Tyrosine Hydroxylase Deficiency

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Tyrosine hydroxylase is an autosomal recessive condition. It is believed that this condition is caused by mutations to the TH gene. This gene is responsible for providing the body with instructions to make the enzyme Tyrosine hydroxylase, this is an important enzyme that helps with the functionality of the nervous system and breaking down substances to produce new substances. This enzyme in particular is important in producing the family of hormones called catecholamines made by your adrenal glands on the top of your kidneys, one example of these hormones is dopamine. Tyrosine is an amino acid; these amino acids can be built up by the body into proteins as well as being utilised in other ways. Tyrosine can be converted to dopamine; this molecule transmits signals to the body to influence muscle movement and emotional behaviour. Dopamine can then produce other hormones such as Norepinephrine and Epinephrine, these have a variety of functions in the nervous system.

A mutation to the TH gene means that tyrosine cannot be broken down by the enzyme or  that tyrosine can be broken down into tyrosine in smaller quantities, this impacts the level of  dopamine, norepinephrine and epinephrine produced, furthermore less production of these hormones will impact emotions and movement in your body. Symptoms for this condition can vary, they can range from mild to quite severe.

Tyrosine Hydroxylase deficiency was first described in medical literature in 1970. Since then, fewer than 50 cases have been reported. It is unclear if this condition is more common amongst males or females or with specific ethnicities.

Symptoms for this condition can vary and they can be quite specific to the individual diagnosed with this condition. There are three types of this condition, in the severe form, symptoms will be expressed from early infancy, in moderate and mild cases, symptoms may not present until later childhood. Some of the more common signs and symptoms of this condition are:

Mild subgroup:

• Abnormal gait [walking]
• Lack of coordination
• Tight/stiff leg muscles
• Tremors when holding objects

Symptoms may become worse as the day goes on; this is known as diurnal fluctuation. Without treatment, children may rely on a wheelchair for mobility as this condition progresses and impacts your motor function [muscle movement].

Moderate subgroup:

• Abnormal gait [walking]
• Tendency to walk on tip toes
• Involuntary muscle spasms
• Delays in speech development
• Abnormal eye movement

Children are sometimes referred to as having ‘infantile parkinsonism’ as symptoms tend to reflect those of Parkinson’s disease.

Severe subgroup:

• Poor control of voluntary muscle movement
• Increased muscle tone [hypertonia]
• Increased heart rate
• Increased blood pressure
• Increased sweating
• Inability to control bowels and bladder
• Floppy head due to muscle weakness [hypotonia]
• Drooping of eye lids
• Difficulty swallowing
• Constipation
• Behavioural problems
• Abnormal brain function
• Lethargy
• Mental health conditions such as depression and anxiety

A diagnosis of this condition is made through an evaluation of the individual and the presentation of their symptoms, analysing the cerebrospinal fluid through a lumbar puncture, this is carried out to detect elevated or depleted levels of substances that may be circulating in this fluid associated with this condition. A lumbar puncture involves inserting a small needle into an area on your neck or spine to drain some of the fluid for analysis, this has been described as slightly painful however it is worth baring the pain if possible as this is a good way to confirm diagnosis.

Another method is to diagnose this condition through genetic testing, this would be carried out if it is believed that you have this condition or if you have a family history of this condition.

Tyrosine Hydroxylase deficiency is managed by replacing the dopamine lost through the error in metabolism. Infants are generally treated with low levels of a medication called Levodopa. This drug is capable of crossing the blood brain barrier after ingestion and being absorbed by the gut in the brain, Levodopa can easily be converted to dopamine by the body. This drug may be given in duel therapy, commonly this second drug is Carbidopa, this is to effectively ensure that enough of these medications can cross the blood brain barrier and be converted into dopamine. Treatment does vary person to person and your health care professional will help to establish the best line of treatment for you and what dose is needed.

Tyrosine Hydroxylase deficiency can only be passed on to a child if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. A person who has a copy of the faulty gene is known as a carrier.

If both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.

If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.

Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inheritedTyrosine Hydroxylase deficiency. They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.

If you have previously had a child with Tyrosine Hydroxylase deficiency and go on to have further children, their New-born Screening test should be carried out within 24-48 hours of birth.

References are available on request. Please contact us on 0800 652 3181 or by emailing contact@metabolicsupport.org.

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This information about metabolic diseases is provided by Metabolic Support UK and is intended for educational purposes only. It should not be used for diagnostic or treatment purposes.

Should you require more detailed information please contact Metabolic Support UK by email (contact@metabolicsupport.org) or by telephone ( 0800 652 3181). For specific medical information regarding a disorder or individual please contact your GP or Paediatrician. Metabolic Support UK accepts no responsibility for any errors or omissions nor does Metabolic Support UK assume any liability of any kind for the content of any information contained within this summary or any use that you may make of it.