ApoE Genotyping: The Test

ApoE Genotyping: The Test

Also known as: ApoE cardiac risk; ApoE 2 mutations; APOE4 genotype

Formal name: Apolipoprotein E genotyping

Related tests:
Cardiac Risk Assessment; HDL Cholesterol; LDL Cholesterol; Lipid Profile; Triglycerides; Tau/Aß42

• At a Glance
• Test Sample
• The Test
• Common Questions
• Ask Us
• Related Pages

The Test

1. How is it used?
2. When is it ordered?
3. What does the test result mean?
4. Is there anything else I should know?

How is it used?

In Cardiovascular Disease (CVD)

The test for ApoE is not widely used and it's clinical usefulness is still being researched. When it is ordered, it may be used in combination with other lipid tests that evaluate risk for CVD, such as cholesterol levels and lipoprotein electrophoresis. It may sometimes be used to check for and help diagnose a genetic component to a lipid abnormality.

Testing for ApoE may sometimes be ordered to help guide lipid treatment. In cases of high cholesterol and triglyceride levels, statins are usually considered the treatment of choice to decrease the risk of developing CVD. However, there is a wide variability in the response to these lipid-lowering drugs that is in part influenced by the Apo E genotype.

Though appropriately responsive to a low fat diet, people with ApoE e4 may be less likely than those with ApoE e2 to respond to statins by decreasing their levels of LDL-C and may require adjustments to their treatment plans. At present, the clinical utility of this type of information is yet to be totally understood. Dietary adjustment and statin drugs are the preferred agents for lipid-lowering therapy. Apo E genotyping may be used to provide supplemental information.

ApoE testing may also be ordered occasionally to help diagnose type III hyperlipoproteinemia in a person with symptoms that suggest the disorder and to evaluate the potential for the condition in other family members.

In Alzheimer's Disease

ApoE genotyping is also sometimes used as an adjunct test to help in the diagnosis of probable late onset Alzheimer's disease (AD) in symptomatic adults. It is called susceptibility or risk factor testing because it indicates whether there is an increased risk of AD but is not specifically diagnostic of AD. If a patient has dementia, the presence of ApoE4 may increase the likelihood that the dementia is due to AD, but does not prove that it is. There are no clear-cut tests to diagnose Alzheimer's disease during life. Physicians can, however, make a reasonably accurate clinical diagnosis of AD by ruling out other potential causes of dementia and checking for a genetic predisposition to AD with ApoE genotyping as supplemental information in conjunction with Tau/Aß42 testing.

^ Back to top

When is it ordered?

• ApoE genotyping is sometimes ordered when a patient has significantly elevated cholesterol and triglyceride levels that do not respond to dietary and exercise lifestyle changes.
• When family members have ApoE e2/e2 and a doctor wants to see if the person might be at a higher risk for early heart disease.
• When someone has yellowish skin lesions called xanthomas and the doctor suspects Type III hyperlipoproteinemia.

ApoE genotyping is also sometimes ordered as an adjunct test when patients have symptoms of progressive dementia, such as decreasing intellectual ability and language and speech skills, memory loss, and personality and behavioral changes that are starting to interfere with daily living. After non-AD causes, such as overmedication, vascular dementia (caused by strokes), and thyroid disease, have been ruled out, ApoE genotyping may help determine the probability that dementia is due to Alzheimer’s disease.

^ Back to top

What does the test result mean?

People with ApoE e2/e2 alleles are at a higher risk of premature vascular disease, but they may never develop disease. Likewise, they may have the disease and not have e2/e2 alleles because it is only one of the factors involved. ApoE genotyping adds additional information and, if symptoms are present, e2/e2 can help confirm type III hyperlipoproteinemia.

Those who have ApoE e4/e4 are more likely to have atherosclerosis. People who have symptoms of late onset Alzheimer's disease (AD) AND have one or more ApoE e4 copies of the e4 gene are more likely to have AD. However, it is not diagnostic of AD and should NOT be used to screen asymptomatic people or their family members. Many of those who have e4 alleles will never develop AD. Even in symptomatic people, only about 60% of those with late onset AD will have ApoE e4 alleles.

ApoE e3 has "normal" lipid metabolism, thus may not have any genotype impact.

^ Back to top

Is there anything else I should know?

Although ApoE genotyping is being used clinically by Alzheimer's experts, the most it can provide at this time is additional information about a patient with dementia. A definite diagnosis of Alzheimer's disease can only be made by examining a patient's brain tissue after their death.

ApoE genotyping is not available in every laboratory. If your doctor recommends this test, your specimen will need to be sent to a reference lab, and results may take longer to return than they would from a local laboratory.

Alterations in lipid concentrations do not lead directly to vascular disease or atherosclerosis. Other factors, such as obesity, diabetes, and hypothyroidism, also play a role in whether a person actually develops disease.

ApoE4 - The Ancestral Allele | For ApoE4 carriers interested in primal diets and science

ApoE4 - The Ancestral Allele | For ApoE4 carriers interested in primal diets and science

with 2 comments

Googling for the rate of APOE4 among Native Americans, I found this paper on omega-3 fats and ApoE4:

The most recent statistics indicate that dietary intake of omega-3 PUFA is insufficient in >95% of Americans.

