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Emma Childrens Hospital and Department of Clinical Chemistry, Academic Medical Center, University of Amsterdam, 1105 AZ Amsterdam [A. B. P. v. K., J. H., R. M., L. Z., H. R. W., F. B., D. J. R., A. H. v. G.], and Slingeland ziekenhuis, 7009 BL Doetinchem [E. W. M.], the Netherlands
| ABSTRACT |
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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. | INTRODUCTION |
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An important determinant in predicting the toxicity as well as the efficacy of 5FU might be the activity of DPD. DPD is the initial and rate-limiting enzyme in the catabolism of the pyrimidine bases uracil and thymine, but also of the pyrimidine analogue 5FU. It has been reported that >80% of the administered 5FU is catabolized by DPD (5) . Furthermore, a correlation has been observed between the pretreatment activity of DPD in PBM cells and the systemic clearance of 5FU in cancer patients (6) . The pivotal role of DPD in chemotherapy using 5FU has been shown in cancer patients with a complete or near-complete deficiency of this enzyme. These patients suffered from severe toxicity, including death, after the administration of 5FU (7, 8, 9, 10, 11) . It was shown that a number of these patients were genotypically heterozygous for a mutant DPD allele (9, 10, 11, 12) . To date, 17 variant DPYD alleles have been identified in pediatric patients suffering from a complete or near-complete DPD deficiency, or in tumor patients with decreased DPD activity (12, 13, 14, 15) . Analysis of the prevalence of the various mutations among DPD patients has shown that the splice-site mutation, IVS14+1G>A, was by far the most common one (52%; Ref. 13 ).
To date, the frequency of the splice site mutation IVS14+1G>A in the normal population is not known. On the basis of population analysis of the DPD activity, the frequency of heterozygotes has been estimated to be as high as 3% (6) . Such individuals might be at risk of developing severe toxicity after the administration of 5FU. Furthermore, only a small number of cases have been reported regarding lethal toxicity after the administration of 5FU. In none of these cases have the molecular mechanisms underlying the 5FU-induced death been resolved. In this paper, we describe a simple genotyping procedure to test for the presence of the IVS14+1G>A mutation. Furthermore, we describe the first patient with lethal toxicity, after the administration of 5FU, who proved to be homozygous for the IVS14+1G>A mutation.
| MATERIALS AND METHODS |
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Analysis of Pyrimidine Bases.
The concentrations of the pyrimidine bases, uracil and thymine in plasma, were determined using reversed-phase HPLC combined with diode-array detection, as described before (16)
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Culture Conditions of Human Fibroblasts.
Fibroblasts were cultured from skin biopsies obtained from controls and the index patient. Biopsies were incubated at 37°C in HAM-F10 medium, supplemented with 20 mM HEPES and 15% (v/v) FCS in 25-cm2 cell-culture flasks until an adequate number of proliferating cells was obtained. Subsequently, cells were cultured in HAM-F10 medium supplemented with 20 mM HEPES and 10% (v/v) FCS. Fibroblasts were harvested with 0.25% (w/v) trypsin, and after washing the cells once with PBS and twice with 0.9% (w/v) NaCl, the cells were collected by centrifugation (175 x g at 7°C for 5 min), and the supernatant was discarded. The pellets were stored at -80°C.
Isolation of Human PBM Cells and Granulocytes.
PBM cells were isolated from 15 ml EDTA-anticoagulated blood by centrifugation over Lymphoprep, and the cells from the interface were collected and treated with ice-cold NH4Cl to lyse the contaminating erythrocytes, as described before (17)
. The pellet of the centrifugation step over Lymphoprep containing the granulocytes and erythrocytes was diluted with 7 ml of supplemented PBS [9.2 mM Na2HPO4, 1.3 mM NaH2PO4, 140 mM NaCl, 0.2% (w/v) BSA, 13 mM sodium citrate, and 5 mM glucose (pH 7.4)] and centrifuged at 800 x g at room temperature for 10 min. To lyse the erythrocytes, the pellet was resuspended in 7 ml of ice-cold ammonium chloride solution (155 mM NH4Cl, 10 mM KHCO3, and 0.1 mM EDTA) and kept on ice for 5 min. After the addition of 10 ml of ice-cold supplemented PBS, the solution was centrifuged at 250 x g at 4°C for 10 min. The pellet was collected and subjected to another lysis step as described above. The pellet containing the granulocytes was washed once more with supplemented PBS, and the final cell pellet was frozen in liquid nitrogen and stored at -80°C until further analysis.
Determination of the DPD Activity.
