The TH gene provides instructions for making the enzyme tyrosine hydroxylase, which is important for normal functioning of the nervous system. Tyrosine hydroxylase takes part in the first step of the pathway that produces a group of hormones called catecholamines. This enzyme helps convert the protein building block (amino acid) tyrosine to a catecholamine called dopamine. Dopamine is also known as a neurotransmitter because it transmits signals between nerve cells in the brain to help control physical movement and emotional behavior. Other catecholamines called norepinephrine and epinephrine are produced from dopamine. Norepinephrine and epinephrine are involved in the autonomic nervous system, which controls involuntary body processes such as the regulation of blood pressure and body temperature.
More than two dozen mutations in the TH gene have been found to cause dopa-responsive dystonia. This condition is characterized by a pattern of involuntary muscle contractions (dystonia), tremors, and other uncontrolled movements and usually responds to treatment with a medication called L-Dopa. Most TH gene mutations that cause this condition change single protein building blocks (amino acids) in the tyrosine hydroxylase enzyme, resulting in a decrease in functional enzyme. A reduction in normal tyrosine hydroxylase enzyme leads to a decrease in the production of dopamine, which causes the movement problems characteristic of dopa-responsive dystonia. The amount of functional enzyme that is produced is associated with the severity of the signs and symptoms. Less functional enzyme leads to more severe symptoms.
More than 20 mutations in the TH gene have been identified in people with tyrosine hydroxylase (TH) deficiency. These mutations result in reduced activity of the tyrosine hydroxylase enzyme. As a result, the body produces less dopamine, norepinephrine, and epinephrine. These catecholamines are necessary for normal nervous system function, and changes in their levels contribute to the abnormal movements, nervous system dysfunction, and other neurological problems seen in people with TH deficiency.
Dopa-responsive dystonia is sometimes considered a mild form of tyrosine hydroxylase deficiency. It is uncertain whether they are two separate disorders or part of the same disease spectrum.
Certain common TH variations (polymorphisms) modify catecholamine production, which affects the risk of developing conditions associated with regulation of the autonomic nervous system. These TH gene polymorphisms affect the extent to which blood pressure increases with stress and may increase the risk of high blood pressure (hypertension).
One TH gene polymorphism has been associated with sudden infant death syndrome (SIDS). SIDS is a major cause of death in babies younger than 1 year. It is characterized by sudden and unexplained death, usually during sleep. The polymorphism, called allele *9.3, is the most common TH gene polymorphism among people of European descent and has been identified in a larger percentage of babies who die from SIDS than in other babies. This version of the gene may affect the regulation of breathing or awakening in infants.
- tyrosine 3-monooxygenase