Genetic background of clinical homogeneity of phenylketonuria in Poland

Phenylketonuria in Portugal: Genotype-phenotype correlations using molecular, biochemical, and haplotypic analyses

Background

The impairment of the hepatic enzyme phenylalanine hydroxylase (PAH) causes elevation of phenylalanine levels in blood and other body fluids resulting in the most common inborn error of amino acid metabolism (phenylketonuria). Persistently high levels of phenylalanine lead to irreversible damage to the nervous system. Therefore, early diagnosis of the affected individuals is important, as it can prevent clinical manifestations of the disease.

Methods

In this report, the biochemical and genetic findings performed in 223 patients diagnosed through the Portuguese Neonatal Screening Program (PNSP) are presented.

Results

Overall, the results show that a high overlap exists between different types of variants and phenylalanine levels. Molecular analyses reveal a wide mutational spectrum in our population with a total of 56 previously reported variants, most of them found in compound heterozygosity (74% of the patients). Intragenic polymorphic markers were used to assess the haplotypic structure of mutated chromosomes for the most frequent variants found in homozygosity in our population (p.Ile65Thr, p.Arg158Gln, p.Leu249Phe, p.Arg261Gln, p.Val388Met, and c.1066‐11G>A).

Conclusion

Our data reveal high heterogeneity at the biochemical and molecular levels and are expected to provide a better understanding of the molecular basis of this disease and to provide clues to elucidate genotype–phenotype correlations.

Molecular Analysis of PKU-Associated PAH Mutations: A Fast and Simple Genotyping Test

Neonatal screening for phenylketonuria (PKU, OMIM: 261600) was introduced at the end of the 1960s. We developed a rapid and simple molecular test for the most frequent phenylalanine hydroxylase (PAH, Gene ID: 5053) mutations. Using this method to detect the 18 most frequent mutations, it is possible to achieve a 75% detection rate in Italian population. The variants selected also reach a high detection rate in other populations, for example, 70% in southern Germany, 68% in western Germany, 76% in Denmark, 68% in Sweden, 63% in Poland, and 60% in Bulgaria. We successfully applied this confirmation test in neonatal screening for hyperphenylalaninemias using dried blood spots and obtained the genotype in approximately 48 h. The method was found to be suitable as second tier test in neonatal screening for hyperphenylalaninemias in neonates with a positive screening test. This test can also be useful for carrier screening because it can bypass the entire coding sequence and intron–exon boundaries sequencing, thereby overcoming the questions that this approach implies, such as new variant interpretations.

Prevalence of tetrahydrobiopterine (BH4)-responsive alleles among Austrian patients with PAH deficiency: comprehensive results from molecular analysis in 147 patients

Phenylketonuria (PKU, MIM 261600) is an autosomal recessive disorder caused by mutations of the phenylalanine hydroxylase gene (PAH, GenBank U49897.1, RefSeq NM_000277). To date more than 560 variants of the PAH gene have been identified. In Europe there is regional distribution of specific mutations. Due to recent progress in chaperone therapy, the prevalence of BH4-responsive alleles gained therapeutic importance. Here we report the mutational spectrum of PAH deficiency in 147 unrelated Austrian families. Overall mutation detection rate was 98.6 %. There was a total of 62 disease-causing mutations, including five novel mutations IVS4 + 6T>A, p.H290Y, IVS8-2A>G, p.A322V and p.I421S. The five most prevalent mutations found in patients were p.R408W, IVS12 + 1G>A, p.R261Q, p.R158Q and IVS2 + 5G>C. Neonatal phenylalanine levels before treatment were available in 114/147 patients. Prediction of BH4-responsiveness in patients with full genotypes was exclusively made according to published data. Among the 133 patients needing dietary treatment, 28.4 % are expected to be BH4 "non-responsive", 4.5 % are highly likely BH4-responsive, 35.8 % are probably BH4-responsive while no interpretation was possible for 31.3 %. The mutation data reflect the population history of Austria and provide information on the likely proportion of Austrian PKU patients that may benefit from BH4-therapy.

A limited spectrum of phenylalanine hydroxylase mutations is observed in phenylketonuria patients in western Poland and implications for treatment with 6R tetrahydrobiopterin

Phenylketonuria (PKU) is an autosomal recessive defect in hepatic metabolism of phenylalanine, which is secondary to mutations in the phenylalanine hydroxylase (PAH) gene. Sixty-seven ethnically Polish PKU patients, followed at the Outpatient Department of Pediatrics and Developmental Medicine in Poznan, Poland, were assessed for mutations in the PAH gene. Two mutations were identified in 61 of 67 patients and a single mutation was identified in the remaining six patients. The four most prevalent mutations (p.R408W, 68%; c.1066-11G>A, 6%; c.1315+1G>A, 5.2%; c.822-832delGCCCATGTATA, 3.7%) accounted for 83% of the mutant alleles. Fifteen additional mutations were identified of which most (13/15) were observed in an individual patient. Before knowledge of PAH genotypes, 19 patients were challenged with a 20 mg kg(-1) dose of 6R tetrahydrobiopterin (BH(4)) and serum phenylalanine concentration was monitored in hospital over 24 h. Two patients responded to the BH(4) challenge with a reduction of serum phenylalanine concentration >30% from baseline. PAH genotypes of the two responsive patients would have been predicted, as they contained mutations recognized as BH(4) responsive, whereas the 17 patients who were unresponsive would have been predicted as their mutations were either recognized as non-responsive or were highly deleterious frame-shift mutations. Overall, only 7.5% (5/ 67) of patients had PAH mutations recognized as responsive to co-factor therapy. Among the PKU patients from western Poland, PAH mutations responsive to BH(4) therapy are poorly represented; therefore, genotyping may be useful for identifying candidate patients likely to respond to BH(4) before physiological challenge.

