Silent but Not Harmless: A Synonymous SLC5A5 Gene Variant Leading to Dyshormonogenic Congenital Hypothyroidism

BACKGROUND: Congenital iodide transport defect (ITD) is an uncommon cause of dyshormonogenic congenital hypothyroidism characterized by the absence of active iodide accumulation in the thyroid gland. ITD is an autosomal recessive disorder caused by loss-of-function variants in the sodium/iodide symporter (NIS)-coding SLC5A5 gene. OBJECTIVE: We aimed to identify, and if so to functionally characterize, novel ITD-causing SLC5A5 gene variants in a cohort of five unrelated pediatric patients diagnosed with dyshormonogenic congenital hypothyroidism with minimal to absent (99m)Tc-pertechnetate accumulation in the thyroid gland. METHODS: The coding region of the SLC5A5 gene was sequenced using Sanger sequencing. In silico analysis and functional in vitro characterization of a novel synonymous variant were performed. RESULTS: Sanger sequencing revealed a novel homozygous synonymous SLC5A5 gene variant (c.1326A>C in exon 11). In silico analysis revealed that the c.1326A>C variant is potentially deleterious for NIS pre-mRNA splicing. The c.1326A>C variant was predicted to lie within a putative exonic splicing enhancer reducing the binding of splicing regulatory trans-acting protein SRSF5. Splicing minigene reporter assay revealed that c.1326A>C causes exon 11 or exon 11 and 12 skipping during NIS pre-mRNA splicing leading to the NIS pathogenic variants p.G415_P443del and p.G415Lfs*32, respectively. Significantly, the frameshift variant p.G415Lfs*32 is predicted to be subjected to degradation by nonsense-mediated decay. CONCLUSIONS: We identified the first exonic synonymous SLC5A5 gene variant causing aberrant NIS pre-mRNA splicing, thus expanding the mutational landscape of the SLC5A5 gene leading to dyshormonogenic congenital hypothyroidism.