The activity of DPD was determined in a reaction mixture containing 35 mM potassium phosphate (pH 7.4), 2.5 mM MgCl2, 1 mM DTT, 250 µM NADPH, and 25 µM [4-14C]-thymine (17)
. Separation of radiolabeled thymine from radiolabeled dihydrothymine was performed isocratically [50 mM NaH2PO4 (pH 4.5) at a flow rate of 2 ml/min] by high-performance liquid chromatography on a reversed-phase column (Alltima C18; 250 x 4.6 mm; 5-µm particle size; Alltech Associates Inc., Deerfield, IL) and protected by a guard column (Supelguard LC-18-S; 5-µm particle size; 20 x 4.6 mm; Supelco, Bellefonte, PA) with online detection of the radioactivity, as described before (17)
. Protein concentrations were determined with a copper-reduction method using bicinchoninic acid, essentially as described by Smith et al. (18)
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PCR Amplification of Exon 14.
DNA was isolated from purified granulocytes by standard procedures. PCR amplification of exon 14 and its flanking intronic regions was carried out using the primer sets DPD14f and DPD14r as specified in Table 1
. Amplification of exon 14 was carried out in a 50-µl reaction mixture containing 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 2 mM MgCl2, 10 pmol each primer, 200 µM each deoxynucleotide triphosphate, and 2 units of Taq polymerase. After initial denaturation for 5 min at 95°C, amplification was carried out for 35 cycles (1 min 95°C, 1 min 55°C, 1 min 72°C). The PCR product was separated on 1% agarose gels, visualized with ethidium bromide, and purified using a Qiaquick Gel Extraction kit or used for direct sequencing.
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Sequence Analysis.
Sequence analysis of genomic fragments amplified by PCR was carried out on an Applied Biosystems model 377 automated DNA sequencer using the dye-terminator method.
| RESULTS |
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Uracil and Thymine Levels in Plasma.
To investigate whether the lethal toxicity after the administration of 5FU might have been caused by a partial or complete deficiency of DPD, plasma was collected for determination of the levels of uracil and thymine. Strongly elevated concentrations of uracil (80 µM; controls, <0.2 µM; n = 20) and thymine (17.5 µM; controls, <0.2 µM; n = 20) were detected in plasma which is indicative of a complete deficiency of DPD.
DPD Activity in PBM Cells and Fibroblasts.
DPD activity was determined in PBM cells isolated from a blood sample obtained during pancytopenia. Morphological examination of the isolated PBM cells on a cytospin preparation showed the presence of only lymphocytes and some residual thrombocytes. Hardly any activity of DPD (0.09 nmol/mg/h) was detected in the PBM cells of the patient when compared with that observed in controls (10.0 ± 3.4 nmol/mg/h; n = 22). Surprisingly, a low but significant activity of DPD (0.56 nmol/mg/h) was detected in thrombocytes when compared with controls (1.7 ± 0.6 nmol/mg/h; n = 22). Because the patient received thrombocyte transfusions, we feel that the very low DPD activity detected in PBM cells of the patient is most likely attributable to contamination of the PBM cell fraction by thrombocytes of the donor. No activity of DPD (<0.009 nmol/mg/h; controls, 0.89 ± 0.56 nmol/mg/h; n = 21) could be detected in cultured fibroblasts established from a skin biopsy of the patient.
Sequence Analysis of the DPD Gene.
To investigate whether or not the common IVS14+1G>A mutation in the DPYD gene might underlie the complete DPD deficiency in the tumor patient, two intron-specific primers were used for amplification of exon 14 and its flanking intron sequences (19)
. Sequence analysis showed that the tumor patient was homozygous for the IVS14+1G>A mutation, which changes the invariant splice donor site from GT to AT.
RFLP Analysis of the IVS14+1G>A Mutation.
