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Author ORCID Identifier

https://orcid.org/0000-0001-6428-766X

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Public Health

Year Degree Awarded

2022

Month Degree Awarded

February

First Advisor

J. Richard Pilsner

Second Advisor

Alexander Suvorov

Third Advisor

Carrie J. Nobles

Fourth Advisor

Brian W. Whitcomb

Subject Categories

Environmental Public Health

Abstract

Infertility, which is the inability to achieve clinical pregnancy within 12 months of unprotected sexual intercourse, affects 15% of all couples globally. Emerging evidence shows, just as females, males are equally likely to contribute to couple infertility, suggesting the relevance of male factor in couple reproductive success. The potential factors thought to drive male infertility include male preconception environment and aging. Thus, this dissertation focused on two (DNA methylation and small non-coding RNAs (sncRNAs)) of the three major potential epigenetic mediators that occur during spermatogenesis and sperm maturation prior to conception using human and mice model.

First, to explore the current state of knowledge on the susceptibility of sperm epigenetics to environmental insults and its potential reproductive and downstream effects, we provided a comprehensive review of male preconception environment and sperm epigenetics. We found that sperm epigenome can be modified by lifestyle such as diet, and exercise with limited studies related to ubiquitous environmental endocrine disruptive compounds like phthalates.

To better understand the impact of phthalate on sperm epigenome and early development, adult male mice were exposed to low and high doses of DEHP and sperm epigenome and embryonic transcriptome profiles were assessed. Consistent with previous findings in human, we found that DEHP exposure modified DNA methylomes in F0 sperm and F1 embryo; altered F1 embryonic transcriptome at developmental genes. Beyond spermatogenesis, sperm also undergo epididymal maturation which has been shown to play a central role in sperm function. This maturation is mediated by extracellular vesicles, the cargo of which has be reported to show sensitivity to nutritional factors and may convey paternal experience to subsequent generation. To evaluate the potential influence of phthalate exposures, we recruited 96 men undergoing infertility treatment as part of the Sperm Environmental Epigenetics and Development Study (SEEDS). We found a total of 23 sncRNAs including the expression levels of microRNA (miRNA), transfer RNA fragments (tRFs) and piwi-interacting RNA (piRNA) that showed alteration due to concentrations of urinary phthalate metabolites. These sncRNAs targeted genomic regions found to be important in biological functions related to development, indicating likely role in early embryonic development.

Lastly, preconception male age has been linked to a wide array of reproductive and offspring phenotypes. Recent evidence shows such phenotypes among younger men are not negligible which suggest the potential variation in underlying biological clock. Interestingly, DNA methylation-based clock has emerged as important estimator of biological aging in somatic tissues. Thus, this dissertation did not only develop sperm-based DNA methylation clock but also investigated its environmental susceptibility among the general population of men (n = 379) planning to conceive as part of the Longitudinal Investigation of Fertility and the Environment (LIFE) study. We found select metabolites were associated with ~ 4 months increase in sperm epigenetic clock which resulted in a suggestive positive trend with the total metabolite mixture, indicating younger men can now carry an older population of sperm.

Overall, the evidence presented by this dissertation shows that paternal adult exposure to select phthalates and phthalate alternatives alters sperm methylome and persists through fertilization to influence genes involved in development in mice. In humans, phthalates are associated with altered sncRNA profiles in seminal plasma extracellular vesicles and with increased biological aging of sperm. This highlights the relevance of male preconception period in couple reproductive and offspring outcomes. Additional studies are required to replicate our findings across human and mice models.

DOI

https://doi.org/10.7275/27094474

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Available for download on Wednesday, February 01, 2023

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