Molecular biology of phenylketonuria

Importance of diet in maternal phenylketonuria

Diet has long been recognized as the primary treatment modality for individuals with phenylketonuria (PKU) during infancy and childhood. Recent findings from the Maternal PKU Collaborative Study clearly indicate that dietary restriction of phenylalanine is also necessary to prevent the adverse effects of an elevated plasma phenylalanine concentration during pregnancy, which include microcephaly, physical anomalies, and mental retardation.

New approaches to the treatment of phenylketonuria

Phenylketonuria (PKU) is the most common of all aminoacidopathies and is caused by autosomal recessive deficiency of the hepatic phenylalanine hydroxylase system. The diagnosis of PKU should be multifactorial and based on a protein overload test that reveals increased plasma phenylalanine levels during the ingestion of a normal diet, a phenylalanine tolerance test, and in vitro and in vivo activity of the liver enzyme. An individualized diagnosis that characterizes the severity of the disease in each patient provides objective and effective criteria for the dietary treatment of each particular case.

The influence of mutations of enzyme activity and phenylalanine tolerance in phenylalanine hydroxylase deficiency

The phenylalanine hydroxylase (PAH) deficiency trait is heterogeneous with a continuum of metabolic phenotypes ranging from classical phenylketonuria (PKU) to mild hyperphenylalaninaemia (MHP). More than 200 mutations in the PAH gene are associated with PAH deficiency. From theoretical considerations or in vitro expression studies each mutation has a particular influence on enzyme activity, which explains the variation in dietary tolerance for phenylalanine (Phe). This paper gives a summary of the effect of each type of mutation on PAH activity and illustrates how the combination of mutations (the genotype) is associated with the Phe tolerance (the metabolic phenotype). Mutations within a population generally include a few prevalent mutations and a high number of rare mutations. The particular distribution of mutations implies that many PAH-deficient patients carry the same mutation combination, enabling the establishment of genotype-phenotype correlations by comparing clinical parameters in patients with identical genotypes. Because certain mutations always cause MHP irrespective of the mutation on the second allele, mutation typing of hyperphenylalaninaemic neonates will differentiate between PKU and MHP. In addition, genotyping will provide a tool for precise diagnosis of the metabolic phenotype of the neonate with PKU and thereby permit earlier implementation of dietary therapy better tailored to each individual patient.

The phenylketonuria locus: current knowledge about alleles and mutations of the phenylalanine hydroxylase gene in various populations

The hyperphenylalaninemic disorders of classic phenylketonuria (PKU), mild phenylketonuria, and hyperphenylalaninemia (HPA), result from a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH) or its cofactor (tetrahydrobiopterin). Use of the complementary DNA of this enzyme has allowed the establishment of a restriction fragment length polymorphism (RFLP) haplotype-analysis system. This haplotype analysis system provides the means for determination of mutant PAH alleles in most affected families and is the basis for mutational analysis of the PKU locus. This review is focused on two major areas of current PKU research: (1) the use of DNA haplotype analysis in the study of the population genetics of PAH deficiency, and (2) the study of genotypes, and their various combinations, as a means of explaining and predicting the phenotypic variability observed for the disorders of PAH deficiency.

Molecular analysis of PKU haplotypes in the population of southern Poland

Out of a population of 138,598 infants born in southern Poland between 1987 and 1989, and screened for phenylketonuria (PKU), 28 cases were ascertained probands and their parents were isolated and eight polymorphic restriction sites were analyzed within the phenylalanine hydroxylase gene region. Twenty-one different haplotypes (HT) were revealed, five of them representing new categories. The most common haplotypes among those carrying normal alleles were: HT1 (27.3%) and HT4 (11.4%). Within the group of haplotypes with mutant alleles the most frequent was HT2 (56.8%), whereas the frequency of this haplotype in other European populations, such as French, Danish and German, ranged from 12% to 24%. HT3, being the most common in Danish (38%), and relatively frequent in the other western European (13-14%) populations, appeared to be very rare in our sample (2.3%). The mutation of codon 408 (exon 12, C----T, Arg----Trp), which has been described to be tightly linked to HT2, was tested on amplified DNA by dot-blot hybridization. This mutation was found in 25 out of 44 proband chromosomes. In one case it was linked to HT5, in the remaining 24 to HT2. Our results confirm molecular heterogeneity of PKU haplotypes, as well as their significant interpopulation variation.

