Type I Tyrosinemia
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Tyrosinemia type I is a
genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders ...
that disrupts the
metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...
of the amino acid
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
, resulting in damage primarily to the
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
along with the
kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
s and
peripheral nerves The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain a ...
. The inability of cells to
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management * Business process, activities that produce a specific s ...
tyrosine can lead to chronic liver damage ending in
liver failure Liver failure is the inability of the liver to perform its normal synthetic and metabolic functions as part of normal physiology. Two forms are recognised, acute and chronic (cirrhosis). Recently, a third form of liver failure known as acute- ...
, as well as
renal disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an inflammatory kidney disease and has several types according to the location of the inflammation. Inflammation can b ...
and
rickets Rickets, scientific nomenclature: rachitis (from Greek , meaning 'in or of the spine'), is a condition that results in weak or soft bones in children and may have either dietary deficiency or genetic causes. Symptoms include bowed legs, stun ...
. Symptoms such as poor growth and
enlarged liver Hepatomegaly is enlargement of the liver. It is a non-specific medical sign, having many causes, which can broadly be broken down into infection, hepatic tumours, and metabolic disorder. Often, hepatomegaly presents as an abdominal mass. Dependi ...
are associated with the clinical presentation of the disease. If not detected via newborn screening and management not begun before symptoms appear, clinical manifestation of disease occurs typically within the first two years of life. The severity of the disease is correlated with the timing of onset of symptoms, earlier being more severe. If diagnosed through
newborn screening Newborn screening (NBS) is a public health program of screening (medicine), screening in infants shortly after birth for conditions that are treatable, but not clinically evident in the newborn period. The goal is to identify infants at risk for ...
prior to clinical manifestation, and well managed with diet and medication, normal growth and development is possible. Tyrosinemia type I is an
autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The firs ...
disorder caused by
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
s in both copies of the
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
encoding the
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
fumarylacetoacetate hydrolase (FAH). FAH is a metabolic enzyme that catalyzes the conversion of fumarylacetoacetate to
fumarate Fumaric acid or ''trans''-butenedioic acid is an organic compound with the formula HO2CCH=CHCO2H. A white solid, fumaric acid occurs widely in nature. It has a fruit-like taste and has been used as a food additive. Its E number is E297. The sa ...
and
acetoacetate Acetoacetic acid ( IUPAC name: 3-oxobutanoic acid, also known as acetonecarboxylic acid or diacetic acid) is the organic compound with the formula CHCOCHCOOH. It is the simplest beta-keto acid, and like other members of this class, it is unstable ...
.  It is expressed primarily in the liver and kidney. Loss of FAH activity results in the accumulation of certain
metabolic intermediate Metabolic intermediates are compounds produced during the conversion of substrates (starting molecules) into final products in biochemical reactions within cells. Although these intermediates are of relatively minor direct importance to cell ...
s in the tyrosine
catabolic pathway Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipid ...
. These compounds are toxic to cells and lead to differential gene expression and
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
in high concentrations. HT1 is diagnosed when elevated levels of
succinylacetone Succinylacetone is a chemical compound that is formed by the oxidation of glycine and is a precursor of methylglyoxal Methylglyoxal (MGO) is the organic compound with the formula CH3C(O)CHO. It is a reduced derivative of pyruvic acid. It is a ...
(SA), one of the metabolites in this pathway, is detected in
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...
and
urine samples A urine test is any medical test performed on a urine specimen. The analysis of urine is a valuable diagnostic tool because its composition reflects the functioning of many body systems, particularly the kidneys and urinary system, and specimens ...
. While there is no cure for tyrosinemia type I, management of the disease is possible utilizing dietary restrictions and
medication Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to medical diagnosis, diagnose, cure, treat, or preventive medicine, prevent disease. Drug therapy (pharmaco ...
s. A diet low in tyrosine and
phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...
is utilized indefinitely once a diagnosis is suspected or confirmed. Additionally, the drug
nitisinone Nitisinone, sold under the brand name Orfadin and Nityr among others, is a pharmaceutical drug, medication used to slow the effects of Tyrosinemia#Types, hereditary tyrosinemia type 1 (HT-1). It is available as a generic medication. Medical use ...
(brand name
Orfadin Nitisinone, sold under the brand name Orfadin and Nityr among others, is a medication used to slow the effects of hereditary tyrosinemia type 1 (HT-1). It is available as a generic medication. Medical uses Nitisinone is used to treat heredita ...
) is prescribed and continued indefinitely in order to combat liver and kidney damage, promoting normal function of these organs. Prior to the development of nitisinone, dietary restrictions and liver transplantation were the only forms of treatment for HT1. Tyrosinemia type I is especially prevalent in the Saguenay-Lac Saint-Jean region of
Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
, where the prevalence is 1 in 1,850 births. It is most common among those with French-Canadian ancestry and this frequency of infliction has been attributed to the
founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...
. There are five other known types of tyrosinemia, all of which derange the metabolism of tyrosine in the human body. They are distinguished by their symptoms and genetic cause.


