Mutations In Thyroid Peroxidase Gene Research Articles

Functional characterization of thyroid peroxidase missense variants causing thyroid dyshormonogenesis in Asian Indian population.

Thyroid dyshormonogenesis (TDH) is a sub-group of congenital hypothyroidism with recessive inheritance resulting from disease causing variants in thyroid hormone biosynthesis pathway genes, like DUOX2, TG, TPO, SLC5A5, SLC26A4, IYD, DUOXA2, SLC26A7 and SECISBP2. Thyroid peroxidase (TPO) is a crucial enzyme involved in thyroid hormone biosynthesis and is one of the frequently mutated genes in patients with TDH. The purpose of the study is to describe the insilico and functional characterization of novel variants in TPO gene identified in patients with thyroid dyshormonogenesis. We performed exome sequencing in Indian patients with TDH. In the current study, we describe the results of patients with TPO gene mutations. Exome sequencing results were further analysed by sanger sequencing, computational studies and invitro functional studies such as immunofluorescence and enzyme assay. We identified nine biallelic disease-causing variants in the TPO gene in twelve patients from nine unrelated Indian families. Eight of the nine variants were novel. No recurrent variants were identified. Computational analysis of six missense variants showed that these amino acid substitutions caused changes in non-covalent interactions with the adjacent residues that may affect the TPO protein structure and function. In-vitro experimental data using immunofluorescence assay showed that these variants did not affect the plasma membrane localization of the TPO protein but caused significant loss of TPO enzymatic activity compared to wild type. Our study revealed multiple novel pathogenic variants in TPO gene in Indian patients, thereby expanding the genotype spectrum. Functional studies helped us to reveal the pathogenicity of the missense variants.

High-resolution melt curve analysis: An approach for variant detection in the TPO gene of congenital hypothyroid patients in Bangladesh.

TPO (Thyroid Peroxidase) is known to be one of the major genes involved in congenital hypothyroid patients with thyroid dyshormonogenesis. The present study aims to validate high-resolution melting (HRM) curve analysis as a substitute method for Sanger sequencing, focusing on the frequently observed non-synonymous mutations c.1117G>T, c.1193G>C, and c.2173A>C in the TPO gene in patients from Bangladesh. We enrolled 36 confirmed cases of congenital hypothyroid patients with dyshormonogenesis to establish the HRM method. Blood specimens were collected, and DNA was extracted followed by PCR and Sanger sequencing. Among the 36 specimens, 20 were pre-sequenced, and variants were characterized through Sanger sequencing. Following pre-sequencing, the 20 pre-sequenced specimens underwent real-time PCR-HRM curve analysis to determine the proper HRM condition for separating the three variations from the wild-type state into heterozygous and homozygous states. Furthermore, 16 unknown specimens were subjected to HRM analysis to validate the method. This method demonstrated a sensitivity and specificity of 100 percent in accurately discerning wild-type alleles from both homozygous and heterozygous states of c.1117G>T (23/36; 63.8%), c.1193G>C (30/36; 83.3%), and c.2173A>C (23/36; 63.8%) variants frequently encountered among 36 Bangladeshi patients. The HRM data was found to be similar to the sequencing result, thus confirming the validity of the HRM approach for TPO gene variant detection. In conclusion, HRM-based molecular technique targeting variants c.1117G>T, c.1193G>C, and c.2173A>C could be used as a high throughput, rapid, reliable, and cost-effective screening approach for the detection of all common mutations in TPO gene in Bangladeshi patients with dyshormonogenesis.

SAT491 Development of Thyroidal Nodule in Congenital Hypothyroid Girl With Thyroid Peroxidase Gene Mutation

