Silent mutations in the phenylalanine hydroxylase gene as an aid to the diagnosis of phenylketonuria

Genetic analysis carried out on blood-spots of phenylalanine hydroxylase-deficient newborns detected by northeastern Italian neonatal screening

The aim of this work was to perform genetic analysis on 18 different blood-spot samples collected from neonates detected as hyperphenylalaninemic by Northeastern Italian screening program. DNA was extracted from blood-spots. Exons/introns of PAH gene were amplified by polymerase chain reaction (PCR), and PCR products were purified and sequenced with both forward and reverse primers. The most frequent mutations were IVS12nt1g>a (16.7%) and R408W, P281L and L48S (all together 11.1%). As expected, compound heterozygosity was the usual finding; homozygosity was found only in two patients with R158Q and IVS2nt5g>c mutations. The V230I mutation was reported for the first time in Italy. We found six previously described polymorphisms (V245V, IVS4nt47c>t, IVS2nt19t>c, IVS3nt-22c>t, IVS5nt-54a>g, and E280>Q280). To our knowledge, four genotypes were not previously described: R158Q/V230I present in one patient with classical PKU; and L48S/R408Q, A403V/IVS2nt-13t>g, and G272X/V230I present in patients showing HPA phenotype. Most of the mutations were located in the exons 12 and 7 and in exon/intron 2 (83.3% detection of total mutations in PKU or HPA patients of Northeastern Italy). From a practical viewpoint, the genetic analysis of blood-spots collected on Guthrie cards for neonatal screening for PKU could be a simple method to establish the genotype of neonates. Consequently, the genotype/phenotype correlation could lead to a more accurate diagnosis and prognosis for families.

Phenylalanine hydroxylase gene in psychiatric patients: screening and functional assay of mutations

BACKGROUND

Reports relating phenylalanine kinetics and metabolism to psychiatric disorders led us to undertake the comprehensive screening of the phenylalanine hydroxylase (PAH) coding region and functional testing of discovered mutations in a sample of psychiatric patients and healthy control subjects.

METHODS

Genomic DNA from psychiatric patients and control subjects was assayed for sequence variants in all PAH coding regions and splice junctions. In vivo functional analysis of mutations was conducted by assessing the kinetics and conversion to tyrosine of a standardized phenylalanine dose and by measuring fasting pterin levels.

RESULTS

A known missense mutation was observed in a schizoaffective subject, and a novel missense mutation was discovered in four subjects with schizophrenia and one normal subject. The schizoaffective patient heterozygous for the known A403V mutation showed the lowest rate of phenylalanine kinetics and lowest conversion to tyrosine in the patient sample. The four schizophrenic patients heterozygous for the novel K274E mutation showed significantly decreased phenylalanine kinetics, reduced conversion to tyrosine, and increased synthesis of the PAH cofactor tetrahydrobiopterin compared with schizophrenic subjects without the mutation.

CONCLUSIONS

The study findings suggest that larger scale studies are warranted to test the relationship of the PAH genotype with a psychiatric phenotype.

Mutation and haplotype analysis of phenylalanine hydroxylase alleles in classical PKU patients from the Czech Republic: identification of four novel mutations

Mutations, haplotypes, and other polymorphic markers in the phenylalanine hydroxylase (PAH) gene were analysed in 133 unrelated Czech families with classical phenylketonuria (PKU). Almost 95% of all mutant alleles were identified, using a combination of PCR and restriction analysis, denaturing gradient gel electrophoresis (DGGE), and sequencing. A total of 30 different mutations, 16 various RFLP/VNTR haplotypes, and four polymorphisms were detected on 266 independent mutant chromosomes. The most common molecular defect observed in the Czech population was R408W (54.9%). Each of the other 29 mutations was present in no more than 5% of alleles and 13 mutations were found in only one PKU allele each (0.4%). Four novel mutations G239A, R270fsdel5bp, A342P, and IVS11nt-8g-->a were identified. In 14 (5.1%) alleles, linked to four different RFLP/VNTR haplotypes, the sequence alterations still remain unknown. Our results confirm that PKU is a heterogeneous disorder at the molecular level. Since there is evidence for the gene flow coming from northern, western, and southern parts of Europe into our Slavic population, it is clear that human migration has been the most important factor in the spread of PKU alleles in Europe.

