IGSF1 Deficiency Leads to Reduced TSH Production Independent of Alterations in Thyroid Hormone Action in Male Mice.

Loss of function mutations in IGSF1/Igsf1 cause central hypothyroidism. Igsf1 knockout mice have reduced pituitary thyrotropin-releasing hormone receptor, Trhr, expression, perhaps contributing to the phenotype. Because thyroid hormones negatively regulate Trhr, we hypothesized that IGSF1 might affect thyroid hormone availability in pituitary thyrotropes. Consistent with this idea, IGSF1 coimmunoprecipitated with the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in transfected cells. This association was impaired with IGSF1 bearing patient-derived mutations. Wild-type IGSF1 did not, however, alter MCT8-mediated thyroid hormone import into heterologous cells. IGSF1 and MCT8 are both expressed in the apical membrane of the choroid plexus. However, MCT8 protein levels and localization in the choroid plexus were unaltered in Igsf1 knockout mice, ruling out a necessary chaperone function for IGSF1. MCT8 expression was low in the pituitary and was similarly unaffected in Igsf1 knockouts. We next assessed whether IGSF1 affects thyroid hormone transport or action, by MCT8 or otherwise, in vivo. To this end, we treated hypothyroid wild-type and Igsf1 knockout mice with exogenous thyroid hormones. T4 and T3 inhibited TSH release and regulated pituitary and forebrain gene expression similarly in both genotypes. Interestingly, pituitary TSH beta subunit (Tshb) expression was consistently reduced in Igsf1 knockouts relative to wild-type regardless of experimental condition, whereas Trhr was more variably affected. Although IGSF1 and MCT8 can interact in heterologous cells, the physiological relevance of their association is not clear. Nevertheless, the results suggest that IGSF1 loss can impair TSH production independently of alterations in TRHR levels or thyroid hormone action.

TRH Action Is Impaired in Pituitaries of Male IGSF1-Deficient Mice.

Loss-of-function mutations in the X-linked immunoglobulin superfamily, member 1 (IGSF1) gene cause central hypothyroidism. IGSF1 is a transmembrane glycoprotein of unknown function expressed in thyrotropin (TSH)-producing thyrotrope cells of the anterior pituitary gland. The protein is cotranslationally cleaved, with only its C-terminal domain (CTD) being trafficked to the plasma membrane. Most intragenic IGSF1 mutations in humans map to the CTD. In this study, we used CRISPR-Cas9 to introduce a loss-of-function mutation into the IGSF1-CTD in mice. The modified allele encodes a truncated protein that fails to traffic to the plasma membrane. Under standard laboratory conditions, Igsf1-deficient males exhibit normal serum TSH levels as well as normal numbers of TSH-expressing thyrotropes. However, pituitary expression of the TSH subunit genes and TSH protein content are reduced, as is expression of the receptor for thyrotropin-releasing hormone (TRH). When challenged with exogenous TRH, Igsf1-deficient males release TSH, but to a significantly lesser extent than do their wild-type littermates. The mice show similarly attenuated TSH secretion when rendered profoundly hypothyroid with a low iodine diet supplemented with propylthiouracil. Collectively, these results indicate that impairments in pituitary TRH receptor expression and/or downstream signaling underlie central hypothyroidism in IGSF1 deficiency syndrome.

IGSF1 variants in boys with familial delayed puberty.

UNLABELLED: The immunoglobulin superfamily member 1 (IGSF1) gene encodes a plasma membrane glycoprotein mainly expressed in pituitary and testes. Loss-of-function mutations in IGSF1 cause an X-linked syndrome of central hypothyroidism (CeH), macroorchidism, and delayed puberty (delayed rise of testosterone, but normal timing of testicular growth). As this syndrome was discovered in patients with CeH, it is unknown whether IGSF1 mutations might also cause delayed puberty without CeH. We therefore determined the prevalence of IGSF1 sequence variants in 30 patients with an apparent X-linked form of constitutional delay of growth and puberty (CDGP). In four families, we discovered three novel variants of unknown clinical significance (VUCSs), with possible pathogenicity predicted by in silico analysis. However, the genotype did not fully cosegregate with CDGP, all three VUCSs showed normal plasma membrane expression in transfected HEK293 cells, and no other features of the IGSF1 deficiency syndrome were observed in family members carrying the VUCSs. The observation of hyperprolactinemia in two carriers remains unexplained. CONCLUSION: There is insufficient evidence to conclude that the three observed VUCSs in IGSF1 are associated with CDGP, making it unlikely that IGSF1 mutations are a prevalent cause of CDGP.

Three novel IGSF1 mutations in four Japanese patients with X-linked congenital central hypothyroidism.

CONTEXT: Congenital central hypothyroidism (C-CH) is a rare disease. We investigated the molecular basis of unexplained C-CH in 4 Japanese boys. PATIENTS AND METHODS: C-CH was diagnosed by low free T4 and/or T3 and low basal TSH concentrations. We used whole-exome sequencing of one patient with C-CH to identify potential disease-causing mutations. Thereafter, PCR direct sequencing was performed to Identify genetic defects underlying C-CH in 3 more patients. We then assessed the effects of mutations identified in the Ig superfamily, member 1 (IGSF1), gene on protein expression and membrane trafficking. RESULTS: All patients had congenital hypothyroidism, and 2 had definitive prolactin deficiency. Two patients were detected by neonatal screening. The other patients were diagnosed by short stature and failure to thrive. We identified a novel nonsense variant in IGSF1 by whole-exome sequencing in patient 1, which was confirmed by PCR direct sequencing (p.R1189X). PCR direct sequencing identified the identical nonsense mutation in patient 2. Patients 3 and 4 harbored distinct missense (p.V1082E) or nonsense (p.Q645X) mutations in IGSF1. The mothers of patients 1, 3, and 4 were heterozygous for these mutations. The R1189X mutant, which lacks the transmembrane domain, failed to traffic to the plasma membrane. V1082E could be observed at the cell surface, but at greatly diminished levels relative to the wild-type form of the protein. The severely truncated Q645X mutant could not be detected by Western blot. CONCLUSION: Our findings provide additional genetic evidence that loss-of-function mutations in IGSF1 cause an X-linked form of C-CH and variable prolactin deficiency.