What is hereditary tyrosinemia type 1 (HT1)?

HT1 is an inherited genetic disorder which occurs due to a deficiency of the enzyme fumarylacetoacetate hydrolase (FAH). An FAH deficiency interferes with the breakdown of an amino acid called tyrosine. Our bodies break down protein into individual amino acids during the typical digestive process.

One such process is the conversion of phenylalanine to tyrosine. But without FAH, people with HT1 cannot break down these amino acids, which leads to abnormal accumulation of tyrosine and its metabolites in the liver. This causes severe health problems if left untreated.

Upon diagnosis, people should be referred to a metabolic specialist for ongoing supervision of their treatment.

Incidence, genetics and how to test for tyrosinemia type 1 (HT1):

HT1 is a rare disorder occurring in about 1 in every 100,000 births in the United States and Europe. HT1 occurs more often in Norway, Finland, and in the province of Quebec, Canada.

HT1 is inherited as a “recessive genetic disorder”, in which both parents are carriers of the disordered gene, although they do not display symptoms of the disorder. With each pregnancy in affected couples, there is a 1 in 4 (25%) chance that their baby will carry the disorder.

A specialist in genetics can help explain the details of the inheritance of HT1 and the risks to future babies if both parents are carriers of the gene. HT1 can be screened through newborn blood spot screening test. This happens via a heel prick in the first week after birth.

Symptoms of heriditary tyrosinemia type 1 (HT1):

If HT1 is not effectively managed symptoms can include: 

• Kidney failure
• Weak and brittle bones (rickets)
• Numbness in the legs or arms
• Breathing difficulties
• Abdominal pain

Management of heriditary tyrosinemia type 1 (HT1):

Although there is currently no cure for HT1. it can be effectively managed with lifelong adherence to both the medication nitisinone (also known as NTBC) and a special diet. The principles of the special diet for HT1 consists of the following:

• Avoidance of foods high in protein such as meat, fish, eggs, cheese, milk, nuts, seeds, bread, pasta and tofu as these foods are naturally high in tyrosine
• A measured intake of tyrosine individually calculated and monitored by your health care professional
• Regular daily consumption of protein substitutes as medically prescribed to ensure requirements for protein and energy are met without providing additional tyrosine and phenylalanine. Sometimes a supplement of additional phenylalanine maybe needed as advised by a health care professional.
• Intake of foods naturally low in protein such as fruit and many vegetables.
• Specially manufactured low protein foods such as pasta, rice, milk replacers can also provide an important source of energy and increase variety in the diet.
• Lifelong monitoring to ensure optimal blood levels of phenylalanine and tyrosine. The medical team will regularly assess blood samples and recommend dietary adjustments accordingly and as nutritional requirements change depending on the life stage.

*There are other types of tyrosinemia (types II and III). These have different, less severe symptoms and different treatment approaches.