For many patients with KMT2B-dystonia, a small part of one chromosome (specifically chromosome 19) is missing or “deleted”. For other patients, the genetic problem is due to a fault or “spelling mistake” in the gene. In either scenario, these genetic changes mean that correct amount of normally functioning protein (known as KMT2B protein) is not made. We do not understand why mutations in KMT2B cause dystonia, but it may be due to this proteins important role in switching on genes that are thought to be important in controlling voluntary movements.
In most cases, the KMT2B mutation is only found in the affected person, and is usually not found on testing the mother and father. This is considered a “de novo” mutation that has occurred at an early stage of development before a baby is born. Rarely, genetic defects in KMT2B are inherited from a parent who may not be affected with dystonia, but carries the gene change without showing symptoms.