THE VICIOUS CYCLE OF OBESITY: DO EPIGENETICS PLAY A ROLE?
Epigenetics as a new mechanism to explain why obesity is transgenerational..
AIMS
This website will focus on
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

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The novel idea of epigenetics in obesity
The role of altered epigenetic mechanisms in developmental induction of obesity, and
how these processes may make a significant contribution to the obesity epidemic
To assess the effect of maternal and paternal obesity on the health of the future
offspring
Complications of pregnancy associated diseases in obesity and how epigenetics have
a role in these
This site was made by a group of University of Edinburgh Biomedical Sciences Honours
Students who studied this subject over 8 weeks as part of the Reproductive Systems course.
This website has not been peer reviewed.
We certify that this website is our own work and that we have authorisation to use all the
content (e.g. figures) used in website.
We would like to thank Dr Marian Aldhous for her help and guidance throughout this project.
Word Count: 5948
INTRODUCTION
While the effects of smoking and alcohol on the developing foetus are well established, the
role of parental obesity is sometimes overlooked.[1,2] As obesity affects 15-20% of pregnant
women in the UK, its impact on pregnancy and the offspring is important to understand.[3]
Obesity is Hereditary
Both maternal and paternal obesity have been linked to an increased risk of obesity and
obesity related health problems (Fig.1) in the offspring through epigenetic changes.[4,5,6]
Maternal obesity increases the risk of almost all pregnancy complications, such as premature
labour, gestational diabetes and pre-eclampsia, which in turn increase the offspring’s risk of
developing obesity.[7]
Obesity seems to be a vicious cycle, often transmitted from generation to generation.
However, the underlying mechanism remains elusive and is an area of much research, with
proposed epigenetic mechanisms at the forefront. Understanding the role of epigenetics in
obesity could allow us to treat and prevent obesity at the foetal stage, thus offering a possible
solution for the worldwide epidemic of obesity.
The Effect of Parental Nutrition On Offspring
According to Barker’s hypothesis, exposure to an adverse intrauterine environment
predisposes to developing disease later in life. [8] Studies on the Dutch Famine of 1944 show
this. Children whose mothers suffered from under-nutrition during the last trimester of
pregnancy were born smaller than average and had lower rates of obesity later in life, while
children whose mothers were malnourished only in early pregnancy were born normal-sized
but had high rates of later life obesity. This indicates that an adverse intrauterine environment
causes metabolic adaptations that have long-term health impacts.[9,10] Furthermore
maternal over-nutrition, and recently also paternal obesity, have been shown to affect the
long-term health of offspring.[10]
Epigenetics And Its Mechanisms
Epigenetics, meaning “on top of genetics,” refers to changes in gene expression that are not
related to changes of the DNA sequence.[11] It is a naturally occurring process but can be
influenced by the environment, resulting in epigenetic modifications. These modifications are
mediated by three potential mechanisms (Fig.3):
1. DNA Methylation
A methyl-group is added to the carbon-5 of a cytosine base by a methyltransferase enzyme,
turning it into a 5-methylcytosine (Fig.2). This occurs at CpG sites throughout the genome,
where a cytosine is located next to a guanine – promoter regions of genes are especially rich
in these sites. Methylation prevents transcription factors from binding to the gene and
therefore silences it.[12]
2. Histone Modification
Histones can be modified by methylation, acetylation, phosphorylation and ubiquination of
their tails. These modifications control how condensed the chromatin around the histones is,
which controls whether transcription machinery can bind to the DNA and allow gene
expression.[13] Histone modifications and DNA methylation often act together.[12]
3. Non-coding MicroRNA’s
Small, single-stranded, non-coding pieces of microRNA may control the structure and
expression of mRNAs and therefore be able to silence genes. However, this is poorly
understood.[12]
Genomic Imprinting
Genomic imprinting is a subset of epigenetics that is controlled by the epigenetic mechanisms
described above. Usually every gene is functionally diploid, with one allele inherited from
each parent and their transcription is controlled together. However, a small proportion of
genes (~100 in humans) are imprinted.[14] This means that either the maternal or paternal
allele is silenced, and only one allele is expressed. Imprinting may have evolved in response
to parental conflict; the paternally imprinted genes tend to be growth promoting and
encourage distribution of resources to the foetus, whereas maternally imprinted genes tend
to limit growth to conserve the mother’s own resources.[15]
Emerging evidence suggests that imprinting may be altered by parental obesity and this could
explain the trans-generational effect of obesity – a concept that will be explored in this
website.
