Tyrosinaemia type 3

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Tyrosinaemia is a group of genetic disorder characterised by different disruptions in the multistep process that breaks down the amino acid tyrosine, a building block of most proteins. If untreated, tyrosine and its by-products build up in tissues and organs, which can lead to serious health problems.

Tyrosinaemia type 3 is caused by faults in the HBD gene. This gene is responsible for creating the enzyme 4-hydroxyphenylpyruvate dioxygenase, one of the enzymes required for the multi-step process that breaks down tyrosine. As a consequence, tyrosine concentrations increase largely.

Tyrosinemia type 3 is the least frequent form of Tyrosinaemia with less than 20 cases reported. However, the current treatment of another disorder in tyrosine metabolism, Tyrosinemia type 1 (1:100.000), involves the medical product NTBC. NTBC is a strong inhibitor of the HBD enzyme, which is proximal of the real enzymatic defect in Tyrosinemia type 1. In this way treatment with NTBC creates a pseudo-Tyrosinemia type 3.

Only a few cases of Tyrosinemia type 3 have been reportedMost of the reported patients presented with developmental delay, cognitive problems and/or behavioural problems such as ADHD.

In addition some patients have been reported to present with neurological symptoms during infancy, however it is still not completely sure if these symptoms are really the result of Tyrosinemia type 3:

• Seizures, small head size (microcephaly)
• Lack of muscle control (ataxia)
• Involuntary shakiness or quivering (tremor)
• And/or weak muscles (hypotonia)

Unfortunately, all these symptoms are not really specific and your child may show or have shown none of these symptoms.

Tyrosinaemia is diagnosed based on blood tests and urine tests

There is no cure for Tyrosinaemia Type 3, treatment is lifelong and mostly includes:

• special diet and medication:
• Limited high protein foods
• Measured amounts of tyrosine (protein) containing foods
• A protein substitute. Sometimes extra phenylalanine is needed
• Low protein foods

These foods are high in tyrosine (protein) and must be avoided:

• meat
• fish
• eggs
• cheese
• bread
• pasta
• nuts
• seeds
• soya
• Quorn
• tofu

In babies, a restricted amount of tyrosine (protein) is given from breast milk or measured amounts of infant formula. The amount given will be monitored regularly by your specialist metabolic dietitian.

A protein substitute, with all necessary parts of protein except for, is essential for metabolic control. In this way, it will help to meet your baby’s protein, energy, vitamin and mineral requirements. It is available on prescription.

There are many low protein foods. These include fruit, many vegetables and low protein foods such as bread and pasta which are available on prescription. They provide a source of energy and variety in the diet.

Tyrosinaemia type 3 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 Tyrosinaemia type 3 is inherited. They can also tell you about genetic testing for the rest of your family.

References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing contact@metabolicsupportuk.org [Resource Library No: TAP022].