VARIANCE OF THE AMYLOID BETA PEPTIDE AS A METRIC FOR THE DIAGNOSIS OF ALZHEIMER\u27S DISEASE

Alzheimer’s disease (AD) is the most prevalent neurodegenerative disorder associated with aging. AD is by far the best understood and most studied neurodegenerative disease. Substantial advances have been made over the last decade, however it is debatable how much closer we are to a clinically useful therapy. A long standing goal in the AD field has been to improve the accuracy of early detection, with the assumption that the ability to intervene earlier in the disease process will lead to a better clinical outcome. Major facets of this effort have been the continued development and improvement of AD biomarkers, with a strong focus on developing imaging modalities. AD is accompanied by two pathological hallmarks in the brain: extracellular neuritic plaques composed of the beta-amyloid peptide (Aβ) and intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein. Evidence of Aβ as the driving force behind the progression of AD (the amyloid cascade hypothesis) was first published by Hardy & Higgins in 1992, and this peptide has been the focus of therapeutic and diagnostic testing for decades. Significant technological advances in recent years now allow imaging of amyloid pathology in vivo. These methods evaluate Aβ burden in a living person, and could potentially serve as both a biomarker, and as a diagnostic tool to detect disease. Pittsburgh Compound B (PiB) is currently the best studied of these imaging agents, however, our current knowledge of the quantitative relationship between PiB binding and amyloid pathology in the brain is limited. A better understanding of how these variables relate to one another is essential for the continued development of reliable diagnostic biomarkers for AD. We analyzed increasingly insoluble pools of Aβ to quantify their relative contributions to the overall Aβ burden, and to determine if any of these measures could be used to predict disease status. We found that the amount of PiB binding in a cortical region of the brain could distinguish cases of mild cognitive impairment (MCI) when corrected to the amount of PiB binding in the cerebellum. As the Aβ peptide ages, the amino acid aspartate may spontaneously convert to an isoaspartate residue through a succinimide intermediary. The presence of iso-Asp Aβ has been used to indicate the presence of aged plaques in AD and Down syndrome cases. We sought to investigate the potential relationship between levels of ‘aged’ Aβ in the plasma as indicated by iso-Asp Aβ and disease state, as a potential biomarker for the presence of AD pathology. We found that AD cases had lower levels of all forms of Aβ in plasma when standardized to the group average, and that plasma levels of Aβ and iso-Asp Aβ were reversed between disease groups. A follow up study is required, however, these initial data are a promising step towards utilizing aged iso-Asp Aβ plasma levels as a potential biomarker to indicate disease state