Signs and symptoms

Type 1 tyrosinemia typically presents in infancy as failure to thrive and
hepatomegaly Hepatomegaly is enlargement of the liver. It is a non-specific sign (medicine), medical sign, having many causes, which can broadly be broken down into infection, hepatic tumours, and metabolic disorder. Often, hepatomegaly presents as an abdomin ...
. The primary effects are progressive liver and kidney dysfunction. The liver disease causes cirrhosis, conjugated hyperbilirubinemia, elevated AFP, hypoglycemia and coagulation abnormalities. This can lead to
jaundice Jaundice, also known as icterus, is a yellowish or, less frequently, greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving ...
,
ascites Ascites (; , meaning "bag" or "sac") is the abnormal build-up of fluid in the abdomen. Technically, it is more than 25 ml of fluid in the peritoneal cavity, although volumes greater than one liter may occur. Symptoms may include increased abdo ...
and hemorrhage. There is also an increased risk of
hepatocellular carcinoma Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer in adults and is currently the most common cause of death in people with cirrhosis. HCC is the third leading cause of cancer-related deaths worldwide. HCC most common ...
.The kidney dysfunction presents as
Fanconi syndrome Fanconi syndrome or Fanconi's syndrome (, ) is a syndrome of inadequate reabsorption in the proximal renal tubules of the kidney. The syndrome can be caused by various underlying congenital or acquired diseases, by toxicity (for example, from t ...
: Renal tubular acidosis, hypophosphatemia and aminoaciduria. Cardiomyopathy, neurologic and dermatologic manifestations are also possible. The urine has an odor of cabbage or rancid butter. The presentation of symptoms of tyrosinemia type 1 in terms of timing is broken into three categories: acute, sub-acute, and chronic. The acute classification typically is presented clinically between birth and 6 months of age. The common presentation in an acute case is synthetic liver failure, marked by the lack of formation of
coagulation factors Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of coagulation ...
in
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...
. Patients are prone to infections at this stage accompanied by
fever Fever or pyrexia in humans is a symptom of an anti-infection defense mechanism that appears with Human body temperature, body temperature exceeding the normal range caused by an increase in the body's temperature Human body temperature#Fever, s ...
,
vomiting Vomiting (also known as emesis, puking and throwing up) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteritis, pre ...
, increased tendency to bleed, and
diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...
along with bloody feces as manifestations of
sepsis Sepsis is a potentially life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage of sepsis is followed by suppression of the immune system. Common signs and s ...
. Other symptoms include
enlarged liver Hepatomegaly is enlargement of the liver. It is a non-specific medical sign, having many causes, which can broadly be broken down into infection, hepatic tumours, and metabolic disorder. Often, hepatomegaly presents as an abdominal mass. Dependi ...
,
jaundice Jaundice, also known as icterus, is a yellowish or, less frequently, greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving ...
, and excess abdominal fluid. Sub-acute cases present between 6 months and the first year of life and the severity of
liver disease Liver disease, or hepatic disease, is any of many diseases of the liver. If long-lasting it is termed chronic liver disease. Although the diseases differ in detail, liver diseases often have features in common. Liver diseases File:Ground gla ...
is lessened to an extent. Again, synthetic function of the liver in terms of blood coagulation factors is impaired in addition to enlargement of the liver and spleen. The infant may also display a
failure to thrive Failure to thrive (FTT), also known as weight faltering or faltering growth, indicates insufficient weight gain or absence of appropriate physical growth in children. FTT is usually defined in terms of weight, and can be evaluated either by a low ...
as their growth is limited by the disease. This growth impairment can manifest itself in
rickets Rickets, scientific nomenclature: rachitis (from Greek , meaning 'in or of the spine'), is a condition that results in weak or soft bones in children and may have either dietary deficiency or genetic causes. Symptoms include bowed legs, stun ...
, which is the softening of bones. The final classification, chronic HT1, is detected with presentations occurring after one year of life. The course of the disease up to this point can lead to different ailments affecting the liver.
Cirrhosis Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, chronic liver failure or chronic hepatic failure and end-stage liver disease, is a chronic condition of the liver in which the normal functioning tissue, or parenchyma, is replaced ...
,
liver failure Liver failure is the inability of the liver to perform its normal synthetic and metabolic functions as part of normal physiology. Two forms are recognised, acute and chronic (cirrhosis). Recently, a third form of liver failure known as acute- ...
, or cancer of the liver may present as a result of chronic liver disease. Additional symptoms common in this classification include
cardiomyopathy Cardiomyopathy is a group of primary diseases of the heart muscle. Early on there may be few or no symptoms. As the disease worsens, shortness of breath, feeling tired, and swelling of the legs may occur, due to the onset of heart failure. A ...
,
renal disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an inflammatory kidney disease and has several types according to the location of the inflammation. Inflammation can b ...
, and acute neurological crises.


