Tyrosinemia

Tyrosinemia

Overview

In tyrosinemia, the body doesn't have an enzyme it needs [called fumarylacetoacetate hydrolase (FAH)] to metabolize tyrosine. Metabolism is a process in which our bodies break down substances as we use them for energy; in this case tyrosine. Tyrosine is an amino acid that is found in most proteins. When people with tyrosinemia break down protein, abnormal toxic break down products of tyrosine build up in their bodies. This causes progressive damage to the liver and kidneys, but mainly the liver. This is because the liver is normally the primary place tyrosine is metabolized.

Tyrosinemia is hereditary; in order to have the disease, a child must get a mutation in the gene for tyrosinemia from each parent. In families where both parents carry a mutation, there is a one in four risk that a child will have tyrosinemia. There is now a genetic test available, so that couples at high risk of being carriers can determine their risk of having a child with tyrosinemia. This is a very rare disease; only about one person in 100,000 has it.

Tyrosinemia type I is a rare autosomal recessive genetic metabolic disorder characterized by lack of the enzyme fumarylacetoacetate hydrolase (FAH), which is needed for the final break down of the amino acid tyrosine. Failure to properly break down tyrosine leads to abnormal accumulation of tyrosine and its metabolites in the liver, potentially resulting in severe liver disease. Tyrosine may also accumulate in the kidneys and central nervous system.

Symptoms and physical findings associated with tyrosinemia type I appear in the first months of life and include failure to gain weight and grow at the expected rate (failure to thrive), fever, diarrhea, vomiting, an abnormally enlarged liver (hepatomegaly), and yellowing of the skin and the whites of the eyes (jaundice). Tyrosinemia type I may progress to more serious complications such as severe liver disease, cirrhosis, and hepatocarcinoma if left untreated. Treatment with nitisinone and a low-tyrosine diet should begin as soon as possible after the diagnosis is confirmed.

Symptoms

Symptoms associated with tyrosinemia type I often vary greatly from person to person. Infants with tyrosinemia type I typically present with either the acute or chronic form of the disorder.

The acute form of tyrosinemia type I is present at birth (congenital) or during the first months of life. This form of the disorder is more common and severe than the chronic form. Infants with the acute form exhibit rapid onset of symptoms usually beginning with failure to gain weight and grow at the expected rate (failure to thrive). Additional early symptoms include fever, diarrhea, bloody stools (melena), and vomiting. Affected infants may also exhibit an abnormally enlarged liver (hepatomegaly), a tendency to bruise easily, jaundice, lethargy, and/or irritability. Some affected infants may develop a distinctive, cabbage-like odor.

Eventually infants with the acute form of tyrosinemia type I experience developmental delays, an abnormally enlarged spleen (splenomegaly), and accumulation of fluid (edema) in the abdomen (ascites). The disorder may rapidly progresses to acute life-threatening liver failure and blood clotting abnormalities (coagulopathy).

The chronic form of tyrosinemia type I occurs less frequently than the acute form and is characterized by a more gradual onset and less severe expression of the symptoms. Symptoms of tyrosinemia type I may not become apparent in infants with the chronic form of the disorder until after six months of age. Failure to thrive is often the first symptom. Additional symptoms include developmental delays and progressive scarring and impaired function (cirrhosis) of the liver resulting in chronic liver failure.

Many infants with tyrosinemia type I develop kidney (renal) abnormalities such as renal Fanconi syndrome, a rare disorder characterized by kidney dysfunction that often leads to progressive softening and weakening of the bone structure (rickets). Fanconi syndrome is also associated with episodes of vomiting, dehydration, weakness, and fever.

Approximately 40 percent of affected infants also experience episodes of disease affecting many nerves (polyneuropathy) often following a minor infection. These episodes, which may be referred to as neurological crises, are associated with severe pains in the legs and stomach, increased muscle tone (hypertonia), vomiting, obstruction of the intestines (ileus), an irregular heartbeat (tachycardia), and high blood pressure (hypertension). Some affected individuals may also exhibit self-mutilating behavior (e.g., biting one’s tongue or grinding the teeth) during these episodes. Neurological crises and respiratory failure may occur.

