Endocrine

, Volume 48, Issue 1, pp 36–46

Epigenetic effects of paternal diet on offspring: emphasis on obesity

Authors

  • Yuriy Slyvka
    • Department of Biomedical Sciences, HCOMOhio University
    • The Diabetes InstituteOhio University
  • Yizhu Zhang
    • Department of Biomedical Sciences, HCOMOhio University
    • Department of Biomedical Sciences, HCOMOhio University
    • Program in Molecular and Cell BiologyOhio University
    • The Diabetes InstituteOhio University
Review

DOI: 10.1007/s12020-014-0328-5

Cite this article as:
Slyvka, Y., Zhang, Y. & Nowak, F.V. Endocrine (2015) 48: 36. doi:10.1007/s12020-014-0328-5

Abstract

Overnutrition, obesity, and the rise in associated comorbidities are widely recognized as preventable challenges to global health. Behavioral, metabolic, and epigenetic influences that alter the epigenome, when passed on to offspring, can increase their risk of developing an altered metabolic profile. This review is focused on the role of paternal inheritance as demonstrated by clinical, epidemiological, and experimental models. Development of additional experimental models that resemble the specific epigenetic sensitive situations in human studies will be essential to explore paternally induced trans-generational effects that are mediated, primarily, by epigenetic effects. Further elucidation of epigenetic marks will help identify preventive and therapeutic targets, which in combination with healthy lifestyle choices, can diminish the growing tide of obesity, type 2 diabetes, and other related disorders.

Keywords

EpigeneticMetabolic diseaseObesityPaternal dietTrans-generational

Introduction

According to recent data, 35.7 % of adults in the United States are obese [1]. There is no significant difference in prevalence between men and women at any age. More than 37 million men and almost 41 million women aged 20 and over are obese in the US alone. In addition, 16.9 % of US children and adolescents are obese. In 1999–2000, 27.5 % of men were obese, and by 2009–2010, the prevalence had increased to 35.5 %. Among women, 33.4 % were obese in 1999–2000 with no significant change in 2009–2010 (35.8 %). The prevalence of obesity among boys increased from 14.0 % in 1999–2000 to 18.6 % in 2009–2010. There was no significant change among girls: the prevalence was 13.8 % in 1999–2000 and 15.0 % in 2009–2010 [2]. The obesity rate of US adults in 1960–1962 was 13.4 %. The rate tripled over the next 50 years [3]. A similar threefold increase in the prevalence of overweight and obese children and adolescents has been observed over the same time span. Worldwide, it is reported that there are greater than 155 million overweight and around 40 million obese children and adolescents [4]. These increases in obesity in recent decades have occurred too rapidly to be explained completely by genomic DNA mutation or selection. This suggests the involvement of other causes including epigenetic modifications of gene expression [5], in addition to and, possibly, as a result of, caloric imbalance imposed by lifestyle choices that may include high food consumption and low physical activity. Epigenetic modifications can occur within the lifespan of numerous individuals within a population and thus be transmitted immediately to a large number of offspring in the next generation, unlike genomic events that spread slowly through a population.

Intergenerational transmission of obesity risk occurs between parents and children [6] and between grandparents and grandchildren [7]. Based on studies in twins, it has been established that genetic inheritance contributes to 40–75 % of obesity cases [8, 9]. This includes monogenic, multigenic, and epigenetic contributions.

Below, we present an overview of epigenetic regulation and transmission of adaptable markers that may alter metabolic processes in offspring, with an emphasis on obesity and paternal inheritance. Related clinical and epidemiological studies are summarized, as are experimental data from animal models.

Epigenetics: definitions and mechanisms

Definitions

Epigenetics is defined as changes in gene expression that occur without altering the DNA sequence and can be transmitted through mitosis and/or meiosis [10, 11]. All cells in the individual organism contain the same DNA sequence but not all express the same genes or accomplish the same functions. Epigenetics can be thought of as those processes that regulate gene expression in a given cell leading to its cellular transcriptome and phenotype. The sum of the chemical modifications of the DNA templates in the organism that lead to changes in gene expression constitutes the epigenome. Epigenetic modification is a continual process, and some changes may be reversible.

