What are the genetic changes related to spinocerebellar ataxia type 3 ?
Mutations in the ATXN3 gene cause SCA3. The ATXN3 gene provides instructions for making an enzyme called ataxin-3, which is found in cells throughout the body. Ataxin-3 is involved in a mechanism called the ubiquitin-proteasome system that destroys and gets rid of excess or damaged proteins. The molecule ubiquitin is attached (bound) to unneeded proteins, which tags them to be broken down (degraded) within cells. Ataxin-3 removes the ubiquitin from these unwanted proteins just before they are degraded so that the ubiquitin can be used again. Researchers believe that ataxin-3 also may be involved in regulating the first stage of protein production (transcription).  The ATXN3 gene mutations that cause SCA3 involve a DNA segment known as a CAG trinucleotide repeat. This segment is made up of a series of three DNA building blocks (cytosine, adenine, and guanine) that appear multiple times in a row. Normally, the CAG segment is repeated 12 to 43 times within the gene. Most people have fewer than 31 CAG repeats. In people with SCA3, the CAG segment is repeated more than 50 times. People who have 44 to 52 CAG repeats are described as having an "intermediate repeat." These individuals may or may not develop SCA3. People with 75 or fewer repeats tend to first experience signs and symptoms of SCA3 in mid-adulthood, while people with around 80 repeats usually have signs and symptoms by their teens.  An increase in the length of the CAG segment leads to the production of an abnormally long version of the ataxin-3 enzyme that folds into the wrong 3-dimensional shape. This nonfunctional ataxin-3 enzyme cannot remove ubiquitin from proteins that are no longer needed. As a result, these unwanted proteins, along with ubiquitin and ataxin-3, cluster together to form clumps (aggregates) within the nucleus of the cells. It is unclear how these aggregates affect cell function, because they are found in healthy cells as well as those that die.  Nerve cells (neurons) and other types of brain cells are most affected by mutations in the ATXN3 gene. SCA3 is associated with cell death in the part of the brain that is connected to the spinal cord (the brainstem), the part of the brain involved in coordinating movements (the cerebellum), and other areas of the brain. This condition is also associated with the death of neurons in the spinal cord. Over time, the loss of cells in the brain and spinal cord cause the signs and symptoms characteristic of SCA3.