Affected infants may also experience enlargement (hypertrophy) of the partition that separates the left and right ventricles of the heart and, in some children, of the left ventricular wall (hypertrophic cardiomyopathy). In addition, affected infants and children are at a greater risk than the general population to develop a form of liver cancer known as hepatocellular carcinoma.

Treatment of affected children with nitisinone and a low-tyrosine diet has improved survival to over 90% and resulted in normal growth, improved liver function, prevention of cirrhosis, correction of kidney disease and improvement in rickets.

Causes

Tyrosinemia is caused by mutations in the fumarylacetoacetate hydrolase (FAH) gene that is responsible for the production of the FAH enzyme. Deficiency of this enzyme leads to an accumulation of fumarylacetoacetate and accumulation of tyrosine and its metabolites in the liver, kidney, and central nervous system eventually causing tyrosinemia type I.

Tyrosinemia type I is inherited as an autosomal recessive genetic condition.

Recessive genetic disorders occur when an individual inherits two copies of an abnormal gene for the same trait, one from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.

Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.

Affected populations

Tyrosinemia type I affects males and females in equal numbers. The prevalence has been estimated to be 1 in 100,000 to 120,000 births worldwide. In Quebec, Canada, the birth prevalence is estimated to be 1/16,000. The estimated prevalence in the Saguenay-Lac Saint-Jean region of Quebec is one in 1,850 births. In Norway, the birth prevalence is estimated to be 1 in 60,000 births.

Disorders with Similar Symptoms

Signs and symptoms of the following disorders can be similar to those of tyrosinemia type I. Comparisons may be useful for a differential diagnosis:

Acute intermittent porphyria (AIP) is one of a group of inherited metabolic disorders known as the porphyrias. AIP is characterized by the deficiency of the enzyme porphobilinogen deaminase (PBG-D), also known as uroporphyrinogen I-synthase. Symptoms may include severe abdominal pain, nausea, vomiting, and constipation. Neurological symptoms similar to tyrosinemia type I may also be present including pain in arms and legs, muscle weakness, rapid heart rate, and increased blood pressure. Affected individuals may also experience hallucinations and seizures. AIP is inherited as an autosomal recessive genetic condition (For more information on this disorder, choose “acute intermittent porphyria” as your search term in the Rare Disease Database.)

Galactosemia is a rare, inherited disorder of carbohydrate metabolism that affects the body’s ability to convert galactose (a sugar contained in milk, including human mother’s milk) to glucose (a different type of sugar). Galactose is converted to glucose by a series of three enzyme reactions. Galactosemia is caused by a deficiency of galactose-1-phosphate uridyl transferase enzyme which is vital to this process. Infants with galactosemia appear normal at birth, but within a few days or weeks lose their appetites (anorexia) and start vomiting excessively. Yellowing of the skin and the whites of the eyes (jaundice), enlargement of the liver (hepatomegaly), failure to gain weight and grow at the expected rate (failure to thrive), and, ultimately, accumulation of fluid in the abdominal cavity (ascites) and other swelling (edema) may also occur. Galactosemia is inherited as an autosomal recessive genetic condition. (For more information on this disorder, choose “galactosemia” as your search term in the Rare Disease Database.)

Hereditary fructose intolerance (HFI) is a rare, inherited inability to digest fructose (fruit sugar) or its precursors (sugar, sorbitol and brown sugar). The disorder develops because of a deficiency or lack of the enzyme fructose-1-phosphate aldolase, resulting in an accumulation of fructose-1-phosphate in the liver, kidney, and small intestine. Infants with HFI usually develop a strong dislike for sweets and fruit. Soon after fructose is added to the diet of an infant with HFI, symptoms usually become apparent. These may include prolonged vomiting, failure to thrive, and yellowing of the skin and the whites of the eyes (jaundice). Additional symptoms include enlargement of the liver (hepatomegaly) and a tendency towards gastrointestinal bleeding because of abnormalities in the body’s ability to form blood clots. (For more information on this disorder, choose “hereditary fructose intolerance” as your search term in the Rare Disease Database.)