Epigenetic mechanisms: crafting the epigenome

The main epigenetic mechanisms that are currently known to regulate gene expression in humans and mammals are summarized in Table 1 [5, 1122].
Table 1

Mechanisms of epigenetic regulation

Mechanism

Details and examples

DNA methylation

Occurs predominantly in the fifth carbon of cytosines that are followed by a guanine. Adenosine can also be methylated

Histone modifications

These include methylation, acetylation, ubiquitination and sumoylation of lysine, phosphorylation of serine and threonine, and methylation of arginine

DNA-associated nuclear proteins

These include proteins that are critical components of chromatin remodeling complexes, several classes of effector proteins that facilitate different types of histone modifications, and insulator proteins

Genomic imprinting

This limits expression of a gene to one of the two parental alleles

Non-coding RNAs

These include microRNAs, picoRNAs, and long non-coding RNAs, which can bind to and regulate multiple mRNAs, and possibly prions

Non-covalent mechanisms

Examples are physical alterations in nucleosomal positioning via nucleosome remodelers or replacement of canonical histone proteins with specialized histone variants such as H3.3 and H2A.Z

Impact of environment on epigenetic patterns

Specific epigenetic patterns condition the accessibility of chromatin to transcription factors, facilitating the distinction between genes that are to be expressed to various extents and genes that are to be silenced, transiently, or permanently [23]. These include covalent modification of histones by phosphorylation, methylation, acetylation, and ubiquitination. Factors in both the internal and external environment direct the epigenetic programming of gene expression. These factors include dietary composition and caloric intake, physical activity, social stressors, environmental toxicants, medication, hypoxia, inflammation, aging, metabolic and hormonal disorders, and type and level of psychosocial interactions [14, 15, 24]. Susceptibility to epigenetic modification increases during certain critical windows of development which include the preconception period of gametogenesis, pre-implantation embryo development [2527], in utero gestation, puberty, and advanced age [7, 11, 14, 15, 28, 29].

Epigenetic modifications can be heritable

Epigenetic chromatin modifications can be propagated mitotically or meiotically; the latter results in stable inheritance of metabolic traits. Although many sperm chromatin modifications are erased post-fertilization, some persist into the embryonic stage, supporting the hypothesis that paternal epigenetic modifications can be transmitted to subsequent generations. Trans-generational transmission of epigenetic changes via the paternal line has been shown to reside in the mature spermatozoa through nuclear siRNAs, PIWI-interacting RNAs, the pattern of cytosine methylation of sperm DNA, and acetylation of lysine residues in nucleohistones and in the chromatin structure [15, 19, 22, 3033].

Most histones are replaced by protamines at the end of spermatogenesis with approximately 10 and 1 % histone retention in human and mouse sperm, respectively. A majority of transcription factor binding sites are located in the 1 % of the human genome that comprises the intragenic and intergenic accessible regions [34]. The retained histone methylation patterns in sperm are transmitted to offspring, with high expression of paternal basic housekeeping genes in sperm and early embryo, and low expression of genes that regulate differentiation [35]. Operant mechanisms identified to date in mammals include distribution of histone H3 variants and retention of specific methylation and acetylation patterns on histones, especially H3K27me3 and H3K4me2/3, and H4K12ac [3537]. Genes bearing high H3K4me2/3 are significantly enriched in paternal loci expressed in 4-cell and 8-cell human embryos [32], while H3K27me3 plays a role in paternal transmission of the repressed state [35]. Retention of paternal H3K27me3 methylation patterns in early embryos promotes repression of genes involved in differentiation and helps maintain totipotency in propagating cells [35]. Of note, the methylation patterns are highly conserved among fertile men, but alterations in the methylation status at specific loci for transcription factors and promoters of genes involved in embryo development have been observed in infertile men [38]. When taken together, these data support an effect of paternal trans-generational transmission of histone-encoded epigenetic information on phenotypic variation in offspring.

