IGSF1 (Immunoglobulin Superfamily 1) gene defects cause central hypothyroidism and macroorchidism. However, the pathogenic mechanisms of the disease remain unclear. Based on a patient with a full deletion of IGSF1 clinically followed from neonate to adulthood, we investigated a common pituitary origin for hypothyroidism and macroorchidism, and the role of IGSF1 as regulator of pituitary hormone secretion. The patient showed congenital central hypothyroidism with reduced TSH biopotency, over-secretion of FSH at neonatal minipuberty and macroorchidism from 3 years of age. His markedly elevated inhibin B was unable to inhibit FSH secretion, indicating a status of pituitary inhibin B resistance. We show here that IGSF1 is expressed both in thyrotropes and gonadotropes of the pituitary and in Leydig and germ cells in the testes, but at very low levels in Sertoli cells. Furthermore, IGSF1 stimulates transcription of the thyrotropin-releasing hormone receptor (TRHR) by negative modulation of the TGFβ1-Smad signaling pathway, and enhances the synthesis and biopotency of TSH, the hormone secreted by thyrotropes. By contrast, IGSF1 strongly down-regulates the activin-Smad pathway, leading to reduced expression of FSHB, the hormone secreted by gonadotropes. In conclusion, two relevant molecular mechanisms linked to central hypothyroidism and macroorchidism in IGSF1 deficiency are identified, revealing IGSF1 as an important regulator of TGFβ/Activin pathways in the pituitary.
A unique missense TRHR defect identified in a consanguineous family is associated with central hypothyroidism in homozygotes and hyperthyrotropinemia in heterozygotes, suggesting compensatory elevation of TSH with reduced biopotency. The I131T mutation decreases TRH binding and TRHR-Gq coupling and signaling.
Central congenital hypothyroidism (CCH) is an underdiagnosed disorder poorly described in childhood and adolescence. Congenital defects in thyroid-stimulating hormone (TSH) synthesis, secretion or bioactivity may lead to a state of 'regulatory' hypothyroidism expressed through aberrantly low or normal TSH levels and low thyroxine (T4), a hormonal pattern undetectable by TSH-based neonatal screening programs for congenital hypothyroidism (CH) implemented in most countries worldwide. CCH is more prevalent than previously thought, reaching 1 in 16,000 neonates in countries consistently identifying CCH through T4-based CH screening strategies. Neonatal detection and early treatment of CCH would prevent the risk of developing mental retardation secondary to late diagnosis of infantile hypothyroidism. CCH is frequently associated with other pituitary defects causing life-threatening situations (like e.g. adrenocorticotropic hormone deficiency) which could benefit from the early detection of CCH, avoiding considerable morbidity and mortality. CCH is not easy to identify clinically, and therefore few children are investigated for the disorder. The current knowledge on the genetic bases of CCH is also scarce. At the hypothalamic level no gene defects causing CCH have yet been identified in humans, but pituitary (thyrotrope)-selective genes encoding the TSH-releasing hormone (TRH) receptor (TRHR), the TSH β-subunit (TSHB) and, recently, the immunoglobulin superfamily factor 1 (IGSF1) are genes involved in isolated central hypothyroidism. Moreover, central hypothyroidism is a complex condition where many regulatory signals are implicated and converge to finely modulate the activity of the hypothalamic-pituitary-thyroid axis. This review focuses on novel pathogenic mechanisms and their implications to understand human CCH and improve the identification and the therapeutic handling of this elusive disease in the pediatric age.
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