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Table 1.  
APOE4 Noncarriers and Carriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementiaa
APOE4 Noncarriers and Carriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementiaa
Table 2.  
APOE4 Noncarriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementia by Postmortem Assessment of Neuritic Plaque Densitya
APOE4 Noncarriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementia by Postmortem Assessment of Neuritic Plaque Densitya
Table 3.  
APOE4 Carriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementia by Postmortem Assessment of Neuritic Plaque Densitya
APOE4 Carriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementia by Postmortem Assessment of Neuritic Plaque Densitya
Table 4.  
Primary NP Diagnosis for No to Sparse CERAD Neuritic Plaque Density in APOE4 Carriers and Noncarriersa
Primary NP Diagnosis for No to Sparse CERAD Neuritic Plaque Density in APOE4 Carriers and Noncarriersa
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Beach  TG, Monsell  SE, Phillips  LE, Kukull  W.  Accuracy of the clinical diagnosis of Alzheimer disease at National Institute on Aging Alzheimer Disease Centers, 2005-2010. J Neuropathol Exp Neurol. 2012;71(4):266-273.
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Beekly  DL, Ramos  EM, Lee  WW,  et al; NIA Alzheimer’s Disease Centers.  The National Alzheimer’s Coordinating Center (NACC) database: the Uniform Data Set. Alzheimer Dis Assoc Disord. 2007;21(3):249-258.
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Morris  JC, Weintraub  S, Chui  HC,  et al.  The Uniform Data Set (UDS): clinical and cognitive variables and descriptive data from Alzheimer Disease Centers. Alzheimer Dis Assoc Disord. 2006;20(4):210-216.
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Folstein  MF, Folstein  SE, McHugh  PR.  “Mini-mental state”. A practical method for grading the cognitive state of patients for the clinician. J Psychiatr Res. 1975;12(3):189-198.
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Morris  JC.  The Clinical Dementia Rating (CDR): current version and scoring rules. Neurology. 1993;43(11):2412-2414.
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Mirra  SS, Heyman  A, McKeel  D,  et al.  The Consortium to Establish a Registry for Alzheimer’s Disease (CERAD), part II: standardization of the neuropathologic assessment of Alzheimer’s disease. Neurology. 1991;41(4):479-486.
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NACC Researchers Data Dictionary. The neuropathology data set. https://www.alz.washington.edu/NONMEMBER/NP/rdd_np.pdf. Accessed July 10, 2015.
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Braak  H, Alafuzoff  I, Arzberger  T, Kretzschmar  H, Del Tredici  K.  Staging of Alzheimer disease-associated neurofibrillary pathology using paraffin sections and immunocytochemistry. Acta Neuropathol. 2006;112(4):389-404.
PubMedArticle
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Vellas  B, Carrillo  MC, Sampaio  C,  et al; EU/US/CTAD Task Force Members.  Designing drug trials for Alzheimer’s disease: what we have learned from the release of the phase III antibody trials: a report from the EU/US/CTAD Task Force. Alzheimers Dement. 2013;9(4):438-444.
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Crary  JF, Trojanowski  JQ, Schneider  JA,  et al.  Primary age-related tauopathy (PART): a common pathology associated with human aging. Acta Neuropathol. 2014;128(6):755-766.
PubMedArticle
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Duyckaerts  C, Braak  H, Brion  J-P,  et al.  PART is part of Alzheimer disease. Acta Neuropathol. 2015;129(5):749-756.
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Jellinger  KA, Alafuzoff  I, Attems  J,  et al.  PART, a distinct tauopathy, different from classical sporadic Alzheimer disease. Acta Neuropathol. 2015;129(5):757-762.
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Jack  CR  Jr.  PART and SNAP. Acta Neuropathol. 2014;128(6):773-776.
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Corder  EH, Saunders  AM, Risch  NJ,  et al.  Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease. Nat Genet. 1994;7(2):180-184.
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Talbot  C, Lendon  C, Craddock  N, Shears  S, Morris  JC, Goate  A.  Protection against Alzheimer’s disease with APOE epsilon 2. Lancet. 1994;343(8910):1432-1433.
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Suri  S, Heise  V, Trachtenberg  AJ, Mackay  CE.  The forgotten APOE allele: a review of the evidence and suggested mechanisms for the protective effect of APOE ɛ2. Neurosci Biobehav Rev. 2013;37(10, pt 2):2878-2886.
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Dugger  BN, Hentz  JG, Adler  CH,  et al.  Clinicopathological outcomes of prospectively followed normal elderly brain bank volunteers. J Neuropathol Exp Neurol. 2014;73(3):244-252.
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Johnson  KA, Sperling  RA, Gidicsin  CM,  et al; AV45-A11 study group.  Florbetapir (F18-AV-45) PET to assess amyloid burden in Alzheimer’s disease dementia, mild cognitive impairment, and normal aging. Alzheimers Dement. 2013;9(5)(suppl):S72-S83.
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Serrano-Pozo  A, Qian  J, Monsell  SE, Frosch  MP, Betensky  RA, Hyman  BT.  Examination of the clinicopathologic continuum of Alzheimer disease in the autopsy cohort of the National Alzheimer Coordinating Center. J Neuropathol Exp Neurol. 2013;72(12):1182-1192.
PubMedArticle
Original Investigation
October 2015

