ISSN 1022-7954, Russian Journal of Genetics, 2017, Vol. 53, No. 6, pp. 712–718. © Pleiades Publishing, Inc., 2017.
Original Russian Text © P. Gundorova, A.A. Stepanova, T.V. Bushueva, E.Yu. Belyashova, R.A. Zinchenko, S.S. Amelina, S.I. Kutsev, A.V. Polyakov, 2017, published in Genetika,
2017, Vol. 53, No. 6, pp. 732–739.
Genotyping of Patients with Phenylketonuria from Different Regions
of Russia for Determining BH4 Responsiveness
P. G u n d o r ov a
*, A. A. Stepanova
, T. V. Bushueva
, E. Yu. Belyashova
R. A. Zinchenko
, S. S. Amelina
, S. I. Kutsev
, and A. V. Polyakov
Research Centre for Medical Genetics, Moscow, 115478 Russia
Scientific Center of Children’s Health, Ministry of Health of the Russian Federation, Moscow, 119296 Russia
Medical and Genetic Counseling, Orenburg Regional Clinical Hospital No. 2, Orenburg, 460035 Russia
Pirogov Russian National Research Medical University, Moscow, 117997 Russia
Rostov State Medical University, Rostov-on-Don, 344022 Russia
Received July 13, 2016; in final form, August 8, 2016
Abstract⎯To date, the efficacy of the phenylalanine hydroxylase (PAH) cofactor is proved for the treatment
of both BH4-dependent hyperphenylalaninemia and phenylketonuria patients with mutations in the PAH
gene. Since the patient’s response depends on the presence of residual PAH enzyme activity, it is advisable to
search for mutations in the PAH gene to identify the potential responders and nonresponders to therapy. Four
hundred thirty-five phenylketonuria patients from 13 regions of the Russian Federation were genotyped in
order to identify responders and nonresponders to tetrahydrobiopterin (BH4) therapy. According to the
results of this study, the number of probable nonresponders to the BH4 treatment exceeds 50% owing to a
higher overall allelic frequency of “severe” PAH gene mutations. Responder patients with two “mild” muta-
tions in the PAH gene were identified (1.6%).
Keywords: hyperphenylalaninemia, diet therapy, residual PAH activity, tetrahydrobiopterin, sapropterin,
Phenylketonuria (PKU) is an autosomal-recessive
disorder caused by disturbed phenylalanine (Phe)
metabolism. The abnormal phenylalanine hydroxy-
lase (PAH) responsible for Phecleavage results in the
activation of abnormal pathways of its metabolism,
resulting in the formation of toxic metabolites. Their
accumulation causes an irreversible CNS disease and
cognitive impairments in children with subsequent
development of severe forms of cognitive decline.
Hyperphenylalaninemia (HPA) is characterized by
some increase in blood Phe level (normal level is
under 2 mg/dL). A level of Phe level from 10 to
15mg/dL is classified as mild PKU, from 15 to
20 mg/dL is medium severe, and more than 20 mg/dL
is severe PKU. Mutations in the PAH gene result in a
decreased activity or complete absence of phenylala-
nine hydroxylase and phenylketonuria development.
Defects in the PAH gene are responsible for 98% of all
HPA and PKU cases. Mutations in the genes involved
in the synthesis and metabolism of PAH cofactor tet-
rahydrobiopterin (BH4), including the PTS, GCH1,
QDPR, and PCBD1 genes, cause the development of
hyperphenylalaninemia with a deficit in type A, B, C,
and D tetrahydrobiopterins, respectively (2% of all
HPA and PKU) [1, 2].
The patients with mutations in the genes involved
in BH4 synthesis and metabolism are directed to sub-
stitutive therapy with sapropterin (BH4 pharmacolog-
ical analog). Recently, it was demonstrated that such
treatment was also effective for the patients with muta-
tions in the PAH gene with a residual PAH activity.
A significant decrease in blood phenylalanine level of
PKU patients receiving synthetic analogs of phenylal-
anine hydroxylase cofactor was reported. Small doses
of preparation might significantly extend the diet and
improve the quality of life of patients with PKU.
Moreover, the higher the residual enzymatic activity,
the better the efficacy of treatment is observed [3–5].
On the other hand, the treatment appears to be inef-
fective in the cases of classic PKU in patients with two
severe mutations, for instance, R408W homozygotes.
Cofactor administration caused a clinically insignifi-
cant effect in the case of null-close residual activity of
PAH. Together with the enhancement of reaction rate
of phenylalanine conversion to tyrosine catalyzed by
phenylalanine hydroxylase, BH4 also stabilizes the
enzyme and affects protein folding, which determines