Detection of beta-ureidopropionase deficiency with HPLC-electrospray tandem mass spectrometry and confirmation of the defect at the enzyme level

The pyrimidine bases uracil and thymine are degraded via the consecutive action of three enzymes to beta-alanine and beta-aminoisobutyric acid, respectively. To date, a number of patients have been described with a deficiency of dihydropyrimidine dehydrogenase and dihydropyrimidinase, the first two enzymes of the pyrimidine degradation pathway. In this study, we demonstrate that the first patient presenting with N-carbamyl-beta-amino aciduria, due to a deficiency of beta-ureidopropionase, was easily diagnosed at the metabolite level using HPLC-tandem mass spectrometry. Urinary analysis showed strongly elevated levels of N-carbamyl-beta-alanine and N-carbamyl-beta-aminoisobutyric acid, with normal or moderately increased levels of the pyrimidine bases and the dihydropyrimidines, respectively. The deficiency of beta-ureidopropionase was confirmed by measuring all three enzymes of the pyrimidine degradation pathway. No activity of beta-ureidopropionase could be detected in a liver biopsy of the patient, while a normal activity of dihydropyrimidine dehydrogenase and dihydropyrimidinase was present. Thus, HPLC-tandem mass specrometry proved to be a powerful tool for the initial diagnosis of patients with deficiency of beta-ureidopropionase.

Clinical implications of dihydropyrimidine dehydrogenase (DPD) deficiency in patients with severe 5-fluorouracil-associated toxicity: identification of new mutations in the DPD gene

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-fluorouracil (5FU), and it is suggested that patients with a partial deficiency of this enzyme are at risk for developing a severe 5FU-associated toxicity. To evaluate the importance of this specific type of inborn error of pyrimidine metabolism in the etiology of 5FU toxicity, an analysis of the DPD activity, the DPD gene, and the clinical presentation of patients suffering from severe toxicity after the administration of 5FU was performed. Our study demonstrated that in 59% of the cases, a decreased DPD activity could be detected in peripheral blood mononuclear cells. It was observed that 55% of patients with a decreased DPD activity suffered from grade IV neutropenia compared with 13% of patients with a normal DPD activity (P = 0.01). Furthermore, the onset of toxicity occurred, on average, twice as fast in patients with low DPD activity as compared with patients with a normal DPD activity (10.0 +/- 7.6 versus 19.1 +/- 15.3 days; P < 0.05). Analysis of the DPD gene of 14 patients with a reduced DPD activity revealed the presence of mutations in 11 of 14 patients, with the splice site mutation IVS14+1G-->A being the most abundant one (6 of 14 patients; 43%). Two novel missense mutations 496A-->G (M166V) and 2846A-->T (D949V) were detected in exon 6 and exon 22, respectively. Our results demonstrated that at least 57% (8 of 14) of the patients with a reduced DPD activity have a molecular basis for their deficient phenotype.

Pitfalls in the diagnosis of patients with a partial dihydropyrimidine dehydrogenase deficiency

BACKGROUND

Dihydropyrimidine dehydrogenase (DPD) catalyzes the degradation of thymine, uracil, and the chemotherapeutic drug 5-fluorouracil. To identify patients suffering from complete or partial DPD deficiency and to identify pitfalls that can preclude the proper diagnosis of patients with partial DPD deficiency, a sensitive and accurate assay is necessary.

METHODS

The activity of DPD was measured using [4-(14)C]thymine followed by separation of substrate and products with reversed-phase HPLC with on-line detection of the radioactivity.

RESULTS

Complete baseline separation of radiolabeled thymine and all degradation products was achieved within 15 min. The detection limit for dihydrothymine was 0. 4 pmol. In lymphocytes, the DPD activity deviated from linearity at low protein concentrations (<0.2 g/L). Profoundly decreased activity of DPD was detected in the peripheral blood mononuclear cells (PBM cells) of two tumor patients when measured at low protein concentrations. Low DPD activity comparable to that observed in obligate heterozygotes was initially detected in PBM cells, containing substantial amounts of myeloid cells, from a patient suffering from 5-fluorouracil toxicity. However, after the patient experienced full clinical recovery, normal DPD activity was observed in the PBM cells. No significant differences in DPD activity were observed between exponentially growing fibroblasts and those at confluence. The range of DPD activities of obligate heterozygotes overlaps the range of DPD activities of controls.

CONCLUSIONS

The low activity of DPD measured in PBM cells containing myeloid cells or that measured at a low protein concentration in the assay mixture is not indicative of heterozygosity for a mutant DPD allele. Although fibroblasts are suitable to establish a complete deficiency of DPD, unambiguous detection of heterozygotes is not possible.

A radiochemical assay for beta-ureidopropionase using radiolabeled N-carbamyl-beta-alanine obtained via hydrolysis of [2-(14)C]5, 6-dihydrouracil

A radiochemical assay was developed to measure the activity of beta-ureidopropionase in human liver homogenates which is based on the detection of the reaction product (14)CO(2) by liquid scintillation counting. Radiolabeled N-carbamyl-beta-alanine was prepared within 15 min by a simple hydrolysis of [2-(14)C]5, 6-dihydrouracil under alkaline conditions at 37 degrees C. The enzymatic reaction proved to be linear with time up to at least 3.5 h and protein concentrations up to at least 1 mg/ml. Human beta-ureidopropionase obeyed Michaelis-Menten kinetics with an apparent Km for N-carbamyl-beta-alanine of 15.5 +/- 1.9 microM. The assay proved to be very accurate and sensitive with an intraassay coefficient of variation of 2% and a detection limit of 28 pmol for the product CO(2).

