Identification of a Novel Mutation in the Carboxy-Terminus of the Sodium/Iodide Symporter in a Patient with Congenital Hypothyroidism

NICOLA, JP; MARTIN, M; SIGNORINO, M; TESTA, G; SOBRERO, G; MUÑOZ, L; MIRAS, M; MASINI-REPISO, AM

Boston

Congreso; ENDO 2016; 2016

Endocrine Society

I- transport defect (ITD) is an autosomal recessive disorder caused by the inability of the thyroid cell to actively accumulate I-, which leads to dyshormonogenic congenital hypothyroidism. The diagnostic criteria for ITD include a variable degree of goiter, low to absent thyroid radioiodide uptake, and low I- saliva-to-serum ratio. However, the clinical presentation of the resulting phenotypes varies from euthyroid to severe hypothyroidism.Active I- accumulation?the first step in the biosynthesis of thyroid hormones?is mediated by the Na+/I- symporter (NIS), an integral plasma membrane glycoprotein located on the basolateral surface of thyrocytes. To date, fifteen different loss-of-function mutations in the gene encoding NIS have been identified as causes of ITD.We aimed to analyze the presence of NIS gene mutations in a pediatric patient suspected of ITD on the basis of severely reduced 99mTc-pertechnetate accumulation in a eutopic thyroid gland. The index patient showed abnormally high TSH level during neonatal screening (64 µIU/ml). Ten days after birth, diagnostic confirmation of congenital hypothyroidism was achieved by measuring serum TSH 203 µIU/ml, FT4 1.6 ng/dl, T4 8.7 µg/dl, and T3 121 ng/dl. Slightly increased serum thyroglobulin concentration was evidenced (84 ng/ml). Thyroid autoantibodies were negative. Ultrasound showed a normal-sized gland. Thyroid hormone supplementation was started immediately after diagnostic.Genomic DNA was extracted from peripheral blood mononuclear cells. All 15 exons of the gene encoding NIS were examined using PCR and Sanger sequencing. The analysis revealed a previously unidentified homozygous G>A transition at nucleotide +1682 in exon 14 resulting in a glutamic acid instead of a glycine at position 561 (G561E). The genetic analysis was approved by the ethics committee of the Hospital de Niños de la Santísima Trinidad.The hemaglutinin (HA)-tagged human NIS cDNA wild-type (WT) or mutated (G561E) was transiently transfected into non-polarized Cos-7 cells which do not express NIS endogenously. Surprisingly, cells transfected with G561E NIS displayed 125I- uptake levels similar to those of cells expressing WT NIS. Flow cytometry analysis using an anti-HA antibody directed against an extracellular HA tag engineered onto the amino terminus of NIS in non-permeabilized cells, showed that the levels of G561E NIS at the plasma membrane were similar to those of WT NIS.Although the mechanism by which G561E mutation impairs NIS activity remains unknown, we hypothesized that the negative charge of the Glu residue may interfere the recognition of the dileucine sorting motif (L562L563) located in NIS carboxy-terminus by adaptor proteins, thus affecting NIS basolateral plasma membrane sorting in polarized cells. Further evaluation of G561E NIS in polarized cells is likely to provide novel evidence regarding NIS targeting to the plasma membrane.