Phenylalanine hydroxylase gene mutations in the United States: report from the Maternal PKU Collaborative Study

The major cause of hyperphenylalaninemia is mutations in the gene encoding phenylalanine hydroxylase (PAH). The known mutations have been identified primarily in European patients. The purpose of this study was to determine the spectrum of mutations responsible for PAH deficiency in the United States. One hundred forty-nine patients enrolled in the Maternal PKU Collaborative Study were subjects for clinical and molecular investigations. PAH gene mutations associated with phenylketonuria (PKU) or mild hyperphenylalaninemia (MHP) were identified on 279 of 294 independent mutant chromosomes, a diagnostic efficiency of 95%. The spectrum is composed of 71 different mutations, including 47 missense mutations, 11 splice mutations, 5 nonsense mutations, and 8 microdeletions. Sixteen previously unreported mutations were identified. Among the novel mutations, five were found in patients with MHP, and the remainder were found in patients with PKU. The most common mutations were R408W, IVS12nt1g-->a, and Y414C, accounting for 18.7%, 7.8%, and 5.4% of the mutant chromosomes, respectively. Thirteen mutations had relative frequencies of 1%-5%, and 55 mutations each had frequencies < or = 1%. The mutational spectrum corresponded to that observed for the European ancestry of the U.S. population. To evaluate the extent of allelic variation at the PAH locus within the United States in comparison with other populations, we used allele frequencies to calculate the homozygosity for 11 populations where >90% ascertainment of mutations has been obtained. The United States was shown to contain one of the most heterogeneous populations, with homozygosity values similar to Sicily and ethnically mixed sample populations in Europe. The extent of allelic heterogeneity must be a major determining factor in the choice of mutation-detection methodology for molecular diagnosis in PAH deficiency.

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.

Multiplex genotype determination at a DNA sequence polymorphism cluster in the human immunoglobulin heavy-chain region

We have developed a method for multilocus genotype determination. The method involves using restriction fragment length polymorphisms (RFLPs) for allele discrimination. If a polymorphism is not an RFLP, it is converted into an RFLP during the polymerase chain reaction (PCR). After amplification and restriction enzyme digestion, samples are analyzed by sequential gel loading during electrophoresis. The efficiency of this method was demonstrated by determining the genotypes of 108 semen samples at seven DNA sequence polymorphic sites identified in the human immunoglobulin heavy-chain variable region. It was shown that more than 1000 PCR products could be easily analyzed per day per investigator. To show the reliability of this method, some of the typing results were confirmed by DNA sequence analysis. By computer simulation, most (98%) polymorphisms were shown to be natural or convertible (by changing 1 bp close to or next to each polymorphic site) RFLPs for the commercially available 4-base cutters.

DNA sequence polymorphisms in exonic and intronic regions of the human phenylalanine hydroxylase gene aid in the identification of alleles

Five sequence polymorphisms at the phenylalanine hydroxylase (PAH) gene locus were observed to be in tight association with specific alleles of this locus. Since these polymorphisms can be detected using polymerase chain reaction (PCR) methodology, application of a combination of these polymorphisms reduces the effort involved in PAH DNA haplotype analysis, which is needed for population genetic analysis or diagnosis of the disease status. In addition our results indicate the evolution of haplotype 3, 4 and 7 PAH alleles from a common ancestor, whereas PAH haplotypes 5, 6, and 11 arose from another common ancestor allele. These data reveal that two of the polymorphisms investigated originated before the separation of races.

