Phenylketonuria (PKU) Case Report

Phenylketonuria (PKU)

• Phenylketonuria (PKU) is covered in chapter 19.
• William L. Anderson and Steven M. Mitchell are the authors.

Case Report

• Penny Urick, a 22-year-old G1P0 woman in her first pregnancy, sought routine prenatal care.
• Mrs. Urick stated that she was generally healthy, took no medications, and had no known medical allergies.
• Her medical record revealed a history of phenylketonuria (PKU).
• Mrs. Urick said she had PKU as a child but was fine now after being on a special diet until age 18 and having no problems after stopping it.
• Mrs. Urick was concerned that her baby might inherit the disorder and need to be on the PKU diet.
• Mrs. Urick described the PKU diet as terrible and restrictive, involving potatoes, cereal, constant monitoring, and a daily supplement.
• Mrs. Urick met with a dietician constantly, which was expensive.
• Blood tests were frequently required.
• Mrs. Urick stopped the diet at 18 because her doctor thought it would be fine, and she hadn't had any trouble.
• Mrs. Urick expressed interest in learning about treatment changes for her baby if it had PKU.
• She was curious about tetrahydrobiopterin (BH4) therapy, which she found on the Web.

Diagnosis

• The standard diagnostic test for PKU is the Guthrie bacterial inhibition assay.
• The Guthrie test detects the presence of high levels of phenylalanine (PA), phenylpyruvate, or phenyllactate by using the growth of a strain of bacteria on a specially prepared agar plate.
• Beta-2-thienylalanine will inhibit the growth of Bacillus subtilis (ATCC 6051) on minimal culture media, but if PA, phenylpyruvate, or phenyllactate is added to the medium, then growth is restored.
• These compounds exist in excess in the blood or urine of patients with PKU and can be detected using the Guthrie test
• If a suitably prepared sample of blood or urine is applied to the seeded agar plate, then the growth of the bacteria in the test will be a positive indicator for PKU in the patient.

Epidemiology

• Accurate data regarding the incidence and prevalence of PKU is sparse despite the widespread screening of newborns for phenylketonuria (PKU) since the 1960s.
• Existing data shows that Mrs. Urick's PA levels ought to be controlled as tightly as possible during her pregnancy.
• How closely dietary restriction should be controlled in non-pregnant adults; which other treatments are most effective and available are currently under debate.
• In 1994, the Council of Regional Networks for Genetic Services provided some of the most convincing data regarding the epidemiology of PKU.
• The 1994 data were limited due to lack of uniformity of criteria used by individual states to screen for PKU and non-PKU hyperphenylalaninemia (HPA) in addition to considerable variability in the actual data reported.
• In the United States, the total incidence of phenylalanine hydroxylase (PAH) deficiency of varying degrees is approximately 1 in 10,000 births, and the prevalence of the heterozygous condition is roughly 2%.
• The incidence of frank PKU (serum PA levels >1 mM) is 1 in 13,500 to 19,000 births.
• The reporting of the incidence of non-PKU HPA varies widely; current composite estimates suggest an incidence of 1 in 48,000 among newborns.
• HPA is less common in blacks, Asians, and Hispanics, with a higher incidence in white and Native Americans.

Clinical Manifestations

• Mental retardation is the hallmark of PKU, but other neurological manifestations are often present.
• The clinical course of PKU is progressive and exhibits considerable variability among individuals.
• Newborns initially appear normal but fail to attain early developmental milestones, including failure to roll by 4 months, failure to sit by 6 months, failure to walk by 1 year, and delayed language development.
• These delays are usually noticeable by 6 months.
• All patients with untreated PKU will be mentally retarded and cannot be accurately diagnosed until after age 6 years but can be strongly suspected based on poor language skills and late motor milestones.
• Mental dysfunction worsens during early childhood, with increasing dietary consumption of PA and continuing myelination during brain development.
• Commonly, patients eventually develop hyperactivity, abnormalities of gait, disturbances in posture and stance, seizures, and severe mental retardation, including autisticlike syndrome.
• Once brain maturation is complete, some physicians think it stabilizes, but other investigators documented neurological difficulties in adults who do not strictly adhere to a PA-restricted diet.
• There is a recommendation for all affected individuals to remain on the diet for their lifetime.
• The precise cause of brain damage in patients with PKU is unclear and likely multifactorial but most probably reflects the accumulation of aromatic metabolites of PA in neural tissues.
• Due to accumulation of phenylacetate, patients often have a “mousy” odor to their skin, hair, and urine.
• Hypopigmentation results from decreased availability of tyrosine and competitive inhibition of tyrosinase by an overabundance of PA.

