Alzheimer's Disease Genetics
By Report by the US Dept of Health and Human Services •
August 1, 1997
Genes play a complex and not yet fully understood role in all living things. Their part in Alzheimer's disease (AD) is no exception. The more researchers learn about AD, the more they become aware of the important function genes play in the development of AD. Recent excitement has centered around the discovery of the relationship between the apolipoprotein E (apoE) gene and AD.
Like recipes, genes provide instructions about how to make something, indicating what ingredients go in and in what order. But, the environment (things outside the body like food, the air we breathe, or chemicals we are exposed to) and processes inside the body determine which ingredients are available and in what forms and quantities. Along with environmental influences, genes and processes inside the body combine to do more than just determine eye and hair color and other traits inherited from our parents. For example, genes ensure that we have two hands and can use them to do things, like play the piano. In almost every case, nature (genes) and nurture (including the physical and chemical environment) work together to shape all living things. Genes alone are not all-powerful. Most genes can do little until spurred on by other substances. Although they are necessary in their own right, genes basically wait inside the cell's nucleus (control center) for other molecules to come along and read their messages. Each of these messages is used to build a certain protein. Genes may build a protein correctly or incorrectly, depending on the content of the DNA (deoxyribonucleic acid) message. A gene can produce a faulty protein if it has one or more mutations (defects) in its DNA. Faulty proteins can lead to cell malfunction, disease, and death.
Graphical Representation –- Anatomy of Genes
Shows a cell, mitochondria, the cell membrane, a chromosome, the DNA double helix, DNA chains, linked sequence pairs of bases, paired bases, and the four bases (cytosine, adenine, quanine, and thymine). Within the nucleus of every human cell, two long, thread-like DNA strands encode the instructions for making all proteins needed for life. Each cell holds more than 50,000 different genes found on 46 chromosomes of tightly coiled DNA. Each DNA strand bears four types of coding molecules or bases. The sequence of bases in a gene is the code for making a protein.
Alzheimer's Disease: Not a Single-Gene Disorder
Diseases such as cystic fibrosis, muscular dystrophy, and Huntington's disease are single-gene disorders. If a person inherits the gene that causes one of these disorders, he or she surely will get the disease, unless it is prevented by other means. AD, on the other hand, is not a single-gene disorder. More than one gene mutation can cause AD, and genes on multiple chromosomes are involved. Sometimes, two genes--one from each parent--are needed for a person to get the disorder. The two basic types of AD are familial and sporadic. Familial AD (FAD) is a rare form of AD, affecting less than 10 percent of AD patients. It is associated with gene mutations on chromosomes 1, 14, and 21. FAD is the result of a certain inheritance pattern called autosomal dominant. In this pattern, all offspring in the same generation have a 50/50 chance of developing AD if 1 of their parents had it. FAD occurs in younger people (usually before age 60) than sporadic AD does.
ApoE in Sporadic Alzheimer's Disease
Sporadic AD usually occurs later in life, is far more common than FAD, and appears to be related to the apoE gene found on chromosome 19. ApoE comes in several different forms or alleles, but three occur most frequently. People inherit one allele (apoE2, apoE3, or apoE4) of the apoE gene from each parent. People with both apoE3 and apoE4 alleles (E3/E4) are affected by both alleles. Having one or two copies of the E4 allele increases a person's risk of getting AD. That is, having the E4 allele is a risk factor for AD. But, it does not mean that AD is certain. Having one or two E4 alleles of the apoE gene increases a person's risk of AD, but not to 100 percent. Some people with two copies of the E4 allele (the highest risk group) have not developed the disease, and others with no E4s have. Scientists have yet to determine the exact degree of risk of AD for any given person based on apoE status.
Medical tests are designed for various purposes. Some tests can indicate susceptibility (the risk or likelihood of getting a disease); some help confirm diagnoses, and others assist in planning or monitoring treatment. In an effort to prevent disease, physicians test some people without symptoms to predict who might develop a given medical problem. For people with AD symptoms, doctors try to rule out other disorders and determine, as accurately as possible, what is causing the symptoms. If no other cause is found, AD is diagnosed.
A blood test is available to identify which apoE alleles a person has, because apolipoprotein also is associated with an already well-studied condition, heart disease. However, this blood test cannot tell people whether they will develop AD, or when. Instead of a yes or no answer, the best information a person can get from this genetic assessment for apoE is maybe or maybe not. Although some people want to know whether they will get AD later in life, this type of prediction is not yet possible. In fact, some researchers believe that apoE tests or other screening measures may never be able to predict AD with 100 percent accuracy. In the research setting, apoE testing is a tool that can identify study volunteers who may be at risk of getting AD. In this way, researchers can look for early brain changes. This test also helps researchers compare the effectiveness of treatments for patients with different apoE statuses. Several researchers believe that the apoE test is most useful for studying AD risk in large groups of people and not for determining one person's individual risk. Predictive screening in otherwise healthy people will be useful when effective ways to treat or prevent AD are available.