Abstract Disclosure: J. Lee: None. A. Oh: None. H. Han: None. Background: Reduced thyroxine synthesis in dyshormonogenesis induces increased thyrotropin (TSH) secretion through reduced negative feedback of thyroxine. Consequently, the affected patients are born with goiter or develop postnatal goiter, when L-thyroxine (LT4) supplementation is delayed. Cases of thyroid cancer have been reported in congenital hypothyroidism (CH) due to dyshormonogenesis, usually of follicular histology. Clinical Case: A 22-year lady has been taking LT4 since 8-years, under the diagnosis of goitrous CH. Her mother recalled neck swelling at newborn period. However, normal newborn screening test prevented neonatal goiter from further investigation. Initial test at 8-year of age revealed normal thyroid function without thyroid auto-antibodies, diffuse enlarged thyroid gland with increased uptake on 99mTc scan, and diffuse thyroid enlargement with heterogeneous echogenicity on ultrasonography (US). The goiter size waxed and waned according to the drug compliance. The highest TSH level (normal 0.17-4.05 μIU/mL) was 8.68 μIU/mL even without medication for 8 months by her own judgement. At 11.5-years, US found isoechoic solid nodule of 1.8 x 1.7 x 1.6 cm with peripheral halo in the inferior aspect of right thyroid, which was demonstrated to be benign follicular lesion favoring nodular hyperplasia on fine needle aspiration biopsy. With good drug compliance, the nodule size decreased to 1.1 x 0.9 cm at 18-years, and peripheral calcification appeared around the isoechoic solid nodule previously reported without change in size at 22-years. NGS panel for CH reported compound heterozygous variants in TPO gene. One pathogenic variant from asymptomatic father is c.2757del;p.(Met921Trpfs*53), the other likely pathogenic variant from asymptomatic mother is c.1580G>T;p.(Trp527Leu). Conclusion: This case is demonstrating development of thyroid nodule even with mild increased TSH level during LT4 medication in dyshormonogenic CH caused by TPO mutation. Regular follow-up and compliance to LT4 might be important to avoid thyroid tissue stimulation and possibly nodule formation by elevated TSH. Presentation Date: Saturday, June 17, 2023

Screening and Functional Analysis of TPO Gene Mutations in a Cohort of Chinese Patients With Congenital Hypothyroidism.

BackgroundsAs a crucial enzyme in thyroid hormone synthesis, the genetic defective thyroid peroxidase (TPO) was one of the main genetic factors leading to congenital hypothyroidism (CH).MethodsMutations in the TPO gene were screened and identified in 219 patients with CH from northwest China by using high-throughput sequencing and bioinformatics analysis. The biological function of detected variants was studied by in vitro experiments and homology modeling.ResultsNineteen rare variants, including seven novel ones, were detected in 17 of 219 patients (7.8%). Most cases were detected with one single heterozygous variant, and only two patients were detected with multiple variants, i.e., compounds for (1) IVS7-1G>A, p.Ala443Val, and p.Arg769Trp and (2) p.Asn592Ser and p.Asn798Lys. The biological function of the four missense mutations (i.e., p.Ala443Val, p.Arg769Trp, p.Asn592Ser, and p.Asn798Lys) they carried were further studied. Experimental data showed that these four mutations did not affect the protein expression level of the TPO gene but remarkably reduced the peroxidase activity toward guaiacol oxidation, retaining 8–32% of activity of the wild-type protein. The comparison of the predicted 3-D structures of wild-type and mutant TPO proteins showed that these four amino acid substitutions changed the non-covalent interactions of studied residues that might alter the structure and function of the TPO protein.ConclusionThis study was the first to analyze the TPO mutation spectrum of patients with CH in northwest China. Our data indicated that the TPO mutation was not a common reason to cause CH in China. The functional data may help to clarify the structure-function relationship of the TPO protein and provide further evidence for the elucidation of the genetic etiology of CH.

A Novel Homozygous Mutation of Thyroid Peroxidase Gene Abolishes a Disulfide Bond Leading to Congenital Hypothyroidism.

Congenital hypothyroidism (CH) is the most prevalent congenital endocrine disorder and causes mental retardation. A male Japanese patient with first cousin marriage parents was diagnosed as CH at 10 months. He was born before introduction of mass screening for CH. With continuous thyroid hormone replacement therapy, normal thyroid hormone status was maintained until adulthood. Genetic screening of next-generation sequencing was performed at the age of 52 years, and we identified a new homozygous thyroid peroxidase (TPO) gene mutation (GRCh38.p13, chromosome 2 at position 1493997, c.1964 G>T, p.Cys655Phe). TPO is an important enzyme to produce thyroid hormone. As demonstrated by a homology analysis of TPO proteins among different species, cysteine 655 residue is highly conserved, suggesting an important role in maintaining TPO function and structure. An in silico study with three-dimensional structure of the novel mutation was performed and suggested that the mutation abolished disulfide bond between cysteines at positions 598 and 655. An in vitro functional analysis using HEK293 cells revealed that TPO activity of the mutant was significantly impaired compared with that of the wild type. Furthermore, study of immunohistochemistry showed that localization of TPO in cells did not differ between the wild type and the mutant. In conclusion, this single disulfide bond loss mutation of a new TPO homozygous mutation, p.Cys655Phe, reduced TPO activity and caused congenital hypothyroidism without affecting subcellular localization of TPO proteins.

