GENETICS OF DEMENTIA

Many people with dementia are concerned that they have inherited the condition and that they may in turn pass it on to their children. Also, family members of people with dementia are sometimes concerned that they might be more likely to develop dementia themselves.

salehaThe term ‘dementia’ describes a set of symptoms which include loss of memory, mood changes, and problems with communication and reasoning. These symptoms occur when the brain is damaged by certain diseases, including Alzheimer’s disease and damage caused by a series of small strokes.

Dementia is progressive, which means the symptoms will gradually get worse. How fast dementia progresses will depend on the individual person and what type of dementia they have. Each person is unique and will experience dementia in their own way.

What causes dementia?

There are several diseases and conditions that result in dementia. These include:

  • Alzheimer’s disease − The most common cause of dementia. During the course of the disease the chemistry and structure of the brain change, leading to the death of brain cells. Problems of short-term memory are usually the first noticeable sign.
  • Vascular dementia − If the oxygen supply to the brain fails due to vascular disease, brain cells are likely to die and this can cause the symptoms of vascular dementia.
  • Dementia with Lewy bodies − This form of dementia gets its name from tiny abnormal structures that develop inside nerve cells. Their presence in the brain leads to the degeneration of brain tissue.
  • Fronto-temporal dementia (including Pick’s disease) − In fronto-temporal dementia, damage is usually focused in the front part of the brain. At first, personality and behavior changes are the most obvious signs.

Genes and Inheritance:

Characteristics that we have inherited from our parents are passed down to us in the form of thousands of genes, the basic units of inheritance. Genes are made from DNA and are found packed within each cell of our bodies on structures called chromosomes. We have 23 pairs of chromosomes and we inherit two copies of each gene, one from each parent.

Our individual qualities (e.g. height) reflect the complicated effects of many different genes. Both simple and complex (multi-gene) patterns of inheritance are seen in dementia.

The diseases we may experience are also greatly influenced by our environment and lifestyle, which act to modify the effects of our genetic inheritance.

Genes and Alzheimer’s disease:

Alzheimer’s disease is the most common form of dementia and, of all the main types of dementia, the genetics of Alzheimer’s is the best understood. We can consider the disease to have two forms: the rare early onset Alzheimer’s disease, where first symptoms appear before the age of 65; and the much more common late onset Alzheimer’s disease, where typically the first symptoms develop after this age. These two types of Alzheimer’s disease generally have different patterns of genetic inheritance.

Early onset Alzheimer’s disease:

This form of Alzheimer’s tends to cluster within families, sometimes with several generations affected, in which case it is called familial disease. In some of these cases, early onset Alzheimer’s is caused by mutations in one of three genes. These three genes are the amyloid precursor protein gene (APP) and two presenilin genes (PSEN-1 and PSEN-2). People with any of these extremely rare mutations tend to develop Alzheimer’s disease in their 30s or 40s.

Late onset Alzheimer’s disease:

Late onset Alzheimer’s disease is much more common than early onset Alzheimer’s disease and its inheritance follows a more complex pattern. A small but growing number of genes have now been identified which affect – to different degrees – the chances of developing late onset Alzheimer’s.

The gene with the greatest known influence on the risk of developing late onset Alzheimer’s disease is called apolipoprotein E (APOE). This gene is found on chromosome 19 and comes in three forms, which by convention are named with the Greek letter epsilon (ε):

  • APOE ε2
  • APOE ε3
  • APOE ε4

Genetic testing and Counseling:

Anyone who is worried about inheriting a form of dementia and who has a relative with the condition should speak to their GP. Although scientists are discovering more and more about the genetics of late onset Alzheimer’s disease, there are no approved tests for this condition.

However, if you have more than one family member affected by early onset Alzheimer’s, and particularly if your family members first showed signs of the disease between the ages of 30 and 50, you may be referred to a regional genetics clinic.

Here you will be given more information and an opportunity to discuss the risk to yourself and other family members.