Specific genetic deficiencies of the A and B isoenzymes of monoamine oxidase are characterized by distinct neurochemical and clinical phenotypes.

Monoamine oxidase (MAO) exists as two isoenzymes and plays a central role in the metabolism of monoamine neurotransmitters. In this study we compared the neurochemical phenotypes of previously described subjects with genetically determined selective lack of MAO-A or a lack of both MAO-A and MAO-B with those of two subjects with a previously described X chromosome microdeletion in whom we now demonstrate selective MAO-B deficiency. Mapping of the distal deletion breakpoint demonstrates its location in intron 5 of the MAO-B gene, with the deletion extending proximally into the Norrie disease gene. In contrast to the borderline mental retardation and abnormal behavioral phenotype in subjects with selective MAO-A deficiency and the severe mental retardation in patients with combined MAO-A/MAO-B deficiency and Norrie disease, the MAO-B-deficient subjects exhibit neither abnormal behavior nor mental retardation. Distinct neurochemical profiles characterize the three groups of MAO-deficient patients. In MAO-A-deficient subjects, there is a marked decrease in deaminated catecholamine metabolites and a concomitant marked elevation of O-methylated amine metabolites. These neurochemical changes are only slightly exaggerated in patients with combined lack of MAO-A and MAO-B. In contrast, the only biochemical abnormalities detected in subjects with the MAO-B gene deletion are a complete absence of platelet MAO-B activity and an increased urinary excretion of phenylethylamine. The differences in neurochemical profiles indicate that, under normal conditions, MAO-A is considerably more important than MAO-B in the metabolism of biogenic amines, a factor likely to contribute to the different clinical phenotypes.

Activity of pyrimidine degradation enzymes in normal tissues.

In this study, we measured the activity of dihydropyrimidine dehydrogenase (DPD), dihydropyrimidinase (DHP) and beta-ureidopropionase (beta-UP), using radiolabeled substrates, in 16 different tissues obtained at autopsy from a single patient. The activity of DPD could be detected in all tissues examined, with the highest activity being present in spleen and liver. Surprisingly, the highest activity of DHP was present in kidney followed by that of liver. Furthermore, a low DHP activity could also be detected in 8 other tissues. The highest activity of beta-UP was detected in liver and kidney. However, low UP activities were also present in 8 other tissues. Our results demonstrated that the entire pyrimidine catabolic pathway was predominantly confined to the liver and kidney. However, significant residual activities of DPD, DHP and beta-UP were also present in a variety of other tissues, especially in bronchus.

Determination of ITPase activity in erythrocyte lysates obtained for determination of TPMT activity.

The indication for the determination of both thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase is identical (i.e., adverse drug reactions toward mercaptopurines). Therefore, we tested whether or not our standard procedure to prepare erythrocyte lysates for measurement of TPMT activity, which includes treatment with Chelex 100 (a chelating resin), was suitable for the measurement of ITPase activity. It also was tested to see if ITPase activity differs in EDTA and Heparin anti-coagulated blood samples. We found that there was no difference between the ITPase activity in erythrocyte lysates prepared from EDTA or Heparin anti-coagulated blood. Treatment with a chelating resin or omission of magnesium from the assay procedure resulted in decreased and nearly absent ITPase activity, respectively. We conclude that untreated erythrocyte lysates obtained for determination of TPMT activity are suitable for determination of ITPase activity. However, after treatment with Chelex 100 the erythrocyte lysates become unsuitable for determination of ITPase activity.

Determination of 5-fluorouracil in plasma with HPLC-tandem mass spectrometry.

In this article, we describe a fast and specific method to measure 5FU with HPLC tandem-mass spectrometry. Reversed-phase HPLC was combined with electrospray ionization tandem mass spectrometry and detection was performed by multiple-reaction monitoring. Stable-isotope-labeled 5FU (1,3-15N2-5FU) was used as an internal standard. 5FU was measured within a single analytical run of 16 min with a lower limit of detection of 0.05 microM. The intra-assay variation and inter-assay variation of plasma with added 5FU (1 microM, 10 microM, 100 microM) was less then 6%. Recoveries of the added 5FU in plasma were > 97%. Analysis of the 5FU levels in plasma samples from patients with the HPLC tandem mass spectrometry method and a HPLC-UV method yielded comparable results (r2 = 0.98). Thus, HPLC with electrospray ionization tandem mass spectrometry allows the rapid analysis of 5FU levels in plasma and could, therefore, be used for therapeutic drug monitoring.

