Genetic and Pharmacological Targeting of Transcriptional Repression in Resistance to Thyroid Hormone Alpha

Background: Thyroid hormones act in bone and cartilage via thyroid hormone receptor alpha (TR alpha). In the absence of triiodothyronine (T3), TR alpha interacts with co-repressors, including nuclear receptor co-repressor-1 (NCoR1), which recruit histone deacetylases (HDACs) and mediate transcriptional repression. Dominant-negative mutations of TR alpha cause resistance to thyroid hormone alpha (RTH alpha; OMIM 614450), characterized by excessive repression of T3 target genes leading to delayed skeletal development, growth retardation, and bone dysplasia. Treatment with thyroxine has been of limited benefit, even in mildly affected individuals, and there is a need for new therapeutic strategies. It was hypothesized that (i) the skeletal manifestations of RTH alpha are mediated by the persistent TR alpha/NCoR1/HDAC repressor complex containing mutant TR alpha, and (ii) treatment with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) would ameliorate these manifestations. Methods: The skeletal phenotypes of (i) Thra1(PV/+) mice, a well characterized model of RTH alpha; (ii) Ncor1(Delta ID/Delta ID) mice, which express an NCoR1 mutant that fails to interact with TR alpha; and (iii) Thra1(PV/+)Ncor1(Delta ID/Delta ID) double-mutant adult mice were determined. Wild-type, Thra1(PV/+), Ncor1(Delta ID/Delta ID), and Thra1(PV/+)Ncor1(Delta ID/Delta ID) double-mutant mice were also treated with SAHA to determine whether HDAC inhibition results in amelioration of skeletal abnormalities. Results: Thra1(PV/+) mice had a severe skeletal dysplasia, characterized by short stature, abnormal bone morphology, and increased bone mineral content. Despite normal bone length, Ncor1(Delta ID/Delta ID) mice displayed increased cortical bone mass, mineralization, and strength. Thra1(PV/+)Ncor1(Delta ID/Delta ID) double-mutant mice displayed only a small improvement of skeletal abnormalities compared to Thra1(PV/+) mice. Treatment with SAHA to inhibit histone deacetylation had no beneficial or detrimental effects on bone structure, mineralization, or strength in wild-type or mutant mice. Conclusions: These studies indicate treatment with SAHA is unlikely to improve the skeletal manifestations of RTH alpha. Nevertheless, the findings (i) confirm that TR alpha 1 has a critical role in the regulation of skeletal development and adult bone mass, (ii) suggest a physiological role for alternative co-repressors that interact with TR in skeletal cells, and (iii) demonstrate a novel role for NCoR1 in the regulation of adult bone mass and strength.