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March 2001, Volume 15, Number 3, Pages 398-405
Table of contents    Previous  Article  Next   [PDF]
Original Manuscript
MDR1 expression in poor-risk acute myeloid leukemia with partial or complete monosomy 7
M M van den Heuvel-Eibrink1,2, E A C Wiemer1, M J de Boevere1, R M Slater3,4, E M E Smit3, M M van Noesel2, B van der Holt5, M Schoester1, R Pieters2 and P Sonneveld1

1Department of Hematology, University Hospital and Erasmus University, Rotterdam, The Netherlands

2Department of Pediatric Oncology/Hematology, University Hospital and Erasmus University, Rotterdam, The Netherlands

3Department of Clinical Genetics, University Hospital and Erasmus University, Rotterdam, The Netherlands

4Department of Cell Biology and Genetics, University Hospital and Erasmus University, Rotterdam, The Netherlands

5Department of Statistics, University Hospital and Erasmus University, Rotterdam, The Netherlands

Correspondence to: M M van den Heuvel-Eibrink, Dept of Oncology/Hematology, Rm Sp 2429, Sophia Children's Hospital, PO Box 2060, 3000 CB Rotterdam, The Netherlands; Fax: +31-10-463 6801

Abstract

Expression of the multidrug resistance (MDR1) phenotype, encoded by the MDR1 gene, is an adverse prognostic factor for CR and survival in acute myeloid leukemia (AML). Other prognostic factors, such as specific cytogenetic abnormalities, have been identified in AML. We have investigated the expression of the MDR1 gene in untreated AML patients with monosomy 7 (n = 12), and partial deletions (n = 7) of the long arm of chromosome 7 (respectively -7/7q-), because of the extremely bad prognosis associated with these cytogenetic abnormalities and because of the fact that the MDR1 gene is located on chromosome 7q21.1.The findings were compared with the level of MDR1 expression in a group of 42 other AML patients, matched for age with favourable, neutral or complex cytogenetic abberations. MDR1 mRNA expression, as measured by the RNase protection assay was significantly higher in the -7/7q- group vs other AML patients (median 1.3 vs 0.1 arbitrary units, P = 0.02). Protein expression of MDR1 in the -7/7q- group, as determined with the monoclonal antibody MRK16, was found to be similar to the levels found in the control group. With a functional rhodamine retention assay using the modulator PSC833, increased MDR1 activity was observed in the -7/7q- group as compared to the control group of patients (P = 0.05). Considering the higher MDR1 mRNA expression and equal or slightly elevated level of protein expression of MDR1, we studied the presence of MDR1 genes in this group of -7/7q- patients. Fluorescence in situ hybridization (FISH) studies, using a specific MDR1 probe revealed no loss of an MDR1 allele in any of the deleted q- arms of the seven patients with 7q-, whereas all monosomy 7 patients lacked one MDR1 gene homologue. To determine whether there was selective loss of the MDR1 gene in the -7/7q- patients, the genetic polymorphism of the MDR1 gene was used. Both allelic variants (G and T) were represented in the -7/7q- and in the control group, showing a predominance for GT at position 2677 of the MDR1 gene in the control group. In the 12 monosomy 7 patients loss of the MDR1 allele was random. Methylation studies of the CpG island of the MDR1 gene revealed no hypermethylation in any of the -7/7q- patients. We conclude that MDR1 expression in -7/7q- AML patients is upregulated at transcriptional, but not at translational level, suggesting that mechanisms other than MDR1 are responsible for the poor prognosis in these patients. Leukemia (2001) 15, 398-405.

