A Genetically-Engineered Thyroid Gland Built for Selective Triiodothyronine Secretion.

Thyroid hormones (thyroxine, T, and triiodothyronine, T) 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 T) and the C-terminal CholinEsterase-Like (ChEL) domain (responsible for synthesis of most T, which can also be generated extrathyroidally by T deiodination). Using CRISPR/Cas9-mediated mutagenesis, we engineered a knock-in of secretory ChEL into the endogenous locus. Secretory ChEL acquires Golgi-type glycans and is properly delivered to the thyroid follicle lumen, where T is first formed. Homozygous knock-in mice are capable of thyroidal T synthesis but largely incompetent for T synthesis such that T-to-T conversion contributes little. Instead, T production is regulated thyroidally by thyrotropin (TSH). Compared to mice with conventional hypothyroidism (low serum T and T), the body size of ChEL-knock-in mice is larger; although, these animals with profound T deficiency did exhibit a marked elevation of serum TSH and a large goiter, despite normal circulating T 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 T versus T action, reflecting key considerations in patients receiving thyroid hormone replacement therapy.