Timothy G. Jenkins

The sperm epigenome and potential implications for the developing embryo

Recent work in the field of male fertility has yielded significant increases in our understanding of the sperm epigenome and its potential role in embryonic development. These new findings have enabled a broad classification of a normal epigenetic state in the male gamete and have provided insight into the possible etiologies of some idiopathic male infertility cases. Histone retention and modification, protamine incorporation into the chromatin, DNA methylation, and spermatozoal RNA transcripts appear to play important roles in the epigenetic state of mature sperm. These epigenetic factors may reveal a historical record of spermatogenesis, portend future functions in embryogenesis, and help to elucidate mechanism of pluripotency. In contrast to the once held dogma regarding the importance of the paternal epigenome, the unique epigenetic landscape in sperm appears to serve more than the gamete itself and is likely influential in the developing embryo. In fact, growing evidence suggests that mature sperm provide appropriate epigenetic marks that drive specific genes toward activation and contribute to the pluripotent state of the embryonic cells. Although not definitive, the current literature provides evidence for the role of the sperm epigenome in the embryo. Future work must be focused on the characterization of epigenetic abnormalities commonly found in individuals with compromised fertility to further establish this role. Additionally, studies should target the effects of environment and aging on the sperm epigenetic program and subsequent fertility loss to determine the etiology of aberrant epigenetic profiles.

Age-Associated Sperm DNA Methylation Alterations: Possible Implications in Offspring Disease Susceptibility

Recent evidence demonstrates a role for paternal aging on offspring disease susceptibility. It is well established that various neuropsychiatric disorders (schizophrenia, autism, etc.), trinucleotide expansion associated diseases (myotonic dystrophy, Huntingtons, etc.) and even some forms of cancer have increased incidence in the offspring of older fathers. Despite strong epidemiological evidence that these alterations are more common in offspring sired by older fathers, in most cases the mechanisms that drive these processes are unclear. However, it is commonly believed that epigenetics, and specifically DNA methylation alterations, likely play a role. In this study we have investigated the impact of aging on DNA methylation in mature human sperm. Using a methylation array approach we evaluated changes to sperm DNA methylation patterns in 17 fertile donors by comparing the sperm methylome of 2 samples collected from each individual 9–19 years apart. With this design we have identified 139 regions that are significantly and consistently hypomethylated with age and 8 regions that are significantly hypermethylated with age. A representative subset of these alterations have been confirmed in an independent cohort. A total of 117 genes are associated with these regions of methylation alterations (promoter or gene body). Intriguingly, a portion of the age-related changes in sperm DNA methylation are located at genes previously associated with schizophrenia and bipolar disorder. While our data does not establish a causative relationship, it does raise the possibility that the age-associated methylation of the candidate genes that we observe in sperm might contribute to the increased incidence of neuropsychiatric and other disorders in the offspring of older males. However, further study is required to determine whether, and to what extent, a causative relationship exists.

The paternal epigenome and embryogenesis： poising mechanisms for development

The scope of paternal contributions during early embryonic development has long been considered limited. Dramatic changes in chromatin structure throughout spermatogenesis have been thought to leave the sperm void of complex layers of epigenetic regulation over the DNA blueprint, thus leaving the balance of that regulation to the oocyte. However, recent work in the fields of epigenetics and male factor infertility has placed this long-held, and now controversial dogma, in a new light. Elegant studies investigating chromatin and epigenetic modifications in the developing sperm cell have provided new insights that may establish a more critical role for the paternal epigenome in the developing embryo. DNA methylation, histone tail modifications, targeted histone retention and protamine incorporation into the chromatin have great influence in the developing sperm cell. Perturbations in the establishment and/or maintenance of any of these epigenetic marks have been demonstrated to affect fertility status, ranging in severity from mild to catastrophic. Sperm require this myriad of chromatin structural changes not only to serve a protective role to DNA throughout spermatogenesis and future delivery to the egg, but also, it appears, to contribute to the developmental program of the future embryo. This review will focus on our current understanding of the epigenetics of sperm. We will discuss sperm-specific chromatin modifications that result in genes essential to development being poised for activation early in embryonic development, the disruption of which may result in reduced fecundity.

Paternal aging and associated intraindividual alterations of global sperm 5-methylcytosine and 5-hydroxymethylcytosine levels.

