Thyroid Hormone Receptor α1 Mutants Impair B Lymphocyte Development in a Mouse Model

Background: Mutations of the thyroid hormone receptor α (THRA) gene cause resistance to thyroid hormone (RTHα). RTHα patients exhibit very mild abnormal thyroid function test results (serum triiodothyronine can be high-normal to high; thyroxine normal to low; thyrotropin is normal or mildly raised) but manifest hypothyroid symptoms with growth retardation, delayed bone development, and anemia. Much has been learned about the in vivo molecular actions in TRα1 mutants affecting abnormal growth, bone development, and anemia by using a mouse model of RTHα (Thra1^PV/+ mice). However, it is not clear whether TRα1 mutants affect lymphopoiesis in RTHα patients. The present study addressed the question of whether TRα1 mutants could cause defective lymphopoiesis. Methods: We assessed lymphocyte abundance in the peripheral circulation and in the lymphoid organs of Thra1^PV/+ mice. We evaluated the effect of thyroid hormone on B cell development in the bone and spleen of these mice. We identified key transcription factors that are directly regulated by TRα1 in the regulation of B cell development. Results: Compared with wild-type mice, a significant reduction in B cells, but not in T cells, was detected in the peripheral circulation, bone marrow, and spleen of Thra1^PV/+ mice. The expression of key transcription regulators of B cell development, such as Ebf1, Tcf3, and Pax5, was significantly decreased in the bone marrow and spleen of Thra1^PV/+ mice. We further elucidated that the Ebf1 gene, essential for lineage specification in the early B cell development, was directly regulated by TRα1. Thus, mutations of TRα1 could impair B cell development in the bone marrow via suppression of key regulators of B lymphopoiesis. Conclusions: Analysis of lymphopoiesis in a mouse model of RTHα showed that B cell lymphopoiesis was suppressed by TRα1 mutations. The suppressed development of B cells was, at least in part, via inhibition of the expression of key regulators, Ebf1, Tcf3, and Pax5, by TRα1 mutations. These findings suggest that the mutations of the THRA gene in patients could lead to B cell deficiency.