IMPRINTING IN OBESITY
The Role of Obese Parents
It is becoming increasingly apparent that whilst pre-determined genetics and post-natal
environment play a role in determining an offspring’s risk of obesity, epigenetic reprogramming taking place during foetal development, also has a role. [5,16,17] The ill-effects
of in utero undernourishment on foetal development and life-long health has been well
studied. [18,19] It has brought to light the notion that nutritional availability in utero
influences foetal programming. [19]
Maternal obesity increases the availability of nutrients for developing foetuses, thus they take
up more nutrients.[20]Underlying mechanisms affecting foetal growth during pregnancy as a
result of maternal obesity are likely to involve the dysregulation of glucose, insulin and lipid
transfer to the foetus. [21] Animal studies have shown that foetuses over-nourished in utero
display altered glucose-stimulated-insulin secretion in pancreatic islet cells and altered
orexigenic peptides in the hypothalamus. [22,23] This suggests that prenatal nutrition,
particularly glucose, is sufficient to programme epigenetic alterations in the foetus, with
regard to glucose tolerance and appetite control.
Though the effects of maternal nutrition on foetal development is better documented,
evidence is emerging that suggests there is a paternal contribution to heritable epigenetics.
This is mediated through exposure of developing sperm to environmental insults at timewindows of particular vulnerability. [24] This theory supports a recent mouse experiment
whereby obesity related phenotypes were worse in mice born to two obese parents
compared to one. [25]
Imprinting and the Developing Baby
The phenomenon of imprinting, creating functionally haploid genes, is thought to have a
strong role in the growth-determinence of offspring both pre- and post-natally. [26] An innate
problem of being functionally haploid is an increased vulnerability of imprinted genes to
epigenetic dysregulation in response to alternate exposures, such as parental obesity. [27]
The imprinting marks on the parental chromosomes forming a foetus are removed and
remarked at fertilisation. [28] However, there is a build up of evidence suggesting not all of
these parental marks are erased, thus some imprinting marks may be trans-generational. This
provides a mechanism by which the epigenetic methylations contributing to parental obesity
phenotypes may be passed on from parent to child. [24,25]
The maternal and paternal genomes have differing but balancing drives for allocating
maternal resources to the developing foetus in utero. These drives are mediated through
differentially imprinted genes affecting placental development and nutrient transport
capacity. The paternal genome maximises allocation of maternal-foetal resources and the
maternal genome conserves. This balance is facilitated through regulation of differing
imprinting centres controlling different imprinting genes, coding for placental and foetal
growth factors (Table 1). [22]
Other Epigenetically Modifiable Genes
Whilst Imprinted genes are at the forefront of research into the transgenerational phenotype
of obesity, other non-imprinted genes have been shown to undergo differential methylation
in response to parental obesity (Table 2).
However, as these genes are functionally diploid, they are less vulnerable to adverse
environments and so perhaps less intrinsic to the recent increase in the prevalence of obesity.
Balancing Imprinting
An imprinted balance of particular consequence is the Insulin Growth Factor 2 (IGF2)/H19
gene and the IGF2 Receptor gene. [27] The expression of these genes is controlled by
imprinting centres on Chromosome 11 and 6 respectively. [29,30] IGF2 is maternally
imprinted and stimulates placental and foetal growth. The IGFR and H19 genes are paternally
imprinted and restrict maternal resources and foetal growth. A mismatch in the balance of
these genes, especially as IGF2 and H19 share a common Imprinting Centre, is proposed to
have a role in the deregulation of foetal metabolic programming that contributes to the transgenerational obesity phenotype. [27]
The importance of this imprinting regulation is demonstrated in extreme medical conditions,
such as Beckwith-Wiedemann Syndrome, caused by perturbations in the control of imprinted
genes (Fig.7). These conditions have devastating effects on growth and development. [31]
The concept that parents may capture messages from their external environment as
epigenetic modifications combined with emerging notions that imprinting marks are not as
“wiped clean” as originally thought, provides a mechanism by which imprinting may
contribute to the trans-generational phenotype of obesity. The epigenetic imprinting
perturbations have a lifelong effect in the programming of body-weight and manifestation of
metabolic diseases. [20,24]
MATERNAL OBESITY
The effect of maternal obesity on pregnancy outcomes is well documented. However,
understanding the exact mechanism by which obesity affects the offspring presents
challenges. One theory considers that epigenetic modifications in obese mothers that may
cause permanent changes in foetal programming of adult disease. This could be a mechanism
possibly linking maternal obesity to childhood obesity, pre-eclampsia and gestational
diabetes (GDM).