Characterizing Apolipoprotein E ε4 Carriers and Noncarriers With the Clinical Diagnosis of Mild to Moderate Alzheimer Dementia and Minimal β-Amyloid Peptide Plaques

Author Affiliations
  • 1National Alzheimer’s Coordinating Center, University of Washington, Seattle
  • 2Department of Epidemiology, University of Washington, Seattle
  • 3Banner Sun Health Research Institute, Sun City, Arizona
  • 4Arizona Alzheimer’s Consortium, Phoenix, Arizona
  • 5Department of Neurology, Mayo Clinic, Scottsdale, Arizona
  • 6Department of Pathology, University of Washington, Seattle
  • 7Banner Alzheimer’s Institute, Phoenix, Arizona
  • 8University of Arizona, Phoenix
  • 9Arizona State University, Phoenix
  • 10Translational Genomics Research Institute, Phoenix, Arizona
JAMA Neurol. 2015;72(10):1124-1131. doi:10.1001/jamaneurol.2015.1721
Abstract

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.

Introduction

β-Amyloid peptide (Aβ) plaques are a cardinal neuropathologic feature of Alzheimer disease1 (AD), and clinical trials targeting this neuropathologic condition are under way. Recent positron emission tomographic (PET) studies have shown that more than one-third of apolipoprotein E ε4 (APOE4) noncarriers with the clinical diagnosis of mild to moderate Alzheimer dementia may not meet the criteria for significant cerebral amyloidosis,2 and 20% to 30% of all individuals with the clinical diagnosis of AD may not meet cliniconeuropathologic diagnostic criteria for AD.3 Further studies are needed to clarify the extent to which these findings are related to low PET tracer affinity for neuritic and/or diffuse Aβ plaques or accurately reflect low levels of Aβ accumulation in the cerebral cortex. If the levels of Aβ accumulation are low, these studies could also help clarify the extent to which patients with a clinical diagnosis of mild to moderate Alzheimer dementia and minimal Aβ accumulation may have appreciable neurofibrillary degeneration or a primary neuropathologic diagnosis other than AD.

The aims of this study were to (1) determine the percentage of APOE4 noncarriers with the primary clinical diagnosis of mild to moderate Alzheimer dementia near the end of life and minimal AD plaques as assessed by consensus histopathologic scoring; (2) explore potential associations between Aβ plaques and appreciable neurofibrillary degeneration in both APOE4 noncarriers and carriers with the primary clinical diagnosis of mild to moderate Alzheimer dementia and minimal Aβ plaques, as well as their demographic and clinical features; and (3) determine the frequency of a primary neuropathologic diagnosis other than AD in APOE4 carriers and noncarriers with minimal Aβ plaques. Although previous studies4 have sought to confirm this finding in an autopsy cohort, those investigations have not looked at APOE4 carriers and noncarriers separately, which is an important feature necessary to emulate the imaging studies.