Sperm histones are significantly enriched at the promoters for microRNAs (miRNAs) [32]. MiRNAs can regulate DNA methyltransferases, histone deacetylases, and acetyl transferases, enzymes which help regulate DNA structure and gene transcription [39, 40]. MiRNAs interact with the 3′ UTR of specific target mRNAs to induce their translational repression, degradation, or deadenylation [40]. It is known that the circulating miRNA profile of obese men differs from that of non-obese men, including miRNAs associated with genes that regulate adipocyte development and function, as well as markers for acute and chronic inflammation [41]. As sperm miRNAs have been shown to respond to diet [19] and can be transmitted to the developing embryo at fertilization, influencing offspring phenotype [31], this is another potential epigenetic mechanism for effects of paternal high fat diet (HFD).

Diet affects epigenetic marks in sperm

Diet-induced obesity in C57Bl6 mice is associated with increased reactive oxygen species and DNA damage in sperm [42], resulting in germline effects including poor fertilization rates and impaired embryo development and implantation [4244]. Binder et al. [45] found embryos of obese male mice had impaired mitochondrial function and blastocysts developed with a decreased inner cell mass:trophectoderm ratio, resulting in decreased rates of implantation and impaired fetal development. Paternal HFD has been shown to result in altered expression of regulatory microRNAs in sperm and global hypomethylation of sperm DNA [46]. In addition, several studies provide clear evidence that HFD-induced obesity in C57BL6 male mice results in subfertility in both male and female offspring for two generations [4648]. Diminished reproductive and gamete functions are transmitted through the first generation paternal line to both sexes of the second generation and via the first generation maternal line to second-generation males [47]. Interestingly, these effects can be reversed by paternal dietary fat reduction and exercise [4850]. The Tet family proteins, Tet1 and Tet3, have been shown to play roles in chromatin demethylation following fertilization in humans and mice, and progeny of paternal Tet1 knockout mice exhibit developmental abnormalities, with specific hyper-methylated sites observed in both sperm and embryos [51, 52]. However, the complete set of changes in the epigenetic code due to paternal diet and obesity and the mechanisms for their reversal remains to be determined.

In humans, increased BMI in males is associated with decreased blastocyst development and live birth rates after in vitro fertilization (IVF) [53]. Increased reactive oxygen species in sperm, increased seminal fluid neopterin, a marker of reproductive tract macrophage activation, decreased sperm counts and serum testosterone, and increased serum estradiol are found in men with BMI >25 kg m−2 [54]. The impacts of obesity on male fertility, sperm function, and molecular composition are summarized in an earlier review [55].

Dietary exposure can influence subsequent generations

When considering the scope of environmental effects, it is necessary to differentiate direct paternal germline effects (individual, F0 generation) from those that are multigenerational, affecting both parent and first generation offspring (F0 and F1) or trans-generational. A true paternal trans-generational effect would be manifested in offspring from sperm produced in a gonadal environment that has not been exposed to persistent dietary modification, i.e., F2 and beyond [18, 30]. This is in contrast to inheritance from the maternal founder where true trans-generational effects manifest in the F3 generation [56, 57].

Many studies of the epigenetic effects of obesity have centered on factors modified in the affected individual during his personal development [14, 15]. Fewer studies have focused on transmission of epigenetic changes from parent to child and possible mechanisms of this, including formation of metastable epialleles and genomic imprinting. A majority of reported multigenerational and trans-generational epigenetic phenomena have been related to maternal transmission, while the paternal epigenome has been comparatively neglected [5, 30, 31, 58, 59]. The importance of studying paternal epigenetic effects lies in the fact that 50 % of autosomal genes are inherited from each parent, and expression of the single functional allele of an imprinted gene that is silenced in one allele, is parent-of-origin dependent. From this point of view, it is important and timely to consider the epigenetic effects of paternal diet, especially the overnutrition that has arisen with certain modern life styles and may well be having potent health effects on subsequent generations [5, 60].

Epidemiological and clinical findings

Epidemiological and clinical studies provided the first evidence of paternal multigenerational and trans-generational epigenetic effects. Different effects based on age and sex of offspring and status of fathers were observed.

