TRANSPORT DEFECT-CAUSING NIS MUTANTS UNCOVER A CRITICAL TRYPTOPHAN-ACID MOTIF REQUIRED FOR PLASMA MEMBRANE TRANSPORT
Mariano Martín1, Carlos Modenutti2, Victoria Peyret1, Romina C. Geysels1, Carlos E. Bernal Barquero1, Malvina Signorino3, Graciela Testa3, Patricia Papendieck4, Ana María Masini-Repiso1, Ana Elena Chiesa4, Mirta Beatriz Miras3, Marcelo Adrián Martí2, Juan Pablo Nicola1
Introduction: I- transport defect (ITD) is an autosomal recessive disorder characterized by impaired thyroidal I- accumulation due to loss-of-function mutations in the Na+/I- symporter (NIS)-coding SLC5A5 gene. Objectives: To characterize novel homozygous (p.G561E) and compound heterozygous (p.G543R and p.L562M) SLC5A5 mutants found in two ITD-suspected patients on the basis of non-detectable I- accumulation in an eutopic thyroid gland. Methods: SLC5A5 gene coding region was PCR-amplified and subjected to Sanger sequencing. In silico and functional in vitro studies of novel NIS variants were performed. Results: Functional studies revealed that G561E and L562M markedly reduces I- uptake, when the proteins are heterologously expressed in non-thyroid epithelial cells, because their targeting to the plasma membrane is severely impaired. G543R NIS was previously reported as non-functional. G561Q NIS, like G561E, is mainly retained in the endoplasmic reticulum (ER). Bioinformatics reveal a fully conserved tryptophan-acidic (WD) motif whose disruption leads to NIS retention in the ER. Computational and biochemical analysis indicate that G561E and L562M impair the recognition of the flanking WD motif by the kinesin light chain (KLC) 2, thus impairing mutant NIS exit from the ER. Moreover, short-hairpin RNA-mediated KLC2 knock-down in FRTL-5 cells reduces NIS expression at the plasma membrane, and consequently NIS-mediated I- accumulation. Conclusion: Newly identified NIS variants negatively impact on the three-dimensional structure of its flanking WD motif severely reducing the interaction with the ER-to-Golgi transport adaptor KLC2, thus impairing NIS maturation beyond the ER and reducing I- accumulation in thyroid follicular cells. Conflict of interest: None declared.