Recently, it was shown that both Bcr and Bcr-Abl can interact with xeroderma pigmentosum group B (XPB/ERCC3), a protein implicated in DNA repair after UV-induced damage. To further analyze the effect of Bcr-Abl on the DNA damage response, we used cell lines stably transfected with the BCR-ABL gene and their parental counterparts (MBA-1 versus MO7E and Bcr-AblT1 versus 4A2+-pZAP) and several assays reflecting DNA repair: the comet assay, a radioimmunoassay for cyclobutane pyrimidine dimers, and clonogenic assays. After exposure to UVC (0.5–5.0 joules m−2), the Comet assay demonstrated greater efficiency of DNA repair in the BCR-ABL-positive cells (both MBA-1 and Bcr-AblT1) when compared with their parental counterparts. Furthermore, there was less production of the UV-induced DNA adduct—cyclobutane pyrimidine dimers—as well as a more rapid rate of disappearance of these adducts and greater UV survival (clonogenic assays) in MBA-1 cells as compared with MO7E cells. Apoptosis (annexin V-FITC/propidium iodide staining) was markedly reduced in the BCR-ABL-positive cells. These results indicate that BCR-ABL confers enhanced resistance to UV radiation-induced damage and increased efficiency of DNA repair and that these changes are associated with a protective antiapoptotic effect.
The BCR-ABL oncogene is the molecular hallmark of Ph2 -positive CML and results from a reciprocal translocation [t(9,22)(q34;q11)] between chromosomes 9 and 22 (1, 2). The major protein products (p190Bcr-Abl and p210Bcr-Abl) are characterized by elevated levels of tyrosine kinase activity, which contributes to the transforming capability of BCR-ABL (3, 4, 5, 6, 7, 8). There is now cogent data supporting the central role of Bcr-Abl in the pathogenesis of Ph-positive leukemias. For example, p190BCR-ABL or p210BCR-ABL-bearing transgenic mice develop acute lymphocytic leukemia or a CML-like myeloproliferative disorder (9, 10, 11). In addition, therapeutic application of the Bcr-Abl-specific kinase inhibitor—STI 571—has yielded substantial responses in patients with Ph-positive leukemias (12, 13).
The Bcr-Abl protein has been linked to a variety of cytoplasmic signaling pathways (Refs. 14, 15, 16, 17, 18, reviewed in Ref. 19). Interestingly, Abl resides both in the cytoplasm and the nucleus (15, 20), and Bcr—although originally localized to the cytoplasm—is now known to have a nuclear fraction as well (15, 21, 22). In the nucleus, the Bcr protein was found to associate with condensed DNA such as metaphase chromatin, as well as the highly condensed heterochromatin in interphase cells (21). Of importance in this regard, it has recently been shown (through yeast two-hybrid screening) that both Bcr and Bcr-Abl can interact with the XPB protein (23, 24). XPB is known to be critical in the DNA repair process. Indeed, patients with xeroderma pigmentosum develop serious skin damage and tumors after exposure to UV light, and protection from sunlight is critical in these individuals (reviewed in Ref. 25). Abl, through its interaction with the ATM protein product (ATM being the gene mutated in ataxia telangectasia, a disorder characterized by hypersensitivity to ionizing radiation), has also been linked to the DNA damage response process (26, 27). Further, Abl tyrosine kinase is a downstream target of phosphorylation and activation by the ATM kinase after ionizing radiation (27), and both proteins appear to mediate signals required for the assembly of Rad51 and Rad52 recombination complex (28). (Rad51 and Rad52 play a key role in DNA repair of double-strand DNA breaks that may result from drug- or γ-irradiation-induced damage.)
Attenuation of DNA repair in BCR-ABL-positive cells has been found by some investigators (24, 29, 30) but not by others (31, 32). Slupianek et al. (32) recently suggested that in response to cytotoxic drugs, Bcr-Abl actually enhances repair of double-stranded DNA breaks. To additionally assess the impact of Bcr-Abl on DNA repair, we compared UVC-induced DNA damage and repair in BCR-ABL-transfected cell lines and their parental controls using several distinct assays (33, 34, 35, 36, 37, 38, 39, 40, 41, 42): (a) single-cell gel electrophoresis (Comet assay) [used to detect incision events that are part of the post-UVC DNA repair process (33)]; (b) RIA to measure specific DNA damage, i.e., CPDs (34, 35); and (c) clonogenic assays to measure cell survival (36). Our results suggest that in the presence of Bcr-Abl, UVC-induced DNA damage is decreased and repair is facilitated. Furthermore, the more efficient DNA damage response was associated with a significant reduction in UVC-induced programmed cell death.
