Nowadays, the prevalence of diabetes mellitus is increasing extremely rapidly worldwide, which is now considered a pandemic non-communicable disease. According to the Diabetes Atlas published by the International Diabetes Federation( 1 ) on World Diabetes Day in 2013, 382 million people are suffering from diabetes and the number will rise to 592 million by 2035, which implies that one-tenth of the world's population will endure diabetes. In particular, more than 21 million infants inherited diabetes from their mothers during pregnancy in 2013. It is estimated that diabetes caused 5·1 million deaths in 2013, i.e. a person died from the disease every 6 s. In summary, diabetes is becoming a more and more severe problem for our society.
Unfortunately, the pathogenesis of diabetes has not been clearly understood yet. However, it is generally believed that genes together with adult lifestyle factors are the major risks of diabetes. There is substantial evidence that the prenatal and early-postnatal nutrition play a key role in determining susceptibility to diabetes in later life( Reference Slatkin 2 ). Little is known about the molecular mechanisms underlying the interaction between maternal diet and ageing. However, the hypothesis that epigenetic mechanisms may link such imbalanced nutrition with altered disease risk has been widely accepted in recent years. Epigenetics is one of the major mechanisms explaining the theory of Developmental Origins of Health and Disease (DOHaD), which focuses on the association between perinatal nutrition and late-onset diseases, such as obesity, insulin resistance, impaired glucose tolerance and type 2 diabetes mellitus (T2DM)( Reference Gluckman, Hanson and Buklijas 3 ). Therefore, epigenetics is likely to be an important molecular basis of malnutrition during early life and glucose metabolism disorders in later life (Fig. 1).
Epigenetic modifications of early-life nutrition and glucose metabolism. Traditionally, genes together with some adult lifestyle factors determine the risk of developing some metabolic diseases such as obesity, type 2 diabetes mellitus (T2DM), impaired glucose tolerance (IGT) and insulin resistance in later life. However, there is now substantial evidence that prenatal and early-postnatal nutrition play a key role in determining susceptibility to these diseases. Epigenetics is likely to be the important molecular basis of malnutrition during early life and glucose metabolism disorders in later life. A colour version of this figure can be found online at http://www.journals.cambridge.org/bjn.
An overview of epigenetics
Traditionally, epigenetics has been used to explain the phenotypic events that cannot be explained by genetic mechanisms. The term ‘epigenetics’ was first proposed by Waddington in 1942, and defined as a branch of biology that studied the causal interactions between genes and their products( Reference Waddington 4 ). In 2006, the definition of epigenetics was updated and considered as a mechanism that could affect gene expression without altering the nucleotide sequence. Most importantly, it could be inherited between generations steadily by mitosis and meiosis through cell differentiation( Reference Holliday 5 ). Epigenetic conditions can illustrate the reason why an organism produces many different cell types during its development, despite the fact that most of the cells in a multicellular organism share the same genetic information. Epigenetic modifications can regulate gene expression reversibly. The three major epigenetic processes are DNA methylation, histone modification and non-coding RNA. DNA methylation was the first recognised and the most well-characterised epigenetic modification in the 1970s( Reference Holliday and Pugh 6 ). Histone modification can influence gene expression by altering chromatin structure( Reference Turner 7 ). Non-coding RNA, such as microRNA and long non-coding RNA, are also included in epigenetic modifications( Reference Aguilera, Fernandez and Munoz 8 ). Epigenetic processes play a critical role in normal development and differentiation in mammals( Reference Fraga 9 ). Epigenetics was defined by Skinner et al. ( Reference Skinner, Manikkam and Guerrero-Bosagna 10 ) in 2010 as ‘molecular factors and processes around DNA that are mitotically stable and regulate genome activity independent of DNA sequence’. Although the definition of epigenetics has been updated constantly, the three core features are always reserved, namely (1) without any alterations in the DNA sequence, (2) heritability and (3) plasticity and reversibility. In the present review, we focus on DNA methylation.
What is DNA methylation?
