Epigenetics is the study of heritable changes in gene activity or phenotype that are not caused by changes in the DNA sequence.[1] Environmental epigenomics reveals the intricate communication between the epigenome and environment, including both exogenous (such as nutritional and chemical exposures) and endogenous (such as immune status and levels of hormones) factors.[2] It has been well established that various factors such as stress and nutrition during pregnancy can influence fetal development. Furthermore, it was recently shown that paternal experiences can also be transmitted through neuroepigenetic marks.[3] The greatest advantage in studying paternal influence on the fetal epigenome is that complex confounding factors such as maternal behaviours and variability in the intrauterine environment can be excluded. Paternal experiences and exposure to diverse environmental conditions alter the fetal epigenome, affecting subsequent neurodevelopment of the offspring and possibly leading to disorders and disease that manifest in childhood or later on in the course of the offspring’s life.
| Fetal Neurodevelopment |
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| Harryman, W. Integral Options Cafe. Retrieved from http://integral-options.blogspot.ca/2010/06/wiring-brain-what-is-neurodevelopmental.html |
1.1 Introduction
The epigenome is dynamic and susceptible to dysregulation during early development stages such as gametogenesis, fertilization, embryogenesis, and following during the maintenance of somatic cells and development of germ cells that give rise to the next generation.[4] While mature sperm cells do not have the machinery required for chromatin remodelling, they are able to undergo heritable changes in spermatozoa RNA populations, methylation patterns, histone modifications, and sperm microRNA (miR) content as responses to environmental challenges during spermatogenesis.[5] Recent rodent transgenerational models and human epidemiological studies provide evidence for transmission of paternal experiences to offspring and show that epigenetic components are important factors affecting neuropsychiatric disease risk.
| Epigenetic Transformation of DNA |
| An overview of epigenetic mechanisms. |
| Mouse model for epigenetic influences |
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| Candidate mechanisms underlying how environmental conditions on the paternal mouse lead to offspring phenotypic alterations. Rando, OJ. (2012, Nov 9). Cell. Retrieved from http://www.cell.com/cell/abstract/S0092-8674(12)01235-4 |
1.2 Lifestyle factors
1.2a Smoking
It has been well recognized that maternal smoking before and during pregnancy leads to adverse health outcomes in the child, including pulmonary and cardiovascular disorders, and problems with human fertility, reproduction and early development. Paternal cigarette smoking at the time of conception also causes harm at an epigenetic level, causing DNA damage in the child’s cord blood.[6] It was found that cigarette smoke alters miRNA expression in human spermatozoa which targeted epigenetic compounds important in DNA methylation and histone modification to be the mechanism.[7]
1.2b Diet
In a rodent model of a dietary challenge study, it was demonstrated that male mice given a low-protein diet before reaching sexual maturity showed different RNA content and chromatin packaging of sperm as compared to controls. The paternal mice on a low-protein diet also gave rise to offspring with altered DNA methylation at specific liver CpG islands, especially at a key lipid regulator Ppara, and elevated expression of hepatic genes involved in cholesterol and lipid metabolism.[8] In addition, a high-fat paternal diet results in changes in global methylation and microRNA content in sperm and further led to metabolic and health problems in subsequent generations.[9] These studies thus indicate that the father’s diet during male gametogenesis can influence the metabolic processes and phenotypic outcomes such as body weight in the offspring.
| Diet and epigenetic changes |
| The transgenerational effects of diet on the human epigenome. |
1.2c Alcohol
In the rat model, it was shown that chronic alcohol consumption by the paternal rat resulted in a reduction in mRNA expression of cytosine methyltransferase in sperm.[10] This decreased expression could lead to hypomethylated DNA and further have transgenerational genetic influences in the offspring.[10] In addition, chronic alcohol intake in men is associated with hypomethylation of two typically hypermethylated and imprinted genes, H19 and intergenic differentially-methylated region (IG-DMR).[11]
| DNA Methylation |
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| The mechanism of cytosine methyltransferase. pubs.niaaa.nih.gov. Retrieved from http://pubs.niaaa.nih.gov/publications/arcr351/images/zakhari01.png |
1.3 Stress
In neurodevelopment and affective disorders such as autism, schizophrenia and depression, the hypothalamic-pituitiary-adrenal (HPA) axis is found to be hyperreactive or hyporeactive resulting in stress dysregulation.[12-13] Recently, it was shown that paternal stress significantly contributes to offspring HPA axis dysregulation. Paternal mice exposed to 6 weeks of chronic stress before breeding induced epigenetic reprogramming of germ cells in male mice, with a significantly altered sperm miRNA content (which are normally found at low basal levels), particularly nine specific miRs which increased in paternal stress groups.[14] Offspring from males with stress exposure (either throughout puberty or in adulthood) showed significantly reduced stress reactivity. Furthermore, gene array analysis of offspring stress regulatory brain regions showed enhanced expression of glucocorticoid receptor responsive genes in the paraventricular nucleus (PVN) which is associated with a tighter control of CRF neurons.[14] In addition, expression of gene sets associated with CREB activity, CREB-binding protein, and six members of the miR-154 family was found to be enriched in the bed nucleus of stria terminals (BNST).[14] Thus, this study demonstrated that male exposure to even mild stress, either throughout puberty or in adulthood, can produce long-term changes in male germ cells through an epigenetic reprogramming of germ cells and results in transmission of an offspring HPA stress axis dysregulation phenotype.
