CBD - Causes of corticobasal degeneration

Corticobasal degeneration (CBD) is caused by a progressive loss of brain cells.

It’s one of a group of conditions called '4 repeat tauopathies', characterised by an excess amount of an abnormal protein called tau protein.

In a healthy brain, tau protein is found in low levels, but in CBD the levels increase, forming clumps that are thought to kill nearby brain cells.

This shrinks parts of the brain responsible for movement, speech and higher thought processes, such as planning and understanding.

The symptoms of CBD are similar to those of another condition called progressive supranuclear palsy (PSP). However, in PSP the pattern of brain damage is slightly different. It's possible that both CBD and PSP are two related syndromes that have the same underlying causes.

It's not known what leads to the over-production of the tau protein and the resulting death of brain cells.

Genetic link

One theory is that CBD could be the result of a mutation in the gene responsible for producing the tau protein. This is known as the MAPT gene (microtubule-associated protein tau gene).

A genetic mutation occurs when the instructions found in all living cells become scrambled in some way. This leads to one or more of the body's processes not working as it should.

There's evidence that the genetic mutations affecting the MAPT gene are responsible for conditions with similar symptoms, such as some types of dementia and PSP.

However, there's still no firm evidence to support the theory that mutations in the MAPT gene are responsible for CBD.

Also, as CBD doesn't run in families, it would appear that the condition isn't wholly caused by the genes a person inherits from their parents.

One or more environmental factors may trigger CBD in people who are susceptible to it.

Possible factors could include:

a virus or other type of infectious agent, which may slowly infect the brain over many years
an unidentified neurotoxin (poison that damages the brain and nerve cells) that may be present in the environment