A comparison of the effect of bcr/abl breakpoint specific phosphothiorate oligodeoxynucleotides on colony formation by bcr/abl positive and negative, CD34 enriched mononuclear cell populations

Targeting oncogenic fusion genes in leukemias and lymphomas by RNA interference

Leukemias and lymphomas are often characterized by non-random chromosomal translocations that, at the molecular level, induce the activation of specific oncogenes or create novel chimeric genes. They have frequently been regarded as optimal targets for gene-silencing approaches because of the large body of evidence that these single abnormalities directly initiate and maintain the malignant process. Herein, we discuss RNA interference (RNAi)-based approaches for targeting the fusion sites of chromosomal translocations as a future treatment option in leukemias and lymphomas.

BCR-ABL as a target for novel therapeutic interventions

The BCR-ABL oncogene is the result of a reciprocal translocation between the long arms of chromosome 9 and 22 t(9; 22). There is good experimental evidence demonstrating that BCR-ABL is the single causative abnormality in chronic myeloid leukaemia (CML), making it a unique model for the development of molecular targets. In addition to CML, BCR-ABL transcripts can be found in a minority of acute lymphoblastic leukaemias and very rarely in acute myeloid leukaemia (AML). Elucidating the molecular mechanisms and downstream pathways of BCR-ABL has led to the design of several novel therapeutic approaches. In this review, molecular targeting of BCR-ABL will be discussed based on the inhibition of protein tyrosine kinase activity, antisense strategies and immunomodulation.

Effect of antisense oligonucleotides on CD34+ cells from chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a hematopoietic stem cell disorder characterized by a specific hybrid gene BCR-ABL (formed as a result of t(9;22)). This leads to two possible mRNA usually present in leukemic cells, either B2A2 or B3A2. Targeting these mRNA by antisense oligonucleotides (AS) might offer the opportunity to decrease leukemic growth. We have tested the ability of AS to inhibit the in vitro proliferation of CD34 positive (CD34+) blood cells from 16 patients with newly diagnosed CML. CD34+ cells were isolated by an immunomagnetic technique and incubated for 16 to 18 hours with an 18 mer AS (0.25 mM). Sense oligonucleotides served as controls. The effects of AS were evaluated by clonogenic test (production of CFU-GM). Moreover, colonies were picked out and studied by RT-PCR to analyse the presence of BCR-ABL transcript. For nine patients with B3A2 transcript, the median inhibition of CFU-GM formation at day 14 was 64.0 +/- 11.2% (68.0 +/- 11.4% at day 21) and for the seven patients with a B2A2 transcript: 59.0 +/- 11.4% (72.5 +/- 12.0% at day 21). AS showed no effect on CD34+ cells from three normal volunteer donor cells. However, for every patient studied, colonies picked out remained BCR-ABL positive with the RT-PCR technique.

Oligonucleotide therapeutics for hematologic disorders

During the last decade, the catalogue of known genes responsible for cell growth, development, and neoplastic transformation has expanded dramatically. Attempts to translate this information into new therapeutic strategies for both hematologic and non-hematologic diseases have accelerated at a rapid pace as well. Inserting genes into cells which either replace, or counter the effects of disease causing genes has been one of the primary ways in which scientists have tried to exploit this new knowledge. Strategies to directly downregulate gene expression have developed in parallel with this approach. The latter include triple helix forming oligonucleotides (ODN) and 'antisense' ODN. The latter have already entered clinical trials for a variety of disorders. In this monograph, we review the use of these materials in the treatment of hematologic diseases, particularly myelogenous leukemias. Problems and possible solutions associated with the use of ODN will be discussed as well.

