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Original Investigation
April 22, 2019

Association of Cortical β-Amyloid Protein in the Absence of Insoluble Deposits With Alzheimer Disease

Author Affiliations
  • 1Rush Alzheimer’s Disease Center, Rush University Medical Center, Chicago, Illinois
  • 2Department of Neurological Sciences, Rush University Medical Center, Chicago, Illinois
  • 3Pacific Northwest National Laboratory, Richland, Washington
  • 4Department of Behavioral Sciences, Rush University Medical Center, Chicago, Illinois
  • 5Department of Pathology, Rush University Medical Center, Chicago, Illinois
  • 6Center for Translational and Computational Neuroimmunology, Columbia University Medical Center, New York, New York
  • 7Cell Circuits Program, Broad Institute, Cambridge, Massachusetts
JAMA Neurol. 2019;76(7):818-826. doi:10.1001/jamaneurol.2019.0834
Key Points

Question  How is cortical β-amyloid protein in the absence of insoluble deposits associated with classic features of Alzheimer disease?

Findings  In this study of 148 older individuals without β-amyloid deposits, β-amyloid protein was associated with faster cognitive decline, although not in episodic memory. No association of protein with paired helical filament tau tangle density was observed, and neither apolipoprotein ε4 nor a polygenic Alzheimer risk score was associated with β-amyloid protein.

Meaning  Cortical soluble β-amyloid is neurotoxic in aging; the lack of protein association with paired helical filament tau tangles, episodic memory decline, or strong genetic drivers of deposited β-amyloid suggests an underlying neuropathologic change that differs from that of AD.

Abstract

Importance  β-Amyloid deposits are a pathologic hallmark of Alzheimer disease (AD). However, the extent to which cortical β-amyloid protein in the absence of insoluble deposits is associated with classic features of AD appear to be unknown.

Objective  To examine the associations of cortical β-amyloid protein in the absence of insoluble deposits with cognitive decline, neurofibrillary tangles, other age-associated neuropathologic conditions, and APOE.

Design, Setting, and Participants  This analysis combines data from 2 community-based clinicopathologic cohort studies of aging. The Religious Orders Study started in 1994, and the Rush Memory and Aging Project started in 1997. Both studies are ongoing. Participants without known dementia were enrolled and agreed to annual clinical evaluations and brain donation after death. Primary analyses focused on individuals without β-amyloid deposits. Data analyses occurred in mid-September 2018.

Main Outcomes and Measures  β-Amyloid protein abundance was measured by targeted proteomics using selected reaction monitoring. β-Amyloid deposits were detected using immunohistochemistry. Other neuropathologic indices were quantified via uniform structured evaluation. Linear mixed models were used to examine the association of β-amyloid protein with cognitive decline. Regression models examined the protein associations with neuropathologic outcomes and the APOE genotype.

Results  By mid-September 2018, 3575 older persons were enrolled, and 1559 had died and undergone brain autopsy. Proteomic data were collected in 1208 individuals, and 5 with missing cognitive scores were excluded. Of the remaining 1203, primary analyses focused on 148 individuals (12.3%) without β-amyloid deposits. In this group, the mean (SD) age at death was 87.0 (7.0) years, and 84 individuals (56.8%) were women. In the absence of β-amyloid deposits, we did not observe an association of β-amyloid protein with decline in episodic memory, but the protein was associated with faster rates of decline in processing speed (mean [SE] change, −0.014 [0.005]; P = .008) and visuospatial abilities (mean [SE] change, −0.013 [0.005]; P = .006). We did not observe protein association with paired helical filament tau tangle density. The protein was associated with amyloid angiopathy (odds ratio, 1.38 [95% CI, 1.15-1.67]; P < .001) but no other brain pathology. The associations with cognitive decline were unchanged after controlling for amyloid angiopathy. Neither APOE ε4 nor a polygenic Alzheimer risk score was associated with β-amyloid protein.

Conclusions and Relevance  Cortical β-amyloid protein was associated with faster cognitive decline in the absence of β-amyloid deposits, which supports the role of cortical soluble β-amyloid as a neurotoxic agent in aging. The lack of protein association with paired helical filament tau tangles, episodic memory decline, or strong genetic drivers of deposited β-amyloid suggests an underlying neuropathologic change that may differ from that of AD.

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