Methods
Study Sample

The study sample comprised research participants from the 34 past and present National Institute on Aging–sponsored AD centers who were assessed with the Uniform Data Set (UDS)5 and had their data uploaded to the National Alzheimer Coordinating Center (NACC) between September 1, 2005, and September 1, 2012. Research using the NACC database was approved by the University of Washington institutional review board. Patients had provided written informed consent. Analysis of information in the limited data set was performed from October 9, 2012, to March 20, 2015.

The UDS contains clinical and demographic information on participants with cognitive impairment due to AD and other conditions, as well as those with normal cognition. Detailed descriptions of UDS data have been published.6 The UDS participants may also consent to autopsy, in which case neuropathologic features are assessed by consensus guidelines and recorded using a standardized neuropathologic assessment form, which is submitted to the NACC. Although NACC has made several revisions to its neuropathologic assessment form, the consensus guidelines for assessment of neuritic and diffuse plaques, as well as for neurofibrillary degeneration, were constant throughout the present study period.

Study inclusion criteria were applied in the following manner: of the 2288 UDS participants with an autopsy informed consent form, 1834 died within 24 months of their last clinical assessment. Of these, 794 had a primary diagnosis of probable or possible Alzheimer dementia. Limiting the analytic sample to those with a Mini-Mental State Examination (MMSE)7 score of 16 to 26 reduced the sample to 230 participants. Removing 30 participants who were missing APOE4 genotype information resulted in a study population of 100 APOE4 carriers (11 [11.0%], 80 [80.0%], and 9 [9.0%] with the APOE 4/4, 3/4, and 2/4 genotypes, respectively) and 100 APOE4 noncarriers (86 [86.0%] and 14 [14.0%] with the APOE 3/3 and 2/3 genotypes, respectively). See the eFigure in the Supplement for a more detailed sample size derivation.

Participants were further characterized in terms of sex, educational level, age (at symptom onset, last clinical evaluation, and death), years from onset of cognitive symptoms to last evaluation, and years from onset of cognitive symptoms to death.

Clinical measures of interest included the MMSE and the Clinical Dementia Rating Sum of Boxes (CDR-SOB) scores.8 The CDR grades participants’ cognitive and functional abilities in 6 domains: memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care. The evaluator, incorporating input from the coparticipant, evaluates impairment in each domain as none (0), questionable or very mild (0.5), mild (1), moderate (2), or severe (3). The scores for each domain are summed to create an SOB score ranging from 0 to 18, with higher scores indicating more severe impairment.

The frequency of Aβ plaques was assessed using the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) neuritic plaque score template.9 In the NACC Neuropathology Data Set Coding Guidebook,10neuritic plaques are defined as plaques with argyrophilic, thioflavin S–positive or tau-positive dystrophic neurites with or without dense Aβ cores. This 4-level rating was dichotomized into none or sparse (minimal plaques) vs moderate or frequent. Diffuse Aβ plaque frequency also was graded using the CERAD plaque template. Neurofibrillary degeneration was assessed by Braak staging.11

As part of the NACC standardized form, the neuropathologist estimates the most likely primary cause of dementia. The frequencies of these conditions were summarized for APOE4 carriers and noncarriers with minimal neuritic plaques and Braak stages 0 to II, as well as Braak stages III to IV.

Neurochemical Analyses

In post hoc neurochemical analyses, we sought to further characterize soluble and fibrillar Aβ levels with greater sensitivity than is obtained using the consensus histopathologic methods, thus clarifying the extent to which they were free of increased concentrations of Aβ. Tissue samples from the 50 participants with no or sparse Aβ plaques were requested from the individual AD centers. Samples from 22 participants were provided by the AD centers. Parietal tissue was available for 19 APOE4 noncarriers and 3 carriers, and temporal tissue was available for 18 APOE4 noncarriers and 3 carriers.