MATERIALS AND METHODS
We used the human hematopoietic cell line, MO7E, and its stably transfected p210BCR-ABL derivative, MBA-1 [kindly provided by Dr. John E. Dick (University of Toronto, Toronto, Ontario, Canada) and described by Sirard et al. (37)]. We also used the human fibroblast cell line, 4A2+, and its derivatives transfected with either empty vector (pZAP) or human p210BCR-ABL-pZAP (Bcr-AblT1) [kindly provided by Dr. Jean Y. J. Wang (University of California-San Diego, La Jolla, CA) and described by Renshaw et al. (38)]. The MBA-1 cells were maintained in RPMI 1640 (BioWhittaker, Walkersville, MD) containing 15% FCS, whereas the growth factor-dependent parent cell line, MO7E, was cultured in RPMI 1640 containing 10% FCS plus 100 units/ml GM-CSF (Amgen, Thousand Oaks, CA). The 4A2+ cell line and its derivatives were cultured in DMEM (BioWhittaker) containing 10% iron-supplemented FCS. All cells were grown at 37°C in a 5% CO2 incubator.
MO7E and MBA-1 cells were seeded in 6-well tissue culture plates at a concentration of 3 × 105 cells/well in 1.5 ml of media and irradiated with 254 nm of UV light (UVC) [UVXL-1000; Fisher Scientific, Pittsburgh, PA] ranging from 0.5 to 10.0 joules/m2 (Jm−2). Controls were exposed to no UVC. For the comet assays (see below), the cells were allowed to incubate for different lengths of time after irradiation (15 min to 24 h) and subsequently harvested. The 4A2+ derivatives were seeded 1 day before treatment under similar conditions as above. For these cells, the media was removed before UV treatment and replaced before additional incubation of the cells at 37°C. For the RIA, MO7E and MBA-1 cells were irradiated with 10 Jm−2 UVC then harvested at different time points (0–48 h) after irradiation.
Single-Cell Gel Electrophoresis (Comet) Assay.
The Comet assay is used to detect DNA breaks, which are reflected by the migration of DNA (the comet tail) from the nucleus toward the anode (39, 40, 41). After UVC exposure, these DNA breaks occur as part of the DNA NER process. [Generally, at low doses, UVC radiation does not induce DNA breaks as a manifestation of damage in of itself (41).] Therefore, after UVC irradiation, longer comet tails reflect more DNA repair (41). The assay was performed in accordance with previously published methods (33) and as modified by Lemay and Wood (42) using the COMET Assay Kit (Trevigen, Inc., Gaithersburg, MD). After harvesting, the cells were washed with PBS and then resuspended at a concentration of 1 × 105 cells/ml in PBS. The cells were combined at a ratio of 1:10 with molten, low melting point agarose kept at 37°C (Trevigen, Inc.). Sixty μl of the mixture were immediately pipetted and evenly spread unto Comet slides that are specially coated with high melting point agarose, which serves as an anchor for the cells/low melting point agarose mixture (Trevigen, Inc.). The slides were kept at 4°C for 10–20 min to allow the agarose to solidify, then immersed in prechilled lysis solution (Trevigen, Inc.) for 30–60 min at 4°C. Subsequently, the slides were immersed into freshly prepared alkali solution (300 mm NaCl and 1 mm EDTA) for 20 min. The slides were transferred to an electrophoresis chamber, and fresh alkali solution was poured to sufficiently cover the slides. Electrophoresis was done at 300 mA, 20 V (constant voltage) for 30–40 min. Subsequently, the slide was rinsed briefly in Tris-EDTA (TE) solution [10 mm Tris (pH 7.5) and 1 mm EDTA] and then fixed in 100% ice-cold ethanol for 5 min. The samples were air-dried at room temperature, and the cells were viewed by staining with 40 μl of Sybr green (Trevigen, Inc.) diluted 1:10,000 in TE. The cells were viewed under a fluorescence microscope (excitation/emission at 494/521 nm; Olympus Corporation, Lake Success, NY). For quantification, we used a visual scoring system under fluorescence microscopy, which was a modification of the method of Collins et al. (43). Two hundred comets on each slide were classified by an observer blinded to the conditions on the slide. Relative tail fluorescence intensity was scored as 1+ (none, 25%), 2+ (low, 5–20%), 3+ (medium, 20–40%), 4+ (high, 40–95%), and 5+ (total, > 95%). Length of tail was similarly scored on a 1+ to 5+ scale. Fluorescence intensity score was multiplied by length of tail score, giving a final score of 1–25 for each cell. Thus, the total score for 200 comets could range from 1 (all undamaged) to 5000 (all maximally damaged). Scores on different slides were compared by the Mann-Whitney test. Scoring was performed in triplicates.