DNA methylation is the first discovered and one important epigenetic modification. It is one of the best-studied epigenetic modifications in the context of altered environment. It is a biochemical process involving the covalent addition of a methyl group at the 5′ position of cytosine in DNA. This normally occurs on a cytosine followed by a guanine, known as a CpG dinucleotide (the p denotes the intervening phosphate group). It is catalysed by DNA methyltransferases, and S-adenosylmethionine is the methyl donor( Reference Bird 11 ). DNA methylation typically occurs in the context of CpG dinucleotides. CpG dinucleotides are not randomly distributed throughout the genome, but are often grouped in clusters at the 5′ regulatory regions of many genes, called CpG islands. Most CpG dinucleotides are methylated, but those located in CpG islands are usually unmodified( Reference Gardinergarden and Frommer 12 ). However, hypermethylation of these CpG islands has a specific effect on reducing gene expression and repressing transcription, while their hypomethylation is related to transcriptional activation( Reference Reik and Dean 13 ) (Fig. 2). DNA methylation is a common modification in mammalian genomes. It constitutes a stable epigenetic symbol that can stably alter the expression of genes and transmit through DNA replication as cells divide and differentiate from embryonic stem cells into specific tissues( Reference Bird 11 ). DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation( Reference Kacem and Feil 14 ), suppression of repetitive elements and carcinogenesis( Reference Jaenisch and Bird 15 ).
Molecular mechanisms of DNA methylation. DNA methylation is a biochemical process involving the covalent addition of a methyl group at the 5′ position of cytosine in DNA. It is catalysed by DNA methyltransferases and S-adenosylmethionine (SAM) is the methyl donor. Hypermethylation or hypomethylation of some DNA can alter gene transcription. A colour version of this figure can be found online at http://www.journals.cambridge.org/bjn.
Evidence of early-life nutrition and glucose metabolism from human cohorts
Fetal programming hypothesis
In 1977, the association between the quality of early-life environment and the future risk of disease in later life was first proposed by Forsdahl( Reference Forsdahl 16 ), who discovered that infant mortality rate was positively associated with an increased risk of CVD in middle age. Subsequently, Barker et al. ( Reference Barker, Winter and Osmond 17 ) found an inverse relationship between birth weight and increased CVD mortality in 1989. The effects of early-life nutrition on the increased risk of metabolic diseases were most clearly shown in studies of the Dutch famine during the winter of 1944. These studies showed that individuals whose mothers were exposed to the famine in the late stage of gestation had lower birth weight and an increased risk of obesity, CVD, insulin resistance and hypertension in later life compared with unexposed individuals( Reference Painter, Roseboom and Bleker 18 ). What is more, apart from nutrition deficiency in early life, overnutrition is also associated with a significantly increased susceptibility to metabolic diseases such as obesity, hypertension, atherosclerosis, insulin resistance and diabetes mellitus( Reference Curhan, Willett and Rimm 19 ). It can involve some organs related to glucose metabolism, including liver, pancreas, adipose tissue and skeletal muscle through different mechanisms (Fig. 3). Nutrition during early life has long-term phenotypic effects. This idea was first proposed by professor Barker and Hales in the 1990s, known as ‘fetal programming hypothesis’( Reference Hales, Barker and Clark 20 ). After more than 20-year long-term research, it has now been accepted worldwide.
Fetal programming hypothesis in relation to glucose metabolism. Nutrition during early life has long-term phenotypic effects along with ageing, such as impaired glucose metabolism. This idea was first proposed by professor Barker and Hales in the 1990s, known as ‘fetal programming hypothesis’. It has been widely accepted that the essence of this hypothesis is epigenetic mechanism, such as DNA methylation. It can involve some organs related to glucose metabolism, including liver, pancreas, adipose tissue and skeletal muscle through different mechanisms. A colour version of this figure can be found online at http://www.journals.cambridge.org/bjn.
