Introduction to Phenylketonuria

Phenylketonuria (PKU) is an inherited disorder caused by a mutation on the PAH gene on chromosome 12. Since the pattern of inheritance of this disorder is autosomal recessive, each of the 2 parents should carry one copy the mutated gene, for the child to inherit the disorder.

PAH structure

The PAH gene codes for Phenylalanine Hydroxylase (PAH) which is responsible for the catabolism of the amino acid Phenylalanine (Phe). The PAH exists as a tetramer.

The mutation of the PAH will result in the inability or reduced ability of the enzyme to convert Phenylalanine to Tyrosine (Tyr). The concentration of Phenylalanine will build up in the body and eventually this will poison the neurons as they are sensitive to Phenylalanine levels.

If still left untreated, Phenylalanine can build up to harmful levels in the body, causing intellectual disability and other severe health problems.

The Normal Situation 

Graphic representation of PKU

Studies have proven that another form of PKU exists due to inactive coenzymes. The main inactive coenzyme causing PKU is (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4). PAH together with BH4 catalyses the hydroxylation of L-Phe to L-Tyr.L- Tyr is then converted to Fumarate and then form CO2 and H2O. This metabolism mainly occurs in the liver.

Not much is known about cellular regulations of BH4 but several studies have shown that some patients suffering from PKU have decreased levels of tetrahydrobiopterin in their blood serum. Therefore, any mutation or change in structures of BH4 can lead to a rare form of Phenylketonuria. 

Structure of BH4

References

• Phenylketonuria - Genetics Home Reference. 2012. Phenylketonuria - Genetics Home Reference. [ONLINE] Available at: http://ghr.nlm.nih.gov/condition/phenylketonuria. [Accessed 28 December 2012].
• Phenylketonuria: What causes it? . 2012. Phenylketonuria: What causes it? . [ONLINE] Available at: http://www.yourgenesyourhealth.org/pku/cause.htm. [Accessed 28 December 2012].
• Basic PKU Biochem. 2012. Basic PKU Biochem. [ONLINE] Available at: http://www.uic.edu/classes/phar/phar332/Clinical_Cases/aa%20metab%20cases/  PKU%20Cases/PKU%20biochem%20intro.htm. [Accessed 28 December 2012].
• Neurowiki2012 - Phenylketonuria. 2013. Neurowiki2012 - Phenylketonuria. [ONLINE] Available at: https://neurowiki2012.wikispaces.com/Phenylketonuria#Ref12. [Accessed 13 January 2013].

Symptoms of Phenylketonuria

"Phenylalanine plays a role in the body's production of melanin, the pigment responsible for skin and hair color. Therefore, infants with the condition often have lighter skin, hair, and eyes than brothers or sisters without the disease.

Other symptoms may include:
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

If the condition is untreated or foods containing phenylalanine are not avoided, a "mousy" or "musty" odor may be detected on the breath and skin and in urine. The unusual odor is due to a build up of phenylalanine substances in the body."

Adapted from

• Phenylketonuria - Symptoms, Diagnosis, Treatment of Phenylketonuria - NY Times Health Information . 2013. Phenylketonuria - Symptoms, Diagnosis, Treatment of Phenylketonuria - NY Times Health Information . [ONLINE] Available at: http://health.nytimes.com/health/guides/disease/phenylketonuria/overview.html. [Accessed 13 January 2013].

Diagnosis of Phenylketonuria

A PKU test is conducted a day or two after the baby is born. In the test, a nurse or lab technician collects a few drops of blood from the baby's heel or the bend in the baby's arm using a needle or lancet. A laboratory tests the blood sample for certain metabolic disorders, including PKU. The test isn't done before the baby is 24 hours old or before the baby has ingested some protein in the diet for accuracy in results. If the baby isn't delivered in a hospital or are discharged soon after the birth, a newborn screening with pediatrician or family doctor is compulsory.