FITC-conjugated Annexin V/PI Staining.
To identify cells in the early and late stages of apoptosis, we used the Annexin V-FITC Apoptosis Detection Kit (BD PharMingen, San Diego, CA). Briefly, cells exposed to UV radiation were harvested, washed twice with cold PBS, then resuspended in 1× binding buffer (BD PharMingen) at a concentration of 1 × 106 cells/ml. One hundred μl of the cell suspension were transferred to a 5-ml polypropylene tube, and 5 μl each of PI (50 μg/ml stock) and annexin V-FITC were added simultaneously. The cells were gently mixed and incubated at room temperature in the dark for 15 min. Three hundred μl of 1× binding buffer were added to each tube, and the cells were analyzed immediately by flow cytometry (FACScan, Becton Dickinson, Franklin Lakes, NJ).
DNA phosphoproducts such as CPDs are induced by UV irradiation and can be measured by RIA (34, 35). The CPD specificity is derived from the specificity of the antiserum, which was raised against triplet-sensitized UVB-irradiated DNA (containing only CPDs). The methods used to characterize this RIA include enzymatic photoreactivation, digestion with T4 endo V, and mobility shift assays using synthetic oligonucleotides containing CPDs exclusively. For the RIA, 2–5 μg of heat-denatured sample DNA were incubated with 5–10 pg of poly(dA dT) (labeled to >5 × 108 cpm/μg by nick translation with 32P-dTTP) in a total volume of 1 ml of solution [10 mm Tris (pH 7.8), 150 mm NaCl, 1 mm EDTA, and 0.15% gelatin (Sigma, St. Louis, MO)]. Antiserum was added at a dilution that yielded 30–60% binding to labeled ligand. (The antisera were raised against DNA that was dissolved in 10% acetone and irradiated with UVB light under conditions that have been shown to produce CPDs exclusively.) After incubation overnight at 4°C, the immune complex was precipitated with goat antirabbit immunoglobulin (Calbiochem, San Diego, CA) and carrier serum from nonimmunized rabbits (University of Texas M. D. Anderson Cancer Center, Science Park/Veterinary Division, Bastrop, TX). After centrifugation, the pellet was dissolved in NCS tissue solubilizer (Amersham, Arlington Heights, IL), mixed with ScintiSafe (Fisher, Pittsburgh, PA) containing 0.1% glacial acetic acid, and the 32P quantified by liquid scintillation spectrometry. (Under these conditions, antibody binding to an unlabeled competitor inhibits antibody binding to the radiolabeled ligand.) Sample inhibition is extrapolated through a standard (dose response) curve to determine the number of photoproducts in 106 bases. For standard, we used double-stranded salmon testes DNA (Sigma) irradiated with increasing doses of UVC radiation and heat denatured, aliquoted, and kept frozen at −20°C. Rates of photoproduct induction for the standards were previously determined using nonimmunological enzymatic and biochemical techniques and determined to be 8.1 CPDs/megabase/Jm (2). These details, as well as those concerning the specificities of the RIAs, are described by Mitchell (34, 35).
Clonogenic survival curves were used to assess UVC radiation sensitivity. MO7E and MBA-1 cells were exposed to no radiation (control) or 0.5, 1.0, 5.0, and 10.0 Jm−2 UVC, and the clonogenic assay was performed as described previously (36). Briefly, immediately after irradiation, the cells were cultured in 0.8% methylcellulose (Fluka Chemical Corp., Ronkonkoma, NY), 10% FCS and RPMI 1640 in 1% (vol/vol) methylcellulose. [For the MO7E cells, the medium was supplemented with 250 units of GM-CSF (Amgen)]. The culture mixture was placed in 35-mm Petri dishes (Nunc, Inc., Naperville, IL) in duplicate and maintained at 37°C with 5% CO2 in air in a humidified atmosphere. Colonies were counted after 7 days using an inverted microscope. A colony was defined as a cluster of >40 cells.
We examined DNA damage and repair as reflected by three assays: comet assay; a RIA for CPDs/megabase DNA; and clonogenic assays. Apoptosis was assessed by annexin V-FITC/PI staining followed by FACS.
