. The effect of diverse stressors on the epigenetic status and its transgenerational inheritance is summarized from a mechanistic viewpoint. The consequences of transgenerational epigenetic inheritance are presented, focusing on the knowledge about its stability, and in relation to genetically fixed mutations, recombination, and genomic rearrangement. We conclude with an outlook on the importance of transgenerational inheritance for adaptation to changing environments and for practical applications. This article is part of a Special Issue entitled "Epigenetic control of cellular and developmental processes in plants".     

Transgenerational epigenetic inheritance in health and disease

Over the past century, patterns of phenotypic inheritance have been observed that are not easily rationalised by Mendel's rules of inheritance. Now that we have begun to understand more about non-DNA based, or 'epigenetic', control of phenotype at the molecular level, the idea that the transgenerational inheritance of these epigenetic states could explain non-Mendelian patterns of inheritance has become attractive. There is a growing body of evidence that abnormal epigenetic states, termed epimutations, are associated with disease in humans. For example, in several cases of colorectal cancer, epimutations have been identified that silence the human mismatch repair genes, MLH1 and MSH2. But strong evidence that the abnormal epigenetic states are primary events that occur in the absence of genetic change and are inherited across generations is still absent.     

Transgenerational inheritance of longevity: epigenetic mysteries abound

Transgenerational inheritance of epigenetic characteristics in plants has been reported, whereas nongenetic persistence of complex phenotypes in animals is controversial. A recent report by Anne Brunet and colleagues describes a fascinating example of persistence across generations of extended life span in worm and explores whether epigenetic mechanisms account for the longevity.     

Transgenerational inheritance of acquired epigenetic signatures at CpG islands in mice

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Sex-specificity in transgenerational epigenetic programming

Prenatal programming of the epigenome is a critical determinant in offspring outcome and stands at the interface between environment and genetics. Maternal experiences such as stress and obesity are associated with a host of neurodevelopmental and metabolic diseases, some of which have been characterized into the second and third generations. The mechanism through which determinants such as maternal diet or stress contribute to disease development likely involves a complex interaction between the maternal environment, placental changes, and epigenetic programming of the embryo. While we have begun to more fully appreciate and explore the epigenome in determination of disease risk, we know little as to the contribution embryo sex makes in epigenetic regulation. This review discusses the importance of sex differences in the transmission and inheritance of traits that are generated in the prenatal environment using models of maternal stress and diet.     

Fatty-acid-mediated hypothalamic inflammation and epigenetic programming

A high-fat diet is the main environmental cue that has been studied in the hypothalamus since the discovery of its connection with hypothalamic inflammation. Current evidence shows hypothalamic inflammation as a likely mechanism for the dysregulation on the homeostatic control of energy balance, which leads to metabolic alterations and obesity. Although this mechanism seems to be reversible when set during adulthood, we argue whether dietary fatty acids, during critical periods of development, could affect hypothalamic function permanently and set an increased susceptibility to obesity. We found few experimental studies that looked at programming induced by different fatty acids on the hypothalamus. They clearly showed a connection between maternal fat diet, hypothalamic inflammation and metabolic alterations in the offspring. We found that not only a high-fat diet but also a normolipidic diet with unbalanced quantities of different fatty acids produced diverse inflammatory responses on the hypothalamus. Therefore, strategies of manipulating dietary fatty acids in pregnant and lactating women may have great impact on the population's future health. However, more research is still needed on the effects of fatty acids and the hypothalamic inflammation on programming.     

Transgenerational inheritance of chronic adolescent stress: Effects of stress response and the amygdala transcriptome

Adolescent stress can impact health and well‐being not only during adulthood of the exposed individual but even in future generations. To investigate the molecular mechanisms underlying these long‐term effects, we exposed adolescent males to stress and measured anxiety behaviors and gene expression in the amygdala—a critical region in the control of emotional states—in their progeny for two generations, offspring and grandoffspring. Male C57BL/6 mice underwent chronic unpredictable stress (CUS) for 2 weeks during adolescence and were used to produce two generations of offspring. Male and female offspring and grandoffspring were tested in behavioral assays to measure affective behavior and stress reactivity. Remarkably, transgenerational inheritance of paternal stress exposure produced a protective phenotype in the male, but not the female lineage. RNA‐seq analysis of the amygdala from male offspring and grandoffspring identified differentially expressed genes (DEGs) in mice derived from fathers exposed to CUS. The DEGSs clustered into numerous pathways, and the “notch signaling” pathway was the most significantly altered in male grandoffspring. Therefore, we show that paternal stress exposure impacts future generations which manifest in behavioral changes and molecular adaptations.     

Induction of dedifferentiation, genomewide transcriptional programming, and epigenetic reprogramming by extracts of carcinoma and embryonic stem cells

Functional reprogramming of a differentiated cell toward pluripotency may have long-term applications in regenerative medicine. We report the induction of dedifferentiation, associated with genomewide programming of gene expression and epigenetic reprogramming of an embryonic gene, in epithelial 293T cells treated with an extract of undifferentiated human NCCIT carcinoma cells. 293T cells exposed for 1 h to extract of NCCIT cells, but not of 293T or Jurkat T-cells, form defined colonies that are maintained for at least 23 passages in culture. Microarray and quantitative analyses of gene expression reveal that the transition from a 293T to a pluripotent cell phenotype involves a dynamic up-regulation of hundreds of NCCIT genes, concomitant with down-regulation of 293T genes and of indicators of differentiation such as A-type lamins. Up-regulated genes encompass embryonic and stem cell markers, including OCT4, SOX2, NANOG, and Oct4-responsive genes. OCT4 activation is associated with DNA demethylation in the OCT4 promoter and nuclear targeting of Oct4 protein. In fibroblasts exposed to extract of mouse embryonic stem cells, Oct4 activation is biphasic and RNA-PolII dependent, with the first transient rise of Oct4 up-regulation being necessary for the second, long-term activation of Oct4. Genes characteristic of multilineage differentiation potential are also up-regulated in NCCIT extract-treated cells, suggesting the establishment of "multilineage priming." Retinoic acid triggers Oct4 down-regulation, de novo activation of A-type lamins, and nestin. Furthermore, the cells can be induced to differentiate toward neurogenic, adipogenic, osteogenic, and endothelial lineages. The data provide a proof-of-concept that an extract of undifferentiated carcinoma cells can elicit differentiation plasticity in an otherwise more developmentally restricted cell type.     

Maternal influences on epigenetic programming of the developing hypothalamic-pituitary-adrenal axis

Parental and environmental factors during the prenatal and postnatal periods permanently affect the physiology and metabolism of offspring, potentially increasing disease risk later in life. Underlying mechanisms are being elucidated, and effects on a number of organs and metabolic pathways are likely involved. In this review, we consider effects on the developing hypothalamic-pituitary-adrenal (HPA) axis, which may represent a common pathway for developmental programming. The focus is on prenatal and early postnatal development, during which the HPA axis may be programmed in a manner that affects health for a lifetime. Programming of the HPA axis involves, at least in part, epigenetic remodeling of chromatin, leading to alterations in the expression of genes in many organs and tissues involved in HPA activation and response, including the hippocampus and peripheral tissues. Examples of developmental epigenetic modifications affecting the HPA axis as well as target tissues are provided.