Quantification of total apolipoprotein E and its isoforms in cerebrospinal fluid from patients with neurodegenerative diseases

Background The human APOE gene, which codes for apolipoprotein E (apoE), has three major polymorphic alleles: epsilon 2, epsilon 3, and epsilon 4 that give rise to amino acid substitutions. APOE-epsilon 4 is a strong risk factor of sporadic Alzheimer's disease (AD) but the reason why is still unknown despite intense research for more than 20 years. The aim of the study was to investigate if the concentrations of total apoE and the specific apoE isoforms in cerebrospinal fluid (CSF) differ between various neurodegenerative diseases and control individuals, as well as among the APOE genotypes. Methods Quantification of total apoE and specific apoE isoforms (E2, E3, and E4) in CSF was performed using high-resolution parallel reaction monitoring mass spectrometry. In total, 1820 individuals were involved in the study including clinically diagnosed AD patients (n = 228), cognitively unimpaired (CU) patients (n = 896), and patients with other neurodegenerative disorders (n = 696). Follow-up data was available for 100 individuals, assessed at two time points. Subjects were dichotomized based on an A beta(42/40) CSF concentration ratio cut-off into A beta positive (A beta+, < 0.091) and A beta negative (A beta-, > 0.091) groups. Results Even though there was a significant increase of total apoE in the amyloid beta-positive (A beta+) group compared with amyloid beta-negative (A beta-) individuals (p < 0.001), the magnitude of the effect was very small (AUC = 0.55). Moreover, CSF total apoE concentrations did not differ between A beta- CU controls and clinically diagnosed AD patients. There was a difference in concentration between isoforms in heterozygous individuals in an isoform-dependent manner (E2 < E3 < E4) (p < 0.001, AUC = 0.64-0.69), and these associations remained when dichotomizing the samples into A beta+ and A beta- groups (p < 0.01, AUC = 0.63-0.74). In the cohort with follow-up samples, neither total apoE nor isoform-specific apoE concentrations differed between the two time points (p > 0.05). Conclusions The results indicate that neither the concentrations of total apoE nor the different apoE isoforms in CSF are associated with APOE-epsilon 4 carrier status, A beta status, or clinical dementia diagnoses.