[Monogenic and syndromic symptoms of morbid obesity. Rare but important]. / Monogene und syndromale Krankheitsbilder bei morbider Adipositas. Selten aber wichtig.

BACKGROUND: Monogenic and syndromic obesity are rare diseases with variable manifestation. Therefore diagnosis is difficult and often delayed. OBJECTIVES: The purpose of this work was to develop a clinical diagnostic algorithm for earlier diagnosis. MATERIAL AND METHODS: Available publications for clinical symptoms and molecular defects of monogenic and syndromic obesity cases were evaluated. RESULTS: Monogenic and syndromic obesity can be expected in cases with early manifestation before the age of 5 years and a BMI above 40 or above the 99th percentile. Syndromic cases are mostly associated with a low IQ and dwarfism. Monogenic cases are associated with additional endocrine defects. Measurement of serum leptin proves the treatable leptin deficiency. Sequencing of the melanocortin-4 receptor gene (MC4R) allows diagnosis of the most frequent monogenic form of obesity. Treatment with a melanocyte-stimulating hormone (MSH) analog can be expected in the future. Early treatment of children with Prader-Willi syndrome can prevent severe obesity. CONCLUSION: Because in some cases treatment is available, monogenic and syndromic obesity should be diagnosed early. Based on the disease symptoms, serum leptin, and MC4R sequencing, a diagnostic algorithm is proposed, which can be used to diagnose cases of morbid obesity.

Severe early-onset obesity, adrenal insufficiency and red hair pigmentation caused by POMC mutations in humans.

Sequential cleavage of the precursor protein pre-pro-opiomelanocortin (POMC) generates the melanocortin peptides adrenocorticotrophin (ACTH), melanocyte-stimulating hormones (MSH) alpha, beta and gamma as well as the opioid-receptor ligand beta-endorphin. While a few cases of isolated ACTH deficiency have been reported (OMIM 201400), an inherited POMC defect has not been described so far. Recent studies in animal models elucidated a central role of alpha-MSH in the regulation of food intake by activation of the brain melanocortin-4-receptor (MC4-R; refs 3-5) and the linkage of human obesity to chromosome 2 in close proximity to the POMC locus, led to the proposal of an association of POMC with human obesity. The dual role of alpha-MSH in regulating food intake and influencing hair pigmentation predicts that the phenotype associated with a defect in POMC function would include obesity, alteration in pigmentation and ACTH deficiency. The observation of these symptoms in two probands prompted us to search for mutations within their POMC genes. Patient 1 was found to be a compound heterozygote for two mutations in exon 3 (G7013T, C7133delta) which interfere with appropriate synthesis of ACTH and alpha-MSH. Patient 2 was homozygous for a mutation in exon 2 (C3804A) which abolishes POMC translation. These findings represent the first examples of a genetic defect within the POMC gene and define a new monogenic endocrine disorder resulting in early-onset obesity, adrenal insufficiency and red hair pigmentation.

Mutations in LHX3 result in a new syndrome revealed by combined pituitary hormone deficiency.

Combined pituitary hormone deficiency (CPHD) has been linked with rare abnormalities in genes encoding transcription factors necessary for pituitary development. We have isolated LHX3, a gene involved in a new syndrome, using a candidate-gene approach developed on the basis of documented pituitary abnormalities of a recessive lethal mutation in mice generated by targeted disruption of Lhx3 (ref. 2). LHX3, encoding a member of the LIM class of homeodomain proteins, consists of at least six exons located at 9q34. We identified a homozygous LHX3 defect in patients of two unrelated consanguineous families displaying a complete deficit in all but one (adrenocorticotropin) anterior pituitary hormone and a rigid cervical spine leading to limited head rotation. Two of these patients also displayed a severe pituitary hypoplasia, whereas one patient presented secondarily with an enlarged anterior pituitary. These LHX3 mutations consist of a missense mutation (Y116C) in the LIM2 domain at a phylogenetically conserved residue and an intragenic deletion predicting a severely truncated protein lacking the entire homeodomain. These data are consistent with function of LHX3 in the proper development of all anterior pituitary cell types, except corticotropes, and extrapituitary structures.