Functional analysis of thyroid peroxidase gene mutations resulting in congenital hypothyroidism.

Thyroid peroxidase (TPO) is essential for thyroid hormone biosynthesis. TPO mutations might lead to congenital hypothyroidism. In the present study, we analysed the function of a compound heterozygous TPO mutation in a Chinese family. We studied a 23-year-old Chinese girl with a history of growth retardation and severe constipation from the age of 3months, who was diagnosed as having congenital hypothyroidism. Genomic DNA was extracted from peripheral blood samples obtained from the patient's family members. The genomic DNA was sequenced to detect mutations in a panel of genes associated with congenital hypothyroidism. Bioinformatic analysis and structural modelling predicted the potential disease-causing potential mutant genes and the microstructure of the mutant protein, respectively. Western blotting and ELISA were used to measure protein expression, and guaiacol oxidation assay measured the TPO activity of the mutant protein. We identified a compound heterozygous mutation (c.C1993T, c.T2473C) in the TPO gene. Bioinformatic analysis predicted that the TPO mutations were potentially disease causing. Structural modelling predicted damage to the microstructure of the mutant TPO protein. Western blotting and ELISA showed reduced protein levels of the mutant TPO protein compared with that of the wild-type protein. The mutant TPO protein showed weaker activity compared with that of the wild-type protein. A novel compound heterozygous mutation of TPO gene was identified in a Chinese family. This mutation might alter the extracellular microstructure of TPO, and decrease its expression and the activity, resulting in congenital hypothyroidism.

Mild TPO deficiency characterized by progressive goiter and normal serum TSH level.

Mild thyroid peroxidase (TPO) deficiency is rare and can be extremely occult. This study aimed to replenish the phenotypic and genetic spectrum of mild TPO deficiency. Four unrelated patients with progressive goiter were described in this study. Genes associated with congenital hypothyroidism were analyzed and in vitro functional experiments were conducted to evaluate the residual TPO enzyme activities of each mutant. The four patients (age: 5-27 years old) were characterized by progressive goiter, discordant alteration in thyroid hormones with free triiodothyronine (FT3) to free thyroxine (FT4) ratio ranging from 0.557 to 1.012, two with slightly elevated TSH level and two with normal TSH level. Six different mutations of TPO gene were identified including three novel mutations (p.Glu337Lys, p.Ala544Val, and p.Glu641Lysfs∗21). Two mutants (p.Asp224del and p.Ala544Val) with residual TPO activity of 41 and 65% may explain the mild TPO-deficient picture in our study. After levothyroxine (L-T4) therapy, three patients showed gradual decline of FT3 to FT4 ratio and two patients showed reduced thyroid size. Patients with mild TPO deficiency can present with progressive goiter, normal TSH level, and largely reserved TPO activities.

Novel genetic variants in the TPO gene cause congenital hypothyroidism

Background: Mutations in the dual oxidase maturation factor 2 (DUOXA2) and thyroid peroxidase (TPO) genes have been reported to cause goitrous congenital hypothyroidism (GCH). The aim of this study was to determine the genetic basis of GCH in affected children. Methods: Thirty children with GCH were enrolled for molecular analysis of the DUOXA2 and TPO genes. All subjects underwent clinical examination and laboratory testing. Genomic DNA was extracted from peripheral blood leukocytes, and Sanger sequencing was used to screen for DUOXA2 and TPO gene mutations in the exon fragments amplified from the extracted DNA. Family members of those patients with mutations were also enrolled and evaluated. Results: Analysis of the TPO gene revealed six genetic variants, including two novel heterozygous mutations, c.1970T> C (p.I657T) and c.2665G> T (p.G889X), and four mutations that have been reported previously (c.670_672del, c.2268dup, c.2266T> C and c.2647C> T). Three patients harbored the same mutation c.2268dup. The germline mutations from four unrelated families were consistent with an autosomal recessive inheritance pattern. Conversely, no mutations in the DUOXA2 gene were detected. Conclusion: Two novel inactivating mutations (c.1970T> C and c.2665G> T) in the TPO gene were identified. The c.2268dup mutation occurred frequently. No mutations in the DUOXA2 gene were detected in this study.

Functional analysis of thyroid peroxidase gene mutations detected in patients with thyroid dyshormonogenesis.