Grandparental diet is linked to sex-specific phenotype in offspring

There is compelling evidence that epigenetic marks, including DNA methylation, vary between tissues, individuals, and disease conditions in humans. It is likely that changing circumstances within the individual or over several generations can recruit silent alleles back into the active genome and contribute to the reversibility of adaptive or acquired changes. A recent study in obese men showed changes in circulating microRNAs that target VEGF, an adipocyte mitogen, that was reversible following weight loss [41].

Very interesting trans-generational effects were described based on tracking three cohorts born in 1890, 1905, and 1920 in Överkalix parish in Northern Sweden until death or 1995. Access to food for parents and grandparents during their slow growth period (SGP) was determined by referring to historical data on harvests and food prices, records of local community meetings, and general historical facts. The age of SGP was defined as 8–10 years for girls and 9–12 years for boys [7, 61, 62]. When the father (p = 0.046) was exposed to a famine during his SGP, the child was protected against cardiovascular causes of death. Furthermore, if the paternal grandfather lived through a famine during his SGP, it tended to protect the grandchild from diabetes (p = 0.09). If the paternal grandfather had access to a surfeit of food during the SGP, the grandchildren had a fourfold over-risk for death of diabetes mellitus according to the point estimate (p = 0.01). The authors concluded a nutrition-linked mechanism through the male line appeared to influence the risk for cardiovascular and diabetes mellitus mortality in both genders in subsequent generations [61].

An association was also established between overall longevity and food availability during the paternal grandfather’s SGP [62] whereby increased food availability to the grandfather decreased survival of the grandchild. Analysis of the three-generation Överkalix parish data by sex of the grandchildren shows striking sex-specific effects; the paternal grandfather’s food supply was only linked to the mortality risk ratios of grandsons, while paternal grandmother’s food supply was only associated with the granddaughters’ mortality risk ratio. The absence of an association in the reverse paternal grandparent/grandchild pairings provides an important internal control for paternal-line social economic confounders, as the presence or absence of the trans-generational effect involves transmission through the same fathers [7]. Analysis of these results was not affected by the grandchild’s own childhood circumstances. Differences in grandparental diet during other time periods of childhood did not have male-line trans-generational effects. Effects of paternal diet on offspring longevity were also observed. However, these effects were dependent on offspring childhood circumstances. Good food availability for fathers corresponded with decreased life span in daughters when data were not adjusted for offspring circumstances, but when adjusted, the decreased life span was seen only in sons [7].

Paternal and maternal BMI affect BMI of offspring

A study of 127 healthy children (63 girls and 64 boys) showed BMI at 6–18 months of age is a strong predictor of BMI at 4 years. Protein intake at 17–18 months and at 4 years, energy intake at 4 years, and the father’s, but not the mother’s, BMI were independent contributing factors [63]. A study of the complete birth population in Norway between 1967 and 1998 identified 67,795 trios of father–mother–firstborn child. Both maternal and paternal birth weight correlated positively with offspring birth weight. There was an almost linear increase in offspring birth weight as paternal birth weight increases, within categories of maternal birth weight [64]. However, parental BMI at the time of conception was not considered. Others have found that paternal BMI (parental BMIs determined at 28 weeks gestation) had no effect on offspring weight, length, or BMI at birth but was correlated with offspring length at 1 year and offspring weight and BMI at 1 year and 2 years. The authors concluded that paternal BMI has effects on offspring BMI that are independent but additive to effects of maternal BMI [65].

Analysis of 9,377 offspring and their parents from the 1958 British Birth Cohort Study indicated both maternal and paternal BMI positively associated with offspring BMI at age 11 years and did not diminish at 44–45 years in both sexes. These associations remained after adjustment for multiple lifestyle and socioeconomic factors [66]. Excessive gains in paternal BMI in early childhood (7–11 years) and adulthood (16–33 years) were associated with higher BMI and an increased risk of offspring being overweight or obese [67]. Interestingly, parental obesity may more than double the risk of adult obesity regardless of offspring obesity status before the age of 10 [6]. Finally, the combination of increased paternal BMI with diabetes and other signs of metabolic syndrome further increases risk of BMI >95th percentile in offspring [68]. These studies did not consider gender-based assortative effects.