Studies have shown alterations in DNA methylation and histone modifications dependent on
maternal diet. This suggests that epigenetic analysis may be useful to identify individual
vulnerability to later obesity and metabolic disease. One of the biggest challenges will be to
discover ways that maternal metabolism alters chromatin structure in the foetus through
epigenetic events. Determining this could lead to a revolutionary way to tackle obesity from
inside the womb.
POTENTIAL EPIGENETIC MECHANISMS
Obese mothers mean obese children…
So Where do Epigenetics Fit In?
Epigenetic mechanisms might mediate some associations of maternal obesity with offspring
outcomes (Fig.9). DNA methylation in response to over-nutrition may affect metabolic
imprinting of genes that control energy homeostasis. DNA methylation is environmentally
responsive and could provide a viable explanation for the developmental origins of disease.
DNA methylation is proposed as a mechanism in relation to developmental obesity. A study
(Fig.10) generated an example of methyl-dependent epigenetic modification, where two sets
of genetically identical pregnant mice were given Bisphenol A (BPA). [32] BPA is proposed to
disrupt epigenetic reprogramming of the mice-specific Agouti gene. One set of mice received
BPA plus a normal mouse diet and the other set received BPA plus a methyl-rich diet. The
mouse with the BPA plus a normal mouse diet had its agouti gene completely unmethylated
and its phenotype was obese, prone to cancer, diabetes and a yellow coat. The mice with the
BPA and supplemented diet had its agouti gene methylated, was normal weight and had a
brown coat. This is a prime example of epigenetics working via DNA methylation changes.
What Evidence is there for Epigenetic Modification occurring in Obese Mothers?
Leptin
Several energy homeostasis genes such as those for Leptin, SOCS3 and Glucose Transporters
are regulated by DNA methylation and histone modifications. Leptin is a strong candidate for
DNA methylation studies to verify whether epigenetic mechanisms are affected by glucose
metabolism dysregulation during pregnancy. The study by Bouchard et al has shown that
placental Leptin gene DNA methylation levels were correlated with glucose levels in women
with impaired glucose tolerance. One hypothesis is that maternal hyperglycaemia leads to
foetal Leptin gene DNA demethylation which leads to higher mRNA levels and higher leptin
levels, possibly promoting leptin resistance and obesity development. [33] This is largely
speculative but does provide a potential epigenetic mechanism to explain the detrimental
health effects associated with maternal obesity in pregnancy.
IGF2
The IGF2 gene (maternally imprinted) plays an important role in the regulation of growth
during gestation and is highly regulated by its methylation status. The Dutch Famine is
probably the best human example of the differences in IFG2 DNA methylation. [34] An
increased level of IGF2 protein in cord blood is associated with low levels of IGF2 methylation,
which is a stronger association in children of obese women. [35] This was shown in a study
when infants in the heaviest weight category at 1 years old had higher percentages of
methylation of IGF2 regulator H19 in cord blood. [36] Higher circulating IGF2 protein levels
have been associated with obesity in adults.
RXRA
Epigenetic DNA methylation, specifically CpG methylation, of specific gene promoters in
neonates has been linked to child obesity; a study found that greater methylation of retinoid
X receptor-α gene (RXRA) in children at birth was associated with higher adiposity in later
childhood. [37] In addition to RXRA, eNOS, SOD1, IL8 and PI3KCD in umbilical cord tissue have
all been positively correlated to DXA-determined fat mass.
Histone Modification
Foetal histone H3 serves as an epigenetic mark for active or inactive chromatin. A study has
shown that a significant in utero exposure of a caloric-dense high-fat maternal diet in
primates induces site-specific alterations in foetal hepatic H3 acetylation. [38] The results
obtained suggest that a caloric-dense maternal diet leading to obesity epigenetically alters
foetal chromatin structure in primates via histone modifications.
