Engineering substrate selectivity in the human sodium/iodide symporter (NIS)

Iodide (I^-) uptake mediated by the Na⁺/I^- symporter (NIS) is the first step in the biosynthesis of the thyroid hormones, of which I^- is an essential constituent. NIS couples the inward transport of I^- against its electrochemical gradient to the inward translocation of Na⁺ down its electrochemical gradient. NIS also transports oxyanions (XO₄ ^-s), such as perrhenate (ReO₄ ^-) and the environmental pollutant perchlorate (ClO₄ ^-). Furthermore, NIS is the basis for radioiodide (¹³¹I^-) therapy for thyroid cancer (administered after thyroidectomy), the most effective targeted internal radiation cancer therapy available. ¹³¹I^- selectively targets remnant malignant cells and metastases expressing NIS, causing only minor side effects. There is great interest in expressing NIS exogenously, by gene transfer, in extrathyroidal cancers to render them susceptible to destruction by ¹³¹I^-. This approach, however, would also harm patients’ thyroids. Therefore, a strategy is needed for killing non-thyroidal cancer cells exogenously expressing NIS while protecting the thyroid. Addressing this need, we present here an engineered double mutant, L253P/V254F (PF)-NIS, which selectively transports XO₄ ^-s but not I^-. We used cryo-EM to determine the structure of PF-NIS with ReO₄ ^- and Na⁺ ions bound to it at a 2.58 Å resolution, and showed that high concentrations of non-radioactive I^- protect WT-NIS-expressing cells from radioactive ¹⁸⁶ReO₄ ^-, whereas PF-NIS-expressing cells are killed. Thus, PF-NIS could potentially be used, together with ^186/188ReO₄ ^- and non-radioactive I^-, to treat non-thyroidal cancers while safeguarding the thyroid. This study establishes a framework for developing therapies using NIS molecules engineered to have selective substrate specificities to extend the clinical use of NIS beyond thyroid cancer.