Abstract 2064 RTHa Mutations Alter Thyroid Hormone Receptor Interactions with Nuclear Corepressor 1

Thyroid hormone receptors (TRs), members of the nuclear receptor super family, are transcription factors responsible for binding thyroid hormone (T3) and activating or repressing the expression of target genes to regulate metabolism, cell growth, and energy expenditure. Intracellular dynamics of TR, particularly its compartmentalization and mobility within the cell, are known to regulate its transcriptional role. Three TR isoforms, TRa1, TRb1, and TRb2, are abundant differentially in human tissues, with TRa1 observed predominantly in cells of the central nervous system, intestines, bones, and cardiac tissue. Recent characterization of TRa1 involvement in Resistance to Thyroid Hormone Syndrome a (RTHa), a rare genetic disorder, has initiated research efforts to elucidate the underlying mechanism of pathogenesis. Several mutations have been identified in RTHa patients with varying degrees of symptom severity. Among others, missense mutations at alanine 263 have been observed with considerable frequency. A263V and A263S substitutions were identified in several families with the condition, with the valine mutation displaying increased severity in skeletomuscular and neuropsychological development delays. Certain RTHa mutants were previously demonstrated to have increased affinity for Nuclear Corepressor 1 (NCoR1), a coregulator of TRa1, and are proposed to form more stable repressor complexes compared to wild type TRa1. To determine whether NCoR1 promotes nuclear retention of RTHa mutants, we cotransfected HeLa cells with expression plasmids for mCherry-TRa1 wild-type or RTHa mutants and GFP-NCoR1. Using fluorescence microscopy, we compared Nuclear-to-Cytoplasmic localization ratios of wild-type TRa1 to the A263V and A263S mutants in the presence of NCoR1 and its non-TR-binding counterpart NCoRΔID, as a control. We observed a significant increase in the nuclear retention of A263V with the overexpression of NCoR1 (p = 0.00196), relative to wild-type TRa1, and high variability in nuclear localization between replicates of A263S. Our results suggest that the observed deviations in the localization of RTHa mutants relative to wild-type TRa1 may, in part, be caused by changes in the stability of the repressor complex, providing further insight into RTHa pathogenesis on a molecular level. This work was supported by Grant 2R15DK058028 from the National Institutes of Health (to L.A.A.). The student is supported by the Beckman Scholars Program.