A recessive mutation in the APP gene with dominant-negative effect on amyloidogenesis

Abstract

beta-Amyloid precursor protein (APP) mutations cause familial Alzheimer's disease with nearly complete penetrance. We found an APP mutation [alanine-673-->valine-673 (A673V)] that causes disease only in the homozygous state, whereas heterozygous carriers were unaffected, consistent with a recessive Mendelian trait of inheritance. The A673V mutation affected APP processing, resulting in enhanced beta-amyloid (Abeta) production and formation of amyloid fibrils in vitro. Co-incubation of mutated and wild-type peptides conferred instability on Abeta aggregates and inhibited amyloidogenesis and neurotoxicity. The highly amyloidogenic effect of the A673V mutation in the homozygous state and its anti-amyloidogenic effect in the heterozygous state account for the autosomal recessive pattern of inheritance and have implications for genetic screening and the potential treatment of Alzheimer's disease.

Fig. 1

Analysis of APP gene and APP processing. (A) APP gene analysis by restriction fragment length polymorphism of 169—base pair (bp) polymerase chain reaction (PCR) products amplified from homozygous (III-16), heterozygous (II-11), and control subjects. In the absence of the A673V mutation, the enzyme HpyCH4V generates two fragments of 91 and 78 bp. The mutation abolishes the restriction site, and the PCR product remains uncut. (B) sAPPβ:sAPPα ratio in conditioned media from CHO cells transfected with wild-type or A673V-mutated APP and fibroblasts of the proband and four controls. Error bars represent means ± SD. (C) APP carboxy-terminal fragments C99 and C83 (arrowheads) in CHO cells transfected with wild-type (lane 2) or A673V-mutated (lane 3) APP and fibroblasts from a control (lane 4) and the proband (lane 5), as shown by immunoblot analysis. Lane 1 corresponds to from nontransfected CHO cells. (D) Densitometric analysis of immunoblots, showing a significant increase in the C99:C83 ratio (P = 0.0001) in cells carrying the A673V mutation. Error bars represent means ± SD.

Fig. 2

Short-time kinetics of Aβ assembly and disassembly, determined by laser light scattering. (A) Course of the light intensity scattered by solutions of Aβ1-40_wt (blue), Aβ1-40_mut (red), and their equimolar mixture (green). The corresponding exponential fits are indicated by full lines. (B) Particle size distribution of Aβ1-40_wt (blue), Aβ1-40_mut (red), and the peptide mixture (green) immediately after sample preparation (time 0) and after 24 hours. (C) Short-time dissolution kinetics of 48-hour-aged peptide aggregates after fivefold dilution with buffer. a.u., arbitrary units.

Fig. 3

Aggregation properties of mutated and wild-type Aβ peptides. (A to F) Electron micrographs of aggregates generated by Aβ1-40_wt, Aβ1-40_mut, and equimolar mixtures after 72 hours [(A) to (C), negative staining] and 20 days incubation [(D) to (F), positive staining]. (G to L) Polarized light microscopy of Aβ aggregates stained with Congo red after 72 hours [(G) to (I)] and 20 days [(J) to (L)]. (M to O) Electron micrographs of negatively stained aggregates generated by Aβ1-42_wt (M), Aβ1-42_mut (N), and equimolar mixtures (O) after 5 days incubation. The peptide mixture contains mainly amorphous material (O), whereas wild-type and mutated Aβ1-42 are assembled in fibrillary structures. Scale bars indicate 250 nm [(A) to (F)], 50 μm [(G) to (L)], and 125 nm [(M) to (O)].

Fig. 4

Physicochemical and biological properties of mutated and wild-type Aβ peptides. (A) Size exclusion chromatograms of Aβ1-40_wt, Aβ1-40_mut, and equimolar peptide mixture aggregates after treatment with 1 M urea for 24 hours. Monomeric species (arrow) are only seen in the peptide mixture. (B and C) Binding of wild-type and mutated Aβ1-40 (B) or Aβ1-6 (C) to amyloid fibrils of Aβ1-40_wt determined by surface plasmon resonance. Solutions of Aβ1-40 (1 μM) or Aβ1-6 (500 μM) were injected onto Aβ1-40_wt fibrils immobilized on the sensor chip for the time indicated by the bars. (D) Viability of human neuroblastoma cells after 24 hours exposure to 5 μM Aβ1-42_wt, Aβ1-42_mut, and the equimolar mixture. Error bars represent SD of the mean of eight replicates. *P = 0.026, **P < 0.001.