Classical phenylketonuria in Bulgaria: RFLP haplotypes and frequency of the major mutations

Phenylketonuria mutations and their relation to RFLP haplotypes at the PAH locus in Czech PKU families

A detailed study of the mutant phenylalanine hydroxylase (PAH) gene from the eastern part of the Czech Republic (Moravia) is reported. A total of 190 mutant alleles from 95 phenylketonuria (PKU) families were analyzed for 21 prevalent Caucasian mutations and restriction fragment length polymorphism/variable number of tandem repeats (RFLP/VNTR) haplotypes. Eighty per cent of all mutant alleles were found to carry 11 mutations. The most common molecular defect was the mutation R408W (55.3%), with a very high degree of homozygosity (34.6%). Each of four other mutations (R158Q, R243X, G272X, IVS12nt1) accounted for more than 3% of PKU alleles. Rarely present were mutations IVS10nt546 (2.6%), R252W (2.6%), L48S (2.1%), R261Q (1.6%), Y414C (1.0%) and 165T (0.5%). Mutations that have been predominantly described in southern Europe (IVS7nt1, A259V, Y277D, R241H, T278N) were not detected. A total of 14 different mutant haplotypes were observed. Three unusual genotype-haplotype associations were identified (R158Q on haplotypes 2.3 and 7.8 and R252W on haplotype 69.3). There was a strong association between the mutation R408W and haplotype 2.3 (54.7%). Heterogeneity was found at mutations R408W (haplotypes 2.3 and 5.9), R158Q (haplotypes 4.3, 2.3 and 7.8) and IVS10nt546 (haplotypes 6.7 and 34.7). The molecular basis of PKU in the Moravian area appears to be relatively homogeneous in comparison with other southern and western European populations, thus providing a good starting point for prenatal diagnosis and early clinical classification.

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.

The phenylketonuria G272X haplotype 7 mutation in European populations

We have compiled data on the frequencies of the phenylketonuria G272X mutation in European populations. This mutation occurs north of the Alps. It has a particularly high frequency in the Oslo Fjord region of Norway with the adjacent Bohuslän region of Sweden. An intermediate frequency was noted in a separate area, the eastern part of Germany with the adjacent western part of Czechoslovakia. The G272X mutation was associated with phenylalanine hydroxylase haplotype 7, except for one case with haplotype 3. Genealogical studies going back eight to nine generations revealed no common source for this mutation, but there was some geographical convergence to the Bohuslän region. These findings suggest a single origin for this mutation, with at least one founding population in south-eastern Norway/adjacent Sweden.

Haplotype distribution at the phenylalanine hydroxylase locus in PKU families from the Moravian area of Czechoslovakia

The analysis of 21 families affected with classical phenylketonuria (PKU) from the Moravian area of Czechoslovakia has revealed 12 different RFLP haplotypes. Nine and eight haplotypes were associated with the normal and with the mutant alleles, respectively. Most normal alleles are associated with haplotype 1 (42.9%). Almost 80% of all mutant alleles are confined within only three haplotypes (1, 2 and 4). There was a strong association between haplotype 2 and the Czech mutant alleles (61.9% of the mutant alleles compared with 4.8% of the normal alleles). There was linkage disequilibrium between this haplotype and the R408W mutation in exon 12. Two mutant haplotypes 7 were found and in both cases they were tightly linked with G272ter mutation. Our finding is in agreement with observations in other Eastern European countries. These data provide further support for the theories of the spread of the R408W mutation from east to west in European populations.

Multiple origins for phenylketonuria in Europe

Phenylketonuria (PKU), a disorder of amino acid metabolism prevalent among Caucasians and other ethnic groups, is caused primarily by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). PKU is a highly heterogeneous disorder, with more than 60 molecular lesions identified in the PAH gene. The haplotype associations, relative frequencies, and distributions of five prevalent PAH mutations (R158Q, R261Q, IVS10nt546, R408W, and IVS12n1) were established in a comprehensive European sample population and subsequently were examined to determine the potential roles of several genetic mechanisms in explaining the present distribution of the major PKU alleles. Each of these five mutations was strongly associated with only one of the more than 70 chromosomal haplotypes defined by eight RFLPs in or near the PAH gene. These findings suggest that each of these mutations arose through a single founding event that occurred within time periods ranging from several hundred to several thousand years ago. From the significant differences observed in the relative frequencies and distributions of these five alleles throughout Europe, four of these putative founding events could be localized to specific ethnic subgroups. Together, these data suggest that there were multiple, geographically and ethnically distinct origins for PKU within the European population.