OBJECTIVE To evaluate the relative intraindividual changes in sperm 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) levels associated with age and to compare the levels of 5-hmC in mature human sperm to blood DNA. DESIGN Prospective research study. SETTING University-based andrology and in vitro fertilization (IVF) laboratory. PATIENT(S) Fifteen known fertile sperm donors, 22 other known fertile controls, and 41 male blood donors from a general population tissue bank. INTERVENTION(S) None. MAIN OUTCOME MEASURE(S) Measurements of global 5-mC and 5-hmC levels via an enzyme-linked immunosorbent assay (ELISA)-based assay. RESULT(S) Global sperm 5-mC levels exhibit a statistically significant increase with age, and a similar trend was seen in 5-hmC levels. On average, in the age ranges we analyzed, 5-mC increased by 1.76% per year, and 5-hmC, though more variable, increased by approximately 5% per year. Additionally, we found that 5-hmC levels in sperm are 32.59% of that found in blood DNA. CONCLUSION(S) Global sperm DNA methylation patterns are stable over short periods of time but increase with age, which raises important questions regarding the risks of advanced paternal age. Additionally, as we would predict for a transcriptionally quiescent cell type, 5-hmC levels are statistically significantly lower in human sperm than in blood DNA.

Aberrant sperm DNA methylation predicts male fertility status and embryo quality

OBJECTIVE To evaluate whether male fertility status and/or embryo quality during in vitro fertilization (IVF) therapy can be predicted based on genomewide sperm deoxyribonucleic acid (DNA) methylation patterns. DESIGN Retrospective cohort study. SETTING University-based fertility center. PATIENT(S) Participants were 127 men undergoing IVF treatment (where any major female factor cause of infertility had been ruled out), and 54 normozoospermic, fertile men. The IVF patients were stratified into 2 groups: patients who had generally good embryogenesis and a positive pregnancy (n = 55), and patients with generally poor embryogenesis (n = 72; 42 positive and 30 negative pregnancies) after IVF. INTERVENTION(S) Genomewide sperm DNA methylation analysis was performed to measure methylation at >485,000 sites across the genome. MAIN OUTCOME MEASURE(S) A comparison was made of DNA methylation patterns of IVF patients vs. normozoospermic, fertile men. RESULT(S) Predictive models proved to be highly accurate in classifying male fertility status (fertile or infertile), with 82% sensitivity, and 99% positive predictive value. Hierarchic clustering identified clusters enriched for IVF patient samples and for poor-quality-embryo samples. Models built to identify samples within these groups, from neat samples, achieved positive predictive value ≥ 94% while identifying >one fifth of all IVF patient and poor-quality-embryo samples in each case. Using density gradient prepared samples, the same approach recovered 46% of poor-quality-embryo samples with no false positives. CONCLUSION(S) Sperm DNA methylation patterns differ significantly and consistently for infertile vs. fertile, normozoospermic men. In addition, DNA methylation patterns may be predictive of embryo quality during IVF.

Dynamic alterations in the paternal epigenetic landscape following fertilization

Embryonic development is a complex and dynamic process with frequent changes in gene expression, ultimately leading to cellular differentiation and commitment of various cell lines. These changes are likely preceded by changes to signaling cascades and/or alterations to the epigenetic program in specific cells. The process of epigenetic remodeling begins early in development. In fact, soon after the union of sperm and egg massive epigenetic changes occur across the paternal and maternal epigenetic landscape. The epigenome of these cells includes modifications to the DNA itself, in the form of DNA methylation, as well as nuclear protein content and modification, such as modifications to histones. Sperm chromatin is predominantly packaged by protamines, but following fertilization the sperm pronucleus undergoes remodeling in which maternally derived histones replace protamines, resulting in the relaxation of chromatin and ultimately decondensation of the paternal pronucleus. In addition, active DNA demethylation occurs across the paternal genome prior to the first cell division, effectively erasing many spermatogenesis derived methylation marks. This complex interplay begins the dynamic process by which two haploid cells unite to form a diploid organism. The biology of these events is central to the understanding of sexual reproduction, yet our knowledge regarding the mechanisms involved is extremely limited. This review will explore what is known regarding the post-fertilization epigenetic alterations of the paternal chromatin and the implications suggested by the available literature.