Keywords

acute myeloid leukemia (AML); multidrug resistance (MDR); P-glycoprotein (P-gp); MDR1; chromosome 7 deletions

Introduction

Intrinsic or acquired drug resistance is a major cause of treatment failure in acute myeloid leukemia (AML). Resistance to anthracyclines, such as daunorubicin, doxorubicin, vinca alkaloids and epipodophyllotoxines, is associated with the classical multidrug resistance phenotype (MDR1). In cell lines that are cross-resistant to these drugs, the ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp) is expressed.1,2,3,4 This acts as a transmembrane drug efflux pump and is encoded by the MDR1 gene, located on chromosome 7 at band q21.1.5,6,7 In leukemic blast cells, P-gp expression is associated with a lower intracellular retention of cytostatic drugs and a relative resistance to these agents.8,9,10,11 In addition, other proteins are associated with multidrug resistance, including the lung resistance protein (LRP) located on chromosome 16p13.2 and the 190 kDa multidrug resistance-associated protein (MRP1), located on chromosome 16p13.1.12,13,14

In AML, MDR1 expression was identified as an independent adverse prognostic factor with respect to complete response to induction treatment and survival.15,16,17,18,19,20,21,22 Also, other prognostic factors such as specific cytogenetic abnormalities, age, CD34 expression and white blood count at diagnosis have been identified.15,16,17,18,19,20

AML patients with -7/7q- have an extremely poor outcome, which is independent of the above mentioned clinical and immunological prognostic factors.15,16,17,18,19,20,21,22 It is not known why and how the (partial) loss of a chromosome 7 affects the sensitivity of these AML cells to chemotherapy.23,24,25 The breakpoint of partial chromosome 7 deletions is close to the 7q21.1 site of the MDR1 gene in many patients. Therefore, we have attempted to investigate whether the poor response to therapy of AML patients with -7/7q- is associated with an altered regulation of the MDR1 gene, and whether selective loss of one MDR1 allele is involved in this process.

Patients and methods

Patients

Routine cytogenetic studies on bone marrow samples of newly diagnosed AML patients revealed 19 patients to have a deletion of the long arm of chromosome 7 or monosomy 7. Cytogenetic analysis had been carried out by standard techniques, and the findings described according to the international nomenclature.26 Forty-two other karyotyped AML patients, matched for age, FAB, and WBC, were taken as controls. Written informed consent was obtained to perform these studies, according to the Helsinki agreement. Morphologic classification was performed according to the French-American-British (FAB) criteria.27 In the -7/7q- group the FAB classifications were M0 (n = 1), M1 (n = 4), M2 (n = 5), M4 (n = 3), M5 (n = 4), M7 (n = 2). The FAB classifications of the control group were M1 (n = 9), M2 (n = 15), M3 (n = 2), M4 (n = 9), M5 (n = 7), and M7 (n = 1). The -7/7q- group included 12 patients with a monosomy 7, and 7 with a 7q- karyotype. The cytogenetic abnormalities in the control group were favorable in six patients (t(8;21), inv(16), t(15;17)), unfavorable in four patients (t(9;22), 5q-, and 11q23 with mixed lineage leukemia (MLL) rearrangements), and normal or other karyotypes in 33 patients. The median age of the -7/7q- group of patients was 54 years; the median age of the control group was 55 years.

All patients were treated according to the protocols of the Dutch-Belgian Hemato-Oncology Cooperative Group (HOVON 4/4a, respectively HOVON 29). Induction therapy consisted of daunorubicin (45 mg/m2 for 3 days), cytosine arabinoside (200 mg/m2 for 7 days, and 2 g/m2 for 6 days), and amsacrine (120 mg/m2 for 3 days) (HOVON 4/4a). In HOVON 29, induction treatment consisted of cytosine arabinoside (200 mg/m2 for 7 days), idarubicine (12 mg/m2 for 3 days), followed by amsacrine (120 mg/m2 for 3 days and cytosine arabinoside (2 g/m2) for 6 days. Sixteen of 19 patients with the -7/7q- karyotype received standard induction chemotherapy as compared to 36 out of the 42 in the control group. Three (19%) of sixteen -7/7q- AML patients, as compared to 22/36 (61%) of the controls, achieved complete remission (P = 0.007). A group of a 104 normal healthy blood donors was, after informed consent, used as a pilot study to assess the genetic polymorphism of the MDR1 gene (peripheral blood mononuclear cells) in the normal population.

