Christine M. Hulette

Correlation of Alzheimer Disease Neuropathologic Changes With Cognitive Status: A Review of the Literature

Abstract Clinicopathologic correlation studies are critically important for the field of Alzheimer disease (AD) research. Studies on human subjects with autopsy confirmation entail numerous potential biases that affect both their general applicability and the validity of the correlations. Many sources of data variability can weaken the apparent correlation between cognitive status and AD neuropathologic changes. Indeed, most persons in advanced old age have significant non-AD brain lesions that may alter cognition independently of AD. Worldwide research efforts have evaluated thousands of human subjects to assess the causes of cognitive impairment in the elderly, and these studies have been interpreted in different ways. We review the literature focusing on the correlation of AD neuropathologic changes (i.e. &bgr;-amyloid plaques and neurofibrillary tangles) with cognitive impairment. We discuss the various patterns of brain changes that have been observed in elderly individuals to provide a perspective forunderstanding AD clinicopathologic correlation and conclude that evidence from many independent research centers strongly supports the existence of a specific disease, as defined by the presence of A&bgr; plaques and neurofibrillary tangles. Although A&bgr; plaques may play a key role in AD pathogenesis, the severity of cognitive impairment correlates best with the burden of neocortical neurofibrillary tangles.

Neuropathology of nondemented aging: Presumptive evidence for preclinical Alzheimer disease

OBJECTIVE To determine the frequency and possible cognitive effect of histological Alzheimers disease (AD) in autopsied older nondemented individuals. DESIGN Senile plaques (SPs) and neurofibrillary tangles (NFTs) were assessed quantitatively in 97 cases from 7 Alzheimers Disease Centers (ADCs). Neuropathological diagnoses of AD (npAD) were also made with four sets of criteria. Adjusted linear mixed models tested differences between participants with and without npAD on the quantitative neuropathology measures and psychometric test scores prior to death. Spearman rank-order correlations between AD lesions and psychometric scores at last assessment were calculated for cases with pathology in particular regions. SETTING Washington University Alzheimers Disease Research Center. PARTICIPANTS Ninety-seven nondemented participants who were age 60 years or older at death (mean=84 years). RESULTS About 40% of nondemented individuals met at least some level of criteria for npAD; when strict criteria were used, about 20% of cases had npAD. Substantial overlap of Braak neurofibrillary stages occurred between npAD and no-npAD cases. Although there was no measurable cognitive impairment prior to death for either the no-npAD or npAD groups, cognitive function in nondemented aging appears to be degraded by the presence of NFTs and SPs. CONCLUSIONS Neuropathological processes related to AD in persons without dementia appear to be associated with subtle cognitive dysfunction and may represent a preclinical stage of the illness. By age 80-85 years, many nondemented older adults have substantial AD pathology.

Hypothesis: Microtubule Instability and Paired Helical Filament Formation in the Alzheimer Disease Brain Are Related to Apolipoprotein E Genotype

A genetic classification of Alzheimer disease(s) (AD) is presented. We describe a potential metabolic process in individuals who inherit apolipoprotein E-epsilon 4 (APOE4, gene; apoE4, protein) alleles, leading to increased risk and earlier age of onset of late-onset Alzheimer disease. Apolipoprotein E-epsilon 3 (apoE3) binds to tau protein, possibly slowing the initial rate of tau phosphorylation and self-assembly into paired helical filaments (PHFs); apoE4 does not bind tau. Tau promotes microtubule assembly and stabilizes microtubules; hyperphosphorylated tau does not bind, thereby destabilizing microtubules. Hyperphosphorylated tau may self-assemble into PHFs. Over time a bias toward destabilization of microtubules and the formation of neurofibrillary tangles may occur in individuals who inherit APOE4 alleles, leading to a shorter functional neuronal life span. This hypothesis focuses attention on two important aspects of AD research design: (1) Although the inheritance of APOE4 is associated with increased risk and decreased age of onset, apoE4 does not directly cause the disease. Our data point to the absence of an important function of apoE3 or apoE2 in individuals who do not inherit these alleles as the genetically relevant metabolic factor. This has important implications for design of experiments directed toward understanding the relevant neuronal metabolism. (2) Should this hypothesis be proven and confirmed, targets for pharmaceutical therapy designed to mimic the metabolic function of apoE3 or apoE2 become a realistic preventive strategy.

