Congenital hypothyroidism

TOPIC: Dyshormonogenesis and congenital hypothyroidism

Title: Mutations in the iodotyrosine deiodinase gene and hypothyroidism.

Authors: Moreno JC, Klootwijk W, van Toor H, Pinto G, D'Alessandro M, L'ger A, Goudie D, Polak M, Gr'ters A, & Visser TJ.

Reference: New England Journal of Medicine 358: 1811-1818, 2008

Summary

Context

DEHAL1 has been identified as the gene predicted to encode the thyroidal iodotyrosine deiodinase that deiodinates mono- and diiodotyrosines in order to recycle iodide for thyroid hormone synthesis.

Objective

To screen patients with a clinical phenotype suggestive for a defective iodotyrosine deiodinase for mutations in the DEHAL1 gene.

Design, Setting and Patients

Clinical and genetic characterization of 2 sporadic patients and 2 siblings with hypothyroidism and features suggestive of an iodotyrosine deiodinase defect. The patients presented with goitrous hypothyroidism. The radioiodine uptake and the organification of iodide were found to be normal. All patients have consanguineous parents.

Main Outcome Measures

Mutational analysis of the coding exons of the DEHAL1 gene, and determination of enzymatic activity of DEHAL1 mutations in vitro .

Results

Mutational analysis identified homozygous mutations in the DEHAL1 gene. Patient 1 had a missense mutation (c.301C>T; p.R101W). The siblings (patients 2 & 3) were found to have a deletion of 3 base pairs that results in deletion of phenylalanine 105 and insertion of leucine (c.315-317delCAT; p.delF106insL107); the remainder of the reading frame is preserved. Patient 3 had a missense mutation (c.347T>C; p.I116T). The three identified mutations are located in the nitro-reductase domain of the enzyme, R101W also affects the flavin mononucleotide (FMN)-binding domain. The 2 sporadic patients had normal TSH levels at neonatal screening; the siblings were not evaluted by neonatal testing. All 4 patients developed severe hypothyroidism & large goiters later in infancy/childhood. Two patients developed mental impairment because of delayed onset of therapy with levothyroxine. Serum diiodotyrosine levels (measured in 2 patients) were very high. The ability to deiodinate iodotyrosines in vitro by the mutant proteins was absent or severely reduced, and they did not respond to stimulation of the enzymatic activity with flavin mononucleotide.

Conclusions

Mutations in the DEHAL1 gene lead to defective deiodination of iodotyrosines through partial or complete inactivation of the intrathyroidal dehalogenase. Infants with DEHAL1 gene defects may have normal thyroid function at birth. Therefore, they may be missed by neonatal screening programs for congenital hypothyroidism.

Commentary

A thyroidal iodotyrosine dehalogenase was first characterized by Jean Roche and his colleagues in France in 1952. This enzyme deiodinates mono- and di-iodotyrosine, but not T4, and the released iodide is reused for hormone synthesis. For a historical review of the contributions of Jean Roche to thyroid physiology see a review article by Fragu (How the field of thyroid endocrinology developed in France after World War II. Bulletin de l'Histoire de la M'decine 77:393, 2003). Next, it was shown that this enzyme is dependent on flavin mononucleotide (FMN). In 2001, Moreno et al. cloned several thyroid-restricted genes by serial analysis of gene expression (SAGE). One of these genes was referred to as dehalogenase 1 (DEHAL1) based on homology to nitro-reductases, and predicted to encode the iodotyrosine dehalogenase. Analysis of the peptide sequence of the DEHAL1 protein demonstrated a nitro-reductase domain and, consistent with older biochemical data, a FMN-binding domain. Although it seemed likely that the protein encoded by the DEHAL1 gene does indeed correspond to the iodotyrosine deiodinase, formal functional or genetic proof was still pending.

Patients thought to harbour a defect in iodotyrosine deiodination were first reported by McGirr and Hutchison who described an inbred family of itinerant Scottish tinkers with 12 hypothyroid goitrous subjects After administration of radioiodine, these individuals secreted a large fraction of a labeled compound that was not corresponding to T4 or T3. John Stanbury, in collaboration with colleagues from the Netherlands, studied a hypothyroid man with a large goiter and, using chromatographic techniques, the investigators were able to demonstrate that this patient lost large amounts of mono- and diiodotyrosine in the urine. Several other patients with a similar phenotype were identified subsequently.

In the study discussed here, Moreno et al. identified 4 patients with mutations in the DEHAL1 gene. The biallelic mutations are predicted to disrupt the nitro-reductase domain of the enzyme. p.R101W also affects the FMN-binding domain, and p.delF106insL107 is in close vicinity to this domain. Functional characterization in transfected cells revealed that the mutations resulted in severe reduction or complete inactivation of the enzymatic activity in terms of its deiodination capacity, and the mutants can not be stimulated with FMN. These findings provide definitive proof that the DEHAL1 gene is encoding the long-sought thyroidal iodotyrosine deiodinase, and that it is essential for normal iodide metabolism in thyroid follicular cells.

From a clinical perspective, the study is of importance and interest for several reasons. The 2 patients who were evaluated for congenital hypothyroidism as neonates had normal TSH levels at that time. This indicates that patients with this defect may potentially be missed on neonatal screening. Severe hypothyroidism, associated with the development of large goiters, developed only later in infancy or childhood. It seems likely that the neonatal phenotype is influenced by the nutritional iodine intake during pregnancy. Similarly, the development of overt hypothyroidism and/or goiter is expected to be modified by iodine intake later in life. Characterization of the iodotyrosine deiodinase DEHAL1 underscores that thyroid cells have developed an intricate system to efficiently use and recycle iodide in order to avoid any losses of this usually scarce trace element, which is essential for thyroid hormone synthesis. In the future, it will be of interest to determine whether a subset of patients with acquired non-autoimmune goiter and hypothyroidism could have milder defects in the DEHAL1 gene.

Summary and commentary prepared by Peter Kopp (Related to Chapters 2 & 15 of TDM)