Tyrosinemia type II is an autosomal recessive genetic disorder caused by a deficiency of the enzyme tyrosine aminotransferase. Affected children do not have liver abnormalities. Symptoms can include excessive tears from the eyes, eye pain and redness as well as painful skin lesions on the palms and soles and sensitivity to light (photophobia). Developmental delay and intellectual disability are sometimes present. Symptoms improve with a low phenylalanine and tyrosine diet.

Tyrosinemia type III is an autosomal recessive genetic disorder caused by a deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase. Affected children do not have liver abnormalities. This condition is extremely rare and few children have been described, but symptoms can include loss of balance and coordination (ataxia), skin and eye abnormalities and developmental delay. Symptoms improve with a low phenylalanine and tyrosine diet.

Diagnosis

A diagnosis of tyrosinemia type I is made based upon a thorough clinical evaluation, a detailed patient history, and specialized tests. A diagnosis of tyrosinemia type I may be suspected in infants who display failure to thrive and an enlarged liver (hepatomegaly) during the first three months of life. The diagnosis is likely when tyrosine metabolites and succinylacetone are detected in the urine. It is also possible to make the diagnosis based on decreased activity of FAH in liver tissue or cultured fibroblasts, but this test is not readily available. Molecular genetic testing for FAH gene mutations is available to confirm the diagnosis.

Tyrosinemia type I may also be diagnosed through newborn screening programs. Succinyl acetone can be measured on the newborn blood spot by tandem mass spectroscopy. Most states in the U.S. screen every newborn for tyrosinemia type 1. Early detection is important because prompt identification and treatment may prevent the development of serious problems during infancy.

Carrier testing and prenatal diagnosis by DNA analysis are available if the specific gene-causing mutation has been identified in the family. Next Generation DNA sequencing techniques, like exome sequencing, whole genome sequencing (WGS) can help in identifying the mutations responsible for the disease. Prenatal diagnosis is also possible by detection of succinyl acetone and DNA analysis in amniotic fluid.

Standard Therapies

Treatment

In 2017, Nityr (nitisinone tablets) was approved by the U.S. Food and Drug Administration (FDA) for the treatment of hereditary tyrosinemia type 1. Nityr is manufactured by Cycle Pharmaceuticals.

The FDA approved the orphan drug Orfadin, a capsule and oral suspension formulation of nitisinone, to treat tyrosinemia type I in 2002. Nitisinone was developed by Swedish Orphan International Biovitrum AB and is marketed by Sobi, Inc.

These drugs should only be prescribed by physicians experienced in treating tyrosinemia type I since the correct dose must be adjusted for each patient according to specific biochemical tests and to the weight. Access to a nutritionist skilled in managing children with inborn errors of metabolism requiring a low protein diet is an important part of therapy. Blood tests should be monitored regularly to maintain the correct dose for the patient.

Nitisinone must be used in conjunction with a diet restricted in the amino acids tyrosine and phenylalanine. Treatment with nitisinone and dietary management should begin as soon as possible after the diagnosis is confirmed.

Infants with tyrosinemia type I are placed on a low protein diet that contains limited amounts of phenylalanine and tyrosine. Some affected infants have exhibited an improvement of liver and kidney abnormalities with dietary management alone. However, progression to cirrhosis, liver failure and potential hepatocellular carcinoma is still possible. Physicians often recommend that affected individuals observe a strict diet using special medical foods throughout their lifetime.

Liver transplantation may be required for affected infants who have already developed end-stage liver failure by the time of diagnosis, have evidence of liver cancer (hepatocellular carcinoma), or do not respond to nitisinone therapy. In some children, liver transplantation improves kidney function.

Genetic counseling is recommended for affected individuals and their families. Other treatments are symptomatic and supportive.

Type of Doctor Department :  biochemical geneticist, A hepatologist or gastroenterologist, and A hematologist.