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PHENYLKETONURIA (PKU)
BACKGROUNDER
What is PKU?
Phenylketonuria (pronounced fen-il-Key-to-New-ree-uh), or PKU, is a rare genetic metabolic
disorder that, if left unmanaged in infants and children, can result in severe mental retardation.
Individuals with PKU are born with a deficiency in the activity of the enzyme called
phenylalanine hydroxylase (PAH). With little or no PAH activity, they are unable to metabolize
an essential amino acid, called phenylalanine (Phe), which is found in most foods including
meat, eggs, dairy, nuts, pasta, bread and all fruits and vegetables. (Note: An amino acid is one
that humans obtain from their diet and cannot produce on their own.)
What are the health risks for children and adults living with PKU?
When Phe cannot be metabolized by the body, abnormally-high levels of it accumulate in the
blood and are toxic to the brain. Complications of PKU can include:
ƒ
Mental retardation; e.g., IQ loss
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Mood disorders; e.g., depression, irritability, phobias, anxiety, hypochondria
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Irregular motor functioning; e.g. spasticity, tremor
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Abnormal behavior; e.g., Attention Deficit and Hyperactivity Disorder (ADHD), agoraphobia
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Brain/executive function abnormalities; e.g., slower thinking, slower communication
between areas of the brain, and inhibition of working memory as measured by EEG and
brain scans
How is PKU diagnosed?
All newborn babies in the United States are screened for some serious or life-threatening
conditions, including PKU. As a result of newborn screening efforts implemented in the 1960s
and early 1970s, most PKU patients in the developed world are diagnosed at birth.
Who has PKU? Is it a common disorder?
It is estimated that approximately12,000 infants in the United States are born with PKU1, and
about 50,000 people younger than 40 have the disorder in the developed world. It is one of the
most common genetic metabolic disorders.
If two people with the recessive PKU gene conceive a child, there is a one in four chance that
their child will be diagnosed with PKU.
How do patients and healthcare providers manage PKU?
To manage their disorder, PKU patients must lower their blood Phe levels in order to avoid
complications. Currently, blood Phe levels only can be lowered through a Phe-restricted diet,
which requires patients to monitor their daily intake of a prescribed amount of Phe. A Pherestricted diet is supplemented with vitamin and mineral replacement formulas and speciallyproduced low-Phe foods.
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How long does a PKU patient have to follow a strict, low-Phe diet?
Based on multiple studies that show low blood Phe levels are crucial for healthy brain and
neuropsychological functioning, experts advocate that PKU patients follow the diet for life.
Researchers also believe that previously untreated adults with PKU can benefit from reducing
their blood Phe levels.2
Do patients have difficulty following a strict diet?
Compliance with diet is a major problem for PKU patients, particularly for older children,
adolescents and adults3. The issues causing poor compliance include very limited food choices,
social limitations of following a strict diet, poor palatability of formulas, availability and cost of
specially-produced low-Phe foods, family beliefs that diet can be discontinued, and getting
patients to keep diet records.4
Is diet enough to manage PKU?
The Phe-restricted diet can prevent severe mental retardation, yet studies now show that
managing PKU entirely through diet may not produce completely normal mental and
neurological results for patients. Dietary restrictions – although beneficial in lowering Phe
intake/levels – do not have the ability to remedy the underlying enzymatic defect that exists in
individuals with PKU. As a result, PKU patients managed with a Phe-restricted diet can show
higher incidences of lower IQ scores, executive functioning deficits, depression, anxiety,
phobias, self-image disturbances, and behavioral and school problems.5
What kind of research is being done for PKU?
BioMarin Pharmaceutical Inc. has completed clinical studies of its investigational drug,
sapropterin dihydrochloride, or sapropterin. If approved by the FDA, the brand name for this
treatment is expected to be Kuvan™ and it would be the first and only FDA-approved
prescription drug to treat PKU by lowering Phe levels and increasing Phe tolerance. Increasing
Phe tolerance may enable patients to follow a more relaxed diet and may address some of the
shortcomings of a highly-restricted diet. BioMarin is exploring other approaches to the treatment
of PKU, in particular for patients who may not benefit from sapropterin.
Where can I learn more about PKU?
•
Visit www.PKU.com. Register as a member to take advantage of discussion forums.
•
Visit National PKU News at www.pkunews.org for up-to-date, accurate news and information
on PKU.
•
Contact the offices of [insert local EAP investigator] at [insert phone number] to learn
more about PKU.
References
1
Consensus Development Conference on Phenylketonuria (PKU): Screening and Management, NIH 2000
Diet Intervention Guidelines for Adults with Untreated PKU, 2000; Dolan, B.; Koch, R.; Bekins, C.; Schuett, V.
3
Walter 2002
4
Wappner, 1999
5
Arnold 2004; Azen 1996; Bhat 2005; Feldmann, et al, 2005; Gassio et al, 2005; Leuzzi, et al, 2004; Netley 1984; Smith and
Knowles, 2000; White et al, 2001; White et al, 2002; Wiersema et al, 2005
2
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