Deficits in omega-3s have been shown to contribute to inflammatory signaling, apoptosis, and neuronal dysfunction in all cause dementia, including Alzheimer’s disease.

DHA (22:6[n-3]), specifically, is a critical contributor to cell structure and function in the nervous system, and a recently identified DHA-derived messenger, neuroprotecting D1 (NPD1) has been found to regulate brain cell survival and to promote non-amyloidogenic processing of amyloid precursor protein, thus protecting against Alzheimer’s disease by inhibiting formation of β-amyloid.

Studies utilizing omega-3 supplementation to improve cognitive function in elders, however, have had mixed outcomes, an inconsistency which newly published research indicates is related to ApoE genotype. ApoE ε4 carriers have not been able to benefit from omega-3s. This article discusses why and what can be done to enable carriers of the ApoeE ε4 allele to receive the neuroprotective benefits of omega-3s.

The important thing for us is the dietary recommendations.  Some highlights:

ApoE ε4 carriers are the canaries in the mine of the Western way of life.

Individuals with this genetic heritage cannot afford the “normal” level of dietary and lifestyle insults typical of life in the modern industrialized world because the ApoE ε4 allele magnifies the risks inherent in the Western diet and lifestyle.
…
Despite the disproportionately high prevalence of ApoE ε4, cardiovascular disease and diabetes among Native Americans, and the Pima Indians, specifically, research examining a Native American rural population in nearby New Mexico clearly shows that carrying the ApoE ε4 allele does not increase the risk for any of these conditions in people eating a low fat diet and following an active lifestyle.

Another important point the paper makes is that while O3s provide many benefits, they are also vulnerable to oxidative damage.  Depending on the body’s redox state, O3s can be neurotrophic (good for the brain) or neurotoxic (not so good).

The paper seems to conflate a low fat diet with a plant-centered, unprocessed one.  While it has some great information on omega-3s, it doesn’t have much to answer other key primal / e4 fat questions like whether saturated fats are good (as in primal) or bad (because of differences in lipid metabolism for e4s).

ApoE and HDL, and heart and cerebrovascular disease: LDL-apheresis therapy

Association of ApoE and HDL-C with cardiovascular and cerebrovascular disease:
potential benefits of LDL-apheresis therapy,
Clinical Lipidology, Future Medicine

June 2009, Vol. 4, No. 3, Pages 311-329 , DOI 10.2217/clp.09.21

Review

Association of ApoE and HDL-C with cardiovascular and cerebrovascular disease: potential benefits of LDL-apheresis therapy

Patrick M Moriarty

ApoE forms a lipid–protein complex with HDL-cholesterols (HDL-C) and remnant lipoproteins and is an important regulator of cholesterol and lipid clearance, transport and distribution. In the CNS, ApoE is strictly bound to HDL.

Unlike ApoE2 or ApoE3, the ApoE4 isoform is associated with both coronary artery disease and Alzheimer’s disease. 

HDL-C levels may possess a U-shaped association with vascular diseases and HDL-C size might reflect an alteration in function.

Inflammation plays a key role in coronary artery disease and Alzheimer’s disease.

Elevated inflammatory markers such as C-reactive protein and serum amyloid A are associated with both diseases. Serum amyloid A, similar to ApoE, binds to HDL-C and may alter the lipoproteins size and function.

Familial hypercholesterolemia (FH) is a genetic disorder resulting in elevated plasma levels of LDL-cholesterol (LDL-C), xanthomas and premature coronary artery disease. FH patient’s plasma contains decreased levels of HDL-C with increased levels of ApoE4 and ApoE-bound HDL.

LDL-apheresis therapy lowers LDL-C and is designated for FH patients resistant to pharmacotherapy.

LDL-apheresis also lowers inflammatory HDL-C, ApoE4, and a host of inflammatory markers such as C-reactive protein and serum amyloid A. LDL-apheresis, adjunct to reducing cholesterol, may provide additional benefit to patients with cardiovascular and cerebrovascular diseases.

Full Text

PDF (2053 KB)

PDF Plus (2177 KB)

HDL, apoE4 and Alzheimer’s Disease » Alzheimer's Association | Blog

HDL Cholesterol and Alzheimer’s Disease » Alzheimer's Association | Blog

"This finding further advances the idea that the interplay between cholesterol, cholesterol-carrying proteins such as apoE and HDL, and beta-amyloid may be critical in the development of Alzheimer’s disease. "

HDL, apoE4 and Alzheimer’s Disease » Alzheimer's Association | Blog

According to researchers at Columbia University, people with high levels of HDL cholesterol (the “good” form) are 60 percent less likely to develop AD.

The researchers followed 1,130 seniors with no history of memory loss or dementia and measured their cholesterol levels every 18 months for four years. When the researchers compared the cholesterol levels of study participants with and without Alzheimer’s, they found that those with the highest HDL counts, greater than 55 mg/dL, had about a 60 percent reduced risk of developing the disease compared to those whose levels were less than 39 mg/dL.

Their findings support the theory that high levels of HDL cholesterol are correlated with lower incidence of AD. The study was published earlier this week in the Archives of Neurology and sheds more light on the interactions between cholesterol and AD.

Apolipoprotein E (apoE), as readers of this blog will recall, participates in the mobilization and distribution of cholesterol among various tissues of the body, including the brain. In humans, there are three common isoforms of apoE: apoE2, apoE3 and apoE4.

