Transgenerational epigenetic inheritance
Transgenerational epigenetic inheritance is the transmission of epigenetic markers from one organism to the next (i.e., from parent to child) that affects the traits of offspring without altering the primary structure of DNA (i.e. the sequence of nucleotides):168—in other words, epigenetically. The less precise term "epigenetic inheritance" may cover both cell–cell and organism–organism information transfer. Although these two levels of epigenetic inheritance are equivalent in unicellular organisms, they may have distinct mechanisms and evolutionary distinctions in multicellular organisms.
Environmental factors can induce the epigenetic marks (epigenetic tags) for some epigenetically influenced traits, while some marks are heritable, thus leading some to consider that with epigenetics, modern biology no longer rejects the inheritance of acquired characteristics (Lamarckism) as strongly as it once did.
Four general categories of epigenetic modification are known:
- self-sustaining metabolic loops, in which a mRNA or protein product of a gene stimulates transcription of the gene; e.g. Wor1 gene in Candida albicans;
- structural templating in which structures are replicated using a template or scaffold structure on the parent; e.g. the orientation and architecture of cytoskeletal structures, cilia and flagella, prions, proteins that replicate by changing the structure of normal proteins to match their own;
- chromatin marks, in which methyl or acetyl groups bind to DNA nucleotides or histones thereby altering gene expression patterns; e.g. Lcyc gene in Linaria vulgaris described below;
- RNA silencing, in which small RNA strands interfere (RNAi) with the transcription of DNA or translation of mRNA; known only from a few studies, mostly in Caenorhabditis elegans.
Inheritance of epigenetic marks
Although there are various forms of inheriting epigenetic markers, inheritance of epigenetic markers can be summarized as the dissemination of epigenetic information by means of the germline. Furthermore, epigenetic variation typically takes one of four general forms, though there are other forms that have yet to be elucidated. Currently, self-sustaining feedback loops, spatial templating, chromatin marking, and RNA-mediated pathways modify epigenes of individual cells. Epigenetic variation within multicellular organisms is either endogenous or exogenous. Endogenous is generated by cell–cell signaling (e.g. during cell differentiation early in development), while exogenous is a cellular response to environmental cues.
Removal vs. retention
In sexually reproducing organisms, much of the epigenetic modification within cells is reset during meiosis (e.g. marks at the FLC locus controlling plant vernalization), though some epigenetic responses have been shown to be conserved (e.g. transposon methylation in plants). Differential inheritance of epigenetic marks due to underlying maternal or paternal biases in removal or retention mechanisms may lead to the assignment of epigenetic causation to some parent of origin effects in animals and plants.
In mammals, epigenetic marks are erased during two phases of the life cycle. Firstly just after fertilization and secondly, in the developing primordial germ cells, the precursors to future gametes. During fertilization the male and female gametes join in different cell cycle states and with different configuration of the genome. The epigenetic marks of the male are rapidly diluted. First, the protamines associated with male DNA are replaced with histones from the female's cytoplasm, most of which are acetylated due to either higher abundance of acetylated histones in the female's cytoplasm or through preferential binding of the male DNA to acetylated histones. Second, male DNA is systematically demethylated in many organisms, possibly through 5-hydroxymethylcytosine. However, some epigenetic marks, particularly maternal DNA methylation, can escape this reprogramming; leading to parental imprinting.
In the primordial germ cells (PGC) there is a more extensive erasure of epigenetic information. However, some rare sites can also evade erasure of DNA methylation. If epigenetic marks evade erasure during both zygotic and PGC reprogramming events, this could enable transgenerational epigenetic inheritance.
Recognition of the importance of epigenetic programming to the establishment and fixation of cell line identity during early embryogenesis has recently stimulated interest in artificial removal of epigenetic programming. Epigenetic manipulations may allow for restoration of totipotency in stem cells or cells more generally, thus generalizing regenerative medicine.
