Molecular pathogenesis of neonatal hypothyroidism

In patients with congenital hypothyroidism (CH), the autosomal recessive inheritance of mutations of thyroid peroxidase, thyroglobulin and the NIS and pendrin genes encoding for sodium iodide transporters has been identified. CH due to thyroid dysgenesis was considered to be a sporadic disease, but recently, inheritable defects of thyroid development have been described. The autosomal recessive inheritance of mutations of the thyroid-stimulating hormone receptor gene was recognized in patients with CH and thyroid hypoplasia, while autosomal dominant mutations of the Pax-8 gene were described in patients with thyroid dysgenesis. In addition, analysis of mutations of the beta-thyrotropin gene has resulted in a new understanding of the pathogenesis of central CH. Molecular genetic studies in patients with CH detected by newborn screening will provide the information necessary for genetic counselling and may help to explain the less favourable outcome present in 5-10% of the patients.     

Linkage analysis identifies the thyroglobulin gene region as a major locus for familial congenital hypothyroidism

Congenital hypothyroidism affects 1/3000-4000 newborns and it has been estimated that 10-20% are familial cases with an autosomal recessive mode of inheritance. Previous studies of mostly individual cases have led to the identification of mutations in a number of genes, indicating that it is a genetically heterogeneous disease, but no major gene has been identified. In the present investigation, a population-based sample of 23 families with autosomal recessive congenital hypothyroidism, but no signs of goitre, were subject to linkage analysis. When markers located close to the thyroglobulin gene on chromosome 8q24 were used in a two-point analysis allowing for heterogeneity, a Z(max) of 4.10 was obtained with the microsatellite marker D8S557, indicating heterogeneity with 43% of the families being linked. A multipoint analysis using the markers D8S557 and D8S1835 gave a Z(max) of 3.51, assuming homogeneity. There was significant evidence of heterogeneity with 44.5% of the families being linked. The results indicate that a gene in 8q24 is a common cause of familial congenital hypothyroidism. Since thyroglobulin is essential for thyroid physiology, the gene encoding this protein is the obvious candidate for mutation analysis in the linked families.     

Familial hypothyroidism caused by a nonsense mutation in the thyroid-stimulating hormone beta-subunit gene.

Hereditary hypothyroidism caused by thyroid-stimulating hormone (TSH) deficiency is a rare autosomal recessive disease. Affected individuals show symptoms of severe mental and growth retardation that can be prevented by early administration of exogenous thyroid hormone. In this paper, we describe two related Greek families with three children affected by congenital TSH-deficient hypothyroidism. Sequence analysis of the TSH beta-subunit gene (TSHB) showed that the mutation responsible for the hypothyroidism in these families is a nonsense mutation in exon 2. This mutation is a G-to-T transversion at nucleotide 94 that destroys the only TaqI site in the TSHB-coding region and gives rise to a novel 8.5-kb TaqI fragment. Restriction analysis showed that the three affected children are homozygous for the 8.5-kb allele and that the four parents and two unaffected children are heterozygous. This mutation gives rise to a truncated peptide which includes only the first 11 of 118 amino acids of the mature TSHB peptide.     

Genetic and linkage analysis of familial congenital hypothyroidism: exclusion of linkage to the TSH receptor gene

Congenital hypothyroidism affects 1/3000– 4000 newborns. The causes of this group of disorders are still largely unknown. Although most cases are sporadic, some families have several affected children and/or consanguineous parents, suggesting autosomal recessive inheritance. Furthermore, there is a murine strain (hyt) with congenital hypothyroidism and autosomal recessive inheritance, whose phenotype appears to be identical with the corresponding human disease. In the hyt mouse, the disease is caused by a mutation in the thyroid-stimulating hormone receptor (TSHR) gene, making this gene a likely candidate also for the human disease. The human TSHR gene was mapped on radiation hybrid panels and closely linked flanking markers D14S287 and D14S616 were identified. On the Genebridge 4 panel, D14S287 was found to be located 8.5 cR (corresponding to 2.3 cM) proximal to the TSHR gene, and D14S616 was found to be located 4.4 cR (1.2 cM) distal to the TSHR gene. These markers were analyzed in 23 families, most of them with two or more children affected by congenital hypothyroidism and some with appreciable consanguinity of the parents. Assuming homogeneity, the two-point lod score at θ = 0.1 was –4.8 for D14S287 and –5.8 for D14S616, and thus linkage to the TSHR gene was excluded. Even when the data were analyzed with allowance for heterogeneity, there was no evidence of linkage. Our conclusion is that if mutation of the TSHR gene causes familial congenital hypothyroidism in humans, it affects only a small proportion of the cases. Received: 8 July 1996     