Early-life nutrition and altered epigenetic modifications
Epigenetic events are crucial for early development. However, it can be influenced by some environmental factors, potentially programming the genome for later adverse health outcomes. Gestation is the critical time window for maternal nutrition to affect the offspring. Early-life nutrition can induce persistent DNA methylation. More specifically, both undernutrition and overnutrition will bring about epigenetic modifications during early life, including embryonic development and the neonatal period. A number of clinical studies have shown that people born during the famine in The Netherlands still exhibited a lower methylation level of insulin-like growth factor 2 (IGF2) after 60 years, compared with those who were not exposed to prenatal famine( Reference Tobi, Lumey and Talens 21 ). It indicates that epigenetic modifications of some special genes affected by restricted periconceptional maternal nutrition can be persistent for decades. Another study has shown that children whose mothers took folic acid had 4·5 % higher methylation in the IGF2 gene differentially methylated region (DMR) than those whose mothers did not take folic acid. It also indicates an inverse independent association between IGF2 DMR and birth weight, which means that periconceptional folic acid use is associated with epigenetic changes in IGF2 in the child that may affect the intra-uterine programming of growth and development with consequences for health and disease throughout life( Reference Steegers-Theunissen, Obermann-Borst and Kremer 22 ). A recent cohort study has demonstrated that retinoid X receptor-α (RXRA) methylation had an independent association with sex-adjusted childhood fat mass, which could explain more than 25 % of the variance in childhood adiposity. Higher methylation of RXRA was associated with lower maternal carbohydrate intake in early pregnancy. These studies suggested that prenatal development is a substantial risk of metabolic diseases, such as diabetes, obesity and the metabolic syndrome( Reference Godfrey, Sheppard and Gluckman 23 ).
Enlightenment of monozygotic twins
Since epigenetic mechanisms are partly under genetic control( Reference Chong and Whitelaw 24 ), the role of epigenetic factors in disease aetiology could be confounded by genetic factors in unrelated individuals. Monozygotic (MZ) twins are well matched on their DNA sequence, thus providing an ideal model for epigenetic research. Type 1 diabetes mellitus shows an approximately 40 % concordant rate in MZ twins, suggesting the role of environmental factors and epigenetic modifications in the aetiology of the disease. Stefan et al. ( Reference Stefan, Zhang and Concepcion 25 ) identified eighty-eight CpG sites displaying significant methylation changes in type 1 diabetes mellitus-discordant MZ twin pairs. Using a well-matched MZ twin cohort, Zhao et al. ( Reference Zhao, Goldberg and Bremner 26 ) demonstrated that global DNA methylation in peripheral blood leucocytes was significantly associated with insulin resistance. On average, a 10 % increase in global DNA methylation was associated with a 4·55-unit increase in the homeostasis model assessment (HOMA) value. Some observational human and experimental animal studies have indicated that low birth weight was associated with an increased risk of metabolic diseases, such as obesity and T2DM. Although genetic factors might contribute to this association, studies of MZ twins showed that the twin who was lighter at birth had a more adverse metabolic profile in adulthood compared with its genetically identical co-twin, who was heavier at birth( Reference Grunnet, Vielwerth and Vaag 27 ). A population of twelve MZ twin pairs discordant for T2DM recognised a considerable non-genetic contribution to glucose metabolic disturbances in genetically identical individuals( Reference Vaag, Henriksen and Madsbad 28 ). An exploratory approach showed that the site of umbilical cord insertion into the placenta in monochorionic twins has the strongest positive association with methylation in all IGF2/H19 DMR( Reference Loke, Galati and Morley 29 ), which was crucial for prenatal growth. In addition, the epigenetic state at this locus was environmentally labile. A study of 53- to 80-year-old MZ twin pairs discordant for T2DM found that increased variation in repetitive LINE1 DNA sequence methylation was associated with more phenotypic dissimilarity such as BMI and 2 h plasma glucose. T2DM-related genes, including PPARGC1α encoding PPARγ co-activator 1α (PGC-1α) in muscle and hepatocyte nuclear factor-4α (HNF4α) in adipose tissue, have increased methylation in T2DM individuals( Reference Ribel-Madsen, Fraga and Jacobsen 30 ). A recent study on peri/postnatal epigenetic twins that contained a longitudinal cohort of 250 pairs of twins and their mothers confirmed that twins exhibit a wide range of epigenetic discordance at birth, and that within-pair birth-weight discordance correlated with epigenetic discordance in genes associated with glucose and lipid metabolism( Reference Loke, Novakovic and Ollikainen 31 ). Therefore, all these lines of evidence support an epigenetic mechanism for the DOHaD and fetal programming hypothesis.
