Date of Award

Summer 8-19-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Biochemistry

Advisor

Ewelina Bolcun-Filas

Second Committee Member

Gregory Cox

Third Committee Member

Beth Dumont

Additional Committee Members

Martin Pera

Karissa Tilbury

Abstract

In humans and mice, the ovarian reserve (OR) is established during a lengthy process that starts during early embryogenesis with germ cell specification and culminates in the first weeks after birth when primordial follicles (PF) are formed. OR establishment is an important process which influences the fertile lifespan and fecundity of the individual. Fetal oocyte attrition (FOA) has been identified as a critical developmental event that regulates how many oocytes survive and contribute to the final OR. In addition to FOA, OR size and quality also depend on efficiency of meiotic recombination. Chromosome asynapsis and unrepaired meiotic double-strand breaks (DSB) lead to the exclusion of defective oocytes from the final OR by a checkpoint mechanism. There is limited understanding of how genetic factors modify these mechanisms and determine the size and quality of the OR. LINE-1 retrotransposon elements have previously been identified as playing a critical role in FOA in female mice. Transposable elements are capable of insertion anywhere in the genome by generating DSBs followed by insertional mutagenesis which can affect expression of genes near insertion sites. The mechanism by which LINE-1 activity affects the OR remains unknown. High levels of LINE-1 expression are thought to cause additional DSBs during meiotic recombination and negatively affect oocyte survival. We hypothesized that mouse strains with large OR might exhibit increased survival during FOA and/or quality checkpoint. Survey of the OR size among eight inbred strains, used as the founders for Genetically Diverse mice, revealed that NOD/ShiLt females have the largest and highest quality OR compared to other inbred strains including C57BL/6 strain. Here we show greater oocyte survival in the FOA window of elimination in NOD/ShiLtJ than in C57BL/6J ovaries which results in more oocytes included in the OR. To investigate one possible cause of FOA evasion, we determined LINE-1 expression levels in ovaries during prenatal stages, finding lower overall levels in NOD/ShiLtJ than in C57BL/6J ovaries. These observations support a role for LINE-1 as a regulator of OR size and suggest that unknown genetic factors acting in the NOD/ShiLtJ strain regulate LINE-1 activity and oocyte survival during development. Thus, identifying the NOD/ShiLtJ strain as a good model to investigate regulation of FOA and LINE-1 activity in female germline. Supported by R01 HD093778.

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