Liver

The liver is the organ affected most by Tyrosinemia Type I due to the high level of expression of the gene for fumarylacetoacetate hydrolase (FAH) in liver cells. The production of blood coagulation factors by the liver is disrupted, causing hemophiliac-like symptoms. Acute liver failure is common, especially in early life. Additionally, the synthesis of
albumin Albumin is a family of globular proteins, the most common of which are the serum albumins. All of the proteins of the albumin family are water- soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Alb ...
in the liver may be defective, therefore leading to
hypoalbuminemia Hypoalbuminemia (or hypoalbuminaemia) is a medical sign in which the concentration, level of human serum albumin, albumin in the blood is low. This can be due to decreased production in the liver, increased loss in the gastrointestinal tract or ki ...
. As the disease progresses,
cirrhosis Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, chronic liver failure or chronic hepatic failure and end-stage liver disease, is a chronic condition of the liver in which the normal functioning tissue, or parenchyma, is replaced ...
is common. This can lead to a
fatty liver Fatty liver disease (FLD), also known as hepatic steatosis and steatotic liver disease (SLD), is a condition where excess fat builds up in the liver. Often there are no or few symptoms. Occasionally there may be tiredness or pain in the upper r ...
and the development of tumors in areas affected by this scarring of liver tissue. These scars are known as nodules. There is a 37% chance of developing a hepatocellular carcinoma (HCC) for untreated patients.


Renal and neurological manifestations

Many patients display impaired kidney function and neurological symptoms. In addition to liver cells, kidney cell expression involves expression of the gene for FAH. Kidney failure is a potential result of impaired kidney function, but the most common symptom associated with renal dysfunction is hypophosphatemic rickets. Neurological manifestations are characterized by acute neurological crises due to overaccumulation of
porphyrin Porphyrins ( ) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (). In vertebrates, an essential member of the porphyrin group is heme, w ...
. These crises are characterized by
porphyria Porphyria ( or ) is a group of disorders in which substances called porphyrins build up in the body, adversely affecting the skin or nervous system. The types that affect the nervous system are also known as Porphyria#Acute porphyrias, acute p ...
. They typically follow an infection. Patients can present with a variety of varied symptoms including
paresthesia Paresthesia is a sensation of the skin that may feel like numbness (''hypoesthesia''), tingling, pricking, chilling, or burning. It can be temporary or Chronic condition, chronic and has many possible underlying causes. Paresthesia is usually p ...
s,
abdominal pain Abdominal pain, also known as a stomach ache, is a symptom associated with both non-serious and serious medical issues. Since the abdomen contains most of the body's vital organs, it can be an indicator of a wide variety of diseases. Given th ...
, pain-induced seizures, and can result in
self-mutilation Self-harm refers to intentional behaviors that cause harm to oneself. This is most commonly regarded as direct injury of one's own skin tissues, usually without suicidal intention. Other terms such as cutting, self-abuse, self-injury, and se ...
in response to this pain. Episodes can last for 1–7 days and can lead to
neuropathy Peripheral neuropathy, often shortened to neuropathy, refers to damage or disease affecting the nerves. Damage to nerves may impair sensation, movement, gland function, and/or organ function depending on which nerve fibers are affected. Neuropa ...
.


