Sequencing of the variant thyroxine-binding globulin (TBG)-Quebec reveals two nucleotide substitutions

Thyroxine-binding globulin (TBG) is a liver glycoprotein that transports thyroid hormone in serum. In 1987 a variant TBG was discovered in an infant born in Quebec, following an investigation prompted by the finding of low blood thyroxine (T4) level on screening for neonatal hypothyroidism. This variant, TBG-Quebec, has cathodal shift on isoelectric focusing, reduced affinity for thyroxine, and markedly reduced stability. The latter property of the variant molecule is probably responsible for the partial TBG deficiency. We now report the results of sequencing of the entire coding region and exon-intron junctions of TBG-Quebec, which revealed two nucleotide substitutions; one, located in exon 3, changes the normal codon 283 of TTG (leucine) to that of TTT (phenylalanine), and the other, in exon 4, results in the replacement of the normal histidine-331 (CAT) by tyrosine (TAT). Allele-specific amplification (ASA) confirmed the cosegregation of the two nucleotide substitutions with the TBG-Quebec phenotype in individual members of this family. The substitution in codon 283, but not that in codon 331, has been previously described and, when occurring alone, does not alter the properties of the gene product. Thus, it appears that the replacement of histidine-331 by tyrosine is responsible for the observed altered properties of TBG-Quebec. However, the question of whether substitution of both amino acids is necessary for expression of the variant phenotype has yet to be answered.