This lends a molecular basis to the foetal origins of adult disease hypothesis as proposed in
Barker’s Hypothesis. [9,39] Obesity in the mother can lead to gene-specific alterations in the
histones of her baby, which may explain the vulnerability of the offspring to later obesity and
metabolic disease.
Bariatric Surgery
Further evidence of potentially modifiable epigenetic factors in obesity mothers came from a
study where 113 obese mothers were given biliopancreatic bypass surgery and their offspring
were observed up to 18 years of age. The outcome demonstrated that pre-pregnancy bariatric
surgery lowered the risks of obesity related diseases in offspring. [40]
Targeting the obesity epidemic could begin in the womb
If obesity in the mother does influence the methylation process this could predispose
offspring to many health problems. In conclusion, increased adiposity in pregnancy leads to
many complications. We still do not know if DNA methylation status is an important mediator
between maternal obesity and pregnancy. Further research is needed to show that these
epigenetic modifications influence protein expression and if they occur at conception or postconception.
GESTATIONAL DIABETES
GDM is defined as glucose intolerance which begins or is first detected in pregnancy. [41] As
high pregravid BMI is a major risk factor for developing GDM, it is becoming a more common
problem with rising obesity rates. It has many well understood complications for both mother
and baby such as macrosomia, shoulder dystocia and pre-eclampsia. [42] These complications
are suggested to be directly related to the hyperglycaemia experienced in GDM.
As well as negatively affecting foetal growth and parturition, in utero hyperglycaemia has
long-term consequences on the offspring’s anthropometric and metabolic development. A
longitudinal study found that at eight years old, the children of GDM mothers were 30%
heavier than expected for their height, suggesting that exposure to GDM may predispose
them to obesity later in life. [43] Other studies show that such offspring are also at increased
risk of developing diabetes; resulting therefore in a vicious cycle. [44]
Maternal obesity is a risk factor for pregnancy complications regardless of diabetes status,
and is thought to affect the epigenome of the offspring. It remains unclear whether GDM has
an impact independent to obesity. The majority of studies tend to look at the effects of
maternal obesity and gestational diabetes in conjunction, and therefore few papers directly
addressing the impact of GDM on imprinting and the offspring’s epigenome exist.
It is hypothesised that exposure to GDM can affect the offspring’s epigenome, leading to
abnormal development. GDM may lead to disruption of glucose transport through alterations
in DNA methylation levels. One study found reduced mRNA and protein expression of GLUT1,-2 and -3 in pre-implantation mouse embryos from GDM mothers. It is thought that the
hyperglycaemic environment downregulates glucose transport causing a fall in free
intracellular glucose, which may lead to glucose deprivation in critical times of foetal
development. [45]
Imprinted Genes in the Offspring
Evidence of GDM affecting the expression of imprinted genes exists. A study investigated the
effect of GDM on the epigenome by collecting cord blood and chorionic villi (placental) tissue
from newborns of insulin dependent GDM mothers, dietetically treated GDM mothers and
non-GDM mothers. [46] This tissue was then used to study the methylation levels at the DMRs
of various genes. Multivariate ANOVA models were used to control for confounding factors
and therefore the observed effects are unlikely to be caused by the higher maternal BMI in
the GDM group. The only significant difference between GDM and non-GDM offspring was
the hypomethylation of the maternally imprinted MEST gene, showing relaxed imprinting at
the maternal allele. Overexpression of MEST has been shown to lead to adipocyte
hypertrophy and an increase in fat mass. [47] This hypomethylation of MEST was also found
in the blood of obese adults, supporting the idea that GDM may program an increased risk of
adult obesity in utero. [46]
Imprinted Genes in the Placenta
Abnormal methylation of imprinted genes was also observed in another study. [45] The
methylation of H19 was decreased and PEG3 increased in the placenta of diabetic mice at mid
gestation. The expression of the paternally imprinted H19 was increased, which could lead to
decreased birth weight in the offspring. In this study, these abnormal methylation and
expression patterns were seen only in the placenta and not the foetus. This dysregulation of
the H19/IGF2 imprinting region has links to obesity which are explored in the paternal obesity
section of website. The non-imprinted LEP gene and its receptor were also abnormally
expressed by the GDM placenta. The evidence suggests that these are also important factors
in the foetal programming of obesity.
