Partial iodide organification defect caused by a novel mutation of the thyroid peroxidase gene in three siblings

Three siblings with goitre and latent to mild hypothyroidism were suspected of having thyroid peroxidase (TPO) abnormality. Direct sequencing of their genomic DNAs showed two novel mutations of the TPO gene, one of which was G1687T (Gly533Cys; exon 9) and the other 1808-13del (Asp574/Leu575del; exon 10). The two mutations were compound heterozygous, as the former was found in their father's DNA as heterozygous, and the latter was found in DNA from their mother, also as heterozygous. As Gly533 and Asp574/Leu575 were well-conserved amino acids in the peroxidase superfamily, Gly533Cys- and Asp574/Leu575del-TPOs were thought to be affected structurally or functionally. In expression studies using CHO-Kl cells and mRNAs introduced with individual mutations, both mutated TPO proteins were expressed at the same molecular size as wild-type TPO and had enzyme activity, although Gly533Cys-TPO was slightly lower in efficiency of expression and more degenerative than wild-type TPO.

We examined the localization of both mutated TPOs. Gly533Cys-TPO was located on the endoplasmic reticulum (ER) and nuclear envelope but not on the plasma membrane, whereas Asp574/Leu575del-TPO was located not only on the ER and nuclear envelope but also on the plasma membrane, as wild-type TPO. Nevertheless, only one point differed between Asp574/Leu575del- and wild-type TPOs: the mutated TPO was expressed on the plasma membrane surface at less than half the rate of wild-type TPO.

Gly533Cys-TPO synthesized almost no thyroid hormone because of its defective localization on the apical membrane surface of thyrocytes, whereas Asp574/Leu575del-TPO performed thyroid hormone synthesis at a rate of less half that of wild-type TPO. In cotransfection experiments using three combinations of wild-type and G1687T-mRNAs, wild-type and 1808-13del-mRNAs, and G1687T-, 1808-13del-mRNAs, the three kinds of mRNAs were considered to have no influence on cell surface TPO expression of another mRNA when a 50%-maximal amount of each mRNA was transfected. When a larger amount of each mRNA was transfected, the former two combinations showed the level of cell surface TPO expression obtained from the saturating amount of wild-type mRNA, whereas the last combination of mutated mRNAs covered only about half of the expression level.

Defective thyroid hormone production resulting from the abnormal TPOs was at a level that caused latent hypothyroidism when the patients were born. With their growth, thyroid hormone volume gradually became inadequate and their thyroid gland enlarged compensatorily.