A NEW ISOLEUCINE SUBSTITUTION SF VAL-20 IN TRANSTHYRETIN TETRAMERS SELECTIVELY IMPAIRS DIMER-DIMER CONTACTS AND CAUSES SYSTEMIC AMYLOIDOSIS

The most frequent form of inherited amyloidoses is associated with mut ations in the transthyretin (TTR) gene coding for 127-amino acid resid ues of four identical, noncovalently linked subunits that form a pair of dimers In the plasma protein complex, Amyloid fibrils containing th e variant and to a lesser extent the wild-type form of the TTR molecul e are deposited ire various organs, including peripheral nerves and th e myocardium, with polyneuropathy and cardiomyopathy as major clinical manifestations. So far, more than 40 distinct amino acid substitution s distributed throughout the TTR sequence over 30 positions have been found to be correlated with an increased amyloidogenicity of TTR. Most of these amyloidogenic amino acid substitutions are suspected to alte r the conformation and stability of the monomer. Mere we identify and characterize by protein and DNA analysis a novel amyloidogenic Val-20 to Ile mutation in a German three-generation family, The index patient suffered from severe amyloid cardiomyopathy at the age of 60. Conform ational stability and unfolding behavior of the Ile-20 monomer in urea gradients was found to be almost indistinguishable from that of wild- type TTR. Hn contrast, tetramer stability was significantly reduced in agreement with the expected change in the interactions between tile t wo opposing dimers via the side chain of Ile-20. Our observations prov ide strong evidence far the view that amyloidogenic amino acid substit utions in TTR facilitate the conversion of tetrameric TTR complexes in to those conformational intermediates of the TTR folding pathway that have an intrinsic amyloidogenic potential.