ApoE4 differs from apoE3, the most common isoform of apoE. A single e4 allele is sufficient to increase the risk of developing atherosclerosis, and also Alzheimer’s disease.

The e4 allele results in slightly elevated plasma LDL cholesterol levels and a small but significant decrease in plasma HDL levels. HDL is one of the major carriers of protein in and out of the brain, and also binds to beta-amyloid.

This finding further advances the idea that the interplay between cholesterol, cholesterol-carrying proteins such as apoE and HDL, and beta-amyloid may be critical in the development of Alzheimer’s disease.

This study has important strengths. It is a prospective cohort study designed for the diagnosis of cognitive decline that has complete clinical and neuropsychological evaluation at each interval.
Guidelines recommend that men raise HDL levels that are less than 40 mg/dL and that women increase HDL numbers less 50 mg/dL. An HDL of 60 mg/dL or higher is optimal.

Michael S. Rafii, M.D., Ph.D.
Associate Medical Director, ADCS Medical Core

This post originally appeared in Alzheimer’s Insights, an ADCS Blog.
* Association of Higher Levels of High-Density Lipoprotein Cholesterol in Elderly Individuals and Lower Risk of Late-Onset Alzheimer Disease. Christiane Reitz et al., Arch Neurol. 2010;67(12):1491-1497.

Alzheimer's Disease Genetics Fact Sheet | National Institute on Aging

Alzheimer's Disease Genetics Fact Sheet | National Institute on Aging

Scientists don't yet fully understand what causes Alzheimer's disease. However, the more they learn about this devastating disease, the more they realize that genes* play an important role in its development. Research conducted and funded by the National Institute on Aging (NIA) at the National Institutes of Health and others is advancing the field of Alzheimer's disease genetics.
*Terms in bold are defined at the end of this fact sheet.

The Genetics of Disease

Some diseases are caused by a genetic mutation, or permanent change in one or more specific genes. If a person inherits from a parent a genetic mutation that causes a certain disease, then he or she will usually get the disease. Sickle cell anemia, cystic fibrosis, and early-onset familial Alzheimer's disease are examples of inherited genetic disorders.
In other diseases, a genetic variant may occur. This change in a gene can sometimes cause a disease directly. More often, it acts to increase or decrease a person's risk of developing a disease or condition. When a genetic variant increases disease risk but does not directly cause a disease, it is called a genetic risk factor.

Alzheimer’s Disease Genetics

Alzheimer's disease is an irreversible, progressive brain disease. It is characterized by the development of amyloid plaques and neurofibrillary tangles, the loss of connections between nerve cells, or neurons, in the brain, and the death of these nerve cells. There are two types of Alzheimer's—early-onset and late-onset. Both types have a genetic component.

DNA, Chromosomes, and Genes Click here for a larger version The nucleus of almost every human cell contains a “blueprint” that carries the instructions a cell needs to do its job. The blueprint is made up of DNA (deoxyribonucleic acid), which is present in long strands that would stretch to nearly 6 feet in length if attached end to end. The DNA is packed tightly together with proteins into compact structures called chromosomes. Normally, each cell has 46 chromosomes in 23 pairs, which are inherited equally from a father and a mother. The DNA in nearly all cells of an individual is identical. Each chromosome contains many thousands of segments, called genes. People inherit two copies of each gene from their parents, except for genes on the X and Y chromosomes, which, among other functions, determine a person's sex. The genes “instruct” the cell to make unique proteins that, in turn, dictate the types of cells made. Genes also direct almost every aspect of the cell's construction, operation, and repair. Even slight changes in a gene can produce a protein that functions abnormally, which may lead to disease. Other changes in genes may increase or decrease a person's risk of developing a particular disease.

Early-Onset Alzheimer's Disease

Early-onset Alzheimer's disease occurs in people age 30 to 60. It is rare, representing less than 5 percent of all people who have Alzheimer's. Some cases of early-onset Alzheimer's have no known cause, but most cases are inherited, a type known as familial Alzheimer's disease (FAD).
Familial Alzheimer's disease is caused by any one of a number of different single-gene mutations on chromosomes 21, 14, and 1. Each of these mutations causes abnormal proteins to be formed. Mutations on chromosome 21 cause the formation of abnormal amyloid precursor protein (APP). A mutation on chromosome 14 causes abnormal presenilin 1 to be made, and a mutation on chromosome 1 leads to abnormal presenilin 2.
Scientists know that each of these mutations plays a role in the breakdown of APP, a protein whose precise function is not yet known. This breakdown is part of a process that generates harmful forms of amyloid plaques, a hallmark of the disease. A child whose mother or father carries a genetic mutation for FAD has a 50/50 chance of inheriting that mutation. If the mutation is in fact inherited, the child almost surely will develop FAD.
Critical research findings about early-onset Alzheimer's have helped identify key steps in the formation of brain abnormalities typical of Alzheimer's disease. They have also led to the development of imaging tests that show the accumulation of amyloid in the living brain. In addition, the study of Alzheimer's genetics has helped explain some of the variation in the age at which the disease develops.
NIA-supported scientists are continuing this research through the Dominantly Inherited Alzheimer Network (DIAN), an international partnership to study families with a genetic mutation that causes early-onset Alzheimer's disease. By observing the biological changes that occur in these families long before symptoms appear, scientists hope to gain insight into how and why the disease develops in both its early- and late-onset forms. In addition, scientists are attempting to develop tests that will enable diagnosis of Alzheimer's before clinical signs and symptoms appear, as it is likely that early treatment will be critical as therapies become available.

