What causes Alzheimer’s Disease?

Safiya Zaloum

Alzheimer’s disease is a type of dementia. Dementia describes an ongoing decline in brain functioning. It is a syndrome which means it describes a group of symptoms, and some of these symptoms are memory loss, difficulty with language and difficulties doing daily activities. Alzheimer’s is the most common type of dementia – more than 520,000 people in the UK currently have Alzheimer’s disease.

Alzheimer’s disease causes a number of changes in a person, caused by the physical changes in the brain. It is a progressive disease – affecting more and more of the brain over time. The first sign of Alzheimer’s disease is usually problems with memory – losing items around the house or forgetting people’s names for example.

What happens in the brain in Alzheimer’s Disease?

The brain of a person with Alzheimer’s disease looks different to the brain of a person who does not have Alzheimer’s disease. The disease begins in the brain long before the person affected or the people around them notice any symptoms. This is called the prodromal phase, where the disease process is happening in the brain but there are no signs of it, and it is usually only identified in a research setting. The two main processes which scientists have identified in the brain of people with Alzheimer’s are the formation of plaques and tangles. The plaques and tangles accumulate in the brain and stop the nerves from working properly. They spread throughout the brain, affecting more and more areas, leading to the symptoms of the affected person to get worse.

These plaques are made of a protein called beta-amyloid, and they accumulate in the brain. Usually the amyloid precursor protein which is part of nerve cells in the brain, is broken down into smaller soluble proteins that get absorbed in the brain. However, amyloid can be cut into different smaller proteins by different enzymes, leaving a small protein called beta-amyloid that is not soluble. The beta-amyloid clusters together to form plaques and disrupts communication between nerves.

Another protein called tau is also involved in Alzheimer’s disease. Tau usually helps things move around in your cells, but in Alzheimer’s disease it cannot do its job properly and tangles up instead. These tau tangles disrupt the transport systems in the brain. Tau usually helps maintain the structure of neurons and their transport systems, which help to carry nutrients and other materials to cells in the brain. In Alzheimer’s disease, the tau proteins change shape and collapse into tangles.

Both plaques and tangles are toxic to the brain cells, causing them to die. Without the nerve cells and other supporting cells in the brain, we lose some of our functions, depending on which area of the brain is affected.

What are the causes of Alzheimer’s Disease?

It isn’t clear what exactly causes Alzheimer’s disease, but there are some contributing factors. The older you are, the more likely you are to develop Alzheimer’s. If you have a family history of the condition, or are a smoker, are obese or have diabetes, you are also more likely to get Alzheimer’s disease. None of these mean you will definitely get Alzheimer’s disease when you are older, but they all increase your risk.

Some genes are associated with Alzheimer’s disease. A gene called APOE can have a role which contains instructions for a protein that helps to carry cholesterol around the body. APOE2 is thought to have a protective effect against Alzheimer’s, whereas APOE4 increases a person’s risk of getting Alzheimer’s. Most people have APOE3 which neither increases nor decreases your risk. None of these genes means you will or won’t get Alzheimer’s, but they contribute to the chance a person will or will not develop Alzheimer’s.

There are other genes associated with early-onset Alzheimer’s; this variant of Alzheimer’s is strongly linked to your genes. Mutations in the genes APP and PSEN1 and 2 cause production of the abnormal amyloid protein that plays a role in Alzheimer’s disease. As these mutated genes directly cause production of the beta-amyloid, the person is predisposed to develop Alzheimer’s disease earlier in their life. The early-onset Alzheimer’s tends to run in families.

The causes of Alzheimer’s aren’t fully understood, but it is thought to be a combination of environmental factors and genes. Although there is not yet a cure for Alzheimer’s disease, there are some treatments which help those affected. Some drugs are available which help people with Alzheimer’s continue to be able to do their daily activities for longer and make some symptoms better. Other therapies such as art, music, and physical therapy can help people with Alzheimer’s disease to live a better quality of life. Scientists continue to work to learn more about what causes Alzheimer’s and to find more effective treatments and eventually, a cure.