RNase protection assay

For RNA and protein studies in the AML patients, mononuclear cells from bone marrow or blood were freshly isolated and separated by Ficoll-Isopaque centrifugation (Nycomed, Oslo, Norway). All samples contained more than 85% of blasts. In normal subjects, blood nucleated cells were used as a control. Total RNA was isolated, using TRISOLV extraction (Biotecx, Houston, TX, USA) as originally described by Chomczynski et al.28 Quantitative detection of MDR1 and MRP1 gene transcripts was performed by the RNase protection assay. The assay was done with the RPA II kit (Ribonuclease Protection Assay Kit; Ambion, Austin, TX, USA), a modification of the method described by Zinn et al.29 Ten mug of total RNA were hybridized with 32P-CTP-RNA probes under standard conditions, followed by RNase-A/RNase T1 treatment. For RNase protection, an MDR1-specific mRNA antisense RNA probe was obtained by transcription of a 302 nucleotide cDNA fragment (nucleotide positions 3498-3801) with SP6 RNA polymerase (Ambion).30

The MRP1-specific probe is complementary to sequences at the 5' end of the MRP1 mRNA (nucleotides 240-484).7,13 A human glyceraldehyde-3-phosphate dehydrogenase (GAPDH) probe was included in all RNase protection assays as a control for RNA integrity and recovery. The mRNA levels of MDR1 and MRP1 were quantitated by scanning the films with the ultrascan XL-laser densitometer (LKB, Uppsala, Sweden) and data were analyzed with the gelscan XL software package.31,32 The colchicin-resistant KB 8-5 and KB 8 cell lines, and the drug-sensitive KB 3-1 parental cell line were used in each experiment as positive and negative controls.33 The signal obtained with a 10 mug total RNA sample of KB 8-5 cells was assigned an arbitrary expression level of 30 arbitrary units (AU), and the level in the KB8 cells was related to the level of the KB 8-5 cells (3 AU). The cell lines GLC4 and GLC4/ADR were used as negative and positive controls for MRP1 expression levels,12 in the different experiments. The MRP1 mRNA levels were expressed in units relative to the expression of MRP1 in GLC4/ADR which was arbitrarily set at 100 U. The calculated MRP1 expression of GLC4 was set to 4 U.34 The MDR1 and MRP1 mRNA levels were standardized according to the amount of GAPDH mRNA. All individual experiments included torulla yeast RNA as a control for specific hybridization of the probes to the mRNA samples.

Analysis of P-glycoprotein in bone marrow samples

Analysis of the expression of P-glycoprotein: For measurement of the expression of P-gp, cells were incubated (at room temperature) with monoclonal anti-P-gp antibody, MRK 16 moAb (Kamiya Biomedical Company, Tukwila, WA, USA) at a concentration of 12.5 mug/ml or an isotype-matched control antibody mIgG2a (Sigma, St Louis, MO, USA) at a concentration of 10 mug/ml. Cell-bound antibodies were detected by fluorescein isothiocyanate (FITC)-labeled rabbit anti-mouse immunoglobulin antibodies (DAKO, Glostrup, Denmark).

Results were given as the ratio of the mean of cell-associated fluorescence of cells incubated with the anti-P-gp antibody divided by the mean of cell-associated fluorescence of cells incubated with the control mIgG2a antibody.

As controls in each experiment, the drug-sensitive 8226 S and the drug-resistant 8226 D6 cells were included to measure expression of P-gp. The mean of the ratio of the MRK16 expression of the negative control cell line 8226 S was 1.32 ± 0.29 (mean ± s.d.; n = 59). The mean of the ratio of the MRK16 expression of the positive control cell line 8226 D6 was 30.23 ± 5.01. The ratio of the expression was measured in the total population of blasts and also in the CD34-positive cells, when a subpopulation of more then 10% CD34-positive cells was present.

Analysis of the function of P-glycoprotein: For measurement of the function of P-glycoprotein, the fluorescent molecule rhodamine was used as a P-gp substrate. Therefore, cells were incubated for 1 h at 37°C at 5% CO2 in the absence or presence of 2 muM PSC 833. After this incubation, 200 ng/ml rhodamine 123 (Sigma) was added to the cells. A sample was taken at t = 0 min to correct for background fluorescence and at t = 90 min to measure intracellular rhodamine accumulation.

