A goitrous subject with structural abnormality of thyroglobulin

Two different mutations in the thyroid peroxidase gene of a large inbred Amish kindred: power and limits of homozygosity mapping

Approximately 10% of newborns with congenital hypothyroidism are unable to convert iodide into organic iodine. This iodide organification defect has a prevalence of 1 in 40,000 newborns and may be caused by defects in the thyroid peroxidase enzyme (TPO), the hydrogen peroxide-generating system, the TPO substrate thyroglobulin, or inhibitors of TPO. We identified a high incidence of severe hypothyroidism due to a complete iodide organification defect in the youngest generation of five nuclear families belonging to an inbred Amish kindred. Genealogical records permitted us to trace their origin to an ancestral couple 7-8 generations back and to identify an autosomal recessive pattern of inheritance. Initial studies of homozygosity by descent using two polymorphic markers within the TPO gene showed no linkage to the phenotype. In fact, 4 of 15 affected siblings from 2 of the nuclear families were heterozygous, resulting in homozygosity values of 73% and 53% in affected and unaffected family members, respectively. A genome-wide homozygosity screen using DNA pools from affected and unaffected family members localized the defect to a locus close to the TPO gene. Linkage analysis using 4 additional polymorphic markers within the TPO gene reduced the number of homozygous unaffected siblings to zero without altering the percent homozygosity initially found in the affected. Sequencing of the TPO gene revealed 2 missense mutations, E799K and R648Q. TPO 779K was found in both alleles of the 11 affected homozygotes, both mutations were present in each of the 3 affected compound heterozygotes, and there were no TPO mutations in 1 subject with hypothyroidism of different etiology. These results demonstrate the power of the DNA pooling strategy in the localization of a defective gene and the pitfalls of linkage analysis when 2 relatively rare mutations coexist in an inbred population.

Congenital euthyroid goitre with impaired thyroglobulin transport

A case of congenital goitre with defective thyroglobulin (Tg) synthesis was studied from the clinical, biochemical and morphological perspectives. The patient, 5.5-year-old boy, who was clinically euthyroid, showed a positive perchlorate discharge test (37.2%). However, the iodination system seemed to be normal since radioiodine uptake into the thyroid was very high, and inspection of the H2O2-generating system using thyroid slices and an assay for peroxidase activity in microsomes showed no abnormalities. On the other hand, virtually no Tg was detected in the serum, and the amount of Tg in thyroid tissue, estimated with gel electrophoresis, was below 10% of the normal value, the quality of Tg being unchanged. Morphological observations demonstrated the presence of Tg in the markedly distended rough endoplasmic reticulum of the cytoplasm of follicular cells and a lack of Tg in the colloid of the follicular lumen. These results suggest that the thyroid is defective in Tg synthesis, probably associated with impaired transport of Tg from the cells to the lumen.

Thyroid peroxidase activity in euthyroid and mild hypothyroid patients with Hashimoto's thyroiditis

Thyroid peroxidase (TPO) activity was measured spectrophotometrically according to Hosoya's guaiacol method. The mean TPO activity in ten patients with Hashimoto's thyroiditis was 19.8 +/- 7.6 (mean +/- SE) in an arbitrary unit, which was not significantly different from the normal value in seven normal thyroid tissues (33.7 +/- 5.4). The ten patients were divided into two groups, euthyroid and mild hypothyroid, on the basis of their basal serum TSH. In the euthyroid group, TPO activity (8.17 +/- 1.3) was significantly less than the normal tissue (p less than 0.01). In the hypothyroid group, TPO activity (27.64 +/- 13.8) was almost similar to the normal tissue. A positive correlation was obtained between TPO activity and serum TSH in ten patients with Hashimoto's thyroiditis (r = 0.85, p less than 0.01). It was concluded that TPO activity is significantly decreased in Hashimoto's thyroiditis even when the thyroid function was still within normal range, but the activity might be restored to normal in hypothyroid patients by the stimulation of elevated TSH.

