The Effect of Developmental Iron Deficiency on Gene Expression, Tet Proteins, and Dna Hydroxymethylation In the Rodent Brain

Abstract

Fetal-neonatal iron deficiency (ID) has a lasting negative impact on neurodevelopment, resulting in significant cognitive, socio-emotional, and learning and memory deficits in adulthood, as well as increased risk for neuropsychiatric disease. Given that ID is the most common micronutrient deficiency worldwide, and that pregnant women and young children are disproportionately affected, it presents a significant public health concern. Preclinical models have demonstrated that the developing central nervous system (CNS) is particularly affected by ID, and that the deleterious neurodevelopmental effects and neuropsychiatric risks that follow are associated with dysregulation of CNS gene expression. Dysregulated genes map to signaling pathways and networks critical for neurodevelopment and neuronal function, suggesting that these critical functions are compromised by ID. If developmental ID is corrected by iron repletion within a critical period, correction of neurodevelopmental deficits is possible. However, if iron repletion occurs outside of the critical period, the phenotypic and gene expression changes persist into adulthood despite correction of the deficiency. While changes in gene expression can be understood as the proximate cause of the ID neurocognitive phenotype, it is still unclear what the ultimate cause is. As such, there is a gap in our understanding of how developmental ID establishes and maintains gene expression changes in the CNS. A potential mechanism by which iron could enact these changes is through Ten-Eleven Translocation (TET) enzymes, a family of iron-dependent hydroxylases that generate the epigenetic modification 5-hydroxymethylcytosine (5hmC), or DNA hydroxymethylation. Epigenetic modifications such as DNA hydroxymethylation have the ability to stably influence gene expression throughout the lifespan, and are known to be labile to environmental influences. Of particular relevance, 5hmC is more abundant in the brain than any other tissue, and it increases in enrichment as neurodevelopment progresses, particularly in genes critical for neuronal development and function. The central hypothesis of my thesis research is that dysregulation of TET enzymatic activity and 5hmC by fetal-neonatal ID drives gene expression changes in brain that contribute to the long-term neurocognitive phenotype of developmental ID. To test this hypothesis, the following aims were proposed: 1) Determine the effect of fetal-neonatal ID on TET activity and 5hmC in two regions of the developing rat brain, the hippocampus and the cerebellum, and 2) Determine whether treatment of developmental ID with dietary iron repletion can reverse the changes to this epigenetic system. Completion of these aims contributes to the long-term goal of understanding the cellular and molecular underpinnings of CNS dysfunction and increased neuropsychiatric disease risk following developmental ID. Because the standard therapy of iron repletion incompletely rescues the neurodevelopmental phenotype of ID, there is a need for better therapeutic options. By better understanding the underlying mechanisms of ID-related hippocampal dysfunction, it may be possible to identify new therapeutic targets for more effective treatment of iron deficiency.