Cellular mechanisms may allow for co-transmission of some epigenetic marks. During replication, DNA polymerases working on the leading and lagging strands are coupled by the DNA processivity factor proliferating cell nuclear antigen (PCNA), which has also been implicated in patterning and strand crosstalk that allows for copy fidelity of epigenetic marks. Work on histone modification copy fidelity has remained in the model phase, but early efforts suggest that modifications of new histones are patterned on those of the old histones and that new and old histones randomly assort between the two daughter DNA strands. With respect to transfer to the next generation, many marks are removed as described above. Emerging studies are finding patterns of epigenetic conservation across generations. For instance, centromeric satellites resist demethylation. The mechanism responsible for this conservation is not known, though some evidence suggests that methylation of histones may contribute. Dysregulation of the promoter methylation timing associated with gene expression dysregulation in the embryo was also identified.
Whereas the mutation rate in a given 100-base gene may be 10−7 per generation, epigenes may "mutate" several times per generation or may be fixed for many generations. This raises the question: do changes in epigene frequencies constitute evolution? Rapidly decaying epigenetic effects on phenotypes (i.e. lasting less than three generations) may explain some of the residual variation in phenotypes after genotype and environment are accounted for. However, distinguishing these short-term effects from the effects of the maternal environment on early ontogeny remains a challenge.
Contribution to phenotypes
The relative importance of genetic and epigenetic inheritance is subject to debate. Though hundreds of examples of epigenetic modification of phenotypes have been published, few studies have been conducted outside of the laboratory setting. Therefore, the interactions of genes and epigenes with the environment cannot be inferred despite the central role of environment in natural selection. Experimental methodologies for manipulating epigenetic mechanisms are nascent (e.g.) and will need rigorous demonstration before studies explicitly testing the relative contributions of genotype, environment, and epigenotype are feasible.
Studies concerning transgenerational epigenetic inheritance in plants have been reported as early as the 1950s. One of the earliest and best characterized examples of this is b1 paramutation in maize. The b1 gene encodes a basic helix-loop-helix transcription factor that is involved in the anthocyanin production pathway. When the b1 gene is expressed, the plant accumulates anthocyanin within its tissues, leading to a purple coloration of those tissues. The B-I allele (for B-Intense) has high expression of b1 resulting in the dark pigmentation of the sheath and husk tissues while the B' (pronounced B-prime) allele has low expression of b1 resulting in low pigmentation in those tissues. When homozygous B-I parents are crossed to homozygous B', the resultant F1 offspring all display low pigmentation which is due to gene silencing of b1. Unexpectedly, when F1 plants are self-crossed, the resultant F2 generation all display low pigmentation and have low levels of b1 expression. Furthermore, when any F2 plant (including those that are genetically homozygous for B-I) are crossed to homozygous B-I, the offspring will all display low pigmentation and expression of b1. The lack of darkly pigmented individuals in the F2 progeny is an example of non-Mendelian inheritance and further research has suggested that the B-I allele is converted to B' via epigenetic mechanisms. The B' and B-I alleles are considered to be epialleles because they are identical at the DNA sequence level but differ in the level of DNA methylation, siRNA production, and chromosomal interactions within the nucleus. Additionally, plants defective in components of the RNA-directed DNA-methylation pathway show an increased expression of b1 in B' individuals similar to that of B-I, however, once these components are restored, the plant reverts to the low expression state. Although spontaneous conversion from B-I to B' has been observed, a reversion from B' to B-I (green to purple) has never been observed over 50 years and thousands of plants in both greenhouse and field experiments.
Examples of environmentally induced transgenerational epigenetic inheritance in plants has also been reported. In one case, rice plants that were exposed to drought-simulation treatments displayed increased tolerance to drought after 11 generations of exposure and propagation by single-seed descent as compared to non-drought treated plants. Differences in drought tolerance was linked to directional changes in DNA-methylation levels throughout the genome, suggesting that stress-induced heritable changes in DNA-methylation patterns may be important in adaptation to recurring stresses. In another study, plants that were exposed to moderate caterpillar herbivory over multiple generations displayed increased resistance to herbivory in subsequent generations (as measured by caterpillar dry mass) compared to plants lacking herbivore pressure. This increase in herbivore resistance persisted after a generation of growth without any herbivore exposure suggesting that the response was transmitted across generations. The report concluded that components of the RNA-directed DNA-methylation pathway are involved in the increased resistance across generations.