Late-Onset Alzheimer's Disease

Most cases of Alzheimer's are the late-onset form, which develops after age 60. The causes of late-onset Alzheimer's are not yet completely understood, but they likely include a combination of genetic, environmental, and lifestyle factors that influence a person's risk for developing the disease.
The single-gene mutations directly responsible for early-onset Alzheimer's disease do not seem to be involved in late-onset Alzheimer's. Researchers have not found a specific gene that causes the late-onset form of the disease. However, one genetic risk factor does appear to increase a person's risk of developing the disease. This increased risk is related to the apolipoprotein E (APOE) gene found on chromosome 19. APOE contains the instructions for making a protein that helps carry cholesterol and other types of fat in the bloodstream. APOE comes in several different forms, or alleles. Three forms—APOE ε2, APOE ε3, and APOE ε4—occur most frequently.

• APOE ε2 is relatively rare and may provide some protection against the disease. If Alzheimer's disease occurs in a person with this allele, it develops later in life than it would in someone with the APOE ε4 gene.
• APOE ε3, the most common allele, is believed to play a neutral role in the disease—neither decreasing nor increasing risk.
• APOE ε4 is present in about 25 to 30 percent of the population and in about 40 percent of all people with late-onset Alzheimer's. People who develop Alzheimer's are more likely to have an APOE ε4 allele than people who do not develop the disease.

Dozens of studies have confirmed that the APOE ε4 allele increases the risk of developing Alzheimer's, but how that happens is not yet understood. These studies also help explain some of the variation in the age at which Alzheimer's disease develops, as people who inherit one or two APOE ε4 alleles tend to develop the disease at an earlier age than those who do not have any APOE ε4 alleles.
APOE ε4 is called a risk-factor gene because it increases a person's risk of developing the disease. However, inheriting an APOE ε4 allele does not mean that a person will definitely develop Alzheimer's. Some people with one or two APOE ε4 alleles never get the disease, and others who develop Alzheimer's do not have any APOE ε4 alleles.
Using a relatively new approach called genome-wide association study (GWAS), researchers have identified a number of genes in addition to APOE ε4 that may increase a person's risk for late-onset Alzheimer's, including BIN1, CLU, PICALM, and CR1. Finding genetic risk factors like these helps scientists better understand how Alzheimer's disease develops and identify possible treatments to study.

Genetic Testing

Although a blood test can identify which APOE alleles a person has, it cannot predict who will or will not develop Alzheimer's disease. It is unlikely that genetic testing will ever be able to predict the disease with 100 percent accuracy because too many other factors may influence its development and progression.
At present, APOE testing is used in research settings to identify study participants who may have an increased risk of developing Alzheimer's. This knowledge helps scientists look for early brain changes in participants and compare the effectiveness of treatments for people with different APOE profiles. Most researchers believe that APOE testing is useful for studying Alzheimer's disease risk in large groups of people but not for determining any one person's specific risk.
In doctors' offices and other clinical settings, genetic testing is used for people with a family history of early-onset Alzheimer's disease. However, it is not generally recommended for people at risk of late-onset Alzheimer's.

Epigenetics: Nature Meets Nurture Scientists have long thought that genetic and environmental factors interact to influence a person's biological makeup, including the predisposition to different diseases. More recently, they have discovered the biological mechanisms for those interactions. The expression of genes (when particular genes are “switched” on or off) can be affected—positively and negatively—by environmental factors, such as exercise, diet, chemicals, or smoking, to which an individual may be exposed, even in the womb. Epigenetics is an emerging frontier of science focused on how and when particular genes are expressed. Diet and exposure to chemicals in the environment, among other factors, throughout all stages of life can alter a cell's DNA in ways that affect the activity of genes. That can make people more or less susceptible to developing a disease later in life. There is some emerging evidence that epigenetic mechanisms contribute to Alzheimer's disease. Epigenetic changes, whether protective, benign, or harmful, may help explain, for example, why one family member develops the disease and another does not. Research supported by the National Institutes of Health continues to explore this avenue. Source: National Human Genome Research Institute, NIH, www.genome.gov The epigenome can “mark” DNA in two ways, both of which play a role in turning genes off or on. The first occurs when certain chemical tags called methyl groups attach to the backbone of a DNA molecule. The second occurs when a variety of chemical tags attach to the tails of histones, which are spool-like proteins that package DNA neatly into chromosomes. This action affects how tightly DNA is wound around the histones.

Research Questions

Discovering all that we can about the role of Alzheimer's disease risk-factor genes is an important area of research. Understanding more about the genetic basis of the disease will help researchers:

• Answer a number of basic questions—What makes the disease process begin? Why do some people with memory and other thinking problems develop Alzheimer's while others do not?
• Determine how risk-factor genes may interact with other genes and lifestyle or environmental factors to affect Alzheimer's risk in any one person.
• Identify people who are at high risk for developing Alzheimer's so they can benefit from new interventions and treatments as soon as possible.
• Focus on new prevention and treatment approaches.

