Elucidating the Molecular Determinants of Αβ Aggregation with Deep Mutational Scanning.

Despite the importance of A beta aggregation in Alzheimer's disease etiology, our understanding of the sequence determinants of aggregation is sparse and largely derived from in vitro studies. For example, in vitro proline and alanine scanning mutagenesis of A beta(40) proposed core regions important for aggregation. However, we lack even this limited mutagenesis data for the more disease-relevant A beta(42). Thus, to better understand the molecular determinants of A beta(42) aggregation in a cell-based system, we combined a yeast DHFR aggregation assay with deep mutational scanning. We measured the effect of 791 of the 798 possible single amino acid substitutions on the aggregation propensity of A beta(42). We found that similar to 75% of substitutions, largely to hydrophobic residues, maintained or increased aggregation. We identified 11 positions at which substitutions, particularly to hydrophilic and charged amino acids, disrupted A beta aggregation. These critical positions were similar but not identical to critical positions identified in previous A beta mutagenesis studies. Finally, we analyzed our large-scale mutagenesis data in the context of different A beta aggregate structural models, finding that the mutagenesis data agreed best with models derived from fibrils seeded using brain-derived A beta aggregates.