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

A quantitative urea-based amyloid b (Ab)-sodium dodecyl sulfate-polyacrylamide gel electrophoresis with Western immunoblot (Ab-SDS-PAGE/immunoblot) reveals highly conserved and disease-specific Ab peptide patterns (Ab 1-37, 1-38, 1-39, 1-40, 1-42) in Alzheimer’sdisease (AD)patientsandnondementedcontrols. 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 Ab-SDSPAGE/immunoblot and Ab1-42-enzyme linked immunosorbent assay (ELISA). Both diagnostic groups were discriminated significantly by absolute levels of Ab1-42 and ratios of Ab1-42/40, 1-42/38, 1-42/39. Preanalytical freezing of CSF led to a highly significant loss of all Ab peptide species. This effect was most pronounced for Ab1-42 and completely prevented by SDS-heat denaturation prior to freezing. Prolonged storage of SDS-heat denatured CSF led to a selective loss of Ab1-42 and impaired the discrimination of diagnostic groups as measured by Ab-SDS-PAGE/immunoblot. Neither freezing nor storage significantly affected absolute Ab1-42 levels as determined by Ab1-42ELISA, but both impaired the discrimation of diagnostic groups. Hence, we suggest immediate analysis of samples for Ab1-42-ELISA, analysis after a short freezing interval for Ab-SDS-PAGE/immunoblot, and avoidance of prolonged storage intervals. Remarkably, Ab-SDS-PAGE/immunoblotmeasured threefoldhigher levels ofAb1-42 inCSF than Ab1-42-ELISA. In summary, our results indicate carrier-mediated epitope masking of Ab1-42.