Inhibition of bcr-abl oncogene expression by novel deoxyribozymes (DNAzymes)

Deoxyribozymes, or DNA enzymes (DNAzymes), are novel nucleic acids that have the ability to bind to specific sequences of RNA, and to cleave the target site catalytically. DNAzymes are smaller and more efficient enzymatically than ribozymes (RZs), which are catalytic nucleic acids synthesized from ribonucleotides. We have designed three DNAzymes that specifically target the two variants of the p210 bcr-abl gene (splice 1, b3a2; splice 2, b2a2) and the p190 variant (ela2). The cleavage sites for these DNAzymes are located 5 nucleotides (nt) 5' from the fusion site for b3a2, and only 1 nt 5' from the fusion sites for b2a2 and e1a2. We have shown in cell-free in vitro cleavage assays that these DNAzymes efficiently cleave their respective substrates. Mutated DNAzymes, in which only one critical base has been altered, do not cleave these targets. We have used a serum-resistant cytofectin (GS 2888; Gilead) to transfect the DNAzymes into target K562 cells, which express p210bcr-abl. In short-term transfection assays, the DNAzymes specifically inhibited p210bcr-abl protein expression by K562 cells by about 40%, and inhibited cell growth by more than 50% in a 6-day liquid culture assay. We have also transfected freshly isolated CD34+ bone marrow cells from patients with CML with the DNAzymes, which specifically inhibited the growth of bcr-abl-positive CFU-Mix colonies by 53-80%. The potential advantages of anti-bcr-abl DNAzymes over RZs include the following: DNAzymes are much less expensive to synthesize; they are more resistant to serum; and the anti-b2a2 DNAzyme cleaves at a site only 1 nt away from the fusion site, whereas its hammerhead RZ counterpart cleaves this target at a site 8 nt 3' to the fusion site, well within abl exon 2. DNAzymes are novel RNA-cleaving molecules that may significantly improve our ability to inhibit bcr-abl oncogene expression in Ph-positive target cells.

Myb targeted therapeutics for the treatment of human malignancies

For the past several years, we have been engaged in developing a therapeutically effective strategy for disrupting gene function with reverse complementary, or so called 'antisense', oligodeoxynucleotides (ODN). This pursuit has focused on finding appropriate diseases in which to apply this approach, and suitable gene targets. Of the genes that we have targeted for disruption using the antisense ODN strategy (Clevenger et al., 1995; Gewirtz and Calabretta, 1988; Ratajczak et al., 1992c; Small et al., 1994) one that has been of particular interest, and one where therapeutically motivated disruptions are now in clinical trial, is the myb gene (reviewed in Lyon et al., 1994). These trials involve treatment of human leukemias. These diseases are a logical choice for developing oncogene targeted therapies because of easy access to tissues, and the abundance of knowledge about the cell and molecular biology of these diseases. Nevertheless, as will be touched on below, other malignancies have also been examined as models for Myb targeted therapy with some surprisingly encouraging results. Finally, while we have focused our efforts on the ODN strategy, I will allude briefly to other strategies for disrupting Myb function with therapeutic intent.

BCR-ABL Antisense Oligodeoxynucleotide In Vitro Purging and Autologous Bone Marrow Transplantation for Patients With Chronic Myelogenous Leukemia in Advanced Phase

BCR-ABL antisense oligodeoxynucleotides (ODN) have provided evidence of antileukemia effect when tested in vitro against Philadelphia-positive (Ph-pos) cells and in vivo when injected into leukemic mice. On the basis of the results obtained in vitro at diagnosis, eight patients with chronic myelogenous leukemia (CML) were selected and submitted to autologous bone marrow transplantation (ABMT) with bone marrow (BM) cells purged in vitro with junction-specific (J-sp) BCR-ABL antisense ODN at the time of transformation in accelerated phase or during second chronic phase. Mononuclear BM cells were treated in vitro for 24 or 72 hours with 150 μg/mL of antisense ODN yielding a median recovery of 47.6% mononuclear cells, 48.8% CD34+ cells, and 20.3% clonogenic cells. After a conditioning regimen including busulphan and etoposide, the reinfused treated cells allowed engraftment and hematologic reconstitution in all patients. Evaluation of the antileukemic effect by standard cytogenetic analysis and fluorescence in situ hybridization showed a complete karyotypic response in two cases and a minimal or no response in the other six. The patient autografted in second chronic phase died in blast crisis 7 months after ABMT; of the seven patients autografted in transformation, three developed blast crisis 21 to 39 months after reinfusion, one died from unrelated BMT complications 30 months after ABMT, and three are in persistent second chronic phase 14 to 26 months after autograft. The low toxicity of the protocol and the hemopoietic reconstitution observed in all patients make this approach feasible; the marked karyotypic response observed in some patients and the duration of the second chronic phase show that ODN-mediated BM purging and autograft is a promising treatment for this high-risk group of CML.