PAX8 mutations associated with congenital hypothyroidism caused by thyroid dysgenesis.

Permanent congenital hypothyroidism (CH) is a common disease that occurs in 1 of 3,000-4,000 newborns. Except in rare cases due to hypothalamic or pituitary defects, CH is characterized by elevated levels of thyroid-stimulating hormone (TSH) resulting from reduced thyroid function. When thyroid hormone therapy is not initiated within the first two months of life, CH can cause severe neurological, mental and motor damage. In 80-85% of cases, CH is associated with and presumably is a consequence of thyroid dysgenesis (TD). In these cases, the thyroid gland can be absent (agenesis, 35-40%), ectopically located (30-45%) and/or severely reduced in size (hypoplasia, 5%). Familial cases of TD are rare, even though ectopic or absent thyroid has been occasionally observed in siblings. The pathogenesis of TD is still largely unknown. Although a genetic component has been suggested, mutations in the gene encoding the receptor for the thyroid-stimulating hormone (TSHR) have been identified in only two cases of TD with hypoplasia. We report mutations in the coding region of PAX8 in two sporadic patients and one familial case of TD. All three point mutations are located in the paired domain of PAX8 and result in severe reduction of the DNA-binding activity of this transcription factor. These genetic alterations implicate PAX8 in the pathogenesis of TD and in normal thyroid development.

Development of a pediatric differentiated thyroid carcinoma registry within the EuRRECa project: rationale and protocol.

Background: Although differentiated thyroid carcinoma (DTC) is the most frequent endocrine pediatric cancer, it is rare in childhood and adolescence. While tumor persistence and recurrence are not uncommon, mortality remains extremely low. Complications of treatment are however reported in up to 48% of the survivors. Due to the rarity of the disease, current treatment guidelines are predominantly based on the results of small observational retrospective studies and extrapolations from results in adult patients. In order to develop more personalized treatment and follow-up strategies (aiming to reduce complication rates), there is an unmet need for uniform international prospective data collection and clinical trials. Methods and analysis: The European pediatric thyroid carcinoma registry aims to collect clinical data for all patients ≤18 years of age with a confirmed diagnosis of DTC who have been diagnosed, assessed, or treated at a participating site. This registry will be a component of the wider European Registries for Rare Endocrine Conditions project which has close links to Endo-ERN, the European Reference Network for Rare Endocrine Conditions. A multidisciplinary expert working group was formed to develop a minimal dataset comprising information regarding demographic data, diagnosis, treatment, and outcome. We constructed an umbrella-type registry, with a detailed basic dataset. In the future, this may provide the opportunity for research teams to integrate clinical research questions. Ethics and dissemination: Written informed consent will be obtained from all participants and/or their parents/guardians. Summaries and descriptive analyses of the registry will be disseminated via conference presentations and peer-reviewed publications.

Ethnicity and comorbidities in an overweight and obese multiethnic childhood cohort in Berlin.

AIM: This study aims to analyse the association between ethnicity, elevated metabolic parameters and metabolic syndrome (MS) in a multiethnic cohort of overweight to obese children and adolescents. METHODS: For 1053 patients, standard deviation of body mass index (BMI-SDS) was calculated and metabolic parameters (fasting blood glucose, fasting insulin, homeostasis model assessment-IR, lipids, blood pressure) were measured. MS was defined by WHO criteria. Bivariate and multivariate analyses were performed. Adjusted differences in BMI-SDS and metabolic parameters between different migration groups were assessed with linear regression models. The risk for MS was calculated with multiple logistic regression models. RESULTS: Forty-eight per cent of the children were German, 25% Turkish and 27% had another ethnicity. Concerning weight status, 23% are overweight, 31% obese and 46% extremely obese with higher rates among the immigrant population. Multivariable models indicate significant associations between elevated metabolic parameters and higher BMI-SDS values. Overall prevalence of MS was 32.3%. MS was detected significantly more often among Turkish patients (40.4%) compared to Germans (27.3%; p=0.02). Logistic regression analysis showed a greater risk for MS with older age (OR=1.09; p=0.003) and Turkish ethnicity (OR=1.62; p=0.02). CONCLUSION: Nearly all patients had symptoms of MS, and 40% had MS showing that this highly health-threatening condition is quite common. Therefore, effective therapy and prevention efforts must be developed for this high risk group. More migration-specific research regarding insulin resistance, MS and Type 2 DM is needed.