Thyroid peroxidase (TPO) is the key enzyme in the biosynthesis of thyroid hormones. We aimed to identify the spectrum of mutations in the TPO gene leading to hypothyroidism in the population of West Bengal to establish the genetic etiology of the disease. 200 hypothyroid patients (case) and their corresponding sex and age matched 200 normal individuals (control) were screened depending on their clinical manifestations. Genomic DNA was isolated from peripheral blood samples and TPO gene (Exon 7 to Exon 14) was amplified by PCR. The PCR products were subjected to sequencing to identify mutations. Single nucleotide changes such as Glu 641 Lys, Asp 668 Asn, Thr 725 Pro, Asp 620 Asn, Ser 398 Thr, and Ala 373 Ser were found. Changes in the TPO were assayed in vitro to compare mutant and wild-type activities. Five mutants were enzymatically inactive in the guaiacol and iodide assays. This is a strong indication that the mutations are present at crucial positions of the TPO gene, resulting in inactivated TPO. The results of this study may help to develop a genetic screening protocol for goiter and hypothyroidism in the population of West Bengal.

A common thyroid peroxidase gene mutation (G319R) in Turkish patients with congenital hypothyroidism could be due to a founder effect

The most common congenital endocrine disorder is congenital hypothyroidism (CH), which can lead to mental retardation if untreated. Majority of the patients have been found to have defects in thyroid development and migration disorders (dysgenesis), and the remaining ones have thyroid hormone synthesis defects (dyshormonogenesis). One of the most common mechanisms to cause dyshormonogenesis is a defect in the thyroid peroxidase (TPO) enzyme. In familial cases, mutations in the TPO gene are fairly prevalent. To date, more than 80 mutations have been identified, which result in variably decreasing TPO bioactivities. Clinical manifestations of TPO defects are typically permanent CH and with or without goiter. In this report, we presented two children with CH who were born to consanguineous parents and were homozygous carriers of a missense (G319R) TPO mutation, the mutation segregated with the disease status in the families confirming its pathogenicity. G319R mutation seemed to be a common cause of CH in Turkish population, which could originate from a common founder ancestor. Moreover, our results also confirmed the phenotypic variability associated with different TPO mutations.

Minimally Invasive Follicular Thyroid Carcinoma Developed in Dyshormonogenetic Multinodular Goiter Due to Thyroid Peroxidase Gene Mutation

The occurrence of thyroid carcinoma in patients with congenital hypothyroidism (CH) caused by dyshormonogenesis is very rare, and has only been reported in one patient harboring mutations in the thyroid peroxidase (TPO) gene. We report on a 29-year follow-up of two consanguineous siblings with CH due to total iodide organification defect who also had sensorineural hearing loss. Molecular analysis revealed a novel biallelic mutation of the TPO gene in which phenylalanine substitutes serine at codon 292 (c.875C>T, p.S292F) in exon 8. Despite early initiation, adequate doses of levothyroxine treatment and consequently normal thyrotropin (TSH) levels, the proposita developed a huge multinodular goiter (MNG) and underwent total thyroidectomy due to tracheal compression. Pathological examination revealed a unifocal follicular thyroid carcinoma without vascular invasion in the left lobe of the thyroid gland. Our finding of follicular thyroid carcinoma arising from dyshormonogenetic MNG in a patient without elevated serum TSH levels indicates that genetic and environmental factors other than TSH level might be involved in the development of thyroid carcinoma in dyshormonogenetic MNG. Despite the rare occurrence of thyroid carcinoma in dyshormonogenetic MNG, we recommend long-term follow-up and regular neck ultrasound imaging to prevent delayed diagnosis of thyroid carcinoma.

The role of Na/I symporter, thyroid peroxidase and type 1 iodothyronine deiodinase gene in the pathogenesis of hashimoto's thyroiditis coexistent with papillary thyroid carcinoma

Objective To investigate the role of Na/I symporter (NIS),thyroid peroxidase (TPO) and type 1 iodothyronine deiodinase (DIOI) mutation in the pathogenesis of Hashimoto' s thyroiditis (HT) coexistent with papillary thyroid carcinoma (PTC).Methods Thirty-four HT patients with PTC were selected as subjects and 26 cases of HT as controls.The genomic DNA of the specimens was extracted.By using polymerase chain reaction (PCR),interesting fragments were obtained and sequenced.Results Among 34 patients of HT with PTC,no PCR products were found with the mutation in the NIS gene.Ten out of 34 patients and 2 out of 26 controls had 6 TPO point mutations and 1 insertion mutation,and 5 of out of 34 patients and 1 out of 26 controls had 3 DIOI point mutations.Statistical analysis showed there was significance in mutation positivity between HT with PTC and HT.In addition,mutation positivity had significant difference between patients with different thyroid function in HT and PTC group ( P ＜ 0.05 ).Conclusion It is possible that TPO and DIOI gene mutation had a potential molecular biological link with the two diseases. Key words: Na/I symporter; Thyroid peroxidase; Hashimoto's thyroiditis; Papillary thyroid carcinoma

Minimal Invasive Follicular Thyroid Carcinoma Developed in Dyshormonogenetic Multinodular Goiter Due to Thyroid Peroxidase Gene Mutation.