Role of the placenta in developmental programming in the context of obesity and diabetes
The placenta performs multiple functions required for bringing forth new life. It is not only a very important organ to provide nutrients for the developing fetus, it also exchanges a wide range of endocrine signals, cytokines and growth factors with the mother and the fetus, thereby regulating intra-uterine development. It is also well known that the placenta have a high and prolific expression of imprinted genes, and recent research have suggested that epigenetic modifications such as DNA methylation in imprinting and non-imprinting genes can have an important impact on the health of the offspring. Modified patterns of placental DNA methylation have been reported in human pregnancies. For example, it has been shown that the maternally imprinted mesoderm specific transcript (MEST) gene is hypomethylated in the placenta obtained from the newborns of mothers with gestational diabetes mellitus (GDM). In addition, decreased blood MEST methylation was also observed in adults with morbid obesity, suggesting that intra-uterine exposure to GDM has long-lasting effects on the epigenome of the offspring( Reference El Hajj, Pliushch and Schneider 32 ). Another study showed that maternal glycaemia was associated with DNA methylation changes in leptin and adiponectin genes, suggesting that these epigenetic adaptations are probably functional and may have impacts on the long-term regulation of newborns’ energy metabolism( Reference Houde, Hivert and Bouchard 33 ).
Evidence of early-life nutrition and glucose metabolism from animal models
In addition to the evidence from studies of human cohorts, the association between dietary changes during specific windows of development and epigenomic modifications in later life has also been reported in several animal models. In the present review, we focus on the most widely studied challenges of nutrition, including maternal energy restriction (protein deficiency), maternal excessive energy (high fat feeding) and paternal malnutrition (protein deficiency and high-fat diet). We describe some well-known epigenetic changes that have been identified from animal models of obesity, insulin resistance and diabetes (see Table 1).
Summary of the relevant studies showing the effects of nutrients on DNA methylation in animal models
LPD, low-protein diet; GR, glucocorticoid receptor; HNF4α, hepatocyte nuclear factor-4α; HMGCR, hepatic activation of 3-hydroxy-3-methylglutaryl-CoA reductase; IGF2/H19, insulin-like growth factor-2/H19; SD, Sprague–Dawley; Gch1, GTP cyclohydrolase 1; MOR, μ-opioid receptor; HFD, high-fat diet; Il13ra2, IL-13 receptor-α2; Pik3ca and Pik3r1, phosphatidylinositol 3-kinase subunits.
Maternal energy restriction
Protein restriction is frequently used as a model for maternal malnutrition. A low-protein (LP) diet is associated with impaired fetal growth, the development of obesity, insulin resistance and diabetes in the offspring( Reference Bocock and Aagaard-Tillery 34 ). Many epigenetic modifications have been reported in offspring exposed to a maternal LP diet. One of the first studies showing the association between nutritional imbalances during intra-uterine development and epigenetic modifications was that feeding a LP diet (9 % casein v. 18 % casein) to rats during pregnancy resulted in global DNA hypermethylation in the liver of the fetuses( Reference Rees, Hay and Brown 35 ). Recent studies confirmed that maternal LP feeding during pregnancy might also result in locus-specific changes in DNA methylation. For example, feeding pregnant rats a protein-restricted diet induced hypomethylation of the glucocorticoid receptor (GR) and PPARα promoters in the liver of juvenile and adult offspring( Reference Burdge, Slater-Jefferies and Torrens 36 ). Also in Wistar rats, methylation of the PPARα gene was 20·6 % lower, with expression being 10·5-fold higher, and that of the GR gene was 22·8 % lower, with expression being 200 % higher in pups of dams that were fed a protein restriction diet throughout pregnancy( Reference Lillycrop, Phillips and Jackson 37 ). The HNF4α gene has been implicated in the aetiology of T2DM. Sandovici et al. ( Reference Sandovici, Smith and Nitert 38 ) observed that maternal LP diet during pregnancy and lactation could lead to progressive epigenetic silencing of the entire HNF4α locus in rat pancreatic islets, which weakened the promoter–enhancer interaction and resulted in a permanent reduction in HNF4α expression. Maternal protein undernutrition during pregnancy and lactation in Balb/c mice affected the balance between food intake and energy expenditure in adults. Moreover, this nutritional stress resulted in the removal of methyls at CpG sites located in the promoter of leptin, which was associated with DNA hypomethylation of the leptin promoter in adipose tissue( Reference Jousse, Parry and Lambert-Langlais 39 ). Interestingly, the DNA methylation of the LP diet is not restricted to rodents. Male offspring of maternal LP diet of Meishan sows during pregnancy and lactation demonstrated higher GR binding to the mitochondrial DNA promoter, which was accompanied by lower cytosine methylation and hydroxymethylation on mitochondrial DNA promoter( Reference Vo, Revesz and Sohi 40 ). Similarly, piglets with maternal LP diet during pregnancy and lactation showed significantly lower body weight and liver weight at weaning. Hepatic activation of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) gene transcription in LP piglets was associated with promoter hypomethylation( Reference Cong, Jia and Li 41 ).