A pivotal role for beta-aminoisobutyric acid and oxidative stress in dihydropyrimidine dehydrogenase deficiency?

Dihydropyrimidine dehydrogenase (DPD) constitutes the first step of the pyrimidine degradation pathway in which the pyrimidine bases uracil and thymine are catabolised to beta-alanine and beta-aminoisobutyric acid (beta-AIB), respectively. The mean concentration of beta-AIB was approximately 5- to 8-fold lower in urine of patients with a DPD deficiency, when compared to age-matched controls. Comparable levels of 8-hydroxydeoxyguanosine (8-OHdG) were present in urine from controls and DPD patients at the age <2 year. In contrast, slightly elevated levels of 8-OHdG were detected in urine from DPD patients with an age >2 year, suggesting the presence of increased oxidative stress.

Genetic analysis of the first 4 patients with beta-ureidopropionase deficiency.

beta-Ureidopropionase is the third enzyme of the pyrimidine degradation pathway and it catalyses the irreversible hydrolysis of N-carbamyl-ss-aminoisobutyric acid or N-carbamyl-ss-alanine to beta-aminoisobutyric acid or ss-alanine, ammonia, and CO2. Analysis of the beta-ureidopropionase gene (UPB1) of the first 4 patients presenting with a complete enzyme deficiency, revealed the presence of 2 splice-site mutations (IVS1-2A>G and IVS8-1G>A) and one missense mutation (A85E). RT-PCR analysis of the complete beta-ureidopropionase cDNA suggested that both splice-site mutations lead to a variety of alternative splice variants, with deletions of a single or several exons. The alanine at position 85 was not conserved in other eukaryotic beta-ureidopropionase protein sequences.

Imbalance in the nucleotide pools of myeloid leukemia cells and HL-60 cells: correlation with cell-cycle phase, proliferation, differentiation, and transformation.

Myeloblasts from the blood of patients with chronic myeloid leukemia (CML) in a blastoid crisis were shown to have an imbalance in the ribonucleotide pools compared with normal blood neutrophils. This imbalance includes decreased ratios of purine:pyrimidine, adenine:guanine, and uracil:cytosine nucleotides as well as an increased relative concentration and a changed composition of the uridine diphosphate (UDP) sugars, with relatively more UDP-N-acetylhexosamines. Similar, more prominent deviations were found in HL-60 promyelocytic leukemia cell line cells. We have used HL-60 cells to investigate the relationships between these changes in the ribonucleotide pools and myelocyte proliferation, maturation, and/or transformation to the malignant state. When HL-60 cells were separated by elutriation centrifugation into fractions enriched in G1, S phase, or G2 + M, we found only differences in the amount of nucleotides per cell (G2 + M greater than S phase greater than G1) corresponding with the increase in cell volume but not in the qualitative composition of the nucleotides. Therefore, throughout this study, the nucleotide content of all cells was calculated per unit of cell volume. When HL-60 cells were induced to myeloid differentiation with dimethyl sulfoxide, proliferation stopped after 3 days. After 6 days, 70-90% of the cells had matured into cells capable of nitro blue tetrazolium reduction upon stimulation with phorbol myristate acetate. During the maturation process, the mean volume of the HL-60 cells decreased, and the nucleotide content and the purine:pyrimidine and adenine:guanine nucleotide ratios increased. The composition of the UDP sugars changed dramatically, with a decrease of UDP-N-acetylhexosamines and an increase of UDP-hexoses. Similar changes were detected in HL-60 cells that stopped proliferating without dimethyl sulfoxide-induced maturation, except that the UDP sugar composition showed an increase of UDP-N-acetylhexosamines and a decrease of UDP-hexoses. Careful examination of these results indicates that the decreased ratio of purine:pyrimidine nucleotides and the decreased ratio of uracil:cytosine nucleotides observed in CML myeloblasts may be regarded as specific changes caused by transformation of myelocytes to the malignant state. The increased amount of UDP-N-acetylhexosamines and total UDP sugars in the CML cells may also be connected with the transformation process. All other deviations in the nucleotide pattern of transformed myelocytes in comparison to that of mature, normal neutrophils can be explained by the state of proliferation and/or immaturity of CML myeloblasts and HL-60 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of a cDNA encoding an isoform of human CTP synthetase.

