A Novel Mutation in the TBG Gene Producing Partial Thyroxine-Binding Globulin Deficiency (Glencoe) Identified in 2 Families.

BACKGROUND:Thyroxine-binding globulin (TBG) is the major thyroid hormone transport protein in serum. Located on the long arm of the X chromosome, TBG (SERPINA7) gene mutations most commonly produce inherited partial TBG deficiency (TBG-PD). OBJECTIVE:We report a novel TBG variant associated with TBG-PD identified in 2 different families of Ashkenazi origin residing in greater Chicago. METHODS:Family 1: The proband was 12.6 years old when she presented for delayed puberty and was placed on L-T₄. Although her serum TSH normalized, her serum T₄ remained low. Affected family members had low total T₄ and T₃, but a normal free T₄ index, even when serum TSH concentrations were normal. Family 2: A 71-year-old male presented with a history of a nonfunctioning pituitary adenoma and normal pituitary axes except for low total T₄ and T₃. His brother had a similar thyroid phenotype. RESULTS:Following direct DNA sequencing, both index patients were found to carry a missense mutation in the TBG gene (c.751T>G) producing p.V215G. The proposita of family 1 was heterozygous and the proband in family 2 was hemizygous for the mutation. Isoelectric focusing showed no alteration in the TBG isoforms and in vitro expression demonstrated a TBG with reduced affinity for T₄. CONCLUSIONS:We report a novel mutation in the TBG gene in 2 unrelated families that produces a molecule with reduced affinity for T₄ resulting in low serum T₄. However, the physical properties of the mutant molecule remained unaltered as determined by isoelectric focusing.

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