Major Alzheimer's Genetics Research Efforts Underway As Alzheimer's disease genetics research has intensified, it has become clear that scientists need many genetic samples to make further progress. The National Institute on Aging supports several major genetics research programs. The Alzheimer's Disease Genetics Study is gathering and analyzing genetic and other information from 1,000 or more families in the United States with two or more members who have late-onset Alzheimer's. The Alzheimer's Disease Genetics Consortium is a collaborative effort of geneticists to collect and conduct GWAS with more than 10,000 samples from thousands of families around the world with members who do and do not have late-onset Alzheimer's. The Dominantly Inherited Alzheimer Network (DIAN) is an international research partnership studying early-onset familial Alzheimer's disease in biological adult children of a parent with a mutated gene. The National Cell Repository for Alzheimer's Disease (NCRAD) is a national resource where clinical information and DNA samples are stored and made available for analysis by qualified researchers. The participation of volunteers is a critical part of Alzheimer's disease genetics research. The more genetic information that researchers can gather and analyze from a wide range of individuals and families, the more clues they will have for finding additional risk-factor genes. To learn more about the Alzheimer's Disease Genetics Study or to volunteer, contact NCRAD toll-free at 1-800-526-2839 or visit www.ncrad.org.

For More Information

Alzheimer’s Disease Education and Referral (ADEAR) Center
P.O. Box 8250
Silver Spring, MD 20907-8250
1-800-438-4380 (toll-free)
www.nia.nih.gov/alzheimers

The National Institute on Aging's ADEAR Center offers information and publications for families, caregivers, and professionals on diagnosis, treatment, patient care, caregiver needs, long-term care, education and training, and research related to Alzheimer's disease. Staff members answer telephone, email, and written requests and make referrals to local and national resources. The ADEAR website provides free, online publications in English and Spanish; email alerts; a clinical trials database; the Alzheimer's Disease Library database; and more.

Additional information about genetics in health and disease is available from the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health. Visit the NHGRI website at www.genome.gov.

The National Library of Medicine's National Center for Biotechnology Information also provides genetics information at www.ncbi.nlm.nih.gov.

Alzheimer's Association
225 North Michigan Avenue
Floor 17
Chicago, IL 60601-7633
1-800-272-3900 (toll-free)
1-866-403-3073 (TTY/toll-free)
www.alz.org

Alzheimer's Foundation of America
322 Eighth Avenue, 7th floor
New York, NY 10001
1-866-232-8484 (toll-free)
www.alzfdn.org

Glossary

• Allele—A form of a gene. Each person receives two alleles of a gene, one from each biological parent. This combination is one factor among many that influence a variety of processes in the body. On chromosome 19, the apolipoprotein E (APOE) gene has three common forms or alleles: ε2, ε3, and ε4.
• Apolipoprotein E (APOE) gene—A gene on chromosome 19 involved in making a protein that helps carry cholesterol and other types of fat in the bloodstream. The APOE ε4 allele is considered a risk-factor gene for Alzheimer's disease and appears to influence the age at which the disease begins.
• Chromosome — A compact structure containing DNA and proteins present in nearly all cells of the body. Chromosomes carry genes, which direct the cell to make proteins and direct a cell's construction, operation, and repair. Normally, each cell has 46 chromosomes in 23 pairs. Each biological parent contributes one of each pair of chromosomes.
• DNA (deoxyribonucleic acid)—The hereditary material in humans and almost all other organisms. Almost all cells in a person's body have the same DNA. Most DNA is located in the cell nucleus.
• Gene—A basic unit of heredity. Genes direct cells to make proteins and guide almost every aspect of cells' construction, operation, and repair.
• Genetic mutation—A permanent change in a gene that can be passed on to children. The rare, early-onset familial form of Alzheimer's disease is associated with mutations in genes on chromosomes 1, 14, or 21.
• Genetic risk factor—A change in a gene that increases a person's risk of developing a disease.
• Genetic variant—A change in a gene that may increase or decrease a person's risk of developing a disease or condition.
• Genome-wide association study (GWAS)—A study approach that involves rapidly scanning complete sets of DNA, or genomes, of many individuals to find genetic variations associated with a particular disease.
• Protein—A substance that determines the physical and chemical characteristics of a cell and therefore of an organism. Proteins are essential to all cell functions and are created using genetic information.

Alzheimer’s Disease Education & Referral (ADEAR) Center
A Service of the National Institute on Aging
National Institutes of Health
U.S. Department of Health and Human Services

Publication Date: June 2011
Page Last Updated: April 9, 2012

Why Does apoE4 Make Alzheimer's More Likely? - Dana Foundation

Why Does apoE4 Make Alzheimer's More Likely? - Dana Foundation

By Jim Schnabel July 07, 2011
 
For the common form of Alzheimer’s that strikes in old age, the greatest risk factor is, of course, old age. But since 1993, Alzheimer’s researchers have puzzled over another major risk factor, a variant of the apoE gene known as apoE4. Compared with people who have other common apoE variants, those with one copy of apoE4 have three times the Alzheimer’s risk and develop symptoms about five years earlier; while those with two apoE4 copies have roughly 12 times the risk, and develop symptoms about a decade earlier.