A full-length cDNA clone encoding an isoform of human CTP synthetase (type II) was isolated. A 1761-nucleotide open reading frame which corresponds to a protein of 586 amino acids with a predicted molecular mass of 65678 Da was identified. The predicted protein sequence showed 74% identity with the translation product of a previously identified human CTP synthetase cDNA clone (type I). The function of the human cDNA encoding type II CTP synthetase was verified by successful complementation of the cytidine-requiring CTP synthetase deficient mutant JF618 of Escherichia coli. The gene encoding type II CTP synthetase has been localized on chromosome Xp22.

cDNA cloning, genomic structure and chromosomal localization of the human BUP-1 gene encoding beta-ureidopropionase.

A full-length cDNA clone encoding human beta-ureidopropionase was isolated. A 1152-nucleotide open reading frame which corresponds to a protein of 384 amino acids with a calculated molecular weight of 43¿ omitted¿158 Da, surrounded by a 5'-untranslated region of 61 nucleotides and a 3'-untranslated region of 277 nucleotides was identified. The protein showed 91% similarity with the translation product of the rat beta-ureidopropionase cDNA. Expression of the human cDNA in an Escherichia coli and eukaryotic COS-7 expression system revealed a very high beta-ureidopropionase enzymatic activity, thus confirming the identity of the cDNA. Since human EST libraries from brain, liver, kidney and heart contained partial beta-ureidopropionase cDNAs, the enzyme seems to be expressed in these tissues, in agreement with the expression profile of this enzyme in rat. Using the human cDNA as a probe a genomic P1 clone could be isolated containing the complete human beta-ureidopropionase gene. The gene consist of 11 exons spanning approximately 20 kB of genomic DNA. Fluorescence in situ hydridization localized the human beta-ureidopropionase gene to 22q11.2.

Studies on the incorporation of precursors into purine and pyrimidine nucleotides via 'de novo' and 'salvage' pathways in normal lymphocytes and lymphoblastic cell-line cells.

The incorporation of radioactive precursors into purine and pyrimidine nucleotides via 'de novo' and 'salvage' pathways was measured in normal lymphocytes, resting as well as proliferating, and lymphoblastic cell-line cells (MOLT-3). Lymphocytes stimulated with anti-CD3 were taken as actively proliferating lymphocytes (35% in the S-phase, 40 h after stimulation). The incorporation of the precursors in the purine and pyrimidine ribonucleotides was measured by a combination of anion-exchange high-performance liquid chromatography (HPLC) and on-line radioactivity measurement. The actively proliferating normal lymphocytes and MOLT-3 cells incorporated 30-500 times more of the various precursors in the ribonucleotides compared to normal resting lymphocytes. The imbalance in the nucleotide pool found in proliferating normal and lymphoblastic cells was reflected in the incorporation pattern of the various precursors. The activities of the branch-point enzymes IMP dehydrogenase and CTP synthetase most likely determine the differences in the composition of the nucleotide pools between resting and proliferating cells.

Eniluracil treatment completely inactivates dihydropyrimidine dehydrogenase in colorectal tumors.

PURPOSE: To determine the effect of eniluracil on colorectal tumor dihydropyrimidine dehydrogenase (DPD) activity. PATIENTS AND METHODS: Patients who were to undergo primary colorectal tumor resection received oral eniluracil 10 mg/m(2) twice daily for 3 days before surgery. Mononuclear cells were obtained before the start of eniluracil and on the morning of surgery, to measure DPD activity, protein, and mRNA. Plasma uracil was also measured at these two time points to assess the effect of eniluracil on pyrimidine accumulation. DPD activity, protein, and mRNA were also assessed in colorectal tumors and adjacent normal mucosa of patients who received eniluracil and untreated control patients. RESULTS: DPD activity in tumors from 10 untreated patients ranged from 30 to 92 pmol/min/mg of protein. In contrast, there was no detectable tumor DPD activity in 10 patients who received eniluracil. A similar pattern was observed in mononuclear cells, where median pretherapy activity was 366.5 pmol/min/mg of protein (range, 265 to 494 pmol/min/mg of protein) and was undetectable immediately before surgery. Plasma uracil changed from a median less than 0.2 micromol/L before therapy to 27.76 micromol/L before surgery. No difference in DPD protein or mRNA was observed between pretherapy and presurgery mononuclear cell samples or between treated and untreated tumor samples. CONCLUSION: This study provides definitive evidence that eniluracil completely inactivates DPD activity in human solid tumors. The increased plasma uracil and decreased DPD activity are consistent with systemic inactivation of the enzyme. The mechanism of inactivation is at the catalytic level, because no changes in DPD protein or mRNA were observed. Treatment with eniluracil will eliminate DPD activity as a source of pharmacokinetic fluorouracil variability or resistance in human colorectal cancer.

