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P J Waters

Publications and source records attributed to P J Waters.

At least 19 recordsLinked to original sources
PubMed2001

Homomeric and heteromeric interactions between wild-type and mutant phenylalanine hydroxylase subunits: evaluation of two-hybrid approaches for functional analysis of mutations causing hyperphenylalaninemia.

Phenylketonuria (PKU) is caused by mutations in the phenylalanine hydroxylase gene (PAH), while mutations in genes encoding the two enzymes (dihydropteridine reductase, DHPR, and pterin-4-alpha-carbinolamine dehydratase, PCD) required for recycling of its cofactor, tetrahydrobiopterin (BH(4)), cause other rarer disease forms of hyperphenylalaninemia. We have applied a yeast two-hybrid method, in which protein--protein interactions are measured by four reporter gene constructs, to the analysis of six PKU-associated PAH missense mutations (F39L, K42I, L48S, I65T, A104D, and R157N). By studying homomeric interactions between mutant PAH subunits, we show that this system is capable of detecting quite subtle aberrations in PAH oligomerization caused by missense mutations and that the observed results generally correlate with the severity of the mutation as determined by other expression systems. The mutant PAH subunits are also shown in this system to be able to interact with wild-type PAH subunits, pointing to an explanation for apparent dominant negative effects previously observed in obligate heterozygotes for PKU mutations. Based on our findings, the applications and limitations of two-hybrid approaches in understanding mechanisms by which PAH missense mutations exert their pathogenic effects are discussed. We have also used this technique to demonstrate homomeric interactions between wild-type DHPR subunits and between wild-type PCD subunits. These data provide a basis for functional studies on HPA-associated mutations affecting these enzymes.

Alleles↗
PubMed2001

Degradation of mutant proteins, underlying "loss of function" phenotypes, plays a major role in genetic disease.

Many Mendelian monogenic disorders are caused by loss of the function of a single protein. This can result from rapid degradation of the mutant protein by cellular proteases, which reduces the steady-state concentration of the protein within the cell. The susceptibility of a protein to such proteolytic breakdown depends upon its kinetics of monomer folding and oligomer assembly and upon the intrinsic (thermodynamic) stability of its functional native-state conformation. Other cellular proteins, notably molecular chaperones, promote correct protein folding and assembly and thus provide some protection against degradation. An accumulation of recent evidence indicates that premature or accelerated degradation of mutant proteins, provoked by aberrations in their conformation, occurs in various subcellular compartments and represents a significant and prevalent pathogenic mechanism underlying genetic diseases. Inter-individual variability in proteolytic and folding systems can in part explain why "simple monogenic diseases" often display inconsistent genotype-phenotype correlations which show these disorders to be in reality quite complex. Protein folding and degradation may also be modulated artificially using exogenous small molecules. The identification or design of compounds which can interact specifically with particular target proteins, and which in so doing can exert beneficial effects on protein folding, assembly and/or stability, is beginning to open up a new and remarkably promising avenue for the treatment of diverse genetic disorders.

Genetic Diseases, Inborn↗
PubMed2000

PAHdb: a locus-specific knowledgebase.

PAHdb is an online relational locus-specific "mutation database" (http://www.mcgill.ca/pahdb) for the human phenylalanine hydroxylase gene (symbol PAH) and its associated phenotypes (protein, metabolic, clinical). When combined with associated information (population distribution of allele, haplotype association, etc.) PAHdb functions as a knowledgebase. From the outset, and in the absence of raw data (e.g., sequence gels), PAHdb has instead been an annotated repository of information about mutations maintained by a team of curators. It is also disease-oriented, being focused on a variant phenotype (hyperphenylalaninemia (HPA) and its most important form of disease, phenylketonuria (PKU)) resulting from primary dysfunction of the PAH enzyme (EC 1.14.16.1); it is "patient friendly" in that it contains information for those personally involved with HPA/PKU (MIM# 261600). PAHdb also serves its community through direct interaction.

Alleles↗
PubMed2000

Characterization of phenylketonuria missense substitutions, distant from the phenylalanine hydroxylase active site, illustrates a paradigm for mechanism and potential modulation of phenotype.

Missense mutations account for 48% of all reported human disease-causing alleles. Since few are predicted to ablate directly an enzyme's catalytic site or other functionally important amino acid residues, how do most missense mutations cause loss of function and lead to disease? The classic monogenic phenotype hyperphenylalaninemia (HPA), manifesting notably as phenylketonuria (PKU), where missense mutations in the PAH gene compose 60% of the alleles impairing phenylalanine hydroxylase (PAH) function, allows us to examine this question. Here we characterize four PKU-associated PAH mutations (F39L, K42I, L48S, I65T), each changing an amino acid distant from the enzyme active site. Using three complementary in vitro protein expression systems, and 3D-structural localization, we demonstrate a common mechanism. PAH protein folding is affected, causing altered oligomerization and accelerated proteolytic degradation, leading to reduced cellular levels of this cytosolic protein. Enzyme specific activity and kinetic properties are not adversely affected, implying that the only way these mutations reduce enzyme activity within cells in vivo is by producing structural changes which provoke the cell to destroy the aberrant protein. The F39L, L48S, and I65T PAH mutations were selected because each is associated with a spectrum of in vivo HPA among patients. Our in vitro data suggest that interindividual differences in cellular handling of the mutant, but active, PAH proteins will contribute to the observed variability of phenotypic severity. PKU thus supports a newly emerging paradigm both for mechanism whereby missense mutations cause genetic disease and for potential modulation of a disease phenotype.

