Domenico Vladimiro Libri

Genetics and phenomics of hypothyroidism due to TSH resistance.

The resistance to thyrotropin (TSH) action is the disease associated with molecular defects hampering the adequate transmission of TSH stimulatory signal into thyroid cells. The defect may in principle affect every step along the cascade of events following the binding of TSH to its receptor (TSHR) on thyroid cell membranes. After the description of the first family affected with loss-of-function (LOF) TSHR mutations in 1995, there is now evidence that TSH resistance is a disease with a broad range of expressivity going from severe congenital hypothyroidism (CH) with thyroid hypoplasia to mild hyperthyrotropinemia (hyperTSH) associated with an apparent euthyroid state. More severe forms occur in patients with disrupting biallelic TSHR mutations and follow a recessive pattern of inheritance. Differential diagnosis in these cases includes the exclusion of other causes of thyroid dysgenesis, such as mutations in thyroid transcription factors. More mild forms may instead occur in patients with monoallelic TSHR defects following a dominant mode of inheritance. In these cases we described the dominant negative effect exerted by some LOF mutants on the activity of the wild-type TSHR. Differential diagnosis involves the exclusion of mild hypothyroidism in autoimmune thyroid disease or pseudohypoparathyroidism associated with genetic or epigenetic defects at the GNAS locus. This review will focus on the prevalence of TSHR mutations, on the molecular mechanisms leading to TSH resistance and on the variable clinical expression of this disease.

New understandings of the genetic basis of isolated idiopathic central hypogonadism

Idiopathic hypogonadotropic hypogonadism is a rare disease that is characterized by delayed/absent puberty and/or infertility due to an insufficient stimulation of an otherwise normal pituitary-gonadal axis by gonadotrophin-releasing hormone (GnRH) action. Because reduced or normal luteinizing hormone (LH)/follicle-stimulating hormone (FSH) levels may be observed in the affected patients, the term idiopathic central hypogonadism (ICH) appears to be more appropriate. This disease should be distinguished from central hypogonadism that is combined with other pituitary deficiencies. Isolated ICH has a complex pathogenesis and is fivefold more prevalent in males. ICH frequently appears in a sporadic form, but several familial cases have also been reported. This finding, in conjunction with the description of numerous pathogenetic gene variants and the generation of several knockout models, supports the existence of a strong genetic component. ICH may be associated with several morphogenetic abnormalities, which include osmic defects that, with ICH, constitute the cardinal manifestations of Kallmann syndrome (KS). KS accounts for approximately 40% of the total ICH cases and has been generally considered to be a distinct subgroup. However, the description of several pedigrees, which include relatives who are affected either with isolated osmic defects, KS, or normo-osmic ICH (nICH), justifies the emerging idea that ICH is a complex genetic disease that is characterized by variable expressivity and penetrance. In this context, either multiple gene variants or environmental factors and epigenetic modifications may contribute to the variable disease manifestations. We review the genetic mechanisms that are presently known to be involved in ICH pathogenesis and provide a clinical overview of the 227 cases that have been collected by the collaborating centres of the Italian ICH Network.

Frequent TSH Receptor Genetic Alterations with Variable Signaling Impairment in a Large Series of Children with Nonautoimmune Isolated Hyperthyrotropinemia

CONTEXT Heterozygous mutations in the TSH receptor gene (TSHR) are associated with partial TSH resistance, characterized by isolated nonautoimmune hyperthyrotropinemia (NAHT). The prevalence and management of this condition is controversial. OBJECTIVE Our objective was to investigate the prevalence and clinical impact of TSHR alterations in a large series of pediatric patients with NAHT and to dissect their mechanism of action. DESIGN AND SETTING For this prospective multicenter study, clinical data and samples were collected in the clinical units and conveyed to a centralized laboratory for analysis. PATIENTS Subjects included 153 unrelated patients with NAHT aged <18 yr. Exclusion criteria included thyroid dysgenesis or major associated congenital defects. MAIN OUTCOME MEASURES Parameters of thyroid function, TSHR gene analysis, and TSHR functional assays were evaluated. RESULTS The frequency of heterozygous nonpolymorphic TSHR variations was 11.8%. We identified seven previously unknown variations: a frameshift (p.Q33PfsX46), one intronic (g.IVS4+2A→G), and five novel missense (p.P162L, p.Y466C, p.I583T, p.I607T, and p.R609Q) variations. The missense variations variably affected TSHR membrane expression and G(s) and/or G(q/11) signaling. Several variations cosegregated with NAHT in the affected families. Parameters of thyroid function were similar between affected and unaffected family members. CONCLUSIONS Nonpolymorphic alterations in the TSHR gene are commonly associated with isolated NAHT in young patients, thus configuring partial TSH resistance as the most frequent inheritable cause of isolated NAHT. The identification of TSHR defects may thus be helpful for a tailored management of subclinical hypothyroidism. We provide further evidence that besides the well-known defects in G(s) signaling, TSHR genetic alternations found in NAHT may frequently impair the G(q/11) pathway.