Babies may also have additional tests to confirm the diagnosis, including more blood tests and urine tests. The parent and baby may also undergo genetic testing to identify any gene mutation leading to PKU.

A simple method utilized to detect phenylketonuria was the Gunthrie Test which employs the use of a bacterial inhibition assay. This bacterial requires phenylalanine for growth. The agar also contains a nonmetabolizable competitive inhibitor set at a certain concentration that the amount of phenylalanine in normal will not support noticeable growth.

Reference

• Phenylketonuria: Tests and diagnosis - MayoClinic.com. 2013. Phenylketonuria: Tests and diagnosis - MayoClinic.com. [ONLINE] Available at: http://www.mayoclinic.com/health/phenylketonuria/DS00514/DSECTION=tests-and-diagnosis. [Accessed 13 January 2013].
• Guthrie test definition - Medical Dictionary definitions of popular medical terms easily defined on MedTerms. 2013. Guthrie test definition - Medical Dictionary definitions of popular medical terms easily defined on MedTerms. [ONLINE] Available at: http://www.medterms.com/script/main/art.asp?articlekey=11390. [Accessed 17 January 2013].

Heel Prick Method - Video

Biochemistry behind Phenylketonuria

Someone suffering from PKU is incapable of metabolizing Phenylalanine which can lead to great toxicity for neuronal cells. There are two main pathways in which Phenylalanine can be metabolized. First is the most common path whereby Phenylalanine (Phe) is oxidized to Tyrosine (Tyr). The second or the minor pathway is Phe transamination to phenylpyruvate and subsequent further metabolism to Phenylacetylglutamate, O-Hydroxyphenylacetate and Phenyl Lactate. The metabolites in the minor pathway will be released into the bloodstream and brought to the brain.

Diagram depicting metabolism pathways of Phenylalanine

When the PAH is mutated and unable to convert Phenylalanine to Tyrosine resulting in Phe levels buiding up, the minor pathway takes over the major pathway. The minor pathway leads to a rise in levels of Phenylacetylglutamate which is known to be substance that is very toxic and on its own, can lead to alteration in mental status and cognitive impairments when accumulated. In addition, studies have shown that other substances created in the minor pathway including Phenyl lactate, can also lead to growth retardations by causing deficit of myelin in the cerebral hemispheres and the cerebellum and therefore, it has been directly linked to the mental retardations in PKU.

A mutation in the cofactor of PAH tetrahydropterin (BH4) can also cause PKU. This is called malignant PKU. This can also be caused by defective synthesis of the BH4. This deterioration cannot be removed by limiting the phenylalanine dietary intake. Consequence of such problem is defection in neurotransmission.

Reference

• Neurowiki2012 - Phenylketonuria. 2013. Neurowiki2012 - Phenylketonuria. [ONLINE] Available at: https://neurowiki2012.wikispaces.com/Phenylketonuria. [Accessed 13 January 2013].
• Malignant phenylketonuria due to defective syn... [Isr J Med Sci. 1985] - PubMed - NCBI. 2013.  [ONLINE] Available at: http://www.ncbi.nlm.nih.gov/pubmed/3874852. [Accessed 25 January 2013].

Treatment for PKU : Diet restrictions

Treatment

The main concept in Phenylketonuria(PKU) treatment is to reduce the level of phenylalanine(phe)  in blood for normal brain development. An individual’s blood phe level depends on dietary intake of phenylalanine and the residual activity of phenyalanine hydroxylase(PHA).

Dietary management

The goals of this management is to prevent excessive accumulation of phe by controlling the intake of natural proteins at the same time allowing for normal growth and micronutrient biochemical status. usually through phe-free protein substitutes to account for the reduced protein intake.
Patients with PKU should adhere to a semi-synthetic diet is used which comprises of:

• Foods with low phe content such as many fruits and vegetables
• Weighed amounts of foods containing medium amounts of phe (e.g. broccoli, potato).
• Protein substitutes e.g. Phe-free amino acid mixtures to provide normal or supra-normal total protein intake
• vitamins, minerals and trace elements.