Radiochemical assay for determination of dihydropyrimidinase activity using reversed-phase high-performance liquid chromatography

A radiochemical assay was developed to measure the activity of dihydropyrimidinase (DHP) in human liver homogenates. The method is based on the separation of radiolabeled dihydrouracil from N-carbamyl-beta-alanine by HPLC with on-line detection of radioactivity combined with detection of 14CO2 by liquid scintillation counting. The assay was linear with time and protein concentration. The minimum amount of radiolabeled products which could be determined proved to be 12 pmol using a purified stock solution of [2-(14)C]-5,6-dihydrouracil. This highly sensitive assay is especially suitable to identify patients with a dihydropyrimidinase deficiency.

Genotype and phenotype in patients with dihydropyrimidine dehydrogenase deficiency

Dihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal recessive disease characterised by thymine-uraciluria in homozygous deficient patients and has been associated with a variable clinical phenotype. In order to understand the genetic and phenotypic basis for DPD deficiency, we have reviewed 17 families presenting 22 patients with complete deficiency of DPD. In this group of patients, 7 different mutations have been identified, including 2 deletions [295-298delTCAT, 1897delC], 1 splice-site mutation [IVS14+1G>A)] and 4 missense mutations (85T>C, 703C>T, 2658G>A, 2983G>T). Analysis of the prevalence of the various mutations among DPD patients has shown that the G-->A point mutation in the invariant splice donor site is by far the most common (52%), whereas the other six mutations are less frequently observed. A large phenotypic variability has been observed, with convulsive disorders, motor retardation and mental retardation being the most abundant manifestations. A clear correlation between the genotype and phenotype has not been established. An altered beta-alanine, uracil and thymine homeostasis might underlie the various clinical abnormalities encountered in patients with DPD deficiency.

Heterozygosity for a point mutation in an invariant splice donor site of dihydropyrimidine dehydrogenase and severe 5-fluorouracil related toxicity

Dihydropyrimidine dehydrogenase (DPD) is responsible for the breakdown of the widely used antineoplastic agent 5-fluorouracil (5-FU), thereby limiting the efficacy of the therapy. It has been suggested that patients suffering from 5-FU toxicities due to a low activity of DPD are genotypically heterozygous for a mutant allele of the gene encoding DPD. In this study we investigated the cDNA and a genomic region of the DPD gene of a cancer patient experiencing severe toxicity following 5-FU treatment for the presence of mutations. Although normal activity of DPD was observed in fibroblasts, the DPD activity in leucocytes of the cancer patient proved to be in the heterozygous range. Analysis of the DPD cDNA showed heterozygosity for a 165bp deletion that results from exon skipping. Sequence analysis of the genomic region encompassing the skipped exon showed that the tumour patient was heterozygous for a G-->A point mutation in the invariant GT splice donor sequence in the intron downstream of the skipped exon. So far, the G-->A point mutation has also been found in 8 out of 11 patients suffering from a complete deficiency of DPD. Considering the frequent use of 5-FU in the treatment of cancer patients, the severe 5-FU-related toxicities in patients with a low activity of DPD and the high frequency of the G-->A mutation in DPD deficient patients, analysis of the DPD activity and screening for the G-->A mutation should be routinely carried out prior to the start of the treatment with 5-FU.

Subcellular localization of dihydropyrimidine dehydrogenase

Although dihydropyrimidine dehydrogenase (DPD) has been purified and characterized from liver tissues of various mammals conflicting data exist on its subcellular localization. To determine the localization of DPD we prepared crude subcellular fractions of a rat liver homogenate by means of differential centrifugation. In the fractions obtained (heavy mitochondrial, light mitochondrial, microsomal and cytosolic) the activities of different marker enzymes were measured as well as the activity of DPD. These results showed that almost all of the activity of DPD was located in the cytosolic fraction. To exclude any particulate-associated DPD, a light mitochondrial fraction was subsequently subjected to equilibrium density gradient centrifugation. The distribution profile of the activity of DPD and the various marker enzymes indicated that DPD from rat liver was exclusively located in the cytosol since no significant activity of DPD could be detected in any subcellular organelle.

Identification and tissue-specific expression of a NADH-dependent activity of dihydropyrimidine dehydrogenase in man

Homogenates of human liver and human fibroblasts were able to convert thymine into dihydrothymine in the presence of NADH whereas almost no NADH-dependent activity could be detected in human lymphocytes. The different tissue distribution of the NADH-dependent activity suggests that different types of human dihydropyrimidine dehydrogenase exist. Both types of human liver dihydropyrimidine dehydrogenase showed a comparable affinity towards thymine, NADH and NADPH. Only a ten-fold lower Vmax value was observed for the NADH-dependent enzyme. During partial purification of the NADPH-dependent enzyme, on a 2', 5' - ADP Sepharose 4B column, the NADH-dependent activity was completely lost. Neither type of activity was retained on a 5' - AMP Sepharose 4B column.

Elutriation of T-lymphoblastic cells and cell cycle analysis of fractions

Elutriation of human lymphoblastic cells has to be performed at a flow rate of 13 ml/min at a temperature of 10 degrees C to retain a high cell viability. Cell cycle analysis of elutriated cells on the basis of their DNA contents is inaccurate. The combined detection of BrdUrd incorporated into the DNA and analysis of the DNA content offers an alternative. After elutriation, fractions of cells with increased percentages of either G1 or S phase cells can be selected on the basis of the median cell size as established by a coulter counter channelyzer.