A new locus for a dominant form of multinodular goiter on 3q26.1-q26.3

A mass screening program for congenital hypothyroidism has markedly improved prognosis of children with congenital hypothyroidism and also revealed several cases with unknown pathogenesis. We here report two independent Japanese multigeneration families with multinodular goiter (MNG) with euthyroidism and with high TSH. The propositi, 3- and 8-year-old girls in two families, were found during a mass screening. An autosomal dominant pattern of inheritance was suggested in both families. The clinical examinations suggested impaired hormonogenesis but discarded known defects in iodine transport, organification, deficiency of hydrogen peroxide, and thyroid peroxidase. Linkage analysis of the two families including 10 members each using 343 microsatellite markers mapped a single locus independently at D3S1618 (theta = 0) on 3q26.1-q26.3 with a two-point LOD score 3.62 (1.81 for each family) and multipoint LOD score of 3.61 (1.80 for each family). Haplotype inspection delimited an 18-cM interval between D3S1565 and D3S3686.     

The C105fs114X is the prevalent thyrotropin beta-subunit gene mutation in Argentinean patients with congenital central hypothyroidism

BACKGROUND: Congenital isolated thyrotropin (TSH) deficiency is an unusual condition characterized by low levels of thyroid hormones and TSH, usually presenting early typical signs of severe hypothyroidism. Five different beta-TSH mutations have been described so far. While 4 of them affect only consanguineous families, a frameshift mutation in exon 3 (C105fs114X) has been found also in nonconsanguineous families. OBJECTIVE: The aim of the present study was to characterize beta-TSH mutations in Argentinean patients with congenital central hypothyroidism (CCH) and to emphasize the importance of early biochemical and molecular diagnosis of this disorder. PATIENTS AND METHODS: We investigated 8 Argentinean children (3 boys, 5 girls) from 7 unrelated families with CCH based upon low levels of T(4) and T(3), and low basal and stimulated TSH levels. Mutation characterizations for the beta-TSH gene were performed by PCR amplification followed by sequence and restriction enzyme analysis with SNABI in the patients, 9 parents and in 100 newborn children. RESULTS: All patients presented the same homozygous mutation in exon 3 of the beta-TSH gene (C105fs114X), the 9 studied parents were heterozygous for the same mutation and 1 carrier was found in the 100 studied newborns. CONCLUSION: Our findings show that the C105fs114X mutation is prevalent in our population and may constitute a hot spot at codon 105 in the beta-TSH gene. Since this mutation is easily demonstrable by a SNABI digestion in DNA amplified from dried blood spots, its investigation would be indicated in patients in our milieu with clinical and biochemical features of CCH, allowing early L-thyroxine (LT(4)) replacement and genetic counseling of the family.     

A novel hypothyroid dwarfism due to the missense mutation Arg479Cys of the thyroid peroxidase gene in the mouse

Recently, we found a novel dwarf mutation in an ICR closed colony. This mutation was governed by a single autosomal recessive gene. In novel dwarf mice, plasma levels of the thyroid hormones, T3 and T4, were reduced; however, TSH was elevated. Their thyroid glands showed a diffuse goiter exhibiting colloid deficiency and abnormal follicle epithelium. The dwarfism was improved by adding thyroid hormone in the diet. Gene mapping revealed that the dwarf mutation was closely linked to the thyroid peroxidase (Tpo) gene on chromosome 12. Sequencing of the Tpo gene of the dwarf mice demonstrated a C to T substitution at position 1508 causing an amino acid change from arginine (Arg) to cysteine (Cys) at codon 479 (Arg479Cys). Western blotting revealed that TPO protein of the dwarf mice was detected in a microsomal fraction of thyroid tissue, but peroxidase activity was not detected. These findings suggested that the dwarf mutation caused a primary congenital hypothyroidism by TPO deficiency, resulting in a defect of thyroid hormone synthesis.     

