Helton Estrela Ramos

Clinical and Molecular Analysis of Thyroid Hypoplasia: A Population-Based Approach in Southern Brazil

BACKGROUND Congenital hypothyroidism (CH) is mainly due to developmental abnormalities leading to thyroid dysgenesis (TD). TD encompasses very distinct morphologic subtypes of disease. This study examined and compared the phenotype in TD variants and searched for genetic alterations in sporadic thyroid hypoplasia (TH), the most misdiagnosed form of CH. This was a longitudinal study over a 14-year period (1990-2004). METHODS A continuous series of 353 children with TD was identified using thyroid function tests [thyroxine (T4) and TSH], scintigraphy, and ultrasound as diagnostic tools. Individual phenotypes were analyzed in 253 children with TD. Mutations in the most likely candidate genes were studied in 35 cases of TH. RESULTS The overall birth prevalence of permanent CH was 1:4795. Ectopy represented 37% of all cases of permanent primary CH, dyshormonogenesis 28%, agenesis 24%, hypoplasia 10%, and hemiagenesis 1%. The lowest screening T4 level and the highest TSH level were in the agenetic group, followed by TH. The TH group had an improvement in the thyroid function showing less-severe phenotype with aging. In the molecular analysis, one patient was identified with a mutation in the PAX8 gene (155G>C; R52P); four patients had a heterozygous G>C substitution in position -569; two patients showed a (234C>A; P52T) or (2181C>G; D727E) polymorphic variants of the TSH-R gene; and one patient presented a novel heterozygous nonsynonymous substitution, 293G>A; S98N, in the NKX2.5 gene. CONCLUSIONS The prevalence of CH was within the previously reported range of 1:3000-4000. Ectopy was the most common etiology. Clinical analysis revealed distinct hormonal patterns in TH subgroup when compared with other variants of TD, with genetic abnormalities identified only in few cases in the TSH-R, PAX8, and NKX2.5 genes.

Thyroid hormone receptor subtypes and their interaction with steroid receptor coactivators

Thyroid hormone (TH) is required for normal growth, development, and metabolism in metazoans. To influence this broad range of physiologic actions, TH is necessarily involved in the regulation of a multitude of genes in virtually every tissue. The diversity of gene expression regulation in response to TH is mediated through specific intranuclear TH receptors (TRs) and other nuclear coregulators. This chapter reviews TRs and nuclear coregulators, specifically coactivators, based on in vivo data from knockout (KO) mouse models.

Extreme phenotypic variability of thyroid dysgenesis in six new cases of congenital hypothyroidism due to PAX8 gene loss-of-function mutations

CONTEXT Within the last two decades, heterozygous loss-of-function PAX8 mutations have been reported in patients with a wide degree of thyroid gland dysfunction and growth despite the presence of identical mutations. OBJECTIVES To search for PAX8 mutations in a cohort of patients with congenital hypothyroidism (CH) and various types of thyroid gland defects. DESIGN A cross-sectional study was conducted in a cohort of patients. SETTING The French neonatal screening program was used for recruiting patients. PATIENTS A total of 118 patients with CH, including 45 with familial and 73 with sporadic diseases, were included in this study. The thyroid gland was normal in 23 patients had hypoplasia, 25 had hemithyroid agenesis, 21 had athyreosis, and 21 had ectopy. RESULTS We found four different PAX8 mutations (p.R31C, p.R31H, p.R108X, and p.I47T) in ten patients (six patients with CH and four family members), two with sporadic and eight with familial diseases. Imaging studies performed in the index cases showed ectopic thyroid gland (n=2), hypoplasia (n=2), eutopic lobar asymmetry (n=1), and eutopic gland compatible with dyshormonogenesis (n=1). The previously reported p.R31C and the novel p.I47T PAX8 mutations are devoid of activity. CONCLUSION Four different PAX8 mutations were detected in six index patients with CH (ten total subjects). The p.R31C, p.R31H, and p.R108X mutations have been reported. The novel p.I47T PAX8 mutation presented loss of function leading to CH. Thyroid ectopy was observed in two cases of PAX8 (p.R31H) mutation, a finding that has not been reported previously. We observed a high inter-individual and intra-familial variability of the phenotype in PAX8 mutations, underlining that population genetic studies for CH should include patients with various clinical presentations.

New Cases of Isolated Congenital Central Hypothyroidism Due to Homozygous Thyrotropin Beta Gene Mutations: A Pitfall to Neonatal Screening

BACKGROUND Congenital central hypothyroidism (CCH) is a rare condition that is often diagnosed in late childhood in countries where neonatal screening programs rely solely on detecting thyrotropin (TSH) elevation. TSHbeta gene mutation is one of the causes of CCH. We describe two cases of c.Q49X mutation and three cases of c.C105Vfs114X mutation in exon 3 of the TSH beta-subunit gene. SUMMARY We found two different TSHbeta gene mutations in two families. In one family, we identified a missense mutation in exon 3 leading to a premature stop at position 49 (c.Q49X) in the two affected twins. In the other family, the three affected siblings had a 313delT nucleotide deletion leading to a frame shift responsible for premature termination at codon 114 (c.C105Vfs114X); neonatal screening showed very low TSH levels in all three patients. The presence of inappropriately low TSH levels at birth in the three affected members of the second family raises questions about the value of the TSH level for CCH screening. CONCLUSIONS The marked phenotypic variability in patients with the c.Q49X mutation suggests modulation by interacting genes and/or differences in the genetic background. TSHbeta gene mutations should be suspected in neonates with inappropriately low TSH levels.

Pregnancy in women heterozygous for MCT8 mutations: risk of maternal hypothyroxinemia and fetal care.