Gray matter from the temporal and parietal lobes (200 mg each) was gently homogenized in 1600 µL of 20mM TRIS, 5mM EDTA (pH 7.8), plus protease inhibitor cocktail (complete protease inhibitor cocktail tablets; Roche) using a Teflon tissue homogenizer (10 strokes). The homogenates were centrifuged at 45 000 rpm (250 000g) for 1 hour at 4°C in a titanium rotor (TLA 50.4 Ti rotor; Beckman Coulter); the supernatant was then recovered and saved as the soluble fraction. The TRIS-insoluble pellet was rehomogenized in 1200 µL of 90% glass-distilled formic acid and centrifuged at 45 000 rpm (250 000g) for 1 hour at 4°C in a Beckman TLA 50.4 Ti rotor. The supernatant was then dialyzed (1000-Da molecular weight cutoff) in deionized water twice (1 hour, each 4 L) followed by 3 changes in 0.1M of ammonium bicarbonate (1 hour, each 4 L), flash frozen in dry ice and ethanol, and lyophilized. The lyophilized proteins were reconstituted in 1000 µL of 5M guanidine hydrochloride, 50mM TRIS (pH 8.0), with protease inhibitor cocktail; shaken for 3 hours at 4°C; and centrifuged as described above, with the supernatant saved as the insoluble fraction. Total protein level in the soluble and insoluble fractions was determined with a protein assay (Pierce Micro BCA protein assay kit; Thermo Fisher Scientific Inc). Both Aβ40 and Aβ42 levels were measured with enzyme-linked immunoassay kits (Life Technologies Corp) following the manufacturer’s instructions: Aβ40 KHB3481 kit (minimal detectable dose <6 pg/mL) and Aβ42 KHB3441 kit (minimal detectable dose <10 pg/mL). In addition, we used the Aβ42 ultrasensitive KHB3544 kit (minimal detectable dose <1 pg/mL). For the purpose of this study and based on our previous laboratory experience, total Aβ of less than 1000 pg/mg was considered a low or negligible Aβ level, total Aβ of 1000 to 100 000 pg/mg was considered a moderate level of Aβ, and a total Aβ level of greater than 100 000 pg/mg was considered severe Aβ consistent with AD.

Statistical Analysis

First, characteristics of APOE4 carriers and noncarriers were summarized and compared. Next, stratifying by APOE4 carrier status, differences in clinical and neuropathologic features were explored for individuals with and without significant levels of Aβ plaques. All statistical comparisons were made using nonparametric Wilcoxon rank sum tests for continuous measures and χ2 tests for categorical characteristics; for comparisons involving small frequencies (<5 in at least 1 category), the Fisher exact test was performed. Because this was a descriptive study, an α level of .05 was applied, and all P values are presented without adjustment for multiple comparisons. Analyses were performed using SAS, version 9.3 (SAS Institute Inc).

Results

With values reported as mean (SD), the APOE4 carriers and noncarriers had 15.1 (3.1) and 14.9 (2.9) years of education, MMSE scores of 20.6 (3.2) and 20.3 (3.2), and CDR-SOB scores of 8.0 (3.3) and 8.5 (3.9), respectively. They were fairly evenly represented by men (51 [51.0%] vs 59 [59.0%]) and women (49 [49.0%] vs 41 [41.0%]). Compared with APOE4 noncarriers, APOE4 carriers were slightly younger at the onset of symptoms (75.8 [11.7] vs 78.5 [9.9] years), last clinical evaluation (83.3 [7.4] vs 85.2 [9.6] years), and death (84.1 [7.4] vs 86.0 [9.5] years) (Table 1). There were no statistically significant differences for years from onset to last evaluation or death, demographic characteristics, or cognitive performance.

Of the 200 APOE4 carriers and noncarriers, 70 individuals (35.0%) had a primary neuropathologic diagnosis other than AD. Of those 70 persons, 11 (15.7%) had a primary diagnosis of AD present but did not meet diagnostic criteria for AD and 7 (10.0%) had a diagnosis of normal brain. Thus, 25.7% of the population met the neuropathologic criteria for a neurodegenerative disease other than AD.

