Effect of apolipoprotein E allele epsilon4 on the initial phase of amyloid beta-protein accumulation in the human brain

Abstract

Deposition of amyloid ss-protein (Ass), a hallmark of Alzheimer's disease, occurs to some extent in the brains of most elderly individuals. We sought to learn when Ass deposition begins and how deposition is affected by apolipoprotein E allele epsilon4, a strong risk factor for late-onset Alzheimer's disease. Using an improved extraction protocol and specific enzyme-linked immunosorbent assay, we quantified the levels of Ass40 and Ass42 in the insoluble fractions of brains from 105 autopsy cases, aged 22 to 81 years at death, who showed no signs of dementia. Ass40 and Ass42 were detected in the insoluble fractions from all of the brains examined; low levels were even found in the brains of patients as young as 20 to 30 years of age. The incidence of significant Ass accumulation increased age-dependently, with Ass42 levels beginning to rise steeply in some patients in their late 40's, accompanied by much smaller increases in Ass40 levels. The presence of the apolipoprotein E epsilon4 allele was found to significantly enhance the accumulation of Ass42 and, to a lesser extent, that of Ass40. These findings strongly suggest that the presence of epsilon4 allele results in an earlier onset of Ass42 accumulation in the brain.

Figure 1.

Levels of Aβ in the insoluble fractions of human brains of various age. Aβ40 (open squares) and Aβ42 (shaded circles) were extracted from the insoluble fractions of 105 normal brains using a guanidine-HCl extraction protocol and quantified by ELISA. The values obtained are plotted as function of age at the time of the patient’s death. Both Aβ40 and Aβ42 were detected in all brains tested, even in those from patients as young as 20 to 30 years of age. The incidence of significant accumulation of Aβ increased age-dependently. Note that y axis is a log scale, and thus levels of Aβ42 increased much more steeply than those of Aβ40.

Figure 2.

Correlation between levels of insoluble Aβ40 and Aβ42 in brain. Aβ40 and Aβ42 levels among individuals were well correlated, suggesting their coordinate increase.

Figure 3.

Representative Western blots of Aβ extracted from the insoluble fractions of normal brains. Samples containing <5 pmol of Aβ40 or Aβ42/g tissue were probed using monoclonal antibodies, BA27 (specific for Aβ40) (A) and BC05 (specific for Aβ42) (B). Two or three distinct Aβ monomer bands at 3 to 4 kd were labeled with both antibodies. A broad band at ∼6 kd represents a sodium dodecyl sulfate-stable Aβ dimer and was observed in most cases. Synthetic Aβ1-40 or Aβ1-42 (10 pg) serving as controls are shown in the left-most lane.

Figure 4.

Effect of the apoE ε4 allele on levels of insoluble Aβ42 (A) and Aβ40 (B) in normal human brains. The dashed lines indicate the threshold for significant Aβ accumulation (5 pmol/g tissue). Patients were subdivided into seven age groups, and the effect of the ε4 allele was assessed by analyzing Aβ accumulation in the presence (n = 28) (closed symbols) or absence (n = 77) (open symbols) of the allele. The levels of insoluble Aβ42 and Aβ40 were found to be statistically related to the presence of ε4 allele.