Although genetic inheritance is important when describing phenotypic outcomes, it cannot entirely explain why offspring resemble their parents. Aside from genes, offspring come to inherit similar cultural and environmental factors established by previous generations. It is necessary to consider both cultural and environmental factors because either factor can influence the other (i.e. cultural norms can influence environmental conditions and environmental conditions can influence cultural norms) and can consequently effect the health of subsequent generations. One environment that all human offspring commonly share for nine months is their mother’s womb. Considering the duration of the fetal stages of development, the environment of the mother’s womb can have long lasting effects on the health of offspring. An example of how the environment within womb can affect the health of an offspring can be explained by considering the consequences of the Dutch hunger winter and the role of induced transgenerational epigenetic inherited diseases. A number of studies suggest the existence of transgenerational epigenetic inheritance in humans, which includes the Dutch famine of 1944–45. During the Dutch hunger winter, the offspring born during the famine were smaller than those born the year before the famine and the effects could last up to two generations. Moreover, these offspring were found to have an increased risk of glucose intolerance in adulthood. Differential DNA methylation has been found in adult female offspring who had been exposed to famine in utero, but it is unknown whether these differences are present in their germline. It is hypothesized that inhibiting the PIM3 gene may have caused slower metabolism in later generations, but causation has not been proven, only correlation. The phenomenon is sometimes referred to as Dutch Hunger Winter Syndrome. Furthermore, the increased rates of metabolic diseases, cardiovascular diseases, and other increased risk factors to the health of F1 and F2 generations during the Dutch hunger winter is a known phenomenon called “fetal programming,” which is caused by exposure to harmful environmental factors in utero. Another study hypothesizes epigenetic changes on the Y chromosome to explain differences in lifespan among the male descendants of prisoners of war in the American Civil War.
The Överkalix study noted sex-specific effects; a greater body mass index (BMI) at 9 years in sons, but not daughters, of fathers who began smoking early. The paternal grandfather's food supply was only linked to the mortality RR of grandsons and not granddaughters. The paternal grandmother's food supply was only associated with the granddaughters' mortality risk ratio. When the grandmother had a good food supply was associated with a twofold higher mortality (RR). This transgenerational inheritance was observed with exposure during the slow growth period (SGP). The SGP is the time before the start of puberty, when environmental factors have a larger impact on the body. The ancestors' SGP in this study was set between the ages of 9-12 for boys and 8–10 years for girls. This occurred in the SGP of both grandparents, or during the gestation period/infant life of the grandmothers, but not during either grandparent's puberty. The father's poor food supply and the mother's good food supply were associated with a lower risk of cardiovascular death.
The loss of genetic expression which results in Prader–Willi syndrome or Angelman syndrome has in some cases been found to be caused by epigenetic changes (or "epimutations") on both the alleles, rather than involving any genetic mutation. In all 19 informative cases, the epimutations that, together with physiological imprinting and therefore silencing of the other allele, were causing these syndromes were localized on a chromosome with a specific parental and grandparental origin. Specifically, the paternally derived chromosome carried an abnormal maternal mark at the SNURF-SNRPN, and this abnormal mark was inherited from the paternal grandmother.
Similarly, epimutations on the MLH1 gene has been found in two individuals with a phenotype of hereditary nonpolyposis colorectal cancer, and without any frank MLH1 mutation which otherwise causes the disease. The same epimutations were also found on the spermatozoa of one of the individuals, indicating the potential to be transmitted to offspring.
