Scientists do not yet fully understand what causes Alzheimer’s disease (AD). However, the more they learn about AD, the more they become aware of the important function genes play in the development of this devastating disease.
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 will usually get the disease. Alzheimer’s disease, on the other hand, is not caused by a single gene. More than one gene mutation can cause AD, and genes on multiple chromosomes are involved.
The two basic types of Alzheimer’s disease are familial and sporadic.
Familial Alzheimer’s disease (FAD) is a rare form of Alzheimer’s disease, affecting less than 10 percent of AD patients. All FAD is early-onset, meaning the disease develops before age 65. It is caused by gene mutations on chromosomes 1, 14, and 21. Even if one of these mutated genes is inherited from a parent, the person will almost always develop early-onset AD. This inheritance pattern is referred to as autosomal dominant inheritance. In other words, all offspring in the same generation have a 50/50 chance of developing FAD if one of their parents had it.
The majority of Alzheimer’s disease cases are sporadic, meaning they have no known cause. Because this type of Alzheimer’s disease usually develops after age 65, it often is referred to as late-onset AD. Sporadic AD shows no obvious inheritance pattern; however, in some families, clusters of cases have been seen. Although a specific gene has not been identified as the cause of sporadic AD, genetics does appear to play a role in the development of this form of AD. Researchers have identified an increased risk of developing sporadic AD related to the apolipoprotein E (apoE) gene found on chromosome 19. This gene codes for a protein that helps carry cholesterol in the bloodstream. ApoE comes in several different forms, or alleles, but three occur most frequently: apoE2 (E2), apoE3 (E3), and apoE4 (E4).
In diagnosing Alzheimer’s disease, apoE testing is not a common practice. The only definite way to diagnose Alzheimer’s disease is by viewing a sample of a person’s brain tissue under a microscope to determine if there are plaques and tangles present.
However, through a complete medical evaluation (including a medical history, laboratory tests, neuropsychological tests, and brain scans), doctors can diagnose Alzheimer’s disease correctly up to 90 percent of the time.