Phenylketonuria Phenylketonuria is an autosomal recessive
metabolic genetic disorder characterized by homozygous or compound heterozygous mutations
in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional.:541
This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid
tyrosine . When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate
, which can be detected in the urine. Untreated PKU can lead to intellectual disability,
seizures, and other serious medical problems. The mainstream treatment for classic PKU patients
is a strict PHE-restricted diet supplemented by a medical formula containing amino acids
and other nutrients. In the United States, the current recommendation is that the PKU
diet should be maintained for life. Patients who are diagnosed early and maintain a strict
diet can have a normal life span with normal mental development. However, recent research
suggests that neurocognitive, psychosocial, quality of life, growth, nutrition, bone pathology
are slightly suboptimal if diet is not supplemented with amino acids.
As an autosomal recessive disorder, two PKU alleles are required for and individual to
exhibit symptoms of the disease. Carriers a single PKU allele do not exhibit symtoms
of the disease but appear to be protected to some extent against the fungal toxin ochratoxin
A. This accounts for the persistence of the allele in certain populations in that it confers
a selective advantage—in other words, being a heterozygote is advantageous.
History Phenylketonuria was discovered by the Norwegian
physician Ivar Asbjørn Følling in 1934 when he noticed hyperphenylalaninemia (HPA) was
associated with intellectual disability. In Norway, this disorder is known as Følling's
disease, named after its discoverer. Dr. Følling was one of the first physicians to apply detailed
chemical analysis to the study of disease. His careful analysis of the urine of two affected
siblings led him to request many physicians near Oslo to test the urine of other affected
patients. This led to the discovery of the same substance he had found in eight other
patients. He conducted tests and found reactions that gave rise to benzaldehyde and benzoic
acid, which led him to conclude that the compound contained a benzene ring. Further testing
showed the melting point to be the same as phenylpyruvic acid, which indicated that the
substance was in the urine. His careful science inspired many to pursue similar meticulous
and painstaking research with other disorders. It was recently suggested that PKU may resemble
amyloid diseases, such as Alzheimer's disease and Parkinson's disease, due to the formation
of toxic amyloid-like assemblies of phenylalanine. Screening and presentation
PKU is commonly included in the newborn screening panel of most countries, with varied detection
techniques. Most babies in developed countries are screened for PKU soon after birth. Screening
for PKU is done with bacterial inhibition assay (Guthrie test), immunoassays using fluorometric
or photometric detection, or amino acid measurement using tandem mass spectrometry (MS/MS). Measurements
done using MS/MS determine the concentration of Phe and the ratio of Phe to tyrosine, both
of which will be elevated in PKU. If a child is not screened during the routine
newborn screening test (typically performed 2–7 days after birth, using samples drawn
by neonatal heel prick), the disease may present clinically with seizures, albinism (excessively
fair hair and skin), and a "musty odor" to the baby's sweat and urine (due to phenylacetate,
one of the ketones produced). In most cases, a repeat test should be done at approximately
two weeks of age to verify the initial test and uncover any phenylketonuria that was initially
missed. Untreated children are normal at birth, but
fail to attain early developmental milestones, develop microcephaly, and demonstrate progressive
impairment of cerebral function. Hyperactivity, EEG abnormalities, and seizures, and severe
learning disabilities are major clinical problems later in life. A "musty or mousy" odor of
skin, hair, sweat and urine (due to phenylacetate accumulation), as well as a tendency towards
hypopigmentation and eczema, are also observed. In contrast, affected children who are detected
and treated are less likely to develop neurological problems or have seizures and intellectual
disability, though such clinical disorders are still possible.
Pathophysiology Classical PKU
Classical PKU is caused by a mutated gene for the enzyme phenylalanine hydroxylase (PAH),
which converts the amino acid phenylalanine to other essential compounds in the body.
Other non-PAH mutations can also cause PKU. This is an example of non-allelic genetic
heterogeneity. The PAH gene is located on chromosome 12 in the bands 12q22-q24.1. More
than 400 disease-causing mutations have been found in the PAH gene. PAH deficiency causes
a spectrum of disorders, including classic phenylketonuria (PKU) and hyperphenylalaninemia
(a less severe accumulation of phenylalanine). PKU is known to be an autosomal recessive
genetic disorder. This means both parents must have at least one mutated allele of the
PAH gene. The child must inherit both mutated alleles, one from each parent. Therefore,
if both parents are carriers for PKU, there is a 25% chance their child with develop the
disorder, a 50% chance their child will be a carrier, and a 25% chance their child will
neither develop nor be a carrier for the disease. Phenylketonuria can exist in mice, which have
been extensively used in experiments into finding an effective treatment for it. The
macaque monkey's genome was recently sequenced, and the gene encoding phenylalanine hydroxylase
was found to have the same sequence that, in humans, would be considered the PKU mutation.