As there is a lack of research it can be difficult to draw conclusions, but the current evidence
suggests that maternal GDM affects the epigenome of the offspring, including altering the
methylation and expression of the imprinted genes H19 and MEST. It also appears that GDM
can have effects independent to maternal obesity and confers additional risks. GDM is
therefore an important complication of maternal obesity, which may through altered gene
expression cause an increased risk of metabolic disorders to the offspring in adulthood
(Fig.11).
PRE-ECLAMPSIA
What is Pre-eclampsia?
Pre-eclampsia is a pregnancy-associated disease characterized clinically by onset of
hypertension and proteinuria after 18 weeks gestation. During decidualisation, extravillous
trophoblast cells invade into the tunica media of spiral arteries and remodel them. This
creates a low resistance, high flow system for nutrient transport to the growing foetus
(Fig.12). However in pre-eclampsia, this process is inadequate which leads to vasoconstriction
and subsequently to the clinical picture seen. [48]
Links between Pre-eclampsia and Obesity
Obesity is a strong risk factor for pre-eclampsia and obese women have a 2-3 times increased
risk of developing it. This measure is based on pre-pregnancy weight, highlighting that obesity
can affect the foetus and placenta at all stages of their development. [49] This concept is
supported by the fact that obesity increases the risk of both early onset pre-eclampsia (EOPE),
starting before 34 weeks, and late onset pre-eclampsia (LOPE), starting after 34 weeks. [50]
Despite these links, the mechanism triggering pre-eclampsia remains elusive. [51] As
discussed in earlier sections, there is strong evidence that epigenetics may mediate the
changes seen in offspring of obese women and therefore, the same may be true of obesity
and pre-eclampsia.
What Impact May Epigenetics Have On Pre-eclampsia?
The placenta has been shown to contain the highest levels of imprinted genes during
development and therefore it is possible that dysfunction of these genes could result in preeclampsia. Since it has been shown that not all epigenetic marks are removed during
gametogenesis, it is possible that epigenetic variations could be passed from one generation
to the next. [52] This is important, since the placenta derives from embryonic tissue as
opposed to maternal tissue and therefore underlying placental insufficiency in pre-eclampsia
may be due to the trans-generational inheritance of imprints. [50]
To assess whether a link exists between epigenetics and pre-eclampsia, studies of placental
biopsies at delivery have been compared to assess methylation levels. The study found a >5%
difference in the methylation levels between preterm and term patients with pre-eclampsia
and controls, which related to 229 gene loci. As this number of genetic variations is beyond
the scope of this website, we focus on a smaller set of maternally expressed genes with links
to pre-eclampsia. [53]
CDKN1C
CDKN1C is a maternally expressed gene in mice and humans that has been linked to preeclampsia. [50] In mice models when CDKN1C heterozygous females mated with wild type or
CDKN1C heterozygous males, they developed proteinuria and hypertension, suggestive of
pre-eclampsia. Furthermore, the pups without CDKN1C had poor trophoblast invasion and
intermediate trophoblast hyperplasia, reflective of the clinical picture of pre-eclampsia. [54]
Moreover, studies in children up to age nine have identified the impact of this. For every 1%
increase in the methylation of CDKN1C, there was a 2.08% increase in BMI, which was found
statistically significant. [55] This increased methylation can be shown to reflect the mouse
model, since methylation inactivates gene expression, giving them a phenotype similar to the
pups without CDKN1C. [54] Furthermore, obese adults have been shown to have
downregulated expression of CDKN1C in their adipose fat tissue, highlighting that the early
changes seen may carry on into adulthood and alter the genes expression during pregnancy.
[56]However, mice studies have been unsuccessful, as when obese mice were given either a
controlled fat diet or a high fat diet, the high fat diet group failed to conceive. Therefore, the
relationship between obesity and CDKN1C imprinting is difficult to determine. [57]
11B HSD2
11B HSD2-gene is altered by epigenetic mechanisms in obese mothers. [58] Its main function
is to convert active cortisol into inactive cortisone in the placenta (Fig.13), which is critical
since the circulating volume of active glucocorticoids in the maternal blood is 1000x higher
than in the foetus. [59] The placentas of pre-eclampsia patients have been assessed and
shown to possess lower levels of this enzyme compared to controls. [60] This means that
more maternal cortisol is able to cross the placenta, which has been postulated to cause many
long-term foetal problems. [59] In sheep models, transient exposure to exogenous steroid
reduced nephron numbers in the foetus and predisposed to kidney disorders and
hypertension in later life. [61] This may explain the epidemiological finding that the offspring
of pre-eclampsia patients have a higher risk of developing hypertension and pre-eclampsia.