In addition to epimutations to the MLH1 gene, it has been determined that certain cancers, such as breast cancer, can originate during the fetal stages within the uterus. Furthermore, evidence collected in various studies utilizing model systems (i.e. animals) have found that exposure during parental generations can result in multigenerational and transgenerational inheritance of breast cancer. More recently, studies have discovered a connection between the adaptation of male germinal cells via pre-conception paternal diets and the regulation of breast cancer in developing offspring. More specifically, studies have begun to uncover new data that underscores a relationship between transgenerational epigenetic inheritance of breast cancer and ancestral alimentary components or associated markers, such as birth weight. By utilizing model systems, such as mice, studies have shown that stimulated paternal obesity at the time of conception can epigenetically alter the paternal germ-line. The paternal germ-line is responsible for regulating their daughters’ weight at birth and the potential for their daughter to develop breast cancer. Furthermore, it was found that modifications to the miRNA expression profile of the male germline is coupled with elevated body weight. Additionally, paternal obesity resulted in an increase in the percentage of female offspring developing carcinogen-induced mammary tumors, which is caused by changes to mammary miRNA expression.
One study has shown childhood abuse, which is defined as "sexual contact, severe physical abuse and/or severe neglect," leads to epigenetic modifications of glucocorticoid receptor expression. Glucocorticoid receptor expression plays a vital role in hypothalamic-pituitary-adrenal (HPA) activity. Additionally, animal experiments have shown that epigenetic changes can depend on mother-infant interactions after birth. Furthermore, a recent study investigating the correlations between maternal stress in pregnancy and methylation in teenagers/their mothers has found that children of women who were abused during pregnancy were more likely to have methylated glucocorticoid-receptor genes. Thus, children with methylated glucocorticoid-receptor genes experience an altered response to stress, ultimately leading to a higher susceptibility of experiencing anxiety.
Additional studies examining the effects of diethylstilbestrol (DES), which is an endocrine disruptor, have found that the grandchildren (third-generation) of women exposed to DES significantly increased the probability of their grandchildren developing attention-deficit/hyperactivity disorder (ADHD). This is because women exposed to endocrine disruptors, such as DES, during gestation may be linked to multigenerational neurodevelopmental deficits. Furthermore, animal studies indicate that endocrine disruptors have a profound impact on germline cells and neurodevelopment. The cause of DES's multigenerational impact is postulated to be the result of biological processes associated with epigenetic reprogramming of the germline, though this has yet to be determined.
Effects on fitness
Epigenetic inheritance may only affect fitness if it predictably alters a trait under selection. Evidence has been forwarded that environmental stimuli are important agents in the alteration of epigenes. Ironically, Darwinian evolution may act on these neo-Lamarckian acquired characteristics as well as the cellular mechanisms producing them (e.g. methyltransferase genes). Epigenetic inheritance may confer a fitness benefit to organisms that deal with environmental changes at intermediate timescales. Short-cycling changes are likely to have DNA-encoded regulatory processes, as the probability of the offspring needing to respond to changes multiple times during their lifespans is high. On the other end, natural selection will act on populations experiencing changes on longer-cycling environmental changes. In these cases, if epigenetic priming of the next generation is deleterious to fitness over most of the interval (e.g. misinformation about the environment), these genotypes and epigenotypes will be lost. For intermediate time cycles, the probability of the offspring encountering a similar environment is sufficiently high without substantial selective pressure on individuals lacking a genetic architecture capable of responding to the environment. Naturally, the absolute lengths of short, intermediate, and long environmental cycles will depend on the trait, the length of epigenetic memory, and the generation time of the organism. Much of the interpretation of epigenetic fitness effects centers on the hypothesis that epigenes are important contributors to phenotypes, which remains to be resolved.