Results are given as the ratio of the mean intracellular rhodamine fluorescence of cells exposed to PSC 833 divided by the mean intracellular rhodamine fluorescence of cells not exposed to PSC 833.

Fluorescence in situ hybridization (FISH)

Dual-coloured fluorescence in situ hybridization (FISH) was carried out, using standard techniques,35 on metaphases in cytogenetic preparations using a biotin-labeled alpha-satellite probe for chromosome 7 (p7t1) detected with avidin FITC (green), and a digoxine-labeled cosmid probe CHMR6, specific for the MDR1 gene at 7q21.1, detected with Texas red (red). The probes were labeled by standard nick translation using Biotin-16-dUTP according to the manufacturer's instructions (Gibco BRL, Gaithersburg, MD, USA). Between five and 32 metaphases per patient were examined.

Detection of MDR1 polymorphism and allelic expression

The presence and allelic expression of the genetic polymorphism of MDR1 at position 2677 was detected using oligonucleotide hybridization as described by Mickley et al.36,37 The PCR products were dot-blotted to a Zeta Probe blotting membrane (Bio-Rad, Hercules, CA, USA) which was prehybridized for 30 min at 50° C in 5 ´ SSPE, 0.5% SDS, 5 ´ Denhardt's, 50 mug/ml denatured herring sperm DNA. Hybridization, using radiolabeled oligonucleotides as allele-specific probes, was performed for 2 h at 50°C after which the blots were rinsed twice at room temperature in 2 ´ SSPE, 0.1% SDS and subsequently washed for 10 min at 55°C in 5 ´ SSPE, 0.1% SDS. Two 30-bp oligonucleotides, designated HMC3 and HMC4, were used. These oligonucleotides cover residues 2656 to 2685 of the MDR1 gene with HMC3 possessing a G at position 2677 and HMC4 a T at this position. Equal amounts of each control were spotted on both sides of the filter, thus providing the means for an indicator of specificity. Because the hybridizations were performed under identical conditions, with probes labeled to similar specific activities, the signals from the control oligonucleotides were usually similar. Phosphor imager techniques (Image quant) were used to confirm G and T positivity.

PCR

One mug of genomic DNA was use as a template in PCR for 40 cycles to detect genetic polymorphism at the DNA level. To detect allelic expression, 1 mug of total RNA was reverse transcribed and cDNA template was subjected to 40 cycles of PCR. The primers used for DNA and RNA oligonucleotide hybridization are described above.

Methylation-specific PCR (MSP)

The MSP assay is a two-step technique. In the first step DNA is pretreated with bisulfite. Bisulfite induces a chemical modification of the DNA sequence by altering cytosine to uracil (which subsequently are replaced by thymidine in the PCR reaction). In this reaction, all cytosines are converted to uracil, except methylated cytosines (5-methylcytosine), which are resistant to this modification. The second step is a PCR-based amplification of the altered DNA. PCR primers are designed to distinguish methylated from unmethylated DNA, taking advantage of the sequence differences after bisulfite modification.

Bisulfite modification: DNA (1 mug in a volume of 50 mul) is denatured by NaOH (final concentration 0.2 M) for 10 min at 37°C. Thirty microliters of 10 mM hydroquinone (Sigma) and 250 mul sodium bisulfite (Sigma) at pH 5, both freshly prepared, are added and mixed. The samples are incubated under a layer of mineral oil at 50°C for 16 h. Modified DNA is purified by using the Wizard DNA purification resin according to the manufacturer (Promega, Madison, WI, USA) and diluted in 50 mul of water. Modification is completed by NaOH (final concentration 0.3 M) treatment for 5 min at room temperature. Then follows an ethanol precipitation. DNA is resuspended in water and used immediately, or stored at -20°C.

PCR amplification: Primer pairs were designed in the 5'-UTR CpG island of the published MDR1 sequence (accession number AC002457, position 141267 sense and 141383 antisense). The primer sequences are: 5'-GGAGGGAGAATTGTATTGGTGGT-3' (unmethylated sense); 5'-GAGAATCGTATTGGCGGC-3' (methylated sense); 5'-CATTAATACCCCAACTACTCTAACCACA-3' (unmethylated anti- sense); 5'-CCCCAACTACTCTAACCGCG-3' (methylated antisense).

