Date

5-2020

Department

School of Health Sciences

Degree

Doctor of Philosophy

Chair

Gary Isaacs

Keywords

Folate, Chronic Nutrient Deficiency, Cognition, Nutrient Epigenetics

Disciplines

Biology | Nutrition

Abstract

Folate is an essential B vitamin that serves as a primary one-carbon carrier in a number of essential cellular reactions; among these reactions are synthesis of purines, thymidylate, and methionine. As a result of these synthesis reactions, folate plays a key role in regulating nucleic acid metabolism, amino acid metabolism, maintenance of DNA stability, and production of S-adenosylmethionine for methylation of nucleic acids, neurotransmitters, phospholipids, histones, and other proteins. As such, folate deficiency is associated with a number of adverse outcomes including various cancers, cardiovascular disease, and cognitive defects. Despite mandatory folate fortification in grains in many developed countries, a number of people groups remain at risk for folate deficiency due to various disease conditions and lifestyle choices such as smoking and alcoholism, many of which arise in early adulthood and persist for the remainder of life. Using a mouse model of folate deficiency beginning post-weaning and persisting through the duration of the adult life, we examine the outcome of chronic folate deficiency in two key tissues: the hippocampus and the liver. The hippocampus was chosen as a tissue of focus due to its central role in cognition. The liver was chosen as the second tissue of focus because of its central role in folate storage and metabolism. Folate deficient mice exhibited memory deficits beginning in early adulthood and persisting through late adulthood. These deficits were linked to differential expression of a number of hippocampal genes in a tissue-specific manner, and enrichment of several transcription factor binding sites were found to be associated with the differentially expressed genes. These genes may represent targets for understanding and treating diseases associated with cognitive decline, such as Alzheimer’s disease. Further, significant site-specific changes in methylation of promoter regions of hepatic genes were observed; however, no significant changes in gene expression were determined, both by global expression analysis using microarrays and by specific expression analysis of a number of tumor suppressor genes by quantitative PCR. Since promoter methylation is often inversely related to gene expression, the lack of expression changes was surprising. This may indicate that many of the differentially methylated promoter regions do not significantly impact gene expression. Gad1, a tumor suppressor gene associated with liver cancer, exhibited significant promoter hypermethylation in response to folate deficiency. Bisulfite sequencing of portions of the Gad1 promoter region showed differential methylation patterns that may indicate that methylation downstream of the transcription start site of this gene has a greater impact on gene expression that methylation in upstream promoter regions. Together, these results show a relative resiliency of the liver in resisting adverse outcomes of chronic folate deficiency, such as differential expression of tumor suppressor genes, despite significant changes in methylation. The mechanisms by which the liver so propitiously resists negative outcomes of this methyl donor deficiency may serve as a template for treating this deficiency in less resilient tissues by comparing liver-specific pathways to similar pathways in other tissues.

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