Scientists believe that many factors influence when Alzheimer's disease  begins and how it progresses. The more they study this devastating disease, the more they realize that genes play an important role. Research conducted and funded by the National Institute on Aging (NIA) at the National Institutes of Health (NIH) and others is advancing our understanding of Alzheimer's disease genetics.
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. A single gene can have many variants. Sometimes, this difference in a gene can cause a disease directly. More often, a variant plays a role in increasing or decreasing 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.
Identifying genetic variants may help researchers find the most effective ways to treat or prevent diseases such as Alzheimer's in an individual. This approach, called precision medicine , takes into account individual variability in genes, environment, and lifestyle for each person.
Alzheimer's disease is an irreversible, progressive brain disease. It is characterized by the development of amyloid plaques and neurofibrillary, or tau, tangles;  the loss of connections between nerve cells (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.
Early-onset Alzheimer's disease  occurs in people age 30 to 60 and represents less than 5 percent of all people with Alzheimer's. Some cases are caused by an inherited change in one of three genes, resulting in a typle known as early-onset familial Alzheimer's disease, or FAD. For others, the disease appears to develop without any specific, known cause.
A child whose biological mother or father carries a genetic mutation for early-onset FAD has a 50/50 chance of inheriting that mutation. If the mutation is in fact inherited, the child has a very strong probability of developing early-onset FAD.
Early-onset FAD  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.
Each of these mutations plays a role in the breakdown of APP, a protein whose precise function is not yet fully understood. This breakdown is part of a process that generates harmful forms of amyloid plaques, a hallmark of the disease.
Critical research findings about early-onset Alzheimer's have helped identify key steps in the formation of brain abnormalities typical of the more common late-onset form of Alzheimer's. Genetics studies have helped explain why the disease develops in people at various ages.
NIA-supported scientists are continuing research into early-onset disease through the Dominantly Inherited Alzheimer Network  (DIAN), an international partnership to study families with early-onset FAD. By observing the Alzheimer's-related brain changes that occur in these families long before symptoms  of memory loss or cognitive issues appear, scientists hope to gain insight into how and why the disease develops in both its early- and late-onset forms.
In addition, an NIA-supported clinical trial in Colombia , South America, is testing the effectiveness of an amyloid-clearing drug in symptom-free volunteers at high risk of developing early-onset FAD.
For more information, see NIA's Early-Onset Alzheimer's Disease: A Resource List. 
Most people with Alzheimer's have the late-onset form of the disease, in which symptoms become apparent in the mid-60s and later. 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 affect a person's risk for developing the disease.
Researchers have not found a specific gene that directly causes the late-onset form of the disease. However, one genetic risk factor—having one form of the apolipoprotein E (APOE) gene on chromosome 19—does increase a person's risk. APOE comes in several different forms, or 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 an APOE ε4 allele 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 regions of interest  in the genome (an organism's complete set of DNA, including all of its genes) that may increase a person's risk for late-onset Alzheimer's to varying degrees. By 2015, they had confirmed 33 regions of interest in the Alzheimer's genome.
A method called whole genome sequencing determines the complete DNA sequence of a person's genome at a single time. Another method called whole exome sequencing looks at the parts of the genome that directly code for the proteins. Using these two approaches, researchers can identify new genes that contribute to or protect against disease risk. Recent discoveries have led to new insights about biological pathways involved in Alzheimer's and may one day lead to effective interventions.
A blood test can identify which APOE alleles a person has, but results 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, researchers believe, because too many other factors may influence its development and progression.
Currently, 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 risk.
Genetic testing is used by researchers conducting clinical trials  and by physicians to help diagnose early-onset Alzheimer's disease. However, genetic testing is not otherwise recommended.
Discovering all that we can about the role of Alzheimer's disease genetic risk and protective factors is an important area of research. Understanding more about the genetic basis of the disease will help researchers to:
Alzheimer’s Disease Education and Referral (ADEAR) Center
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. Visit the ADEAR website to learn more about Alzheimer's and other dementias, find clinical trials, and sign up for email updates .
National Human Genome Research Institute
National Library of Medicine
National Center for Biotechnology Information
Alzheimer's Foundation of America
Alzheimer’s Disease Education & Referral (ADEAR) Center
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National Institutes of Health
U.S. Department of Health and Human Services 2016