Decreased fecundity and sperm DNA methylation patterns

OBJECTIVE To evaluate the relationship between epigenetic patterns in sperm and fecundity. DESIGN Prospective study. SETTING Academic andrology and in vitro fertilization laboratory. PATIENT(S) Twenty-seven semen samples from couples who conceived within 2 months of attempting a pregnancy and 29 semen samples from couples unable to achieve a pregnancy within 12 months. INTERVENTION(S) None. MAIN OUTCOME MEASURE(S) Genomewide assessment of differential sperm DNA methylation and standard semen analysis. RESULT(S) We analyzed DNA methylation alterations associated with fecundity in 124 semen samples, and identified regions of interest in 27 semen samples from couples who conceived within 2 months of attempting a pregnancy and a total of 29 semen samples from couples who were unable to achieve a pregnancy within 12 months. No differences in sperm count, sperm morphology, or semen volume were observed between the patients achieving a pregnancy within 2 months of study time and those not obtaining a pregnancy within 12 months. However, using data from the human methylation 450k array analysis we did identify two genomic regions with statistically significantly decreased (false discovery rate <0.01) methylation and three genomic regions with statistically significantly increased methylation in the failure-to-conceive group. The only two sites where decreased methylation was associated with reduced fecundity are at closely related genes known to be expressed in sperm, HSPA1L and HSPA1B. CONCLUSION(S) Our data suggest that there are genomic loci where DNA methylation alterations are associated with decreased fecundity. We have thus identified candidate loci for future study to verify these results and investigate the causative or contributory relationship between altered sperm methylation and decreased fecundity.

Teratozoospermia and asthenozoospermia are associated with specific epigenetic signatures.

Semen analysis is commonly used as a tool to assess the fertility potential of a male, despite its relatively low predictive power. In this study, we have assessed associations between semen analysis findings (low count, low motility, low viability, poor sperm penetration assay results, poor morphology, and increased DNA damage) and DNA methylation patterns in mature spermatozoa. DNA methylation patterns in the mature spermatozoa are thought to be indicative of patterns in the adult germline stem cells and may offer insight into potential perturbations to cellular pathways involved in spermatogenesis. In this study, sperm DNA methylation at >480,000 CpGs was assessed in 94 men using the Illumina 450k HumanMethylation Array and compared to standard measures of sperm quality. We did not identify any global changes to methylation profiles that were associated with reduced semen parameters. Similarly, we found no significant difference in methylation variability that was associated with any abnormal semen analysis parameter, although sperm displaying abnormal parameters tended to have an increased coefficient of variability, suggesting that, in some samples, this may be a contributing factor. Analysis of methylation at single CpGs and genomic regions did identify associations for low viability and low motility, and to a smaller extent, low count. Interestingly, based on GO Term analysis, differentially methylated regions associated with low viability were over‐represented in regions important in meiosis, spermatogenesis, and genomic imprinting. These results suggest that while there are not global alterations to the sperm methylome associated with semen abnormalites, some viability associated regional alterations do exist that may be indicative of perturbed cellular pathways during spermatogenesis.

Obesity, male infertility, and the sperm epigenome

Obesity is a growing epidemic and a common problem among reproductive-age men that can both cause and exacerbate male-factor infertility by means of endocrine abnormalities, associated comorbidities, and direct effects on the fidelity and throughput of spermatogenesis. Robust epidemiologic, clinical, genetic, epigenetic, and nonhuman animal data support these findings. Recent works in the burgeoning field of epigenetics has demonstrated that paternal obesity can affect offspring metabolic and reproductive phenotypes by means of epigenetic reprogramming of spermatogonial stem cells. Understanding the impact of this reprogramming is critical to a comprehensive view of the impact of obesity on subsequent generations. Furthermore, and perhaps more importantly, conveying the impact of these lifestyle changes on future progeny can serve as a powerful tool for obese men to modify their behavior. Reproductive urologists and endocrinologists must learn to assimilate these new findings to better counsel men about the importance of paternal preconception health, a topic recently being championed by the Centers for Disease Control and Prevention.

Supplementation of cryomedium with ascorbic acid-2-glucoside (AA2G) improves human sperm post-thaw motility

OBJECTIVE To evaluate the efficacy of ascorbic acid-2-glucoside (AA2G) supplementation in cryomedium to improve human sperm motility preservation through the freeze-thaw process. DESIGN Prospective research study. SETTING University-based andrology laboratory. PATIENT(S) One hundred thirty-four patients attending the andrology laboratory for a regular semen analysis between April 2009 and May 2010. INTERVENTION(S) Supplementation of cryomedium with ascorbate or AA2G. MAIN OUTCOME MEASURE(S) Endogenous semen ascorbic acid concentrations, semen lipid peroxidation levels, progressive sperm motility. RESULT(S) AA2G-supplemented samples showed improved sperm motility preservation after freezing compared with control samples. No significant difference was found in motility preservation between control samples and those supplemented with ascorbate. CONCLUSION(S) Our data indicate that AA2G confers a protective effect on human sperm motility preservation through the freeze-thaw cycle. This same effect was not seen with ascorbate. Further investigation is required to elucidate a concrete mechanism of action.