Functional Characterization of Novel Phenylalanine Hydroxylase p.Gln226Lys Mutation Revealed Its Non-responsiveness to Tetrahydrobiopterin Treatment in Hepatoma Cellular Model
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Treatment with tetrahydrobiopterin (BH4) is the latest therapeutic option approved for patients with phenylketonuria (PKU)—one of the most frequent inborn metabolic diseases. PKU or phenylalanine hydroxylase (PAH) deficiency is caused by mutations in the PAH gene. Given that some PAH mutations are responsive to BH4 treatment while others are non-responsive, for every novel mutation that is discovered it is essential to confirm its pathogenic effect and to assess its responsiveness to a BH4 treatment in vitro, before the drug is administered to patients. We found a c.676C>A (p.Gln226Lys) mutation in the PAH gene in two unrelated patients with PKU. The corresponding aberrant protein has never been functionally characterized in vitro and its response to BH4 treatment is unknown. Computational analyses proposed that glutamine at position 226 is an important, evolutionary conserved amino acid while the substitution with lysine probably disturbs tertiary protein structure and impacts posttranslational PAH modifications. Using hepatoma cellular model, we demonstrated that the amount of mutant p.Gln226Lys PAH detected by Western blot was only 1.2% in comparison to wild-type PAH. The addition of sepiapterin, intracellular precursor of BH4, did not increase PAH protein yield thus marking p.Gln226Lys as BH4-non-responsive mutation. Therefore, computational, experimental, and clinical data were all in accordance showing that p.Gln226Lys is a severe pathogenic PAH mutation. Its non-responsiveness to BH4 treatment in hepatoma cellular model should be considered when deciding treatment options for PKU patients carrying this mutation. Consequently, our study will facilitate clinical genetic practice, particularly genotype-based stratification of PKU treatment.
KeywordsBH4-responsiveness Drug responsiveness Mutation characterization Phenylalanine hydroxylase Phenylketonuria, Tetrahydrobiopterin
This work has been funded by grants from the Ministry of Education, Science and Technological Development, Serbia (III41004) and European Commission (EU-FP7-REGPOT-316088) given to SP; the Spanish–Serbian cooperation project funded by the Ministry of Economy and Competitiveness, Spain (PRIAIBSE-2011-1126); and the Ministry of Education, Science and Technological Development, Serbia (451-03-02635/2011-14/14) given to BP and MS.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest.
This study has been approved by the Ethics Committee of the Mother and Child Health Care Institute of Serbia “Dr Vukan Cupic” in Belgrade, Serbia, and has therefore been performed in accordance with the ethical standards laid down in the 1975 Declaration of Helsinki and its later amendments. Informed consent was obtained from all individual participants included in the study and/or their parents or legally authorized representatives.
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