Biochemical Perspectives

• The biochemical basis of Ms. Urick's problem is an inability to dispose of excess dietary PA.
• In mild forms of the condition, this leads to HPA, while in severe cases of PKU, mental retardation results.
• Normal serum PA levels are less than 0.24 mM.
• PA levels increase above 0.9 mM in PKU.
• Normally, PA is not detected in the urine; however, an individual with PKU can excrete as much as 2 g of PA per day in the urine.
• Partial deficiency of PAH results in Non-PKU HPA.
• Serum PA levels are in the range of 0.24-.9 mM in non-PKU HPA.
• Both conditions are autosomal recessive disorders that are the result of different mutations to the PAH enzyme.
• Consequently, severity and clinical presentation varies considerably between individuals.
• There are two different competing pathways available for the catabolism of PA.
• One pathway involves transamination of PA to phenylpyruvate, followed by the phenylpyruvate dehydrogenase reaction to form phenylacetyl-CoA.
• The products of these reactions (phenylpyruvate, phenylacetyl-CoA) and the excretion products phenylacetate and phenyllactate cannot be further metabolized and consequently accumulate in the blood, urine, and tissues.
• PAH (phenylalanine-4-monooxygenase; EC 1.14.16.1) catalyzes the conversion of PA into tyrosine.
• The resulting tyrosine molecule can then be catabolized into fumarate and acetoacetate.
• Both products are nontoxic and can be further catabolized in the citric acid cycle.
• Individuals suffering from HPA and PKU have a defect in the PAH enzyme system, so the alternate transaminase pathway is the only available route.
• PAH reduces and cleaves molecular oxygen into two hydroxyl groups.
• One hydroxyl group is found in the reaction product tyrosine, while the other oxygen atom is used to modify the reaction cofactor BH4, creating the pterin 40-carbinolamine.
• PAH, a nonheme iron-containing enzyme, is a member of a larger BH4-dependent amino acid hydroxylase family.
• Besides PAH, it includes tryosine hydroxylase and tryptophan hydroxylase.
• PAH catabolizes excess dietary PA and synthesizes tyrosine.
• The enzyme family is highly regulated not only by their expression in different tissues but also by reversible phosphorylation of a critical serine residue found in regulatory domains of the three enzymes.
• Regulation is unique for each member of the aromatic amino acid hydroxylase family.
• Problems in any one of these hydroxylation systems can arise from either an inadequate supply of the BH4 cofactor or a defect in the enzyme or its expression.

Biopterin Synthesis and Recycling

• Elevating BH4 levels is a potential therapeutic intervention, raising the possibility of BH4 deficiency as a cause.
• BH4 can be synthesized de novo from GTP and can be salvaged from the pterin-40-carbinolamine product of the hydroxylase reaction.
• There are several possible sources for an error in BH4 production, and patients with most of these possible errors have been identified.
• The specific difficulty in answering Mrs. Urick's question is illustrated by Figure 19-4, which compares the frequency of PKU cases known to be caused by a BH-related problem with reports on effectiveness of BH4 supplementation therapy.
• Defects in biopterin synthesis and the BH4 recycling system account for less than 2% of all PKU cases.
• A significantly higher percentage of PKU and HPA cases will respond favorably (decreased serum PA levels) to BH4 therapy.
• 41% of 41 random patients had PAH mutations that responded to BH4 therapy.
• In a retrospective study of 1919 patients with mild PKU and HPA, greater than 60% responded to BH4 therapy.
• In all newly diagnosed cases of PKU and HPA, serum levels of PA should be monitored following a BH4 challenge to determine if the serum PA levels decrease in response to the BH challenge.
• This BH challenge test should be a routine part of patient management because if the defect is in the PAH enzyme resides in the BH4 binding site and results in a lower affinity for the cofactor, a mechanistic rationale for the proposed BH4 therapy can be justified in more than 2% of patients.

Enzyme Structure

• The active PAH enzyme is a tetramer of identical polypeptides.
• Each polypeptide subunit is composed of three different domains: an amino terminal regulatory domain, the iron-containing catalytic domain, and a carboxyl terminal polymerization domain.
• The crystal structure of the intact enzyme has yet to be elucidated, but the protein database contains several high-resolution human and rat PAH structures that were first truncated.
• The residues involved in catalysis should be reassigned, and the mutation databases should also be reevaluated in light of these dramatic conformational changes.
• Major structural changes in enzyme conformation play a role in the catalytic mechanism, and might also explain the successful use of high levels of BH4 in patient treatment.

Therapy

• Screening for HPA is public policy in the United States and many other countries.
• Unlike most other genetic diseases, PKU is unique in that, once diagnosed, it can be successfully treated.
• In patients with HPA, plasma PA levels are typically normal at birth but quickly rise after the initiation of protein feedings.
• A PA restrictive diet, first introduced by Bickel in 1953, is the primary treatment for HPA.
• Elevated values are confirmed using quantitative amino acid analysis.
• If BH4 levels are normal, then HPA because of a problem with PAH becomes established by exclusion.
• A dietary restriction should begin as soon as possible, as there is considerable evidence that earlier initiation of dietary restriction directly corresponds to higher IQ levels and elevated PA levels affect development of the visual system.
• Current best evidence suggests that tighter metabolic control (low PA levels) correlates directly with improvements in IQ, motor skills, attention span, behavioral skills, and cognitive ability.
• The BH4 effect has been linked to increases in enzyme stability, protection against proteolytic degradation, chaperoning activity, regulation of BH4 synthesis, and increases in PAH expression and stability of its mRNA.
• If the diet is discontinued prior to 6 years of age, there will be a negative effect on performance on IQ tests.
• Supplementation for patients who are restricting PA intake varies with the particular diet that patient is receiving.

Epilogue

• Mrs. Urick maintained good plasma PA levels throughout gestation, carried the baby to term, and delivered a healthy baby girl; she decided to keep following the restrictive diet for a while.