Date Published: January 22, 2019
Publisher: Public Library of Science
Author(s): Polina Gundorova, Anna A. Stepanova, Irina A. Kuznetsova, Sergey I. Kutsev, Aleksander V. Polyakov, Manuel Portolés.
Phenylalanine hydroxylase (PAH) deficiency is responsible for most cases of phenylketonuria (PKU). Furthermore, numerous studies on BH4-sensitive PAH deficiency have been conducted. To date, BH4, a cofactor of PAH, has not been used to treat PKU in Russia.Genotype data of patients with PKU can be used to predict their sensitivity to BH4 therapy. A cohort of 2579 patients with PKU from Russia was analyzed for 25 common PAH gene mutations using custom allele-specific multiplex ligation-dependent probe amplification-based technology. A mutation detection rate of 84.1% chromosomes was accomplished. Both pathogenic alleles were identified in 73.1% of patients. The most frequent pathogenic variants were p.Arg408Trp (50.9%), p.Arg261Gln (5.3%), p.Pro281Leu (3.5%), IVS12+1G>A (3.1%), IVS10-11G>A (2.6%), and p.Arg158Leu (2.4%). The exact boundaries of a PAH exon 5 deletion were defined as EX5del4154ins268 (c.442-2913_509+1173del4154ins268). Severe phenotypes prevailed in the cohort, and classical PKU was observed in 71.8% cases. Due to the genotype-based prediction, 55.9% of the probands were non-responders to the BH4-treatment, and 20.2% were potential responders. Analysis of genotype data is useful to predict BH4 response in PKU patients. The high rate of non-responders among Russian patients was due to the high allele frequency of severe PAH mutations.
Phenylketonuria (PKU, MIM# 261600) is an autosomal recessive disease and an inborn error of amino acid metabolism caused by a deficiency of hepatic enzyme phenylalanine hydroxylase (PAH) (EC 18.104.22.168). This enzyme converts phenylalanine into tyrosine, and the loss of its activity increases the Phe blood concentration, which may lead to a severe developmental delay in infants and cause progressive neurological problems as well as psychiatric symptoms in adults . PKU diagnosis through newborn screening programs  allows early introduction of a Phe-restricted diet therapy, which prevents the neurotoxic effects of phenylalanine and its metabolites. BH4 is a natural cofactor of PAH, and its pharmaceutical formulation (sapropterin dihydrochloride) is an efficacious treatment with no severe adverse events for a subset of PKU patients. PKU patients may or may not respond to this therapy depending on their PAH genotype . The prevalence of PKU varies worldwide. In Caucasians, the prevalence is about 1:10,000 live births, and 1:7000 in Russia .
This study is the first large PKU study in Russia and the biggest PKU study in the world published to date. Common mutations were first selected according to published European studies. Then the panel was completed with repeated mutations from Sanger sequencing data of PKU patients from Russia. Therefore, common mutations specific to Russian patients were identified.
As mentioned above, only 25 frequent mutations were investigated. Although a high diagnostic efficacy was achieved, in some patients, the diagnosis was not established at the molecular genetic level. Next, we plan to perform further investigation of the PAH genotype in patients that did not carry any of the identified pathogenic mutations. Among patients with no frequent mutations identified, we hypothesized that there may be mutations in the BH4 synthesis and metabolism coding genes, which would be identified using a custom NGS panel.