Genetics

Tyrosinemia type I is an autosomal recessive inherited condition. Mutant
allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...
s in the
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
are inherited from both parents. The genetic mutation occurs to the fumarylacetoacetate hydrolase (FAH) enzyme gene, located on
chromosome 15 Chromosome 15 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 15 spans about 99.7 million base pairs (the building material of DNA) and represents between 3% and 3.5% of the total DNA ...
. The most common mutation is IVS12+5(G->A) which is a mutation in the splice site consensus sequence of
intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...
12, therefore affecting
exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...
12. A second allele is the IVS6-1(G-T) mutation. This mutation results in a nonfunctional enzyme. Type 1 tyrosinemia is inherited in an autosomal recessive pattern.


Pathophysiology

Fumarylacetoacetate hydrolase catalyzes the final step in the degradation of
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
- fumarylacetoacetate to fumarate, acetoacetate and succinate. Fumarylacetoacetate accumulates in
hepatocytes A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, bile ...
and proximal renal tubal cells and causes oxidative damage and DNA damage leading to cell death and dysfunctional gene expression which alters metabolic processes like protein synthesis and
gluconeogenesis Gluconeogenesis (GNG) is a metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In verte ...
. The increase in fumarylacetoacetate inhibits previous steps in tyrosine degradation leading to an accumulation of tyrosine in the body. Tyrosine is not directly toxic to the liver or kidneys but causes dermatologic and neurodevelopmental problems.


Tyrosine metabolic pathway

Fumarylacetoacetate hydrolase (FAH) is the final enzyme in the tyrosine metabolic pathway. The mutation of FAH enzyme results in nonfunctional FAH in all cells expressing this gene and thus metabolizing tyrosine is impaired. FAH catalyzes the conversion of fumarylacetoacetate to
fumarate Fumaric acid or ''trans''-butenedioic acid is an organic compound with the formula HO2CCH=CHCO2H. A white solid, fumaric acid occurs widely in nature. It has a fruit-like taste and has been used as a food additive. Its E number is E297. The sa ...
and
acetoacetate Acetoacetic acid ( IUPAC name: 3-oxobutanoic acid, also known as acetonecarboxylic acid or diacetic acid) is the organic compound with the formula CHCOCHCOOH. It is the simplest beta-keto acid, and like other members of this class, it is unstable ...
. Loss of FAH results in the accumulation of upstream compounds in the catabolic pathway. These include maleylacetoacetate (MAA) and fumarylacetoacetate (FAA). MAA and FAA are converted to succinylacetoacetate (SAA) which is then catabolized to succinylacetone (SA). The accumulation of MAA, FAA, and SA in cells inhibits the breakdown of thiol derivatives, leading to post-translational modifications to the
antioxidant Antioxidants are Chemical compound, compounds that inhibit Redox, oxidation, a chemical reaction that can produce Radical (chemistry), free radicals. Autoxidation leads to degradation of organic compounds, including living matter. Antioxidants ...
glutathione Glutathione (GSH, ) is an organic compound with the chemical formula . It is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources ...
. This inhibits the antioxidant activity of glutathione, leading to
reactive oxygen species (ROS) In chemistry and biology, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl radical (OH.), and sing ...
damaging cell components. Over time, the combined effect of accumulation of toxic metabolic intermediates and elevated ROS levels in liver and kidney cells leads to
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
in these tissues which ultimately results in organ failure. Accumulated SA in liver and kidney cells results in its release into the bloodstream, which leads to secondary effects. SA inhibits the enzyme 5-ALA dehydratase which converts aminolevulinic acid (5-ALA) into
porphobilinogen Porphobilinogen (PBG) is an organic compound that occurs in living organisms as an intermediate in the biosynthesis of porphyrins, which include critical substances like hemoglobin and chlorophyll. The structure of the molecule can be described ...
, a precursor to
porphyrin Porphyrins ( ) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (). In vertebrates, an essential member of the porphyrin group is heme, w ...
. Consequently, porphyrin deposits form in the bloodstream and cause
neuropathic pain Neuropathic pain is pain caused by a lesion or disease of the somatosensory nervous system. Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allodynia). It may have continuo ...
, leading to the acute neurological crises experienced by some patients. Additionally. SA can function to inhibit renal tubular function, the synthesis of heme, and the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
. The accumulation of unprocessed
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
itself in the blood stream as a consequence of deficient catabolism can also lead to disruption of hormonal signaling and
neurotransmission Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron ...
. Tyrosine is a precursor molecule required for synthesis of several neurotransmitters and hormones, mainly
Dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. It is an amine synthesized ...
,
norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
, and thryoxine. Excessive synthesis of these molecules due to elevated tyrosine levels can impair physical growth,
motor function Motor control is the regulation of movements in organisms that possess a nervous system. Motor control includes conscious voluntary movements, subconscious muscle memory and involuntary reflexes, as well as instinctual taxes. To control move ...
, and speech development.