Comet Assays Demonstrate More Efficient DNA Repair in BCR-ABL-bearing Cells after UVC Exposure.
MO7E, MBA-1, 4A2+-pZAP, and Bcr-AblT1 (4A2+ bearing BCR-ABL) cells were exposed to UVC radiation. DNA repair at different time points after irradiation was assessed using the Comet assay. Experiments were repeated a total of two to three times for all conditions. After low dosage of UVC, NER is carried out by a single-strand incision on either side of the lesion, and the damaged DNA is then released as a 24–32-nucleotide long fragment (44). The gap is then filled in by DNA synthesis mediated by DNA polymerase. Therefore, after exposure to UVC, the Comet assay measures the level of excision-associated strand breaks, which correlates with the length of the comet tails seen in the Comet assay (41).
After exposure to either 0.5 Jm−2 (Fig. 1) or 5.0 Jm−2 (data not shown), the results were similar. As early as 15 min and continuing through 3 h after treatment, scoring of 200 cells/slide indicated that the BCR-ABL-positive cells (MBA-1) showed longer comet tails (Fig. 1) as compared with the BCR-ABL-negative parent cell line (MO7E; P < 0.05), consistent with greater DNA repair. By 24 h, the vast majority of MBA-1 cells had returned to baseline (P < 0.05). In contrast, <25% of MO7E cells had returned to baseline. The latter results suggest that DNA repair is more rapidly completed and hence more efficient in MBA-1 cells. In addition, the number of MO7E cells remaining at 24 h was significantly less than the number of MBA-1 cells, presumably because many MO7E cells underwent apoptosis (see below). The differences between MO7E and MBA-1 response to UVC described above were also seen when MBA-1 cells were cultured in the presence of GM-CSF (data not shown). A similar pattern of enhanced DNA repair was seen in Bcr-AblT1 compared with parental 4A2+-pZAP (Fig. 1 B).
Annexin V-FITC/PI Staining Demonstrates Less Apoptosis after UVC in BCR-ABL-bearing Cells.
To determine the rate of apoptosis of our cells after UVC irradiation, we stained cells with both annexin V and PI. Annexin V binds to phosphatidylserine, which is normally found on the inner leaflet of the plasma membrane of cells but is flipped to the outer leaflet during the onset of apoptosis (discussed in Refs. 45, 46). PI, which stains DNA, is excluded by intact cells but incorporated by cells that have lost their membrane integrity (45). Using the combination of annexin V-FITC and PI, intact cells are defined as FITC−/PI−, cells in early apoptosis as FITC+/PI−, and cells in late apoptosis as FITC+/PI+ (45).
At 3 h after irradiation (0.5 Jm−2), MBA-1 cells had less apoptosis (∼13%) compared with MO7E cells (∼26%; Fig. 2). At 8 h, apoptosis levels in MBA-1 had not changed significantly while that in MO7E cells increased to 67%. By 24 h and thereafter, most of the MO7E cells, but not the MBA-1 cells, were in late apoptosis. Similar results were seen using 5.0 Jm−2. These observations indicate that: (a) the longer tails in MBA-1 cells at early time points were not attributable to apoptosis; and (b) the rate of UVC-induced apoptosis in MO7E is significantly higher than in the BCR-ABL-positive MBA-1 cells (P < 0.05, Mann-Whitney test).
RIA Demonstrates Less DNA Damage and More Rapid DNA Repair in BCR-ABL-bearing Cells after UVC Exposure (Fig. 3, A and B).
To investigate the repair efficiency of specific UV-induced DNA damage, we used a RIA to look at the differences in induction and repair of CPDs in BCR-ABL-positive versus BCR-ABL-negative cells. We first tested for a repeat effect, e.g., to determine whether the results of repeat 2 are consistently different from the results of repeat 1. This would happen if the experimental conditions were somehow different for each set of repeats. This was tested statistically using a random effects model in which we fitted a linear model to ln(CPD) versus time. There was no evidence of an effect of repeat number: P = 0.134 for MO7E cells and P = 0.832 for MBA-1 cells. Therefore, in all remaining analyses, the repeat number was disregarded. Next, we compared pre-exposure CPDs/megabase DNA for the two cell lines. There was no significant difference (P = 0.147, Mann-Whitney test). In contrast, for each of the postexposure time points, there is a significant difference in UV-induced DNA damage as reflected by CPDs/megabase DNA (P = 0.021, Mann-Whitney test; Fig. 3 A).