Background: The occurrence of thyroid carcinoma in patients with congenital hypothyroidism (CH) caused by dyshormonogenesis is very rare, and has been reported in only one patient harboring mutations in the thyroid peroxidase (TPO) gene. Patient findings: We report on a 29-year follow-up of two consanguineous siblings with CH due to total iodide organification defect who also had sensorineural hearing loss. Molecular analysis revealed a novel biallelic mutation of the TPO gene in which phenylalanine substitutes serine at codon 292 (c.875C>T, p.S292F) in exon 8. Despite early initiation, adequate doses of L-thyroxine treatment and consequently normal TSH levels, the proposita developed a huge multinodular goiter (MNG) and underwent total thyroidectomy due to tracheal compression. Pathological examination revealed a unifocal follicular thyroid carcinoma without vascular invasion in the left lobe of the thyroid gland. Summary: Our finding of follicular thyroid carcinoma arising from dyshormonogenetic MNG in a patient without elevated serum TSH levels indicates that genetic and environmental factors other than TSH level might be involved in the development of thyroid carcinoma in dyshormonogenetic MNG. Conclusions: Despite the rare occurrence of thyroid carcinoma in dyshormonogenetic MNG, we recommend long-term follow-up and regular neck ultrasound imaging to prevent delayed diagnosis of thyroid carcinoma.

Thyroid peroxidase gene mutation in patients with congenital hypothyroidism in isfahan, iran.

Background. Thyroid peroxidase gene (TPO) mutations are one of the most common causes of thyroid dyshormonogenesis in patients with congenital hypothyroidism (CH). In this study, the prevalence of TPO gene mutations in patients with thyroid dyshormonogenesis in Isfahan was investigated. Methods. In this cross-sectional study, genomic DNA of 41 patients with permanent CH due to thyroid dyshormonogenesis was extracted using the salting out method. The 17 exonic regions of the TPO gene were amplified. SSCP technique was performed for scanning of the exonic regions of the TPO gene, except exon 8. DNA sequencing was performed for those with different migration patterns in SSCP by chain termination method. Exon 8 was sequenced directly in all patients. In 4 patients, all fragments were also sequenced. Results. One missense mutation c.2669G > A (NM_000547.5) at exon 15 (14th coding exon) in one patient in homozygous form and seven different single nucleotide polymorphisms (SNPs) in exons 1, 7, 8, 11, and 15 of TPO gene. Conclusion. The TPO gene mutations among CH patients with dyshormonogenesis in Isfahan were less frequent in comparison with other similar studies. It may be due to the presence of other unknown gene mutations which could not be detected by SSCP and sequencing methods.

Mutation analysis of thyroid peroxidase gene in 20 Chinese patients with congenital hypothyroidism

Thyroid peroxidase(TPO) gene was detected in 20 patients with congenital hypothyroidism. An insertion c. 2268insT of TPO gene was found in one of them, and c. 2268insT combined with c. 1477G＞C mutation in another. TPO gene mutation may be related to pathogenesis of congenital hypothyroidism in Chinese. Key words: Congenital hypothyroidism; Thyroid peroxidase gene; Mutation

Mutation of thyroid peroxidase gene in 35 patients with congenital hypothyroidism

To identify thyroid peroxidase (TPO) gene mutations in 35 patients with congenital hypothyroidism. Genomic DNA was isolated from peripheral blood samples of 35 patients with congenital hypothyroidism. All of the 17 exons and flanking introns of TPO gene were amplified by PCR, then the PCR products were sequenced bi-directionally and were analyzed by restriction endonucleases. One patient had compound heterozygous mutations c.961A>G/c.2422delT, one was c.2268insT/c.1477G>A, and three was homozygous mutation c.2268insT. The TPO gene mutation c.961A>G [p. Thr321Ala] was one novel mutation. High frequency mutation in TPO gene was detected in patients with congenital hypothyroidism.