Among APOE4 noncarriers with the primary clinical diagnosis of mild to moderate Alzheimer dementia, 37 (37.0%) had minimal neuritic plaques. Twenty-eight (28.0%) of the noncarriers had both minimal neuritic plaques and minimal diffuse plaques in combination. As indicated in Table 2, APOE4 noncarriers with minimal neuritic plaques had lower Braak stages for neurofibrillary degeneration, on average, compared with those with moderate to frequent neuritic plaques (43.2% vs 95.2% with Braak stages III-VI; P < .001). Only 3 of the 16 individuals (18.8%) with minimal neuritic plaques and Braak stages III to VI had Braak stages V to VI.

Post hoc neurochemical assays were performed for 19 of the 37 APOE4 noncarriers with minimal plaques. In both the parietal and temporal regions, 2 noncarriers (10.5%) had moderate levels of combined soluble and insoluble Aβ. None had high levels of Aβ approximating those seen in neuropathologic AD. Thus, classification of participants using the CERAD neuritic plaque score was similar to that ascertained neurochemically for Aβ peptides.

Compared with APOE4 noncarriers, fewer APOE4 carriers with the clinical diagnosis of mild to moderate Alzheimer dementia had minimal plaque scores: 13 (13.0%) had no or sparse neuritic plaques, and only 4 (4.0%) had both no to sparse neuritic and diffuse plaques. Neurofibrillary degeneration was also less extensive overall for APOE4 carriers with minimal neuritic plaques compared with carriers with moderate to frequent neuritic plaques (6 [46.2%] vs 81 [93.1%] with Braak stages III-VI; P < .001). Full results of these comparisons are presented in Table 3.

One of the 11 APOE4 homozygotes with the clinical diagnosis of mild to moderate Alzheimer dementia had only sparse neuritic plaques. This individual was characterized as having moderate diffuse plaques, Braak stages III to IV, and a primary neuropathologic diagnosis of Lewy body dementia.

In addition, APOE4 carriers with minimal plaques were older than those with moderate to frequent plaques at the onset of symptoms, last clinical evaluation, and death (P = .02, P = .01, and P = .01, respectively). Although this trend was also observed in APOE4 noncarriers, the comparisons were not statistically significant.

In the post hoc analysis, 3 of the 13 APOE4 carriers were analyzed using tissue homogenates and immunochemical assays for soluble and insoluble Aβ. Of these, 1 participant (33.3%) was confirmed to have low tissue Aβ levels and 2 individuals (66.6%) had moderate Aβ levels. None of the participants had Aβ peptides at the level observed in those with neuropathologic AD. Full results are provided in eTables 1 and 2 in the Supplement.

Of the 20 APOE4 noncarriers with minimal neuritic plaques and Braak stages 0 to II, 15 participants (75.0%) received a primary neuropathologic diagnosis other than AD. Two (10.0%) had AD pathologic changes, but the features were considered insufficient to explain the clinical diagnosis of Alzheimer dementia. The remaining 3 participants (15.0%) had no distinct neuropathologic features that could have explained the dementia diagnosis and were determined to have “normal brain.” Table 4 provides a full list of primary neuropathologic diagnoses.

Similarly, of the 7 APOE4 carriers with minimal neuritic plaques and Braak stages 0 to II, 3 participants (42.9%) were considered to have dementia resulting from AD pathologic changes and alterations in the remaining 4 individuals (57.1%) were thought to result from non-AD neuropathologic features.

Finally, substantial neurofibrillary degeneration (Braak stages III-VI) was observed in 16 of 36 APOE4 noncarriers (44.4%) and 6 of 13 of APOE4 carriers (46.2%) with the clinical diagnosis of mild to moderate AD dementia and minimal Aβ plaques. Participants with Braak stages III to VI were a mean of 2 years older at last clinical assessment compared with those with a lower Braak stage. However, among participants with a higher Braak stage, those without Aβ plaques were older than those with moderate to frequent plaques (age at last clinical evaluation; P = .04, Wilcoxon rank sum test).

