Huntington's disease is caused by a faulty gene that runs in families.
Genes and chromosomes
Genes are the instructions for making all parts of the human body and brain. They're made up of DNA and packaged onto strands called chromosomes. We have two copies of all our genes, so our chromosomes are in pairs.
Humans have 46 chromosomes (23 pairs). The faulty gene that causes Huntington's disease is found on chromosome number four.
The normal copy of the gene produces a protein called huntingtin, but the faulty gene contains an abnormal region of what are called CAG repeats. This area is larger than normal and produces a mutant form of huntingtin.
Cells in parts of the brain – specifically, the basal ganglia and parts of the cortex – are very sensitive to the effects of the abnormal huntingtin. This makes them function poorly and eventually die.
The brain normally sends messages through the basal ganglia and cortex to control movement and thinking, as well as motivation. If this part of the brain is damaged, it causes problems with control of movement, behaviour and thinking.
It's still unclear exactly how abnormal huntingtin affects the brain cells and why some are more sensitive than others.
Inheriting Huntington's disease
A parent with the Huntington's disease gene has one good copy of the gene and one faulty copy. Their child will inherit one of these genes. Therefore, there's a 50:50 chance that the child will get the faulty gene and develop Huntington's disease.
However, it's very difficult to predict how old the child will be when they develop the condition if they inherit the abnormal gene, unless it contains a very long CAG repeat (>55).
There's also a 50:50 chance that the child with Huntington's disease will pass the faulty gene on to a child they may have in the future. This pattern of inheritance is called "autosomal dominant".
In around 3% of cases of Huntington's disease, there's no obvious family history of the condition. This could be due to adoption or because relatives with the condition died early from other causes. In rare cases, it's due to a new expansion in the gene.