Two puzzling cases of thyroid dysgenesis.

BACKGROUND: In the last couple of years, different genes that play major roles in embryonic thyroid development have been identified. Several mutations, e.g., NKX2.1, FOXE1 and PAX8, were identified in patients with congenital hypothyroidism due to thyroid dysgenesis. However, the pathophysiology of most cases of thyroid dysgenesis remains unknown. Due to the sporadic occurrence and discordance observed in monozygotic twins, a classic genetic hypothesis for thyroid dysgenesis is improbable. CASE REPORT: We present two pairs of monozygotic twins discordant for thyroid dysgenesis that exemplify these conceptual difficulties. CONCLUSIONS: Identification of the epigenetic differences observed in monozygotic twins discordant for thyroid dysgenesis may be crucial in discovering the pathogenesis of thyroid dysgenesis.

[Update of the S2k guidelines : Surgical treatment of benign thyroid diseases]. / Aktualisierung der S2k-Leitlinie : Operative Therapie benigner Schilddrüsenerkrankungen.

Thyroid resections represent one of the most common operations with 76,140 interventions in the year 2016 in Germany (source Destatis). These are predominantly benign thyroid gland diseases. Recommendations for the operative treatment of benign thyroid diseases were last published by the CAEK in 2010 as S2k guidelines (Arbeitsgemeinschaft der Wissenschaftlichen Medizinischen Fachgesellschaften e.V. [AWMF] 003/002) against the background of increasingly more radical resection procedures. Hemithyroidectomy and thyroidectomy are routinely performed for benign thyroid disease in practice. The operation-specific risks show a clear increase with the extent of the resection. Therefore, weighing-up of the risk-indications ratio between unilateral lobectomy or thyroidectomy necessitates an independent evaluation of the indications for both sides. This principle in particular has been used to update the guidelines. In addition, the previously published recommendations of the CAEK for correct execution and consequences of intraoperative neuromonitoring were included into the guidelines, which in particular serve the aim to avoid bilateral recurrent laryngeal nerve paralysis. Moreover, the recommendations for the treatment of postoperative complications, such as hypoparathyroidism and postoperative infections were revised. The updated guidelines therefore represent the current state of the science as well as the resulting surgical practice.

Implications of proopiomelanocortin (POMC) mutations in humans: the POMC deficiency syndrome.

The recent discovery of the contribution of proopiomelanocortin (POMC)-derived peptides to the regulation of energy homeostasis and exocrine gland secretion in mice aroused new interest in the complex function of the endocrine POMC network. In addition, the first mutations in the gene encoding POMC have been identified in two patients affected by adrenal insufficiency, early onset severe obesity and red hair pigmentation. Therefore, the focus of this brief review will be the detailed discussion of the implications of these new findings in the physiology of the human POMC ligand-receptor system.

How a patient homozygous for a 30-kb deletion of the C4-CYP 21 genomic region can have a nonclassic form of 21-hydroxylase deficiency.