Biosynthesis of thyroid hormone: basic and clinical aspects

Thyroid hormone formation requires the coincident presence of peroxidase, H2O2, iodide, and acceptor protein at one anatomic locus in the cell. The peroxidase enzyme appears to be a protoporphyrin lX containing heme protein, with binding sites for both iodide and tyrosine. It is probable that both iodide and tyrosine are oxidized to free radical forms which unite to form iodotyrosine. The peroxidase is also involved through an uncertain mechanism in iodotyrosine coupling and probably in oxidation of sulfhydryl bonds in thyroglobulin. H2O2 may be supplied by microsomal NADPH-cytochrome c reductase or NADH-cytochrome b5 reductase. Other possible intracellular H2OI generating systems include monoamine oxidase and xanthine oxidase. The usual acceptor for iodide is thyroglobulin, which is currently believed to be iodinated within apical secretory vesicles at the cell border just prior to liberation into the colloid, or possibly after liberation into the colloid. Other soluble an insoluble proteins are also iodinated within the gland. The peroxidase is present in numerous cellular structures, but iodination activity occurs primarily, if not only, at the apical cell border. The controls of iodination are imperfectly known. Thyrotrophin modulation of iodide uptake, H2O2 generation, thyroglobulin synthesis, and peroxidase enzyme level obviously are the main regulations. Many of these actions are thought to involve mediation of adenyl cyclase and subsequent activation of intracellular phosphokinases. Antithyroid drugs of the thiocarbamide group are competitive inhibitors of iodination under some circumstances, but if much iodide is present, they react with the oxidized iodine intermediate and are irreversibly inactivated themselves. Clinical problems involving defective peroxidase function are among the most frequent hereditary defects of thyroid hormone formation. Recognized abnormalities include deficient peroxidase, abnormality in binding of the peroxidase apoprotein to its prosthetic group, and other less well-identified abnormalities in peroxidase structure and function. Peroxidase is typically elevated in thyroid tissue from patients with hyperthyroidism sometimes deficient in cold thyroid nodules, and frequently diminished in tissue from patients with Hashimoto's thyroiditis.

Familial goitre with partial iodine organification defect, lack of thyroglobulin, and high levels of thyroid peroxidase

From a sibship of three sisters having congenital goitre and normal hearing, two had impairment of organification of iodine. S1 (4 years old) had goitre since birth, euthyroidism, and a negative perchlorate test. S2 (15 years old) and S3 (13 years old) were hypothyroid, and had radioiodide discharge after potassium perchlorate administration of 19.8% and 26.1%, respectively. Thyroid tissue was obtained at thyroidectomy. Peroxidase activity, in the thyroidal subcellular particles, was found to be qualitatively normal, but quantitatively increased. In the triiodide assay, the activity was: S1 6912 u, S2 2590 u, and S3 3844 u (normal values 900-1700 u). In the tyrosine-iodinase assay, the activities, expressed as nmoles of iodide incorporation per gram of tissue, were S1 1046, S2 471 (normal values 220-410). The activity of the thyroidal NADPH-cytochrome c reductase, an enzyme possibly involved in hydrogen peroxide generation, was: S1 0.084, S2 0.047, and S3 0.005 (normal values 0.018 muEq/min/mg). No thyroglobulin was detected by analytical ultracentrifugation, polyacrylamide gel electrophoresis, or double immunodiffusion in agar of the supernatant fractions. In patient S2, whose gland was labelled in vivo with 125I, 60% of the total radioactivity of the gland (pooled nodular and paranodular specimens) was in a particulate iodoprotein that was solublilized by trypsin, deoxycholate or digitonin. In the soluble fraction there were two iodoproteins: iodalbumin, and a second iodoprotein similar to the solubilized particulate iodoprotein. It is postulated that absence of the normal thyroidal receptor protein might be in some cases a cause of iodine organification defect.

Deficient cytochrome b5 reductase activity in nontoxic goiter with iodide organification defect

A 37-yr-old woman with nontoxic goiter is presented. The thyroid 131I uptake at 3 and 24 hr were, respectively, 77.1% and 81.4% dose. Thiocyanate discharged 65.5% of the accumulated 131I in 30 min. In vitro organification of iodine in the thyroid homogenate from the patient was impaired and it was restored to normal by the addition of H2O2, glucose, and glucose oxidase system, FAD, or reduced cytochrome b5. Riboflavin, FMN, oxidized cytochrome b5, oxidized or reduced cytochrome c, NAD(H), and NADP(H) were ineffective in the reaction. The microsomal NADH-cytochrome b5 reductase activity was definitely low in the patient's thyroid. It was augmented to a normal level by incubation of the microsomes with FAD for 30 min or more. The activities of thyroid peroxidase, G6-PD, 6-PGD, catalase, protease, and NADPH-cytochrome c reductase were within normal limits. The major thyroid protein was normal thyroglobulin which could be readily iodinated in the presence of H2O2 and horse radish peroxidase. These findings suggest the correlation of an iodide organification defect with a cytochrome b5 reductase deficiency. Administration of high doses of FAD led to the restoration of thyroidal iodide organification mechanism associated with an increased thyroid hormone production and to a marked decrease of the goiter. Riboflavin was given without effect even at a high dosage level. Consequently, it seems likely that the deficient cytochrome b5 reductase activity in this patient is due to a defect in the biosynthesis of FAD, the coenzyme of the reductase, from riboflavin.