New Insights into Iodide Transport Defects (ITDs) from the Characterization of a Heterozygous NIS Missense Variant...

The Na⁺/I^- symporter (NIS) is the plasma membrane (PM) protein that actively mediates iodide (I^-) transport into the thyroid gland. Pathogenic variants in the SLC5A5 gene cause iodide transport defects (ITDs). A heterozygous G288S NIS variant was identified in a Spanish family in which female carriers developed thyroid dysfunction during pregnancy. Here, we characterized the functional significance of the G288S variant and other substitutions at residue 288 of human NIS. Human NIS (hNIS) expression and maturation were analyzed by immunoblotting, its subcellular localization was analyzed by immunofluorescence and flow cytometry, and its activity was analyzed by radioiodide uptake assays. The G288S variant does not affect hNIS maturation, membrane trafficking, or I^- uptake capacity, but significantly reduces I^- affinity while preserving substantial transport activity. In contrast, substitutions introducing charged residues (arginine, aspartic acid, or glutamic acid) or proline severely disrupted NIS maturation, plasma membrane targeting, and iodide transport. Because the variant was identified in heterozygosity, we evaluated residue 288 substitutions under heterozygous-like conditions. Co-expression of the patient-derived G288S variant with WT NIS produced an intermediate apparent K_m without reducing V_max compared with WT, consistent with a modest co-expression-dependent kinetic effect rather than a strong dominant-negative mechanism. In contrast, the severely disruptive G288E substitution reduced cell-surface NIS expression under co-expression conditions, providing proof-of-principle evidence that severe alteration of residue 288 can impair NIS plasma membrane delivery. These findings highlight residue 288 as a key determinant of hNIS functionality and underscore the need to carefully evaluate heterozygous SLC5A5/NIS variants, as they may become clinically relevant under conditions of increased physiological iodine demand and contribute to partial iodide transport impairment.