Doctor of Philosophy
Bérubé, Nathalie G.
ATRX is an ATP-dependent chromatin remodeler required to safeguard genomic integrity. Conditional deletion of Atrx in the mouse embryonic forebrain and anterior pituitary in AtrxFoxg1Cre mice phenocopies mouse models of progeria which display increased DNA damage, coupled with reduced lifespan, growth and subcutaneous fat. These mice also have severely low circulating levels of insulin like growth factor 1 (IGF-1) and (T4) which have been reported in models of premature aging. Based on evidence that Igf1 is activated by the ligand-bound thyroid hormone receptor, I tested whether T4 supplementation could restore IGF-1 levels and ameliorate premature aging phenotypes in AtrxFoxG1Cre mice. However, restoration of normal serum T4 levels failed to rescue circulating IGF-1 levels, growth defects or reduced lifespan. This could be explained in part by an unexpected deletion of ATRX in hepatocytes where ATRX promotes thyroid hormone receptor mediated Igf1 expression. In the second half of this thesis, I demonstrated that AtrxFoxG1Cre mice are also characterized by nervous system hypomyelination, another aging-related phenotype. Myelin is produced by oligodendrocytes and ensures rapid propagation of action potentials necessary for higher cognitive functioning. Systemic thyroxine supplementation improved myelin levels by promoting oligodendrocyte differentiation. However, myelin restoration was incomplete, as the number of oligodendrocyte precursor cells (OPCs) was still diminished, pointing to additional roles of ATRX in the generation or maintenance of these cells. Using directed inactivation of Atrx in postnatal OPCs or neurons, I established that ablation of ATRX in the former, but not the latter, results in hypomyelination. ATRX-null OPCs displayed a more plastic state in vitro and in vivo, allowing a shift from a strict oligodendrocyte differentiation program toward ectopic astrogliogenesis, ultimately leading to hypomyelination. Mechanistically, I provide evidence that ATRX associates with the histone deacetylase HDAC3 in OPCs to promote transcription of the Olig2 fate specification gene. Collectively this study identified thyroxine-dependent and -independent effects of Atrx gene activation on premature aging-like phenotypes in mice.
Summary for Lay Audience
ATRX is a protein that alters the way DNA is packaged in cells and this function is necessary to prevent DNA damage and ensure the proper development of many different cell types and tissues. Mouse models can be used to study the function of ATRX in development by genetically removing it. Deletion of ATRX from the mouse brain and pituitary caused symptoms that appear in other mouse models of premature aging. These abnormalities included a reduction in growth, fat, glucose levels and lifespan. ATRX deficient mice also exhibited low levels of the thyroid hormone thyroxine and insulin-like growth factor 1 (IGF-1), as seen in other premature aging models and both of which are necessary for growth and development. Others have reported that thyroid hormone can increase IGF-1 levels to promote growth and development. With this in mind, we thought that supplementing ATRX deficient mice with thyroid hormone may rescue these harmful developmental symptoms by increasing IGF-1 levels. However, due to inherent problems with genetic manipulation in mice, we found that ATRX was unexpectedly absent in the liver. The liver is the primary site for production of IGF-1 and thyroid hormone was unable to increase IGF-1 levels due to the absence of ATRX in the liver. Excitingly, thyroid hormone treatment rescued hypomyelination in ATRX-deficient mice. The cell type in the brain that produces myelin is the oligodendrocyte which insulates neurons to speed up electrical current in the brain. However, the rescue in myelin was only partial, leading us to investigate whether ATRX is important in oligodendrocytes to produce myelin. Using another mouse model, ATRX was specifically deleted in oligodendrocytes. In this model, we found that ATRX is necessary for the development of oligodendrocytes and in its absence, these cells become another brain cell type called astrocytes, ultimately leading to reduced myelin in the brain. Collectively, this study has identified a role for ATRX in the prevention of premature aging, both dependent and independent of the thyroid hormone thyroxine.
Rowland, Megan E., "Thyroxine-Dependent and -Independent Effects on Premature Aging and Myelination in ATRX Mutant Mice" (2020). Electronic Thesis and Dissertation Repository. 6825.