RFLP haplotyping and mutation analysis of the phenylalanine hydroxylase gene in Dutch phenylketonuria families

Restriction fragment length polymorphism haplotyping of mutated and normal phenylalanine hydroxylase (PAH) alleles in 49 Dutch phenylketonuria (PKU) families was performed. All mutant PAH chromosomes identified by haplotyping (n = 98) were screened for eight of the most predominant mutations. Compound heterozygosity was proven in 40 kindreds. Homozygosity was found for the IVS/2nt1 mutation in 5 families, and for the R158Q and IVS10nt546 mutations in one family each. All patients from these families suffer from severe PKU, providing additional proof that these mutations are deleterious for the PAH gene. Genotypical heterogeneity was evident for mutant haplotype 1 (n = 27) carrying the mutations R261Q (n = 12), E280K (n = 4, P281L (n = 1) and unknown (n = 10), and likewise for mutant haplotype 4 (n = 30) carrying the mutations R158Q (n = 13), Y414C (n = 1) and unknown (n = 16). Mutant haplotype 3 (n = 20), in tight association with mutation IVS12nt1, appeared to be in strong linkage disequilibrium (LDE) with its normal counterpart allele (n = 4). Mutant haplotype 6 (n = 4), in tight association with the IVS10nt546 mutation, showed moderate LDE with its counterpart allele (n = 1). The distribution of the mutant PAH haplotypes 1, 3 and 4 among the Dutch PKU population resembles that in other Northern and Western European countries, but it is striking that mutant haplotype 2 and its associated mutation R408W is nearly absent in The Netherlands, in strong contrast to its neighbouring countries.

Analysis of exon 7 of the human phenylalanine hydroxylase gene: A mutation hot spot?

Complete sequence analysis of 194 human phenylalanine hydroxylase genes from PKU patients originating from West Germany and Bulgaria revealed 13 different mutations within exon 7 of the gene. Four of these mutations (T238P: ACT-->CCT; L242F:CTC-->TTC; R252G:CGG-->GGG; and 1043 delta 11: nt 1043-nt 1053 deleted) have so far not been described in the literature. Including these new mutations at least 21 different gene lesions and one sequence polymorphism exist for exon 7. Despite this large number unbiased calculation of the mutation frequency/exon size ratio does not provide conclusive evidence that exon 7 is a hot spot for disease causing mutations. Extensive screening during our experiments also failed to demonstrate the existence of excessive polymorphism in this part of the gene. It might therefore be speculated that the functional importance of the highly conserved exon 7 sequence accounts for the clustering of observed mutations which result in clinically manifest PKU. In addition we report our experience in regard to the resolution capacity of denaturing gradient gel electrophoresis (DGGE), a nonradioactive technique for the rapid screening of unknown mutations in exon 7.

Molecular basis of phenylketonuria and related hyperphenylalaninemias: mutations and polymorphisms in the human phenylalanine hydroxylase gene

Mutations in the human phenylalanine hydroxylase gene producing phenylketonuria or hyperphenylalaninemia have now been identified in many patients from various ethnic groups. These mutations all exhibit a high degree of association with specific restriction fragment-length polymorphism haplotypes at the PAH locus. About 50 of these mutations are single-base substitutions, including six nonsense mutations and eight splicing mutations, with the remainder being missense mutations. One splicing mutation results in a 3 amino acid in-frame insertion. Two or 3 large deletions, 2 single codon deletions, and 2 single base deletions have been found. Twelve of the missense mutations apparently result from the methylation and subsequent deamination of highly mutagenic CpG dinucleotides. Recurrent mutation has been observed at several of these sites, producing associations with different haplotypes in different populations. About half of all missense mutations have been examined by in vitro expression analysis, and a significant correlation has been observed between residual PAH activity and disease phenotype. Since continuing advances in molecular methodologies have dramatically accelerated the rate in which new mutations are being identified and characterized, this register of mutations will be updated periodically.