A case of nonclassic (NC) 21-hydroxylase deficiency, with a moderately elevated 17-hydroxyprogesterone level (145 nmol/L in filter paper blood spot), was detected in newborn screening. The newborn's phenotype was female, with no sign of virilization. Confirmatory diagnosis revealed elevated serum levels of 17-hydroxyprogesterone and of 21-desoxycortisol, whereas cortisol, PRA, and electrolytes were normal. Hydrocortisone substitution was considered at the age of 6 months, when virilization became obvious. For clinical reasons, this case had to be classified as late-onset congenital adrenal hyperplasia (CAH) with unusually early manifestation. However, the diagnosis of classic 21-hydroxylase deficiency was obtained by Southern blotting studies, showing that she was homozygous for the 30-kb deletion, including the 3' end of CYP21P pseudogene, the C4B gene, and the 5' end of the functional CYP21 gene. Further studies, using PCR and sequencing, were conducted to explain the discrepancy between this genotype, usually associated with a classic salt-wasting form, and the girl's phenotype. Typically, patients homozygous for the 30-kb deletion encoding classic CAH possess a unique CYP21P/21 hybrid gene with the junction site located after the third exon, yielding a nonfunctional pseudogene. The girl in question, however, was heterozygous for the 8-bp deletion, suggesting that the chimeric pseudogene on one allele had a junction site before the third exon. She was compound heterozygous for a 30-kb deletion encoding classic CAH on the paternal allele, and a 30-kb deletion encoding NC CAH on the maternal allele. This novel maternal CYP21P/21 hybrid gene is characterized by a junction site before intron 2 and differs from the normal CYP21 gene only by the P30L mutation in exon 1 and the promoter region of the CYP21P pseudogene. Because the P30L mutation has been described to result in an enzyme with 30-60% activity of the normal P450c21 enzyme, and the CYP21P promoter reduced the transcription to 20% of normal, this puzzling phenotype of a NC CAH with early onset may be fully explained by the genotype of the patient and considered as an intermediate form between the simple virilizing and NC form.

Mutations of the human thyrotropin receptor gene causing thyroid hypoplasia and persistent congenital hypothyroidism.

The pathogenesis of congenital hypothyroidism due to thyroid dysgenesis is still unknown. A point mutation in the TSH receptor (TSHR) of the hypothyroid hyt/hyt mouse invoked the TSHR as a candidate gene for congenital hypothyroidism. Therefore, we screened for mutations in the TSHR gene in patients with congenital hypothyroidism and hypoplasia of the gland. In one girl detected in neonatal screening with the confirmed diagnosis of permanent congenital hypothyroidism with reduced thyroid volume, two novel mutations in the TSHR gene were identified. Single strand conformational polymorphism and subsequent DNA sequencing studies of a fragment of the TSHR gene showed that the patient is a compound heterozygote for 2 loss of function mutations in exon 10 of the TSHR gene. In the mutant maternal allele, 18 nucleotides (positions 1217-1234) are deleted, and 4 novel bp are inserted, resulting in a frame-shift and premature termination of the coding sequence. Transfection studies showed that this truncated TSHR was trapped intracellularly and completely lacked cell surface expression. The paternal gene harbors a missense mutation at nucleotide position 1170, leading to the exchange of the highly conserved C-390 for a W residue. This alteration resulted in a drastic loss of affinity and potency of TSH acting at the mutant compared to the wild-type receptor. In contrast to the published loss of function mutations of the TSHR leading to euthyroid hyperthyrotropinemia, the two new mutations lead to persistent congenital hypothyroidism and defective organ development. Further studies will have to analyze to what extent TSHR mutations are involved in the pathogenesis of congenital hypothyroidism as opposed to other genetic or environmental factors.

Severe congenital hyperthyroidism caused by a germ-line neo mutation in the extracellular portion of the thyrotropin receptor.

Gain of function mutations in the TSH receptor (TSHR) have been identified as the molecular basis for congenital and acquired forms of autonomous thyroid function. Herein, we report the molecular characterization of a case of severe congenital hyperthyroidism with a history of hyperthyroidism in the paternal aunt and the paternal grandmother, who were both found to be heterozygous for a mutation (R528H) located in exon 10 of the TSHR gene. Functional expression of the mutant TSHR-R528H in COS-7 cells, however, did not result in constitutive activity of the TSHR. Subsequent analysis of exons 1-9 led to the detection of an additional heterozygous mutation (S281N) in the patient, but not in other family members. Interestingly, the latter mutation is located in the extracellular domain of the TSHR, and functional studies revealed a marked increase in basal cAMP levels when the mutant receptor was expressed in COS-7 cells. To address the question of whether both mutations were present on the same allele, a double mutant TSHR (S281N/R528H) was generated and characterized. These functional studies in conjunction with RT-PCR analysis of thyroid tissue obtained from subtotal thyroidectomy performed at the age of 6 yr revealed that the patient bears two distinct mutations on different alleles: the familial paternal R528H mutation to be regarded as a polymorphism and a de novo mutation (S281N) on the maternal allele accounting for the clinical picture. Thus, the main conclusions to be drawn from this case are 1) a search for mutations in cases of congenital nonautoimmune hyperthyroidism should not remain restricted to exon 10 of the TSHR gene, because germ-line gain of function mutations of the TSH receptor can be located outside of the transmembrane core of the receptor; and 2) this case illustrates the necessity for careful functional characterization of any novel mutation before a causal relationship to hyperthyroidism can be established.