Recent advances in understanding the molecular basis of primary congenital hypothyroidism

Primary congenital hypothyroidism is characterized by low levels of circulating thyroid hormones and raised levels of thyrotropin at birth. It can be either permanent or transitory. Most permanent cases (80-85%) result from alterations in the formation of the thyroid gland during embryogenesis (thyroid dysgenesis), and several were shown recently to be produced by mutations in genes responsible for the development of thyroid follicular cells (TITF1, TITF2, PAX8 and TSHR). Less frequently, congenital hypothyroidism is determined by defects in thyroid hormone synthesis (hormonogenesis defects). The latter are usually associated with goiter. Recently, the molecular mechanisms of two forms of hormonogenesis defects (iodine transport defects and Pendred syndrome) were elucidated.     

牛蜘蛛腿综合征: 一种骨骼系统畸形遗传疾病

牛蜘蛛腿综合征(Arachnomelia syndrome, AS)是主要在欧洲瑞士褐牛和德系西门塔尔牛群体中出现的一种以骨骼畸形为病理特征的先天致死性遗传病, 呈孟德尔隐性遗传。文章通过对AS的发现历史、病理特点、遗传规律以及AS在欧洲瑞士褐牛和德系西门塔尔牛两个品种中分子机理研究进展进行综述, 并从骨骼发育、比较基因组学两方面对西门塔尔牛AS的定位区段进行候选基因的预测, 为以后筛查影响该病的基因及其突变以及建立检测方法提供一定的依据。     

Evaluation of complex inheritance involving the most common Bardet-Biedl syndrome locus (BBS1)

Bardet-Biedl syndrome (BBS) is a genetic disorder with the primary features of obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation, and hypogenitalism. Patients with BBS are also at increased risk for diabetes mellitus, hypertension, and congenital heart disease. BBS is known to map to at least six loci: 11q13 (BBS1), 16q21 (BBS2), 3p13-p12 (BBS3), 15q22.3-q23 (BBS4), 2q31 (BBS5), and 20p12 (BBS6). Although these loci were all mapped on the basis of an autosomal recessive mode of inheritance, it has recently been suggested-on the basis of mutation analysis of the identified BBS2, BBS4, and BBS6 genes-that BBS displays a complex mode of inheritance in which, in some families, three mutations at two loci are necessary to manifest the disease phenotype. We recently identified BBS1, the gene most commonly involved in Bardet-Biedl syndrome. The identification of this gene allows for further evaluation of complex inheritance. In the present study we evaluate the involvement of the BBS1 gene in a cohort of 129 probands with BBS and report 10 novel BBS1 mutations. We demonstrate that a common BBS1 missense mutation accounts for approximately 80% of all BBS1 mutations and is found on a similar genetic background across populations. We show that the BBS1 gene is highly conserved between mice and humans. Finally, we demonstrate that BBS1 is inherited in an autosomal recessive manner and is rarely, if ever, involved in complex inheritance.     

Mutation analysis of the MKKS gene in McKusick-Kaufman syndrome and selected Bardet-Biedl syndrome patients

McKusick-Kaufman syndrome comprises hydrometrocolpos, polydactyly, and congenital heart defects and overlaps with Bardet-Biedl syndrome, comprising retinitis pigmentosa, polydactyly, obesity, mental retardation, and renal and genital anomalies. Bardet-Biedl syndrome is genetically heterogeneous with three cloned genes ( BBS2, BBS4, and MKKS) and at least three other known loci ( BBS1, BBS3, and BBS5). Both McKusick-Kaufman syndrome and Bardet-Biedl syndrome are inherited in an autosomal recessive pattern, and both syndromes are caused by mutations in the MKKS gene. However, mutations in MKKS are found in only 4%-11% of unselected Bardet-Biedl syndrome patients. We hypothesized that an analysis of patients with atypical Bardet-Biedl syndrome and McKusick-Kaufman syndrome (Group I; 15 probands) and patients with Bardet-Biedl syndrome who had linkage results inconsistent with linkage to the other loci (Group II; 12 probands) could increase the MKKS mutation yield. Both mutant alleles were identified in only two families in Group II. Single (heterozygous) sequence variations were found in three Group I families and in two Group II families. Combining these results with previously published data showed that only one mutant allele was detected in nearly half of all patients screened to date, suggesting that unusual mutational mechanisms or patterns of inheritance may be involved. However, sequencing of the BBS2 gene in these patients did not provide any evidence of digenic or "triallelic" inheritance. The frequency of detected mutations in MKKS in Group II patients was 24%, i.e., six times higher than the published rate for unselected BBS patients, suggesting that small-scale linkage analyses may be useful in suitable families.     

