A Genetically-Engineered Thyroid Gland Built for Selective Triiodothyronine Secretion

Thyroid hormones (thyroxine, T₄, and triiodothyronine, T₃) are indispensable for sustaining vertebrate life, and their deficiency gives rise to a wide range of symptoms characteristic of hypothyroidism, affecting 5-10% of the world's population. The precursor for thyroid hormone synthesis is thyroglobulin (Tg), a large iodoglycoprotein consisting of upstream regions I-II-III (responsible for synthesis of most T₄) and the C-terminal CholinEsterase-Like (ChEL) domain (responsible for synthesis of most T₃, which can also be generated extrathyroidally by T₄ deiodination). Using CRISPR/Cas9-mediated mutagenesis, we engineered a knock-in of secretory ChEL into the endogenous <i>TG</i> locus. Secretory ChEL acquires Golgi-type glycans and is properly delivered to the thyroid follicle lumen, where T₃ is first formed. Homozygous knock-in mice are capable of thyroidal T₃ synthesis but largely incompetent for T₄ synthesis such that T₄-to-T₃ conversion contributes little. Instead, T₃ production is regulated thyroidally by thyrotropin (TSH). Compared to <i>cog/cog</i> mice with conventional hypothyroidism (low serum T₄ and T₃), the body size of ChEL-knock-in mice is larger; although, these animals with profound T₄ deficiency did exhibit a marked elevation of serum TSH and a large goiter, despite normal circulating T₃ levels. ChEL knock-in mice exhibited a normal expression of hepatic markers of thyroid hormone action but impaired locomotor activities and increased anxiety-like behavior, highlighting tissue-specific differences in T₃ versus T₄ action, reflecting key considerations in patients receiving thyroid hormone replacement therapy.