Common variants at 7p21 are associated with frontotemporal lobar degeneration with TDP-43 inclusions

Frontotemporal lobar degeneration (FTLD) is the second most common cause of presenile dementia. The predominant neuropathology is FTLD with TAR DNA-binding protein (TDP-43) inclusions (FTLD-TDP). FTLD-TDP is frequently familial, resulting from mutations in GRN (which encodes progranulin). We assembled an international collaboration to identify susceptibility loci for FTLD-TDP through a genome-wide association study of 515 individuals with FTLD-TDP. We found that FTLD-TDP associates with multiple SNPs mapping to a single linkage disequilibrium block on 7p21 that contains TMEM106B. Three SNPs retained genome-wide significance following Bonferroni correction (top SNP rs1990622, P = 1.08 × 10−11; odds ratio, minor allele (C) 0.61, 95% CI 0.53–0.71). The association replicated in 89 FTLD-TDP cases (rs1990622; P = 2 × 10−4). TMEM106B variants may confer risk of FTLD-TDP by increasing TMEM106B expression. TMEM106B variants also contribute to genetic risk for FTLD-TDP in individuals with mutations in GRN. Our data implicate variants in TMEM106B as a strong risk factor for FTLD-TDP, suggesting an underlying pathogenic mechanism.

Alzheimer's disease is associated with reduced expression of energy metabolism genes in posterior cingulate neurons

Alzheimers disease (AD) is associated with regional reductions in fluorodeoxyglucose positron emission tomography (FDG PET) measurements of the cerebral metabolic rate for glucose, which may begin long before the onset of histopathological or clinical features, especially in carriers of a common AD susceptibility gene. Molecular evaluation of cells from metabolically affected brain regions could provide new information about the pathogenesis of AD and new targets at which to aim disease-slowing and prevention therapies. Data from a genome-wide transcriptomic study were used to compare the expression of 80 metabolically relevant nuclear genes from laser-capture microdissected non-tangle-bearing neurons from autopsy brains of AD cases and normal controls in posterior cingulate cortex, which is metabolically affected in the earliest stages; other brain regions metabolically affected in PET studies of AD or normal aging; and visual cortex, which is relatively spared. Compared with controls, AD cases had significantly lower expression of 70% of the nuclear genes encoding subunits of the mitochondrial electron transport chain in posterior cingulate cortex, 65% of those in the middle temporal gyrus, 61% of those in hippocampal CA1, 23% of those in entorhinal cortex, 16% of those in visual cortex, and 5% of those in the superior frontal gyrus. Western blots confirmed underexpression of those complex I–V subunits assessed at the protein level. Cerebral metabolic rate for glucose abnormalities in FDG PET studies of AD may be associated with reduced neuronal expression of nuclear genes encoding subunits of the mitochondrial electron transport chain.

Ganglioside-induced differentiation-associated protein-1 is mutant in Charcot-Marie-Tooth disease type 4A/8q21

We previously localized and fine-mapped Charcot Marie Tooth 4A (CMT4A), the autosomal recessive, demyelinating peripheral neuropathy, to chromosome 8. Through additional positional cloning, we have identified a good candidate gene, encoding ganglioside-induced differentiation-associated protein-1 (GDAP1). We found three different mutations in four different Tunisian families—two nonsense and one missense mutation. How mutations in GDAP1 lead to CMT4A remains to be understood.

CD36, a class B scavenger receptor, is expressed on microglia in Alzheimer's disease brains and can mediate production of reactive oxygen species in response to β-amyloid fibrils

A pathological hallmark of Alzheimers disease is the senile plaque, composed of beta-amyloid fibrils, microglia, astrocytes, and dystrophic neurites. We reported previously that class A scavenger receptors mediate adhesion of microglia and macrophages to beta-amyloid fibrils and oxidized low-density lipoprotein (oxLDL)-coated surfaces. We also showed that CD36, a class B scavenger receptor and an oxLDL receptor, promotes H(2)O(2) secretion by macrophages adherent to oxLDL-coated surfaces. Whether CD36 is expressed on microglia, and whether it plays a role in secretion of H(2)O(2) by microglia interacting with fibrillar beta-amyloid is not known. Using fluorescence-activated cell sorting analysis and immunohistochemistry, we found that CD36 is expressed on human fetal microglia, and N9-immortalized mouse microglia. We also found that CD36 is expressed on microglia and on vascular endothelial cells in the brains of Alzheimers disease patients. Bowes human melanoma cells, which normally do not express CD36, gained the ability to specifically bind to surfaces coated with fibrillar beta-amyloid when transfected with a cDNA encoding human CD36, suggesting that CD36 is a receptor for fibrillar beta-amyloid. Furthermore, two different monoclonal antibodies to CD36 inhibited H(2)O(2) production by N9 microglia and human macrophages adherent to fibrillar beta-amyloid by approximately 50%. Our data identify a role for CD36 in fibrillar beta-amyloid-induced H(2)O(2) production by microglia, and imply that CD36 can mediate binding to fibrillar beta-amyloid. We propose that similar to their role in the interaction of macrophages with oxLDL, class A scavenger receptors and CD36 play complimentary roles in the interactions of microglia with fibrillar beta-amyloid.

