In vivo assessment of mutations in the phenylalanine hydroxylase gene by phenylalanine loading: characterization of seven common mutations

Beneficial Effects of Slow-Release Large Neutral Amino Acids after a Phenylalanine Oral Load in Patients with Phenylketonuria

The mainstay of phenylketonuria treatment is a low protein diet, supplemented with phenylalanine (Phe)-free protein substitutes and micronutrients. Adhering to this diet is challenging, and even patients with good metabolic control who follow the dietary prescriptions in everyday life ignore the recommendations occasionally. The present study explores the ability of slow-release large neutral amino acids (srLNAAs) to prevent Phe increase following a Phe dietary load. Fourteen phenylketonuric patients aged ≥13 years were enrolled in a 6-week protocol. Oral acute Phe loads of 250 and 500 mg were added to the evening meal together with srLNAAs (0.5 gr/kg). Phe and tyrosine were dosed before dinner, 2h-after dinner, and after the overnight fast. After oral Phe loads, mean plasma Phe remained stable and below 600 µmol/L. No Phe peaks were registered. Tyrosine levels significantly increased, and Phe/Tyrosine ratio decreased. No adverse events were registered. In conclusion, a single oral administration of srLNAAs at the dose of 0.5 gr/kg is effective in maintaining stable plasma Phe during acute oral loads with Phe-containing food and may be added to the dietetic scheme in situations in which patients with generally good adherence to diet foresee a higher than prescribed Phe intake due to their commitments.

Phenylalanine hydroxylase deficiency in south Italy: Genotype-phenotype correlations, identification of a novel mutant PAH allele and prediction of BH4 responsiveness

We investigated the mutation spectrum of the phenylalanine hydroxylase gene (PAH) in a cohort of patients from 33 Italian PKU families. Mutational screening of the known coding region, including conventional intron splice sites, was performed by direct sequencing of the patients' genomic DNA. Thirty-three different disease causing mutations were identified in our patient group, including 19 missense, 6 splicing, 3 nonsense, 5 deletions, with a detection rate of 100%. The most prevalent mutation was the IVS10-11G>A, accounting for 12.1% of PKU alleles studied. Other frequent mutations were: p.R261Q (9.1%), p.P281L (7.6%), and p.R408W (6.1%). We also identified one novel missense mutation, p.H290Q. A spectrum of 31 different genotypes was observed and a genotype based predictions of BH4-responsiveness were assessed. Among all genotypes, 13 were predicted to be BH4-responsive represented by thirteen PKU families. In addition, genotype-phenotype correlations were performed. This study reveals the importance of a full genotyping of PKU patients and the prediction of BH4-responsiveness, not only because of the definitive diagnosis and prediction of the optimal diet, but also to point out those patients that could benefit from new therapeutic approach. They may potentially benefit from BH4 therapy which, combined with a less strict diet, or eventually in special cases as monotherapy, may contribute to reduce nutritional deficiencies and minimize neurological and psychological dysfunctions.

Phenylketonuria: translating research into novel therapies

Phenylketonuria (PKU) is an inborn error of metabolism of the amino acid phenylalanine. It is an autosomal recessive disorder with a rate of incidence of 1 in 10,000 in Caucasian populations. Mutations in the phenylalanine hydroxylase (PAH) gene are the major cause of PKU, due to the loss of the catalytic activity of the enzyme product PAH. Newborn screening for PKU allows early intervention, avoiding irreparable neurological damage and intellectual disability that would arise from untreated PKU. The current primary treatment of PKU is the limitation of dietary protein intake, which in the long term may be associated with poor compliance in some cases and other health problems due to malnutrition. The only alternative therapy currently approved is the supplementation of BH4, the requisite co-factor of PAH, in the orally-available form of sapropterin dihydrochloride. This treatment is not universally available, and is only effective for a proportion (estimated 30%) of PKU patients. Research into novel therapies for PKU has taken many different approaches to address the lack of PAH activity at the core of this disorder: enzyme replacement via virus-mediated gene transfer, transplantation of donor liver and recombinant PAH protein, enzyme substitution using phenylalanine ammonia lyase (PAL) to provide an alternative pathway for the metabolism of phenylalanine, and restoration of native PAH activity using chemical chaperones and nonsense read-through agents. It is hoped that continuing efforts into these studies will translate into a significant improvement in the physical outcome, as well as quality of life, for patients with PKU.