To better understand whether the substantial neurofibrillary degeneration could be driving the cognitive impairment observed in individuals with mild Aβ, we performed a post hoc analysis of the persons in the same age group in the NACC database who were cognitively unimpaired (defined as a diagnosis of normal cognition and an MMSE score of >26) at their last evaluation, died within the next 24 months, and had an assessment of plaque density and Braak staging. We found that 101 of the 155 cognitively unimpaired persons (65.2%) with known Braak stage had no more than sparse neuritic plaques. Thirty-five (34.6%) of the 101 cognitively unimpaired persons with no more than sparse plaques were in Braak stages III to VI. Although this is a smaller percentage than for those with the clinical diagnosis of mild to moderate Alzheimer dementia and no more than sparse plaques (22 [44.9%]; P = .10, χ2 test), 34.6% indicates that this is still a frequent abnormality in individuals with normal cognition.

Discussion

Thirty-seven (37.0%) of APOE4 noncarriers with a primary clinical diagnosis of mild to moderate Alzheimer dementia had minimal AD plaque scores. A much smaller number (13 [13.0%]) of APOE4 carriers had minimal AD plaque scores. Neurochemical assays for soluble and insoluble Aβ peptides were then applied to brain samples from 22 of the 50 brain donors with minimal AD plaque scores, confirming the low level of Aβ in most of these samples of cerebral cortex. These findings support the results of recent PET imaging studies2,12 suggesting that many participants who meet clinical criteria for mild to moderate Alzheimer dementia do not appear to have high levels of Aβ accumulation in the cerebral cortex.

Almost half (45.0%) of the participants with mild to moderate Alzheimer dementia (APOE4 noncarriers and carriers) and minimal Aβ plaque level scores had topographically extensive cerebral neurofibrillary degeneration (Braak stages III-VI). This combination of AD pathologic features long has been reported in the literature13 under several names, including tangle-only dementia or tangle-predominant dementia, to reflect discordance between the 2 cardinal histopathologic features of AD. Indeed, this combination of low levels of Aβ plaques yet moderate to extensive neurofibrillary degeneration was recognized in the recent National Institute on Aging–Alzheimer’s Association1 guidelines for neuropathologic evaluation of AD; the panel of experts recommended that this constellation of features not be reported as AD and that care be taken to exclude other tauopathies. Subsequently, a new diagnostic category for this combination of neuropathologic features, extended to include nondemented individuals with less-extensive neurofibrillary degeneration, has been proposed (primary age-related tauopathy)13 and has engendered significant debate over whether this is distinct from AD or a variant manifestation of AD.11,14,15 Although the data are limited, if we assume PET imaging showing insignificant cerebral Aβ and neuropathologic evaluation showing minimal AD plaque scores are equivalent, then some suspected nonamyloid pathology might be tangle-only dementia or primary age-related tauopathy in cases with less severe cognitive impairment.16

Inheritance of the APOE2 allele is associated with a lower risk of Alzheimer dementia,17,18 suggesting that APOE2 may confer protection against AD.19 Our sample of APOE2 carriers is too small to resolve this issue; however, we did not observe a higher frequency of APOE2 in Braak stages III to VI (23%) vs Braak stages 0 to II (37%) in the presence of minimal plaques.