[62] Obesity in these children is also more common which may reflect animal studies showing
that rats exposed in utero to high corticosteroid levels had an increased body weight and a
30-40% increase in adipose tissue size, supporting the idea that 11B HSD2 changes may lead
to alteration in the foetus. [63,64]
PATERNAL OBESITY
The in utero environment impacts and infant’s life and can predispose them to disease. For a
long time it was thought that this was solely due to maternal factors with the male only
supplying a set of chromosomes. However, a relatively new concept surrounding paternal
obesity and epigenetics has emerged.
In Drosophila it was found that dietary interventions in males could change the body
composition of the offspring. High levels of dietary sugar lead to obese offspring and also
alterations in gene expression, suggesting epigenetic modifications occurred. [10]
This captivating new concept of paternal obesity affecting the offspring and the maternal
environment that the offspring is exposed to is explored here.
WHY DO FATHERS MATTER?
Paternal Obesity Effect on Offspring Health
Recent research has shown that fathers contribute more than just genetic information to their
offspring. The health of the father at the time of conception impacts a set of genes termed
imprinted genes. Due to the growing prevalence of obesity a lot of work has gone into
observing the effects of paternal obesity on general offspring health.
Adipocytes are lipid-storing fat cells that can be affected by changes in metabolism and
hormones in early infancy, acquiring excess fat leading to childhood obesity. At this time
period insulin is an important regulator of fat accumulation. Variations in the Insulin gene
(INS) have been shown to affect insulin secretion. During foetal development INS expression
is controlled by the paternal allele. This indicates a link between paternal gene expression and
insulin secretion which in turn controls weight. It has been shown that with a class III INS gene
the infant has lower insulin production and a higher risk of DMII. The most common class of
the INS gene in the population is class I which predisposes the offspring to obesity. A variable
number tandem repeat in the 5’ region of the INS gene alters not only the INS gene, but also
IGF2 which is a well researched paternally expressed gene. [65]
Investigations into the IGF2 signalling pathway have brought to light roles of IGF2 in glucoseregulated metabolism and energy expenditure. These roles have effects on foetal growth and
size. [66]
Paternal obesity has been suggested to significantly reduce DNA methylation at three points:
Mesoderm-Specific Transcript (MEST), Neuronatin (NNAT) and Paternal Expressed Gene 3
(PEG3). It is important to note that in this experiment the researchers controlled for maternal
age, smoking, education, newborn’s gender and race. [4]
Looking into offspring from a cross of male B6 mice (susceptible to high fat diet induced
obesity) and female PWK mice (resistant to high fat diet induced obesity) gave insight into the
role of PEG3 and IGF2. These offspring (Offspring set 1) were compared to F1 generation of
female B6 mice and male PWK mice (Offspring set 2), shown in table 3. Not only were set 1
offspring more sensitive to high-fat diet-induced obesity, they also had a down regulation of
PEG3 and IGF2. This implicates that IGF2 and PEG3 genes have a function in the paternal
transmission of high-fat diet-induced obesity. [67]
An environment where there is abundant or insufficient food available during a male’s slow
growing phase (a period of time before puberty were environmental factors have a greater
impact on the body) affects offspring health. If there is insufficient food offspring have a lower
risk of cardiovascular mortality. If there is abundant food the risk of diabetes is increased. [68]
Paternal obesity has also been implicated in disorders previously thought to be solely
maternally controlled, such as autism and Asperger’s syndrome. A study found that paternal
obesity increased the risk of offspring being on the Autistic Spectrum by 73%, and doubled
the risk of offspring having Asperger’s syndrome. This study took into account maternal
obesity, sociodemographic and lifestyle factors and found that paternal obesity was a larger
risk factor for Autism and Asperger’s than maternal obesity. [69]
Paternal obesity could impact multiple generations. A study in mice in which the fathers were
on a high fat diet and their female offspring were on a control diet (limiting environmental
effects on the results) showed negative outcomes in blastocyst development, cumulus cells,
oocytes and ovaries. Female offspring had significantly higher trophoectoderm cell number
and a lower proportion of the blastocyst was inner cell mass compared to females whose
fathers were fed a control diet. Cumulus cells and the ovary had increased expression of
glucose transporters. Females showed a subfertility phenotype with impaired embryo
development and quality along with raised lipid content seen in cumulus-oocyte complexes.