Twelve of fourteen surface epitopes of receptor-bound human chorionic gonadotropin (hCG) being antibody-inaccessible suggest an extensive involvement of the long extracellular domain of the hCG receptor.

On the surface of the free (receptor-unbound) form of hCG, we have previously identified 14 topographically distinct epitopes (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here we report that only two of them, i.e. the (adjacent) beta 3 and beta 5 epitopes, can be recognized by 125iodine-labeled monoclonal antibodies when hCG was specifically bound to the rat testis hCG receptor. The exclusive accessibility of precisely these two surface epitopes indicates that hCG assumes a defined rather than a stochastic orientation in its receptor-bound state. The inaccessibility of 12 of 14 epitopes is consistent with the idea that the 341 residues long extracellular domain of the recently cloned hCG receptor (MacFarland et al. (1989) Science 245, 494-499) is the ligand binding domain. It is proposed that the extracellular domain is folded in a way that a cavity is formed large enough to accommodate hCG. Thereby, a considerable portion of the total surface of hCG is covered, as reflected by the masking of most of its epitopes.

Number and topography of epitopes of human chorionic gonadotropin (hCG) are shared by desialylated and deglycosylated hCG.

A previously established map of the surface epitopes of human chorionic gonadotropin (hCG) served as template for the present study in which we investigated the antigenic surfaces of two glycosylation variants of hCG, i.e. desialylated hCG (asialo-hCG) and deglycosylated hCG (degly-hCG). This map allocates five epitopes to the alpha subunit, five to the beta subunit and four alpha beta epitopes to structures formed only by the alpha/beta heterodimer holo-hCG (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here it is described that both variants complied with this template: each of the 14 distinct monoclonal antibodies with which the epitopes of hCG were defined reacted with radiolabeled asialo-hCG and degly-hCG as well and generally bound degly-hCG with greater affinity than hCG. Moreover, every combination of capture and radiolabeled detection antibody that was either compatible or incompatible on unlabeled hCG was so also on unlabeled asialo-hCG and degly-hCG. It thus appears that alterations of the carbohydrate structure of hCG can be associated with a change in affinity between some antibodies and their respective epitopes but not with a loss of an epitope or with a change in the topographical relationships of the 14 epitopes.

The gene for the thyrotropin receptor (TSHR) as a candidate gene for congenital hypothyroidism with thyroid dysgenesis.

According to the central role of the TSH receptor for thyroid function and growth the gene for the TSH receptor is a possible candidate gene for mutations which result in an impairment of thyroid growth and function (Vassart and Dumont 1992). First evidence for the role of TSH receptor defects in the pathogenesis of congenital thyroid disorders was elucidated by the presence of activating germline mutations leading to congenital hyperthyroidism (Duprez et al., 1994). After the finding of partial loss-of-function mutations leading to hyperthyrotropinemia (Sunthornthepvarakul et al., 1995) it was speculated that a more severe phenotype with hypothyroidism and hypoplasia of the gland (thyroid dysgenesis) would be the result, if complete loss-of-function mutations like the isoleucine167 to asparagine mutation would occur in a homozygote or compound heterozygote state. The screening of TSHR gene mutations by SSCP in a well defined cohort of 100 children with congenital hypothyroidism (CH), diagnosed and followed since 1978 in the Childrens Hospital of Berlin, revealed one patient with hypoplasia of the thyroid to be positive for two compound heterozygote inactivating mutations of the TSHR gene, indicating thereby that the clinical approach to define phenotypes of interest could be helpful to understand the fundamental process of thyroid development.

Congenital hyperthyroidism.