Most of the patients who undergo postmortem evaluation in AD centers have evidence of cognitive impairment due to AD or other conditions and subsequently donate their brains have severe dementia at the time of their last visit. In a post hoc analysis, we analyzed data from NACC participants with severe Alzheimer dementia (MMSE score of <16 or unable to participate in testing owing to cognitive impairment). Among the 19 participants with mild Aβ, 6 cases (32%) had Braak stages III to VI. This percentage is similar to that observed in people with normal cognition in the NACC database (34.6%), and is consistent with those reported by Dugger at al20 who found that 37.9% of individuals who were clinically normal at death met NIA-Reagan criteria for intermediate probability of AD with Braak stage III or above. Together, these findings lend support to the suggestion that moderate to severe tauopathy may not be related to dementia but may simply be a manifestation of healthy aging. Differences in the proportion of patients in the earlier and more advanced stages of dementia with no or sparse plaques could be related to subsequent plaque deposition in the advanced stages of AD, slower clinical progression (such that fewer individuals reached the advanced dementia stages in their lifetime), or other factors. Regardless, current clinical trials using either soluble or fibrillar Aβ-modifying treatments seem unlikely to benefit participants in this situation, even if such treatments are shown to be effective in Aβ-positive individuals. Development of new treatment strategies will be required to accommodate this pathologic heterogeneity within the clinical diagnosis of mild to moderate AD dementia.

The APOE4 carriers with higher plaque densities were statistically significantly younger than were APOE4 carriers with lower plaque densities at the time of death. The possibility that less Aβ burden is associated with a longer life expectancy in APOE4 carriers (or in noncarriers, in whom the differences were not statistically significant) would need to be confirmed in an independent cohort.

A previous analysis performed on the NACC database by Serrano-Pozo et al4 produced similar results for participants with mild to moderate AD who were autopsied; the investigators found that approximately 14% of these participants did not have Aβ abnormality. We used an overlapping but larger subject pool, examined pathologic distributions within APOE4 strata, showed the particularly high prevalence of APOE4 noncarriers with a clinical diagnosis of mild to moderate Alzheimer dementia and minimal Aβ abnormality, and performed a substudy demonstrating the absence of appreciable fibrillar or soluble Aβ. Our findings in APOE4 noncarriers also are consistent with previously published florbetapir F18 PET findings.21,22

The present study had several strengths, including the use of a well-characterized sample of participants evaluated at multiple institutions, standardized clinical and neuropathologic forms, and a sensitivity analysis investigating whether participants with minimal evidence of Aβ plaques might have had preferential increases in soluble or insoluble Aβ that had not been detected. There are also limitations to our study. First, there are potential differences in staining protocols across the AD centers. Some of the autopsies were performed using silver staining methods, thioflavin T, or Congo red, which may be less sensitive for detecting diffuse Aβ plaques than immunohistochemistry. Unfortunately, the staining method or methods used are not available in the NACC neuropathology database, so we were unable to account for this protocol variation. We did, however, reexamine tissue from multiple sites with standardized immunohistochemical methods for Aβ and phosphorylated tau and found considerably good agreement in identifying the presence of Aβ.

Second, some data were missing. A total of 162 MMSE scores were indicated as missing owing to a cause other than cognitive impairment; however, it is possible that this group of participants did, in some way, affect the study sample. Only 1 of 200 participants (0.5%) was missing data on Braak stage. There were, however, 30 individuals (13.0%) who were excluded from our study owing to missing APOE data. Although it is possible that these participants could have different pathologic distributions than those included, they met all of the same clinical study inclusion criteria.

Finally, there is potential for selection bias in that all participants survived long enough to enter this study and also provided consent for autopsy. Thus, our study sample may not be representative of the general population although they are likely similar to older adults who would volunteer for observational research studies and/or clinical trials. Previous clinicopathologic studies23 using NACC data have not found it necessary to adjust for selection bias due to participants’ decision to undergo autopsy.

Conclusions

We found that 25.0% of patients with a clinical diagnosis of mild to moderate Alzheimer dementia, including 37.0% of APOE4 noncarriers and 13.0% of APOE4 carriers, have low cerebral cortical Aβ accumulation and that almost half of APOE4 noncarriers and carriers with minimal levels of Aβ have extensive neurofibrillary degeneration. These findings suggest that a nonamyloidogenic variant resembling the clinical phenotype of AD may be more common than previously expected among research participants with mild to moderate Alzheimer dementia, particularly in APOE4 noncarriers, and they provide additional evidence to suggest that these individuals may not respond to treatments that target either fibrillar or soluble Aβ. Additional research is needed to further characterize the longitudinal natural history, as well as the clinical, biological, and genetic features of patients with mild to moderate Alzheimer dementia with low levels of Aβ and to develop strategies other than Aβ-modifying agents to treat and prevent this dementia.