Recombinant heteromeric phenylalanine monooxygenase and the oxygenation of carbon and sulfur substrates

Objectives

The aim of this investigation was to provide in-vitro enzyme kinetic data to support the hypothesis that the in-vivo heterozygous dominant phenotype for phenylalanine monooxygenase (hPAH) was responsible for the S-oxidation polymorphism in the metabolism of S-carboxymethyl-l-cysteine reported in humans. Using a dual-vector expression strategy for the co-production of wild-type and mutant human hPAH subunits we report for the first time the kinetic parameters (Km, Vmax, CLE) for the C-oxidation of l-phenylalanine and the S-oxidation of S-carboxymethyl-l-cysteine in homomeric wild-type, heteromeric mutant and homomeric mutant hPAH proteins in vitro.

Methods

A PROTM dual-vector bacterial expression system was used to produce the required hPAH proteins. Enzyme activity was determined by HPLC with fluorescence detection.

Key findings

The heteromeric hPAH proteins (I65T, R68S, R158Q, I174T, R261Q, V338M, R408W and Y414C) all showed significantly decreased Vmax and CLE values when compared to the homomeric wild-type hPAH enzyme. For both substrates, all calculated Km values were significantly higher than homomeric wild-type hPAH enzyme, with the exception of I65T, R68S and Y414C heteromeric hPAH proteins employing l-phenylalanine as substrate.

Conclusions

The net outcome for the heteromeric mutant hPAH proteins was a decrease significantly more dramatic for S-carboxymethyl-l-cysteine S-oxidation (1.0–18.8% of homomeric wild-type hPAH activity) when compared to l-phenylalanine C-oxidation (25.9–52.9% of homomeric wild-type hPAH activity) as a substrate. Heteromeric hPAH enzyme may be related to the variation in S-carboxymethyl-l-cysteine S-oxidation capacity observed in humans.

Molecular Diagnosis of Phenylketonuria: From Defective Protein to Disease-Causing Gene Mutation

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism, with an average incidence of 1/10000 in Caucasians. PKU is caused by more than 500 mutations in the phenylalanine hydroxylase gene (PAH) which result in phenylalanine hydroxylase (PAH) enzyme deficiency. Two approaches, in vitro expression analysis of mutant PAH and genotype-phenotype correlation study, are used for the assessment of severity ofPAHmutations. It has been shown that there is a significant correlation between mutantPAHgenotypes and PKU phenotypes. As a result, the molecular diagnosis is completely shifted toward the detection of mutations in the phenylalanine hydroxylase gene. The study of the molecular basis of PKU in Serbia included identification of the spectrum and frequency ofPAHmutations in Serbian PKU patients and genotype-phenotype correlation analysis. By using both PCR-RFLP and »broad range« DGGE/DNA sequencing analysis, the mutation detection rate reached 97%. Thus, the base for molecular diagnosis, genetic counseling and selection of BH4-responsive PKU patients in Serbia was created.

Impact of neonatal protein metabolism and nutrition on screening for phenylketonuria

OBJECTIVES

Early blood phenylalanine (Phe) elevation after birth enables screening for and anticipation of the diagnosis of phenylketonuria. The differential impact of factors involved in this phenomenon, however, has not been elucidated. To solve this question, phenotype, genotype, dietary Phe intake, timing of blood collection, and Phe metabolism were retrospectively analyzed in 21 phenylketonuria newborns and prospectively in 1.

PATIENTS AND METHODS

Patients were assigned to 1 of 4 classes of phenylalanine hydroxylase (PAH) deficiency (severe, moderate, mild, and benign) on the basis of their Phe tolerance. Phe ingested, tolerated, and released from endogenous catabolism was assessed.

RESULTS

From birth to screening test, the amount of Phe tolerated ranged from 704 to 1620 mg, according to the class of PAH deficiency. The amount of Phe ingested ranged only from 204 to 405 mg, whereas the endogenous Phe breakdown ranged from 812 to 1534 mg, resulting in a rate of Phe catabolism ranging from 262 to 341 mg/day, regardless of the class of PAH deficiency.