How does apoE4 exert such a profound risk-worsening, disease-accelerating effect? It now appears that it does so via multiple biological mechanisms, and this complexity has slowed progress in the field. Moreover, the apoE gene codes for apolipoprotein-E, which carries “lipid” (fat) molecules in the brain, and lipid biochemistry is notoriously tricky. “Many people have been discouraged from pursuing investigations in this area, because it’s technically difficult,” says Thomas Wisniewski, a neurologist and apoE researcher at New York University’s Langone Medical Center.

The good news is that after nearly two decades of research on apoE4’s effects in Alzheimer’s, researchers appear to be zeroing in on the ones that matter, and are getting close to clinical trials of drugs that could reverse those effects.

ApoE4 enhances amyloid beta clumping

In 1992, Wisniewski reported evidence of apoE proteins in a variety of amyloid protein deposits in the brain and the body. He termed apoE a “pathological chaperone protein” because its frequent presence with aggregated proteins suggested that it somehow triggered or enhanced their aggregation.
The year after Wisniewski’s paper appeared, researchers in Allen Roses’s laboratory at Duke University reported that apoE proteins bound tightly to the Alzheimer’s-linked protein amyloid-beta (A-beta), and that Alzheimer’s patients, compared with healthy people of the same age, were much more likely to have the apoE4 variant. In their initial sample, about half of patients were apoE4 carriers, while only about 15 percent of age-matched controls were apoE4 carriers. The apoE4-carrying patients also had heavier A-beta deposits than other patients did, not only in their brain matter but also in cerebral blood vessels.
In lab dish experiments, Wisniewski and others soon determined that apoE somehow made A-beta proteins more likely to stick together in long, plaque-making aggregates called fibrils. “The presence of any apolipoprotein-E tended to enhance fibril formation, and the E4 variant was most likely to do that,” says Wisniewski.
But is that really how apoE4 worsens Alzheimer’s?

ApoE4 impairs microglial clearance of A-beta

As the main transporters of lipids in the brain, apoE proteins perform an important function in bringing these essential building blocks to cell membranes and nerve fibers undergoing development or repair.
“ApoE2 and apoE3 [the other two common variants] form big particles and can accept lots of lipids and are very good transporters of lipids throughout the brain, but apoE4 is not as good at this; it forms smaller particles and it is intrinsically more labile [more likely to be degraded],” says Gary Landreth, an apoE and Alzheimer’s researcher at Case Western Reserve University.
ApoE4’s reduced lipid-carrying efficiency appears to leave the brain broadly more vulnerable to the stresses of aging or acute injuries. Following head traumas and strokes, says Wisniewski, “apoE4 carriers tend to do less well.” This reduced lipid-carrying efficiency also helps to explain why apoE4 is moderately associated with atherosclerosis:  In the bloodstream, apoE4 does a relatively poor job of grabbing fat molecules and transporting them for proper disposal.
But it turns out that apoE, when covered with lipids, also transports A-beta, because A-beta as it aggregates develops a strong affinity for lipids. For A-beta, this ride often ends within microglial cells, where the protein and its aggregates are digested by strong proteolytic enzymes. ApoE4’s reduced lipid-carrying capacity thus seems to translate into a reduced A-beta removal capacity. Landreth’s lab reported this in Neuron in 2008, and showed that when they used a drug to enhance apoE production in aged “Alzheimer’s mice,” the mice lost most of their A-beta plaques and memory deficits.
Landreth’s and also David Holtzman’s laboratories at Washington University are now separately doing further tests in animal models, to try to confirm and optimize this A-beta clearing effect using different classes of apoE-boosting drugs. “This is now the primary focus of our lab,” says Landreth. “And I would argue that the more apoE you have in the brain, the less amyloid you have, and that’s now consistent with a very substantial literature.”

Will more apoE4 only make things worse?

There are other hypotheses about apoE4’s principal role in Alzheimer’s, and according to these, boosting apoE4 levels in an Alzheimer’s patient would make things worse, not better. Berislav V. Zlokovic’s group at the University of Rochester, for example, reported in 2008 that all apoE variants serve to slow down another important clearance process, in which A-beta is pushed out of the brain into the bloodstream. Zlokovic’s group found that apoE4 slowed down this process more than apoE3 did.
“Zlokovic’s really good, and there’s no reason to question that work,” says Landreth. “But we still don’t know what fraction of A-beta clearance occurs through its export into the periphery, through this vascular mechanism, and how much is intrinsic due to this proteolytic degradation in the brain.” In other words, more apoE might still be beneficial on balance through its proteolysis-enhancing effect, even if it also reduces A-beta clearance via the bloodstream.
A more radical hypothesis is that apoE4 is actively toxic, so that boosting it in patients would be disastrous. This hypothesis comes from the Gladstone Institute, affiliated to the University of California–San Francisco, where Karl Weisgraber, Robert Mahley and Yadong Huang anchor one of the longest-running apoE research programs.
In 2001, Huang, then a postdoc in Mahley’s lab, reported that apoE proteins could be split by enzymes within neurons to form toxic fragments, and that apoE4 was more likely than other variants to be rendered into such fragments. Transgenic mice that overproduced these fragments developed significant cognitive deficits and even neurofibrillary tangles—amyloid fibrils made of tau protein, which are a marker of severe neuronal stress—like those seen in Alzheimer’s patients.
More recently, Weisgraber’s lab has found evidence, both from lab-dish and mouse studies, that apoE4 can undergo a structural collapse, and in this abnormal “molten globule” form puts stress on the astrocyte cells that make it—effectively shortening their lifespans, and the lifespans of the neurons they service. “Perhaps with age or a second hit such as ischemia [lack of oxygen due to stroke] or a blow to the head, the astrocytes cannot support neurons any longer, and neurons themselves [under stress] start to produce apoE, and that leads to the production of these toxic fragments,” Weisgraber says.
In collaboration with drug-maker Merck, Weisgraber and his colleagues have found compounds that inhibit the apoE-fragment-producing enzyme, as well as compounds that bind to apoE4 and help keep it in a functional structure more like apoE3’s. Merck shelved the project after a recent merger, but Weisgraber says that his group is now trying to get the rights back and restart preclinical development with independent funding. “These are two viable targets that we are going to push forward,” he says.

