Sodium/Iodide Symporter Mutant V270E Causes Stunted Growth but No Cognitive Deficiency.

Iodide (I(-)), an essential constituent of the thyroid hormones, is actively accumulated in the thyroid by the Na(+)/I(-) symporter (NIS), a key plasma membrane protein encoded by the slc5a5 gene. Mutations in slc5a5 cause I(-) transport defects (ITDs), autosomal-recessive disorders in which I(-) accumulation is totally or partially impaired, leading to congenital hypothyroidism. The characterization of NIS mutants has yielded significant insights into the molecular mechanism of NIS. This study aimed to determine the basis of a patient's ITD clinical phenotype, by sequencing her slc5a5 gene. Genomic DNA was purified and the slc5a5 gene sequence determined. Functional in vitro studies were performed to characterize the V270E NIS mutant. The index patient was diagnosed with hypothyroidism with minimal radioiodide uptake in a normally located, although enlarged, thyroid gland. We identified a new NIS mutation: V270E. The patient had the compound heterozygous NIS mutation R124H/V270E. R124H NIS has been characterized previously. We show that V270E markedly reduces I(-) uptake via a pronounced (but not total) impairment of the protein's plasma membrane targeting. Remarkably, V270E is intrinsically active. Therefore, a negative charge at position 270 interferes with NIS cell surface trafficking. The patient's minimal I(-) uptake enabled sufficient thyroid hormone biosynthesis to prevent cognitive impairment. A nonpolar residue at position 270, which all members of the SLC5A family have, is required for NIS plasma membrane targeting.