Gender-based differences in obesity inheritance

Very promising results concerning the mechanisms of paternal epigenetic effects on offspring were obtained from the newborn epigenetics cohort study [21]. They suggest a link between increased paternal BMI and hypomethylation at the imprinted insulin-like growth factor-2 (IGF-2), but not H19, differentially methylated regions in DNA isolated from umbilical cord blood leukocytes. A tendency toward an increase in methylation at these two sites was shown in newborns of obese mothers, demonstrating a possible maternal influence on these two paternally inherited alleles. Transcription of IGF-2 may be affected as a result, leading to alterations in metabolic homeostasis later in life.

A study based on 226 healthy trios reported assortative weight gain in mother–daughter and father–son pairs. Large differences in BMI were found among the daughters grouped according to mothers’ category of BMI, but not their sons, and among the sons grouped according to their fathers’ BMI, but not their daughters. The risks of obesity at 8 years were ten-fold greater in girls or six-fold greater in boys if the same-sex parent was obese [69]. In contrast, a case-controlled study of 10–12 year old children in India concluded that maternal obesity mainly passes to boys and paternal obesity to girls [70]. However, several larger studies failed to confirm any gender-based concordance between parent and offspring [7173]. It is not clear if the disparate results are due to sample size, geographic locale, genetic background, or other factors. The study by Whitaker et al. [73] based on pooled data from 4,432 families (7,078 children) did show a stronger maternal effect overall. This study also showed a graded increase in childhood obesity when both parents had an elevated BMI and as BMI increased from normal to overweight, then obese and severely obese.

Longitudinal studies have shown that father’s total and percentage body fat were predictors of baseline percentage body fat and changes in body fat of premenarcheal girls over a 2.7-year period starting at age 7½ [74]. Severity of obesity in both genders at age 15 is correlated strongly with both paternal and maternal BMI [75], determined when the child is between 3–18 years of age. Weight and length of 912 European American children from birth to 35 years and their parental BMI showed that maternal BMI has a stronger influence on offspring BMI during infancy and early childhood than paternal BMI [76].

Gender-related effects on offspring epigenome

Chen et al. [77] analyzed the relationship between paternal BMI and birth weight, ultrasound measurements of fetal growth and umbilical cord hormone levels including cortisol, aldosterone, renin activity, and fetal glycated serum protein in a birth cohort of 899 father/mother/child triads. Paternal BMI correlated significantly with birth weight and perinatal biparietal diameter, head circumference, abdominal diameter, abdominal circumference, and pectoral diameter measured in male offspring. There were no significant correlations between paternal BMI and these parameters in female offspring. Cord blood cortisol level was also associated with paternal BMI in male offspring only. The authors concluded that a sex-specific trans-generational effect of paternal BMI on fetal cortisol secretion may represent a risk factor for cardiovascular disease in male offspring in later life [77].

Different phenotypic effects of genes inherited from the paternal versus maternal side, which predispose to Type 1 diabetes mellitus, provide additional evidence of possible sex-related epigenetic effects. Inheritance of the Class I insulin (INS) VNTR allele from the father increases the risk of early onset obesity by a factor of 1.8. Maternal transmission does not have this effect [78]. Analysis of the transmission of specific INS VNTR alleles in 1,316 families demonstrated that a non-transmitted Class III allele can prevent the predisposition to Type 1 diabetes that is usually conferred by inheritance of the 814 Class I allele. This effect is observed only with paternal and not with maternal transmission [79]. In the case of Type 2 diabetes, maternal transmission of the INSVNTR genotype follows Mendelian law, but paternal transmission shows a clear excess of Class III allele [80]. The selective impact of paternal origin of the INSVNTR in these three metabolic disorders supports classification of the INS VNTR as imprinted.