Diagnosis

Beyond the identification of physical clinical symptoms outlined above, the definitive criterion for diagnostic assessment of Tyrosinemia Type I is elevated succinylacetone (SA) in
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...
and
urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
. Elevated SA levels are not associated with any other known medical condition, so there is minimal risk of misdiagnosis. Quantitation of tyrosine levels is also used as a diagnostic but is less reliable due to high false positive and false negative rates. Newborns are not generally screened for HT1 due to rarity of the condition and lack of apparent symptoms at time of birth. However, prompt assessment upon the manifestation of physical symptoms such as
fever Fever or pyrexia in humans is a symptom of an anti-infection defense mechanism that appears with Human body temperature, body temperature exceeding the normal range caused by an increase in the body's temperature Human body temperature#Fever, s ...
,
vomiting Vomiting (also known as emesis, puking and throwing up) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteritis, pre ...
, increased tendency to bleed,
diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...
along with bloody feces, and
jaundice Jaundice, also known as icterus, is a yellowish or, less frequently, greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving ...
is critical for improving long term prognosis.


Management

The primary treatment for type 1 tyrosinemia is
nitisinone Nitisinone, sold under the brand name Orfadin and Nityr among others, is a pharmaceutical drug, medication used to slow the effects of Tyrosinemia#Types, hereditary tyrosinemia type 1 (HT-1). It is available as a generic medication. Medical use ...
and restriction of tyrosine in the diet. Nitisinone inhibits the conversion of 4-OH phenylpyruvate to homogentisic acid by
4-Hydroxyphenylpyruvate dioxygenase 4-Hydroxyphenylpyruvate dioxygenase (HPPD), also known as α-ketoisocaproate dioxygenase (KIC dioxygenase), is an Fe(II)-containing non-heme oxygenase that catalyzes the second reaction in the catabolism of tyrosine - the conversion of 4- ...
, the second step in tyrosine degradation. By inhibiting this enzyme, the accumulation of the fumarylacetoacetate is prevented. Previously, liver transplantation was the primary treatment option and is still used in patients in whom nitisinone fails. Clinical treatment of HT1 relies on medications and strict regulation of diet. Nitisinone and dietary restrictions that decrease the amount of
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
and phenylalaine absorbed from the
GI tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
during protein
digestion Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into th ...
are used in combination as therapeutic measures that control the disease state if they are continued indefinitely. If not, there is a lack of control over the disease, resulting in continued liver and kidney damage, contributing to
organ failure Organ dysfunction is a condition where an organ does not perform its expected function. Organ failure is organ dysfunction to such a degree that normal homeostasis cannot be maintained without external clinical intervention or life support. It i ...
and
death Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...
. In this case, a liver transplant may be required. Levels of SA are monitored throughout treatment in order to assess treatment effectiveness.


Diet

The prescribed diet for treatment of HT1 is low in protein. Patients received amino acid supplements lacking tyrosine and phenylalanine, most often by drinking a specially engineered formula, in order to acquire sufficient protein. It is recommended that tyrosine levels remain below 500 μmol/L. Phenylalnine is the precursor to tyrosine. The ideology behind maintaining low tyrosine levels is two-fold. Firstly, it prevents the toxic metabolic intermediates from accumulating as a result of the dysfunctional tyrosine metabolic pathway. Prior to the introduction of nitisinone, this was the main treatment measure. Secondly, the mechanism of action of nitisinone is prevention of any tyrosine metabolism, thus it is important to prevent tyrosine from accumulating. Dietary protein consumption while taking nitisinone can also lead to side effects affecting the ocular system, which are easily reversed by removing protein from the diet.