Inherited epigenetic marks may be important for regulating important components of fitness. In plants, for instance, the Lcyc gene in Linaria vulgaris controls the symmetry of the flower. Linnaeus first described radially symmetric mutants, which arise when Lcyc is heavily methylated. Given the importance of floral shape to pollinators, methylation of Lcyc homologues (e.g. CYCLOIDEA) may have deleterious effects on plant fitness. In animals, numerous studies have shown that inherited epigenetic marks can increase susceptibility to disease. Transgenerational epigenetic influences are also suggested to contribute to disease, especially cancer, in humans. Tumor methylation patterns in gene promoters have been shown to correlate positively with familial history of cancer. Furthermore, methylation of the MSH2 gene is correlated with early-onset colorectal and endometrial cancers.
Putatively adaptive effects
Experimentally demethylated seeds of the model organism Arabidopsis thaliana have significantly higher mortality, stunted growth, delayed flowering, and lower fruit set, indicating that epigenes may increase fitness. Furthermore, environmentally induced epigenetic responses to stress have been shown to be inherited and positively correlated with fitness. In animals, communal nesting changes mouse behavior increasing parental care regimes and social abilities that are hypothesized to increase offspring survival and access to resources (such as food and mates), respectively.
Inherited epigenetic effects on phenotypes have been well documented in bacteria, protists, fungi, plants, nematodes, and fruit flies. Though no systematic study of epigenetic inheritance has been conducted (most focus on model organisms), there is preliminary evidence that this mode of inheritance is more important in plants than in animals. The early differentiation of animal germlines is likely to preclude epigenetic marking occurring later in development, while in plants and fungi somatic cells may be incorporated into the germ line.
It is thought that transgenerational epigenetic inheritance can enable certain populations to readily adapt to variable environments. Though there are well documented cases of transgenerational epigenetic inheritance in certain populations, there are questions to whether this same form of adaptability is applicable to mammals. More specifically, it is questioned if it applies to humans. As of late, most of the experimental models utilizing mice and limited observations in humans have only found epigenetically inherited traits that are detrimental to the health of both organisms. These harmful traits range from increased risk of disease, such as cardiovascular disease, to premature death. However, this may be based on the premise of limited reporting bias because it is easier to detect negative experimental effects, opposed to positive experimental effects. Furthermore, considerable epigenetic reprogramming necessary for the evolutionary success of germlines and the initial phases of embryogenesis in mammals may be the potential cause limiting transgenerational inheritance of chromatin marks in mammals.
In addition, some epigenetic variability may provide beneficial plasticity, so that certain organisms can adapt to fluctuating environmental conditions, but the exchange of epigenetic information between generations can result in epigenetic aberrations, which are epigenetic traits that deviate from the norm. Therefore, the offspring of the parental generations may be predisposed to specific diseases and reduced plasticity due to epigenetic aberrations. Though the ability to readily adapt when faced with a new environment in species that can quickly reproduce may be beneficial to certain populations, species with long generational gaps may not benefit from such an ability. If a species with a longer generational gap does not appropriately adapt to the anticipated environment, then the reproductive fitness of the offspring of that species will be diminished.
Furthermore, patterns in epigenetic inheritance and the evolutionary implications of the epigenetic codes in living organisms are connected to both Lamarck's and Darwin's theories of evolution. For example, Lamarck postulated that environmental factors were responsible for modifying phenotypes hereditarily, which supports the constructs that exposure to environmental factors during critical stages of development can result in epimutations in germlines, thus augmenting phenotypic variance. In contrast, Darwin’s theory claimed that natural selection strengthened a populations ability to survive and remain reproductively fit by favoring populations that are able to readily adapt. This theory is consistent with intergenerational plasticity and phenotypic variance resulting from heritable adaptivity.
Life history patterns may also contribute to the occurrence of epigenetic inheritance. Sessile organisms, those with low dispersal capability, and those with simple behavior may benefit most from conveying information to their offspring via epigenetic pathways. Geographic patterns may also emerge, where highly variable and highly conserved environments might host fewer species with important epigenetic inheritance.
Humans have long recognized that traits of the parents are often seen in offspring. This insight led to the practical application of selective breeding of plants and animals, but did not address the central question of inheritance: how are these traits conserved between generations, and what causes variation? Several positions have been held in the history of evolutionary thought.