CONTEXT Monocarboxylate transporter 8 (MCT8 or SLC16A2) mutations cause X-linked Allan-Herndon-Dudley syndrome. Heterozygous females are usually asymptomatic, but pregnancy may modify thyroid function and MCT8 is expressed in the placenta, suggesting that maternal and fetal abnormalities might develop even in the absence of MCT8 fetal mutation. Genetic counseling is so far based on X-linked transmission, and prenatal diagnosis is rarely performed. OBJECTIVE To describe thyroid function and the prenatal diagnosis in pregnant mothers harboring heterozygous MCT8 mutations and management of the persistent maternal hypothyroxinemia. Patients Two women heterozygous for MCT8 mutations (c.1690G>A and c.1393-1G>C) were monitored throughout pregnancy. METHODS Prenatal diagnosis included sex determination, direct MCT8 sequencing, and familial linkage analysis. Ultrasonography and hormonal assays for maternal thyroid function evaluation were performed serially during pregnancy. Neonatal thyroid hormonal status was assessed. RESULTS None of the three fetuses (two males and one female) carried MCT8 mutations. One of the two heterozygous mothers revealed gestational hypothyroxinemia, prompting early levothyroxine (l-T₄) therapy until delivery. The second heterozygous mother showed normal thyroid function but was preventively traited by l-T₄ and all of the three neonates had normal thyroid hormone levels and thyroid gland at birth, suggesting advantages of prenatal care and/or compensatory mechanisms. CONCLUSION Heterozygous MCT8 women should be monitored for requirement of l-T₄ therapy to prevent fetal and neonatal hypothyroidism and to avoid risk of potential cognitive delay due to gestational hypothyroxinemia. Moreover, when the disease-causing mutation is known and/or the first child is affected, prenatal diagnosis for male fetuses should be assessed early for MCT8 mutations by direct sequencing.

Functional characterization of the novel sequence variant p.S304R in the hinge region of TSHR in a congenital hypothyroidism patients and analogy with other formerly known mutations of this gene portion

Abstract Context: Thyroid dysgenesis may be associated with loss-of-function mutations in the thyrotropin receptor (TSHR) gene. Objectives: The aim of this study was to characterize a novel TSHR gene variant found in one patient harboring congenital hypothyroidism (CH) from a cohort of patients with various types of thyroid defects. Materials and methods: This cross-sectional cohort study involved 118 patients with CH and their family members, including 45 with familial and 73 with sporadic diseases. The thyroid gland was normal in 23 patients, 25 patients had hypoplasia, 25 hemithyroid agenesis, 21 had athyreosis, and 21 had ectopy. Genomic DNA was extracted, and 10 exons of the TSHR gene were amplified and sequenced. Mutations in other candidate genes were investigated. Ortholog alignment was performed, and TSHR functional assays were evaluated. Results: We identified one previously unknown missense variation in the hinge region (HinR) of the TSHR gene (p.S304R) in one patient with thyroid hypoplasia. This variant is conserved in our ortholog alignment. However, the p.S304R TSHR variant presented a normal glycosylation pattern and signal transduction activity in functional analysis. Conclusion: We report the ocurrence of a novel nonsynonymous substitution in the HinR of the large N-terminal extracellular domain of the TSHR gene in a patient with thyroid hypoplasia. In contrast with four others in whom TSHR mutations of the hinge portion were previously identified, the p.S304R TSHR variation neither affected TSH binding nor cAMP pathway activation. This TSHR gene variant was documented in a CH patient, but the current data do not support its role in the clinical phenotype.

Síndrome de resistência ao hormônio tireoidiano

Resistance to thyroid hormone (RTH) is a syndrome characterized by elevated serum thyroid hormone (TH) levels and elevated or inappropriately normal thyrotropin levels. In general, patients exhibit TH resistance in the pituitary and peripheral tissues. The phenotype of RTH is variable; the affected individuals are clinically euthyroid or even hypothyroid depending on the severity of the mutation, the variable hyposensitivity to TH among individuals as well as in different tissues. In almost all cases the genetic basis of RTH lies in mutation of the carboxyl-terminus of the ss-thyroid hormone receptor. RTH is a dominant disorder, except in one family; most individuals are heterozygous for the mutant allele. New standard techniques and genetically engineered mouse model systems have increased our understanding on TH receptor action, in particular, how mutant thyroid receptors from RTH patients can block wild-type thyroid receptor function (dominant negative activity), and how the mutant receptors can differently affect various tissues and individuals.

Molecular Insights into the Possible Role of Kir4.1 and Kir5.1 in Thyroid Hormone Biosynthesis

Introduction: Thyroid morphogenesis is a complex process. Inwardly rectifying potassium (Kir) genes play a role in hormone release, cell excitability, pH and K+ homeostasis in many tissues. Objectives: To investigate the thyroid developmental expression of three members, Kir4.1, Kir4.2 and Kir5.1, in mice. To postulate the K+ channel role in thyroid hormone secretion. Material and Methods: Quantitative RT-PCR analysis of Kir4.1, Kir4.2 and Kir5.1 in mice of different stages (E13.5-E18.5). Results: mRNA for Kir4.1, Kir4.2 and Kir5.1 were identified and increased with age in mice. Both Kir4.1 and Kir4.2 genes are better expressed after E16.5. Kir4.2 greatly increases from E13.5 to E16.5 (p ≤ 0.05). Conclusion: Quantitative PCR shows that the mouse thyroid presents increased expression for Kir channels during development. The role of Kir in thyroid morphogenesis and differentiation might be understood in future studies. We speculate that thyroglobulin trafficking might be modulated by Kir4.1/5.1.