Objective: Thyroxine binding globulin (TBG) is the most important thyroid hormone transporter in humans and is encoded by the SERPINA7 gene located on chromosome Xq22.2. By analyzing the genes of TBG-deficient patients, we aim to find a new molecular basis for the possible etiology of the disease.
Design and Methods: 10 groups of subjects were enrolled in the pediatric department of the First Affiliated Hospital of Zhejiang University School of Medicine from 2016.1-2017.12 for the diagnosis of TBG deficiency. SERPINA7 gene was detected by PCR and whole-genome sequencing. Totally, six mutations were found, one was a deletion mutation and the other five were missense mutations. Analyzed the conserved and pathogenicity of these mutations using the online program UCSC Genome Brower, MutationTaster, and PolyPhen-2, and analyzed the three-dimensional structure of the variant proteins using Swiss-Model. Cloned the wild type and mutant cDNA into pcDNA3.1(+) plasmid vector, and then transfected the HEK-293T cells for 48h, measured TBG levels in cell culture medium by ELISA. 6 single mutations and 7 groups of complex mutations were analyzed to evaluate the effect on TBG secretion.
Results: Identified 10 patients who carried complex hemizygous mutation (c.271G>A/c.909G>T, c.275T>C/c.909G>T, c.631G>A/ c.909G>T, c.880C>T/ c.909G>T, c.927delC/ c.909G>T, c.271G>A/ c.631G>A/ c.909G >T). 1 deletion mutation (c. 927delC, p. I310Ffs10) and 4 single nucleotide polymorphism (SNP) (c.271G>A, p.E91K, c.631G>A, p.A211T, c.880C>T, p.R294C, c.909G>T, p.L303F), 1 possible pathogenic mutation (c.275T>C, p.I92T). One of the 10 patients carried a single hemizygous mutation c.909G>T, p.L303F; 3 patients carried a composite hemizygous mutation c.631G>A, p.A211T/c.909G>T, p.L303F; 2 cases carried c.275T>C, p.I92T/c.909G>T, p.L303F; the other 4 cases were c.271G>A, p.E91K/ c.909G>T, p.L303F, c. 880C>T, p.R294C /c.909G>T, p.L303F and c.909G>T, p.L303F /c. 927delC, p. I310Ffs10. One case is a multiple complex hemizygous mutation c.271G>A, p.E91K/c.631G>A, p.A211T /c.909G>T, p.L303F. In vitro expression analysis showed that TBG secretion caused by a single missense mutation was only slightly reduced or not decreased, while the composite mutation resulted in a significant decrease in TBG secretion, and the deletion mutation resulted in complete TBG deficiency. In addition, the more mutations carried, the greater the effect on TBG secretion.
Conclusion: We believe that the polymorphism of the SERPINA7 gene can be pathogenic, especially has multiple SNP sites at the same time, which could lead to decreased expression. The more SNPs are carried, the lower the TBG and the higher the likelihood of complete TBG.
Keywords: TBG deficiency; SERPINA7 gene; mutation; SNP
19 Sep 2019 - 21 Sep 2019