The Amyloid Cascade Hypothesis: A Conclusion in Search of Support

The amyloid cascade hypothesis as the etiological underpinning of Alzheimer disease (AD) is supported by a large body of literature, the most influential of which are genetic studies of the 1980s and 1990s. Other evidence includes the neuropathology of Down syndrome, apparent toxicity of oligomeric amyloid-β (Aβ), interactions with apolipoprotein E, and the analogy of cardiac amyloidosis. However, there is considerable phenotypic heterogeneity among the rare familial AD kindreds, which tempers extrapolation to sporadic AD. Oligomer biology is still in the theoretical realm, with no clinical validation. Apolipoprotein E support for the amyloid cascade and other inferences from the literature are somewhat circular in their logic. Analogy with amyloidoses might also consider secondary amyloidosis, driven by systemic inflammation and treated by treating the underlying etiology. Much of the remaining literature supporting the amyloid cascade is dominated by hypothesis-generating studies. Importantly, we now have a developing evidence base from controlled clinical trials that can potentially inform the issue of Aβ as a cause or driver of disease in sporadic AD. Emerging data provide clear evidence of target engagement. Clinical outcome, however, has been either marginally positive or similar to placebo. Assuming these findings hold, it appears that Aβ neither drives nor mitigates the disease process.