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What is it?

Alzheimer‘s disease (AD) is an irreversible form of dementia characterised by memory loss, a progressive decline in intellectual ability, deteriorating language and speech skills, and by personality and behavioural changes that eventually interfere with daily living. According to Alzheimer's Australia there are over 320,000 people with dementia in Australia and it is predicted that this will grow to almost 900,000 by 2050. Approximately 55% of these will have Alzheimer's disease.

Although AD mimics some changes found in the brain as we age, it is not a normal part of the aging process. It causes nerve cell injury and death and is characterised by the build-up of senile plaques and neurofibrillary tangles (twisted protein fragments that clog nerve cells) in the brain. The destruction of nerve cells also decreases levels of acetylcholine and other neurotransmitters (chemicals necessary for communication between nerve cells) in the brain. Over time, AD results in decreased interaction between different areas of the brain.

Relationship with ageing

The risk of having AD and other dementias increases greatly with age. About 10% to 12% of the population will have dementia by the time they are over 65 years old, with the risk increasing to 50% for those who reach the age of 100. Most of the time AD is ‘late onset,’ beginning after the age of 65, and sporadic (does not seem to be family-related). ‘Early onset’ AD, starting before the age of 65, is more rare and more likely to be family-related. (It accounts for about 5% to 10% of all AD cases).

Genetic relationship

Alzheimer‘s disease appears to be caused by a variety of factors; although many of the factors are not yet well understood, some have a genetic component. We know that there are a few rare cases, associated with early onset Alzheimer‘s disease, where there is a clear genetic inheritance within specific families. These are families where members inherit genetic faults on chromosomes 21, 14 or 1. In these cases half of the children of the person with Alzheimer‘s disease are likely to have the genetic defect. All those who inherit the gene will go on to develop Alzheimer‘s disease.

Another gene, ApoE, has been associated with an increased susceptibility to early onset AD. This gene directs the production of apolipoprotein E, a protein that forms part of the body’s lipoproteins (such as HDL cholesterol) and is involved in lipid transportation and clearing dietary fats from the body. The ApoE gene normally exists in three forms: e2, e3 and e4. Everyone has two copies of the ApoE gene, in some combination of the three forms. ApoE e4 has been associated with an increased risk of AD.

There is a higher incidence of Alzheimer’s disease in those with Down syndrome and in families who have siblings and/or parents with AD and/or several generations of family members with AD. Researchers have been aware of a connection between Down syndrome and Alzheimer‘s disease since the 1940s. People with Down syndrome, who inherit an extra copy of chromosome 21, develop the same 'plaques' and 'tangles' in their brains as people with Alzheimer‘s disease.

Last Review Date: November 6, 2017

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