Sarah Lopes
Human Genetics
Mariluci Bladon
Due: 5/14/12
Phenylketonuria
Phenylketonuria (PKU) is a genetic disorder involving mutation of the phenylalanine hydroxylase (PAH)
gene, rendering the liver enzyme PAH nonfunctional and causing inability to metabolize the essential
amino acid phenylalanine (PHE). When PHE accumulates in the body unmetabolized, it is harmful to the
central nervous system and causes brain damage. A diagnosis of PKU is given when blood levels of PHE
exceed 12 mg/dl on a regular diet.1
PKU is an autosomal recessive disorder, meaning that offspring must inherit the mutated gene from
both parents in order to be affected. There is a 25% chance of inheriting the disease when both parents
are carriers. Occurrence varies among ethnic groups and geographic regions. In the US, occurrence is
approximately 1 in 16,000 births. There are varying levels of severity, the worst being Classic PKU, in
which severe brain damage occurs when PHE is not avoided. Milder forms are variant PKU or non-PKU
hyperphenylalaninemia. In these forms of the disease, the PAH gene mutation allows some PAH enzyme
function. Blood PHE levels do not climb as high in these cases.2
Effects of PKU include mental retardation or cognitive impairment, depression, delayed social skills,
hyperactivity/ADHD, microcephaly (small head), involuntary jerking movements and seizures, eczema, a
“mousy” or musty odor, unusual positioning of hands, and light skin and hair.* Because phenylalanine
plays a role in the body’s production of melanin, those afflicted with PKU tend to have pale skin and
hair, but not as extreme as in albinism.3
In affected babies, Blood PHE levels will start to rise within 24 hours of starting to drink breast milk or
formula, so testing must be performed as soon as possible to prevent possible damage. PKU testing
within the first few days after birth is now a standard procedure in North America. This practice was
instituted in the mid 1960s4. The baby must be drinking milk or formula for at least 24 hours in order to
produce accurate results. Blood is collected via heel stick and tested for PHE levels. Less than 2 mg per
deciliter is considered normal.5
Newborns found to be affected by PKU must be fed with a PHE-free formula to prevent damage in the
most critical early stage of development, but may be allowed sparing portions of breast milk to be mixed
with the formula.
The required diet can be very challenging to follow throughout life, and without supplementation of
PHE-free amino acid formulas, cutting out dietary protein will result in nutrient deficiencies. The odor
and taste of these formulas can be undesirable, but they are essential to maintaining good health for
those with PKU. (PKU Toolkit) Fish oil and/or iron supplements can also be beneficial.3
The correlation of dietary protein to the effects of PKU was discovered in 1951.4 The diet necessary for
controlling PKU greatly reduces or eliminates proteins naturally found in food, whether from animal or
plant sources, because they all contain PHE as a building block. The safest foods to eat are non-starchy
vegetables, fruits, natural sweeteners such as sugar and honey, and special low-protein breads and
pastas. All meat products, including steak or beef, pork, poultry, fish and shellfish, must be avoided,
along with eggs, dairy products, legumes, nuts, seeds, beans, and soy. Aspartame, an artificial sweetener
commonly known as NutraSweet, must also be avoided, as it is composed of phenylalanine and aspartic
acid. Starchy foods such as potatoes, corn, peas, and common varieties of breads and pastas contain a
fair amount of protein and should be consumed very sparingly in measured amounts or avoided.
It was once thought that returning to a relatively normal diet was safe once the affected individual
reached childhood or adolescence, but now the standard recommendation is “diet for life”. While the
most important time to follow the diet is in infancy when the brain is developing, and irreversible
mental retardation will most likely result if the diet is not followed, consuming PHE-containing foods
later in life causes problems with paying attention, concentrating, and remembering. Going back “on
diet” typically reverses this effect.2
Maternal PKU must be handled very carefully with a controlled diet in order for the fetus to develop
normally, even when the fetus is heterozygous and therefore unaffected by the disease. If the mother is
not following a PHE-free diet, the fetus is exposed to high enough levels of PHE in utero to cause mental
impairment, low birth weight, heart defects, and microcephaly. In a study involving 53 children born to
22 PKU-affected mothers at the PKU Clinic of Children’s Hospital, Dr. Harvey Levy and Dr. Susan
Waisbren determined that the IQ of the offspring was “significantly correlated with both maternal IQ
and maternal blood phenylalanine level.” Mental retardation in the offspring was found to be consistent
when the maternal PHE level exceeded 18 mg per deciliter.6
In 2005, a breakthrough PKU research paper was published by gene therapist Dr. Savio Woo and his
