Monday Article #22: IN FOCUS: PKU - a genetic metabolic disease and it’s immunological link
A prevalent disease that deserves more recognition (especially for its impact on our immune system)! -
What is PKU?
Phenylketonuria (PKU) is a metabolic disease caused by a genetic mutation on chromosome 12 where the essential amino acid Phenylalanine (Phe) (Figure 1) cannot be converted into Tyrosine (Tyr) due to a faulty enzyme necessary for this conversion. The disorder manifests as developmental delays, seizures, and skin disorders. New-borns receive a screening for phenylalanine blood levels to determine whether PKU is present. Current treatment for PKU consists of a phenylalanine poor diet. There is still ongoing research on the implications PKU has on the immune system. This article will give an overview on PKU, its history, causes, effect on the immune system, symptoms, diagnosis, and treatments.
History of disease & demographics
PKU was first identified in 1934 by the Norwegian doctor Asbjørn Følling. The first treatment for PKU in the form of a Phe-restricted diet was developed in 1951 at Birmingham's Children Hospital. The first paper investigating a link between PKU and the immune system was published in 1967.
As of 2020, 0.45 million individuals globally are affected by PKU, with a global prevalence of 1 : 24,000 births. This disease is most common in Europe and the Middle East. Possible explanations for PKU’s prevalence in these areas are historical migration and consanguinity in Islamic cultures respectively.
Types of PKU
There are 4 known types of PKU, namely, hyperphenylalaninemia, mild, moderate and classic (severe) PKU. The types of PKU are categorised by Phe serum levels in patients.
As mentioned in the introduction, PKU is a genetic disease. This disease is an autosomal recessive disorder where an individual has to inherit 2 recessive copies of an allele to be affected with the disease.
Figure 2 shows the probability for 2 PKU carriers to produce an affected offspring (25% chance).
Figure 2: Genetic cross displaying the autosomal recessive inheritance of the disease
Let us move on to the details pertaining to the mutations that cause PKU. One third of PKU mutations are base deletion (frameshift) mutations which result in the production of a non-functional PAH enzyme, which is unable to bind to Phe and hence, the catabolic hydroxylation reaction does not occur. This base deletion occurs at codon 55. This mutation alters the reading frame so that a stop signal (TAA) is generated in codon 60 of the PAH gene, resulting in the termination of translation and therefore, the gene sequence is not translated completely. Mild and moderate PKU types are caused by this mutation. Splice mutations in intron 12, which are also forms of base deletions, cause classic (severe) PKU.
Two thirds of PKU mutations are missense mutations and these mutations also give rise to the three types of PKU mentioned previously. Missense means that there are only single amino acid changes. This mutation will also lead to the production of a non-functional enzyme which is caused by protein misfolding. Therefore, this will affect the allostery of the enzyme, causing a high protein turnover and a loss of enzymatic function due to an induced conformational change in the active site of the PAH enzyme.
Other missense mutations that give rise to PKU are different from what we just discussed in the sense that these mutations occur on 4 different chromosomes which code to produce the BH4 cofactor essential for the breakdown of Phe. A rare form of PKU, hyperphenylalaninemia results from this mutation and these genes are also autosomal recessively inherited as it is the case with the other types of PKU. The BH4 cofactor will be key in later parts of this article, where the cofactor is involved with the immune system, diagnosis and treatments.
PKU patients experience many symptoms, ranging from skin disorders to behavioural and psychiatric problems.
Phenylketonuria & the immune system
Based on past research, PKU has a few implications on both the innate and adaptive immune system. It was found that there are lower IgA, IgG & IgM levels, where this deficiency would mean patients are generally more susceptible to various infections, ranging from gastrointestinal infections to viral or bacterial infections that would bring about inflammation.
Patients with PKU also experience oxidative stress arising from an imbalance between ROSs (free radicals) and antioxidants. There is a reduction of 𝛃-carotene and CoQ10 which leads to cell damage and apoptosis. 𝛃-carotene decreases cancer rates and heart disease by increasing T-helper cells count and NKC cytotoxicity. This also helps to reduce mutagenesis. CoQ10 prevents lipid peroxidation, indirectly inhibiting cell death. An increase in pro-inflammatory cytokines (IL-6 and IL-1𝛃) leads to dermatitis. Macrophages secrete IL-6 in response to binding of PAMPs to PRRs and an increase of IL-6 is linked to diseases such as rheumatoid arthritis. IL-1𝛃 itself is a mediator and recruits other inflammatory mediators by chemokine release from stromal cells. This induces cell proliferation, differentiation and apoptosis. This interleukin is known to resolve acute inflammation but a prolonged exposure leads to tumour development promotion.
