Sequence of the variant thyroxine-binding globulin of Australian Aborigines. Only one of two amino acid replacements is responsible for its altered properties

Abstract

A form of thyroxine-binding globulin (TBG) with reduced affinity for hormone and increased susceptibility to heat and acid denaturation has been identified in Australian Aborigines (TBG-A). Results of heat denaturation of TBG established that the TBG(A) allele is X linked and has a frequency of 50.9% in Western Australian Aborigines. The sequence of an isolated TBG(A) allele differed at two positions from that of the normal TBG allele (TBG(C)). One substitution was in codon 191, ACA (threonine) rather than GCA (alanine), and the other was in codon 283, TTT (phenylalanine) instead of TTG (leucine). These nucleotide substitutions resulted in the loss of sites for the enzymes Bgl 1 and Tth 111 II, respectively. The nucleotide substitutions in the TBG-A allele were confirmed by digestion of genomic DNA segments amplified using the polymerase chain reaction. The Bgl 1 and Tth 111 II sites were absent in the genes of two Aboriginal men expressing TBG-A and were present in those of three Aboriginal and six Caucasian males expressing TBG-C. The TBG gene of a seventh Caucasian male possessed the Bgl 1 site but had lost the Tth 111 II site; sequencing of this allele revealed only the substitution in codon 283 identical to that in the TBG(A) allele. As the biochemical properties of TBG(Phe-283) expressed by this individual were indistinguishable from normal TBG(Leu-283), we believe that the abnormal properties of TBG-A are due to substitution of alanine for threonine at residue 191.