One of the most enduring mysteries in Alzheimer’s disease (AD) is closer to being resolved due to new work from the lab of Dr. Berislav Zlokovic: how the gene variant APOE4 increases the risk for cognitive decline and dementia.
APOE4 is one of three major genetic variants (also known as alleles) of the apolipoprotein E gene, abbreviated as APOE. Each cell in a human body starts with two APOE alleles, one from each parent; these alleles may be the same or different from one another. Cells make proteins from gene “blueprints”; an APOE4 allele will yield a slightly different APOE protein than an APOE3 allele will yield. The following three points are from the National Institutes of Health regarding the APOE alleles:
- APOE2 is relatively rare and may provide some protection against the disease. If Alzheimer’s disease occurs in a person with this allele, it usually develops later in life than it would in someone with the APOE4 gene.
- APOE3, the most common allele, is believed to play a neutral role in the disease—neither decreasing nor increasing risk.
- APOE4 increases risk for Alzheimer’s disease and is also associated with an earlier age of disease onset. Having one or two APOE4 alleles increases the risk of developing Alzheimer’s. About 25 percent of people carry one copy of APOE 4, and 2 to 3 percent carry two copies.
In the 1990s, scientists recognized that the APOE4 gene variant was overrepresented in patient populations with late-onset AD and identified APOE4 as the common genetic variant associated with the greatest risk of developing the disease in the cohorts studied. (Rare variants of other genes with greater risk impact have been identified, but they are carried only by a very small number of people.)
How does APOE4 increase the risk of developing Alzheimer’s disease? While the APOE protein has many roles in the brain, as well as the rest of the body, it is not well understood how or why the different versions of the APOE protein encoded by different APOE alleles may behave differently in these roles. The new work from Dr. Zlokovic’s laboratory presents the theory that APOE4 may increase AD risk through at least two separate pathways, including one that does not involve amyloid or tau pathology, the hallmarks of AD.
The field has long recognized that people carrying one or two copies of APOE4 have earlier and elevated amyloid-beta deposition in the brain—a precursor of the Alzheimer’s pathology amyloid plaques—and that these carriers develop mild cognitive impairment and dementia at a higher and earlier rate than non-APOE4 carriers. The breakdown of the protective cellular border lining the brain’s blood vessels that keeps the bloodstream and brain apart, known as the blood-brain barrier, has been shown to occur in patients with Alzheimer’s disease. Work in animal models suggests that APOE4 may contribute to the barrier’s break down. A “leaky” blood-brain barrier allows otherwise-blocked substances to pass from the blood into the brain, where they can be toxic and trigger a pathological immune response.
On the whole, the relationships among the observed consequences of carrying the APOE4 variant have been unclear:
- Does the increased amyloid deposition lead to a leakier blood-brain barrier, or does a leakier blood-brain barrier increase amyloid deposition?
- Is early cognitive decline caused by either or both?
These questions remain a matter of continued investigation, and, with support from Cure Alzheimer’s Fund, answers are beginning to emerge.
Dr. Berislav Zlokovic—Cure Alzheimer’s Fund Research Leadership Group member and Director of the Zilkha Neurogenetic Institute—leads a team of scientists at the University of Southern California to investigate how damage to the blood-brain barrier may underlie cognitive decline.
Findings published in Nature suggest that people carrying at least one copy of APOE4 showed earlier and greater damage to the blood-brain barrier than did non-carrier. This level of damage was predictive of future cognitive decline. Further, the team proposed a mechanism by which APOE4 can contribute directly to blood-brain barrier damage. In their paper, the scientists compared results from a series of neuroimaging and biochemistry tests administered over several years to APOE4 carriers (homozygous APOE4/4 and heterozygous APOE3/4) and non-carriers (homozygous APOE3/3).
First, the researchers used advanced brain imaging technology to show that early damage to the blood-brain barrier in individuals with APOE4 occurred in the hippocampus and parahippocampal gyrus, the learning and memory centers of the brain. This damage was present in people with the variant with no memory loss but was more severe in those with signs of cognitive decline.
The scientists also demonstrated that greater blood-brain barrier breakdown in APOE4 carriers occurred independently of the presence of Alzheimer’s disease pathology. Measures of blood-brain barrier damage in specific brain areas predicted cognitive impairment for APOE4 but not APOE3/3 carriers even after accounting for amyloid-beta and tau burden.
The data from the next set of experiments were particularly exciting: they suggest a new potential diagnostic tool to determine susceptibility to cognitive decline for APOE4 carriers before decline is observed clinically.
To this end, the researchers used a test they previously developed to assess the breakdown of the blood-brain barrier from samples of cerebrospinal fluid (CSF)—the liquid around the brain and spinal cord. On the molecular level, the assay measures a biomarker—in this case, a specific protein that indicates injury to specialized cells that help make up the blood-brain barrier, called pericytes. When pericytes are injured, the protein shed’s their surface and becomes loose in the cerebrospinal fluid. Therefore, an increase in levels of this loose protein in a person’s CSF implies the blood-brain barrier has been damaged.
In their investigation, the researchers divided the APOE4 participants into two groups based on initial testing—one with lower levels of the pericyte injury biomarker and the other with higher levels. For each group, cognitive abilities were evaluated and recorded. The participants’ cognition was tested again over several years. The researchers found that initial higher levels of the biomarker were predictive of cognitive decline severity in the later time points. Also, elevated biomarker levels correlated with increased damage to the blood-brain barrier in the hippocampus and parahippocampal gyrus.
The team performed additional experiments to address the underlying mechanisms behind APOE4-linked injury to pericytes and confirmed that APOE4 activates an inflammatory pathway—ultimately speeding up the breakdown of the blood-brain barrier.
These groundbreaking findings made by Dr. Zlokovic and his lab suggest that drugs or other treatments that can protect or improve the blood-brain barrier’s integrity—perhaps by blocking inflammation in pericytes—could protect cognitive health in people carrying APOE4. The Zlokovic lab is pursuing this investigation in its ongoing work.