Does Apolipoprotein E modify the association of cerebral infarcts with Alzheimer's disease?
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Background: Dementia is a disease known to cause chronic deterioration of intellectual functions severe enough to interfere with the ability to perform activities of daily living. Alzheimer’s disease (AD) is the most frequent cause of dementia and is expected to have a substantial impact on the health care system as the Canadian population ages. Current therapies are ineffective at halting disease progression; thus, investigations examining risk factors for AD have become a popular avenue of research. A relationship between cerebrovascular disease and the risk of AD has been established, but the underlying mechanisms on how these morbidities are related remain unclear. The apolipoprotein E gene (ApoE) influences the development of AD with the apolipoprotein E-e4 allele (ApoE-e4) conferring increased risk. The underlying mechanism by which the ApoE-e4 allele influences AD is unclear. Since the ApoE-e4 allele is related to both AD and stroke, the impact of cerebral infarcts on AD may vary by ApoE-e4 allele status. Objective: The objective of this study was to assess if ApoE-e4 allele status modified the relationship between cerebral infarcts and AD. Methods: Secondary data from the Nun Study, a longitudinal clinico-pathologic study of aging representing 678 female participants 75+ years were used for this investigation. AD was diagnosed using criteria for clinical dementia and AD pathology. Dementia was diagnosed using standard criteria, including the Consortium to Establish a Registry for Alzheimer’s Disease battery of neuropsychological tests and performances on activities of daily living. AD pathology was diagnosed using a modified version of the National Institute on Aging and Reagan Institute criteria. Infarcts were identified during gross neuropathologic assessment at autopsy. Logistic regression was used to assess the relationship between AD and the presence, location, and size of cerebral infarcts. Regression models were then stratified by ApoE-e4 allele status to determine if this variable was a significant effect modifier. The relationship of ApoE-e4 allele status with AD, as well as presence of cerebral infarcts, was also explored. All regression models were adjusted for age at death, educational level, and, when appropriate, ApoE-e4 allele status. A sensitivity analysis using different definitions for the outcome AD was performed and showed that varying criteria for AD pathology did not change study results; however, the use of clinical dementia (regardless of pathology) as an outcome did produce significantly different results. Thus, the research questions were repeated using clinical dementia as an outcome. Results: The presence of cerebral infarcts was not significantly associated with AD; this relationship did not change when location and size of infarcts were examined. ApoE-e4 was significantly associated with an increased risk of AD. ApoE-e4 was not associated with presence of cerebral infarcts. ApoE-e4 did not modify the relationship between presence of cerebral infarcts and AD. When the outcome clinical dementia was investigated, presence of cerebral infarcts significantly increased the risk of dementia. This relationship remained when location and size of infarcts were analyzed. ApoE-e4 allele status slightly modified the relationship between presence of cerebral infarcts and dementia. Conclusions: The findings from this study suggest that individuals with severe AD pathology are unlikely to be affected by cerebral infarcts. Future studies should focus on examining levels of severity of AD pathology in relation to cerebral infarcts. Cerebral infarcts appear to have an impact on dementia and this relationship was found to slightly vary by ApoE-e4 status. Future studies are recommended to examine how ApoE interacts with a variety of age-related risk factors to increase the risk of AD.