Offspring phenotype may differ depending on which parent is affected. Low birth weight has been shown to predict later individual development of Type 2 diabetes. Interestingly, in Pima Indians, low birth weight in offspring is associated with paternal diabetes, and it also predicts later development of paternal but not maternal parental diabetes when neither parent is diabetic at the time of the child’s birth [81]. Another study of Pima Indians showed that adult non-diabetic offspring of fathers with early onset of Type 2 diabetes were leaner and had lower early insulin secretion than offspring of either mothers with early onset of Type 2 diabetes or control subjects where neither parent developed diabetes by age 50 years [82]. These findings indicate the important role of paternal heritability in body composition and β-cell dysfunction and indicate a role for epigenetic mechanisms in the predisposition to diabetes.

Stability of epigenetic modifications

Clinical and epidemiological studies regarding the relative contributions of paternal genetics and family environment to body composition and BMI class of offspring have provided conflicting and controversial results. However, several studies of twins reared apart [83] and adopted children [84, 85] clearly support a significant effect of heredity. There is a clear relationship between adoptee weight and BMI of biologic parents, but not BMI of adoptive parents. The authors conclude that childhood family environment alone has little to no effect.

Paternal epigenetic contributions to offspring weight and metabolic profile are complicated by epigenetic marks that may be modified by maternal pre-pregnancy weight and perinatal diet. For example, excess weight gain and hyperglycemia during pregnancy can result in fetal hyperinsulinemia and increased risk of obesity during adolescence. Human studies of epigenetic mechanisms are challenging because of the variety of factors that cannot be controlled yet influence gene expression.

Challenges of human epigenomics

Human studies are complicated by interpersonal and environmental factors, extended timeframes in generational studies, lifestyle and socioeconomic factors, and many ethical considerations that limit the design and may affect the results [58]. To avoid these shortcomings, Lecomte et al. [86] propose that future investigations be designed to more precisely define human cohorts, assay the epigenetic state at multiple time points and in multiple tissues in both parents and offspring, correlate epigenetic changes with differences in gene transcription and phenotype, and apply genome-wide studies of epigenetic marks. Ongoing and future epigenome mapping projects are needed to elucidate the normal variations in the epigenome [34]. We may also turn to animal models to understand the mechanisms of epigenetic modifications as paternal trans-generational messengers, as there is good evidence to support this approach based on studies of environmentally induced maternal inheritance.

Experimental findings

Inherited traits may be epigenomic

Variation in coat color in isogenic fox and mice is among the first described inherited traits demonstrated to be epigenetic [87, 88]. While the agouti mouse inheritance pattern is maternal, the fox star gene expression pattern is transmitted through both maternal and paternal lines. Likewise, both maternal and paternal trans-generational inheritance have been described for the epigenetic state of the murine AxinFu allele. Hypomethylation in the LTR/intron 6 region of this allele results in a kinked tail phenotype [89]. Experimental manipulations of male rodents have also been found to result in metabolic changes in subsequent generations. These include neonatal thyroidectomy, alloxan-induced diabetes, treatment with cyclophosphamide, chromium(III) or methadone, and preconception fasting.

More recently, low paternal dietary folate has been shown to alter the sperm epigenome and gene expression in offspring, possibly via differences in histone or DNA methylation [90].

Effects of perinatal nutrition on metabolism in offspring

Preconception fasting of male mice for 24 h resulted in consistent decreases in serum glucose in male and female offspring compared with those of controls at 10 weeks of age. When fathers were fasted multiple times additional effects were observed. Corticosterone and IGF-1 levels were lower in male offspring, and IGF-1 was higher in female offspring [91].