CONCLUSIONS

The high rate of protein catabolism is the main determinant of neonatal hyperphenylalaninemia. It is sufficient to turn to positive the screening test in severe and moderate PAH deficiency. In mild and benign PAH deficiency, the outcome of screening procedures can be substantially altered by the concurrence of genetic and peristaltic factors. These results imply that the value of blood Phe at the screening test is not fully predictive of the phenylketonuria phenotype, and strengthen concerns regarding the reliability of early screening procedures.

Co-expression of different subunits of human phenylalanine hydroxylase: evidence of negative interallelic complementation

To study the interaction between two different subunits of the heteromeric human phenylalanine hydroxylase (hPAH), present in hyperphenylalaninemic (HPA) compound heterozygous patients, heteroallelic hPAH enzymes were produced. A dual vector expression system was used (PRO Bacterial Expression System) in which each mutant subunit was expressed from a separate compatible vector, with different epitope tags, in a single bacterial host. Experimental conditions were selected in order that each plasmid produced equivalent levels of mutant subunits. In this study, we demonstrated that both subunits were expressed and that the purified heteroallelic enzymes, were catalytically active. As expected, the produced proteins displayed enzymatic activities levels lower than the predicted catalytic activity, calculated by averaging in vitro PAH activities from both alleles, and were strongly dependent on the proteins subunit composition. The obtained data suggest that interactions between the studied hPAH subunits, namely the I65T, R261Q, R270K and V388M, and the wild-type protein occurred. As postulated, this phenomenon could be a source of phenotypic variation in genetic diseases involving multimeric proteins.

How PAH gene mutations cause hyper-phenylalaninemia and why mechanism matters: insights from in vitro expression

Mutations in the human PAH gene, which encodes phenylalanine hydroxylase are associated with varying degrees of hyperphenylalaninemia (HPA). The more severe of these manifest as a classic metabolic disease--phenylketonuria (PKU). In vitro expression analysis of PAH mutations has three major applications: 1) to confirm that a disease-associated mutation is genuinely pathogenic, 2) to assess the severity of a mutation's impact, and 3) to examine how a mutation exerts its deleterious effects on the PAH enzyme, that is, to elucidate the molecular mechanisms involved. Data on expression analysis of 81 PAH mutations in multiple in vitro systems is summarized in tabular form online at www.pahdb.mcgill.ca. A review of these findings points in particular to a prevalent general mechanism that appears to play a major role in the pathogenicity of many PAH mutations. Amino acid substitutions promote misfolding of the PAH protein monomer and/or oppose the correct assembly of monomers into the native tetrameric enzyme. The resulting structural aberrations trigger cellular defenses, provoking accelerated degradation of the abnormal protein. The intracellular steady-state levels of the mutant PAH enzyme are therefore reduced, leading to an overall decrease in phenylalanine hydroxylation within cells and thus to hyperphenylalaninemia. There is considerable scope for modulation of the enzymic and metabolic phenotypes by modification of the cellular handling--folding, assembly, and degradation--of the mutant PAH protein. This has major implications, both for our understanding of genotype-phenotype correlations and for the development of novel therapeutic approaches.

In vitro expression of 34 naturally occurring mutant variants of phenylalanine hydroxylase: correlation with metabolic phenotypes and susceptibility toward protein aggregation

Phenylalanine hydroxylase (PAH) is a homotetrameric enzyme that catalyzes the conversion of phenylalanine to tyrosine, the rate-limiting step of phenylalanine disposal in humans. Primary dysfunction of PAH caused by mutations in the PAH gene results in hyperphenylalaninemia, which may impair cognitive development unless corrected by dietary restriction of phenylalanine. The mechanism(s) by which PAH missense mutations cause enzyme impairment has been studied in detail only in a small number of cases, but existing evidence points to a major role of enhanced proteolytic degradation due to aberrant folding of mutant polypeptides. We have used two heterologous in vitro expression systems (a mammalian cell-free transcription-translation system and the pET system of Escherichia coli) to examine 34 mutations that have been associated with PAH deficiency in the Danish population. These mutations represent a broad range of amino acid substitutions, functional enzyme domains, and metabolic phenotypes. In both systems, residual in vitro activities correlated broadly with metabolic phenotypes, however, with significant discrepancies. Analysis of E. coli extracts by nondenaturing polyacrylamide gel electrophoresis and storage experiments showed that (i) in general, mutations in the N-terminal regulatory domain are associated with relatively stable proteins compared to most mutations in the central catalytic domain, and (ii) for mutations in the catalytic domain, high levels of protein aggregation do not always correspond with a severe phenotype. Our data support and extend previous evidence that PAH mutations exert their pathogenic effects by several distinct mechanisms that may operate individually or in concert.