A PCR primer with a single-base mismatch was used, which introduced a NdeI restriction site in case a G
A point mutation is present in the invariant splice donor site of exon 14, to rapidly screen for the IVS14+1G>A mutation by RFLP (Fig. 1
). As a positive control for the digestion efficiency of NdeI, an additional NdeI restriction site was introduced in the 5' region of the PCR fragment using a forward PCR primer containing an A
T mismatch (Table 1)
. Under these conditions, the undigested PCR fragment has a length of 198 bp. After digestion with NdeI, the wild-type allele will produce two fragments of 181 bp and 17 bp. In contrast, the PCR fragment containing the G
A point mutation in the invariant splice donor site of exon 14 will produce three fragments of 17 bp, 154 bp, and 27 bp after digestion with NdeI. A diagrammatic representation of the PCR amplification of exon 14 and the subsequent analysis of the IVS14+1G>A mutation by RFLP is shown in Fig. 1
. The validity of the concept is demonstrated by analysis of the DPYD gene of a patient homozygous for the IVS14+1G>A mutation, an individual who is obligate heterozygous for the IVS14+1G>A mutation and the wild-type DPYD gene from a control (Fig. 2)
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| DISCUSSION |
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In the present study we have provided unambiguous evidence at the molecular level for homozygosity of a mutated allele of the gene encoding DPD in a tumor patient experiencing lethal toxicity after the administration of 5FU. Analysis of the DPYD gene for the presence of mutations showed that the patient was homozygous for the G
A mutation in the invariant GT splice donor site [IVS14+1G>A]. This mutation leads to the skipping of exon 14 immediately upstream of the mutated splice donor site in the process of DPD pre-mRNA splicing. As a result, the mature DPD mRNA lacks a 165-nt segment encoding the amino acids 581635 (19)
. Apparently, the mutant DPD protein lacking the amino acids 581635 bears no residual activity, because no significant activity of DPD could be measured in PBM cells and fibroblasts of the patient. Persistently increased levels of thymine and uracil in plasma are observed only in cases where the DPD enzyme has been inactivated for >97% of its normal activity (23)
. Thus, the strongly increased levels of thymine and uracil in the plasma of the patient are in line with the presence of a complete deficiency of DPD.
Recently, we showed that the G
A point mutation in the invariant splice donor site is by far the most common one (52%) among patients with a complete deficiency of DPD (13)
. Furthermore, there appears to be some kind of homogeneity for the IVS14+1G>A mutation in Northern Europe (13)
. To date, the frequency of this mutation in the population has not been thoroughly investigated. Screening for the presence of the G
A splice site mutation in only a very limited number of individuals has revealed heterozygosity for this mutation in 1% of the Finnish population (180 alleles analyzed) and none in the British (60 alleles), Japanese (100 alleles), African-American (210 alleles), or Dutch (100 alleles) populations (19
, 24)
. These analyses of the presence of the G
A splice site mutation were performed using RFLP, based on the fact that the G
A point mutation destroys a unique MaeII restriction site, present in the amplified genomic fragment encompassing the skipped exon and its flanking sequences (9
, 19) . Unfortunately, the restriction enzyme MaeII is rather expensive and thus not suitable for use in screening large numbers of individuals on a routine basis. Recently, a RFLP procedure has been developed for the detection of the IVS14+1G>A mutation using a SnaBI restriction site, which was introduced using PCR-mediated site-directed mutagenesis (25)
. A serious drawback of this method, however, is the fact that the SnaBI site will not be present in the homozygous-deficient type, whereas no positive control for the cutting efficiency of the restriction enzyme was present. For these reasons, we have developed a novel genotyping test for the IVS14+1G>A mutation. In our test, a NdeI restriction site would be introduced, with PCR-mediated site-directed mutagenesis, in the invariant GT splice donor site in the intron downstream of exon 14, in the event that a G
A point mutation is present. As a positive control for the restriction efficiency of NdeI, a second NdeI restriction site was introduced in the 5' region of the amplified genomic fragment.
Using this genotyping test for the G
A mutation, we demonstrated that there is a relatively high frequency of the mutated allele in the normal Dutch population, with an allele frequency of 0.91%. Using the Hardy-Weinberg equilibrium and a frequency of heterozygotes of 1.8%, one can estimate the number of individuals homozygous for the G
A mutation to be 1.2 in 10,000. Compared with other frequently occurring inborn errors of metabolism, such as phenylketonuria (1:20,000), the expected prevalence of a DPD deficiency is high. Considering the common use of 5FU in the treatment of cancer patients and the increasing percentage of patients receiving high doses of 5FU in adjuvant therapy, the apparently high prevalence of the IVS14+1G>A mutation in the normal population warrants genetic screening for the presence of this mutation in cancer patients before the administration of 5FU. In this way, the serious and sometimes lethal 5FU-related toxicities encountered in patients heterozygous or homozygous for the IVS14+1G>A mutation might be prevented.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported by the "Stichting Kinder Oncologisch Centrum Amsterdam." ![]()
2 To whom requests for reprints should be addressed, at Academic Medical Center, Laboratory Genetic Metabolic Diseases, F0-224, Meibergdreef 9, 1105 AZ Amsterdam, the Netherlands. Phone: 31-205665958; Fax: 31-206962596; E-mail: a.b.vankuilenburg{at}amc.uva.nl ![]()
3 The abbreviations used are: 5FU, 5-fluorouracil; DPD, dihydropyrimidine dehydrogenase; PBM, peripheral blood mononuclear. ![]()
Received 8/16/00; revised 1/23/01; accepted 1/25/01.
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