Does Estrogen Put the Brakes on Aging in ApoE4-Positive Cells? - AlzForum Alzheimer Research News

Does Estrogen Put the Brakes on Aging in ApoE4-Positive Cells? - AlzForum Alzheimer Research News

	Does Estrogen Put the Brakes on Aging in ApoE4-Positive Cells?

15 February 2013. White blood cells age quickly in post-menopausal women who carry the ApoE4 gene, but estrogen slows that decline, scientists report February 13 in PLoS ONE. Natalie Rasgon at Stanford University School of Medicine, California, and colleagues found that telomeres, the protective chromosome ends that are trimmed during DNA replication, shrank faster in aging women who carry at least one copy of this Alzheimer’s disease (AD) risk allele. Because shorter telomeres reflect more rounds of DNA replication, they are seen as an indirect measure of a cell’s age, though shrinking telomeres can also be an indication of oxidative stress or inflammation. Interestingly, women on hormone replacement therapy maintained longer telomeres. “It appears that ApoE4 carriers are at risk for accelerated cellular aging,” Rasgon told Alzforum. Further, she added, “Women receiving hormone therapy may have [ApoE] genotype-dependent responses.” Some studies point to accelerated cell aging in AD. Patients have shorter white blood cell telomeres (see Honig et al., 2006) that dwindle further as the disease progresses (see Panossian et al., 2003). Other studies suggest this is only true in ApoE4 carriers (see Takata et al., 2012). Rasgon previously reported that post-menopausal women who reported more years—and thus more estrogen exposure—between menarche and menopause have longer telomeres (see Lin et al., 2011), as do women on long-term hormone replacement therapy (see Lee et al., 2005). These were cross-sectional studies. No one had determined if the association holds up over time or if ApoE4 status influences it. First author Emily Jacobs, University of California, San Francisco, and colleagues addressed both questions in a randomized, longitudinal trial. The research team measured the length of telomeres in white blood cells of 63 cognitively healthy, post-menopausal women, all of whom had started hormone replacement therapy at menopause and had been taking it for at least one year—one decade on average. Therapy consisted of either conjugated equine estrogen, estradiol, or either of those plus progesterone. Twenty-four participants carried at least one copy of the ApoE4 allele. Researchers randomized half the participants to continue hormone treatment and the other half to suspend it at the start of the trial. Two years later, the scientists measured the chromosome ends again. ApoE4 carriers lost significantly more base pairs than did non-carriers. When estrogen entered the equation, it further divided the women. On average, those who remained on hormone therapy maintained their telomeres regardless of allele status. However, if the women stopped their treatments, ApoE4 carriers lost an average of 322 base pairs per telomere while non-carriers gained 490. The results imply that ApoE4 speeds up cell aging through loss of telomeres, and that estrogen’s impact depends on allele status. The mechanism of action is still unclear, wrote the authors. The findings jibe with “numerous previous studies [that] suggest ApoE4 promotes oxidative stress and inflammation,” Mark Mattson, National Institute on Aging, Baltimore, Maryland, told Alzforum in an e-mail. Further studies will be needed to determine if the apolipoprotein exerts a direct influence on telomeres, and whether shorter telomeres in immune cells might mediate ApoE4’s effects on the aging brain and other organs, he wrote. Results also indicate that if women with ApoE4 begin estrogen therapy as soon as menopause starts, they may be able to protect themselves against cell decline. “I find that fascinating,” said Phyllis Wise, University of Illinois at Urbana-Champaign. “We are learning that estrogen’s effects are pleiotropic, and this paper suggests they are even broader than we had appreciated before—they can get into the arena of chromosomal structure.” Estrogen’s precise influence on telomeres is unclear, wrote the authors, but previous studies reported that the hormone stimulates expression of part of the telomerase complex and raises the activity of telomerase, the enzyme that adds DNA to telomere ends (see Misiti et al., 2000, and Calado et al., 2009). Estrogen also has antioxidant properties (see Wong et al., 2008), which could indirectly stabilize telomeres. Treatment does not seem to help non-carriers maintain their telomeres. In fact, stopping the treatment actually led to their growth, a phenomenon still under investigation by scientists, wrote the authors. Why might estrogen affect ApoE4 carriers and non-carriers differently? Rasgon and colleagues are unsure, but write that it could relate to ApoE gene expression. A previous study reported that estradiol boosts ApoE translation (see Srivastava et al., 1997). Regardless of the mechanism, these findings may help experts make sense of contradictory evidence about the benefits and dangers of hormone replacement therapy. For example, some studies suggested that it decreases risk for dementia (see ARF related news story), while others indicated the opposite (see ARF related news story). ApoE status seems to define subsets of women who respond differently to estrogen, said Rasgon. “This study moves us a step closer to understanding which groups of women may benefit from hormone therapy versus those who will not.” Because of the small sample size, this study was unable to tease apart whether different types of estrogen replacement have different effects. Rasgon said she plans to address that issue in future work, in addition to testing whether her findings hold up in a larger sample. As of now, results are too preliminary to make clinical recommendations about genetic testing for ApoE4, or about which subgroups should take supplemental estrogen, Rasgon told Alzforum.—Gwyneth Dickey Zakaib. Reference: Jacobs EG, Kroenke C, Lin J, Epel ES, Kenna HA, Blackburn EH, Rasgon NL. Accelerated Cell Aging in Female APOE-ε4 Carriers: Implications for Hormone Therapy Use. PLoS One. 2013;8(2):e54713. Abstract