Germline prokineticin receptor 2 (PROKR2) variants associated with central hypogonadism cause differental modulation of distinct intracellular pathways.

INTRODUCTION Defects of prokineticin pathway affect the neuroendocrine control of reproduction, but their role in the pathogenesis of central hypogonadism remains undefined, and the functional impact of the missense PROKR2 variants has been incompletely characterized. MATERIAL AND METHODS In a series of 246 idiopathic central hypogonadism patients, we found three novel (p.V158I, p.V334M, and p.N15TfsX30) and six already known (p.L173R, p.T260M, p.R268C, p.V274D, p.V331M, and p.H20MfsX23) germline variants in the PROKR2 gene. We evaluated the effects of seven missense alterations on two different prokineticin receptor 2 (PROKR2)-dependent pathways: inositol phosphate-Ca(2+) (Gq coupling) and cAMP (Gs coupling). RESULTS PROKR2 variants were found in 16 patients (6.5%). Expression levels of variants p.V158I and p.V331M were moderately reduced, whereas they were markedly impaired in the remaining cases, except p.V334M, which was significantly overexpressed. The variants p.T260M, p.R268C, and p.V331M showed no remarkable changes in cAMP response (EC50) whereas the IP signaling appeared more profoundly affected. In contrast, cAMP accumulation cannot be stimulated through the p.L173R and p.V274D, but IP EC50 was similar to wt inp.L173R and increased by 10-fold in p.V274D. The variant p.V334M led to a 3-fold increase of EC50 for both cAMP and IP. CONCLUSION Our study shows that single PROKR2 missense allelic variants can either affect both signaling pathways differently or selectively. Thus, the integrity of both PROKR2-dependent cAMP and IP signals should be evaluated for a complete functional testing of novel identified allelic variants.

Central Hypogonadism (ICH). Germline prokineticin Receptor 2 (PROKR2) variants associated with central hypogonadism cause differential modulation of distinct intracellular pathways.

INTRODUCTION Defects of prokineticin pathway affect the neuroendocrine control of reproduction, but their role in the pathogenesis of central hypogonadism remains undefined, and the functional impact of the missense PROKR2 variants has been incompletely characterized. MATERIAL AND METHODS In a series of 246 idiopathic central hypogonadism patients, we found three novel (p.V158I, p.V334M, and p.N15TfsX30) and six already known (p.L173R, p.T260M, p.R268C, p.V274D, p.V331M, and p.H20MfsX23) germline variants in the PROKR2 gene. We evaluated the effects of seven missense alterations on two different prokineticin receptor 2 (PROKR2)-dependent pathways: inositol phosphate-Ca(2+) (Gq coupling) and cAMP (Gs coupling). RESULTS PROKR2 variants were found in 16 patients (6.5%). Expression levels of variants p.V158I and p.V331M were moderately reduced, whereas they were markedly impaired in the remaining cases, except p.V334M, which was significantly overexpressed. The variants p.T260M, p.R268C, and p.V331M showed no remarkable changes in cAMP response (EC50) whereas the IP signaling appeared more profoundly affected. In contrast, cAMP accumulation cannot be stimulated through the p.L173R and p.V274D, but IP EC50 was similar to wt inp.L173R and increased by 10-fold in p.V274D. The variant p.V334M led to a 3-fold increase of EC50 for both cAMP and IP. CONCLUSION Our study shows that single PROKR2 missense allelic variants can either affect both signaling pathways differently or selectively. Thus, the integrity of both PROKR2-dependent cAMP and IP signals should be evaluated for a complete functional testing of novel identified allelic variants.