Isobaric Quantification of Cerebrospinal Fluid Amyloid-β Peptides in Alzheimer's Disease: C-Terminal Truncation Relates to Early Measures of Neurodegeneration.

The amyloid beta (A beta) peptide is the main constituent of the plaques characteristic of Alzheimer's disease (AD). Measurement of A beta 1-42 in cerebrospinal fluid (CSF) is a valuable marker in AD research, where low levels indicate AD. Although the use of immunoassays measuring A beta 1-38 and A beta 1-40 in addition to A beta 1-42 has increased, quantitative assays of other A beta peptides remain rarely explored. We recently discovered novel A beta peptides in CSF using antibodies recognizing the A beta middomain region. Here we have developed a method using both A beta N-terminal and mid-domain antibodies for immunoprecipitation in combination with isobaric labeling and liquid chromatography tandem mass spectrometry (LC-MS/MS) for relative quantification of endogenous A beta peptides in CSF. The developed method was used in a pilot study to produce A beta peptide profiles from 38 CSFsamples. Statistical comparison between CSF samples from 19 AD patients and 19 cognitively healthy controls revealed no significant differences at group level. A significant correlation was found between several larger C-terminally truncated A beta peptides and protein biomarkers for neuronal damage, particularly prominent in the control group. Comparison of the isobaric quantification with immunoassays measuring 4,61-38 or A beta 1-40 showed good correlation (r(2) = 0.84 and 0.85, respectively) between the two analysis methods. The developed method could be used to assess disease-modifying therapies directed at A beta production or degradation.