Prenatal diagnosis of phenylketonuria: a tertiary center experience

Introduction

Phenylketonuria (PKU) is an autosomal recessive disorder associated with disturbances in amino acid metabolism^(1-4). Deficiency of phenylalanine hydroxylase (PAH), the key hepatic cytosolic enzyme controlling the catabolism of phenylalanine, is the biological rationale behind this disorder^(2,5-7). The incidence of PKU is about 1 in 10,000 in the world, while it is around 1 in 4500 in Türkiye⁽⁸⁾. However, the disease seems to be less common in China, Taiwan and India (1 in 11,000-16,000, 1 in 55,071, and 1 in 28,728, respectively) than in the western countries^(1,3,4,9).

The human PAH gene covers 100 kb of genomic DNA on chromosome 12, and a large number of mutations have been described in different populations^(1,3). Untreated phenylketonuria is associated with mental disorders, autism, seizures, skin lesions, and motor deficits^(1-7,10). PKU-related disorders mainly manifest in developmental anomalies, and symptoms often become more severe as the affected child grows⁽¹⁰⁾. Prenatal diagnosis or early diagnosis facilitated under neonatal period screening programs is part of important measures to avoid challenging complications of PKU. On the other hand, phenylketonuria is a curable genetic disorder, and an early phenylalanine-restricted diet not only results in normal development of the child, but also prevents the progression of serious clinical symptoms^(11-13).

Prenatal diagnosis and screening programs for early detec­tion seem to be the appropriate approaches in the management of phenylketonuria⁽¹⁴⁾. PKU neonatal screening is obligatory in Türkiye. In this study, we have evaluated the results of our prenatal diagnosis program, which gives an alternative to the affected families during the prenatal period.

Materials and method

This study included 90 pregnant women from 69 families provided that the first child of the pregnant woman or any other family member was diagnosed with classical phenylketonuria. Families with known mutant PAH alleles were accepted for the “prenatal diagnosis program” of the Hacettepe University, Ankara, Türkiye, between January 2006 and December 2016. The registries of the Division of Perinatology and the Department of Medical Biology were retrospectively used for data withdrawal. Ethical approval (GO19/1064) for the conduct of this study was granted by the Local Ethics Committee of our institution.

Eighteen families who refused to undergo prenatal diagnosis tests were excluded from the study. Seventy-two pregnant women, including one pregnant with twins, from 51 families, underwent chorionic villus sampling (CVS) within the framework of the “prenatal diagnosis program”. Genomic DNA was extracted from the chorionic villus samples. Polymerase chain reaction (PCR) was carried out for the identification of disease-causing mutations.

In the families where the mutations were determined for prenatal diagnosis, the same mutations were analyzed. In our prenatal diagnosis program, we screened the study participants for IVS10 and R261Q mutations of PAH. In the cases where mutations could not be identified, haplotype analyses were carried out using restriction fragment length polymorphism (with the help of restriction enzymes such as MspI, BgIII, XmnI, PvuIIa, and PvuIIb) and variable number tandem repeats.

Results

Of the 73 fetuses examined in the present study, 55 were healthy or disease-free carriers and 18 were diagnosed with the disease. Three families, where the PKU diagnosis of the fetuses was positive, chose to terminate the pregnancy. Fif­teen pregnant women whose fetuses were also diagnosed with phenylketonuria chose to continue their pregnancies, and the neonates with PKU were delivered. Unfortunately, there was one case of misdiagnosis, and the neonate was found to be affected with phenylketonuria, although prenatal diagnosis test was negative. There were no complications due to CVS. Table 1 presents the demographic and gestational findings of the study subjects.

Discussion

The PAH gene, located on chromosome 12, contains 13 exons, and encodes a 452 residue-long monomeric protein^(1-4). Previ­ous studies have reported almost 1000 different mutations for phenylketonuria. This genetic heterogeneity makes prenatal screening and diagnosis of PKU challenging^(1-7). In our study, we also included families with known mutations for prenatal diagnosis of phenylketonuria. However, we encountered a single case of misdiagnosis. We believe that the high-through­put genomic studies will enable us to handle this problem in a much better way.

Phenylketonuria is an autosomal recessive metabolic disorder associated with progressive complications of central nervous system and mental retardation^(8-10). It is a disease that might be eradicated by genetic screening programs (including prenatal diagnosis), and it can be prevented by modulating the environmental factors such as diet^(11-13).

Differences among the types of mutations and the social reactions of the affected families will direct the type of screening and prenatal diagnosis programs for PKU in the different countries. In our study, 18 of 69 families (26.1%) refused prenatal diagnosis, and three of the 18 PKU positive cases (14.3%) as evident from prenatal diagnosis decided to terminate the pregnancy. Understanding the attitudes of the affected families is very important in planning the health policies in this field.

In conclusion, we believe that prenatal diagnosis is a good option for the families affected with phenylketonuria, and CVS is a safe method for this purpose. However, prenatal diagnosis must be limited to the families whose mutations (or haplotypes) are already known in order to prevent unnecessary conflicts with the families.   

Authorship contributions. Conception and design: M. Sinan Beksac. Data collection: Berrak Bilginer, Emine Aydın, M. Sinan Beksac. Analysis and interpretation of data: Gokcen Orgul, Emine Aydin. Drafting the article: Gokcen Orgul, M. Sinan Beksac.  Revising it critically for important intellectual content: M. Sinan Beksac, Engin Yilmaz, Turgay Coskun, A. Nur Cakar, Imran Ozalp.

CORRESPONDING AUTHOR: Mehmet Sinan Beksac E-mail: beksacsinan@gmail.com

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

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