The most common form of hereditary systemic
amyloidosis is familial amyloidotic
polyneuropathy associated with single
amino acid changes in the
plasma protein,
transthyretin. In addition, there are two variants of
transthyretin (Ser6 and Thr109) not associated with familial amyloidotic
polyneuropathy but with familial euthyroid
hyperthyroxinemia, also an autosomal dominant disorder. In these autosomal dominant diseases, most affected individuals are heterozygous and therefore have hybrid forms of the tetrameric plasma
transthyretin. In order to study the structure/function relationships of homozygous variant transthyretins, normal human
transthyretin and five variant transthyretins (Gly6----Ser, Leu58----His, Thr60----Ala, Ile84----Ser, and Ala109----Thr) were produced in Escherichia coli using the expression vector, pCZ11, and site-directed mutagenesis. These recombinant
transthyretin (r-TTR)
proteins showed the correct size (14 kilodaltons) on
sodium dodecyl sulfate-
polyacrylamide gel electrophoresis and Western analysis and self-associated into tetramers as determined by size exclusion chromatography. Recombinant normal, Ser6, and Ala60 r-TTRs had an affinity for
thyroxine indistinguishable from normal human TTR purified from plasma, whereas His58 and Ser84 r-TTRs had significantly reduced affinity. On the other hand, Thr109 r-TTR had a much higher affinity, probably due to its position within the
thyroxine-binding pocket. Expression of mutant transthyretins in E. coli provides the opportunity to study structure/function relationships and
amyloid-forming capabilities induced by single amino acid substitutions in the
transthyretin molecule.