BCR binds to the xeroderma pigmentosum group B protein

Loss of the xeroderma pigmentosum group B protein binding site impairs p210 BCR/ABL1 leukemogenic activity

Previous studies have demonstrated that p210 BCR/ABL1 interacts directly with the xeroderma pigmentosum group B (XPB) protein, and that XPB is phosphorylated on tyrosine in cells that express p210 BCR/ABL1. In the current study, we have constructed a p210 BCR/ABL1 mutant that can no longer bind to XPB. The mutant has normal kinase activity and interacts with GRB2, but can no longer phosphorylate XPB. Loss of XPB binding is associated with reduced expression of c-MYC and reduced transforming potential in ex-vivo clonogenicity assays, but does not affect nucleotide excision repair in lymphoid or myeloid cells. When examined in a bone marrow transplantation (BMT) model for chronic myelogenous leukemia, mice that express the mutant exhibit attenuated myeloproliferation and lymphoproliferation when compared with mice that express unmodified p210 BCR/ABL1. Thus, the mutant-transplanted mice show predominantly neutrophilic expansion and altered progenitor expansion, and have significantly extended lifespans. This was confirmed in a BMT model for B-cell acute lymphoblastic leukemia, wherein the majority of the mutant-transplanted mice remain disease free. These results suggest that the interaction between p210 BCR/ABL1 and XPB can contribute to disease progression by influencing the lineage commitment of lymphoid and myeloid progenitors.

Polymorphisms of ERCC1 genotype associated with response to imatinib therapy in chronic phase chronic myeloid leukemia

DNA repair machinery may contribute to the mechanism of the action in imatinib. We examined the association between the single nucleotide polymorphism (SNP) markers involved in the DNA repair enzyme pathway (ERCC1/2/4/5, XRCC1/2/4/5) and the clinical outcomes following an imatinib therapy in chronic phase chronic myeloid leukemia (CML) patients. A total of 169 Korean patients were included. Of the 19 SNPs from these patients, those with the TT genotype of ERCC1 (rs11615) showed a higher probability of achieving major cytogenetic response [P = 0.002, HR 5.14 (95 % CI 1.83-14.43)], complete cytogenetic response [P = 0.012, HR 3.47 (95 % CI 1.31-9.17)], and major molecular response [P = 0.001, HR 5.71 (95 % CI 2.13-15.30)] than those with CC or CT genotypes. This suggests that SNP markers on ERCC1 may predict the response to imatinib therapy, which proposes the potential involvement of the DNA repair machinery in the mechanism of imatinib action in chronic phase CML.

BCR-ABL: a multi-faceted promoter of DNA mutation in chronic myelogeneous leukemia

The role of the BCR-ABL oncogene in the progression of chronic myeloid leukemia (CML) to blast crisis (BC) is unknown. The appearance of chromosomal aberrations in patients with CML BC has led to many attempts to elucidate a mechanism whereby BCR-ABL affects DNA damage and repair. BCR-ABL-expressing cells have been found to accumulate genetic abnormalities, but the mechanism leading to this genomic instability is controversial. In this study, we review the effects of BCR-ABL on DNA repair mechanisms, centrosomes, checkpoint activation and apoptosis. BCR-ABL has diverse effects on these mechanisms, but which of these effects are necessary for the progression of CML to BC is still unresolved.

Nuclear translocation contributes to regulation of DNA excision repair activities

DNA mutations are circumvented by dedicated specialized excision repair systems, such as the base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR) pathways. Although the individual repair pathways have distinct roles in suppressing changes in the nuclear DNA, it is evident that proteins from the different DNA repair pathways interact [Y. Wang, D. Cortez, P. Yazdi, N. Neff, S.J. Elledge, J. Qin, BASC, a super complex of BRCA1-associated proteins involved in the recognition and repair of aberrant DNA structures, Genes Dev. 14 (2000) 927-939; M. Christmann, M.T. Tomicic, W.P. Roos, B. Kaina, Mechanisms of human DNA repair: an update, Toxicology 193 (2003) 3-34; N.B. Larsen, M. Rasmussen, L.J. Rasmussen, Nuclear and mitochondrial DNA repair: similar pathways? Mitochondrion 5 (2005) 89-108]. Protein interactions are not only important for function, but also for regulation of nuclear import that is necessary for proper localization of the repair proteins. This review summarizes the current knowledge on nuclear import mechanisms of DNA excision repair proteins and provides a model that categorizes the import by different mechanisms, including classical nuclear import, co-import of proteins, and alternative transport pathways. Most excision repair proteins appear to contain classical NLS sequences directing their nuclear import, however, additional import mechanisms add alternative regulatory levels to protein import, indirectly affecting protein function. Protein co-import appears to be a mechanism employed by the composite repair systems NER and MMR to enhance and regulate nuclear accumulation of repair proteins thereby ensuring faithful DNA repair.

