SILENT BUT NOT HARMLESS: A SYNONYMOUS SLC5A5 GENE VARIANT LEADING TO DYSHORMONOGENIC CONGENITAL HYPOTHYROIDISM

EVENTO VIRTUAL

Congreso; XVIII Latin American Thyroid Congress; 2021

Sociedad Latinoamericana de Tiroides

Introduction: Iodide transport defect (ITD) is an autosomal recessive disorder whose hallmark is the inability of the thyroid follicularcell to actively accumulate iodide. ITD is an uncommon cause of dyshormonogenetic congenital hypothyroidism that results frominactivating mutations in the sodium iodide symporter (NIS)-coding gene SLC5A5. Phonotypical manifestations include low to absentthyroid iodide accumulation and, if untreated, variable degree of hypothyroidism, goiter, and even mental retardation. Objective: Toinvestigate the presence of pathogenic variant in SLC5A5 gene in five unrelated patients with low to absent iodide accumulation ina normally located thyroid gland. Methods: SLC5A5 gene was sequenced. In silico and functional in vitro characterization of theSLC5A5 variants was performed using splicing prediction tools and splicing minigene reporter assay, respectively. Results: Patients?SLC5A5 gene sequencing revealed the homozygous synonymous variants c.1,326A>C (exon 11) and c.1,626C>T (exon 13) intwo patients, whereas no variants were identified in the remaining patients. The variant c.1,326A>C (rs73520743) showed an allelefrequency of 0.001103 in the European (non-Finnish) population, while c.1,626C>T (rs45602038) was 0.02997, according to TheGenome Aggregation Database. In silico analysis assessing the effect of variants on splicing regulatory elements using the softwaresESEfinder and HExoSplice revealed that c.1326A>C is potentially deleterious for normal NIS pre-mRNA splicing. The variantc.1326A>C was predicted to lie at a potential exonic splicing enhancer (ESE) motif using ESEfinder software. The motif carryingthe variant c.1326A>C exhibited a decrease in the splicing regulatory protein SRSF5 score to below the threshold level. In addition,the analysis using HExoSplice software revealed that the variant c.1326A>C causes a significant negative effect on the exonic splicingregulatory (ESR) sequence (ΔtESRseq = -0.5932) whose identity overlaps with the ESEfinder-prediced SRSF5 binding site. SplicingpSPL3-based minigene assay in HeLa cells revealed that c.1326A>C causes exon 11 skipping during NIS pre-mRNA splicing leadingto the in-frame NIS deletion variant p.G415_P443del. Conclusions: We identified the exonic synonymous c.1326A>C SLC5A5gene variant causing aberrant NIS pre-mRNA splicing, thus expanding the mutational landscape in the SLC5A5 gene leading todyshormonogenic congenital hypothyroidism.