This study also showed that male mice on a high fat diet had reduced sperm motility
themselves. [70] The detrimental effect on offspring germ cells could further add to the
transgenerational phenotype of obesity.
POTENTIAL EPIGENETIC MECHANISMS
Obesity in men is associated with elevated levels of oestrogen, inflammation and increased
oxidative stress, all of which can alter DNA methylation profiles. Furthermore, a wide variety
of environmental exposures modify DNA methylation patterns in sperm and therefore
influence the offspring.
It has been suggested that the re-establishment of DNA methylation occurs in male
spermatogenesis from primordial germ cell stage to fertilisation via epigenetic modifications.
This leads to altered gene expression in the offspring and therefore could indicate a
transgenerational epigenetic mechanism for obesity. Soubry et al have proposed a ‘Four
Windows of Susceptibility’ hypothesis. These are four stages of vulnerability that male germ
cells are exposed to throughout the reproductive cycle (Fig.17). [6]
IGF2
The most well-established imprinted gene that is influenced by paternal obesity is IGF2, which
as mentioned previously, affects foetal size and growth. IGF2 is a maternally imprinted, 67
amino acid chain plasma protein with a similar primary and tertiary structure to insulin. It
promotes placental nutrient transport and foetal growth and its expression is closely balanced
with another imprinted gene, H19. [71] On the paternal chromosome the H19 promoter
region is hypermethylated and inactive (Fig.18). In balance, although the IGF2 region is not
hypermethylated on the maternal chromosome, it is silenced by the CTCF binding protein.
[72]
Both the foetus and the labyrinth trophoblasts of the placenta produce IGF2 and in utero this
binds to both IGF type1 (IGF1R) and type2 (IGF2R) receptors. [73] IGF1R exerts the biological
effect of IGF2 for foetal growth and development while IGF2R binds and degrades IGF2. The
high prevalence of both receptors on the placenta implies paracrine release of IGF2 influences
placental growth and function. [71]
The NEST birth cohort brought to light an inverse association between paternal BMI and IGF2
methylation in umbilical cords, independent of maternal BMI. Low levels of IGF2 methylation
in offspring correlate with an increase in circulating IGF2 level, demonstrating how
reprogramming of paternal methylation profiles during paternal spermatogenesis can impact
offspring growth.
Oestrogen’s Role
Animal studies have shown IGF2/H19 methylation is controlled by oestrogen, which is
produced by adipocytes. This may expose the mechanism by which paternal obesity causes
hypomethylation of IGF2, though more research to fully establish this is necessary. [6] IGF2
promotes development of foetal pancreatic cells, thus this is thought to be the connection to
offspring’s predisposition to diabetes mellitus. [74]
The erasure and re-establishment of imprinting marks, such as the IGF2 and H19 locus, during
spermatogenesis is emerging as an area vulnerable to disruption, for example due to paternal
obesity. Alterations of IGF2 transcription as a consequence of this could be responsible for
the increased risk for obesity and adult-onset diseases such as DM2 and hypertension that
children of obese parents are exposed to. [72]
Whilst the IGF2 gene is likely to have a role in the trans-generation phenotype of obesity, it is
likely that other paternally expressed genes are involved. This presents an area for new
research to enable further clarity into this complex mechanism.
DISCUSSION & CONCLUSION
Since DNA was identified as a vehicle of trans-generational genetic transfer, DNA alterations
have been central to parental and child inheritance. However the rise of epigenetics has
added a new dimension to our understanding of DNA mechanisms, with environmental
exposure now being implicated. Epigenetics denotes molecular mechanisms independent of
the DNA sequence that refer to heritable but also reversible regulation of gene transcription.
Epigenetic modification of imprinted genes, a small subset of the genome, is thought to affect
offspring growth and development, since only one active copy can be carried to the next
generation, thus making it vulnerable to change. Previous beliefs that complete erasure of
epigenetic marks occurs have been shown to be false, and it is now apparent that some marks
remain and can therefore be inherited. Obesity during pregnancy therefore exposes offspring
to an abnormal environment, altering the epigenome, and consequently influencing their
phenotypic characteristics.