In vitro expression analysis of mutations in phenylalanine hydroxylase: linking genotype to phenotype and structure to function

Mutations in the human phenylalanine hydroxylase gene (PAH) altering the expressed cDNA nucleotide sequence (GenBank U49897) can impair activity of the corresponding enzyme product (hepatic phenylalanine hydroxylase, PAH) and cause hyperphenylalaninemia (HPA), a metabolic phenotype for which the major disease form is phenylketonuria (PKU; OMIM 261600). In vitro expression analysis of inherited human mutations in eukaryotic, prokaryotic, and cell-free systems is informative about the mechanisms of mutation effects on enzymatic activity and their predicted effect on the metabolic phenotype. Corresponding analysis of site-directed mutations in rat Pah cDNA has assigned critical functional roles to individual amino acid residues within the best understood species of phenylalanine hydroxylase. Data on in vitro expression of 35 inherited human mutations and 22 created rat mutations are reviewed here. The core data are accessible at the PAH Mutation Analysis Consortium Web site (http://www.mcgill.ca/pahdb).

Alterations in protein aggregation and degradation due to mild and severe missense mutations (A104D, R157N) in the human phenylalanine hydroxylase gene (PAH)

Phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine; its activity is the major determinant of phenylalanine disposal. Mutations in the corresponding human gene (PAH), which encodes the human hepatic PAH enzyme, result in hyperphenylalaninemia; the resulting phenotypes can range in severity from mild forms of hyperphenylalaninemia with benign outcome to the severe form, phenylketonuria with impaired cognitive development. This paper describes the detailed characterization of two inherited recessive missense mutations in PAH, c.311C-->A (A104D) and [c.470G-->A;c.471A-->C] (R157N), which are associated, respectively, in the homozygous or functionally hemizygous states, with mild and severe metabolic phenotypes. We used three different in vitro PAH expression systems (in Escherichia coli, cell-free rabbit reticulocyte lysates, and human embryonal kidney cells), as well as a unique assay for phenylalanine oxidation in vivo. In each system, we observed alterations of PAH function and physical properties, compared with wild-type enzyme, and differences in relative severity of effects between these two mutations. Pulse-chase experiments showed increased PAH degradation, probably related to observed aberrations in protein folding and altered oligomerization, as a basic mechanism underlying effects of these missense mutations.

The V388M mutation results in a kinetic variant form of phenylalanine hydroxylase

The molecular mechanism underlying the metabolic defect in phenylketonuria (PKU) patients carrying the V388M missense mutation of the phenylalanine hydroxylase (PAH) gene has been characterized. An in vitro prokaryotic expression system has been used to produce both the wild-type and the mutant form of the human PAH (hPAH) protein. The recombinant enzymes, obtained as fusion proteins, were purified by immobilized metal affinity chromatography and recovered in high yields. The wild-type hPAH possessed a high specific activity and its kinetic properties were the same as those reported for the enzyme isolated from human liver and other recombinant wild-type hPAH enzymes. The recombinant V388M mutant form exhibited a reduced specific activity equivalent to 30% of the wild-type hPAH enzyme when assayed using the synthetic cofactor (6-methyltetrahydropterin). Lower values were obtained (23 and 19%) when the mutant enzyme was assayed with the natural cofactor ((6R)-tetrahydrobiopterin) and different concentrations of l-phenylalanine. The enzyme kinetic studies of the V388M mutant protein revealed that this enzyme was a kinetic variant form of hPAH with a reduced affinity for l-phenylalanine and for the natural cofactor ((6R)-tetrahydrobiopterin). The residual activities determined for the V388M form of hPAH were compatible with the phenotype presented by the PKU patients harboring the V388M mutation in the PAH gene.

Relationship among genotype, biochemical phenotype, and cognitive performance in females with phenylalanine hydroxylase deficiency: report from the Maternal Phenylketonuria Collaborative Study

OBJECTIVE

To examine the relationship of phenylalanine hydroxylase (PAH) genotypes to biochemical phenotype and cognitive development in maternal phenylketonuria (PKU).