In addition to protein restriction, some other models of maternal energy restriction such as nutrient restriction can also affect the epigenome and the metabolism of the offspring. In one study, it has been shown that periconceptional undernutrition at least 5 months before conception to days 6 and 7 after conception was associated with a decrease in the adrenal mRNA expression of IGF2 and decreased methylation in the proximal CCCTC-binding factor (CTCF)-binding site in the DMR of the IGF2/H19 gene in the adrenal gland in offspring sheep, and it is associated with decreased expression of IGF2 in offspring along with an increase in the cortisol stress response in females( Reference Zhang, Rattanatray and MacLaughlin 42 ). In the fetal liver of sheep, maternal periconceptional restriction of vitamin B12, folate, and methionine altered DNA methylation of 4 % of the 1400 CpG islands examined, and more than half of these changes were specific to male offspring( Reference Sinclair, Allegrucci and Singh 43 ). Bilateral uterine artery ligation of pregnant rats has been used to generate a model of fetal growth restriction. A genome-wide survey of male Sprague–Dawley rat offspring showed altered DNA methylation of approximately 1400 loci that occurred predominantly in intergenic regions in pancreatic islets. Moreover, one gene in particular, GTP cyclohydrolase 1 (Gch1), showed almost a 3-fold reduction in mRNA expression, associated with hypermethylation at a conserved intergenic site < 45 kb upstream of the gene itself( Reference Thompson, Fazzari and Niu 44 ). More importantly, these changes in DNA methylation may remain stable until adulthood, thus providing a molecular basis for DOHaD.
Maternal energy excess
Maternal energy restriction (LP diets) is not the only experimental model that suggests that maternal diet influences the epigenome of offspring during development. Nowadays, both the incidence of obesity during pregnancy and GDM are increasing along with improved living conditions. Therefore, scientists are increasingly concerned about the effect of overnutrition on human health. There is increasing evidence that energy excess in maternal diet is associated with an altered metabolic phenotype. Recently, studies have shown that maternal high-fat diet might modify DNA methylation and gene expression in the offspring. Maternal high-fat diet during gestation was associated with global and gene-specific promoter DNA hypomethylation, such as the dopamine reuptake transporter, the μ-opioid receptor (MOR) and preproenkephalin in the brain from the first-generation (F1) offspring of C57BL/6J × DBA/2J hybrids. These changes might affect feeding behaviour, which can increase obesity and obesity-associated risk for the metabolic syndrome( Reference Vucetic, Kimmel and Totoki 45 ). During the postnatal period of the same model, the offspring of maternal high-fat diet showed increased binding of methyl CpG-binding protein 2 (MeCP2) to the MOR promoter in reward-related regions of the brain, which can repress the transcription of the MOR gene( Reference Vucetic, Kimmel and Reyes 46 ). A recent study has shown that the offspring whose mothers fed on a high-fat diet during late gestation has increased weight accrual and food intake, exhibiting insulin resistance and hyperlipidaemia. Furthermore, the offspring mice emerged with increased methylation of adiponectin and leptin receptor, and decreased methylation of leptin genes( Reference Khalyfa, Carreras and Hakim 47 ). Modified patterns of placental DNA methylation have also been reported in animal models exposed to an altered maternal environment. In one study, it has been shown that exposure to a high-fat diet during pregnancy triggered global and gene-specific DNA methylation in mouse placenta alterations in a manner dependent on the sex of the conceptus( Reference Gallou-Kabani, Gabory and Tost 48 ). In Wistar rats, it has also been demonstrated that maternal intake of essential fatty acids during pregnancy affected the global levels of placental DNA methylation( Reference Kulkarni, Dangat and Kale 49 ).