Tetrahydrobiopterin-deficient hyperphenylalaninemia A rarer form of hyperphenylalaninemia occurs
when the PAH enzyme is normal, but there is a defect in the biosynthesis or recycling
of the cofactor tetrahydrobiopterin (BH4). BH4 (called biopterin) is necessary for proper
activity of the enzyme PAH, and this coenzyme can be supplemented as treatment. Those who
suffer from PKU as well may also have a deficiency of tyrosine (which is created from phenylalanine
by PAH). These patients must also be supplemented with tyrosine to account for this deficiency.
Dihydrobiopterin reductase activity is needed to replenish quinonoid-dihydrobiopterin back
into its tetrahydrobiopterin form, which is an important cofactor in many reactions in
amino acid metabolism. Those with this deficiency may produce sufficient levels of the enzyme
phenylalanine hydroxylase (PAH), but since tetrahydrobiopterin is a cofactor for PAH
activity, deficient dihydrobiopterin reductase renders any PAH produced unable to use phenylalanine
to produce tyrosine. Tetrahydrobiopterin is also a cofactor in the production of L-DOPA
from tyrosine and 5-Hydroxy-L-Tryptophan from tryptophan, which must also be supplemented
as treatment in addition to the supplements for classical PKU.
Levels of dopamine can be used to distinguish between these two types. Tetrahydrobiopterin
is required to convert phenylalanine to tyrosine, but is also required to convert tyrosine to
L-DOPA via the enzyme tyrosine hydroxylase. L-DOPA in turn is converted to dopamine. Low
levels of dopamine lead to high levels of prolactin. By contrast, in classical PKU (without
dihydrobiopterin involvement), prolactin levels would be relatively normal.
Tetrahydrobiopterin deficiency can be caused by defects in four different genes. These
types are known as HPABH4A, HPABH4B, HPABH4C, and HPABH4D.
Metabolic pathways The enzyme phenylalanine hydroxylase normally
converts the amino acid phenylalanine into the amino acid tyrosine. If this reaction
does not take place, phenylalanine accumulates and tyrosine is deficient. Excessive phenylalanine
can be metabolized into phenylketones through the minor route, a transaminase pathway with
glutamate. Metabolites include phenylacetate, phenylpyruvate and phenethylamine. Elevated
levels of phenylalanine in the blood and detection of phenylketones in the urine is diagnostic,
however most patients are diagnosed via newborn screening.
Phenylalanine is a large, neutral amino acid (LNAA). LNAAs compete for transport across
the blood–brain barrier (BBB) via the large neutral amino acid transporter (LNAAT). If
phenylalanine is in excess in the blood, it will saturate the transporter. Excessive levels
of phenylalanine tend to decrease the levels of other LNAAs in the brain. However, as these
amino acids are necessary for protein and neurotransmitter synthesis, Phe buildup hinders
the development of the brain, causing intellectual disability.
Treatment If PKU is diagnosed early enough, an affected
newborn can grow up with normal brain development, but only by managing and controlling Phe levels
through diet, or a combination of diet and medication. Optimal health ranges (or "target
ranges") are between 120 and 360 µmol/L, and aimed to be achieved during at least the
first 10 years. When Phe cannot be metabolized by the body, abnormally high levels accumulate
in the blood and are toxic to the brain. When left untreated, complications of PKU include
severe intellectual disability, brain function abnormalities, microcephaly, mood disorders,
irregular motor functioning, and behavioral problems such as attention deficit hyperactivity
disorder. All PKU patients must adhere to a special
diet low in Phe for optimal brain development. "Diet for life" has become the standard recommended
by most experts. The diet requires severely restricting or eliminating foods high in Phe,
such as meat, chicken, fish, eggs, nuts, cheese, legumes, milk and other dairy products. Starchy
foods, such as potatoes, bread, pasta, and corn, must be monitored. Infants may still
be breastfed to provide all of the benefits of breastmilk, but the quantity must also
be monitored and supplementation for missing nutrients will be required. The sweetener
aspartame, present in many diet foods and soft drinks, must also be avoided, as aspartame
contains phenylalanine. Supplementary infant formulas are used in
these patients to provide the amino acids and other necessary nutrients that would otherwise
be lacking in a low-phenylalanine diet. As the child grows up these can be replaced with
pills, formulas, and specially formulated foods. (Since Phe is necessary for the synthesis
of many proteins, it is required for appropriate growth, but levels must be strictly controlled
in PKU patients.) In addition, tyrosine, which is normally derived from phenylalanine, must
be supplemented. The oral administration of tetrahydrobiopterin
(or BH4) (a cofactor for the oxidation of phenylalanine) can reduce blood levels of
this amino acid in certain patients. The company BioMarin Pharmaceutical has produced a tablet
preparation of the compound sapropterin dihydrochloride (Kuvan), which is a form of tetrahydrobiopterin.
