Importance: β-Amyloid peptide (Aβ) plaques are a cardinal neuropathologic feature of Alzheimer disease (AD), yet more than one-third of apolipoprotein E ε4 (APOE4) noncarriers with the clinical diagnosis of mild to moderate Alzheimer dementia may not meet positron emission tomographic criteria for significant cerebral amyloidosis.
Objectives: To clarify the percentage of APOE4 carriers and noncarriers with the primary clinical diagnosis of mild to moderate Alzheimer dementia near the end of life and minimal Aβ plaques noted at autopsy and the extent to which these cases are associated with appreciable neurofibrillary degeneration or a primary neuropathologic diagnosis other than AD.
Design, setting, and participants: Data on participants included in this study were obtained from the National Alzheimer Coordinating Center's Uniform Data Set, which comprises longitudinal clinical assessments performed at the AD centers funded by the National Institute on Aging. Neuropathology data are available for the subset of participants who died. A total of 100 APOE4 noncarriers and 100 APOE4 carriers had the primary clinical diagnosis of mild to moderate Alzheimer dementia at their last visit, known APOE4 genotype, died within the ensuing 24 months, and underwent neuropathologic evaluation on autopsy. The study was conducted from September 1, 2005, to September 1, 2012; analysis was performed from October 9, 2012, to March 20, 2015.
Main outcomes and measures: Standardized histopathologic assessments of AD neuropathologic changes were the primary measures of interest in this study, specifically Consortium to Establish a Registry for Alzheimer's Disease neuritic plaque density score, diffuse plaque density score, and Braak stage for neurofibrillary degeneration. The distributions of scores for these measures were the primary outcomes.
Results: Of the 37 APOE4 noncarriers with minimal neuritic plaques, 16 individuals (43.2%) had Braak stages III to VI ratings, and 15 of the others (75.0%) met neuropathologic criteria for other dementia-related diseases. Of the 13 APOE4 carriers with minimal neuritic plaques, 6 individuals (46.2%) had Braak stages III to VI ratings and met neuropathologic criteria for other dementia-related diseases. Similarly, of the 7 APOE4 carriers with minimal neuritic plaques and Braak stages 0 to II, 4 participants (57.1%) were thought to have pathologic changes and alterations resulting from non-AD neuropathologic features.
Conclusions and relevance: In this study, more than one-third of APOE4 noncarriers with the primary clinical diagnosis of mild to moderate Alzheimer dementia had minimal Aβ plaque accumulation in the cerebral cortex and, thus, may show limited or no benefit from otherwise effective anti-Aβ treatment. Almost half of the participants with a primary clinical diagnosis of mild to moderate Alzheimer dementia and minimal Aβ plaque accumulation had an extensive topographic distribution of neurofibrillary degeneration. Additional studies are needed to better understand and provide treatment for patients with this unexpectedly common cliniconeuropathologic condition.