Blending vs. particulate inheritance
Addressing these related questions, scientists during the time of the Enlightenment largely argued for the blending hypothesis, in which parental traits were homogenized in the offspring much like buckets of different colored paint being mixed together. Critics of Charles Darwin's On the Origin of Species, pointed out that under this scheme of inheritance, variation would quickly be swamped by the majority phenotype. In the paint bucket analogy, this would be seen by mixing two colors together and then mixing the resulting color with only one of the parent colors 20 times; the rare variant color would quickly fade.
Unknown to most of the European scientific community, the monk Gregor Mendel had resolved the question of how traits are conserved between generations through breeding experiments with pea plants. Charles Darwin thus did not know of Mendel's proposed "particulate inheritance" in which traits were not blended but passed to offspring in discrete units that we now call genes. Darwin came to reject the blending hypothesis even though his ideas and Mendel's were not unified until the 1930s, a period referred to as the modern synthesis.
Inheritance of innate vs. acquired characteristics
In his 1809 book, Philosophie Zoologique, Jean-Baptiste Lamarck recognized that each species experiences a unique set of challenges due to its form and environment. Thus, he proposed that the characters used most often would accumulate a "nervous fluid." Such acquired accumulations would then be transmitted to the individual's offspring. In modern terms, a nervous fluid transmitted to offspring would be a form of epigenetic inheritance.
Lamarckism, as this body of thought became known, was the standard explanation for change in species over time when Charles Darwin and Alfred Russel Wallace co-proposed a theory of evolution by natural selection in 1859. Responding to Darwin and Wallace's theory, a revised neo-Lamarckism attracted a small following of biologists, though the Lamarckian zeal was quenched in large part due to Weismann's famous experiment in which he cut off the tails of mice over several successive generations without having any effect on tail length. Thus the emergent consensus that acquired characteristics could not be inherited became canon.
Revision of evolutionary theory
Non-genetic variation and inheritance, however, proved to be quite common. Concurrent with the 20th-century development of the modern evolutionary synthesis (unifying Mendelian genetics and natural selection), C. H. Waddington (1905-1975) was working to unify developmental biology and genetics. In so doing, he adopted the word "epigenetic" to represent the ordered differentiation of embryonic cells into functionally distinct cell types despite having identical primary structure of their DNA. Researchers discussed Waddington's epigenetics sporadically - it became more of a catch-all for puzzling non-genetic heritable characters rather than a concept advancing the body of inquiry. Consequently, the definition of Waddington's word has itself evolved, broadening beyond the subset of developmentally signaled, inherited cell specialization.
Some scientists have questioned whether epigenetic inheritance compromises the foundation of the modern synthesis. Outlining the central dogma of molecular biology, Francis Crick succinctly stated, "DNA is held in a configuration by histone[s] so that it can act as a passive template for the simultaneous synthesis of RNA and protein[s]. None of the detailed 'information' is in the histone." However, he closes the article stating, "this scheme explains the majority of the present experimental results!" Indeed, the emergence of epigenetic inheritance (in addition to advances in the study of evolutionary-development, phenotypic plasticity, evolvability, and systems biology) has strained the current framework of the modern evolutionary synthesis, and prompted the re-examination of previously dismissed evolutionary mechanisms.
There has been critical discussion of mainstream evolutionary theory by Edward J Steele, Robyn A Lindley and colleagues, Fred Hoyle and N. Chandra Wickramasinghe, Yongsheng Liu Denis Noble, John Mattick and others that the logical inconsistencies as well as Lamarckian Inheritance effects involving direct DNA modifications, as well as the just described indirect, viz. epigenetic, transmissions, challenge conventional thinking in evolutionary biology and adjacent fields.
- Contribution of epigenetic modifications to evolution
- Överkalix study
- Dutch famine of 1944–45#Legacy
- Transgenerational stress inheritance
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