STI571 reduces NER activity in BCR/ABL-expressing cells

Nucleotide-excision repair (NER) is the most versatile mechanism of DNA repair, recognizing and dealing with a variety of helix-distorting lesions, such as the UV-induced photoproducts cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) photoproducts. We investigated the influence of an anticancer drug, STI571, on the efficacy of NER in removing UV-induced DNA damage. STI571 is used mostly in the treatment of chronic myeloid leukemia and inhibits activity of the BCR/ABL oncogenic tyrosine kinase, which is a hallmark of this disease. NER activity was examined in the BCR/ABL-expressing cell lines K562 and BV173 of myeloid and lymphoid origin, respectively, as well as in CCRF-CEM cells, which do not express BCR/ABL. A murine myeloid parental 32D cell line and its counterpart transfected with the BCR/ABL gene were also tested. NER activity was assessed in the cell extracts by use of an UV-irradiated plasmid as a substrate and by a modified single-cell gel electrophoresis (comet) assay on UV-treated nucleoids. Additionally, quantitative PCR was performed to evaluate the efficacy of the removal of UV-induced lesions from the p53 gene by intact cells. Results obtained from these experiments indicate that STI571 decreases the efficacy of NER in leukemic cells expressing BCR/ABL. Therefore, STI571 may overcome the drug resistance associated with increased DNA repair in BCR/ABL-positive leukemias.

p210 BCR/ABL kinase regulates nucleotide excision repair (NER) and resistance to UV radiation

Both clinical and experimental evidence illustrate that p190 and p210 BCR/ABL oncogenic tyrosine kinases induce resistance to DNA damage and confer an intrinsic genetic instability. Here, we investigated whether BCR/ABL expression could modulate nucleotide excision repair (NER). We found that ectopic expression of p210 BCR/ABL in murine lymphoid BaF3 cell line inhibited NER activity in vitro, promoting hypersensitivity of these cells to ultraviolet (UV) treatment and facilitating a mutator phenotype. However, expression of p210 BCR/ABL in human and murine myeloid cell lines and primary bone marrow cells resulted in the increased NER activity and resistance to UV irradiation. The ABL tyrosine kinase inhibitor STI571 reversed these effects, showing that p210 BCR/ABL tyrosine kinase activity is responsible for deregulation of NER. Hypoactivity of NER in p210 BCR/ABL-positive lymphoid cells was accompanied by the decreased interaction between proliferating cell nuclear antigen (PCNA) and xeroderma pigmentosum group B (XPB); conversely, this interaction was enhanced in p210 BCR/ABL-positive myeloid cells. p190 BCR/ABL did not affect NER in lymphoid and myeloid cells. In summary, our study suggests that p210 BCR/ABL reduced NER activity in lymphoid cells, leading to hypersensitivity to UV and mutagenesis. In contrast, p210 BCR/ABL expression in myeloid cells facilitated NER and induced resistance to UV.

p38 MAPK-mediated activation of NF-kappaB by the RhoGEF domain of Bcr

The oncogenic fusion protein p210 Bcr-Abl is causally associated with virtually all cases of chronic myelogenous leukemia. The wild-type Bcr product has several recognizable structural and functional motifs including a domain that contains guanine nucleotide exchange activity for Rho family GTPases (DH/PH domain). Although this domain is retained within p210 Bcr-Abl, it has no known signaling activities in vivo. Here we report that a fragment of Bcr that encodes the isolated DH/PH domain is a potent activator of the NF-kappaB transcription factor. Within the context of full length Bcr, this activity is regulated by proximal flanking sequences that suppress the DH/PH domain encoded guanine nucleotide exchange activity. NF-kappaB activation by Bcr is not mediated by nuclear translocation, but rather by p38 mitogen-activated protein kinase (MAPK)-dependent modification of the RelA/p65 transactivation domain. Although we were able to demonstrate that Bcr can function as an exchange factor for Cdc42 in vivo, NF-kappaB activation appears to occur via a Cdc42-independent mechanism. These studies constitute direct evidence that the Bcr RhoGEF domain can function in vivo, and identify a new signaling activity that may contribute to the transforming potential of p210 Bcr-Abl.

TFIIH functions are altered by the P210BCR-ABL oncoprotein produced on the Philadelphia chromosome

P210BCR-ABL counteracted against the complementary effect of XPB on DNA repair when ultraviolet (UV)-sensitive 27-1 cells were treated with UV or cisplatin but not with hydrogen peroxide. Wortmannin, an inhibitor of PI3 kinase did not affect its anti-repair effect. Enhanced recruitment of p44 with TFIIH after cisplatin treatment is inhibited by the expression of P210BCR-ABL in a kinase activity-dependent manner. Although purified TFIIH from P210BCR-ABL expressor and non-expressor showed almost no difference in molar ratio of each component, the in vitro activity of TFIIH was decreased by 5-10% in repair assay but was increased by more than two-fold in transcription assay.

