Targeted Next-Generation Sequencing of Congenital Hypothyroidism-Causative Genes Reveals Unexpected Thyroglobulin Gene Variants in Patients with Iodide Transport Defect.

Congenital iodide transport defect is an uncommon autosomal recessive disorder caused by loss-of-function variants in the sodium iodide symporter (NIS)-coding SLC5A5 gene and leading to dyshormonogenic congenital hypothyroidism. Here, we conducted a targeted next-generation sequencing assessment of congenital hypothyroidism-causative genes in a cohort of nine unrelated pediatric patients suspected of having a congenital iodide transport defect based on the absence of 99m Tc-pertechnetate accumulation in a eutopic thyroid gland. Although, unexpectedly, we could not detect pathogenic SLC5A5 gene variants, we identified two novel compound heterozygous TG gene variants (p.Q29* and c.177-2A>C), three novel heterozygous TG gene variants (p.F1542V fs *20, p.Y2563C, and p.S523P), and a novel heterozygous DUOX2 gene variant (p.E1496D fs *51). Splicing minigene reporter-based in vitro assays revealed that the variant c.177-2A>C affected normal TG pre-mRNA splicing, leading to the frameshift variant p.T59S fs *17. The frameshift TG variants p.T59S fs *17 and p.F1542V fs *20, but not the DUOX2 variant p.E1496D fs *51, were predicted to undergo nonsense-mediated decay. Moreover, functional in vitro expression assays revealed that the variant p.Y2563C reduced the secretion of the TG protein. Our investigation revealed unexpected findings regarding the genetics of congenital iodide transport defects, supporting the existence of yet to be discovered mechanisms involved in thyroid hormonogenesis.