Maternal perinatal nutritional status has also been reported to result in metabolic changes that persist across at least two generations of offspring via the paternal lineage [57, 92, 93]. The first experimental evidence for trans-generational transmission of impaired glucose tolerance and reduced birth weight through paternal lineage was demonstrated with in utero 50 % maternal caloric restriction from day 12.5 until delivery in ICR mice [93]. Male offspring from calorically restricted mice exhibited reduced birth weight with impaired glucose tolerance due to β-cell dysfunction. These traits were transmitted to the next generation of offspring via the paternal line alone (low birth weight) or maternal line x paternal line combined (impaired glucose tolerance) [93]. Interestingly, hyperinsulinemia, impaired insulin sensitivity, and increased visceral fat emerged only in the 2nd generation when both parents were undernourished in utero. As discussed above, trans-generational effects of maternal diet are best supported by findings that are carried into the F3 generation, and more studies of this type are needed.

Fewer reports are currently available that extend the observation of phenotypic changes in response to maternal diet on to the F3 generation. Maternal HFD affects female grand-offspring body size in mice [57], and maternal protein restriction results in gender-specific alterations in glucose metabolism in F3 rats [56].

Obesity may be determined before conception

Koza et al. [95] demonstrated that differences in mRNA expression of genes that regulate metabolism may be programmed pre- or peri-conceptually. Only a subset of male C57Bl6/J mice fed a HFD (45–58 % kcal fat), approximately 2/3, becomes obese [94, 95]. This appears to be due to a mechanism that causes a permanent change in energy metabolism, possibly due to differences in adipocyte expression of several imprinted genes and predates consumption of the HFD. Maternal high-fat diet exposure in C57BL/6:129 hybrid mice resulted in an increase in body length and reduced insulin sensitivity that persisted across two generations and was transmitted via both maternal and paternal lineages [92]. The greatest effect was seen in the second generation when both parents had high-fat fed dams. Increased body weight and length occurred in F3 offspring as well, but these traits were transmitted only through the paternal lineage and only to female offspring. PCR analysis of an array of paternally expressed imprinted genes in liver of these female offspring showed greater than 50 % fluctuation in expression of five growth related genes, Magel2, Peg12, Peg10, Ins1, and Rasgrf1 [57].

Yazbek et al. [96] provided additional evidence for paternal trans-generational epigenetic effects on body weight and food intake, using a 58 % fat diet. They focused on the obesity-resistant 6C2d congenic strain which carries the Obrq2aA/J allele on an otherwise C57BL/6J background. Various crosses between 6C2d and C57BL/6J showed that the Obrq2aA/J allele in the paternal or grandpaternal generation was sufficient to inhibit diet-induced obesity and reduce food intake in the normally obesity-susceptible, high food intake C57BL/6J strain. The phenotype was subsequently transmitted, in the absence of the Obrq2aA/J allele, through the paternal but not the maternal lineage to the male offspring with equal strength for at least two generations. Eliminating social interaction between the father and both his offspring and the pregnant dam did not significantly affect food intake levels, demonstrating that transmission, in this case, is biological, possibly involving retained histone modifications in the sperm genome [32], and not socio-environmental.

Consumption of a HFD (41 % fat) for 10 weeks, from age 4–14 weeks, by male Sprague–Dawley rats programmed β-cell dysfunction in their female but not male offspring on regular chow [97]. HFD in rat fathers induced increased body weight, adiposity, impaired glucose tolerance, and insulin secretion. Female offspring had an early onset of impaired insulin secretion and glucose intolerance, despite being maintained on normal chow and having normal adiposity. The authors detected altered expression of 61 pancreatic islet genes in functionally enriched pathways involved in cation and ATP binding, cytoskeleton and intracellular transport, calcium signaling, MAPK, Wnt, and Jak-Stat signaling, apoptosis, and the cell cycle. Hypomethylation of the interleukin 13 receptor alpha-2 gene, which showed the highest fold difference in expression (a 1.76 fold increase) was demonstrated in pancreatic islets of female offspring from HFD fed fathers. This provides important confirmation of an epigenetic mechanism for effects of paternal dietary composition on offspring phenotype.