The PCR mixture contains 1 ´ PCR buffer (16.6 mM ammonium sulfate, 67 mM Tris, pH 8.8, 6.7 mM MgCl2, 10 mM 2-mercaptoethanol), dNTPs (each at 1.25 mM), primers (300 ng each per reaction), and bisufite-modified DNA (50 ng) in a final volume of 50 mul. PCR reactions are hot started at 95°C for 5 min before the addition of 1.25 units of Taq polymerase (Boehringer, Mannheim, Germany). Amplification is carried out for 35 cycles (30 s at 95°C, 30 s at the annealing temperature of 60°C, and 30 s at 72°C), followed by a final 4 min extension at 72°C. Each PCR is loaded on a 6-8% nondenaturing polyacrylamide gel, stained with ethidium bromide and directly visualized under UV illumination.38,39

Statistical analysis

The units of mRNA from MDR1 and MRP1, the MRK16 and IgG ratios, as well as Rho123 retention were compared between the two subgroups (-7/7q vs control) using the Wilcoxon rank-sum test. The Hardy-Weinberg formula was used to evaluate the genetic polymorphism of MDR1 in the normal population.40,41 All reported P values are two-sided and a significant level of alpha = 0.05 was used.

Results

Patient characteristics

Of the 19 untreated AML patients with an abnormality of chromosome 7, 12 had a monosomy 7 (Table 1). In six instances (patients 1-6) this was the sole cytogenetic abnormality. Three patients (patients 8-10) also had an inv(3)(21q26), an association that has been previously reported.42 Patient 7 had two karyotypically independent clones, one with monosomy 7 and one with a ring chromosome 7. In patient 12, the main clone had monosomy 7 and a partial deletion of the remaining chromosome 7 was found. For this study all these patients were grouped together as monosomy 7 patients. Seven patients (patients 13-19) had deletions of the long arm of chromosome 7 (Table 1, Figure 1). Forty-two other AML patients without chromosome 7 abnormalities were used as the control group.

mRNA expression

MDR1 mRNA expression was analyzed in 13/19 patients with a -7/7q- and in 35/42 control patients. In patients with the -7/7q- karyotype, the median MDR1 mRNA expression was 1.3 AU (range 0.05-107), as compared to 0.1 AU (range 0-12.7) in the other combined karyotypes (P = 0.02) (Figure 2). The median MRP1 mRNA expression was 5.7 AU (range 1.1-13.8) in 8/19 AML patients with a (partial) deleted chromosome 7 as compared to 3.0 AU (range 0-31.1) in the control group (P = 0.20) (Table 2).

Protein expression

P-gp expression was analyzed by flow cytometry in 18 of 19 patients with -7/7q-, and the results were compared with 20/42 (for MRK16 expression) and 19/42 (for PSC/Rho modulation) control patients. The selection of the investigated subjects in the control group with flow cytometry was based on availability of viable cell samples. The median PSC/Rho123 retention ratio's were 1.35 (range 1.01-2.34) in -7/7q- samples as compared to 1.18 (range 1.0-1.9) in the other AML patients (P = 0.05). The median value of the MRK16/IgG2a staining ratio of all -7/7q- samples was 1.76 (range 0.82-4.21) as compared to 1.46 (range 0.95-3.04) in the controls (P = 0.17) (Table 3).

Presence of the MDR1 gene

Dual-coloured FISH studies on metaphases from the monosomy 7 patients, showed that only one copy of the MDR1 gene on the remaining chromosome homologue was present. Patient 7 had two karyotypically independent clones, one with monosomy 7 and one copy of the MDR1 gene, and the other clone with two copies of the MDR1 gene due to a ring chromosome 7 and a normal chromosome 7 analogue. Two other monosomy 7 patients had complex variations (patients 11 and 12). For patient 11, additional FISH studies revealed that a marker chromosome was of chromosome 7 origin, which was negative for the MDR1 gene. In patient 12, the main clone had monosomy 7 and the remaining chromosome 7 had undergone structural changes but was shown by FISH to carry the MDR1 gene. A subclone of this patient had a ring 7 instead of monosomy 7. This ring also had retained the MDR1 gene. For the Discussion all of these cases were grouped together as having monosomy 7 with one copy of the MDR1 gene.

