Phenylketonuria 2012

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Revision as of 09:21, 14 August 2012 by Hollizeck (talk | contribs) (Treatment)

Phenylketonuria (PKU) is an autosomal recessive inherited disorder. It is monogenetic and the connected gene (PAH) is located at the chromosome 12, which encodes for the hepatic enzyme phenylalanine hydroxylase(PAH). People who suffer from this disorder have a strongly decreased activity of this enzyme and therefore suffer from the toxic effects of the accumulated phenylalanine(Phe) which leads mostly to mental retardation but also to certain other symptoms. Due to the ongoing development for treatment and adhering to dietary restrictions the patients can avoid most effects of the disease. A overview for the pathway is shown in <xr id="fig:pathway" />. <figure id="fig:pathway">

schematic phenylalanine pathway extract from a patient with PKU

</figure>

History

In 1934 this disorder was first found, but only in the concurrence of hyperphenylalaninemia (HPA) and mental retardation, by Ivar Asbjørn Følling, a Norwegian physicist. Because of this fact in Norway PKU is called Følling's disease. Dr. Følling also found that the patients not only have increased phenylalanine levels but their urine contains phenylpyruvic acid, which is diagnostic.

Symptoms

Most of the Symptoms have their cause in the competitive inhibition of the large neutral amino acid transporter (LNAAT), which is responsible for the transport of large neutral amino acids like Phenylalanine through the Blood-brain-barrier. This LNAAT is saturated by the high concentration of Phe and therefore can not transport any other essential amino acids into the brain. Those missing amino acids are crucial for the neurotransmitter and protein/hormone synthesis. As of this most of the symptoms occurring with PKU are connected with the brain development. There are some other "soft"symptoms as well.

The PAH gene

Phenylalanine hydroxylase is coded by the gene PAH. PAH lies on Chromosome 12 from 103232104bp to 103311381bp. There are over 400 mutations of that gene known that cause complete or partial loss of enzyme activity. Selected (commented) examples can be found in the OMIM database. <figtable id="tab:typesofmutation">

Shows the probability of a type of mutation occuring in the PAH gene
% of mutations genetic mechanism
62 missense
13 deletion
11 splice
6 silent
5 nonsense
2 insertion
<1 deletion or duplication of exon or even gene

</figtable>

Diagnosis

If the disease is recognised and treated within the first days of a newborn, before toxic amounts of phenylalanine can build, patients can avoid the symptoms of PKU. Newborn screenings generally carried out between day 2 and day 5 include a test for PKU. The most simple test is a bacterial inhibition assay (Guthrie test) that uses Bacillus subtilis, a bacteria that needs phenylalanine to grow. More reliable tests employ mass spectrometry, checking not only the level of Phe but also the tyrosine/phenylalanine ratio.

To differentiate patients with high Phe levels due to PKU from patients with BH4 deficiency, a cofactor of PAH, a 'BH4 loading test' can be performed. This also detects the subgroup of PKU patients that can be treated with BH4, since the cofactor also has some chaperone activity.

Treatment

<figure id="fig:catalyse">

Catalysation of phenylalanine by phenylalanine hydroxylase and phenylalanine ammonia lyase. PAH requires the presence of its cofactor, BH4, as well as a Fe+ molecule and oxygen. PAL does not need a cofactor. Taken from Bélanger-Quintana et al. (2011)

</figure> The most effective way to treat the symptoms of PKU is to avoid food rich in phenylalanine. This requires a strict diet excluding all animal protein, dairy products and many vegetable food and including medical food, especially milk powder for newborns low in Phe. A overviwe in the process can be found in <xr id="fig:pathway" /> and <xr id="fig:catalyse" />

10-60 % of the patients respond to treatment with tetrahydrobiopterin (BH4), the cofactor of PAH, or a synthetic alternative. This cofactor possesses chaperone activity and apparently can restore PAH function for certain mutations.


Treatments in development are enzyme substitution therapy and gene therapy. The enzyme phenylalanine ammonia lyase (PAL) catalyses Phe to ammonia and non-toxic trans-cinnamic acid. In mice it lowered the Phe level safely and persistently and is in clinical trials for humans since 2009. Gene therapy so far has managed to temporarily restore PAH activity for up to a year. Since a viral vector containing a functional PAH-gene could be injected in the liver but was not integrated in the genome, effects vanished when the liver regenerated. Reinjection lead to an immune response to the viral vector.

Cross reference

PAH in external databases

The protein sequence of functional human phenylalanine hydroxylase: P00439 at Uniprot)

The gene sequence of functional human phenylalanine hydroxylase: PAH at NCBI

The protein and ligand structure: 1DMW in PDB

Crystal structure of human phenylalanine hydroxylase

Tasks

References

N. Blau, J. Hennermann, U. Langenbeck, U. Lichter-Konecki, Diagnosis, classification, and genetics of phenylketonuria and tetrahydrobiopterin (BH4) deficiencies; Molecular Genetics and Metabolism 104 (2011) S2-S9

A. Bélanger-Quintana, A. Burlina, C. Harding, A. Muntau, Up to date knowledge on different treatment strategies for phenylketonuria; Molecular Genetics and Metabolism 104 (2011) S19-S25