Discovery of plasma protein biomarkers related to Alzheimer’s disease, sex and APOE genotype

The Australian Imaging and Biomarker Lifestyle (AIBL) study of aging is designed to aid the discovery of biomarkers. The current study aimed to discover differentially expressed plasma proteins that could yield a blood-based screening tool for Alzheimers disease. To search for biomarkers and elucidate mechanisms of AD, we immuno-depleted the most abundant plasma proteins and pre-fractionated the remaining proteins by HPLC, prior to two-dimensional gel electrophoresis. The relative levels of approximately 3,400 protein species resolved on the 2D gels were compared using in-gel differential analysis with spectrally resolved fluorescent protein detection dyes (ZdyesTM). Here we report on analysis of pooled plasma samples from an initial screen of a sex-matched cohort of 72 probable AD patients and 72 healthy controls from the baseline time point of AIBL. We report significant changes in variants of apolipoprotein E, haptoglobin, alpha-anti-trypsin, inter-alpha trypsin inhibitor, histidine-rich glycoprotein, and a protein of unknown identity. Alpha-1-anti-trypsin and alpha-1 anti-chymotrypsin demonstrated plasma concentrations that were dependent on APOE e4 allele dose. Our analysis also identified an association with the level of Vitamin D binding protein fragments and complement factor I with sex. We then conducted a validation study on individual samples using a targeted LC-MS/MS assay. This study indicates that a peripheral protein signature has potential to aid in the characterization of AD. We also found significant associations of protein levels with APOE genotype, indicating that differences in genotype influence the circulating abundances of proteins other than ApoE.