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This glossary aims to provide accurate definitions of key terms. Words in italics have their own explanations. If you feel there are terms that should be added to the glossary, please send an e-mail to with your suggestion.

Click one of the letters above to advance the page to terms beginning with that letter.


Adherence search for term

Describes how patients follow (or have steady observance of) any therapeutic regimen agreed between themselves and their healthcare practitioners.

Amino acid search for term

Amino acids are small molecules with a particular structure, with amino- and carboxy-terminals separating a central carbon atom bearing a side chain that distinguishes one amino acid from another. All proteins are made of strings of amino acids linked together, and some amino acids also have important actions in the nervous system. There are about 20 amino acids that have a role in normal human physiology. Most are synthesized within the body, but some are not and must be obtained from the diet (‘essential amino acids’). People with PKU on a phenylalanine-restricted diet can eat no, or little natural protein; they may need medical foods to supply essential amino acids they would otherwise lack.

Aspartame search for term

Aspartame is an artificial sweetener, sold under various brand names. Aspartame releasesphenylalanine when metabolized within the body and should be avoided by people with PKU.

Atypical hyperphenylalaninemia search for term

This term has been used to refer both to patients with tetrahydrobiopterin deficiency but is also used to describe people with mild hyperphenylalaninemia arising from certain mutations of the gene for phenylalanine hydroxylase.

Autosomal recessive inheritance search for term

The manner of inheritance of a gene mutation that does not markedly affect the phenotype of the individual, as the other (non-mutated) copy of the gene provides sufficient activity of the protein encoded by the gene. For example, subjects with one mutated copy and one normal copy of phenylalanine hydroxylase (one parent is a carrier of a gene mutation) do not display a clinical PKU phenotype characterized by hyperphenylalaninemia. Inheritance of two mutated copies (one from each parent) will result in the display of the PKU phenotype.

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BH4 search for term

A shortened name for tetrahydrobiopterin (the cofactor of phenylalanine hydroxylase) that is often used especially in medical research.

Synonyms: tetrahydrobiopterin

BH4 deficiency search for term

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Carrier (of a gene mutation) search for term

A child can only develop classical PKU if it inherits two faulty copies of the gene forphenylalanine hydroxylase, the enzyme that is deficient in people with PKU, from its parents (one copy from each parent). Individuals with one mutation (faulty copy) of the gene do not have PKU, but are said to be carriers of the mutation. Two carriers of a mutation in the gene forphenylalanine hydroxylase (that inhibits the activity of the enzyme) have:

  • A one in four chance of having a child with PKU (two faulty copies of the gene)
  • A one in four chance of having a child free of PKU mutations (no faulty copies of the gene), and
  • A one in two chance of having a child that is a carrier of one or other of the mutations.
Classical PKU search for term

This is usually used to describe patients with mutations of the gene for phenylalanine hydroxylase that lead to very low, or absent, activity of the enzyme. It is likely that the individual will have severe hyperphenylalaninemia (blood levels of phenylalanine typically higher than 1200 μmol/L) and a phenylalanine-restricted diet will be necessary. It is unusual for patients with classical PKU to respond to treatment with sapropterin.

CNS search for term

Central nervous system: the brain and spinal cord.

Codon search for term

The sequence of three nucleotides (bases) in DNA/RNA that codes for an individual amino acid, or provides an instruction to stop (a stop codon).

Cofactor search for term

Some enzymes need another molecule to support their function: a cofactor. Tetrahydrobiopterinis a cofactor to phenylalanine hydroxylase (the enzyme deficient in PKU).

Compliance search for term

Describes how patients follow a treatment regimen in accordance with the advice given to them by their health care professional. Thus, people comply well or badly with things such as medication (e.g. taking the right tablets in the right numbers at the right times), or special diets (e.g. eating the right foods in the right amounts and avoiding other foods, etc.).

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Deletion search for term

A mutation where DNA (one or more base pairs) is removed from the gene.

Development search for term

A broad term to encompass the physical and cognitive changes accompanying growth, usually applied to young people. Optimizing nutrition to maintain normal development is important: iodine deficiency remains a co-factor in the development of thyroid disorders in some countries

Diarrhea search for term

Symptom characterized by an increased number of daily bowel movements and loose and watery stools. Diarrhea can be caused by a variarity of issues: in PKU patients diarrhea can occur when low blood Phe concentrations persist for a longer period of time. A longer period of low phenylalanine concentrations may cause a decrease in growth velocity, skin atrophy (which may be falsely interpreted as eczema), and diarrhea as a sign of the gut not being able to synthesize new cells or having a poor function at that time. On the other hand, diarrhea, likewise vomiting and fever, may cause catabolism and a subsequent increase in blood plenylalanineconcentration.

Dopamine search for term

An important neurotransmitter in the CNS. Dopamine is involved in many CNS functions, including fine control of movement (dopaminergic cells are lost in Parkinson’s Disease) andcognition. Impairment of systems in the forebrain regulated by dopamine through loss of myelinhas been proposed as one mechanism by which long-term hyperphenylalaninemia may damagecognitive function.

Tyrosine is required for the biosynthesis of dopamine. High levels of phenylalanine in the blood inhibit the entry of tyrosine into the brain (phenylalanine and tyrosine each compete for the same large neutral amino acid carrier in the blood-brain barrier). This may be another way by which hyperphenylalaninemia adversely affects the function of dopaminergic systems in the brain.

Dysmorphology search for term

Aberrant physical development, observed on physical examination. Congenital hypothyroidism might lead to dysmorphology in affected newborn.

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Early treated search for term

In the context of PKU, people who were put on a phenylalanine-restricted diet in the early days or weeks of life, in time to mitigate the adverse impact of elevated blood phenylalanine ondevelopment.

Eczema search for term

Eczema is a skin condition characterized by inflammation, dryness, flaking and itching. Typically, formation of vesicles (blisters) is followed by erythema (reddening), edema (swelling), formation of raised areas of skin (papules) and then thickening and scaling of skin. Eczema is a common feature of PKU that was not controlled by the phenylalanine-restricted diet soon after birth.

Enzyme search for term

Enzymes are proteins that act as catalysts for biochemical reactions essential to life.

Epicanthic folds search for term

Skin folds of the upper eyelids covering the inner corners of the eyes.

Epilepsy search for term

Epilepsy is a condition in which uncoordinated electrical activity within the brain leads to unconsciousness and convulsions. This is a common feature of PKU if not controlled by thephenylalanine-restricted diet soon after birth.

Essential amino acids search for term

Amino acids that cannot be synthesized within the body and must be obtained through the diet.Medical foods are often an important source of essential amino acids for patients with PKU.

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Executive function search for term

Term that describes several of the higher functions of the brain. Examples of executive function include exercising will, insight, or judgement, coordinating use of resources (such as memory) to achieve a desired goal, planning tasks, generating strategies or placing complex activities in order to complete a larger task, correcting errors, troubleshooting, adapting behaviour in the light of new information, generating behavioural responses based on actions that are not already well learned, or performing well in situations that require overcoming temptation or force of habit. Studies in populations with iodine deficiency and related thyroid disorder have found reduced executive function, increased depression/anxiety and poor working memory, on average, compared with populations without iodine deficiency and related thyroid disorder.

Exons search for term

Genes include exons and introns in their DNA sequence. Exons represent the parts of the gene that will be represented in the final protein. The portions of RNA corresponding to introns in the gene will be removed in order to produce the final messenger RNA that will be translated into the protein.

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Fluorometric analysis search for term

This is a method used in some countries for newborn screening for PKU and other inherited metabolic disorders. Like other methods, it measures the phenylalanine level in the blood. It has been replaced with tandem mass spectrometry in many countries.

Frame shift search for term

In the synthesis of a protein, DNA is first transcribed to messenger RNA, made of a string of nucleotides (bases). In the genetic code, a set of three adjacent bases (a codon) in a strand of RNA codes for each amino acid used in a protein. Codons also provide other information, such as when to stop. The ‘reading frame’ consists of a sequence of these codons. A frameshift mutationoccurs when a number of base pairs not divisible by three is inserted into or deleted from the DNA. This changes the reading frame from that point onwards. For example, consider the following strand of RNA, with bases shown as a reading frame of six codons ending with a stop codon:


Leucine Tyrosine Serine Lysine Alanine STOP

Now imaging that a single additional base (C)
is inserted (a missense mutation, shown in red):


Phe Isoleucine Glutamine STOP Not transcribed

Følling’s disease search for term

An early term for PKU, named after Asbjorn Følling, who was the first to discover a link between excess phenylalanine and the syndrome of progressive and severe mental retardation we now know as PKU.

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Gene search for term

A sequence of DNA that determines the structure of a particular protein.

Gene therapy search for term

Treatment for a disease that involves changing the patient’s genetic code itself. For example, experimental studies are looking into gene therapy that provides a working copy ofphenylalanine hydroxylase for patients with PKU, who are lacking this enzyme. However, these studies are highly speculative at present and years away from use in the clinical management of PKU.

Genetic heterogeneity search for term

Each individual is unique in terms of their DNA. Genetic heterogeneity is a term used to describe the range of mutations in a particular gene between individuals within a population. Many different gene mutations give rise to PKU, so that the population of people with the disease are said to display a high degree of genetic heterogeneity.

Genotype search for term

The pattern of mutations present in particular genes, at the level of the DNA of an individual.

Glycomacroprotein search for term

A source of protein derived from milk whey that is free of phenylalanine. Glycomacroprotein is sometimes used as a dietary protein supplement (medical food) for people with PKU.

Guideline search for term

In a medical context, guidelines usually refer to documents detailing agreed best practice in a given area of medicine. Guidelines usually contain detailed reviews of the medical evidence supporting particular treatments, with recommendations on their use. Physicians are not absolutely constrained to follow guidelines, however, and may exercise their clinical judgement in individual cases.

Guthrie test search for term

A method of mass newborn screening for PKU that is still in use today in some countries. The test involves applying a drop of blood from a heel prick to filter paper, which is sent to the laboratory. Small circles of paper from within the blood spot are punched out and applied to the surface of an agar gel containing the bacterium, Bactilis subtilis, which requires phenylalanine for growth. The gel also contains β-2-thienylalanine, which suppresses bacterial growth by inhibiting the use of phenylalanine by the bacteria. Increasing the local phenylalanine concentration (i.e. from a blood spot from an infant with hyperphenylalaninemia) overcomes the inhibition of growth and a ring of bacterial growth appears around the positive sample within one day. The diameter of the bacterial colony provides an estimate of the concentration of phenylalanine in the sample.

In many countries, the Guthrie test has been overtaken by newer methods such as Tandem Mass Spectrometr

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Heel prick search for term

Blood for newborn screening for PKU and other diseases is usually taken from the heel of a baby, after pricking it with a needle.

Heterozygous search for term

We each have two copies of each gene. A subject with a particular mutation of one gene, but not the other, is said to be heterozygous for that mutation.

Homozygous search for term

We each have two copies of each gene. A subject with an identical mutation of both genes is said to be homozygous for that mutation.

HPA search for term

A common shortened name for hyperphenylalaninemia.

Hyperphenylalaninemia search for term

Elevated levels of phenylalanine in the blood. The level of blood phenylalanine that divides the normal range of blood phenylalanine from hyperphenylalaninemia in guidelines varies with age.

Hypertonia search for term

Increased muscle tone

Hypotonia search for term

Decreased muscle tone

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Insertion search for term

A mutation where additional DNA (one or more base pairs) is added to the gene

Introns search for term

DNA sequences within a gene that are not represented in the final protein. RNA sequences arising from introns will be removed during processing, to form the messenger RNA that will be used for translation into protein

IQ search for term

“Intelligence Quotient”, a commonly used index of intelligence derived from tests. The IQ is the ratio of a person’s mental age to their physical age, expressed as a percentage.

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Kuvan® search for term

Kuvan® is sapropterin dihydrochloride, a pharmaceutical formulation of tetrahydrobiopterin(soluble tablets) that is approved for use in the management of PKU in many countries.

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Large neutral amino acids search for term

This term described several amino acids according to their chemical structure (large, non-ionized side chains), and usually describes: phenylalanine, tyrosine, tryptophan, valine, isoleucine, leucine, methionine, and histidine. Supplements of LNAA are sometimes prescribed for people with PKU.

Synonyms: LNAA

Large, neutral amino acid carrier search for term

The brain is separated from the rest of the body by a barrier (the ‘blood-brain barrier’) which has many function, including the precise modulation of chemical environment of the CNS. Large, neutral amino acids (including phenylalanine) are essential to the proper function of the brain, for example in the synthesis of important neurotransmitters, such as dopamine, norepinephrine (noradrenaline) or serotonin (5-hydroxytryptamine). LNAA are taken through the blood-brain barrier into the brain on this specific carrier protein, so that they are available for use by nerve cells.

Late treated search for term

In the context of PKU, people who were not put on a phenylalanine-restricted diet in the early days or weeks of life. Late-treated people with PKU almost always display the developmentaldisorders characteristic of PKU (such as severe mental retardation).

Loading test search for term

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Maternal PKU search for term

The developing fetus is exposed to high blood phenylalanine if the mother has PKU. Women with PKU should take steps to control their blood phenylalanine if they are, or plan to become, pregnant. The consequences for the fetus of uncontrolled maternal PKU (blood phenylalanine higher than 1200 μmol/L) include a very high risk (more than 90%) of mental retardation andmicrocephaly. Physical defects (typically facial dysmorphology) and congenital heart disease also occur.

Medical foods search for term

Patients with PKU are usually unable to eat natural sources of protein, due to its phenylalaninecontent. Medical foods (sometimes called protein substitutes) provide a source of protein withoutphenylalanine, but with other amino acids necessary for good nutrition.

Melanin search for term

Melanin is a hormone that causes darkening of the skin, e.g. in response to sunlight. The highphenylalanine levels in people with poorly controlled PKU inhibit the production on melanin within the body. As a result, patients with PKU often have fair skin.

Mental retardation search for term

A lack of normal development of intellectual capacities. Iodine deficiency may lead to severe mental retardation early in life.

Metabolite search for term

A chemical substance produced within the body, typically due to the actions of an enzyme. These are then typically used in further biochemical reactions, broken down further, or excreted via the urine. For example, pterins in urine (measured in the diagnosis of tetrahydrobiopterin deficiency) are metabolites of tetrahydrobiopterin.

Mild hyperphenylalaninemia search for term

Mild hyperphenylalaninemia refers to small elevations in the blood phenylalanine level that are considered insufficiently severe for a diagnosis of PKU and application of the phenylalanine-restricted diet.

Mild PKU search for term

A low or intermediate increase in blood phenylalanine, such as that observed in people withmutations of the gene for phenylalanine hydroxylase that only partially impair its activity.

Missense mutation search for term

A missense mutation is one where a mutation of a single base in a gene causes one amino acidto be replaced by another in the resulting protein. For example, the R408W missense mutation changes the DNA so that a CGG codon (arginine) changes to TGG (tryptophan). This one amino acid substitution effectively abolishes phenylalanine hydroxylase activity in the resulting protein.

Molecular chaperone search for term

A molecular chaperone guards and preserves the structure and function of another molecule, usually a protein. For example, tetrahydrobiopterin is a cofactor for phenylalanine hydroxylase. A subset of patients (usually, but not exclusively, with mild PKU) respond to treatment withtetrahydrobiopterin with an increase in the activity of phenylalanine hydroxylase and a reduction in blood phenylalanine levels. It is believed that the mutation of phenylalanine hydroxylase alters its activity by changing its 3-dimensional structure. Tetrahydrobiopterin is believed to help maintain the structure of the mutated phenylalanine hydroxylase protein in a way that preserves its enzyme activity. It may also prevent the phenylalanine hydroxylase protein from being broken down or inactivated by other enzymes. In this way, it is acting as a ‘molecular chaperone’.

Motor skills search for term

The ability to perform controlled and precise movements of muscle groups. Measurement of motor skills is one of the neuropsychological tests sometimes prescribed for people with PKU to track their development.

Myelin search for term

A sheath, made of specialized cells (oligodendrocytes in the CNS, Schwann cells in the periphery), that covers and insulates nerves and increases the speed of transmission of nerve impulses. Sub-optimal control of blood phenylalanine over the long term has been shown to cause damage to myelin in the brain. It is unclear, however, how this phenomenon relates to the loss of executive function and cognitive function that has been observed in populations with PKU.

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Natural protein search for term

Natural protein is the protein found in everyday foodstuffs, as opposed to the manufactured protein in medical foods.

Neonatal screening search for term
Neuropsychological testing search for term

Long-term hyperphenylalaninemia impairs the function of the brain. Regular neuropsychological testing is necessary to track the development of the brains of young people with PKU, and measures a range of functions of the brain, such as cognitive function, memory, and motor skills.

Neurotoxic search for term

Causing damage to nervous tissues. The long-term elevation of phenylalanine characteristic of uncontrolled or late-treated PKU is neurotoxic to the brain, and causes mental retardation, among other adverse effects.

Newborn screening search for term

The process of testing all babies for certain diseases, such as PKU, so that treatment can be applied in time to protect the child from long-term harm from the disease. Also called neonatal screening.

Non-PKU hyperphenylalaninemia search for term

People with non-PKU hyperphenylalaninemia usually have only mildly impaired phenylalanine hydroxylase. This often arises from a mutation of only one of their two copies of the gene forphenylalanine hydroxylase, or from mutations that only slightly impair the action of the enzyme. They display a mildly elevated level of blood phenylalanine that is not high enough to trigger a diagnosis of PKU and which does not usually require medical intervention.

Nonsense mutation search for term

A nonsense mutation involves changing a codon for an amino acid to a stop codon. This results in a shortened protein that usually has no biological activity. The R111X mutation changes a DNA codon from CGA (arginine) to TGA (stop).

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PEG-PAL search for term

Phenylalanine ammonia lyase (PAL) is an enzyme derived from a plant that breaks downphenylalanine, but does not require tetrahydrobiopterin as a cofactor. PEG-PAL is a pegylated injectable formulation of PAL that is being studied in clinical trials in people with PKU. PEG-PAL is not yet available for prescription.

Phe search for term

Phenylalanine. An essential amino acid usually derived from dietary protein. Patients with PKU lack the ability to convert phenylalanine to tyrosine due to a mutation of the enzyme phenylalanine hydroxylase. The resulting accumulation of phenylalanine in the bloodstream is toxic to the CNS unless levels of phenylalanine are controlled using a phenylalanine-restricted diet or other treatment (seesapropterin).

Phe/Tyr ratio search for term

Much of the body’s tyrosine is usually produced from phenylalanine by the enzyme,phenylalanine hydroxylase. When phenylalanine hydroxylase activity is lacking, as in PKU, the levels of both of these amino acids may be disturbed (high phenylalanine, low tyrosine). This has important implications, particularly for the activity of the brain, which requires access to bothphenylalanine and tyrosine. For some people with PKU, establishing a normal ratio betweenphenylalanine and tyrosine may be an important aspect of treatment with a phenylalanine-restricted diet, medical foods and, in some cases, tyrosine supplementation.

Phenotype search for term

This describes the appearance of an individual in medical terms. For example, someone who responds to treatment with sapropterin with a substantial fall in blood phenylalanine may be described as displaying “the tetrahydrobiopterin-responsive PKU phenotype”.

Phenyl ammonia lyase search for term
Phenylalanine (Phe) level search for term

The level of phenylalanine measured in the blood. The blood phenylalanine level is used to diagnose PKU and to measure the success of the phenylalanine-restricted diet, or of other treatments for PKU. Recommended blood phenylalanine levels vary with age.

Phenylalanine exchange search for term

Phenylalanine exchanges are lists of common foods with information on the amount ofphenylalanine that they contain. These exchanges help people with PKU to plan their diets, by letting them know which foods can be eaten freely (within reason), which foods must be consumed in carefully controlled amounts, and which foods should be avoided altogether.

Phenylalanine hydroxylase search for term

This is an enzyme (EC, gene locus 12q24.1) that converts phenylalanine (absorbed from the diet) to another amino acid, tyrosine. People with PKU have a mutation in their genesfor phenylalanine hydroxylase that reduces or completely blocks its activity. Mutations in both copies of the gene for phenylalanine hydroxylase are required to produce clinical PKU.

Synonyms: PAH

Phenylalanine tolerance search for term

The amount of phenylalanine that a patient can take in with food, without excess phenylalanine appearing in the blood.

Phenylalanine-restricted diet search for term

PKU is characterized by an inability to control levels of phenylalanine in the blood, which builds up to toxic levels when individuals with PKU follow a normal diet. Thus, a special diet is required, which is very low in phenylalanine and which usually involves prescription of medical foods.oods.

Phenylketonuria search for term

An inherited (autosomal recessive) disease caused by a mutation of the gene for the enzyme,phenylalanine hydroxylase, which normally converts phenylalanine to tyrosine. The resulting accumulation of phenylalanine in the bloodstream is toxic to the central nervous system unless levels of phenylalanine are controlled using a special diet or other treatment (see sapropterin, amino acids).

Philtrum search for term

Medial groove in the upper lip that runs from the nose to the top of the lip.

Protein substitute search for term

Patients with PKU are usually unable to eat natural sources of protein, which containsphenylalanine. Protein substitutes (medical foods) provide a source of protein withoutphenylalanine, but with other amino acids necessary for adequate nutrition.

Pterins (in urine) search for term

A group of substances (metabolites) produced during the breakdown of tetrahydrobiopterin in the body. The profile of different pterins found in the urine of patients with tetrahydrobiopterin deficiency is measured to determine which of the enzymes involved in the production oftetrahydrobiopterin is not working properly. See also tetrahydrobiopterin deficiency.

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Sapropterin search for term

A chemical name for a form of tetrahydrobiopterin (BH4): a naturally occurring substance that is essential for the activity of phenylalanine hydroxylase (the enzyme that is deficient in PKU). In a subset of patients with PKU (the tetrahydrobiopterin-responsive phenotype – usually, but not exclusively, patients with milder forms of PKU), treatment with sapropterin increases the activity of phenylalanine hydroxylase and improves control of blood phenylalanine.

Screening test(s) search for term

Screening tests are used for the detection of diseases in populations. All newborn babies in most countries are screened for PKU and other inherited metabolic diseases in the first days or weeks of life. Appropriate therapeutic intervention can then be applied where necessary to prevent long-term adverse consequences of the disease.

Silent mutation search for term

Not all DNA mutations alter the activity of the resulting protein. For example, a codon changing from CAA to CAG would produce glutamine in the final protein in either case, and the protein would be identical for either genotype. This silent mutation has been described for phenylalanine hydroxylase.

Special diet search for term

Patients with PKU must carefully control their intake of phenylalanine and require a special,phenylalanine-restricted diet to do so.

Splice search for term

Interruption of a sequence of DNA base pairs to insert new DNA or to delete existing DNA. The IVS10 splicing mutation is common among some populations with PKU, and severely diminishes the activity of phenylalanine hydroxylase.

Stop codon search for term

The sequence of three nucleotides (bases) in DNA/RNA that provides an instruction to stop transcribing DNA into RNA or producing a protein.

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Tandem mass spectrometry search for term

An advanced technique for mass newborn screening that tests blood from newborn babies for PKU along with a number of other inherited diseases. This is the most advanced screening methodology currently available and has already replaced older methods (Guthrie test, fluorometric screening) in many countries.

Tetrahydrobiopterin search for term

A naturally occurring substance (a cofactor) that is essential for the activity of phenylalanine hydroxylase (the enzyme that is deficient in PKU). A pharmacological formulation oftetrahydrobiopterin is available in Japan.

Tetrahydrobiopterin deficiency search for term

A condition in which gene mutations impair the production of tetrahydrobiopterin, the cofactor for phenylalanine hydroxylase. The activity of any one of several enzymes may be deficient (GTP cyclohydrolase I; 6-pyruvoyl-tetrahydropterin synthase; sepiapterin reductase; pterin-4-alpha-carbinolamine dehydratase 1; dihydropteridine reductase). Some, but not all, forms of tetrahydrobiopterin deficiency feature hyperphenylalaninemia, requiring treatment with aphenylalanine-restricted diet. About 1–2% of cases of hyperphenylalaninemia detected on routine newborn screening are due to tetrahydrobiopterin deficiency.<br/>Tetrahydrobiopterin is also required for the production of key neurotransmitters in the CNS, and some forms of tetrahydrobiopterin deficiency present with symptoms arising from alteredneurotransmitter activity.

Tetrahydrobiopterin loading test search for term

A medical investigation to identify people who respond to treatment with sapropterin. A successful response is determined according to the size of the reduction in blood phenylalanineafter treatment (usually, but not always, a 30% reduction in the level of blood phenylalaninecompared with the level immediately before treatment). Several different loading tests are currently in use at different centres.

Tetrahydrobiopterin responsive search for term

This term is used to describe people with PKU who have responded positively to treatment withsapropterin in a tetrahydrobiopterin loading test. Such people are potentially medically suitable for long-term treatment with sapropterin to help control their blood phenylalanine.

Tyr search for term

The shortened name for tyrosine, an amino acid usually largely provided by the conversion ofphenylalanine by an enzyme, phenylalanine hydroxylase.

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Variant PKU search for term

This is usually used to describe patients with PKU who have some residual activity ofphenylalanine hydroxylase. Blood phenylalanine levels are usually lower than in classical PKU, although a phenylalanine-restricted diet is still usually necessary. People with residual activity ofphenylalanine hydroxylase are more likely to respond to treatment with sapropterin than people with classical PKU.

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White matter search for term

The CNS contains ‘grey matter’, containing the bodies of nerve cells, and ‘white matter’, which is made mainly of nerve fibres (axons). Long-term sub-optimal control of blood phenylalanine has been observed to cause damage to white matter in the brain.

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