Defects in the biosynthesis of BH4 (hyperphenylalaninemia arises from this) have been shown to lead to higher levels of metabolites neopterin (NH2) and biopterin (BH2). Neopterin is a biomarker for monitoring infections, particularly in activating the cell-mediated immune system. The elevated concentration of neopterin in urine can indicate active inflammation, demyelination, metabolic stress, and neurodegenerative diseases. Neopterin is released by macrophages and lymphocytes when stimulated by cytokine interferon-Ɣ produced by T-lymphocytes and natural killer cells. A key note is that interferon-Ɣ influences NH2 secretion. High levels of interferon-Ɣ leads to a proportional increase in Nitric oxide (NO). NO is used to measure oxidative stress and is clear evidence of said inflammation.
Diagnosing PKU early has a significant impact on reducing the effect of the disease. The original diagnosis for PKU was the ferric chloride test, in which a complex is formed with phenylpyruvate. In 1962, the Guthrie test was developed by Robert Guthrie where he used a bacterial inhibition assay which ulitised the bacteria Bacillus subtilis. Another form of diagnosis was the collection of a Dried Blood Spot (DBS) from the infant’s heel as part of neonatal diagnosis. This spot is sent to a lab for processing and analysis. Neonatal screening is completed around day 2-5 after birth.
In the late 90’s, tandem mass spectrometry (MS/MS) was used as a modern diagnostic method. This method was far more accurate and precise than the previous tests as it reduces false positives.
Currently, prenatal diagnosis is currently under investigation as a possible diagnostic method for PKU in the future. Sampling from amniotic fluid cannot be done as the PAH enzyme is only expressed in the liver. However, prenatal diagnosis is only feasible by molecular analysis.
As highlighted in the beginning, PKU is treated with dietary management. This is done by restricting Phe and aspartame intake. Why aspartame? Aspartame is a common sweetener found in F&B products such as diet sodas. In addition, Tyr supplements must be given to PKU patients because Tyr is used for the synthesis of the melanin pigment, giving rise to hair & skin colouration. These supplements help to prevent hypopigmentation due to Tyr deficiency as observed in affected patients.
Tyr is also then further broken down into other catecholamines and this is done by modification of the tyrosine functional groups in a series of reactions. Tyr is catabolised to L-dopa, which mediates neurotrophic factor release by the brain and the CNS. Next, L-dopa could be further degraded into dopamine. Dopamine has a significant role as a chemical messenger; whereby signals are transduced from one neurone to another. Dopamine also has a role in pleasure in which dopamine deficiency has a link to low self-esteem symptoms and neurodegenerative diseases experienced by PKU patients. Consequently, dopamine could be used to synthesise noradrenaline and noradrenaline can be used to synthesise adrenaline. Noradrenaline is the main neurotransmitter of the sympathetic nerves in the cardiovascular (CSV) system. It also has a regulatory role in the CSV system whereby binding to 𝛃1 receptors on sinoatrial nodes increases heart rate. On the other hand, adrenaline is the main hormone secreted by the adrenal medulla in response to fight-or-flight situations. This stresses the importance of providing Tyr supplements although Phe intake has to be restricted.
However, an early established diet does not prevent the occurence of neurophysiological and neuropsychological disabilities. Interestingly, 30% of PKU patients are responsive to BH4 therapy. This implies that the BH4 cofactor, if used in excess, could improve the catalytic activity of the PAH enzyme, thus improving a patient’s tolerance to Phe in their body.
The main takeaway is that there needs to be an improved understanding of PKU and its immunological link. Research in this field is especially lacking, which leaves this area shrouded in mystery. There is also an utmost importance of interdisciplinary research (immunology, biochemistry, genetics, physiology etc.) in understanding, diagnosing and treating PKU.
Content taken from : Phenylketonuria (PKU) Group Research Project for the SM2501 Research Skills for Medical Sciences Course,School of Medicine,Medical Sciences and Nutrition,University of Aberdeen.
[Reservoir of references - available upon request]
Figure 1 : Phenylalanine Amino Acid
Taken from: Biospace. Retrieved 19 March 2022,from https://www.biospace.com/article/-cp8r-synlogic-posts-positive-interim-data-for-synthetic-biotic-pku-treatment/
Figure 2 : PKU Genetic Cross
Illustration created by Eldrian Tho using Google slides.
Figure 3 : Neopterin production
Taken from: Eisenhut, M. (2013). Neopterin in Diagnosis and Monitoring of Infectious Diseases. Journal Of Biomarkers, 2013, 1-10. doi: 10.1155/2013/196432
Figure 4 : A PKU’s patient diet
Taken from: PKU Primer for Adolescents and Adults. New England Consortium of Metabolic Programs. Retrieved 10 March 2022, from https://www.newenglandconsortium.org/pku-primer-for-adolescents-and-adults
This article was prepared by Eldrian Tho Jiat Yang