CONICET | Buscador de Institutos y Recursos Humanos

Introduction: Iodide transport defect (ITD) is an autosomal recessive disorder whose hallmark is the inability of the thyroidfollicular cell to actively accumulate iodide. ITD is an uncommon cause of dyshormonogenesis congenital hypothyroidismthat results from inactivating mutations in the slc5a5 gene ? which encodes the sodium iodide symporter (NIS). The clinicaland biochemical presentation of ITD include low to absent thyroid and salivary iodide accumulation and, if untreated, thepatients develop a variable degree of hypothyroidism, goiter, and even mental retardation. Objectives: To determine if a pediatricpatient with a clinical phenotype of ITD harbors an inactivating mutation in the slc5a5 gene, and if so, to ascertain themolecular mechanisms of the effect of the mutation on the biogenesis and activity of NIS. Methods: The whole coding regionof the slc5a5 was PCR-amplified and subjected to Sanger sequencing, and in silico computational and in vitro functional studiesof a newly identified NIS mutation were performed. Results and conclusions: We report a novel homozygous missenseand loss-of-function mutation in the slc5a5 gene as a cause of ITD in a pediatric patient with dyshormonogenic congenitalhypothyroidism. The patient carries a G>A transition at position +1.682 in exon 14 resulting in a Gly to Glu substitution atresidue 561 (G561E) not previously reported in public reference exome databases. We show that G561E markedly reducesiodide uptake when the protein is heterologously expressed in MDCK-II cells, because targeting of G561E NIS to the plasmamembrane is severely impaired. Replacing G561 with Gln also resulted in severe intracellular retention, suggesting that a bulkyside-chain rather than a negative charge at position 561 interferes with NIS cell surface trafficking. Bioinformatics and biochemicalanalysis indicates that G561E impair the recognition of an adjacent tryptophan-acid domain by the kinesin light chain 2,thus impairing mutant NIS exit from the endoplasmic reticulum and subsequent plasma membrane targeting. Altogether, ourresults indicate that a small residue at position 561 is required for NIS maturation and plasma membrane trafficking. Of note,comparison of slc5a5 gene sequence across different species indicates a high conservation of the kinesin light chain 2-recognizedtryptophan-acid domain.