Medication

Nitisinone Nitisinone, sold under the brand name Orfadin and Nityr among others, is a pharmaceutical drug, medication used to slow the effects of Tyrosinemia#Types, hereditary tyrosinemia type 1 (HT-1). It is available as a generic medication. Medical use ...
is prescribed ultimately to reduce the accumulation of toxic metabolic intermediates, such as succinylacetate, which are toxic to cells. It modifies the function of 4-hydrooxyphenylpyruvate dioxygenase by acting as a
competitive inhibitor Competitive inhibition is interruption of a chemical pathway owing to one chemical substance inhibiting the effect of another by competing with it for binding or bonding. Any metabolic or chemical messenger system can potentially be affected b ...
. 4-hydrooxyphenylpyruvate dioxygenase functions to convert
4-hydroxyphenylpyruvate 4-Hydroxyphenylpyruvic acid (4-HPPA) is an intermediate in the metabolism of the amino acid phenylalanine. The aromatic side chain of phenylalanine is hydroxylated by the enzyme phenylalanine hydroxylase to form tyrosine. The conversion from tyr ...
to
homogentisate Homogentisic acid (2,5-dihydroxyphenylacetic acid) is a phenolic acid usually found in ''Arbutus unedo'' (strawberry-tree) honey. It is also present in the bacterial plant pathogen Xanthomonas campestris pv. phaseoli, ''Xanthomonas campestris'' p ...
as the second enzymatic reaction in the tyrosine catabolic pathway. This prevents the further catabolism of tyrosine. It is recommended that nitisinone treatment begins immediately following a confirmed or suspected case of HT1. It is supplied orally as a capsule or
suspension Suspension or suspended may refer to: Science and engineering * Car suspension * Cell suspension or suspension culture, in biology * Guarded suspension, a software design pattern in concurrent programming suspending a method call and the calling ...
in dose increments of 2 mg, 5 mg, 10 mg, or 20 mg or 4 mg/mL respectively. The starting dose is 1 mg/kg one time daily or 2 mg/kg one time daily for 48 hours if the patient is experiencing acute liver failure. Patient responsiveness to nitisinone is assessed by measuring blood coagulation activity and SA levels in blood and urine. Patients should display a
positive response Electoral system criteria In social choice, the negative response, perversity, or additional support paradox is a pathological behavior of some voting rules where a candidate loses as a result of having too much support (or wins because of inc ...
within 24–48 hours of first dose. Establishment of the long-term dosage will vary from patient to patient. It is recommended that nitisinone levels be maintained at 30-50 μM in the blood stream.


Prognosis

Prior to the development of nitisinone, dietary restrictions and liver transplantation were the only forms of treatment for HT1. A study regarding the efficacy of treatment with nitisinone and dietary restrictions found that 93% of people survived at two years, four years, and six years indicating the prognosis of stabilizing the HT1 disease state is positive.


Epidemiology

Tyrosinemia type I affects males and females in equal numbers. Its prevalence has been estimated to be 1 in 100,000 to 120,000 births worldwide. HT1 is especially prevalent in the Saguenay-Lac Saint-Jean region of
Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
is one in 1,850 births. The elevated frequency of this disorder within individuals of French-Canadian ancestry in Quebec is believed to be due to reduced genetic heterogeneity within the original founder population for the Saguenay-Lac Saint-Jean region. The initial settlement of Saguenay Lac-Saint-Jean (SLSJ) occurred between 1838 and 1911. From a total of 28,656 settlers, 75 percent originated from the neighboring
Charlevoix Charlevoix ( , ) is a cultural and natural region in Quebec, on the north shore of the Saint Lawrence River as well as in the Laurentian Mountains area of the Canadian Shield. This dramatic landscape includes rolling terrain, fjords, headlands ...
region. The settling of the Charlevoix region itself started in 1675 when 599 founders of mostly French descent moved to this region from the Quebec City area. Worldwide, type I tyrosinemia affects about 1 person in 100,000. This type of tyrosinemia is much more common in Quebec, Canada. The overall incidence in
Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...
is about 1 in 16,000 individuals. In the Saguenay-Lac-Saint-Jean region of Quebec, type 1 tyrosinemia affects 1 person in 1,846. The carrier rate has been estimated to be between 1 in 20 and 1 in 31.


