Alzheimer’s disease is a neurodegenerative illness impacting nearly 6 million Americans and 24 million people globally.
Is Alzheimer’s genetic?
There is no exact gene that is responsible for Alzheimer’s disease that is known about yet. It’s likely, however, that a combination of genetic and environmental factors contribute to the likelihood of developing Alzheimer’s. It’s true that family history of the disease makes it more likely that a person will also develop it.
Increasingly, scientists are finding correlations between markers in the genetic code and the likelihood of developing Alzheimer’s disease. There are two types of Alzheimer’s—early-onset and late-onset. According to the National Institute of Aging (NIA) in the United States, both types could have a genetic component.
Most people with Alzheimer’s have the late-onset form of the disease, in which symptoms become apparent in their mid-60s and later. To date, scientists have not found a silver bullet – or rather – one specific gene that directly causes late-onset Alzheimer’s disease. However, the NIA notes that having a genetic variant of the apolipoprotein E (APOE) gene on chromosome 19 does increase a person’s risk. The APOE gene is involved in making a protein that helps carry fats, including cholesterols, in the bloodstream. Characterization of the gene has revealed that APOE comes in several different alleles (different forms). Each person inherits two APOE alleles, one from each biological parent. The NIA notes the following about each allele of APOE:
- The allele APOE ε2 is rare. It’s thought that this version of APOE 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 APOE ε4 gene.
- APOE ε4 increases risk for Alzheimer’s disease and is also associated with an earlier age of disease onset. Having one or two APOE ε4 alleles increases the risk of developing Alzheimer’s. About 25 percent of people carry one copy of APOE ɛ4, and 2 to 3 percent carry two copies.
- While APOE ε4 is called a risk-factor gene because it increases a person’s risk of developing the disease, 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.
- APOE ε3 is the most common variant of the gene. It is not known to influence Alzheimer’s.
The more rare type of Alzheimer’s disease is called Early-onset Alzheimer’s disease. It represents less than 10 percent of all people with the disease. It typically occurs between a person’s 30s and mid-60s. Some cases are caused by an inherited change in one of three genes. According to the NIA, the three single-gene mutations associated with early-onset Alzheimer’s disease are:
- Amyloid precursor protein (APP) on chromosome 21
- Presenilin 1 (PSEN1) on chromosome 14
- Presenilin 2 (PSEN2) on chromosome 1
Mutations in these genes result in the production of abnormal proteins that are associated with the disease. Each of these mutations plays a role in the breakdown of APP – the breakdown of which is part of a process that generates harmful forms of amyloid plaques, a hallmark of Alzheimer’s disease.
The NIA says that a child whose biological mother or father carries a genetic mutation for one of these three genes has a 50/50 chance of inheriting that mutation. Once the mutation is inherited, the person is likely to develop early-onset Alzheimer’s disease.