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Alzheimer's disease (AD) is the most common form of age-related neurodegenerative illness. The defining pathological hallmarks of AD are the presence of neurofibrillary tangles and senile plaques in the brain. Amyloid ? polypeptides (A?) are the major constituents of amyloid plaques and are derived from altered processing of amyloid precursor proteins (APPs). A? consists predominantly of two forms, A? 40 and A? 42 . Although A? 40 is the predominant form, recent evidence suggests that A? 42 is the pathogenic form. SUMMARY This document involves methods and materials related to determining whether or not a mammal (e.g., a human) is susceptible to develop mild cognitive impairment (MCI) or Alzheimer's disease (AD). Identifying at-risk individuals can allow doctors and patients to select treatment and patient care options before the onset of noticeable or impairing symptoms. Analyses of plasma A? have shown that A? 42 is increased in subjects who have PS1/2 or APP717 mutations that cause early onset familial AD. Both A? 42 and A? 40 are elevated in plasma from subjects with Down's syndrome or the Swedish APP mutation, and both forms are also elevated in the unaffected first-degree relatives and extended families of patients with late onset AD. In addition, plasma A? 40 increases with aging over age 65. A? 42 may also increase, but to a lesser extent. Plasma A? 42 is not significantly increased in patients with AD. Many studies, however, have shown that CSF A? 42 is significantly decreased in AD patients and MCI patients. In the Tg2576 transgenic mouse model of AD, both CSF A? and plasma A? levels were found to decrease as A? is deposited in the brain. As described herein, over five hundred normal Caucasian subjects were followed for up to 12 years. The subjects were cognitively tested about every one to two years. From this study, the ratio of plasma A? 42 /A? 40 was found to be a strong predictor of conversion to AD or MCI. In general, this document features a method for determining whether or not a human is susceptible to develop mild cognitive impairment or Alzheimer's disease. The method includes obtaining a plasma sample from the human and determining the A? 42 to A? 40 ratio in the plasma sample, wherein a ratio less than 0.082 indicates that the human is susceptible to develop mild cognitive impairment or Alzheimer's disease. An A? 42 to A? 40 ratio less than 0.05 can indicate that the human is susceptible to develop mild cognitive impairment or Alzheimer's disease within two years. In another embodiment, this document features a method for determining whether or not a human is susceptible to develop mild cognitive impairment or Alzheimer's disease. The method includes (a) determining the A? 42 to A? 40 ratio in a plasma sample from the human, and (b) classifying the human as being or not being susceptible to develop mild cognitive impairment or Alzheimer's disease, wherein the human is susceptible to develop mild cognitive impairment or Alzheimer's disease if the A? 42 to A? 40 ratio in the plasma sample is less than 0.082, and wherein the human is not susceptible to develop mild cognitive impairment or Alzheimer's disease if the A? 42 to A? 40 ratio in the plasma sample is greater than 0.082. The human can be classified as being susceptible to develop mild cognitive impairment or Alzheimer's disease within two years if the A? 42 to A? 40 ratio is less than 0.05. The human can be classified as being susceptible to develop mild cognitive impairment or Alzheimer's disease within four years if the A? 42 to A? 40 ratio is between 0.05 and 0.064. The human can be classified as being susceptible to develop mild cognitive impairment or Alzheimer's disease within ten years if the A? 42 to A? 40 ratio is between 0.064 and 0.082. In another embodiment, this document features a method for determining whether or not a mammal is susceptible to develop mild cognitive impairment or Alzheimer's disease. The method includes determining whether or not the mammal contains plasma with an A? 42 to A? 40 ratio that is less than 0.082, wherein the presence of the ratio indicates that the mammal is susceptible to develop mild cognitive impairment or Alzheimer's disease. The mammal can be a human. The mammal can be a human not diagnosed with mild cognitive impairment or Alzheimer's disease. The method can include determining whether or not the mammal is susceptible to develop mild cognitive impairment. The method can include determining whether or not the mammal is susceptible to develop Alzheimer's disease. The method can include determining whether or not the mammal contains plasma with an A? 42 to A? 40 ratio that is less than 0.064. The method can include determining whether or not the mammal contains plasma with an A? 42 to A? 40 ratio that is less than 0.05. The determining step can include using an enzyme-linked immunosorbant assay. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within two years. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within two years when the A? 42 to A? 40 ratio is less than 0.05. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within four years. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within four years when the A? 42 to A? 40 ratio is between 0.05 and 0.064. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within ten years. The method can include classifying the mammal as being susceptible to develop mild cognitive impairment or Alzheimer's disease within ten years when the A? 42 to A? 40 ratio is between 0.064 and 0.082. The method can include classifying the mammal as not being susceptible to develop mild cognitive impairment or Alzheimer's disease within ten years when the A? 42 to A? 40 ratio is greater than 0.082. The method can include, before the determining step, obtaining a plasma sample from the mammal, wherein the determining step contains determining whether or not the plasma sample contains the A? 42 to A? 40 ratio that is less than 0.082. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
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