A Novel Albumin Gene Mutation (R222I) in Familial Dysalbuminemic Hyperthyroxinemia

Abstract

Context:

Familial dysalbuminemic hyperthyroxinemia, characterized by abnormal circulating albumin with increased T ₄ affinity, causes artefactual elevation of free T ₄ concentrations in euthyroid individuals.

Objective:

Four unrelated index cases with discordant thyroid function tests in different assay platforms were investigated.

Design and Results:

Laboratory biochemical assessment, radiolabeled T ₄ binding studies, and ALB sequencing were undertaken. ¹²⁵I-T ₄ binding to both serum and albumin in affected individuals was markedly increased, comparable with known familial dysalbuminemic hyperthyroxinemia cases. Sequencing showed heterozygosity for a novel ALB mutation (arginine to isoleucine at codon 222, R222I) in all four cases and segregation of the genetic defect with abnormal biochemical phenotype in one family. Molecular modeling indicates that arginine 222 is located within a high-affinity T ₄ binding site in albumin, with substitution by isoleucine, which has a smaller side chain predicted to reduce steric hindrance, thereby facilitating T ₄ and rT ₃ binding. When tested in current immunoassays, serum free T ₄ values from R222I heterozygotes were more measurably abnormal in one-step vs two-step assay architectures. Total rT ₃ measurements were also abnormally elevated.

Conclusions:

A novel mutation (R222I) in the ALB gene mediates dominantly inherited dysalbuminemic hyperthyroxinemia. Susceptibility of current free T ₄ immunoassays to interference by this mutant albumin suggests likely future identification of individuals with this variant binding protein.