Is Dementia Inherited by
Dr. Edward Wild & Mrs. Katy Judd, Dementia Research Centre
Dementia may occur in an individual person without anyone else in the family having been affected – this is called “sporadic” dementia. The majority of cases of dementia are sporadic.
Sometimes dementia runs in families and is caused by genetic faults. Dementia that runs in families is called genetic or familial. Familial dementia affects only a small number of families. For example, less than half of all people with frontotemporal degeneration had a parent with a similar illness.
Inheritance
A gene is like a recipe for a protein, and each cell of our bodies contains two copies of each gene, one inherited from each patient. Genetic diseases can occur when there is a mistake (a mutation) in the gene, resulting in a faulty protein. Depending on the protein, and the nature of the mistake, one or both copies of the gene may need to be faulty to cause a disease.
All the mutations discovered so far that cause dementia are “autosomal dominant”, meaning that only one faulty copy of the gene is needed to develop the disease. The risk of passing the faulty copy to children in autosomal dominant disorders is 50% (1 in 2 chance) for each pregnancy. However, with some mutations, for reasons we don’t fully understand, it is possible to have the gene and still not develop signs of the disease.
In frontotemporal lobar degeneration, the genes that have been identified so far that can be tested for are tau and progranulin.
Does it skip a generation?
There is usually a strong family history of the illness and you may know of cousins, aunts/uncles and grandparents who are affected. The disease may appear to skip a generation if a person with the faulty gene dies of another cause before the illness develops.
If I inherit the gene, will I get the disease?
If you have an abnormal gene, you will probably develop the disease if you live long enough. However, having the abnormal gene does not always predict when you will get symptoms, or how rapidly the disease may progress. A minority of people with an abnormal gene may never get symptoms.
Genetic counselling and testing
The medical team will ask about family history of similar illnesses. If they are concerned that the illness may be inherited, they can test for known mutations. Where tests are available, it often takes several months to get a result. The person’s next of kin would be included in the counselling and testing process. Specific consent is needed to perform genetic tests.
If the precise mutation is not known, researchers may be interested in identifying it, and will need DNA from several affected family members. Identifying a new mutation takes many months or years and is not always possible.
If the precise mutation affecting someone is known, it is sometimes possible to test the person’s adult children to see whether they have inherited it, to give an idea of whether they will develop the disease. Being tested is a very difficult decision with no right or wrong answer, and counselling and support is available throughout the process.
If you are thinking about finding out whether you have inherited a faulty gene, please ask your GP or hospital consultant to refer you to a Regional Genetics Clinic. It will be helpful to discuss this with the medical team caring for you first.
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