Differential sperm histone retention in normozoospermic ejaculates of infertile men negatively affects sperm functional competence and embryo quality

BACKGROUND

The unique epigenetic architecture that sperm cells acquire during spermiogenesis by retaining <15% of either canonical or variant histone proteins in their genome is essential for normal embryogenesis. Whilst heterogeneous levels of retained histones are found in morphologically normal spermatozoa, their effect on reproductive outcomes is not fully understood.

METHODS

Processed spermatozoa (n = 62) were tested for DNA integrity by sperm chromatin dispersion assay, and retained histones were extracted and subjected to dot-blot analysis. The impact of retained histone modifications in normozoospermic patients on sperm functional characteristics, embryo quality, metabolic signature in embryo spent culture medium and pregnancy outcome was studied.

RESULTS

Dot-blot analysis showed heterogeneous levels of retained histones in the genome of normozoospermic ejaculates. Post-wash sperm yield was affected by an increase in H3K27Me3 and H4K20Me3 levels in the sperm chromatin (p < 0.05). Also, spermatozoa with higher histone H3 retention had increased DNA damage (p < 0.05). Spermatozoa from these cohorts, when injected into donor oocytes, correlated to a significant decrease in the fertilisation rate with an increase in sperm histone H3 (p < 0.05) and H3K27Me3 (p < 0.01). An increase in histone H3 negatively affected embryo quality (p < 0.01) and clinical pregnancy outcome post-embryo transfer (p < 0.05). On the other hand, spent culture medium metabolites assessed by high-resolution (800 MHz) nuclear magnetic resonance showed an increased intensity of the amino acid methionine in the non-pregnant group than in the pregnant group (p < 0.05) and a negative correlation with sperm histone H3 in the pregnant group (p < 0.05).

DISCUSSION AND CONCLUSION

Histone retention in spermatozoa can be one of the factors behind the development of idiopathic male infertility. Such spermatozoa may influence embryonic behaviour and thereby affect the success rate of assisted reproductive technology procedures. These results, although descriptive in nature, warrant further research to address the underlying mechanisms behind these clinically important observations.

Insights into the sperm chromatin and implications for male infertility from a protein perspective

Male germ cells undergo an extreme but fascinating process of chromatin remodeling that begins in the testis during the last phase of spermatogenesis and continues through epididymal sperm maturation. Most of the histones are replaced by small proteins named protamines, whose high basicity leads to a tight genomic compaction. This process is epigenetically regulated at many levels, not only by posttranslational modifications, but also by readers, writers, and erasers, in a context of a highly coordinated postmeiotic gene expression program. Protamines are key proteins for acquiring this highly specialized chromatin conformation, needed for sperm functionality. Interestingly, and contrary to what could be inferred from its very specific DNA-packaging function across protamine-containing species, human sperm chromatin contains a wide spectrum of protamine proteoforms, including truncated and posttranslationally modified proteoforms. The generation of protamine knock-out models revealed not only chromatin compaction defects, but also collateral sperm alterations contributing to infertile phenotypes, evidencing the importance of sperm chromatin protamination toward the generation of a new individual. The unique features of sperm chromatin have motivated its study, applying from conventional to the most ground-breaking techniques to disentangle its peculiarities and the cellular mechanisms governing its successful conferment, especially relevant from the protein point of view due to the important epigenetic role of sperm nuclear proteins. Gathering and contextualizing the most striking discoveries will provide a global understanding of the importance and complexity of achieving a proper chromatin compaction and exploring its implications on postfertilization events and beyond. This article is categorized under: Reproductive System Diseases > Genetics/Genomics/Epigenetics Reproductive System Diseases > Molecular and Cellular Physiology.

Assessment of seminal cell-free DNA as a potential contaminate in studies of human sperm DNA methylation

BACKGROUND

Human seminal cell-free DNA (cfDNA) methylation patterns have not yet been thoroughly explored, however, recent work in mouse has suggested that some cfDNA encountered in the epididymis may contaminate DNA methylation studies assessing the mature sperm. Such contamination could clearly prove to be a significant confounder, for many reasons, in epigenetic studies of male factor infertility.

