New Clues on How ApoE4 Affects Alzheimer’s Risk
Alzheimer’s disease is the most common cause of dementia in older adults, and affects more than 5 million Americans. A hallmark of the disease is a toxic protein fragment called beta-amyloid that accumulates in clumps, or plaques, within the brain. Gene variations that cause higher levels of beta-amyloid are associated with a rare type of Alzheimer’s that appears early in life, between age 30 and 60. However, it is the ApoE4 gene variant that is most strongly tied to the more common, late-onset type of Alzheimer’s disease. Inheriting a single copy of ApoE4 from a parent increases the risk of Alzheimer’s disease by about three-fold. Inheriting two copies, one from each parent, increases the risk by about 12-fold.
While common variants of the ApoE gene are strongly associated with the risk of developing late-onset Alzheimer’s disease, the gene’s role in the disease has been unclear. Now, according to an article published online in Nature (16 May 2012), it was discovered that mice having the most risky variant of ApoE damages the blood vessels that feed the brain. The study found that the high-risk variant, ApoE4, triggers an inflammatory reaction that weakens the blood-brain barrier, a network of cells and other components that lines brain’s brain vessels. Normally, this barrier allows nutrients into the brain and keeps harmful substances out.
The ApoE gene encodes a protein that helps regulate the levels and distribution of cholesterol and other lipids in the body. The gene exists in three varieties. ApoE2 is thought to play a protective role against both Alzheimer’s and heart disease, ApoE3 is believed to be neutral, and ApoE4 confers a higher risk for both conditions. Outside the brain, the ApoE4 protein appears to be less effective than other versions at clearing away cholesterol; however, inside the brain, exactly how ApoE4 contributes to Alzheimer’s disease has been a mystery.
The study evaluated several lines of genetically engineered mice, including one that lacks the ApoE gene and three other lines that produce only human ApoE2, ApoE3 or ApoE4. Mice normally have only a single version of ApoE. The authors found that mice whose bodies made only ApoE4, or made no ApoE at all, had a leaky blood-brain barrier. With the barrier compromised, harmful proteins in the blood made their way into the mice’s brains, and after several weeks, authors were able to detect loss of small blood vessels, changes in brain function, and a loss of connections between brain cells.
The study also found that ApoE2 and ApoE3 help control the levels of an inflammatory molecule called cyclophilin A (CypA), but ApoE4 does not. Levels of CypA were raised about five-fold in blood vessels of mice that produce only ApoE4. The excess CypA then activated an enzyme, called MMP-9, which destroys protein components of the blood-brain barrier. Treatment with the immunosuppressant drug cyclosporine A, which inhibits CypA, preserved the integrity of the blood-brain barrier and lessened damage to the brain. An inhibitor of the MMP-9 enzyme had similar beneficial effects. In prior studies, inhibitors of this enzyme have been shown to reduce brain damage after stroke in animal models.