Identification of a mutation in the coding sequence of the human thyroid peroxidase gene causing congenital goiter

Clinical and molecular genetics studies in Pendred's syndrome

A large and highly inbred kindred including patients with incomplete and complete forms of Pendred's syndrome was studied. Blood samples were collected from 42 individuals (23 affected and 19 normal), and serum thyroid hormones, TSH, Tg, and anti-TPO autoantibodies were assayed. Thyroid function studies have indicated euthyroidism in all 42 individuals. The affected subjects, however, had significantly elevated serum Tg levels (19.4 +/- 6.8 ng/dL) as compared with normals (9.6 +/- 2.9 ng/dL). Nineteen subjects had clinical and or ultrasonographic evidence of a multinodular goiter. In addition, 13 individuals had impaired hearing with or without goiter. Computer axial tomography scan studies in six patients confirmed the presence of a defective cochlea (Mondini's cochlear defect) in three of these subjects. It has been suggested that thyroperoxidase (TPO) in patients with Pendred's syndrome might be defective for coupling but could be partially effective for iodide organification. We have investigated possible abnormalities in the TPO gene by Southern blot analysis. Genomic DNA was obtained from peripheral blood leukocytes of 40 subjects (22 affected and 18 normal). DNA samples were digested with five restriction enzymes and hybridized with the pM5 probe (831 bp). Polymorphic fragment patterns obtained with three of the five enzymes employed were equally distributed in normal and affected subjects of this kindred. Lod score analysis did not disclose any linkage of TPO gene polymorphisms with the phenotypic characteristics observed in this family. Our findings may be explained in two different ways. First one might have hitherto undetected mutations in the TPO gene, and, second, the pathology may in fact be due to a genetic defect lying elsewhere.

Defective organification of iodide causing hereditary goitrous hypothyroidism

We present a survey of the current state of knowledge about the prevalence of the syndrome involved in defective organification of iodide, and the mechanism of iodination and coupling catalyzed by the thyroid peroxidase (TPO) enzyme. A brief summary of the recent developments in molecular cloning of TPO and regulation of TPO gene expression is also included. Methods for purification of the enzyme and details about the assessment of TPO activity in tissue are briefly explained. The classification of defective organification of iodide is primarily based on the site of the biochemical defect, being quantitative (TPO absent) or qualitative (TPO structure, localization or apoenzyme are defectives). The presence of TPO inhibitors is also briefly described. The rare possibility of an absent source of peroxide (H2O2) causing defective iodide organification is discussed. Analysis of the 118 reported cases shows that the biochemical classification covers a spectrum of abnormalities and it is likely that further molecular biology studies will increase this heterogeneity as well as refining it. Genetic studies have suggested linkage between the TPO gene polymorphisms and the iodide organification defect and can be of importance for carrier detection and prenatal diagnosis. Neonatal screening for hypothyroidism is likely to expand the number of cases available for DNA analysis and possibly the molecular diagnosis. The importance of the mutations that would affect the histidine (His) residues in the translated protein was recently documented by the finding of a deletion removing part of exon 9 and thus also deleting a proximal His residue. The resulting TPO enzyme was inactive for iodide organification and coupling reaction. It is hoped that in time we will be able to expand our knowledge of the molecular diagnosis of the inborn errors of iodide organification.