Inheriting different versions of a protein called apolipoprotein E (APOE) significantly alter a person's risk of developing the sporadic, late-onset form of Alzheimer's disease. One version of the gene, APOE4, is the strongest genetic risk factor for developing this type of Alzheimer's disease. Whereas inheriting the APOE2 version of the gene appears to decrease the risk. But exactly how these variants alter risk has been controversial among researchers.
Now an animal study led by Massachusetts General Hospital (MGH) investigators in collaboration with researchers at the University of Iowa shows that even low levels of the Alzheimer's-associated APOE4 protein can increase the number and density of amyloid beta (A-beta) brain plaques. The presence of APOE4 also leads to increased plaque-associated damage of brain cells and greater amounts of toxic soluble A-beta within the brain in mouse models of the disease. Conversely, introducing APOE2 into the brains of mice with established plaques actually reduced A-beta deposition, retention, and neurotoxicity, suggesting the potential for gene-therapy-based treatment.
"Using a technique developed by our collaborators at the University of Iowa, we were able to get long-term expression of these human gene variants in the fluid that bathes the entire brain," says Dr. Bradley Hyman, of the MassGeneral Institute for Neurodegenerative Disease (MGH-MIND) and senior author of the report in the Nov. 20 issue of Science Translational Medicine. "Our results suggest that strategies aimed at decreasing levels of APOE4, the harmful form of the protein, and increasing concentrations of protective variant APOE2 could be helpful to patients."
The technique used to deliver APOE proteins into the mouse brains in the study was originally developed by Beverly Davidson, professor of molecular physiology and biophysics at the University of Iowa Roy J. and Lucille A. Carver College of Medicine, with the aim of providing broad delivery of proteins missing in the brains of children affected by lysosomal storage diseases, such as Batten disease.
"We simply adapted that general modality to provide broad delivery of the various APOE isoforms," says Davidson, who also holds appointments in the University of Iowa Health Care Departments of Internal Medicine and Neurology.
The association between the APOE4 variant and increased Alzheimer's risk was first made more than 20 years ago. Subsequent research has established that carrying two copies of the harmful variant increases risk 12 times compared with having two copies of the more common form, APOE3. Inheriting the APOE2 variant, however, appears to cut the risk in half. The extremely rare gene variants that directly cause the familial forms of the disease all participate in the production and deposition of A-beta, but exactly how APOE variants contribute to the process remains poorly understood.
Secreted by certain brain cells, APOE regulates cholesterol metabolism within the brain and can interact with A-beta peptides, suggesting that the different forms of the protein may affect whether and how toxic A-beta plaques form. Previous studies examining the protein's effects have used either mice in which gene expression was knocked out or transgenic animals that expressed human gene variants throughout their lifetimes.
The new study used a different approach to investigate the effects of introducing the variant forms of the protein into brains in which plaque formation had already begun. The researchers directly injected viral vectors carrying genes for one of the three APOE variants or a control protein into the cerebrospinal fluid of a mouse model of Alzheimer's – adult animals in which plaques were well established.
Two months after the vectors had been injected, about 10 percent of the APOE in the brains of the animals that received one of the variants was found to be the introduced human version. At five months after injection, the A-beta plaques in brains of mice that received APOE4 injections were more numerous and significantly denser than those in mice receiving APOE2. The growth of plaques in animals receiving APOE3 was intermediate between that of the other two groups and similar to what was seen in control animals. Levels of A-beta in the blood of mice that received APOE2 were higher than in the other groups, suggesting that the protective variant had increased clearance of A-beta from the brain.
In addition to Davidson, UI research assistant Maria Scheel was also involved in the study.
Editor's note: This story is adapted from a release issued by Massachusetts General Hospital.