Phenylketonuria 2011
Contents
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Summary
Phenylketonuria is a serious metabolic disease which causes several syndromes if untreated in newborns. Most syndromes 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 and 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 |
The enzyme phenylalanine hydroxylase catalyzes the conversion of phenylalanie to tyrosine. If the function of phenylalanine hydroxlase is reduced by a heavy amount 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 provided by 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 of individuals different categories are applied. Individuals with values from 120–600 μmol/L are classified as having mild hyperphenylalaninaemia, 600–1200 μmol/L is classified as mild phenylketonuria and individuals with concentrations above 1200 μmol/L are classified as having the classical phenylketonuria.
Cross-references
See also description of this disease in
Biochemical disease mechanism
Phenylalanine hydroxylase is involved in the phenylalanine pathway. It catalyses the conversion of phenylalanine to tyrosine. It's the major way to reduce the concentration of phenylalanine. In phenylketonuria the function of this protein is reduced by at least ... %. The missing reduction of phenylalaine leads to harmful concentrations of phenylalanine. The enzyme ... is responsible for the transport of large neutral amino acids across the blood brain barrier. Through the high concentration of phenylalanine other large neutral amino acids are less frequently transported to the brain. These missing amino acids cause severe problems in the brain development.
Cross-references
Diagnosis of phenylketonuria
In the last decades diagnosis of phenylketonuria shifed 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 10-14days. 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, of course phenylketonuria.
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 phenylalanine 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 essential amino acids but 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. This effected can be explained with the help of the brain blood barrier. If other large neutral amino acids are present, other than only phenylalanine, they all together compete to enter the brain via the brain blood barrier. Hence, a increased amount of large neutral amino acids might reduce the overall concentration. However, clinical data for this kind of therapy is sparse and further research in this area has to be done.
Phenylalanine ammonia lyase
Gene therapy
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
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's will then have a 25% chance to be affected from phenylketonuria because these individuals inherited both dysfunctional PAH alleles from their parents. Furthermore, there is only a 25% for their offspring's 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 a 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 require 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).
References
- Nenad Blau, Francjan J van Spronsen, Harvey L Levy . Phenylketonuria. Lancet 2010; 376: 1417–27
- PAH gene description by NIH
- ENSEMBL
- PAHdb
- Wikipedia
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% 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.
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.
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
Disease causing mutations
There are 509 disease causing mutations known (HGMD).