 The amount of phe ingested is often calculated using an exchange system. In the UK system 1 ‘exchange’ = 50 mg phe which is approximately 1 g protein

The precise tolerance for phe varies, but for most individuals with PKU it is between 200 and 500mg/day. furthermore, the phe tolerance of the individual may change over time, so careful and continuous monitoring of the individuals whose phe intake is restricted is necessary to avoid both elevations and deficiencies of phe.

References:

• ScienceDirect.com - Paediatrics and Child Health - Phenylketonuria. 2013. ScienceDirect.com - Paediatrics and Child Health - Phenylketonuria. [ONLINE] Available at: http://www.sciencedirect.com/science/article/pii/S1751722210001836. [Accessed 17 January 2013].

Suggested diet - Low in Phenylalanine content

Reference

Vanessa Shaw, Margret Lawson, M, 2001. Clinical paediatric dietetics. 2nd ed. UK: Blackwell UK.

Treatments for PKU: Alternative treatments

New treatments for PKU
As long-term dietary modification is difficult and a great hassle, PKU patients may risk being outcasted by friends and families due to their abnormal dietary needs, there is a need for alternative modes of treatments.


Large neutral amino acids
The large neutral amino acids (LNAA) including phe compete at the blood brain barrier for entry to the brain through the same transporter (LAT1). Increasing the concentration of LNAA in the blood therefore reduces phe entry to the brain. There is a similar mechanism in the gut, and absorbed phe is lower if LNAA are supplemented in generous amounts. It is unlikely that LNAA given as sole treatment without phe restriction could replace diet in childhood but may be a useful approach for adults.

Sapropterin therapy (Tetrahydrobiopterin, BH4, trade name: Kuvan)

Sapropterin is an important cofactor of PAH in the degradation of phe. Due to the lack of PAH expression in PKU patients, the degradation of phe is also reduced as a result. We can reconstitute the lack of PAH expression through Sapropterin therapy. Thus resulting in reduction of phe levels. However this is more likely to work for cases of mild PKU rather than classic PKU as the later PKU may have absolutely no expression of PAH. it has been estimated that 80% of those with mild PKU and 40% of those with classical PKU weill benefit from this treatment. To be sure of the patients' response to this treatment, Genotyping could be used to predict the response but should not be assumed.

Gene therapy

The most promising results come from experiments using recombinant adeno-associated virus vector in which long-term correction without adverse effects has been reported in the mouse model (PKUenu2). There are no human gene therapy studies yet.


Liver Transplant

A liver transplantation can effectively provide a healthy PAH activity in a PKU patient. But the complications of this procedure makes transplantation an unrealistic treatment option for PKU

Future Prospects of research to tackle Phenylketonuria

One promising therapy involves oral administration of recombinant phenylalanine lyase (PAL), an enzyme that degrades phenylalanine in the intestinal tract into trans-cinnamic acid that can be broken down in the liver of people with PAH deficiency. Animal studies and limited human studies have suggested that PAL has the potential to make the PKU diet less restrictive for humans. It has been tested using intraperitoneal and subcutaneous routes. The utility and feasibility of enzyme replacement using PAL continue to be evaluated, though less progress has been made. This avenue is still open to research and lots of trails are underway currently. The key to an effective treatment method would be to have sufficient bioavailability, minimal side effects and consistent effectiveness.

Research on foods and supplements also shows promise for improvong the lives of those affectcted by PKU. Glycomacropeptide (GMP) is a whey protein and the only protein free of Phe; researchers have been able to to successsfully create foods out of GMP that were met with positive reviews by people with PKU who tried them. This GMP food may be made available to the common public commercially in time to come.

Reference

• Patricia Jackson Allen, Judith A Vessey,  2010. Primary care of the Child with a Chronic Condition . 5th ed. United States of America: Mosby Elsevier.

My PKU Life - Life Account of a person living with Phenylketonuria