post-doctoral students Li Chen and Zhiyu Li at Mt Sinai School of Medicine. Dr. Woo’s students claimed
to have cured PKU in mice using a gene insertion technique with the use of a viral enzyme. When the
findings could not be replicated by other researchers, the matter was investigated and the post-doctoral
students were found to have engaged in scientific misconduct. The paper, along with others, was
regretfully retracted by Dr. Woo.7
Much focus has been placed upon enzyme replacement therapy as a treatment for PKU. PAH, the
naturally occurring liver enzyme produced by normal individuals, is unstable and has many complex
functional requirements. PAH has been administered to PKU mice in clinical studies, and although the
initial response was positive, this was short lived, as an immune response quickly rendered the foreign
enzyme ineffective. Phenylalanine ammonia lyase (PAL), an enzyme derived from plant, fungi, or
bacterial sources, appears to be more promising. PAL is a more stable enzyme, and unlike PAH, it does
not require a cofactor to function. It can convert the excess phenylalanine to trans-cinnamic acid and
ammonia, which are both harmless. As one might expect, however, PAL proved to be immunogenic as
well. In fact, the PAL enzyme was effective for no longer than 1 week in PKU mouse models with
repeated dosing.8
PEGylation is a process of attaching polyethylene glycol to a drug or therapeutic protein in order to
camouflage and protect them from degradation by immune response. PEGylated PAL (PEG-PAL) has
been shown to have substantially reduced immunogenicity, and long-term use of PEG-PAL has been
successful in reducing PEH levels in mouse models. Because polyethylene glycol is harmless to the
body, the PAG-PAL compound is injected subcutaneously for direct absorption into the blood.9,10
Phenylalanine hydroxylase (PAH)
Phenylalanine ammonia lyase (PAL)
Sapropterin dihydrochloride (brand name Kuvan) is relatively new drug is now available to help keep
PHE blood levels low. It is a synthetic version of the cofactor tetrahydrobiopterin (BH4). It is intended to
be used in conjunction with a proper diet, rather than to allow consumption of off-limit foods.
Unfortunately, it is not effective for all individuals with PKU. Those for whom the drug is beneficial are
considered to be “BH4-responsive.” Some studies have shown that mild cases are the best candidates,
and effectiveness tapered off over time. Furthermore, PKU News advises: “Even if BH4 treatment is
found to be effective, cost is an issue. For a 10 mg/kg dose, it would cost more than $7,000 yearly for a
1-3 year old child; for a school-age child, more than $14,000; for a teenager/adult, $35-40,000 or more
per year. This does not include any formula that may still be necessary.”11,12,13
Total enzyme replacement therapy may never be feasible for those with PKU, either due to difficulties
with administering the enzymes or high cost. However, the ability to manage PKU and avoid mental
retardation simply by following a protein-restricted diet is quite miraculous.
References:
1. Waisbren, S.E., Doherty, L.B., Bailey, I.V., Rohr, F.J., Levy, H.L. (1988) The New England Maternal PKU
Project: Identification of At-Risk Women. American Journal of Public Health. Vol. 78 No. 7.
2. Phenylketonuria. Genetics Home Reference. Found online at
http://ghr.nlm.nih.gov/condition/phenylketonuria. Retrieved March 15, 2012.
3. Phenylketonuria. U.S. National Library of Medicine. Found online at
www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002150/. Retreived March 15, 2012.
4. Levy, H.L. (1999) Phenylketonuria: Old disease, new approach to treatment. PNAS – Proceedings of
the National Academy of Sciences of the United States of America.
5. Phenylketonuria (PKU) Test. Web MD. Found online at
http://www.webmd.com/parenting/baby/phenylketonuria-pku-test. Retreived March 15, 2012.
6. Levy, H.L., Waisbren, S.E. (1983) Effects of Untreated Maternal Phenylketonuria and
Hyperphenylalaninemia on the Fetus. New England Journal of Medicine. 309: 1269-1274.
7. Stone, J. (2010) US ‘gene therapy’ scientists forced to retract papers. BioNews. 577
8. Shedlovsky, A., McDonald, J. D., Symula, D., and Dove, W.F. (1993) Mouse Models of Human
Phenylketonuria. Genetics Society of America.
9 Sarkissian, C.N., Adams, J., Gámez, A. (2009) Explanation of PEG-PAL. Canadian PKU and Allied
Disorders Inc.
10. Sarkissian, C.N., Gámez, A., Wang, L., Charbonneau, M., Fitzpatrick, P., Lemontt, J.F., Zhao, B.,
Vellard, M., Bell, S.M., Henschell, C., Lambert, A., Tsuruda, L., Stevens, R.C., Scriver, C.R. (2008) Preclinical evaluation of multiple species of PEGylated recombinant phenylalanine ammonia lyase for the
treatment of phenylketonuria. Proc Natl Acad Sci USA 105: 20894-9.
11. Sapropternin. U.S. National Library of Medicine. Found online at
http://www.nlm.nih.gov/medlineplus/druginfo/meds/a608020.html. Retrieved May 7, 2012.
12. Harding, C.O. (2010) New era in treatment for phenylketonuria: Pharmacologic therapy with
sapropterin dihydrochloride. Biologics. 4: 231-236. Dove Medical Press Ltd.
13. Schuett, V. (2003) Will Tetrahydrobiopterin Have A Role In PKU Treatment? National PKU News.
Spring/Summer.