Congenital hyperthyroidism is a very rare disease. But, for each affected child it has to be considered as a serious condition because of the negative impact of hyperthyroidism on fetal and postnatal development. If the manifestation occurs during fetal life tachycardia, cardiac arrhythmia, growth retardation and, most significant, prematurity are the consequences. Postnatal signs of hyperthyroidism are irritability, tachycardia, hypertension, poor weight gain and thyroid enlargement. Even cardiac failure may occur if hyperthyroidism is severe and treatment not adequate which explains the high early mortality rate of 16%. The main complication of persistent hyperthyroidism in the neonatal period and during infancy is craniosynostosis. Severe developmental delay or even mental retardation can be the consequence of inadequate high T4-levels during fetal and neonatal life. Congenital hyperthyroidism was first recognized in infants born to mothers with Graves' disease. The description of transplacental passage of the maternal thyroid stimulating antibodies elucidated the molecular mechanism in this major group of patients with "autoimmune congenital hyperthyroidism". In contrast to this transient, self-limited character of "autoimmune congenital hyperthyroidism", due to the clearance of maternal antibodies from the infant's circulation, some cases of persistent congenital hyperthyroidism without signs of thyroid autoimmunity have been recognized. Activating mutations in the thyroid-stimulating hormone receptor were described recently as the underlying molecular pathogenesis in this group of "non-immune congenital hyperthyroidism". Therefore the possibility of a molecular differential diagnosis of both groups of congenital hyperthyroidism now exists and opens the opportunity of optimal treatment for each patient.

Screening for mutations of the human thyroid peroxidase gene in patients with congenital hypothyroidism.

While congenital hypothyroidism in 80-90% of the affected individuals is caused by thyroid dysgenesis (athyrosis, ectopy or hypoplasia), hypothyroidism in patients with a thyroid gland of normal position and size can be due to regulatory or enzymatic defects of thyroid hormone biosynthesis. Beside defects of thyroglobulinsynthesis, defects of the sodium-iodide-transporter or the TSH-receptor, a defect of the thyroidperoxidase, the key-enzyme of thyroid hormone biosynthesis, can cause a total iodide organification defect and thereby congenital hypothyroidism. We screened 14 of 103 patients (13.6%) with non familial congenital hypothyroidism and a normally developed thyroid gland detected by the newborn screening program with the PCR-SSCP (single-stranded-conformational-polymorphism) technique for mutations in the exons 2, 8, 9, 10 and 14 of the human thyroperoxidase gene, and in which mutations had been described previously in Dutch and Brazilian families with total organification defects. Most of the previously reported mutations were found in exons 8, 9 and 10 which code for the caralytic part of the enzyme. In two patients a GGCC-duplication in exon 8 was detected leading to a premature stop codon in exon 9. While one patient without neonatal goiter was homozygous for this mutation, the second patient was only heterozygous thus demanding another mutation on the second TPO-allel to explain the phenotype. Since the GGCC duplication is easily demonstrable by a NaeI digestion, because it creates a restriction site for this enzyme, screening for this mutation is indicated since it is easy to perform. In contrast to the perchlorate discharge test molecular genetic studies are less invasive, but as useful in making a definitive diagnosis in the individual patient. Furthermore it is the first feasible step to study the etiology and epidemiology of the so far only putative defects of thyroid hormone biosynthesis leading to congenital hypothyroidism.

The use of L-T4 as liquid solution improves the practicability and individualized dosage in newborns and infants with congenital hypothyroidism.

UNLABELLED: Dosage recommendations for the initial therapy of congenital hypothyroidism (CH) in newborns vary between 8 microg/kg/d and 10-15 microg/kg/d. AIM: To evaluate the practicability of LT4 in liquid form and to define the initial dosage for optimal treatment. METHODS: Liquid LT4 solution was administered to 28 consecutive newborns with primary CH. We measured TSH, T3, T4, free T3 and free T4 before therapy and during follow-up up to 2 years. After 2 years a standardized developmental test (Griffith) was performed. RESULTS: The median dosage at start of therapy was 12.3 microg LT4/kg/d and decreased to about 5 microg LT4/kg/d after 9 months. The median time of normalization of TSH (< or =6 mU/l) was 2 weeks. In 21 patients, who received a median starting dosage of 12.7 microg LT4/kg (range 9.8-17.1 microg/kg), TSH levels normalized within a median of 1 week. Seven patients receiving only 10.1 microg LT4/kg normalized their TSH only after a median of 2 months. CONCLUSIONS: Newborns with CH should normalize their TSH within 1-2 weeks. The initial dose necessary to normalize TSH is not lower when a liquid solution is used. The higher dose used in tablets is not due to inefficient absorption, but rather reflects the increased demand for thyroid hormone in the first weeks of life.

