Endocrine Abstracts

We aimed to identify causal mutation(s) in 2 patients (P1 and P2) with thyroid dyshormonogenesis (TD) from a consanguineous Tunisian family. Patient P1 developed TD at age 10; while P2 developed it at a late age (30 years) with no goiter. Scintigraphy showed homogeneous uptake of ¹³¹I in P1 patient. Genetic analysis was performed using candidate gene approach. Thus, sequencing of the 17 exons of the TPO gene revealed only presence of rs4927611 polymorphism (Ala257Ser) at the homozygous state in P1 and P2. Structural modeling of the rs4927611 polymorphism showed that it is rather lying in the entrance of the active site of TPO enzyme. The presence of a hydrophilic residue (Ser) instead of a hydrophobic one (Ala) might influence the substrate selection. Segregation analysis of this polymorphism showed that it was also present in unaffected family members, excluding involvement of TPO gene in TD in this family. In a second step, and in order to target responsible gene, we have performed perchlorate test in patient P1. The test result ruled out any form of defect in iodine organization, and rather suggested a possible defect in iodine transport. We then moved to sequencing of coding region (15exons) and the 5′ and 3′ UTR of NIS gene. No causal mutation was reported in P1 patient. However, we have identified rs7250061 polymorphism (c.699–75C> T) in intron 5 (MAF = the frequency of the minor allele T = 0.1113). ESEfinder program showed that this substitution creates a new splicing enhancer sequence “CAGAAGT” which is recognized by the splicing factor SRSF1 / SRSF1 (IgM-BRCA1) with a score of 3.46589 and 2.87763 respectively therefore significantly higher than the threshold values of 1.956 and 1.867 respectively. MFOLD program showed that the c.699–75C> T substitution has no marked effect on the RNA structure, excluding thus NIS gene in this family.

Search for causing TD gene will be performed using exhaustive approach (genome scan, exome analysis...) since informativeness of the studied family. Such gene identification may help to develop a genetic screening protocol for congenital hypothyroidism in Tunisia.