Cerebrospinal fluid amyloid β peptide patterns in Alzheimer's disease patients and nondemented controls depend on sample pretreatment: Indication of carrier‐mediated epitope masking of amyloid β peptides

Abstract

A quantitative urea‐based amyloid β (Aβ)‐sodium dodecyl sulfate‐polyacrylamide gel electrophoresis with Western immunoblot (Aβ‐SDS‐PAGE/immunoblot) reveals highly conserved and disease‐specific Aβ peptide patterns (Aβ 1‐37, 1‐38, 1‐39, 1‐40, 1‐42) in Alzheimer's disease (AD) patients and nondemented controls. For further standardization of this method, we analyzed cerebrospinal fluid (CSF) of eight probable AD patients and seven nondemented controls using different preanalytical procedures for Aβ‐SDS‐PAGE/immunoblot and Aβ1‐42‐enzyme linked immunosorbent assay (ELISA). Both diagnostic groups were discriminated significantly by absolute levels of Aβ1‐42 and ratios of Aβ1‐42/40, 1‐42/38, 1‐42/39. Preanalytical freezing of CSF led to a highly significant loss of all Aβ peptide species. This effect was most pronounced for Aβ1‐42 and completely prevented by SDS‐heat denaturation prior to freezing. Prolonged storage of SDS‐heat denatured CSF led to a selective loss of Aβ1‐42 and impaired the discrimination of diagnostic groups as measured by Aβ‐SDS‐PAGE/immunoblot. Neither freezing nor storage significantly affected absolute Aβ1‐42 levels as determined by Aβ1‐42‐ELISA, but both impaired the discrimation of diagnostic groups. Hence, we suggest immediate analysis of samples for Aβ1‐42‐ELISA, analysis after a short freezing interval for Aβ‐SDS‐PAGE/immunoblot, and avoidance of prolonged storage intervals. Remarkably, Aβ‐SDS‐PAGE/immunoblot measured threefold higher levels of Aβ1‐42 in CSF than Aβ1‐42‐ELISA. In summary, our results indicate carrier‐mediated epitope masking of Aβ1‐42.