The roles of uridine-cytidine kinase and CTP synthetase in the synthesis of CTP in malignant human T-lymphocytic cells.

The pattern of incorporation of [14C]uridine showed that in MOLT-3 cells an increased proportion of CTP was synthesized via CTP synthetase, compared to proliferating normal human T lymphocytes at a physiological concentration of cytidine (< 0.5 microM). Furthermore, in the proliferating normal human T lymphocytes similar patterns of incorporation of [14C]uridine were observed in the presence of the physiological concentration of cytidine and after addition of 2 microM of cytidine. In contrast, in the MOLT-3 cells after addition of 2 microM of cytidine the proportion of CTP synthesized by conversion of UTP into CTP was substantially decreased, whereas the salvage of cytidine was proportionally increased. We conclude that the reutilization of uridine is a preferred route in the synthesis of CTP for MOLT-3 cells at physiological concentrations of uridine and cytidine, whereas in proliferating normal human T lymphocytes CTP is largely synthesized through reutilization of cytidine. This difference in salvage of pyrimidine ribonucleosides may be exploited for selective chemotherapy.

Prevalence of a common point mutation in the dihydropyrimidine dehydrogenase (DPD) gene within the 5'-splice donor site of intron 14 in patients with severe 5-fluorouracil (5-FU)- related toxicity compared with controls.

Deficiency of dihydropyrimidine dehydrogenase (DPD), the rate-limiting enzyme in 5-fluorouracil (5-FU) catabolism, has been linked to toxic side effects of 5-FU. The most prominent mutation of the DPD gene resulting in severe DPD deficiency is a G to A mutation in the GT 5'-splice recognition site of intron 14 (exon 14-skipping mutation). The corresponding mRNA lacks exon 14, and the enzymatic activity of the translated DPD protein is virtually absent. We developed a reverse transcription-PCR-based assay suitable for routine identification of the exon 14-skipping mutation and screened a control cohort of 851 Caucasian individuals as well as a cohort of 25 cancer patients reported by their physicians to have suffered from WHO grades 3-4 toxicity upon 5-FU chemotherapy. Within the control cohort, in total, eight heterozygotes were detected (0.94%): one heterozygote in 51 healthy donors, (1.96%); five heterozygotes in 572 hospital patients (0.87%); and two heterozygotes in 228 colorectal tumor patients (0.88%). Among the 25 patients with severe 5-FU-related toxicity, 5 (20%) were heterozygous and 1 (4%) was homozygous for the exon 14-skipping mutation. All six patients had experienced WHO grade 4 myelosuppression. Lethal outcome was seen in the homozygous and two of the heterozygous cases. We conclude that carriers of the DPD exon 14-skipping mutation are at significantly increased risk to experience life-threatening myelosuppression upon 5-FU treatment, even when the allelic status is heterozygous. These data lead us to suggest routine testing for the exon 14-skipping mutation before 5-FU treatment.

Lethal outcome of a patient with a complete dihydropyrimidine dehydrogenase (DPD) deficiency after administration of 5-fluorouracil: frequency of the common IVS14+1G>A mutation causing DPD deficiency.

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 from developing a severe 5FU-associated toxicity. In this study, we demonstrated that a lethal toxicity after a treatment with 5FU was attributable to a complete deficiency of DPD. Analysis of the DPD gene for the presence of mutations showed that the patient was homozygous for a G-->A mutation in the invariant GT splice donor site flanking exon 14 (IVS14+1G>A). As a consequence, no significant residual activity of DPD was detected in peripheral blood mononuclear cells. To determine the frequency of the IVS14+1G>A mutation in the Dutch population, we developed a novel PCR-based method allowing the rapid analysis of the IVS14+1G>A mutation by RFLP. Screening for the presence of this mutation in 1357 Caucasians showed an allele frequency of 0.91%. In our view, the apparently high prevalence of the IVS14+1G>A mutation in the normal population, with 1.8% heterozygotes, warrants genetic screening for the presence of this mutation in cancer patients before the administration of 5FU.

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.

Pharmacokinetics of bolus 5-fluorouracil: relationship between dose, plasma concentrations, area-under-the-curve and toxicity.