Kuvan is the first drug that can help BH4-responsive PKU patients (defined among clinicians as
about 1/2 of the PKU population) lower Phe levels to recommended ranges. Working closely
with a dietitian, some PKU patients who respond to Kuvan may also be able to increase the
amount of natural protein they can eat. After extensive clinical trials, Kuvan has been
approved by the FDA for use in PKU therapy. Some researchers and clinicians working with
PKU are finding Kuvan a safe and effective addition to dietary treatment and beneficial
to patients with PKU. Dietary supplementation with large neutral
amino acids(LNAAs), with or without the traditional PKU diet is another treatment strategy. The
LNAAs (e.g. leu, tyr, trp, met, his, iso, val, thr) compete with phe for specific carrier
proteins that transport LNAAs across the intestinal mucosa into the blood and across the blood
brain barrier into the brain. Studies have demonstrated that PKU patients
given daily supplements of LNAAs have decreased plasma phe levels and reduced brain phe concentrations
measured by magnetic resonance spectroscopy. Another interesting treatment strategy for
PKU patients is casein glycomacropeptide (CGMP), which is a milk peptide naturally free of
Phe in its pure form CGMP can substitute the main part of the free amino acids in the PKU
diet and provides several beneficial nutritional effects compared to free amino acids. The
fact that CGMP is a peptide ensures that that the absorption rate its amino acids is prolonged
compared to free amino acids and thereby results in improved protein retention and increased
satiety compared to free amino acids. Another important benefit of CGMP is that the taste
is significantly improved when CGMP substitutes part of the free amino acids and this may
help ensure improved compliance to the PKU diet.
Furthermore, CGMP contains a high amount of the phe lowering LNAAs, which constitutes
about 41 g per 100 g protein and will therefore help maintain plasma phe levels in the target
range. Other therapies are currently under investigation,
including gene therapy and enzyme substitution therapy with phenylalanine ammonia lyase (PAL).
In the past, PKU-affected people were allowed to go off diet after approximately eight,
then 18 years of age. Today, most physicians recommend PKU patients must manage their Phe
levels throughout life. Maternal phenylketonuria
For women with phenylketonuria, it is essential for the health of their children to maintain
low Phe levels before and during pregnancy. Though the developing fetus may only be a
carrier of the PKU gene, the intrauterine environment can have very high levels of phenylalanine,
which can cross the placenta. The child may develop congenital heart disease, growth retardation,
microcephaly and intellectual disability as a result. PKU-affected women themselves are
not at risk of additional complications during pregnancy.
In most countries, women with PKU who wish to have children are advised to lower their
blood Phe levels (typically to between 2 and 6 mg/dL) before they become pregnant, and
carefully control their levels throughout the pregnancy. This is achieved by performing
regular blood tests and adhering very strictly to a diet, in general monitored on a day-to-day
basis by a specialist metabolic dietitian. In many cases, as the fetus' liver begins
to develop and produce PAH normally, the mother's blood Phe levels will drop, requiring an increased
intake to remain within the safe range of 2–6 mg/dL. The mother's daily Phe intake
may double or even triple by the end of the pregnancy, as a result. When maternal blood
Phe levels fall below 2 mg/dL, anecdotal reports indicate that the mothers may suffer
adverse effects, including headaches, nausea, hair loss, and general malaise. When low phenylalanine
levels are maintained for the duration of pregnancy, there are no elevated levels of
risk of birth defects compared with a baby born to a non-PKU mother. Babies with PKU
may drink breast milk, while also taking their special metabolic formula. Some research has
indicated an exclusive diet of breast milk for PKU babies may alter the effects of the
deficiency, though during breastfeeding the mother must maintain a strict diet to keep
her Phe levels low. More research is needed. US scientist announced in June 2010 that they
would be conducting a thorough investigation on the mutation of genes in the human genome.
Their top priority is PKU, as it has become increasingly common, and sufferers often bear
children who will be carriers of the recessive gene, and may themselves live past the age
of sixty. Incidence
The mean incidence of PKU varies widely in different human populations. United States
Caucasians are affected at a rate of 1 in 10,000. Turkey has the highest documented
rate in the world, with 1 in 2,600 births, while countries such as Finland and Japan
have extremely low rates with fewer than one case of PKU in 100,000 births. A 1987 study
from Slovakia reports a Roma population with an extremely high incidence of PKU (one case
in 40 births) due to extensive inbreeding.