Phenylketonuria in Louisiana
Lindsay Burrage
Hans C. Andersson
Background
Phenylketonuria (PKU), a classic
example of an inborn error of metabolism,
was first described by Asbjörn
Fölling in 1934 when he discovered
phenylpyruvic acid in the urine
of two mentally retarded siblings.
Following this discovery, he analyzed
the urine of over 400 mentally retarded
patients and discovered eight other
patients whose urine contained abnormal
quantities of phenylpyruvic acid.
Each of the patients had a similar
clinical presentation or phenotype
with features including mental retardation,
a "mousy" odor, and dermatitis.
Further studies indicated that the
disorder is inherited as an autosomal
recessive trait.
The
exact metabolic defect in PKU was
not elucidated until 1947. After
administering phenylalanine (PHE)
to a group consisting of both normal
controls and affected patients.
Jervis noticed that there was an
increase in the concentration of
tyrosine (TYR) in the blood of the
controls but not in PKU patients,
and he concluded that the hydroxylation
of PHE to TYR was deficient in PKU
patients. In 1953, Jervis first
demonstrated deficient activity
of phenylalanine hydroxylase (PAH)
(EC 1.14.16.1) in the liver of PKU-affected
patients. PAH is primarily active
only in the liver, and enzymatic
activity can only be measured in
this tissue.
The classical and variant forms of PKU are caused by a deficiency of the hepatic phenylalanine hydroxylase (PAH). PAH is a homotetramer that oxidizes PHE to form TYR. This oxidation is a critical step in the pathway of the conversion of PHE to carbon dioxide and water, as well for the formation of TYR which is an essential amino acid in the diet of PKU patients. The gene which codes for the PAH enzyme is mapped to chromosome 12q22-q24.1. The PAH gene is nearly 90 kb in length and consists of 13 exons. About 400 mutations in the PAH gene have been discovered, and although some of these mutations appear to have no affect on the enzyme's function, most have been associated with either classical or variant PKU.
The phenotypic heterogeneity in classical and variant PKU is a result of the vast number of PAH mutations responsible for the disorders. For the most part, particular mutations can be correlated with specific phenotypes. For example, "severe" mutations result in higher plasma PHE levels (classical PKU) than do "mild mutations". Compound heterozygosity for one of the "severe" mutations and one of the "mild" mutations results in intermediate plasma PHE levels. Furthermore, patients that are homozygous for a "mild" mutation or compound heterozygotes for two different "mild" mutations generally have plasma PHE levels characteristic of variant rather than classical PKU. In contrast, there are a few cases in which the mutations cannot be correlated with the patient's Clinical Phenotype.
Clinical
Phenotype
With the advent of newborn screening
methods, hyperphenylalaninemia patients
are generally diagnosed soon after
birth. The blood concentrations
of PHE in untreated classical PKU
patients with normal protein intake
is usually over 1200 mcmol/L (19.8
mg/dL) phenotype . These patients
generally have less than 5% of the
normal enzyme activity. Blood PHE
concentrations in untreated patients
that are slightly elevated compared
to normal (greater than 151 mcmol/L
but less than 1200 mcmol/L or 2.5-
19.8 mg/dL) indicate either a
milder form termed variant PKU or
transient hyperphenylalaninemia
of the newborn. If the slight elevation
of PHE disappears within the first
three months, the patient is presumed
to have transient hyperphenylalaninemia.
Transient hyperphenylalaninemia
is believed to be due to an immature
PAH system, which is developmentally
induced during infancy. Most patients
with variant PKU do not require
treatment unless the plasma PHE
rises above 484-605 mcM (8-10 mg/dL).
All patients with elevated plasma
PHE must be tested for non-PAH forms
of the disease since plasma PHE
levels do not suffice to distinguish
these types.
Treatment
Untreated PKU patients develop moderate
to profound mental retardation and
have a characteristic "mousy"
odor, fair complexion, abnormal
gait and stance, and dermatitis.
The impaired brain functioning in
PKU patients is felt to be due to
the excess of PHE and PHE-metabolites
in the brain, but the exact pathophysiology
remains unclear. The abnormal levels
of PHE prevent the normal transport
of other amino acids, particularly
TYR, across the blood-brain barrier
. As a result, neurotransmitter
synthesis and protein synthesis
in the brain is disrupted. In addition,
the structure of the brain cells
is abnormal, and myelination is
defective.
