Nonfibrillar diffuse amyloid deposition due to a gamma(42)-secretase site mutation points to an essential role for N-truncated A beta(42) in Alzheimer's disease.
Kumar-Singh S., De Jonghe C., Cruts M., Kleinert R., Wang R., Mercken M., De Strooper B., Vanderstichele H., Loefgren A., Vanderhoeven I., Backhovens H., Vanmechelen E., Kroisel P.M., Van Broeckhoven C.
Amyloidogenic processing of the amyloid precursor protein (APP) with deposition in brain of the 42 amino acid long amyloid beta-peptide (A beta(42)) is considered central to Alzheimer's disease (AD) pathology. However, it is generally believed that nonfibrillar pre-amyloid A beta(42) deposits have to mature in the presence of A beta(40) into fibrillar amyloid plaques to cause neurodegeneration. Here, we describe an aggressive form of AD caused by a novel missense mutation in APP (T714I) directly involving gamma-secretase cleavages of APP. The mutation had the most drastic effect on A beta(42)/A beta(40) ratio in vitro of approximately 11-fold, simultaneously increasing A beta(42) and decreasing A beta(40) secretion, as measured by matrix-assisted laser disorption ionization time-of-flight mass spectrometry. This coincided in brain with deposition of abundant and predominant nonfibrillar pre-amyloid plaques composed primarily of N-truncated A beta(42) in complete absence of A beta(40). These data indicate that N-truncated A beta(42) as diffuse nonfibrillar plaques has an essential but undermined role in AD pathology. Importantly, inhibiting secretion of full-length A beta(42 )by therapeutic targeting of APP processing should not result in secretion of an equally toxic N-truncated A beta(42).