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‣ Missense Mutations in the N-Terminal Domain of Human Phenylalanine Hydroxylase Interfere with Binding of Regulatory Phenylalanine

Gjetting, Torben; Petersen, Marie; Guldberg, Per; Güttler, Flemming
Fonte: The American Society of Human Genetics Publicador: The American Society of Human Genetics
Tipo: Artigo de Revista Científica
Português
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Hyperphenylalaninemia due to a deficiency of phenylalanine hydroxylase (PAH) is an autosomal recessive disorder caused by >400 mutations in the PAH gene. Recent work has suggested that the majority of PAH missense mutations impair enzyme activity by causing increased protein instability and aggregation. In this study, we describe an alternative mechanism by which some PAH mutations may render PAH defective. Database searches were used to identify regions in the N-terminal domain of PAH with homology to the regulatory domain of prephenate dehydratase (PDH), the rate-limiting enzyme in the bacterial phenylalanine biosynthesis pathway. Naturally occurring N-terminal PAH mutations are distributed in a nonrandom pattern and cluster within residues 46–48 (GAL) and 65–69 (IESRP), two motifs highly conserved in PDH. To examine whether N-terminal PAH mutations affect the ability of PAH to bind phenylalanine at the regulatory domain, wild-type and five mutant (G46S, A47V, T63P/H64N, I65T, and R68S) forms of the N-terminal domain (residues 2–120) of human PAH were expressed as fusion proteins in Escherichia coli. Binding studies showed that the wild-type form of this domain specifically binds phenylalanine, whereas all mutations abolished or significantly reduced this phenylalanine-binding capacity. Our data suggest that impairment of phenylalanine-mediated activation of PAH may be an important disease-causing mechanism of some N-terminal PAH mutations...

‣ Distinct Missense Mutations of the FGFR3 Lys650 Codon Modulate Receptor Kinase Activation and the Severity of the Skeletal Dysplasia Phenotype

Bellus, Gary A.; Spector, Elaine B.; Speiser, Phyllis W.; Weaver, Christine A.; Garber, Anthony T.; Bryke, Christine R.; Israel, Jamie; Rosengren, Sally S.; Webster, Melanie K.; Donoghue, Daniel J.; Francomano, Clair A.
Fonte: The American Society of Human Genetics Publicador: The American Society of Human Genetics
Tipo: Artigo de Revista Científica
Português
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47.65993%
The fibroblast growth factor–receptor 3 (FGFR3) Lys650 codon is located within a critical region of the tyrosine kinase–domain activation loop. Two missense mutations in this codon are known to result in strong constitutive activation of the FGFR3 tyrosine kinase and cause three different skeletal dysplasia syndromes—thanatophoric dysplasia type II (TD2) (A1948G [Lys650Glu]) and SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans) syndrome and thanatophoric dysplasia type I (TD1) (both due to A1949T [Lys650Met]). Other mutations within the FGFR3 tyrosine kinase domain (e.g., C1620A or C1620G [both resulting in Asn540Lys]) are known to cause hypochondroplasia, a relatively common but milder skeletal dysplasia. In 90 individuals with suspected clinical diagnoses of hypochondroplasia who do not have Asn540Lys mutations, we screened for mutations, in FGFR3 exon 15, that would disrupt a unique BbsI restriction site that includes the Lys650 codon. We report here the discovery of three novel mutations (G1950T and G1950C [both resulting in Lys650Asn] and A1948C [Lys650Gln]) occurring in six individuals from five families. Several physical and radiological features of these individuals were significantly milder than those in individuals with the Asn540Lys mutations. The Lys650Asn/Gln mutations result in constitutive activation of the FGFR3 tyrosine kinase but to a lesser degree than that observed with the Lys540Glu and Lys650Met mutations. These results demonstrate that different amino acid substitutions at the FGFR3 Lys650 codon can result in several different skeletal dysplasia phenotypes.