The Dbl homology domain of BCR is not a simple spacer in P210BCR-ABL of the Philadelphia chromosome

The Dbl homology (DH) domain of BCR in P210BCR-ABL (P210/WT) has been thought to have a negative effect on the activation of BCR-ABL because P185BCR-ABL, in which this region is physically deleted, has stronger biochemical and biological activities. To study the role of the DH domain of BCR in the background of P210/WT, the region was replaced with homologous sequences derived from Dbl (P210/Dbl) or CDC24 (P210/CDC24) or with irrelevant sequences from LacZ (P210/LacZ) or luciferase (P210/Luci). Surprisingly, the abilities to transform Rat1 cells or mouse bone marrow cells and induce growth factor independence in interleukin 3-dependent mouse Ba/F3 cells were retained only in P210/Dbl. However, even P210/Dbl could not achieve the wild type level of surviving potential against genotoxins in Rat1 cells and in Ba/F3 cells. Activation of Akt correlated with the biological changes in Rat1 cells but did not correlate with the biological changes in Ba/F3 cells. The DH domain was not tyrosine-phosphorylated in vitro, nor could we find any differences in peptide mapping between in vitro phosphorylated P210/WT and P210/Dbl. Although functions of the DH domain remain to be discovered, we propose that the DH domain makes positive contributions to P210BCR-ABL.

Reconstitution of recombinant TFIIH that can mediate activator-dependent transcription

BACKGROUND

TFIIH is one of the general transcription factors required for accurate transcription of protein-coding genes by RNA polymerase II. TFIIH has helicase and kinase activities, plays a role in promoter opening and promoter escape, and is also implicated in efficient activator-dependent transcription.

RESULTS

We have established a reconstitution system of recombinant TFIIH using a three-virus baculovirus expression system. The recombinant TFIIH was active in CTD kinase and DNA helicase assays, and showed both basal and activator-dependent transcriptional activities that were indistinguishable from those of HeLa cell-derived TFIIH. Further analyses using recombinant TFIIH confirmed a critical role of TFIIH in activator-dependent transcription. The dose response of TFIIH in activator-dependent transcription suggested that mere recruitment of TFIIH is not sufficient for transcriptional activation. The sensitivity of activator-dependent transcription to nonhydrolysable ATP analogues indicated the importance of the enzymatic activities of TFIIH in transcriptional activation.

CONCLUSIONS

Our results raise a possibility that transcriptional activation by GAL4-VP16 requires enzymatic activities. Recombinant TFIIH reconstituted from this baculovirus system should be useful for analysis of the mechanisms of activation by GAL4-VP16.

Molecular biology of chronic myeloid leukemia

Multistep carcinogenesis is exemplified by chronic myeloid leukemia with clinical manifestation consisting of a chronic phase and blast crisis. Pathological generation of BCR-ABL (breakpoint cluster region-Abelson) results in growth promotion, differentiation, resistance to apoptosis, and defect in DNA repair in targeted blood cells. Domains in BCR and ABL sequences work in concert to elicit a variety of leukemogenic signals including Ras, STAT5 (signal transducer and activator of transcription-5), Myc, cyclin D1, P13 (phosphatidylinositol 3-kinase), RIN1 (Ras interaction/interference), and activation of actin cytoskeleton. However, the mechanism of differentiation of transformed cells is poorly understood. A mutator phenotype of BCR-ABL could explain the transformation to blast crisis. The aim of this review is to integrate molecular and biological information on BCR, ABL, and BCR-ABL and to focus on how signaling from those molecules mirrors the biological phenotypes of chronic myeloid leukemia.

Cytoplasmic and nuclear localization of the 130 and 160 kDa Bcr proteins

Formation of the Bcr-Abl chimeric protein is the molecular hallmark of Philadelphia-positive leukemia. Normal Bcr is a complex protein which has been found in the cytoplasm, has serine kinase activity, and has been implicated in cellular signal transduction. However, we have recently demonstrated that Bcr can also associate with condensed chromatin. Since two major Bcr proteins have been characterized (p160Bcr and p130Bcr), we sought to determine if different forms of Bcr localized to the nucleus vs the cytoplasm. Metabolic labeling and Western blotting experiments were performed using nuclear and cytoplasmic extracts of three human Philadelphia-negative leukemia/lymphoma cell lines (KG-1, HL-60, and Jurkat). Both methodologies showed that p160Bcr and p130Bcr localized to the cytoplasm, but the p130 form predominated in the nucleus. These results suggest that Bcr serves both nuclear and cytoplasmic functions, and that different forms of Bcr may be preferentially involved in these distinct activities.