Genetics of dementia

6. Other conditions

Some other conditions that are not forms of dementia themselves can raise a person's risk of developing dementia. This section covers some of these conditions.

Down's syndrome

People with Down's syndrome are at particular risk of developing dementia. This is usually Alzheimer's disease, which can affect as many as 50 per cent of people with Down's syndrome who live into their 60s. This increased risk may be because most people with Down's have an extra copy of chromosome 21. They therefore have an extra copy of the amyloid precursor protein gene (APP) that is found on that chromosome. APP has been linked to the development of Alzheimer's disease.

Huntington's disease

Huntington's disease is a rare progressive hereditary condition caused by a mutation in the HTT gene on chromosome 4. Symptoms include problems with mental abilities, including sometimes dementia, as well as problems with behaviour and movement. Huntington's is inherited in a single-gene mutation pattern. Someone with Huntington's disease therefore has a 50 per cent chance of passing it on to each child.

Huntington's disease was the first single-gene condition for which genetic testing was widely available, in 1994. Affected families are routinely offered genetic counselling - that is, professional advice and support on the issues surrounding inheritance and genetic testing. Guidelines that were first developed for families with Huntington's disease have now been adapted to counselling for genetic dementias (see 'Genetic testing and counselling' below).