Seven patients (patients 13-19) had deletions of the long arm of chromosome 7 (Table 1). In all instances dual-coloured FISH showed both chromosome 7 homologues to be positive for the MDR1 gene (Figure 1) allelic expression. Hybridization studies with the MDR1-specific primers for position 2677 at 7q21.1 were performed in 12 patients with monosomy 7. In eight cases, one primer hybridized (7x with G, 1x with T), while four of these patients hybridized with two primers (GT). Patients with 7q- expressed a G gene variant in three cases, a T variant in two cases, and a GT MDR1 variant in two cases. Five of these 7q- patients were homozygous for MDR1 since they had been shown to have two copies of the MDR1 gene by FISH (Figure 3). In the control group, 15 patients were examined with oligonucleotide hybridization. Twelve expressed the heterozygous variant, whereas the G variant was found twice and the T variant once (Figure 4). In the peripheral white blood samples of 104 healthy volunteers the G variant was found 43 times, the GT variant 45 times and the T variant 16 times.

Methylation studies of the MDR1 gene

Methylation analysis was performed in all monosomy 7 patients, 5/7 patients with 7q-, and 15/42 of the AML patients of the control group. In all samples the CpG island of the promoter of MDR1 gene was found to be unmethylated (Figure 5).

Discussion

AML patients with -7/7q- have a poor response rate to induction chemotherapy and a short overall survival.15,16,17,18,19,20 Expression of MDR1 in de novo AML patients is also associated with a poor outcome.16,17,18,19,21,22,43 The MDR1 gene is localized on the long arm of chromosome 7 at band 7q21.1.5,6,7 The aim of our study was to investigate whether the extremely poor prognosis of -7/7q- AML patients could in part be due to a modified MDR1 gene expression.

Our data show that the MDR1 expression at the mRNA level in blasts of 19 -7/7q- AML patients, was 13-fold higher than in matched AML patients with other abnormal or normal cytogenetics. However, the increased mRNA level was not reflected in higher protein levels as measured with the monoclonal antibody MRK16, ie levels were similar in -7/7q- patients as compared to the control group. Only a small increase of MDR1 activity was observed in the -7/7q- group (P = 0.05). Dual-coloured FISH studies showed the presence of only one MDR1 gene in the monosomy 7 patients, whereas all 7q- patients revealed both MDR1 genes, even if the breakpoint was very close to band 7q21.1. Apparently, in the -7/7q- patients, P-glycoprotein levels are preserved, irrespective of the number of MDR1 alleles.

A high level of MDR1 mRNA expression in monosomy 7 patients may be explained by retention of the most active MDR1 gene. If the genetic polymorphism of MDR1 is functionally important, upregulation or activation of the remaining most resistant MDR1 allele would be expected. Evidence for this theory was found in a study of Mickley et al37 who described this phenomenon in patients with Burkitt's lymphoma, who lost one of their MDR1 alleles during development to resistant disease, suggesting selection of a drug-resistant clone. However, we observed by analysis of the genetic polymorphism of the MDR1 gene at position 2677, that the loss of one MDR1 allele in patients with monosomy 7 was random, ie both G and T variants were found. The T variant was expressed less frequently, but this was concordant with the random distribution of the G and T variants, using the Hardy-Weinberg formula for distribution of alleles in the normal population. A low incidence of the T variant was also reported by Mickley et al.37 He found 3x a T variant and 15x a G variant in cell lines, and 8x a T variant and 21x a G variant in normal tissue at position 2677 of the MDR1 gene in homozygous cases.

