Functional Characterization of a Novel Iodide Transport Defect (ITD)-Causing Na+/I- Symporter (NIS) Mutation

NICOLA, JP; SAENGER, P; MUZUMDAR, R; RODRIGUEZ-BURITICA, DF; GAMEZ GODOY, JD; CARRASCO, N

Vancouver

Congreso; Pediatric Academic Societies and Asian Society for Pediatric Research Joint Meeting; 2014

Pediatric Academic Societies and Asian Society for Pediatric Research

BACKGROUND: ITD is an uncommon cause of dyshormonogenetic congenital hypothyroidism that results from inactivatingmutations in the NIS gene. Clinical manifestations include low to absent thyroid and salivary iodide uptake and variabledegrees of hypothyroidism, goiter, and mental retardation.OBJECTIVE: To characterize a novel NIS mutation on two sisters with primary hypothyroidism.DESIGN/METHODS: Index patient presented at 7 3/12 years of age with severe primary hypothyroidism, growth failure anddelayed bone age. Thyroid autoantibodies were negative. US showed a multinodular gland. Thyroid uptake scandemonstrated a decreased uptake of 5.4% and absent uptake in salivary glands. Her sibling, at 2.5 years, showed mildhypothyroidism. Both had marked initial elevations of thyroglobulin that showed only mild decrease after treatment. Thegenomic DNA encoding NIS was sequenced in the 2 sisters and parents, and in vitro functional studies of a newly identifiedNIS mutation were carried out.RESULTS: Both patients were compound heterozygous for mutations R124H/V270E. The R124H NIS mutant has previouslybeen identified as a cause of ITD. The newly identified V270E NIS mutation caused a substantial decrease in I- uptake whenexpressed in COS-7 cells. Flow cytometry revealed a severe impairment in targeting of V270E NIS to the plasma membrane.Strikingly, membrane vesicles from V270E NIS-expressing cells transport I- approximately as much as vesicles fromwild-type NIS-expressing cells, indicating that, although trafficking of V270E NIS to the cell surface is impaired, the proteinitself is highly functional. V270D also resulted in intracellular retention, indicating that a negative charge is not tolerated atposition 270. Remarkably, V270Q NIS is targeted to the cell surface.CONCLUSIONS: We demonstrate that the V270E substitution reduces NIS targeting to the plasma membrane in two patientswith congenital hypothyroidism, thus causing a decrease in I- uptake even though V270E NIS is intrinsically active. This mayexplain the unusual phenotype in the 7yr old index patient, who despite severe hypothyroidism is intellectually normal.