In regard to repair rate, we first looked at whether zero-order kinetics (linear kinetics) (fixed number of CPDs/megabase DNA repaired/unit time) or first order (exponential kinetics, fixed proportion of CPDs/megabase DNA repaired/unit time) fit the data better. To do this, curvature was detected using regression analysis of remaining CPDs (linear scale) versus time; a quadratic term (time squared) added to the model was significantly different from zero, indicating deviation from linearity. Therefore, first-order (exponential) kinetics were assumed to apply (as is usually true for biological processes). We compared the repair rates for the two cell lines by comparing the slopes of the lines fitted to ln(CPD)/megabase DNA remaining versus time for each line. Assuming exponential repair of CPDs, the repair rates are significantly faster in the MBA-1 cells (P = 0.009, F test). For MO7E, the half-time for repair (time required to repair half of the CPDs/megabase DNA) is 43 h (95% CI, 27.5–102.2 h), whereas for MBA-1, the half-time for repair is 21 h (95% CI, 16.2–28 h).
Clonogenic Assays Demonstrate that BCR-ABL Confers Decreased Sensitivity to UVC.
UVC survival curves for MBA-1 and MO7E cells are demonstrated in Fig. 4. There was significantly higher clonogenic survival after UVC in BCR-ABL-positive MBA-1 cells than in parental MO7E cells at all dose levels tested (0.1, 0.5, 1.0, 5.0, and 10.0 Jm−2; P < 0.05).
Several lines of evidence suggest involvement of both Bcr and Abl in DNA damage response. For instance, the SH3 domain of Abl binds residues 1373–1382 of the ATM product (26). After ionizing radiation, the Atm kinase phosphorylates Abl; in cells from ataxia telangectasia patients (known to be sensitive to radiation damage), this interaction fails to occur (26, 27). Bcr has also been linked to the DNA repair process, albeit via pathways distinct from those associated with Abl. Indeed, Bcr as well as Bcr-Abl bind to the XPB/ERCC3 protein (23, 24), a subunit of the basal transcription factor TFIIH and a critical player in the UV-induced NER process (reviewed in Ref. 47). Mutations in XPB induce a profound human DNA repair disorder characterized by numerous skin tumors after UV exposure (reviewed in Ref. 25). Although XPB is a nuclear protein and Bcr was initially believed to be localized exclusively in the cytoplasm (15, 21, 48), it has now been demonstrated that Bcr can also be found in the nucleus in close association with condensed chromatin (21, 22). Furthermore, it has been claimed that even a small fraction of Bcr-Abl can be discerned in the nucleus (24). On the basis of these data, we sought to determine the effect of UVC on DNA repair processes in cells stably transfected with BCR-ABL versus their parental counterparts.
Our initial hypothesis was that the presence of BCR-ABL would attenuate the DNA repair process and thus account for the genomic instability that characterizes CML and leads to clonal evolution and blast crisis. The experimental data in this area are still extremely sparse and not all of it is supportive. Indeed, in the only study to examine the impact of BCR-ABL on UV-related DNA repair, Takeda et al. (24) demonstrated that when XPB is coexpressed with transfected p210BCR-ABL, the XPB protein is heavily phosphorylated on its tyrosine residues and is unable to correct a UVC-related DNA repair defect in XPB-null mutants. On the other hand, transfection of BCR-ABL into cells carrying the wild-type XPB gene did not substantially impair DNA repair (24).
Contrary to our expectations, our initial experiments using the Comet assay demonstrated that the DNA repair incision process was enhanced in both hematopoietic cells and fibroblasts transfected with BCR-ABL as compared with parental controls (Fig. 1 and data not shown). More vigorous DNA repair (longer comet tails) were consistently evident for 3 h after UVC exposure. In addition, there was more rapid completion of the DNA repair process at 24 h. To verify these results, a RIA that specifically measures CPDs was used. This assay showed substantially less DNA damage induced by equivalent doses, as well as more rapid NER in BCR-ABL-positive hematopoietic cells as compared with their parental counterparts (Fig. 3, A and B). Because the BCR-ABL-transformed cells and the parental cells (grown in the presence of growth factor) had similar proliferative rates and cell cycle distribution (as determined by FACS; data not shown), these factors could not account for the differences in DNA repair and damage. Finally, the demonstration of increased post-UVC survival of the BCR-ABL-positive cells in clonogenic assays confirmed their enhanced UVC resistance. (These results, although interesting, do not necessarily imply a functional interaction between BCR-ABL and XPB).