New clues on how ApoE4 affects Alzheimer's risk, May 16, 2012 News Release - National Institutes of Health (NIH)

NIH-funded research provides new clues on how ApoE4 affects Alzheimer's risk, May 16, 2012 News Release - National Institutes of Health (NIH)

Embargoed for Release
Wednesday, May 16, 2012
1 p.m. EDT

Contact:
Daniel Stimson
301-496-5751

NIH-funded research provides new clues on how ApoE4 affects Alzheimer's risk

Common variants of the ApoE gene are strongly associated with the risk of developing late-onset Alzheimer's disease, but the gene's role in the disease has been unclear. Now, researchers funded by the National Institutes of Health have found that in mice, having the most risky variant of ApoE damages the blood vessels that feed the brain.
The researchers 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 blood vessels. Normally, this barrier allows nutrients into the brain and keeps harmful substances out.
The study appears today in Nature, and was led by Berislav Zlokovic, M.D., Ph.D., director of the Center for Neurodegeneration and Regeneration at the Zilkha Neurogenetic Institute, Keck School of Medicine, University of Southern California, Los Angeles.
“Understanding the role of ApoE4 in Alzheimer's disease may be one of the most important avenues to a new therapy," Dr. Zlokovic said. "Our study shows that ApoE4 triggers a cascade of events that damages the brain's vascular system," he said, referring to the system of blood vessels that supply the brain.
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.
Dr. Zlokovic and his team studied 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 researchers 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, the researchers were able to detect loss of small blood vessels, changes in brain function, and a loss of connections between brain cells.
"The study demonstrates that damage to the brain's vascular system may play a key role in Alzheimer's disease, and highlights growing recognition of potential links between stroke and Alzheimer's-type dementia," said Roderick Corriveau, Ph.D., a program director at NIH's National Institute of Neurological Disorders and Stroke (NINDS), which helped fund the research. "It also suggests that we might be able to decrease the risk of Alzheimer's disease among ApoE4 carriers by improving their vascular health."
The researchers 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.

Destructive enzymes (shown in green) become more active and weaken the blood-brain barrier in mice that are genetically engineered to produce only human ApoE4 (right), rather than mouse ApoE (left). Image courtesy of Robert Bell, Ph.D., University of Rochester Medical Center, New York.

"These findings point to cyclophilin A as a potential new drug target for Alzheimer's disease," said Suzana Petanceska, Ph.D., a program director at NIH's National Institute on Aging (NIA), which also funded Dr. Zlokovic's study. "Many population studies have shown an association between vascular risk factors in mid-life, such as high blood pressure and diabetes, and the risk for Alzheimer's in late-life. We need more research aimed at deepening our understanding of the mechanisms involved and to test whether treatments that reduce vascular risk factors may be helpful against Alzheimer's."
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.
Dr. Zlokovic's study and others point to a complex interplay between beta-amyloid and ApoE4. On the one hand, beta-amyloid is known to build up in and damage blood vessels and cause bleeding into the brain. On the other hand, Dr. Zlokovic's data suggest that ApoE4 can damage the vascular system independently of beta-amyloid. He theorizes that this damage makes it harder to clear beta-amyloid from the brain. Some therapies under investigation for Alzheimer's focus on destroying amyloid plaques, but therapies designed to compensate for ApoE4 might help prevent the plaques from forming, he said.
This research was funded by grant NS034467 from NINDS, and grants AG023084, AG039452 and AG013956 from NIA.
NINDS (http://www.ninds.nih.gov) is the nation's leading funder of research on the brain and nervous system. The NINDS mission is to reduce the burden of neurological disease — a burden borne by every age group, by every segment of society, by people all over the world.
NIA (http://www.nia.nih.gov) leads the federal government effort conducting and supporting research on aging and the health and well-being of older people. NIA provides information on age-related cognitive change and neurodegenerative disease specifically at its Alzheimer's Disease Education and Referral (ADEAR) Center at http://www.nia.nih.gov/Alzheimers.