PhenylketonuriaSamuel Chan, Jekaterina Davydova

PHM142 Fall 2015Instructor: Dr. Jeffrey Henderson

What is Phenylketonuria (PKU)?• Genetic disorder

• Most common inborn error of amino acid metabolism• Affect phenylalanine (Phe) metabolism

• Phenylketones in urine• Results in a hyperphenylalaninemia (HPA) phenotype

L-Phenylalanine chemical structure

Retrieved from http://www.mpbio.com/images/product-images/molecular-structure/02102623.png

PKU Epidemiology• International Incidence

• Incidence in US is 1:15000• Less common in African-American descent 1:50000

• Incidence in Turkey 1:2600, in Scotland 1:5300• Incidence in Japan 1:125000, in Finland 1:100000

• No correlation with sex/gender

Symptoms • If left undiagnosed or untreated initially :

• Intellectual disability• Delayed development• Neurological problems (ex. Seizures)• Eczema• Musty odour in urine and breath• Lighter skin and hair

Pathophysiology - Genetic• Mutation in phenylalanine hydroxylase (PAH)

gene• Autosomal recessive on chromosome 12• Generally missense mutation

• Mutation related to tetrahydrobiopterin (BH4)• Autosomal recessive mutation in 1 of 5 proteins

• Guanosine triphosphate cyclohydrolase 1 (GTPCH)• 6-pyruvoyl-tetrahydrobiopterin synthase (PTPS)• Sepiapterin Reductase (SR)• Dihydropteridine reductase (DHPR)• Carbinolamine-4a-dehydratase (PCD) Irina et al.

Pathophysiology - MetabolismOdour

Light skin

Schuck et al.

Pathophysiology – Oxidative Stress• Stress induced by phenylketone metabolites• Impaired antioxidant enzyme activity

• Phenylpyruvic acid inhibits glucose-6-phosphate• Impair pentose monophosphate shunt

• High [Phe] impairs catalase, superoxide dismutase, glutathione peroxidase activity

Pathophysiology - Neurotransmitter• Impaired neurotransmitter synthesis

• High [Phe] impair amino acid transport through blood brain barrier• Phe has high affinity for large neutral amino acid transporter• Phe out-competes other amino acids, including Tyr and Trp• Tyr and Trp are precursors for dopamine and serotonin respectively

• High [Phe] impair synthetic enzyme activity• Competitively inhibits tyrosine and tryptophan hydroxylase• Phe metabolites inhibit 5-hydroxytryptophan and dopa decarboxylase

• BH4 is an important co-factor for neurotransmitter synthesis

Retrieved from http://www.biomedcentral.com/1471-2148/4/24/figure/F1?highres=y

Testing and Diagnosis • Made through newborn screening • Involves blood spot or heel prick test (Guthrie Test)• Levels of phenylalanine and tyrosine are measured

• if ratio is greater than 3 between the two PKU• Other tests to rule out possibility of other conditions which

increase phenylalanine levels

Management of PKU• Aim to reduce phenylalanine levels• Phenylalanine levels depend on dietary intake • Mostly managed through diet which involves:

• elimination of high protein foods, such as milk, dairy products, meat, fish, chicken, eggs, beans, and nuts

• special phenylalanine-free formula which contains protein, vitamins, minerals and energy with no phenylalanine

• high consumption of fruits and vegetables• counting phenylalanine content in medium-rich foods

• Pregnant women who have PKU should be extra careful with their phenylalanine intake

New Treatments for PKU

• Large neutral amino acids

• Enzyme replacement therapy

• Sapropterin Therapy• KuvanTM is available on market

Summary Slide• Phenylketonuria is an inborn error of phenylalanine metabolism• Major symptoms include intellectual development and

neurological problems• Due to mutations in Phenylalanine Hydroxylase or BH4 deficiency

• Results in phenylalanine accumulation and production of phenylketones

• Oxidative stress and impaired neurotransmitter synthesis contribute to neurologic problems

• Main diagnostic is newborn screening• Management includes dietary restriction on phenylalanine intake• Pharmacology treatments include large neutral amino acid

supplements, enzyme replacement and Sapropterin therapy

Reference• Cleary, M. A. (2011). Phenylketonuria. Paediatrics and Child Health, 21(2), 61-64.• Guldberg P, Henriksen K, Sipila I, et al. Phenylketonuria in a low incidence population: molecular

characterisation of mutations in Finland. Journal of Medical Genetics. 1995; 32: 976.• Hofman K, Steel G, Kazazian H, Valle D. Phenylketonura in U.S. blacks: molecular analysis of the

phenylalanine hydroxylase gene. American Journal of Human Genetics. 1991; 48: 791.• Ionova I, Vásquez J, Whisett J, et al. Deficient BH4 production via de novo and salvage pathways regulates

NO responses to cytokines in adult cardiac myoctes. American Journal of Physiology – Heart and Circulatory Physiology. 2008; 295: 2178-2187.

• Levy L, Milanowski A, Chakrapani A, et al. Efficacy of sapropterin dihydrochloride (tetrahydrobiopterin, 6R-BH4) for reduction of phenylalanine concentration in patients with phenylketonuria: a phase III randomized placebo-controlled study. Lancet 2007; 370: 504–10.

• Pietz J, Kreis R, Rupp A, et al. Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria. J Clin Invest 1999; 103: 1169–78.

• Réblová K, Kulhánek P, Fajkusová L. Computational study of missense mutations in phenylalanine hydroxylase. Journal of Molecular Modeling. 2015; 21:69-70.

• Sarkissian CN, Gamez A, Wang L, et al. Preclinical evaluation of multiple species of PEGylated recombinant phenylalanine ammonia lyase for the treatment of phenylketonuria. Proc Natl Acad Sci USA 2008; 52: 20894–99.

• Schuck P, Malgarin F, Cararo J, et al. Phenylketonura Physiology: on the Role of Metabolic Alterations. Aging and Disease. 2015; 6(5): 1-10.

• Shintaku H, Ohwada M. Long-term follow-up of tetrahydrobiopterin therapy in patients with tetrahydrobiopterin deficiency in japan. Brain & Development. 2013; 35: 406-410.