Therefore, it is currently unknown whether these genes may contribute to the development of insulin resistance or diabetes in later life. It is certainly interesting to further explore the potential implication and epigenetic mechanisms of these genes on glucose metabolism.
Paternal malnutrition
Apart from the detrimental impacts of maternal malnutrition on abnormal glucose metabolism in offspring, recent studies have shown that paternal malnourished exposures could also affect the phenotype of the offspring. Paternal lifestyle and particular nutrition factors can affect spermatogenesis at the level of germ and sertoli cells( Reference Sharpe 50 ) and the composition of the seminal fluid( Reference Robertson 51 ). Similarly, alterations in paternal diet are also associated with altered DNA methylation in the offspring. As shown by Ng et al. ( Reference Ng, Lin and Laybutt 52 ), paternal exposure to a high-fat diet could induce intergenerational transmission of impaired glucose tolerance and insulin homeostasis in their female offspring. The IL-13 receptor-α2 (Il13ra2) promoter was hypomethylated in female offspring after high fat feeding of their fathers( Reference Ng, Lin and Laybutt 52 ). Offspring of fathers fed on a LP diet in C57BL/6J mice exhibited elevated hepatic expression of many genes involved in glucose metabolism and lipid biosynthesis. Epigenomic bisulphite sequence analysis on DNA from the liver of the offspring revealed numerous modest (about 20 %) changes in cytosine methylation depending on the paternal diet, including a substantial increase in methylation at an intergenic CpG island 50 kb upstream of the PPARα gene( Reference Carone, Fauquier and Habib 53 ). A recent study has shown that paternal prediabetes increased the susceptibility of the offspring to diabetes through gametic epigenetic alterations( Reference Wei, Yang and Wei 54 ). All these results indicate that paternal diet can also induce impaired glucose metabolism in offspring through epigenetic modification mechanisms, especially DNA methylation.
Transgenerational epigenetic effects
Maternal malnutrition during gestation and lactation not only have lifelong effects on F1 offspring, but can also impact the second-generation (F2), despite the fact that there is no further dietary modification to F1. The first direct evidence of transgenerational epigenetic inheritance in mammals came from studies in mouse nucleocytoplasmic hybrids, showing that the cytoplasmic ‘environment’ of the early embryonic genome induces persistent and heritable changes in gene expression and CpG methylation( Reference Roemer, Reik and Dean 55 ). Another study indicated that mice fed on a high-fat diet from adulthood developed T2DM themselves, which was reversible, but their F1 and F2 offspring developed T2DM irreversibly, without further high-fat diet feeding( Reference Gniuli, Calcagno and Caristo 56 ). Similar results on glucose metabolism have been reported in another animal study on offspring of mothers with GDM. Impaired glucose tolerance was found in the F1-GDM mice, whose plasma glucose level was significantly increased, while the fasting insulin level of the F2-GDM was significantly increased. The expression of the imprinted genes IGF2 and H19 was down-regulated in pancreatic islets, caused by abnormal methylation status of the DMR( Reference Ding, Wang and Shu 57 ). Emerging evidence suggests that cross-generation transmission of adverse dietary effects during gestation/lactation is relevant to changes in epigenetics. Burdge et al. ( Reference Burdge, Hanson and Slater-Jefferies 58 ) observed that DNA methylation of hepatic PPARα in both F1 and F2 generations was reduced due to dietary protein restriction during gestation, despite that dietary protein restriction was only applied in one generation and no dietary modification was administered in their offspring. However, not all epigenetic changes are transmittable across generations. For example, diet-induced hypermethylation at agouti viable yellow is not inherited transgenerationally through the female( Reference Waterland, Travisano and Tahiliani 59 ). A recent study uniquely revealed that prediabetes could be inherited transgenerationally through the mammalian germ line by DNA methylation( Reference Wei, Yang and Wei 54 ). In summary, most studies revealed that altered epigenetic modifications during early life could be transmitted to the next generation.