The codon 408 mutation associated with haplotype 2 is predominant in Polish families with phenylketonuria

The incidence of phenylketonuria (PKU) in the western part of Poland is 1 in 5000 live births. Restriction fragment length polymorphism (RFLP) haplotypes at the phenylalanine hydroxylase locus have been analysed in 46 Polish families with PKU. Among 43 fully-informative families 16 RFLP haplotypes were identified. Haplotype 2 is the most frequently (62%) associated with Polish PKU alleles, and the codon 408 mutation is in complete linkage disequilibrium with this haplotype in Poland. This finding is in agreement with observations in other eastern European countries (German Democratic Republic, Czechoslovakia, and Hungary) and in contrast to the genotype distribution observed in western European countries. The present observation suggests the spread of classical PKU, due to the codon 408 mutation associated with haplotype 2, from east to west in European populations. Perhaps more important for genetic counselling, 62% of all PKU chromosomes in the Polish population can now be detected using only one mutant-specific oligonucleotide probe.

Haplotype distribution and molecular defects at the phenylalanine hydroxylase locus in Italy

In order to investigate the molecular basis of phenylketonuria (PKU) in Italy, we characterized the RFLP haplotypes at the phenylalanine hydroxylase gene in 38 unrelated Italian PKU families. The distribution of haplotypes associated with PKU alleles differs from that of other European populations. In particular, haplotypes 1 and 6 are present in 39.7% and 17.6% of the PKU chromosomes, whereas the frequencies of haplotypes 2 and 3 are 5.9% and 2.9%, respectively. The characterization of PKU mutations using the polymerase chain reaction and allele-specific oligonucleotides shows that 1 out of 2 haplotypes 3 carries the splicing mutation and that 2 out of 4 haplotypes 2 carry the missense mutation associated with these haplotypes in North European populations. Our results indicate that the two molecular defects most frequent in Northern Europe represent a minority of PKU mutations in Italy.

Study design and description of patients

A West German multicentre study (eight centres) of PKU was designed in 1976. The subjects of the study are the differential diagnosis, factors influencing the therapeutic outcome, and the extension of dietary therapy into adolescence. Between 1978 and 1984, 165 patients were enrolled, of whom 38 were of non-German nationality. The educational and occupational status of the West German parents were comparable to the population of the Federal Republic of Germany. In the central data bank located at the University Childrens Hospital in Heidelberg, data from recurrent medical examinations and from biochemistry, dietetics, neurology, psychometry and demography were collected. The differential diagnosis of the elevated plasma Phe level in the newborn period resulted in the detection of 2 patients with a PTPS-deficiency, and of 163 with an apo-enzyme defect. Depending upon the magnitude of the Phe levels during the first weeks of life, preliminary treatment groups were formed. They were revised at the age of 6 months with a protein challenge. The levels of Phe during the protein challenge resulted in three types of response. Of these, type III can apparently forgo dietary restrictions resulting in plasma Phe concentrations of around 10 mg/dl. Preliminary results of the whole study are now presented.

Direct detection of a major mutation responsible for phenylketonuria in the population of the Federal Republic of Germany

Forty-six individuals having phenylketonuria (PKU) alleles at the phenylalanine hydroxylase (PAH) locus were tested for the haplotype 2 PKU mutation by allele-specific hybridization following in vitro DNA amplification. Patients and carriers previously shown to have a mutant haplotype 2 PAH allele demonstrated conservation of this mutation. In vitro DNA amplification greatly facilitated this analysis and provides the possibility of population screening for 37% of the mutant German PAH alleles.

RFLPs of the phenylalanine hydroxylase gene in the Italian population

Different mutations of the phenylalanine hydroxylase (PAH) gene leading to phenylketonuria (PKU) have been described associated with specific haplotypes in several European countries. In order to investigate the distribution of DNA haplotypes in Italy, restriction fragment length polymorphism (RFLP) analysis of the PAH gene was performed in nine Italian PKU patients from eight unrelated families, and in the available relatives. The analysis of eight polymorphic sites revealed haplotypes 1 and 6 in association with PKU. This pattern appears to differ from those reported for other European populations. The majority of the 14 PKU subjects studied showed compound heterozygosity for different haplotypes, as observed for other European series. RFLP analysis at the PAH locus allowed us to offer the possibility of prenatal diagnosis to six of the studied families. One prenatal diagnosis was performed and a normal fetus was diagnosed.

Haplotype analysis of classical and mild phenotype of phenylketonuria in the German Democratic Republic

The restriction fragment length polymorphism (RFLP)-haplotypes have been analysed in 16 families from the northern part of the GDR at risk for classical and mild phenylketonuria (PKU). Ten different RFLP haplotypes associated with the normal and mutant phenylalanine hydroxylase (PAH) alleles were identified. Of the 32 mutant alleles analysed, 29 (90.6%) were associated with haplotypes 1, 2, 3 and 4; 53.1% of the mutant alleles were linked with haplotype 2. The distribution of RFLP haplotypes in 16 patients of clinical different PKU phenotypes (classical and mild) is reported.