History

Nitisinone was first used to clinically treat tyrosinemia type I in 1991. Nitisinone was approved by the European Medicine Agency (EMA) under exceptional circumstances in 2005. Originally, nitisinone was developed as a weed-killer by Zeneca Agrochemicals. It was epidemiologically observed that the growth of plants and weeds was inhibited under the bottlebrush plant (Callistemon citrinus). It became clear that neither the shade nor the litterfall of these plants was responsible for the suppression of plant and weed growth. Rather, a substance – which was identified as leptospermone – in the soil under the bottlebrush plant was shown to have bleaching activity on the emerging plants. The allelochemical leptospermone was extracted from the bottlebrush plant and chemically characterized. Leptospermone belongs to the triketone family and inhibits chloroplast development due to a lack of plastoquinone secondary to hepatic 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibition; thus, it served as a blueprint for the synthesis of nitisinone. In 1932,
Grace Medes Grace Medes (November 9, 1886 – December 31, 1967) was an American biochemist, who discovered tyrosinosis—a metabolic disorder today known as tyrosinemia—and studied fatty acid metabolism. She was awarded the Garvan-Olin Medal in 1955 for ...
first described "a new disorder of tyrosine metabolism," She coined the condition "tyrosinosis" after observing 4-hydroxyphenylpyruvate in the urine of a 49-year-old man with myasthenia gravis. She proposed that the metabolic defect in this patient was a deficiency of 4-hydroxyphenylpyruvate dioxygenase, but her case remains puzzling and has since been assigned a separate OMIM number. The first typical patient with hepatorenal tyrosinemia was described in 1956 by Margaret D Baber at Edgware General Hospital in Middlesex, England. Starting the following year, Kiyoshi Sakai and colleagues, at the Jikei University School of Medicine in Tokyo, published 3 reports describing the clinical, biochemical, and pathological findings of a 2-year-old boy with hepatorenal tyrosinemia who was then thought to have an "atypical" case of tyrosinosis. Between 1963 and 1965, Swedish pediatrician Rolf Zetterström and colleagues at the Karolinska Institute in Sweden published the first detailed clinical account of hepatorenal tyrosinemia and its variants. Shortly thereafter, a Canadian group also described the clinical and laboratory findings of hepatorenal tyrosinemia. Both the Scandinavian and Canadian groups suggested that the Japanese patients described earlier by Sakai and colleagues had the same disorder, ie, hepatorenal tyrosinemia. In 1965, doubts emerged that the underlying biochemical cause of hepatorenal tyrosinemia was a defective form of the 4-hydroxyphenylpyruvate dioxygenase enzyme. In 1977, Bengt Lindblad and colleagues at the University of Gothenburg in Sweden demonstrated that the actual defect in causing hepatorenal tyrosinemia involved the fumarylacetoacetate hydrolase enzyme. This was subsequently confirmed using direct enzyme assays.


Research directions

In March 2014, a study successfully used
CRISPR gene editing CRISPR gene editing (; pronounced like "crisper"; an abbreviation for "clustered regularly interspaced short palindromic repeats") is a genetic engineering technique in molecular biology by which the genomes of living organisms may be modified. ...
to correct the FAH gene mutation in
hepatocytes A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, bile ...
in a
mouse model A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mod ...
of human type I tyrosinemia. The study found that a single hydrodynamic injection of a DNA plasmid, encoding
Cas9 Cas9 (CRISPR associated protein 9, formerly called Cas5, Csn1, or Csx12) is a 160 dalton (unit), kilodalton protein which plays a vital role in the immunological defense of certain bacteria against DNA viruses and plasmids, and is heavily utili ...
and a sgRNA, along with a ssDNA repair template cured enough hepatocytes of a FAH gene SNP to the wild-type gene to cure the
weight-loss Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other con ...
symptom of type 1 tyrosinemia in mice. Since human type 1 tyrosinemia is caused by a similar mutation in the same gene, this finding sets a precedent for further research into gene editing treatments for HT1. As of April 2020, two new clinical trials, are underway in the USA for a Mass Spectrometry-based biomarker for the early and sensitive diagnosis of Tyrosinemia type 1 from blood plasma.


References


External links

* * {{Amino acid metabolic pathology Amino acid metabolism disorders Autosomal recessive disorders Rare diseases