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 huntington affects the brain cells and why some are more sensitive than others.