These findings indicate that, while the MDR1 gene is upregulated at the RNA level, the functional expression has not changed. Upregulation of the MDR1 gene as a result of (somatic) mutations was not the focus of our study. Methylation changes of the CpG islands in the promoter region of housekeeping genes is one of the mechanisms by which gene transcription is regulated.44 Especially in human cancers, de novo methylation of the islands usually has a significant (negative) effect on the transcription level of the gene involved. In our study, the methylation analysis of the MDR1 promoter-associated CpG island in 17/19 of our -7/7q- patients and (15/42) control AML patients showed no abnormal methylation pattern in any case. Therefore, we conclude that the transcription of the MDR1 gene is not regulated through methylation changes of the promoter region. Fryxell et al45 suggested in their detailed study that methylation changes upstream of the promoter, in an ALU repeat may be important for transcription regulation, but they did not show a correlation between the methylation level of the ALU repeat and transcription level of the MDR1 gene. The only study that showed a correlation between MDR1 expression and methylation of the MDR1 gene was performed by Kantharidis et al.46 They examined two AML cell lines, of which only one expressed MDR1. However, their conclusions are mainly based on DNA digestion of HpaII sites, which are located just outside the CpG island of the MDR1 gene, and it is not known what the effect of the methylation status of this region will have on the expression of this gene.46

We identified four patients with monosomy 7 and one copy of the MDR1 gene as shown by FISH, who had heterozygous MDR1 (G and T) expression. This phenomenon may be the result of contamination of the purified bone marrow samples with normal cells. An alternative explanation may be that these patients had a leukemia with disomy for chromosome 7, which had not been detected by karyotyping or FISH.

We have demonstrated that MDR1 expression is upregulated in -7/7q- patients at a transcriptional level which is not translated to the protein level. The mechanism for higher expression level was found to be due neither to selective allelic loss of the MDR1 gene nor to methylation changes of the promoter region of MDR1. As P-glycoprotein expression does not follow the upregulation at transcriptional level, we suggest that MDR1 is not a mechanism of drug resistance in poor prognostic AML with -7/7q-.

Acknowledgements

This study was supported by the Foundation of Medical Research Sophia (SSWO), the Kröger Society and the Foundation of Pediatric Oncology Centre Rotterdam (SKOR). The authors express their gratitude to Drs F Baas and Dr P Devilee for providing the MDR1-specific FISH probe and the centromer probe of chromosome 7. They also thank the technicians of the Tumorcytogenetics laboratory, Department of Clinical Genetics, and Department of Cell biology and Genetics for their expert technical assistance.

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Figures

Figure 1  An ideogram of chromosome 7 showing the MDR1 gene at 7q21.1, and the deleted segments involved in the patients with 7q- (patients 12-19). The extent of the deleted region in patient 18 was unclear due to the quality of the metaphases.

Figure 2  RNase protection assay. Analysis of MDR1 expression in control cell lines (KB 8.5, KB 8, KB 3.1) and BM mononuclear cells of 16 of the investigated 48 patients (13 -7/7q- and 35 controls).

Figure 3  Results of the dual-coloured fluorescence in situ hybridization (FISH) study on one of the AML patients with a partial deletion of one chromosome 7, showing the biotin labeled alpha-satellite centromer probe (7pta) for detection of chromosome 7(green) and the digoxinine-labeled (CHMR6) for the MDR1 gene (red). The arrows indicate the chromosomes 7. Two copies of the MDR1 gene are present in this metaphase.

Figure 4  Detection of the genetic polymorphism of the MDR1 gene in blasts of AML patients. The DNA and RNA lanes indicate which alleles are expressed. The oligonucleotides HMC3 and HMC4 represent the hybridization controls for the G and T primer respectively. The box at the bottom side, right of the figure shows the results of initial controls on the procedures (PCR- and RT-).

Figure 5  An example of the MDR1 DNA methylation assay in monosomy 7 (samples 1 and 2 ) and 7q- (samples 3 and 4) patients. All samples show unmethylated DNA in the CpG island of the MDR1 promoter. U, unmethylated; M, methylated.

Tables

Table 1  Karyotype and FAB type of the 19 newly diagnosed AML patients with complete or partial monosomy 7

Table 2  Control group of AML patients without partial or complete monosomy 7

Table 3  MDR1 and MRP1 expression in AML

Received 5 May 2000; accepted 29 September 2000
March 2001, Volume 15, Number 3, Pages 398-405
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