ATP-Binding Cassette Transporters↗
PubMed1999

Monogenic traits are not simple: lessons from phenylketonuria.

The classification of genetic disease into chromosomal, monogenic and multifactorial categories is an oversimplification. Phenylketonuria (PKU) is a classic 'monogenic' autosomal recessive disease in which mutation at the human PAH locus was deemed sufficient to explain the impaired function of the enzyme phenylalanine hydroxylase (enzymic phenotype), the attendant hyperphenylalaninemia (metabolic phenotype) and the resultant mental retardation (cognitive phenotype). In the era of molecular genetics, expectations for a consistently close correlation between the mutant genotype and variant phenotype have been somewhat disappointed, and PKU is used here to illustrate how and why this might be the case. So-called monogenic traits do, indeed, conform to long-accepted ideas about the expression of 'major' loci and their importance in determining parameters of phenotype, but the associated features are as complex, in their own ways, as those in so-called complex traits.

Alleles↗
PubMed1998

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).

Animals↗
PubMed1998

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.

Cell Line↗
PubMed1998

Mutation at the phenylalanine hydroxylase gene (PAH) and its use to document population genetic variation: the Quebec experience.

We describe variation at the PAH locus in the population of Quebec. We successfully analyzed 135 of 141 chromosomes from phenylketonuria (PKU) probands (95.7% of the sample), and eight additional chromosomes from a small number of probands with non-PKU hyperphenylalaninemia (HPA). The full set of chromosomes harboured 45 different PAH mutations: i) seven polymorphisms (IVS2nt19, IVS3nt-22, IVS6nt-55, Q232Q, V245V, L385L, Y414Y); ii) four mutations causing non-PKU HPA (T92I, E390G, R408Q, D415N); iii) 34 mutations causing PKU. Only six mutations (M1V, R261Q, F299C, S349P, R408W and IVS12nt1) occurred in the whole province at relative frequencies > 5%: most are rare and probably identical by descent. By studying associations of mutations with polymorphic haplotype alleles, we found examples of mutations on different haplotypes that were identical by state, but not by descent because they were recurrent mutations (E280K and R408W); and examples of mutations identical both by state and by descent because of intragenic recombination (S67P, G218V, V245A and IVS12nt1). Ten mutations were first described in Quebec and five are still unique there; three of these 'Quebec' mutations are reported here for the first time (c.125A-->T (K42I); [c.470G-->A; c.471A--C] (R157N); c.707nt-55 (IVS6nt-55). The PAH mutations stratify by geographic region and population, their distributions validating hypotheses about European range expansion to North America during three separate phases of immigration and demographic expansion in the Quebec region over the past four centuries. The PAH homozygosity value (j) is 0.06 for the total Quebec sample (0.5-0.08 by regions), and the corresponding homoallelic fraction of mutant PAH genotypes is 24%. These findings are a documentation of genetic diversity in the Quebec population.

Alleles↗
PubMed1997

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.

Alleles↗
PubMed1997

Analysis of phenylalanine hydroxylase genotypes and hyperphenylalaninemia phenotypes using L-[1-13C]phenylalanine oxidation rates in vivo: a pilot study.

Hyperphenylalaninemia (HPA) resulting from deficient activity of phenylalanine hydroxylase (PAH) is caused by mutations in the human PAH gene (McKusick 261600). Herein, we report a noninvasive method to: 1) estimate whole-body phenylalanine oxidation in patients with HPA and 2) compare effects of mutant genotypes on phenotypes. We used oral L-[1-13C]phenylalanine as a substrate and measured 13CO2 formation in the first hour as an index of phenylalanine oxidation rates in: 1) patients with PKU (n = 6), variant phenylketonuria (PKU) (n = 7) and non-PKU HPA (n = 4); 2) obligate heterozygotes (n = 18); and 3) controls (n = 8). PAH mutations were identified by PCR, denaturing gradient gel electrophoresis, and DNA sequencing. Phenylalanine oxidation rates demonstrated a gene dosage effect; oxidation in heterozygotes was intermediate between probands and controls. The three classes of HPA had different mean oxidation rates (PKU < variant PKU < non-PKU HPA). The in vivo phenotype (HPA class or whole-body oxidation rate) did not always correspond to prediction from in vitro expression analysis of the mutation effect on enzyme activity. The findings indicate that the in vivo metrical trait (phenylalanine oxidation rate) is not a simple equivalent of phenylalanine hydroxylation activity (unit of protein phenotype) and, as expected, is an emergent property under the control of more than the PAH locus.