In 1954, Bickel demonstrated that dietary restriction of PHE effectively reduces plasma PHE and urine phenylpyruvate levels. In addition, the mental development of PKU patients on the PHE-restricted diet was improved. Further studies revealed that the PHE-restricted diet prevents mental retardation and other phenotypic effects of the disorder. As a result, dietary therapy has become the standard treatment for PKU. It is recommended that PKU patients maintain plasma PHE levels of between 121-484 mcM or 2-8 mg/dL (normal values are 30-121 mcM or 0.5-2.0 mg/dL) throughout childhood, adolescence, and even adulthood. Various low-PHE products are commercially available for PKU patients particularly synthetic, low-PHE formulas. These formulas are supplemented with free amino acids, vitamins, and minerals, but unfortunately, the poor taste and odor of the formulas contribute to problems with dietary compliance.
Adequately treated PKU patients do not develop mental retardation. Some studies suggest that the IQ of continuously treated PKU patients may be slightly decreased as compared to control subjects. Other studies indicate that strict dietary adherence throughout childhood and adolescence may eliminate most of the differences in test scores between PKU patients and control subjects. According to some studies, the only area where the test scores of PKU patients fall significantly below those of control subjects is mathematics. In contrast, the results of a more recent study indicate that the spatial intelligence of early and continuously treated patients is poor even though verbal intelligence and arithmetic skills are normal. Overall, the I.Q. of PKU patients appears to be significantly affected by delays in the initiation of the low-PHE diet, prolonged exposure to PHE levels above the treatment range, and five or more months of exposure to PHE levels below the target range during the first two years.
Newborn
Screening for the Hyperphenylalaninemias
Since early dietary therapy is necessary
to achieve optimal clinical outcome,
hyperphenylalaninemia patients must
be identified as soon as possible
after birth. In 1961, Dr. Robert
Guthrie developed a method for the
early detection of newborn patients.
His assay utilized the inhibitory
effect of a phenylalanine analogue
on the growth of the bacteria Bacillus subtilis. In the presence of PHE,
the inhibitory effect is overcome,
and the area of growth can be used
to measure the quantity of PHE in
blood. Blood for the assay can readily
be obtained from a filter paper
blood spot taken 48 hours after
birth. In 1965, a fluorometric method,
which also utilizes filter paper
blood spots, was developed. The
fluorometric method relies on the
fact that PHE forms a fluorescent
substance when it is heated with
ninhydrin and L-leucyl-L-alanine.
The amount of fluorescence indicates
the quantity of PHE present in the
sample. This method has replaced
the bacterial inhibition assay in
many states including Louisiana.
The threshold level of PHE for positive
samples is 121-242 mcM or 2-4 mg/dL,
the normal upper limit of plasma
PHE, and all positive samples are
referred for amino acid analysis
for a definitive diagnosis.
Maternal
PKU
The current dietary recommendations
for PKU patients stress dietary
therapy for life, especially for
affected women because high plasma
PHE acts as a fetal teratogen. Nearly
80% of fetuses exposed to high levels
of PHE during development have microcephaly,
mental retardation, and growth retardation.
A smaller percentage are born with
heart defects and other congenital
malformations including dysmorphic
facial features. The level of fetal
abnormalities correlates with the
maternal plasma PHE levels, and
as a result, women who maintain
low plasma PHE (below 605 mcM or
10 mg/dL) have a lower risk of having
a child with maternal PKU. Women
with PKU must maintain proper dietary
control prior to conception because
high levels of PHE have the most
severe detrimental effects in the
first trimester
PKU In Louisiana
Between 1985 and 1999, 67 (.0067%)
of approximately 1,000,000 newborns
in Louisiana were diagnosed with
a form of hyperphenylalaninemia.
Sixty-six of the patients were diagnosed
with either classical PKU or variant
PKU, and the overall incidence of
PKU (classical and variant) in Louisiana
during the study period was 1:16,000.
One patient was diagnosed with 6-pyruvoyl
tetrahydropterin synthase (6-PTS)
deficiency, a non-PAH form of hyperphenylalaninemia
during the study period. No patients
were diagnosed with transient hyperphenylalaninemia
of the newborn. All 67 patients
were identified by the Louisiana
newborn screening program and cared
for by the Tulane Medical School
Human Genetics Program. Of the 66
PKU patients born in Louisiana,
60 (91%) were white, and six (9%)
were non-white. Five of the non-white
patients were African-American,
and one had a mixed racial background
(white and African-American). No
Asian PKU patients were born during
the study period.
Tulane Medical School Human Genetics
Program:
www.tmc.tulane.edu/departments/human_genetics/main.htm
Contact Webmaster I LSUHSC Home I Privacy Policy I Disclaimer
