Molecular Mechanisms of Alzheimer’s Disease Protection by the A673T Allele of Amyloid Precursor Protein [Molecular Bases of Disease]

September 24th, 2014 by Maloney, J. A., Bainbridge, T., Gustafson, A., Zhang, S., Kyauk, R., Steiner, P., van der Brug, M., Liu, Y., Ernst, J. A., Watts, R. J., Atwal, J. K.

Pathogenic mutations in the Amyloid precursor protein (APP) gene have been described as causing early onset familial Alzheimer′s disease (AD). We recently identified a rare APP variant encoding an alanine-to-threonine substitution at residue 673 (A673T) that confers protection against development of AD (1). The A673 residue lies within the β-secretase recognition sequence, and is part of the Aβ peptide cleavage product (position 2 of Aβ). We previously demonstrated that the A673T substitution makes APP a less favorable substrate for cleavage by BACE1. In follow-up studies, we confirm that A673T APP shows reduced cleavage by BACE1 in transfected mouse primary neurons and in isogenic human iPSC-derived neurons. Using a biochemical approach, we show that the A673T substitution modulates the catalytic turnover rate (Vmax) of APP by the BACE1 enzyme, without affecting the affinity (Km) of the APP substrate for BACE1. We also show a reduced level of Aβ1-42 aggregation with A2T Aβ peptides, an observation not conserved in Aβ1-40 peptides. When combined in a ratio of 1:9 Aβ1-42:Aβ1-40 to mimic physiologically relevant mixtures, A2T retains a trend toward slowed aggregation kinetics. Microglial uptake of the mutant Aβ1-42 peptides correlated with their aggregation level. Cytotoxicity of the mutant Aβ peptides was not dramatically altered. Taken together, our findings demonstrate that A673T, a protective allele of APP, reproducibly reduces amyloidogenic processing of APP and also mildly decreases Aβ aggregation. These effects could together have an additive or even synergistic impact on risk of developing AD.