We tend to resemble our parents because we inherit characteristics such as height and eye colour through the genes that they pass on. Many people become concerned that they may have inherited the gene for a disease, and that they may pass it on to their children. Alzheimer's disease is one of many conditions where there is a genetic factor in a small number of families. This information sheet outlines the present state of knowledge about inherited risk.
| Summary | Genes and inheritance |
| Alzheimer's disease genes | Early onset Alzheimer's disease |
| Later onset Alzheimer's disease | What are the pros and cons of genetic testing ? |
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The genetic factors associated with Alzheimer's disease can be summarised as follows:
The basic material of inheritance, DNA, is passed on in the form of genes, and delivered in packages called chromosomes, which are long chains of genetic instructions. We can think of DNA as being the letters of the alphabet, genes as the words they make up and chromosomes the sentences which convey meaning.
We each inherit 23 pairs of chromosomes, one half of each pair from our mother and the other from our father. There are literally millions of combinations of genes we can inherit, and the effect of each gene is not yet known, although scientists worldwide are rapidly expanding our knowledge.
Alzheimer s disease runs in a small number of families, where up to half of the family members are at risk.
Alzheimer's disease is common amongst elderly people, so having one close relative with the disease is not evidence of a family link. Even where there are two close relatives with the disease it is likely to have occurred by chance.
If you have three or more close relatives who developed Alzheimer s disease at an early age, then your doctor will be able to counsel you about genetic testing for familial Alzheimer s disease and refer you to a geneticist where appropriate.
All these genetic faults are associated with early onset of Alzheimer's disease, usually between ages of 35 and 60.
On average half the children of someone with one of these genetic defects inherits it, and probably all those who inherit the gene develop the disease. Those who do not inherit cannot pass on the genetic faults. They do not skip a generation.
Most cases of Alzheimer's develop later in life. Below the age of 65 the risk is approximately 1 in 1000. Over the age of 65 it affects one person in 50. The risk rises to one person on five by the age of 80
There is also a genetic link with many later cases, weaker than the link described above, but not confined to a few families.
The link is with a protein called apolipo-protein E (ApoE), which we all have in the blood and the brain. It comes in three forms, known as ApoE2, ApoE3 and ApoE4, all found in the general population. We each have two copies of the gene, which may be the same as each other or different.
The ApoE risk is very different from familial Alzheimer's. ApoE4 increases the chances of the disease, but does not make it certain. Some other factor, not yet understood, must also contribute.
It is likely that there are many other genes that might contribute to increasing risk of Alzheimer's disease. Genes linked by recent research to late onset Alzheimer's disease include certain apolipoprotein receptors, the serotonin transporter, a small change in the presenilin-1 gene, butyrylcholinesterase K, the HLA gene and a region on chromosome 12.
Much more research remains to be done before geneticists can be sure that these genes are risk factors for Alzheimer's disease and in any case it is quite likely that the degree of risk involved will be individually quite small.
A genetic test for Alzheimer's could:
However, population-wide screening would create problems for individuals and for society.
If you have three or more close family members with early onset of dementia, you may want to be referred to a specialist centre for genetic testing. Your GP will counsel you and help to assess your risk. You should discuss the issues carefully before making a decision.
October 1997
Page Text supplied by The Alzheimer's Disease Society of Great Britain