METHODOLOGY

PAH gene mutations were examined in 222 hyperphenylalaninemic females enrolled in the Maternal PKU Collaborative Study (MPKUCS). A total of 84 different mutations were detected, and complete genotype was obtained in 199 individuals. Based on previous knowledge about mutation-phenotype associations, 78 of the mutations could be assigned to one of four classes of severity (severe PKU, moderate PKU, mild PKU, and mild hyperphenylalaninemia [MHP]). Then, 189 MPKUCS subjects were grouped according to the various combinations of mutation classifications. The sample sizes were large enough for statistical testing in four groups with at least one mutation that completely abolishes enzyme activity. These patients are considered functionally hemizygous.

RESULTS

The biochemical phenotype predicted from the genotype in functionally hemizygous patients was related significantly to the assigned phenylalanine level. Cognitive performance (IQ) was also significantly related to genotype. The IQ of PAH-deficient mothers with a severe PKU mutation in combination with a MHP mutation or a mild PKU mutation was 99 and 96, respectively, whereas the IQ of PKU mothers with two severe PKU mutations or with one severe and one moderate PKU mutation was 83 and 84, respectively. Of the patients with PKU, 92% had been treated during childhood. Those who were untreated or treated late had lower than average IQ scores for their group of mutation combinations. Females with moderate or mild PKU who were treated early and treated for >6 years showed IQ scores 10 points above average for their group.

CONCLUSIONS

The reproductive outcome in maternal phenylketonuria is dependent on prenatal metabolic control and postnatal environmental circumstances. Both factors depend on the intellectual resources of the mother with PKU. The significant relationship among genotype, biochemical phenotype, and cognitive performance observed in the present study is of importance for the development of an optimal strategy for future treatment of females with PKU who plan pregnancy.

Genomics, mutations and the Internet: the naming and use of parts

Mutations are the source of genetic variation and diversity; by their effect, some are neutral, others are pathogenic. In contemporary genetics, mutations appear at the interface between genomics (structural and functional) and genetics (heredity), where they serve gene discovery and mapping (genomics) and generate challenges to modify their phenotypic effects (medical genetics). Assuming the human genome harbours 80,000 transcribed genes each possessing at least 100 different (germline) alleles in a typical population, how then to record and recover data on at least 8 million human alleles? Bioinformatics is the essential resource to create the corresponding accessible digital libraries (genomic and locus-specific mutation databases) for this purpose, a goal to which The HUGO Mutation Database Initiative (Science 279: 10-11, 1998) aspires. Guidelines now exist for naming alleles (Hum Mutat 11: 1-3, 1998). The principles behind the practice are illustrated by PAHdb (http:/(/)www.mcgill.ca/ pahdb), a prototype locus-specific mutation database (NAR 26: 220-225, 1998), and by prototype genomic mutation databases (HGMD (NAR 26: 285-287, 1998), http:/(/)www.uwcm.ac.uk/uwcm/mg/hgmd0.h tml; the EBI mutation database, http:/(/)www2.ebi.ac.uk/mutations/; and OMIM, http:/(/)www.ncbi.nlm. nih.gov/Omim.html).

A model of human phenylalanine metabolism in normal subjects and in phenylketonuric patients

The derivation of a quantitative model of phenylalanine metabolism in humans is described. The model is based on the kinetic properties of pure recombinant human phenylalanine hydroxylase and on estimates of the in vivo rates of phenylalanine transamination and protein degradation. Calculated values for the steady-state concentration of blood phenylalanine, rate of clearance of phenylalanine from the blood after an oral load of the amino acid, and dietary tolerance of phenylalanine all agree well with data from normal as well as from phenylketonuric patients and obligate heterozygotes. These calculated values may help in the decision about the degree of restriction of phenylalanine intake that is necessary to achieve a satisfactory clinical outcome in classical patients and in those with milder forms of the disease.