In conclusion, studies of human cohorts and animal models demonstrated that developmental programming of adult diseases occurs at each nutrition spectrum during the early life, due to maternal energy deprivation, maternal nutritional excess and paternal malnutrition. Although the specific mechanisms leading to aberrant glucose metabolism in each situation are different, current evidence supports that epigenetic modifications, especially DNA methylation, may be an important molecular link between them, and this epigenome can be inherited from one generation to another.
Developmental plasticity and epigenetic reversibility
Generally speaking, plasticity is viewed as the ability that one genotype can produce different phenotypes in response to different environments. The time window of maximal plasticity appears to be during development, which is termed as ‘Developmental plasticity’. It has evolved to provide the best chances of survival and reproductive success to the organism. Further studies illustrated that developmental plasticity could be regulated by epigenetic modifications. It was traditionally considered that epigenetic modifications were static in the control of gene expression. However, it is now being altered by the idea that these epigenomic marks are dynamically induced by some factors. Examples supporting the idea that intervention with drugs in early life can bring long-term benefits to health are presented as follows. Offspring born to dams with energy restriction during gestation and lactation are more likely to develop obesity and insulin resistance( Reference Vickers, Breier and Cutfield 60 ). However, when leptin was given to neonatal mice born to dams that suffered from energy restriction, the offspring mice showed a phenotype close to healthy controls and did not develop obesity in later life even when fed with an energy-dense high-fat diet( Reference Vickers, Gluckman and Coveny 61 ). It suggests that developmental metabolic programming is potentially reversible by an intervention late in the phase of developmental plasticity. The long-lasting effects of early leptin intervention has been shown to be associated with a consistent up-regulation of some critical glucose metabolic genes such as the transcription factor PPARα, whereas a sustained up-regulation of PPARα is regulated by reduced DNA methylation in the promoter region of PPARα ( Reference Gluckman, Lillycrop and Vickers 62 ). Stress responses in the adult rat are programmed early in life by maternal care, and are associated with altered transcription factor binding to the hippocampal GR promoter( Reference Weaver, Cervoni and Champagne 63 ). Interestingly, central infusion of these adult offspring with l-methionine, a precursor to S-adenosylmethionine that serves as the donor of methyl groups for DNA methylation, with altered DNA methylation due to maternal behaviour programming during gestation can be reversed, together with altered nerve growth factor-inducible protein-A binding to the GR promoter and hypothalamic–pituitary–adrenal and behavioural responses to stress( Reference Weaver, Champagne and Brown 64 , Reference Weaver 65 ). These results demonstrate that despite the inherent stability of epigenomic marks established early in life through behavioural programming, they are potentially reversible in the adult brain. Therefore, in view of the reversibility of epigenetic mechanisms, intervention with drugs during early life may ameliorate glucose metabolism disorders in later life if the early disrupting environment has resulted in modified epigenetics, which can generate long-lasting effects.
Conclusions
In summary, the increased risk of aberrant glucose metabolism during later life is not only determined by the genotype, to some extent, it is also distinctly affected by early developmental nutrition. The fundamental mechanism is that early developmental nutrition can alter epigenetic modifications, especially DNA methylation, which, in turn, can alter the expression of some genes related to cell differentiation and glucose metabolism. Pregnancy and lactation are the pivotal periods of growth and development, which are very sensitive to the changes in developmental environments, such as nutrition. Therefore, it is very meaningful to study the epigenetics. It may be that the mechanisms account for the relationship between early-life nutrition and glucose metabolism in later life, which can help us explain the characteristics of the onset of some chronic diseases such as diabetes. More importantly, epigenetics may be utilised as a new theoretical foundation for the aetiology of diabetes mellitus, to explore a novel drug for the prevention and intervention of diabetes from the early stage of life in the future.
Acknowledgements
The present review was supported by the National Key Program of Clinical Science and the National Natural Science Foundation of China (NSFC) (project no. 81170736).
The authors’ contributions are as follows: J. Z. and X. X. contributed to the idea, conception and design of the review; Q. Z. and M. Y. drafted and critically revised the manuscript for important intellectual content. All authors read and approved the final version.
The authors declare that there are no conflicts of interest.
