In vitro studies of amyloid beta-protein fibril assembly and toxicity provide clues to the aetiology of Flemish variant (Ala(692) -> Gly) Alzheimer'sdisease

In a Flemish kindred, an Ala(692) --> Gly amino acid substitution in the am yloid beta -protein precursor (A beta PP) causes a form of early-onset Alzh eimer's disease (AD) which displays prominent amyloid angiopathy and unusua lly large senile plaque cores. The mechanistic basis of this Flemish form o f AD is unknown. Previous in vitro studies of amyloid beta -protein (A beta ) production in HEK-293 cells transfected with cDNA encoding Flemish A beta PP have shown that full-length [A beta (1-40)] and truncated [A beta (5-40 ) and A beta (11-40)] forms of A beta are produced. In an. effort to determ ine how these peptides might contribute to the pathogenesis of the Flemish disease, comparative biophysical and neurotoxicity studies were performed o n wild-type and Flemish A beta (1-40), A beta (5-40) and A beta (11-40). Th e results revealed that the Flemish amino acid substitution increased the s olubility of each form of peptide, decreased the rate of formation of thiof lavin-T-positive assemblies, and increased the SDS-stability of peptide oli gomers. Although the kinetics of peptide assembly were altered by the Ala(2 1) --> Gly substitution, all three Flemish variants formed fibrils, as did the wild-type peptides. Importantly, toxicity studies using cultured primar y rat cortical cells showed that the Flemish assemblies were as potent a ne urotoxin as were the wildtype assemblies. Our results are consistent with a pathogenetic process in which conformational changes in A beta induced by the Ala(21) --> Gly substitution would facilitate peptide adherence to the vascular endothelium, creating nidi for amyloid growth. Increased peptide s olubility and assembly stability would favour formation of larger deposits and inhibit their elimination. In addition, increased concentrations of neu rotoxic assemblies would accelerate neuronal injury and death.