Heterogeneity of mutations in Bulgarian phenylketonuria haplotype 1 and 4 alleles

A study of Bulgarian patients with classical PKU demonstrated that haplotypes 1 and 4 carry a significant number of rare molecular defects resulting from independent mutational events. Differences in mutations associated with these common haplotypes exist even between populations which share a common major PKU mutation. Some amino acid substitutions, previously reported to lead to mild phenylalanine hydroxylase deficiency, were detected in the present study in compound heterozygotes with severe PKU. These findings preclude carrier testing and hyperphenylalaninemia typing by genomic analysis at least in the heterogeneous Bulgarian population.

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.

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

Direct sequencing of the phenylalanine hydroxylase (PAH) gene indicated the existence of silent mutations in codons 232, 245, and 385, linked to specific RFLP haplotypes in several Caucasian populations, namely Germans, Bulgarians, Italians, Turks, and Lithuanians. All three mutations create a new restriction site and can be easily detected on PCR amplified DNA. The usefulness of the silent mutations for diagnostic purposes depends on the haplotype distribution in the target population. The combined analysis of these markers and one or two PKU mutations forms a simple panel of diagnostic tests with full informativeness in a large proportion of PKU families, which helps to avoid the problems of genetic heterogeneity and of prenatal genomic Southern blot analysis.

Aberrant splicing of phenylalanine hydroxylase mRNA: the major cause for phenylketonuria in parts of southern Europe

We report a mutation within the phenylalanine hydroxylase (PAH) gene that causes aberrant splicing of the mRNA and that is in tight association with chromosomal haplotypes 6, 10, and 36. Because of the high frequency of these particular haplotypes in Bulgaria, Italy, and Turkey, it appears to be one of the more frequent defects in the PAH gene causing classical phenylketonuria in this part of Europe. The mutation is a G to A transition at position 546 in intron 10 of the PAH gene, 11 bp upstream from the intron 10/exon 11 boundary. It activates a cryptic splice site and results in an in-frame insertion of 9 nucleotides between exon 10 and exon 11 of the processed mRNA. Normal amounts of liver PAH protein are present in homozygous patients, but no catalytic activity can be detected. This loss of enzyme activity is probably caused by conformational changes resulting from the insertion of three additional amino acids (Gly-Leu-Gln) between the normal sequences encoded by exon 10 and exon 11.

A frameshift mutation in exon 2 of the phenylalanine hydroxylase gene linked to RFLP haplotype 1

A deletion of a single base in codon 55 (exon 2) of the phenylalanine hydroxylase (PAH) gene has been identified by direct DNA sequencing of 94 phenylketonuria (PKU) chromosomes. This mutation alters the reading frame so that a stop signal (TAA) is generated in codon 60 of the PAH gene. Haplotype analysis revealed that all PKU alleles showing the codon 55 frameshift mutation exhibited haplotype 1. In our panel of DNA probes 13% of all mutant haplotype 1 alleles carry this particular mutation. Patients who were compound heterozygotes for this deletion and R408W in exon 12, or the splice mutation in intron 12, were affected by severe PKU. Thus, the clinical data provide additional evidence that haplotype 1 PKU alleles carry molecular defects which confer a null phenotype. In addition, we were able to show that the newly detected mutation occurs on alleles of different ethnic background.

Molecular characterization of PKU allele prevalent in southern Europe and Ireland

A novel substitution has been characterized in the phenylalanine hydroxylase (PAH) gene that is linked exclusively to mutant haplotype 6, which is prevalent in southern Europe but rare in northern and eastern Europe. It is a G-to-A transition in intron 10, 11 bases from exon 11. This substitution creates an additional AG dinucleotide, which may serve as a cryptic splice acceptor site. Individuals who bear this substitution in the homozygous state have a severe PKU phenotype with pretreatment serum phenylalanine levels over 1200 mumol/liter. The frequency and distribution of this substitution among European populations suggests two possible founding populations, one being Middle Eastern and the other Roman. The use of this substitution as a marker to identify PKU chromosomes will be an invaluable aid to carrier screening and prenatal diagnosis in populations where mutant haplotype 6 is prevalent.