ABSTRACT
Dihydropyrimidinase (DHP) is the second enzyme of the pyrimidine degradation pathway and it catalyses the ring opening of 5,6-dihydrouracil and 5,6-dihydrothymine to N-carbamyl-beta-alanine and N-carbamyl-beta-aminoisobutyric acid, respectively. To date, only nine individuals have been reported suffering from a complete DHP deficiency. We report two siblings presenting with strongly elevated levels of 5,6-dihydrouracil and 5,6-dihydrothymine in plasma, cerebrospinal fluid and urine. One of the siblings had a severe delay in speech development and white matter abnormalities, whereas the other one was free of symptoms. Analysis of the DHP gene (DPYS) showed that both patients were compound heterozygous for the missense mutation 1078T>C (W360R) in exon 6 and a novel missense mutation 1235G>T (R412M) in exon 7. Heterologous expression of the mutant enzymes in Escherichia coli showed that both missense mutations resulted in a mutant DHP enzyme without residual activity. Analysis of the crystal structure of eukaryotic DHP from the yeast Saccharomyces kluyveri and the slime mold Dictyostelium discoideum suggests that the W360R and R412M mutations lead to structural instability of the enzyme which could potentially impair the assembly of the tetramer.
Subject(s)
Amidohydrolases/deficiency , Amidohydrolases/biosynthesis , Amidohydrolases/chemistry , Amidohydrolases/genetics , Amino Acid Sequence , Animals , Brain/abnormalities , Child, Preschool , Crystallography, X-Ray , Dictyostelium/enzymology , Enzyme Stability , Escherichia coli/enzymology , Humans , Language Development Disorders/physiopathology , Magnetic Resonance Imaging , Male , Models, Molecular , Molecular Sequence Data , Mutation, Missense , Protein Conformation , Saccharomyces/enzymology , Siblings , Thymine/analogs & derivatives , Thymine/blood , Thymine/cerebrospinal fluid , Thymine/urine , Uracil/analogs & derivatives , Uracil/blood , Uracil/cerebrospinal fluid , Uracil/urineABSTRACT
Adenosine monophosphate, inosine monophosphate, inosine, adenosine, guanosine, adenine, guanine, hypoxanthine, xanthine, uric acid and pyrimidine bases were determined in the CSF of 18 children after simple febrile seizures and in a control group. There was no statistically significant difference between the two groups for any of these metabolites. This suggests that simple febrile seizures neither significantly disturb the metabolism of nucleotides, nucleosides or bases, nor significantly deplete neuron adenosine triphosphate ATP levels.
Subject(s)
Purines/cerebrospinal fluid , Pyrimidine Nucleosides/cerebrospinal fluid , Seizures, Febrile/cerebrospinal fluid , Adenine/cerebrospinal fluid , Adenosine/cerebrospinal fluid , Adenosine Monophosphate/cerebrospinal fluid , Child, Preschool , Cytosine/cerebrospinal fluid , Female , Guanine/cerebrospinal fluid , Guanosine/cerebrospinal fluid , Humans , Hypoxanthine , Hypoxanthines/cerebrospinal fluid , Infant , Inosine/cerebrospinal fluid , Inosine Monophosphate/cerebrospinal fluid , Male , Thymine/cerebrospinal fluid , Uracil/cerebrospinal fluid , Uric Acid/cerebrospinal fluid , Xanthine , Xanthines/cerebrospinal fluidABSTRACT
In the urine of a child with unexplained convulsions large amounts of uracil and thymine were detected by gas chromatography. Identification was performed by coupled gas chromatography-mass spectrometry. Quantitation of the urinary excretion by means of a sensitive high-performance liquid chromatographic (HPLC) method revealed a 1000-fold elevation compared to normal. Serum and cerebrospinal fluid levels of the two pyrimidine bases were about a hundred times higher than normal. In fibroblasts the activity of dihydrothymine dehydrogenase was determined by measuring the conversion of radioactive labelled thymine to dihydrothymine with HPLC of the reaction mixture. In the patient's cells a complete deficiency of dihydrothymine dehydrogenase activity was found. Our patient is the first case described with such a proven enzyme deficiency.
