Causes and Heredity

In Australia, Alzheimers Disease (AD) makes up for approximately 50-70% of cases of dementia, Vascular Dementia (VaD)approximately 20-25% and Dementia with Lewy Bodies (DLB) 10-15%

However, many cases of dementia could be considered as "Mixed dementia" i.e. with features and neuropathological changes of both AD and VaD.

Other causes include: Fronto-temporal dementia, Parkinsons Disease with dementia, Creutzfeldt-Jacob Disease (CJD), Alcohol Related Dementia (Korsakoff’s Syndrome), Huntingtons Disease, and AIDS related dementia.

The cause of most cases of Alzheimer’s disease are unknown. For appr. 2-3 % of the people who have Alzheimer's disease, the cause is genetic.

Hereditary Alzheimers Disease

Today, three genes are known that can be linked with Alzheimer's disease. In each of these genes, up to 50 different mutations have been discovered that may cause Alzheimer's disease. In every family affected by hereditary Alzheimer ’s disease, there is normally only one mutation in a gene that causes the disease.

The three genes are the genes for:

• presenilin 1, which is located on chromosome 14

          (the cause of 50-70 % of hereditary cases)

• presenilin 2, which is located on chromosome 1

(the cause of appr. 10 % of hereditary cases)

• the amyloid precursor protein (APP), located on chromosome 21

(the cause of appr. 3 % of hereditary cases)

Hereditary Alzheimer's disease often begins at an early age.

Predisposed Genes

Various genes can put a person at greater risk of developing Alzheimer's disease without the disease actually being hereditary. These genes are said to cause predisposition to the disease.

The best known of these is the gene for apolipo-protein E. It comes in three different types, numbers 2, 3 and 4. We all have two copies of each gene. Thus, we can have two of the same apolipo-protein E types or a mixture of the three different types. Type Number 2 is partially protective against Alzheimer's disease, while Type Number 4 increases the risk. Type 3 is neutral. It is not possible to make the diagnosis of Alzheimer’s Disease by determining this genetic material, and an examination is normally only done for the presence of the apolipo-protein E gene in conjunction with research studies.

There are also genes on chromosomes 6, 12 and 17 that appear to increase the risk for Alzheimer's disease.

Risk Factors
The lifetime risk increases from 36% to 39% if you have a 1st degree relative with AD. The risk is higher if you have a particular genetic problem (very uncommon).

Gender

Women are at a slightly greater risk for developing Alzheimer's disease than men. That is probably related to the fact that women stop producing estrogen after menopause, whereas men continue to make small amounts of oestrogen throughout life..

Age and Other Risk Factors

Age is the greatest risk factor in developing Alzheimer's disease..

Hardening of the arteries, elevated blood pressure, smoking and elevated cholesterol also contribute to a greater risk for developing Alzheimer's disease. However, none of these factors in themselves can explain the disease.

People with Down syndrome (trisomy 21) are also at greater risk for Alzheimer's disease.