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Article Information

Accepted for Publication: June 4, 2015.

Corresponding Author: Eric M. Reiman, MD, Banner Alzheimer’s Institute, University of Arizona, 901 E Willetta St, Phoenix, AZ 85006 (eric.reiman@bannerhealth.com).

Published Online: August 24, 2015. doi:10.1001/jamaneurol.2015.1721.

Author Contributions: Ms Monsell had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Monsell, Kukull, Beach, Caselli, Montine, Reiman.

Acquisition, analysis, or interpretation of data: Monsell, Roher, Maarouf, Serrano, Beach, Montine, Reiman.

Drafting of the manuscript: Monsell, Reiman.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Monsell, Roher, Maarouf.

Obtained funding: Beach, Montine, Reiman.

Administrative, technical, or material support: Monsell, Roher, Maarouf, Serrano, Beach, Reiman.

Study supervision: Roher, Montine, Reiman.

Conflict of Interest Disclosures: Dr Beach is paid as a consultant by GE Healthcare and Avid Radiopharmaceuticals and performs contracted services for Navidea Biopharmaceuticals. Dr Caselli receives grant funding from Merck. No other disclosures were reported.

Funding/Support: The National Alzheimer Coordinating Center (NACC) database is funded by National Institute on Aging/National Institutes of Health (NIA/NIH) grant U01 AG016976. NACC data are contributed by the NIA-funded Alzheimer disease center grants: P30 AG019610 (principal investigator [PI], Dr Reiman), P30 AG013846 (PI, Neil Kowall, MD), P50 AG008702 (PI, Scott Small, MD), P50 AG025688 (PI, Allan Levey, MD, PhD), P30 AG010133 (PI, Andrew Saykin, PsyD), P50 AG005146 (PI, Marilyn Albert, PhD), P50 AG005134 (PI, Bradley Hyman, MD, PhD), P50 AG016574 (PI, Ronald Petersen, MD, PhD), P50 AG005138 (PI, Mary Sano, PhD), P30 AG008051 (PI, Steven Ferris, PhD), P30 AG013854 (PI, M. Marsel Mesulam, MD), P30 AG008017 (PI, Jeffrey Kaye, MD), P30 AG010161 (PI, David Bennett, MD), P30 AG010129 (PI, Charles DeCarli, MD), P50 AG016573 (PI, Frank LaFerla, PhD), P50 AG016570 (PI, David Teplow, PhD), P50 AG005131 (PI, Douglas Galasko, MD), P50 AG023501 (PI, Bruce Miller, MD), P30 AG035982 (PI, Russell Swerdlow, MD), P30 AG028383 (PI, Linda Van Eldik, PhD), P30 AG010124 (PI, John Trojanowski, MD, PhD), P50 AG005133 (PI, Oscar Lopez, MD), P50 AG005142 (PI, Helena Chui, MD), P30 AG012300 (PI, Roger N. Rosenberg, MD), P50 AG005136 (PI, Thomas J. Montine, MD, PhD), P50 AG033514 (PI, Sanjay Asthana, MD), and P50 AG005681 (PI, John Morris, MD). These grants support data collection. Additional support was provided by grant R01 AG031581 (PIs, Drs Reiman and Caselli), the Arizona Alzheimer’s Consortium (PI, Dr Reiman), and NACC grant U01AG016976.

Role of the Funder/Sponsor: The funding organizations had roles in the design and conduct of the study; interpretation of the data; and preparation, review, or approval of the manuscript but no role in the decision to submit the manuscript for publication.

Additional Contributions: We thank all of the Alzheimer’s disease center participants who volunteered for this study. We also thank the following centers for contributing brain tissue for our sub-study: Massachusetts Alzheimer’s Disease Research Center, Oregon Health and Sciences University, Rush University, University of California at Irvine, University of California at San Diego, University of Southern California, University of Kentucky, and University of Wisconsin.

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