Paternal diet effects on offspring have also been studied with male C57BL/6 mice fed a low-protein diet (11 % rather than 20 % protein, with remaining mass made up with sucrose) [19]. Low-protein diet offspring of both genders exhibited elevated hepatic expression of 445 genes involved in DNA replication and in lipid and cholesterol biosynthesis, as well as decreased hepatic cholesterol, when compared to offspring of control diet males. Epigenomic profiling of offspring livers revealed widespread modest changes in CpG methylation depending on paternal diet, including methylation of a putative enhancer for the key lipid regulator PPARα. Of note is that the gene expression profile of genes that change in offspring is not the same as the genes induced in the parental generation by the different dietary protocols. Differential expression of a number of proliferation related hepatic microRNAs was also observed in offspring from the two diet groups, with up-regulation of miR-21, let-7, miR-199, and miR-98, and down-regulation of miR-210 in the low-protein diet offspring.

More recently, the effects of HFD-induced paternal obesity on the metabolic health of offspring have also been studied in mice. Male C57BL/6 mice (F0) were fed a HFD for 10 weeks to induce obesity without overt diabetes. The F1 female offspring were obese and insulin resistant. Male F1 offspring were insulin resistant and hyperleptinemic but not obese. Altered phenotypes were further transmitted to the F2 generation. Female, but not male, F2 offspring of F1 males exhibited increased adiposity and decreased insulin sensitivity. In contrast, male F2 offspring of F1 females showed increased adiposity and decreased glucose tolerance. Female offspring from this group showed only impaired insulin sensitivity. These differences support the concept of transmission of impaired metabolic health to subsequent generations on the basis of paternal environmental variables and physiology [46].

When the effects of neonatal overnutrition were examined in the ICR-CD1 mouse strain, multiple metabolic derangements were transmitted to the F1 and F2 offspring via the paternal line [98]. Neonatal overnutrition was modeled by adjusting litter size to 8 pups per dam (F0 control), or 4 pups per dam (F0 overnutrition). Overnutritioned males demonstrated accelerated postnatal growth that persisted until adulthood, hypertriglyceridemia, fed and fasting hyperinsulinemia, fasting hyperglycemia, glucose intolerance and insulin resistance. Offspring (F1) derived from mating overnutritioned F0 fathers with external control group mothers also demonstrated fasting hyperglycemia, moderate glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and insulin resistance at 4 months of age. Only mild fasting hyperglycemia and impaired glucose tolerance transferred to the next generation (F2) through the paternal line. Neither F1 nor F2 offspring of overfed fathers exhibited increased body weight, and F1 males displayed a seemingly paradoxical reduction in epididymal fat mass. The fact that the metabolic dysregulation is greatly reduced in second-generation offspring argues strongly for an epigenetic modification of gene expression rather than a change in DNA sequence as the latter would be expected to remain stable across generations [23].

Recent studies in mice compared offspring obtained by in vitro versus in vivo fertilization using sperm from calorically restricted males showed differences in offspring phenotype. Although this raises the possibility that some effects of paternal diet are due to maternal exposure to the males during mating [99] and are still transmitted through the maternal line, the results may also reflect changes introduced by the IVF procedure itself. Finally, diet may also influence the intestinal flora which, in turn, sends signals that alter the testicular environment and possibly the sperm epigenome. Thus, there may be multiple routes whereby environmental factors experienced by parents alter the development of offspring.

Conclusions

Parental diet has been shown in epidemiological, clinical, and experimental arenas to have multiple trans-generational effects on the metabolic profile of offspring through at least two generations. Many of these effects are transmitted through the paternal germ line. It can be expected that the increasing prevalence of diet-induced obesity in parents affects obesity and related metabolic syndromes in the children. Therefore, development of experimental models that resemble the specific epigenetic sensitive situations in human studies is essential. Maternally induced trans-generational effects are mediated by a complex interplay of metabolic, mitochondrial, in utero fetal programming, epigenetic, and social factors, whereas paternally induced trans-generational effects can be studied in a model where they are mediated, primarily, by epigenetic effects. Further elucidation of epigenetic marks will help identify preventive and therapeutic targets, which in combination with healthy lifestyle choices, can diminish the growing tide of obesity, type 2 diabetes, and other related disorders.

Conflict of interest

The authors declare they have no conflict of interest.

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© Springer Science+Business Media New York 2014