The effects of UV, γ-radiation, and cytotoxic drugs on DNA may be substantially different, at least partly because of distinct DNA repair pathways associated with each treatment. Several investigators have examined the impact of BCR-ABL on the sequelae of ionizing radiation. Santucci et al. (29) demonstrated that after γ-irradiation, BCR-ABL-positive cells show decreased survival in clonogenic assays as compared with the BCR-ABL-negative controls. They therefore suggested that BCR-ABL either amplified γ-irradiation-induced DNA damage or inhibited the DNA repair process. Deutsch et al. (30) demonstrated down-regulation of a major DNA double-strand break repair protein (DNA-PK) and induction of a DNA repair defect (as measured by fluorescent in situ hybridization) in BCR-ABL-positive cells. However, Bedi et al. (31) reported that the rate of DNA repair in BCR-ABL-positive cells was not different from that of the controls after ionizing radiation. Furthermore, Slupianek et al. (32) showed that Bcr-Abl enhanced expression and phosphorylation of RAD51, a key player in the repair of drug- and γ-irradiation-induced double-strand breaks (49), and increased recombination repair after exposure to cytotoxic drugs. All of these authors reported that BCR-ABL-bearing cells are resistant to γ-irradiation-induced apoptosis and/or are drug-resistant as compared with their BCR-ABL-negative parental counterparts (29, 30, 31, 32). We also found significantly reduced apoptosis in the BCR-ABL-positive cells after UVC exposure (Fig. 2). Hence, diminished programmed cell death in BCR-ABL-bearing cells is seen after a variety of insults, including ionizing radiation, UVC, and cytotoxic drugs (29, 30, 31, 32). The mechanisms mediating resistance to programmed cell death in BCR-ABL-positive cells may include induction of antiapoptotic genes such as BCL-2 and BCL-xL (50, 51), inhibition of proapoptotic proteins such as Bad (52), up-regulation of cytoprotective signaling pathways, including signal transducers and activators of transcription 5, Akt, nuclear factor-κB (53, 54, 55, 56), as well as prolonged cell cycle arrest at the G2-M checkpoint to allow the cells sufficient time to repair DNA damage (31). In addition, increased resistance to DNA damage and enhanced DNA repair may account for the diminished apoptosis after UVC or chemotherapy exposure.
In conclusion, we demonstrate that in response to UVC irradiation, p210BCR-ABL confers resistance to overt UV damage and accelerates the DNA repair rate. Increased DNA repair may or may not play a key functional role in resistance of Bcr-Abl-bearing cells to DNA damage. These changes are accompanied by a marked diminution in UVC-induced apoptosis. Our results are analogous to those reported by Slupianek et al. (32), who demonstrated that Bcr-Abl facilitates RAD51-associated homologous recombination repair of DNA double-strand breaks occurring after exposure to cytotoxic agents and hence induces drug resistance. Taken together, the literature suggests that Bcr-Abl profoundly alters the response to DNA injury, albeit with different effects depending on the genotoxic trigger and which of several repair pathways are investigated (57). The potential leukemogenic effects of DNA damage from ionizing radiation are well recognized, and recent results of both epidemiological studies and animal models implicate UV irradiation in augmentation of internal lymphoid malignancies (58, 59). Therefore, a role for a BCR-ABL-related alteration in DNA repair in the genomic instability leading to blast crisis may be postulated (24, 29, 30). Superficially, it would appear that more efficient DNA repair would be incompatible with such a role. It is, however, conceivable that this interplay is complex and that more hurried repair may, in the long run, be accompanied by subtle errors in the process. Furthermore, the resistance to apoptosis that is conferred on these cells may make them susceptible to the eventual emergence of aberrations. In the presence of double-strand breaks, it has been previously shown that lack of repair leads to programmed cell death, whereas incorrect repair may lead to chromosomal loss or translocation (60). Progression in CML is driven by the slow accumulation of molecular genetic aberrations that, although inevitable, occurs over a period of years. Additional studies are needed to determine to what extent facilitated but unfaithful repair exists in CML and if such a phenomenon coupled with a stalled apoptotic program may, in the long term, impact genomic integrity in this disorder.
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The abbreviations used are: Ph, Philadelphia; CML, chronic myelogenous leukemia; XPB, xeroderma pigmentosum group B; NER, nucleotide excision repair; CPD, cyclobutane pyrimidine dimer; PI, propidium iodide; GM-CSF, granulocyte macrophage colony-stimulating factor.