The genetic landscape for amyloid beta nucleation accurately discriminates familial Alzheimer's disease mutations
Alzheimer's Disease (AD) is the most common form of dementia and a leading global cause of human mortality and morbidity.  Amyloid plaques of the amyloid beta (Aß) peptide are a universal pathological hallmark of AD and rare mutations in Aß can cause familial forms of AD (fAD).  However, hundreds of additional mutations in Aß of uncertain significance are likely to exist in the human population, making clinical interpretation of genetic variation a difficult challenge even in this short peptide.  Here, we use deep mutagenesis to quantify the effects of all possible single nucleotide variants and thousands of double mutations on the ability of Aß to nucleate amyloid fibrils in vivo. This data provides the first comprehensive view of how mutations alter the nucleation of an amyloid, reveals  a modular organisation of mutational effects in Aß, and demonstrates the importance of charge in preventing nucleation.  Moreover, the in vivo nucleation data, unlike computational predictors, accurately discriminates the known dominant fAD mutations.  These results illustrate how deep mutagenesis can be used to genetically validate assays as discovery platforms.  They also further prioritize nucleation as the critical biochemical event to target in order to prevent and treat AD. Overall design: Systematic measurement of the amyloid nucleation of 15,000 mutations of Aß42