The pharmacokinetics of 5-fluorouracil (5FU) have been related to toxicity and antitumor activity, in particular for continuous infusion schedules, but to a lesser extent for frequently used bolus injections. The use of intensive sampling schedules limits the application of pharmacokinetics to optimize individual dosing or to define the ideal combination with other drugs. We therefore reanalyzed a pharmacokinetic study in order to develop a limited sampling schedule. Patients received escalating doses of 5FU at 500, 600 and 720 mg/m2 as a bolus until toxicity developed. Blood samples were analyzed until 24 h after administration. The area under the concentration time curve from 0-90 min (AUC(0-90)) was strongly correlated with dose and also with toxicity (p = 0.0009). The 5FU concentrations at 30 and 60 min were correlated to the AUC(30-240) and to that of the AUC(0-90) (r2 = 0.970). The use of limited sampling (30, 60, 90 min) in a patient given 353 mg/m2 5FU with severe toxicity at initial dosing at 500 mg/m2 revealed that the AUC(0-90) at 353 mg/m2 was higher than the normal AUC(0-90) for 500 mg/m2. This patient appeared to have an 8-fold lower activity of the 5FU degradation enzyme dihydropyrimidine dehydrogenase. Limited sampling will allow us to define potential aberrant kinetics of pharmacokinetic interaction of 5FU with other drugs being developed for treatment of colorectal cancer.

Nomenclature for human DPYD alleles.

To standardize DPYD allele nomenclature and to conform with international human gene nomenclature guidelines, an alternative to the current arbitrary system is described. Based on recommendations for human genome nomenclature, we propose that each distinct allele be designed by DPYD followed by an asterisk and an Arabic numeral. The number specifies the key mutation and, where appropriate, a letter following the number indicates an additional mutation on the mutant allele. Criteria for classification as a distinct allele are also presented.

The effect of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles.

In this paper we report the effects of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles. MIBG is a structural analogue of noradrenaline and is capable of inhibiting complex I and complex III of the respiratory chain. The results of our studies show that MIBG enhanced both NADH-driven superoxide formation and NADH-driven lipid peroxidation at concentrations that are likely to exist inside mitochondria of the target cells of neuroblastoma patients treated with [131I]MIBG. The effect of MIBG is comparable to that of rotenone (an inhibitor of complex I) rather than that of antimycin (an inhibitor of complex III). These results suggest that the formation of superoxide and lipid peroxidation contributes to the cytotoxicity of [131I]MIBG.

Cyclopentenyl cytosine inhibits cytidine triphosphate synthetase in paediatric acute non-lymphocytic leukaemia: a promising target for chemotherapy.

Cytidine triphosphate (CTP) synthetase is a key enzyme in the anabolic pathways of cytosine and uracil ribonucleotide metabolism. The enzyme catalyses the conversion of uridine triphosphate (UTP) into CTP, and has a high activity in various malignancies, which has led to the development of inhibitors of CTP synthetase for therapeutic purposes. We studied both CTP synthetase activity and ribonucleotide concentrations in leukaemic cells of 12 children suffering from acute non-lymphocytic leukaemia (ANLL), and performed incubation experiments with cyclopentenyl cytosine (CPEC), a nucleoside analogue that is capable of inhibiting CTP synthetase. The CTP synthetase activity in ANLL cells (5.1+/-2.3 nmol CTP/mg/h) was significantly higher compared with granulocytes of healthy controls (0.6+/-0.4 nmol CTP/mg/h, P=0.0002), but was not different from the CTP synthetase activity in non-malignant CD34+ bone marrow cells (5. 6+/-2.4 nmol CTP/mg/h). Major shifts were observed in the various ribonucleotide concentrations in ANLL cells compared with granulocytes: the absolute amount of ribonucleotides was increased with a substantial rise of the CTP (2.4 versus 0.4 pmol/microg protein, P=0.0007) and UTP (8.7 versus 1.6 pmol/microg protein, P=0. 0007) concentrations in ANLL cells compared with granulocytes. Treatment of ANLL cells in vitro with CPEC induced a major depletion (77% with 2.5 microM of CPEC) in the concentration of CTP, whilst the concentrations of the other ribonucleotides remained unchanged. Therefore, the high activity of CTP synthetase in acute non-lymphocytic leukaemic cells can be inhibited by CPEC, which provides a key to a new approach for the treatment of ANLL.

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.