The Coexistence of a Novel Inactivating Mutant Thyrotropin Receptor Allele with Two Thyroid Peroxidase Mutations: A Genotype-Phenotype Correlation

TSH receptor (TSHR) and thyroid peroxidase (TPO) gene mutations occur independently. This is the first report of their coexistence in the same individuals. The objective of the study was to evaluate the genotype-phenotype correlations when mutations in both genes are present alone or together in the same individual. Thirty subjects from an extended Arab kindred underwent clinical investigation and molecular studies of the mutant TSHRs. A novel mutant TSHR was identified, involving four nucleotides at three sites on the same allele, c.267G>T (L89L), c.269/270AG>CT (Q90P), and c.790C>T (P264S). In addition, two known TPO gene mutations, G493S and R540X, were identified. Thirteen heterozygotes for the mutant TSHR allele had mild hyperthyrotropinemia. In nine of theses, the coexistence of a TPO mutation in one allele did not magnify the hyperthyrotropinemia. Homozygotes for the mutant TSHR and a compound heterozygote for the TPO mutations presented frank hypothyroidism. In vitro studies showed increasing loss of function for Q90P less than P264S less than Q90P/P264S TSHR mutants, the latter being that expressed in the subjects under investigation. The two interchangeably used WT TSHR vectors, L87 and V87, although functionally identical, differed in structure and function in the presence of the Q90P mutation. TSHR and TPO gene mutations were identified alone and together in individuals of a consanguineous kindred. Homozygotes for the TSHR and a compound heterozygote for the TPO mutations were hypothyroid. The mild hyperthyrotropinemia of heterozygotes for the mutant TSHR allele was not aggravated by the coexistence of a TPO defect in one allele.

Monoallelic thyroid peroxidase gene mutation in a patient with congenital hypothyroidism with total iodide organification defect

The aim of this study was to identify the genetic defect of a patient with dyshormonogenetic congenital hypothyroidisms (CH) with total iodide organification defect (TIOD). A male child diagnosed with CH during neonatal screening. Laboratory tests confirmed the permanent and severe CH with TIOD (99% perchlorate release). The coding sequence of TPO, DUOX2, and DUOXA2 genes and 2957 base pairs (bp) of the TPO promoter were sequenced. Molecular analysis of patient's DNA identified the heterozygous duplication GGCC (c.1186_1187insGGCC) in exon 8 of the TPO gene. No additional mutation was detected either in the TPO gene, TPO promoter, DUOX2 or DUOXA2 genes. We have described a patient with a clear TIOD causing severe goitrous CH due to a monoallelic TPO mutation. A plausible explanation for the association between an autosomal recessive disorder with a single TPO-mutated allele is the presence of monoallelic TPO expression.

Two novel mutations in the human thyroid peroxidase (TPO) gene: genetics and clinical findings in four children

We report four children originating from two unrelated German families with congenital hypothyroidism (CH) due to mutations in the thyroid peroxidase (TPO) gene. Three female siblings (family 1) were found to be compound heterozygous for two mutations, a known mutation in exon 9 (W527C), and a mutation in exon 8 (Q446H), which has not been described before. In the second family we identified a boy with goitrous CH, who had a novel homozygous mutation in the TPO gene in exon 16 (W873X). All children of family 1 were diagnosed postnatally by newborn screening. The case of the boy of family 2 has already been reported for the in utero treatment of a goiter with hypothyroidism. Our results confirm existing data on the phenotypic variability of patients with TPO gene mutations.

Thyroid Peroxidase Gene Mutations Causing Congenital Hypothyroidism in Three Turkish Families

In Turkey congenital hypothyroidism (CH) occurs with a prevalence of one in 2,736 newborns while the worldwide incidence is one in 3,000-4,000 newborns. 85-90% of these cases are due to dysgenesis of the thyroid gland, whereas defects in thyroid hormone synthesis account for 10-15%. The majority of patients with dyshormonogenesis have a defect in thyroid peroxidase (TPO). To date, more than 60 different mutations have been described in the TPO gene, mostly single nucleotide substitutions. Five children from three consanguineous families were diagnosed with CH on the basis of clinical symptoms and signs--goiter, macroglossia and prolonged jaundice at newborn age. Two different mutations in the TPO gene were identified. Affected children in families I and II had a nonsense mutation in exon 10 (R540X). Genotyping of polymorphic markers within the TPO gene revealed that these families shared a common haplotype, suggesting a founder effect. In the third family, a novel mutation (G319R) in exon 8 was identified.