Adolescent↗
PubMed1995

Decreased sialidase activity in mononuclear leucocytes of type 1 diabetic subjects: relationship to diabetic complications and glycaemic control.

Leucocyte surface sialic acid content influences surface charge, deformability, and leucocyte-endothelial interaction. Abnormal leucocyte structure and function contributes both to microvascular damage and diabetic complications. The aim of this study was to investigate altered leucocyte SA metabolism in diabetic subjects and measure lysosomal sialidase which regulates leucocyte surface sialylation. We examined 26 Type 1 (insulin-dependent) diabetic subjects with retinopathy, 26 Type 1 diabetic subjects without complications, and 38 matched normal control subjects. Sialidase was assayed in freshly prepared sonicates of pure mononuclear leucocytes (MNLs), using the fluorometric substrate 4-methyl-umbelliferyl-N-acetylneuraminic acid. In the subjects with diabetes there was a significant negative correlation between MNL sialidase activity and both HbA1c (rs = 0.37, p = 0.007) and fructosamine (rs = -0.31, p = 0.026). MNL sialidase activity was significantly decreased in diabetic subjects with clinical evidence of complications compared to control subjects. HbA1c was significantly higher (p = 0.036) in diabetic patients with complications compared to those without. The observed decrease in MNL sialidase activity related to diabetic control may be important in the pathogenesis of vascular damage. Diabetes-associated changes in sialylation of functional cell surface glycoconjugates may have important clinical consequences.

Adolescent↗
PubMed1994

Freeze-stable sialidase activity in human leucocytes: substrate specificity, inhibitor susceptibility, detergent requirements and subcellular localization.

Human leucocytes contain a freeze-stable sialidase (neuraminidase; EC 3.2.1.18) activity in addition to the better-characterized lysosomal freeze-labile enzyme. In order to discriminate between the sialidase activities detected with the synthetic fluorimetric substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (MU-Neu5Ac), different tritiated sialoglycoconjugate substrates were prepared. Using this sensitive radioactive assay system, leucocyte sialidase activity towards glycoproteins was shown to be labile to repeated freeze-thawing, but a Triton-stimulated activity towards gangliosides was entirely freeze-stable. Assay conditions were optimized for this freeze-stable ganglioside sialidase activity. Subcellular fractionation of mononuclear leucocytes (MNLs) on Percoll-density gradients showed that this ganglioside sialidase activity was entirely associated with the plasma membrane. Study of the detergent requirements showed that MNLs also demonstrated ganglioside sialidase activity when sodium cholate was present in place of Triton. Cholate-stimulated ganglioside sialidase activity was found to be entirely freeze-stable and localized at the plasma membrane. Studies on whole homogenates of MNLs demonstrated that the Triton-stimulated and cholate-stimulated activities showed similar acidic pH optima at < or = 3.9 and were both strongly inhibited by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid and Cu2+, but not by free N-acetylneuraminic acid, N-(4-nitrophenyl)oxamic acid or heparan sulphate. These results suggest that human MNLs contain, in addition to the lysosomal freeze-labile sialidase, a single sialidase activity which is freeze-stable, ganglioside-specific, plasma membrane-associated and stimulated both by Triton and by cholate.

Animals↗
PubMed1992

Increases in plasma lysosomal enzymes in type 1 (insulin-dependent) diabetes mellitus: relationship to diabetic complications and glycaemic control.

Lysosomal enzymes degrade membrane glycoconjugates, and increased circulating enzyme activity may be an important mechanism in the pathogenesis of diabetic microangiopathy. We have assayed a profile of seven lysosomal enzyme activities (nmol.h-1.ml-1) in platelet-free plasma from 54 Type 1 (insulin-dependent) diabetic subjects (median age 31 years) and 42 matched normal control subjects. A significant increase in median (interquartile range) enzyme activity was measured in diabetic compared to control subjects for beta-D-glucuronidase, 121 (97.7-171) vs 88.8 (62.8-113), p less than 0.001; beta-D-Nacetylglucosaminidase, 693 (568-799) vs 568 (462-686), p less than 0.001; alpha-D-mannosidase, 23.8 (16.7-28.9) vs 14.5 (10.1-20.0), p less than 0.001; and beta-D-galactosidase, 6.94 (6.11-9.99) vs 6.66 (4.78-8.33), p less than 0.04. In contrast, alpha-L-fucosidase, alpha-D-galactosidase and beta-D-mannosidase activities were similar in diabetic and control subjects. None of the enzyme activities differed significantly (p less than 0.05) between 24 diabetic patients with clinical complications and 30 complication-free diabetic patients with similar glycaemic control which does not support the hypothesis that enzyme increases in diabetes arise simply by leakage from damaged tissues. In the diabetic subjects HbA1, median (interquartile range) 9.10 (7.40-10.60), was significantly related to beta-D-glucuronidase (rs = 0.56, p less than 0.001) and beta-D-Nacetylglucosaminidase (rs = 0.55, p less than 0.001). We have therefore demonstrated in diabetic subjects an increase in certain lysosomal glycosidases, that correlates with glycaemic control.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylglucosaminidase↗