Concerns About Confidentiality
ApoE testing, and indeed all genetic testing, raises ethical, legal, and social questions for which we have few answers. ApoE information gathered for research purposes generally can be protected by confidentiality laws. On the other hand, information obtained in apoE testing may not be protected as confidential once it is part of a person's medical records. Thereafter, employers, insurance companies, and other health care organizations could gain access to this information; and discrimination could result. For example, employment opportunities or insurance premiums could be affected. Little is known about how stigma associated with an increased risk for AD may affect people's families and their lives.
Public Policy on ApoE Testing
Scientists, ethicists, and other health professionals joined together in October of 1995 to write a public policy statement about the appropriateness of apoE testing and the role of genetic counseling for AD. Discussions leading to the statement took place at a conference in Chicago, Illinois, sponsored by the National Institute on Aging (NIA) and the Alzheimer's Association. The public policy statement supports the use of apoE testing for diagnostic purposes only in conjunction with other tests during medical evaluations of patients who show AD symptoms. It recommends not using apoE testing as a patient screening (predictive) method. Conference participants said that further research and agreement about confidentiality are needed before they will recommend routine apoE testing.
Depending on the study, research volunteers may have the opportunity, during genetic counseling, to learn the results of their apoE testing. The meaning of these results is complex. Since the results of apoE testing can be hard to understand, and more importantly, devastating to those tested, the NIA and the Alzheimer's Association recommend that research volunteers and their families receive genetic counseling before and after testing. People who learn through testing that they have an increased risk of getting AD may experience emotional distress and depression about the future because there is no effective way to prevent or cure the disease. Through counseling, families can learn about the genetics of AD, the tests themselves, and possible meanings of the results. Due to privacy, emotional, and health care issues, the primary goal of genetic counseling is to help people with AD and their families explore and cope with the consequences of such knowledge.
Many questions remain about the usefulness of apoE testing in non-research settings. Some researchers believe that the best use of apoE testing will be as one in a combination of methods for assessing patients (including family history, neurological tests, needs assessments, etc.) to help doctors make informed treatment recommendations. Experts still do not know how limited information about AD risk can benefit people. Among the issues are privacy and confidentiality policies related to genetic information and AD, and the small number of genetic counselors now trained in neurodegenerative disorders. Learning more about the role of apoE in the development of AD may help scientists identify who would benefit from prevention and treatment efforts. Age, still the most important known risk factor for AD, continues to be associated with the disease even when no known genetic factors are present. Research focusing on advancing age may help explain the role that other genes play in most AD cases. For example, recent research suggests that certain alleles of other as yet unidentified genes also may increase risk in late-onset cases. Scores of AD researchers are studying the genetics of AD. In addition, researchers, ethicists, and health care providers are developing policies about the appropriate use of genetic testing and counseling for AD.
Alternate forms of the same gene. Two or more alleles can shape each human trait. Each person receives two alleles, one from each parent. This combination is one factor among many that influences a variety of processes in the body. On chromosome 19, the apolipoprotein E (apoE) gene has three common forms or alleles: E2, E3, and E4. Thus, the possible combinations in one person are E2/2, E2/3, E2/4, E3/3, E3/4, or E4/4.
A gene on chromosome 19 involved in making apoE, a substance that helps carry cholesterol in the bloodstream. ApoE is considered a "susceptibility" gene for AD and appears to influence the age of onset of the disease. However, it is not the sole cause of AD. No cause and effect relationship exists between a person's apoE status and the development of AD.
Rod-like structures in every cell of the human body. Chromosomes carry genes. All healthy people have 46 chromosomes in 23 pairs. Usually, people receive one chromosome in each pair from each parent.
Basic units of heredity that direct almost every aspect of the construction, operation, and repair of living organisms. Every human cell has from 50,000 to 100,000 genes arranged like beads on a string (chromosome). Each gene is a set of biochemical instructions that tells a cell how to assemble one of many different proteins. Each protein has its own highly specialized role to play in the body.
Permanent changes to genes. Once such change occurs, it can be passed on to children. The relatively rare, early-onset familial AD is associated with mutations in genes on chromosomes 1, 14, and 21.
Cells translate genetic information into specific proteins. Proteins determine the physical and chemical characteristics of cells and therefore organisms. Proteins are essential to all life processes.
U.S Department of Health and Human Services
Public Health Service
National Institutes of Health National Institute on Aging
Published in August 1997
Be the first to comment on this article!