Epigenetic changes are just one possible mechanism for why children of obese parents have
an increased predisposition to obesity and related diseases. Other mechanisms may include
the in utero and postnatal environment, pre-determined genetics and dysregulated nutrient
transfer. However the role of epigenetics, highlighted by ongoing research, appears to be
significant in programming future offspring health. The most established evidence of this
comes from studies of maternal obesity in pregnancy. The Dutch Famine in 1944 showcased
that the offspring of malnourished parents had differences in IGF2 methylation. [8] Animal
models further indicate that epigenetic windows exist where altered methylation can occur,
particularly in IGF2 and RXRA expression. Strong links also exist between maternal obesity
and the epigenetic mediation of pregnancy-associated disorders, such as GDM and preeclampsia. GDM alters methylation and expression of placental and foetal imprinted genes,
particularly H19 in the placenta and MEST in the foetus. This confers an additional risk, on top
of maternal obesity, for the offspring to develop obesity and/or type II diabetes in later life.
The high levels of imprinted genes in the placenta have also suggested an epigenetic role in
pre-eclampsia. Whilst a link between obesity and pre-eclampsia has been established, the
mechanisms behind this require further research to assess how gene expression differences
in the two groups. Current research has focused on the maternal regulation of the in utero
environment and methylation changes that occur during pregnancy. However, new evidence
supporting the existence of oogonial stem cells may expose a new area of maternal epigenetic
reprogramming; whereby changes in gene expression could occur from the primordial germ
cell stage in development. [75]
The paternal influence on offspring is another emerging field in epigenetics. Compelling
evidence indicates that fathers could contribute to epigenetic phenotypic changes in offspring
and the placenta, therefore playing a role in diseases such as pre-eclampsia. The ‘paternal
environment legacy,’ coined by Soubry et al, offers feasible explanations for the epigenetic
inheritance of obesity through the male germ line. [28] Several studies demonstrate this since
significant hypomethylation of IGF2 is seen in newborns with obese fathers, even when
several maternal contributions were accounted for. [6] Males continuously produce germ
cells throughout their reproductive life, thus they are vulnerable to epigenetic modifications
at any post-pubertal point, which can be passed to their offspring. Consequently, more
research is required to understand the potential underlying mechanisms.
Limitations
Since epigenetics is a novel concept, more work is required to grasp its full potential in genetic
and disease programming. This is complicated by the fact that controlling exposures, such as
nutrition, in the postnatal environment is hard. This therefore makes long-term assessment
of specific gene alterations difficult. Currently, heavy reliance on animal models also means
that identified epigenetic alterations may not reflect human findings. Furthermore, since our
understanding is limited, it may be the case that exploring CpG sites only represents a fraction
of the changes occurring in the genome.
Future Research
Epigenetic research could also have implications for new therapeutic targets. Identifying
markers of adverse pregnancy outcomes, such as GDM and pre-eclampsia, would allow earlier
detect and intervention. Furthermore, the reversibility of epigenetic mechanisms could
provide a starting point for breaking the vicious cycle of obesity. By reducing maternal obesity,
potentially through bariatric surgery, we could reduce offspring obesity, thus fighting the
obesity epidemic from the womb. A recent study of metformin use in pregnant, obese women
showed no difference in the birthweight of their offspring compared to controls. However,
since long-term follow-up has not occurred, metformin may improve outcomes for the
offspring in later life.
In males it is hypothesised that epigenetic modifications occurs from the primordial stage of
spermatogenesis. Therefore if oogonial stem cells are proven to regenerate oocytes, research
into potential epigenetic modifications of these cells could alter our understanding of transgenerational obesity.
Conclusion
Obesity and associated diseases are global issues. As it is becoming more and more apparent
that obesity traits may be foetally programmed, it raises concerns that future generations will
be unavoidably obese, thus the obesity burden will continue to rise. Substantial evidence
now demonstrates that gamete development and the foetal environment strongly influence
an individual’s obesity risk and that altered epigenetic regulation of specific genes is central
to this. Both parents contribute to permanent epigenetic marks, and so even if only one
parent is obese, the offspring’s epigenome is altered and the vicious cycle of obesity is
continued.