APOE ε4 genotype-dependent cerebrospinal fluid proteomic signatures in Alzheimer's disease

Background Aggregation of amyloid beta into plaques in the brain is one of the earliest pathological events in Alzheimer's disease (AD). The exact pathophysiology leading to dementia is still uncertain, but the apolipoprotein E (APOE) epsilon 4 genotype plays a major role. We aimed to identify the molecular pathways associated with amyloid beta aggregation using cerebrospinal fluid (CSF) proteomics and to study the potential modifying effects of APOE epsilon 4 genotype. Methods We tested 243 proteins and protein fragments in CSF comparing 193 subjects with AD across the cognitive spectrum (65% APOE epsilon 4 carriers, average age 75 +/- 7 years) against 60 controls with normal CSF amyloid beta, normal cognition, and no APOE epsilon 4 allele (average age 75 +/- 6 years). Results One hundred twenty-nine proteins (53%) were associated with aggregated amyloid beta. APOE epsilon 4 carriers with AD showed altered concentrations of proteins involved in the complement pathway and glycolysis when cognition was normal and lower concentrations of proteins involved in synapse structure and function when cognitive impairment was moderately severe. APOE epsilon 4 non-carriers with AD showed lower expression of proteins involved in synapse structure and function when cognition was normal and lower concentrations of proteins that were associated with complement and other inflammatory processes when cognitive impairment was mild. Repeating analyses for 114 proteins that were available in an independent EMIF-AD MBD dataset (n = 275) showed that 80% of the proteins showed group differences in a similar direction, but overall, 28% effects reached statistical significance (ranging between 6 and 87% depending on the disease stage and genotype), suggesting variable reproducibility. Conclusions These results imply that AD pathophysiology depends on APOE genotype and that treatment for AD may need to be tailored according to APOE genotype and severity of the cognitive impairment.