Neuropathological and neuropsychological changes in "normal" aging: evidence for preclinical Alzheimer disease in cognitively normal individuals.

The presence of diffuse or primitive senile plaques in the neocortex of cognitively normal elderly at autopsy has been presumed to represent normal aging. Alternatively, these patients may have developed dementia and clinical Alzheimer disease (AD) if they had survived. In this setting, these patients could be subjects for cognitive or pharmacologic intervention to delay disease onset. We have thus followed a cohort of cognitively normal elderly subjects with a Clinical Dementia Rating (CDR) of 0 at autopsy. Thirty-one brains were examined at postmortem according to Consortium to Establish a Registry for Alzheimer Disease (CERAD) criteria and staged according to Braak. Ten patients were pathologically normal according to CERAD criteria (1a). Two of these patients were Braak Stage II. Seven very elderly subjects exhibited a few primitive neuritic plaques in the cortex and thus represented CERAD 1b. These individuals ranged in age from 85 to 105 years and were thus older than the CERAD la group that ranged in age from 72 to 93. Fourteen patients displayed Possible AD according to CERAD with ages ranging from 66 to 95. Three of these were Braak Stage I, 4 were Braak Stage II, and 7 were Braak Stage III. The Apolipoprotein E4 allele was over-represented in this possible AD group. Neuropsychological data were available on 12 individuals. In these 12 individuals, Possible AD at autopsy could be predicted by cognitive deficits in 1 or more areas including savings scores on memory testing and overall performance on some measures of frontal executive function.

Tissue-Specific Genetic Control of Splicing: Implications for the Study of Complex Traits

Numerous genome-wide screens for polymorphisms that influence gene expression have provided key insights into the genetic control of transcription. Despite this work, the relevance of specific polymorphisms to in vivo expression and splicing remains unclear. We carried out the first genome-wide screen, to our knowledge, for SNPs that associate with alternative splicing and gene expression in human primary cells, evaluating 93 autopsy-collected cortical brain tissue samples with no defined neuropsychiatric condition and 80 peripheral blood mononucleated cell samples collected from living healthy donors. We identified 23 high confidence associations with total expression and 80 with alternative splicing as reflected by expression levels of specific exons. Fewer than 50% of the implicated SNPs however show effects in both tissue types, reflecting strong evidence for distinct genetic control of splicing and expression in the two tissue types. The data generated here also suggest the possibility that splicing effects may be responsible for up to 13 out of 84 reported genome-wide significant associations with human traits. These results emphasize the importance of establishing a database of polymorphisms affecting splicing and expression in primary tissue types and suggest that splicing effects may be of more phenotypic significance than overall gene expression changes.

Microvascular injury and blood–brain barrier leakage in Alzheimer's disease

Thinning and discontinuities within the vascular basement membrane (VBM) are associated with leakage of the plasma protein prothrombin across the blood-brain barrier (BBB) in Alzheimers disease (AD). Prothrombin immunohistochemistry and ELISA assays were performed on prefrontal cortex. In severe AD, prothrombin was localized within the wall and neuropil surrounding microvessels. Factor VIII staining in severe AD patients indicated that prothrombin leakage was associated with shrinkage of endothelial cells. ELISA revealed elevated prothrombin levels in prefrontal cortex AD cases that increased with the Braak stage (Control=1.39, I-II=1.76, III-IV=2.28, and V-VI=3.11 ng prothrombin/mg total protein). Comparing these four groups, there was a significant difference between control and Braak V-VI (p=0.0095) and also between Braak stages I-II and V-VI (p=0.0048). There was no significant difference in mean prothrombin levels when cases with versus without cerebral amyloid angiopathy (CAA) were compared (p-value=0.3627). When comparing AD patients by APOE genotype (ApoE3,3=2.00, ApoE3,4=2.49, and ApoE4,4=2.96 ng prothrombin/mg total protein) an analysis of variance indicated a difference between genotypes at the 10% significance level (p=0.0705). Tukeys test indicated a difference between the 3,3 and 4,4 groups (p=0.0607). These studies provide evidence that in advanced AD (Braak stage V-VI), plasma proteins like prothrombin can be found within the microvessel wall and surrounding neuropil, and that leakage of the blood-brain barrier may be more common in patients with at least one APOE4 allele.