Medical Grand Rounds

Alzheimer disease prevention: Focus on cardiovascular risk, not amyloid?

Author and Disclosure Information

ABSTRACTAutosomal dominant (early-onset) Alzheimer disease and the much more common sporadic Alzheimer disease share a common pathology but not necessarily a common pathophysiology. Common cardiovascular comorbidities are associated with increased risk for Alzheimer disease and offer opportunities for intervention. Class I evidence for prevention is extremely limited. The overall body of evidence suggests the best time to intervene is in midlife, not in old age.

KEY POINTS

  • Vascular risk factors clearly increase the risk of Alzheimer disease and can be addressed. However, controlled trials in patients with hypertension or with dyslipidemia have had negative results.
  • Risk is lower with a diet high in antioxidants and polyunsaturated fatty acids.
  • Estrogen therapy has had mixed results in observational studies, mostly hinting at lower risk. However, a randomized trial of hormone replacement therapy in late life indicated a higher risk of dementia with estrogen.
  • Physical activity in midlife and in late life was associated with a lower risk of Alzheimer disease in observational studies. Controlled trials were not so positive, but the benefits of exercise may be slowly cumulative.


 

References

Efforts to modify the relentless course of Alzheimer disease have until now been based on altering the production or clearance of beta-amyloid, the protein found in plaques in the brains of patients with the disease. Results have been disappointing, possibly because our models of the disease—mostly based on the rare, inherited form—may not be applicable to the much more common sporadic form.

Ely Lilly’s recent announcement that it is halting research into semagacestat, a drug designed to reduce amyloid production, only cast further doubt on viability of the amyloid hypothesis as a framework for effective treatments for Alzheimer disease.

Because of the close association of sporadic Alzheimer disease with vascular disease and type 2 diabetes mellitus, increased efforts to treat and prevent these conditions may be the best approach to reducing the incidence of Alzheimer disease.

This article will discuss current thinking of the pathophysiology of Alzheimer disease, with special attention to potential prevention and treatment strategies.

THE CANONICAL VIEW: AMYLOID IS THE CAUSE

The canonical view is that the toxic effects of beta-amyloid are the cause of neuronal dysfunction and loss in Alzheimer disease.

Beta-amyloid is a small peptide, 38 to 42 amino acids long, that accumulates in the extracellular plaque that characterizes Alzheimer pathology. Small amounts of extracellular beta-amyloid can be detected in the brains of elderly people who die of other causes, but the brains of people who die with severe Alzheimer disease show extensive accumulation of plaques.

The amyloid precursor protein is cleaved by normal constitutive enzymes, leaving beta-amyloid as a fragment. The beta-amyloid forms into fibrillar aggregations, which further clump into the extracellular plaque. Plaques can occur in the normal aging process in relatively low amounts. However, in Alzheimer disease, through some unknown trigger, the immune system appears to become activated in reference to the plaque. Microglial cells—the brain’s macrophages—invade the plaque and trigger a cycle of inflammation. The inflammation and its by-products cause local neuronal damage, which seems to propagate the inflammatory cycle to an even greater extent through a feed-forward loop. The damage leads to metabolic stress in the neuron and collapse of the cytoskeleton into a neurofibrillary tangle. Once the neurofibrillary tangle is forming, the neuron is probably on the path to certain death.

This pathway might be interrupted at several points, and in fact, much of the drug development world is working on possible ways to do so.

GENETIC VS SPORADIC DISEASE: WHAT ARE THE KEY DIFFERENCES?

The amyloid model is based on early-onset disease, a distinct syndrome with autosomal dominant inheritance. The onset of symptoms usually occurs in the patient’s 40s, but it can occur as early as the 20s or as late as the 50s. The disease is aggressive and almost always follows a 5- to 8-year course to death. In terms of its onset, course, and comorbidities, early-onset Alzheimer disease is very different than the much more common sporadic or late-onset disease found in older adults ( Table 1 ).

Although the autosomal dominant form of the disease accounts for probably only 1% or 2% of all cases of Alzheimer disease, most animal models and hence much of the basic research and drug testing in Alzheimer disease are based on those dominant mutations. The pathology—the plaques and tangles—in Alzheimer disease in older adults is identical to that in younger adults, but the origins of the disease may not be the same. Therefore, the experimental model for one may not be relevant to the other.

Pages

Next Article: