Phenylketonuria 2011

From Bioinformatikpedia

Summary

Phenylketonuria is a serious metabolic disease which causes several symptoms if untreated in newborns. Most symptoms affect the mental abilities of individuals, for example reduced intelligence or hyperactivity. This disease is caused by a defect of the phenylalanine hydroxylating system which has a dramatically reduced activity in affected individuals. Thus, leading to a toxic concentration of phenylalanine. The gene associated with phenylketonuria is PAH which encodes for the protein phenylalanine hydroxylase.

Phenotype

Phenotypes
Delayed mental and social skills
Head size significantly below normal
Hyperactivity
Jerking movements of the arms or legs
Mental retardation
Seizures
Skin rashes
Tremors
Unusual positioning of hands
Fair coloring
Mousy odor of skin, hair, sweat and urine

The enzyme phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine. If the function of phenylalanine hydroxlase is reduced significantly the individuals suffer from phenylketonuria. The amount of phenylalanine in the blood rises to harming concentrations, which leads to several symptoms, which e.g. include mental retardation, hyperactivity and reduced head size. An exhaustive list of symptoms is listed in the attached table. A normal concentration of phenylalanine in the blood ranges from 50–110 μmol/L. Values above that can be interpreted as toxic. Depending on the concentration of phenylalanine in the blood different categories are applied. Individuals with values from 120–600 μmol/L are classified as having mild hyperphenylalaninemia (HPA), 600–1200 μmol/L is classified as mild phenylketonuria and individuals with concentrations above 1200 μmol/L are classified as having the classical phenylketonuria.

The reason for the fair coloring of patients is the missing of tyrosine, which is a precursor to the pigment melanin.

If phenylalanine is over-represented, it will be metabolized to phenylketone. Patients with phenylketonuria excrete the produced phenylketone in urine and sweat. This causes the mousy odor of skin, hair, sweat and urine.

Cross-references

See also description of this disease in

Biochemical disease mechanism

Phenylalanine hydroxylase catalyses the conversion of phenylalanine to tyrosine. It is the major pathway to reduce the concentration of phenylalanine. The reduced activity of phenylalanine hydroxylase in patients with phenylketonuria leads to harmful concentrations of phenylalanine. Phenylalanine is a large neutral amino acid. There is a transporter across the blood-brain barrier for these kind of acids, the large neutral amino acid transporter. Large neutral amino acids compete for the transportation by this enzyme. If phenylalanine is over-represented other amino acids will be missing in the brain. That is especially critical during brain development.


Cross-references

Diagnosis of phenylketonuria

In the last decades diagnosis of phenylketonuria shifted away from a clinical, symptom orientated, diagnosis to a biochemical diagnosis. Due to neonatal screening a diagnosis whether newborns suffer from a form of phenylketonuria can be diagnosed early in life before symptoms develop after 10-14 days. This is done by measuring the concentration of phenylalanine in blood. A standard method known as the "heel prick" test is normally applied to all newborn infants for this purpose. This test simply takes blood from the heel of the infant which is then taken to test against a range of genetic diseases for example cretinism, cystic fibrosis and phenylketonuria of course.

References

  • van Spronsen, F. J. Phenylketonuria: a 21st century perspective. Nat. Rev. Endocrinol. 6, 509–514 (2010)
  • Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27
  • Heel prick test on Wikipedia

Treatment of phenylketonuria

Diet therapy

The most common strategy to reduce phenylalanine concentration in affected individuals is by reducing the intake of phenylalanine rich food. This has to be done from the very first day when phenylketonuria is diagnosed in infants to avoid the commonly known symptoms of phenylketonuria. These dietary products are mainly low protein products. Affected individuals have to avoid food like e.g. meats, fish, eggs, standard bread, most cheeses, nuts, and seeds. In addition there is also a need to avoid drinks which contain aspartame, flour and soya. Also beer or cream liqueurs are not recommended. Recommended food include potatoes, some vegetables, and most cereals products. However, even these products shall be eaten only in a restricted manner. In alternative to natural products the industry provides special low-protein food for affected individuals like e.g. low-protein bread and low-protein pasta.

Glycomacropeptide

This protein contains no traces of the amino acids tyrosine, tryptophan, or phenylalanine. Instead, it is a rich source for all other essential amino acids. Therefore, it is recommended as supplement for the standard diet therapy to ensure a sufficient level of all essential amino acids apart from phenylalanine.

BH4

Phenlyketonuria is not only a result of dysfunctional phenylalanine hydroxlase protein (See chapter about The PAH gene for further details). In addition, there are individuals who do not suffer from the classical phenylketonuria. In these individuals the synthesis or the recycling system of the co-factor tetrahydrobiopterin (BH4) might be affected. In those cases a injection of pharmacological doses of BH4 may help to reduce the concentration of phenylalanine to a normal (non toxic) level.

Large neutral aminoacids

Doses of large neutral amino acids might help to reduce the concentration of phenylalanine in the brain. If a higher concentration of other large neutral amino acids is present, other than only phenylalanine, they all together compete against each other to enter the brain via the brain blood barrier. Thus, an increased amount of large neutral amino acids might reduce the probability that phenylalanine enters the brain which finally results in a overall lower concentration of phenylalanine in the brain to a, hopefully, non-toxic level. However, clinical data for this kind of therapy is sparse and further research in this area has to be done.

Phenylalanine ammonia lyase

This bacterial protein is able to catalyze a transformation of phenylalanine to trans-Cinnamic acid and ammonia without a cofactor requirement. So far, this therapy was successful in mouse models of phenylketonuria.

Gene therapy

Another approach to fight phenylketonuria could be gene therapy. This approach tries to introduce a vector of a functional PAH gene to the DNA. If successful, individuals will be able to express their own phenylalanine hydroxylase protein.

References

  • van Spronsen, F. J. Phenylketonuria: a 21st century perspective. Nat. Rev. Endocrinol. 6, 509–514 (2010)
  • Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27

The PAH gene

Figure 1: Phenylketonuria is inherited in an autosomal recessive fashion. Disclaimer: This file is redistributed from Wikimedia and copyrighted under the GFDL.

The PAH gene, also known as phenylalanine hydroxylase, is located on the long arm of the autosomal chromosome 12 between positions 22 and 24.2 in humans. The precise location is defined from base pairs 103,232,103 to 103,311,380 which results in a total length of 79,277 bps on the chromosome. This gene consists of 13 exons and 12 introns, after the introns of the pre-mature mRNA are spliced away a length of only 2,681 bps is left on the transcript. This means only 3.38% of the original gene size is left on the mature mRNA. However, the full length of a functional phenylalanine hydroxylase protein is after translation 452 residues.


Individuals who suffer from Phenlyketonuria require two mutated alleles of the PAH gene of which the protein product has to be severe dysfunctional in its ability to catalyze the transformation from phenylalanine to tyrosine. This is only possible when both healthy parents carry one dysfunctional allele on their chromosome 12. Their offspring will then have a 25% chance to be affected by phenylketonuria because these individuals inherited both dysfunctional PAH alleles from their parents. Furthermore, there is only a 25% for their offspring to be a non-carrier of a dysfunctional allele and obviously a 50% chance to inherit exactly one dysfunctional allele of the PAH gene.


Protein function

A properly functional phenylalanine hydroxylase protein realizes the transformation from phenylalanine to tyrosine by hydroxylating the substrate, in our case phenylalanine. More precisely, it adds an OH group to the 4th position of the 6-carbon aromatic ring of phenylalanine, thus resulting in a tyrosine.

However, phenylalanine hydroxylase requires three helper molecules for the process which are O2, Fe+2 and tetrahydrobioterin (BH4). BH4 is synthesized from guaninethreephosphate (GTP) in a three step process which requires the enzymes GTP cyclohydrolase I (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTPS) and sepiapterin reductase (SR). During the hydroxylating process of phenylalanine to tyrosine the molecule BH4 is consumed and has to be recycled in order to be reused again in another hydroxylating process. This recycling process is catalyzed by the two enzymes carbinolamie-4adehydratase (PCD) and the NADH-dependent dihydropteridine reductase (DHPR).


Figure 2: Structure of phenylalanine. This amino acid is used by phenylalanine hydroxylase as a substrate. Disclaimer: This file is redistributed from Wikimedia and copyrighted under the public domain.
Figure 3: Structure of tyrosine. This amino acid is the product of the hydroxylation of phenylalanine by phenylalanine hydroxylase. Disclaimer: This file is redistributed from Wikimedia and copyrighted under the public domain.
Figure 4: Schematic process of the phenylalanine hydroxylating system. Disclaimer: This file is redistributed from [Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27] . All rights belong to the creator.

Protein Structure

More to the protein structure can be found here.

References

Mutations

The PAH gene is located on a highly heterogenic locus, by now there are 642 mutations for the PAH gene known (as stated by HGMD, 10th of May 2011). 509 of these mutations could be associated with the disease phenylketonuria (as stated by HGMD, 10th of May 2011).

Mutation types

The 3 most common mutation types for the PAH gene locus are missense, deletion and splice junction mutations with 60%, 13.48% and 10.99% of all mutation types respectively.

Reference

Gene region of mutations

After analyzing the gene regions of known mutations we found out that most mutations are located on exons of the PAH gene. The exons E7, E6, E11 and E3 contain 15.43%, 13.83%, 8.87%, 8.69% and 7.80% respectively of all mutations. In contrast, almost all introns of PAH (with only a few exceptions) contain less than 1% of all known mutations. However this seems to be somehow not surprising since more than 60% of all mutations are of type missense.

Figure 5: Mutation map of the PAH gene. last updated: August 13th, 2007 Disclaimer: This figure is redistributed from http://www.pahdb.mcgill.ca. All rights belong to the creator.

Reference

Position of the mutations on the phenylalanine hydroxylase protein

70% of all disease associated mutations are located on the catalytic domain of phenylalanine hydroxylase. Whereas the regulatory domain and the tetramerisation domain contain 16% and 14% respectively of all disease associated mutations.

Figure 6: This figure shows disease associated mutations on the phenylalanine hydroxylase protein. Positions of mutations are highlighted by green side chains. The shown domains are catalytic domain (blue), regulatory domain (red), tetramerisation domain (lilac). Disclaimer: This figure is redistributed from [Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27] . All rights belong to the creator.


Reference

  • Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27

Mutations by ethnicity

The occurrence of mutations for the PAH locus on chromosome 12 is unevenly distributed among populations. For example most mutations could be found among people with English or German ethnicity with 5.98% and 5.79% respectively. The distribution for the first eight ethnic groups is as follows:

  • English: 5.98%
  • German: 5.79%
  • Spanish: 5.14%
  • American: 4.48%
  • French-Canadian: 4.20%
  • Italian: 4.14%
  • Norwegian: 3.74%
  • Belgian: 3.71%

Reference

Reference sequence

The following links contain the reference sequence of the PAH gene

Neutral mutations

Hyperphenylalaninaemia causing mutations

Phenylketonuria causing mutations

There are 509 disease causing mutations known (HGMD).

Tasks

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