A European multicenter study of phenylalanine hydroxylase deficiency: classification of 105 mutations and a general system for genotype-based prediction of metabolic phenotype

Phenylketonuria (PKU) and mild hyperphenylalaninemia (MHP) are allelic disorders caused by mutations in the gene encoding phenylalanine hydroxylase (PAH). Previous studies have suggested that the highly variable metabolic phenotypes of PAH deficiency correlate with PAH genotypes. We identified both causative mutations in 686 patients from seven European centers. On the basis of the phenotypic characteristics of 297 functionally hemizygous patients, 105 of the mutations were assigned to one of four arbitrary phenotype categories. We proposed and tested a simple model for correlation between genotype and phenotypic outcome. The observed phenotype matched the predicted phenotype in 79% of the cases, and in only 5 of 184 patients was the observed phenotype more than one category away from that expected. Among the seven contributing centers, the proportion of patients for whom the observed phenotype did not match the predicted phenotype was 4%-23% (P<.0001), suggesting that differences in methods used for mutation detection or phenotype classification may account for a considerable proportion of genotype-phenotype inconsistencies. Our data indicate that the PAH-mutation genotype is the main determinant of metabolic phenotype in most patients with PAH deficiency. In the present study, the classification of 105 PAH mutations may allow the prediction of the biochemical phenotype in >10,000 genotypes, which may be useful for the management of hyperphenylalaninemia in newborns.

Human phenylalanine hydroxylase mutations and hyperphenylalaninemia phenotypes: a metanalysis of genotype-phenotype correlations

We analyzed correlations between mutant genotypes at the human phenylalanine hydroxylase locus (gene symbol PAH) and the corresponding hyperphenylalaninemia (HPA) phenotypes (notably, phenylketonuria [OMIM 261600]). We used reports, both published and in the PAH Mutation Analysis Consortium Database, on 365 patients harboring 73 different PAH mutations in 161 different genotypes. HPA phenotypes were classified as phenylketonuria (PKU), variant PKU, and non-PKU HPA. By analysis both of homoallelic mutant genotypes and of "functionally hemizygous" heteroallelic genotypes, we characterized the phenotypic effect of 48 of the 73 different, largely missense mutations. Among those with consistent in vivo expression, 24 caused PKU, 3 caused variant PKU, and 10 caused non-PKU HPA. However, 11 mutations were inconsistent in their effect: 9 appeared in two different phenotype classes, and 2 (I65T and Y414C) appeared in all three classes. Seven mutations were inconsistent in phenotypic effect when in vitro (unit-protein) expression was compared with the corresponding in vivo phenotype (an emergent property). We conclude that the majority of PAH mutations confer a consistent phenotype and that this is concordant with their effects, when known, predicted from in vitro expression analysis. However, significant inconsistencies, both between in vitro and in vivo phenotypes and between different individuals with similar PAH genotypes, reveal that the HPA-phenotype is more complex than that predicted by Mendelian inheritance of alleles at the PAH locus.

Phenylketonuria in Italy: distinct distribution pattern of three mutations of the phenylalanine hydroxylase gene

Phenylketonuria (PKU) is an autosomal recessive disease caused by the deficiency of a liver-specific enzyme, phenylalanine hydroxylase (PAH). The pattern of PAH mutations in Mediterranean populations appears to be different from that observed in northern Europe and Asia. Our aim was to study the molecular basis of PKU in Campania and Calabria, two regions of southern Italy. We studied 99 unrelated alleles, detecting 75.8% of the mutations. Our results show that 57% of all the PKU alleles are caused by three different mutations: IVS10nt-546, R261Q and L48S, which display significant differences in their relative distribution across Italy. A novel mutation, a G-to-T transversion at the codon 257 (G257C), was also identified. This mutation results in a Gly-to-Cys change in the catalytic domain of the protein.

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.

PAH Mutation Analysis Consortium Database: a database for disease-producing and other allelic variation at the human PAH locus

The PAH Mutation Analysis Consortium (81 investigators, 26 countries) is engaged in mutation detection at the human PAH locus. Ascertainment of probands occurs largely through newborn screening for hyperphenylalaninemia. A relational database records allelic variation (disease-producing and polymorphic) at the locus. Information is distributed by Newsletter, diskette (WINPAHDB software stand-alone executable on IBM compatible hardware), and at a 'real' site on the Worldwide Web (http://www.mcgill.ca/pahdb). The database presently records (Sept. 27, 1995) 248 alleles in 798 different associations (with polymorphic haplotype, geographic region and population) along with additional information. The database, as a record of human genetic diversity, at a particular locus, contributes to the study of human evolution and demic expansion; it also has medical relevance.