OBJECTIVES

To explore the nature of seminal cfDNA methylation and the likelihood that it would be retained following standard semen sample processing for epigenetic analysis.

MATERIALS AND METHODS

We assessed 12 semen samples collected at Utah Fertility Center. For each sample, seminal cfDNA was isolated from the sperm pellet. The sperm was split into three aliquots including one exposed to DNase to remove any additional cfDNA (termed "pure sperm"), one not exposed to DNase, and one exposed to DNase but reintroduced to seminal cfDNA. We additionally assessed blood DNA as our benchmark for somatic cell DNA methylation patterns. DNA methylation was measured via Illumina's 850k array and assessed for differential regional methylation.

RESULTS

46,352 differentially methylated regions (FDR > 40) were identified between pure sperm and seminal cfDNA. We found at these sites that the average DNA methylation in cfDNA always fell somewhere between the average methylation in sperm and in blood. We also assessed each sperm treatment groups at all 46,352 regions of interest and found no significant differences at any of these sites.

DISCUSSION AND CONCLUSION

Our data suggest that seminal cfDNA is a clear mixture of both somatic and germline DNA and that cfDNA is not a contaminating feature in sperm DNA methylation studies following standard protocols in human sperm DNA extraction. This article is protected by copyright. All rights reserved.

Small RNAs in Rat Sperm Are a Predictive and Sensitive Biomarker of Exposure to the Testicular Toxicant Ethylene Glycol Monomethyl Ether

Testicular histology and semen parameters are considered the gold standards when determining male reproductive toxicity. Ethylene glycol monomethyl ether (EGME) is a testicular toxicant with well-described effects on histopathology and sperm parameters. To compare the predictivity and sensitivity of molecular biomarkers of testicular toxicity to the traditional endpoints, small RNAs in the sperm were analyzed by next generation RNA-sequencing (RNA-seq). Adult rats were exposed to 0, 50, 60, or 75 mg/kg EGME by oral gavage for 5 consecutive days. Testis histology, epididymal sperm motility, and sperm small RNAs, including microRNAs (miRNAs), mRNA fragments, piwi-interacting RNAs (piRNAs), and tRNA fragments (tRFs), were analyzed 5 weeks after cessation of exposure. Testicular histology showed a significant dose-dependent increase in retained spermatid heads (RSH), while sperm motility declined with increasing dose. RNA-sequencing of sperm small RNAs was used to identify significant dose-dependent changes in percent mRNA fragments (of total reads), percent miRNAs (of total reads), average tRF length, average piRNA length, and piRNA and tRF length-distributions. Discriminant analysis showed relatively low predictivity of exposure based on RSH or motility compared to the average read length of all assigned RNAs. Benchmark dose (BMD) modeling resulted in a BMD of 62 mg/kg using RSH, whereas average read length of all assigned RNAs resulted in a BMD of 47 mg/kg. These results showed that sperm small RNAs are sensitive and predictive biomarkers of EGME-induced male reproductive toxicity.

The most common vices of men can damage fertility and the health of the next generation

Animal and human studies demonstrate that acquired paternal traits can impair both a male's fertility and the health of his offspring, including advanced age, smoking, stress, trauma, under-nutrition, infection, toxin exposure, and obesity. Many of these factors lead to similar changes to neurological, behavioural, and/or metabolic functioning in offspring. The molecular mechanisms that both respond to the paternal environment and act to transmit traits to offspring are beginning to emerge. This review focuses on three vices of men (alcohol consumption, overweight/obesity, and tobacco smoking) that damage fertility and pose risks to offspring health. These vices are not only the three most prevalent but are also leading risk factors for death and disability adjusted life years (DALYs) worldwide. Moreover, given that these vices are predominantly self-inflicted, interventions aimed at mitigating their consequences are readily identified.