[The human chorionic gonadotropin (hCG) receptor: a new class within the family of GTP protein coupled receptors. Epitope mapping of receptor-bound agonistic and antagonistic forms of hCG]. / Der humane Chorion-Gonadotropin (hCG)-Rezeptor: Eine neue Klasse innerhalb der Familie der GTP-Protein-gekoppelten Rezeptoren. Epitop-Mapping an Rezeptor-gebundenen agonistischen und antagonistischen Formen des hCGs.

Human chorionic gonadotropin (hCG) is a member of the glycoprotein hormone family. It is composed of an alpha and a beta subunit, the latter being closely homologous to that of human luteinizing hormone (hLH). HCG and hLH bind to the same receptor. The molecular mass of hCG is 38 kD, up to 30% of which is contributed by the carbohydrate moieties linked to each of the two subunits. Structure-function relationship studies have indicated that both subunits interact with the receptor and that altogether four different peptide domains (two on each subunit) are responsible for high-affinity receptor binding. The carbohydrate units are responsible for expression of agonist activity: deglycosylated hCG (degly-hCG) is unable to induce a biological response (cAMP increase), despite high-affinity binding. We have previously mapped the antigenic surface of hCG and described 14 different epitopes that can be recognized by specific monoclonal antibodies (MCA), i.e. five epitopes on the alpha subunit, five on the beta subunit and four epitopes which are formed through association of the two subunits (conformational or alpha beta epitopes). The number and topography of epitopes was found to be the same on degly-hCG. This framework of immunological coordinates was then applied to probe the orientation of hCG, as well as that of the competitive antagonist degly-hCG in their receptor-bound states. With a receptor-hormone-125I-MCA sandwich approach we found that while hCG still presented two of the 14 epitopes, i.e. beta 3 and beta 5, no epitope was accessible on receptor-bound degly-hCG. This differential pattern thus correlates with the signal transduction-competence of the respective ligand. Overall, these data indicate that most of the surface of hCG is masked by moieties of the receptor. Accordingly, that moiety must itself be folded in a way that it provides a large contact surface. No portion other than the 341-residue long extracellular domain would seem capable of doing that. Computer-assisted secondary structure predictions support this view. This domain is the novel feature that distinguishes glycoprotein-hormone receptors from the other members of the G protein-coupled receptor superfamily.

Alterations of neonatal thyroid function.

Recent progress has been made in understanding the pathogenesis of neonatal thyroid disorders. Autosomal recessive inheritance of mutations of the thyroid peroxidase and thyroglobulin genes has been described in some patients with congenital hypothyroidism (CH) and a family history of CH. Autosomal recessive inheritance of mutations of the thyrotrophin (TSH) receptor gene has also been reported in patients with CH and thyroid hypoplasia, and autosomal dominant mutations of the PAX8 gene have been described in patients with different forms of thyroid dysgenesis. These discoveries are important for patients with CH diagnosed by neonatal screening, as these patients will have normal fertility. The molecular genetic analysis of mutations of the TSH gene in patients with familial and sporadic cases of isolated central CH, who are missed by TSH screening programmes, now enables rapid diagnosis and appropriate therapy in the neonate. In newborn infants with severe non-autoimmune hyperthyroidism, autosomal dominant gain-of-function mutations in the TSH receptor gene have been demonstrated. In these patients, molecular genetic studies are extremely helpful in therapeutic decision making, as early thyroid ablation is the only effective treatment that avoids the sequelae of long-term hyperthyroidism. Molecular genetic studies are therefore useful in the diagnostic work-up of neonatal thyroid alterations.