Defining some relevant terms may be helpful in considering the role of genetics in Alzheimer’s disease (AD).
1. Alleles — 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 biological processes. On chromosome 19, the apolipoprotein E (apoE) gene has three common forms or alleles: E2, E3, and E4. Thus, the possibilities for each person are E2/2, E2/3, E2/4, E3/3, E3/4, or E4/4.
2. ApoE Gene — 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 one’s apoE status and the development of AD.
3. ApoE Genotyping — Testing to find which alleles are present. It involves taking a blood sample and isolating and segmenting DNA from the blood into pieces of specific sizes.
4. Chromosomes — Rod-like structures in every cell of the human body. A cell is the tiny building block that makes up organized tissue and is the fundamental unit of all living things. All healthy people have 46 chromosomes in 23 pairs. Usually, people receive one chromosome in a pair from each parent.
5. Genes — Basic units of heredity that direct almost every aspect of the construction and operation 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 human physiology.
6. Genetic Marker — A tag that denotes a disease and can be passed on or inherited.
7. Genetic Mutations — Permanent changes to genes. Once such change occurs, it can be passed on to children. The relatively rare familial AD is associated with mutations on chromosomes 1, 14, and 21.
Source: Connection Magazine [Volume 5(1), Winter/Spring 1996]