Molecular characterization of phenylketonuria in South Brazil

Phenylketonuria (PKU) is an autosomal recessive disorder due to phenylalanine hydroxylase (PAH) deficiency. The PAH gene, located at 12q22-q24.1, includes about 90kb and contains 13 exons. To date, more than 420 different alterations have been identified in the PAH gene. To determine the nature and frequency of PAH mutations in PKU patients from South Brazil, mutation analysis was performed on genomic DNA from 23 unrelated PKU patients. The 13 exons and flanking regions of the PAH gene were amplified by PCR and the amplicons were analyzed by single strand conformation polymorphism (SSCP). Amplicons that showed abnormal migration patterns were analyzed by restriction endonuclease digestion and/or sequencing. Twenty-two previously reported mutations were identified including R261X, R408W, IVS2nt5g-->c, R261Q, and V388M. Polymorphisms were observed in 48.8% of the PKU patients, the most frequent being IVS2nt19t-->c, V245V, and IVS12nt-35c-->t. In addition, two novel sequence variants were identified: 1378g-->t in the 3(')-untranslated region in exon 13 which may be disease-causing and an intron 12 polymorphism, IVS12nt-15t-->c. The mutation spectrum in the patients from Southern Brazil differed from that observed in patients from other Latin American countries and further defined the molecular heterogeneity of this disease.

Phenylketonuria mutations in Europe

Phenylketonuria (PKU) is heterogeneous. More than 400 different mutations in the phenylalanine hydroxylase (PAH) gene have been identified. In a systematic review of the molecular genetics of PKU in Europe we identified 29 mutations that may be regarded as prevalent in European populations. Comprehensive regional data for these mutations were collated from all available studies. The spectrum of mutations found in individual regions results from a combination of factors including founder effect, range expansion and migration, genetic drift, and probably heterozygote advantage. Common mutations include R408W on a haplotype 2 background in Eastern Europe, IVS10-11G>A in the Mediterranean, IVS12+1G>A in Denmark and England, Y414C in Scandinavia, I65T in Western Europe, and R408W on haplotype 1 in the British Isles. Molecular data from mild hyperphenylalaninemia (MHP) patients are available from a number of countries, but it is currently not possible to calculate relative allele frequencies. The available data on PAH mutations are useful for the understanding of both the clinical features and the population genetics of PAH deficiency in Europe.

Recurrence of the R408W mutation in the phenylalanine hydroxylase locus in Europeans

The relative frequency of the common phenylalanine hydroxylase (PAH) mutation R408W and its associations with polymorphic RFLP, VNTR, and short-tandem-repeat (STR) sites in the PAH gene were examined in many European populations and one representative North American population of defined European descent. This mutation was found to cluster in two regions: in northwest Europe among Irish and Scottish peoples, and in eastern Europe, including the Commonwealth of Independent States. This allele was significantly less frequent in intervening populations. In eastern European populations, the R408W mutation is strongly associated with RFLP haplotype 2, the three-copy VNTR allele (VNTR 3), and the 240-bp STR allele. In northwestern European populations, it is strongly associated with RFLP haplotype 1, the VNTR allele containing eight repeats (VNTR 8), and the 244-bp STR allele. An examination of the linkage between the R408W mutation and highly polymorphic RFLP, VNTR, and STR haplotypes suggests that recurrence is the most likely mechanism to account for the two different major haplotype associations of R408W in Europe.

Population genetics of phenylketonuria

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a large number of mutations at the phenylalanine hydroxylase (PAH) locus, most of which are strongly associated with specific RFLP or VNTR haplotypes. One of the major questions remaining in PKU research is why this apparently maladaptive disorder has been maintained at a frequency of approximately 1 in 10,000 among Caucasians. A growing number of studies have provided evidence that both the relatively high frequency of PKU and the strong mutation/haplotype associations might reflect the existence of multiple founding populations for PKU. Examples of putative founding populations for PKU in both Europe and Asia will be presented. Some PAH mutations are associated with multiple haplotypes, suggesting recurrence. Evidence for and against recurrence as the mechanism responsible for the association of the R408W mutation with RFLP haplotypes 1 and 2 will be discussed.

A simplified method for detection of the mutations predominantly causing cystic fibrosis and phenylketonuria in Polish families

Genomic DNA was isolated from dried blood specimens and subsequently used as a template in simplified PCR-based detection assays of delta F508 mutation of CFTR gene and of R408W mutation of PAH gene in families with cystic fibrosis and phenylketonuria, respectively. Products of amplification of CFTR gene were analyzed in NuSieve agarose gel. The amplification-created restriction site with TaqI digestion was used for detection of the PAH gene mutation.