Molecular analysis of thyroid hormone receptors and retinoic acid receptors: DNA recognition specificity and roles in endocrine disease and cancer

Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) are members of the nuclear hormone receptor family. These ligand-regulated transcription factors bind to specific DNA sequences, called response elements, as homodimers or heterodimers with retinoid-X receptor (RXR) and regulate specific target genes. In the absence of hormone, TR and RAR repress transcription by recruiting corepressor complexes. In the presence of hormone, a coactivator complex is recruited and results in transcriptional activation. TRs and RARs are highly structurally conserved and possess very similar DNA binding domains. Despite these similarities, these receptors bind to different response elements in the genome, regulate different target genes, and control distinct physiological processes.;The work presented in this dissertation identifies the DNA recognition properties of TR and RAR that account for these receptors' response element binding specificity and, subsequently, their distinct physiological responses. While response elements based on the consensus half-site are high affinity binding sites for TR and RAR, they show poor specificity. In contrast, the variations in half-site sequence, flanking sequences, and spacing found in natural TR and RAR target genes generate response elements that are bound in vitro and transcriptionally activated in vivo in a receptor-specific manner. Finally, auxiliary proteins in cells may further contribute to receptor-specific target DNA recognition.;The work presented in this dissertation also examines the roles of RARs and TRs in disease. Mutant RARs are involved in Acute Promyelocytic Leukemia, and mutant TRs have been implicated in Resistance to Thyroid Hormone Syndrome. In Acute Promyelocytic Leukemia, impaired corepressor release from oncogenic x-RAR chimeras has been implicated in blocking proper myeloid differentiation. I found that corepressor splice variants are differentially expressed in NB4 cells, an APL cell line. This work complements other studies from our laboratory that find PML-RARalpha and PLZF-RARalpha to have different affinities for various splice variants. Taken together, it is possible that the aberrant corepressor binding properties of these chimeric receptors may contribute to their oncogenic nature.;And lastly, I examined the molecular differences in RTH mutants that may account for the phenotypic differences between generalized RTH (GRTH) and pituitary-specific RTH (PRTH). I found that while PRTH mutations are predominantly located in the dimerization interface of the receptor, these mutants are not impaired in a manner that correlates with the TRbeta2-specific transcriptional activation defects observed in PRTH and not GRTH. Therefore, altered dimerization of the RTH mutants is not likely to be the main cause for the different phenotypes of these two endocrine syndromes.