Organization and results of mass screening for congenital hypothyroidism in newborn infants in the Cracow province

Screening for congenital hypothyroidism has been carried out in 69,286 newborn children born in the region and city of Krakow in the years 1985-1988. Fourteen cases of congenital thyroid insufficiency were detected indicating the incidence of 1:4816 in this region. In 25 newborns (incidence of 1:1979) a transient hyperthyreotropinemia was diagnosed. It was concluded that the main reason of the appearance of an elevated TSH level in newborns of this region seems to be the deficiency of iodine in the diet and/or environment.     

Spectrum of the GJB2 mutations in Belarussian patients with hearing loss. Findings of pilot genetic screening of hearing impairment in newborns

A total of 111 unrelated probands and their 8 sibs from Grodno oblast (Belarus) with bilateral isolated sensorineural hearing impairment were studied for the presence of mutations in the connexin 26 (GJB2) gene. Mutations were detected in 51 probands (46% of the sample). A significantly higher frequency of the GJB2 gene mutations was observed in familial cases of the disease with the autosomal recessive mode of inheritance (in 78% of families). Detected characteristics of the GJB2 gene mutation spectrum demonstrated that the using the algorithm, which was designed for Russian patients, is optimal for the molecular study of patients from Belarus. In the sample of patients with hearing loss, the highest (among other similar samples studied in the world) allele frequency of c.313_326del14 mutation (7% of all pathological GJB2 alleles) was registered; Polish origin of this deletion was suggested. It was demonstrated that detection of the GJB2 gene mutation on one patient’s chromosome only is insufficient to confirm a molecular genetic diagnosis of hearing loss of the DFNB1 genetic type (autosomal recessive hearing loss caused by the GJB2 gene mutations). Pilot screening for the GJB2 gene mutations in newborns from Grodno oblast was performed. The material from 235 children was studied during the screening; nine heterozygous carriers of the mutation were found. The c.35delG mutation was detected in a homozygous state in a single newborn (hearing loss of moderate severity was subsequently audiologically confirmed in this child).     

RAB23 mutations in Carpenter syndrome imply an unexpected role for hedgehog signaling in cranial-suture development and obesity

Carpenter syndrome is a pleiotropic disorder with autosomal recessive inheritance, the cardinal features of which include craniosynostosis, polysyndactyly, obesity, and cardiac defects. Using homozygosity mapping, we found linkage to chromosome 6p12.1-q12 and, in 15 independent families, identified five different mutations (four truncating and one missense) in RAB23, which encodes a member of the RAB guanosine triphosphatase (GTPase) family of vesicle transport proteins and acts as a negative regulator of hedgehog (HH) signaling. In 10 patients, the disease was caused by homozygosity for the same nonsense mutation, L145X, that resides on a common haplotype, indicative of a founder effect in patients of northern European descent. Surprisingly, nonsense mutations of Rab23 in open brain mice cause recessive embryonic lethality with neural-tube defects, suggesting a species difference in the requirement for RAB23 during early development. The discovery of RAB23 mutations in patients with Carpenter syndrome implicates HH signaling in cranial-suture biogenesis--an unexpected finding, given that craniosynostosis is not usually associated with mutations of other HH-pathway components--and provides a new molecular target for studies of obesity.     

McKusick-Kaufman or Bardet-Biedl syndrome? A new borderline case in an Italian nonconsanguineous healthy family

McKusick-Kaufman syndrome (MKS, OMIM #236700) is a rare syndrome inherited in an autosomal recessive pattern with a phenotypic triad comprising hydrometrocolpos (HMC), postaxial polydactyly (PAP), and congenital cardiac disease (CHD). The syndrome is caused by mutations in the MKKS gene mapped onto chromosome 20p12 between D20S162 and D20S894 markers. Mutations in the same gene causes Bardet-Biedl-6 syndrome (BBS-6, OMIM #209900) inherited in an autosomal recessive pattern. BBS-6 comprises retinitis pigmentosa, polydactyly, obesity, mental retardation, renal and genital anomalies. HMC, CHD, and PAP defects can also occur in BBS-6, and there is a significant clinical overlap between MKS and BBS-6 in childhood. We describe a new borderline case of MKS and BBS syndrome and suggest insights for understanding correlation between MKKS gene mutations and clinical phenotype. Here, we report the results of molecular analysis of MKKS in a female proband born in an Italian nonconsanguineous healthy family that presents HMC and PAP. The mutational screening revealed the presence of two different heterozygous missense variants (p.242A>S in exon 3, p.339 I>V in exon 4) in the MKKS gene, and a nucleotide variation in 5'UTR region in exon 2 (-417 A>C).