| Sperm miRNA content |
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| Chronic stress experienced during puberty or adulthood significantly alters miRNA content in sperm of exposed paternal mice. Rodgers et al. (2013, May 22). The Journal of Neuroscience. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/23699511 |
1.4 Drugs of abuse
1.4a Cocaine
It has been observed that paternal cocaine intake affects working memory in female offspring and in male offspring causes hyperactivity and enhanced perseveration in a T-maze.[15-16] In a rodent study, voluntary paternal administration of cocaine induced an epigenetic reprogramming of the male germ line, resulting in significantly increased mPFC BDNF protein and mRNA levels in male cocaine-sired rats which was consistent with the increased acetylation levels (an epigenetic mechanism) of sperm Bdnf promoter in cocaine sires.[17] The offspring of cocaine-exposed paternal rats also showed resistance to cocaine reinforcement which is accounted by the increased BDNF signalling in the mPFC and possibly in other nuclei which blunts the reinforcing effect of cocaine.[17]
| Increased acetylation of sperm Bdnf promoter as a result of paternal cocaine exposure |
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| Representative sections of testes from control (a-c; saline-exposed) and cocaine-exposed (d-f) rats. Immunohistochemistry reveals increased AcH3 expression in the tubules of cocaine-exposed sires. Vassoler, F. et al. (2013, Jan). Nature Neuroscience. Retrieved from http://www.nature.com/neuro/journal/v16/n1/full/nn.3280.html |
1.4b Methamphetamine
Long-term exposure to methamphetamine in parental mice resulted in an altered DNA methylation pattern in the hippocampus of the offspring.[18] The male offspring of meth-exposed parental mice also exhibited a significantly more sensitive response to cocaine-conditioned reward and hyperlocomotion while both male and female offspring showed cognitive deficits such as a reduced response to fear conditioning.[18]
1.5 Advanced paternal age
Offspring of older fathers have been found to have a broad range of disease outcomes, including increased risk for neural tube defect and cleft palate, intellectual impairments, epilepsy, and bipolar disorder.[19-22] Particularly, advanced paternal age (APA) has been associated with elevated risk for the neurodevelopment disorders autism and schizophrenia.[23] In older fathers, the sperm have undergone many more replications of the genome, increasing its susceptibility to copy error mutations.[23] Interestingly, dysregulation of epigenetic processes occurring during spermatogenesis in older men can also contribute to the association between APA and neurocognitive disorders. In spermatozoa of older rats, changes in chromatin packaging and integrity, as well as hypermethylation in ribosomal DNA were evident.[24-25]
| Old father and son |
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| Nature magazine. Retrieved from http://www.scientificamerican.com/article/older-fathers-pass-on-mor/ |
1.6 Exposure to ionizing radiation
Father mice exposed to radiation resulted in a significant upregulation of the miR-29 family in the exposed male germline and directly affected methylation patterns which was accounted by the downregulated levels of de novo methyltransferases.[26] In addition, long interspersed nuclear elements 1 (LINE1) and short interspersed nuclear elements B2 (SINE B2) were found to be hypomethylated.[26] Such epigenetic changes in the father’s germline cells further led to a global loss of genome-wide methylation in the thymus of the offspring of the exposed male mouse.[26] Moreover, hypomethylation of the repetitive elements was due to a decrease in the expression of lymphoid-specific helicase (LSH), a member of SNF2 family of chromatin remodeling protein and an important element for genome-wide CpG methylation.[26-27] Thus, ionizing radiation experienced by the paternal mouse can have deleterious effects through epigenetic mechanisms, resulting in a suppression of important proteins such as LSH in the offspring and ultimately, genomic instability in the fetus.
1.7 Exposure to environmental toxins
1.7a Exposure to ethylnitrosourea
Dubrova et al. demonstrated that paternal mice exposure to ethylnitrosourea (an alkylating agent that produces specific types of DNA adducts) leads to altered mutation rates in the germline cells of both male and female offspring mice.[28] Particularly, mutation rates were significantly increased at two expanded simple tandem repeat loci and resulted in an overall transgenerational genomic instability.[28] This study suggests that paternal exposure to anti-cancer therapeutics (which involve potent alkylating agents to induce DNA damage in proliferating tumour cells) can result in epigenetic changes and have harmful genetic effects on the fetus.
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