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Many studies have been carried out to identify the factors associated with an increased risk for developing AD. Initial studies focused on those families in which members developed AD at an early age, typically under the age of 60 years. While these early-onset families comprise less than 5% of all cases of AD, they have provided important information about the genetics of AD. Changes in the DNA sequence (termed mutations) in three different genes have been found to cause AD in some of these families. These genes are called amyloid precursor protein (APP), presenilin I (PS1) and presenilin 2 (PS2). The vast majority of cases of AD have an onset after the age of 60 years. These later-onset cases of AD are unlikely to have a mutation in the sequence of the APP, PS1 or PS2 genes. Rather, individuals who develop AD later in life are more likely to have developed the disease due to a combination of environmental and genetic risk factors. A gene, termed apolipoprotein E (ApoE) has been found to be an important risk factor for later onset AD. The APP, PS1 and PS2 genes are all considered to be genes that when altered can cause AD. An important distinction must be made with ApoE. Variation in the DNA sequence of ApoE can increase or decrease the risk that an individual will develop AD. However, it is thought that variation in ApoE alone does not determine whether an individual will develop AD. Therefore, ApoE is often called a susceptibility factor. It is clear that other genes must also play an important role in affecting AD susceptibility. Studies using samples collected and distributed by the National Cell Repository for Alzheimer Disease will help scientists identify the other important genes contributing to AD susceptibility. View information on how to participate in ongoing studies or, for more information about genetics, see the Alzheimer's Disease Education & Referral Center (ADEAR) website.
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In
order to understand the genetics of Alzheimer disease (AD) it is
first necessary to understand the basics of genetics. Most cells
of the body have 46 chromosomes, grouped into 23 pairs. One member
of each chromosome pair is transferred from parent to child during
fertilization. These chromosomes are composed of deoxyribonucleic
acid (DNA) which is the genetic material. Segments or sequences
of DNA consist of genes, which encode many of the characteristics
of each individual, including hair and eye color. Often, the variation
between individuals in these traits is due to variation in the sequence
of the DNA of these genes. 