BCR-ABL Antisense Oligodeoxynucleotide In Vitro Purging and Autologous Bone Marrow Transplantation for Patients With Chronic Myelogenous Leukemia in Advanced Phase

BCR-ABL antisense oligodeoxynucleotides (ODN) have provided evidence of antileukemia effect when tested in vitro against Philadelphia-positive (Ph-pos) cells and in vivo when injected into leukemic mice. On the basis of the results obtained in vitro at diagnosis, eight patients with chronic myelogenous leukemia (CML) were selected and submitted to autologous bone marrow transplantation (ABMT) with bone marrow (BM) cells purged in vitro with junction-specific (J-sp) BCR-ABL antisense ODN at the time of transformation in accelerated phase or during second chronic phase. Mononuclear BM cells were treated in vitro for 24 or 72 hours with 150 μg/mL of antisense ODN yielding a median recovery of 47.6% mononuclear cells, 48.8% CD34+ cells, and 20.3% clonogenic cells. After a conditioning regimen including busulphan and etoposide, the reinfused treated cells allowed engraftment and hematologic reconstitution in all patients. Evaluation of the antileukemic effect by standard cytogenetic analysis and fluorescence in situ hybridization showed a complete karyotypic response in two cases and a minimal or no response in the other six. The patient autografted in second chronic phase died in blast crisis 7 months after ABMT; of the seven patients autografted in transformation, three developed blast crisis 21 to 39 months after reinfusion, one died from unrelated BMT complications 30 months after ABMT, and three are in persistent second chronic phase 14 to 26 months after autograft. The low toxicity of the protocol and the hemopoietic reconstitution observed in all patients make this approach feasible; the marked karyotypic response observed in some patients and the duration of the second chronic phase show that ODN-mediated BM purging and autograft is a promising treatment for this high-risk group of CML.

Oligodeoxyribonucleotide Uptake in Primary Human Hematopoietic Cells Is Enhanced by Cationic Lipids and Depends on the Hematopoietic Cell Subset

The use of antisense oligodeoxyribonucleotides (ODN) is a potential method to switch off gene expression. The poor cellular uptake of ODN in primary cells still is a limiting factor that may contribute to the lack of functional efficacy. Various forms of cationic lipids have been developed for efficient delivery of nucleic acids into different cell types. We examined the two cationic lipids DOTAP and DOSPER to improve uptake of ODN into primary human hematopoietic cells. Using a radiolabeled 23-mer, ODN uptake into blood-derived mononuclear cells could be increased 42- to 93-fold by DOTAP and 440- to 1,025-fold by DOSPER compared with application of ODN alone. DOTAP was also effective for delivery of ODN into leukocytes within whole blood, which may resemble more closely the in vivo conditions. As assessed by fluorescein isothiocyanate–conjugated ODN both cationic lipids enhanced cytoplasmic accumulation of ODN in endosome/lysosome-like structures with a partial shift of fluorescence to the whole cytoplasm and the nucleus following an incubation of 24 hours. ODN uptake by cationic lipids into different hematopoietic cell subsets was examined by dual-color immunofluorescence analysis with subset-specific monoclonal antibodies. We found a cell type–dependent delivery of ODN with greatest uptake in monocytes and smallest uptake in T cells. CD34+ cells, B cells, and granulocytes took up ODN at an intermediate level. Uptake of ODN into isolated CD34+cells could be increased 100- to 240-fold using cationic lipids compared with application of ODN alone. Stimulation of CD34+ cells by interleukin-3 (IL-3), IL-6, and stem cell factor did not significantly improve cationic lipid-mediated ODN delivery. Sequence-specific antisense effects in clonogenic assays could be shown by transfection of bcr-abl oncogene-directed antisense ODN into primary cells of patients with chronic myelogenous leukemia using this established protocol. In conclusion, cationic lipids may be useful tools for delivery of antisense ODN into primary hematopoietic cells. These studies provide a basis for clinical protocols in the treatment of hematopoietic cells in patients with hematologic malignancies and viral diseases by antisense ODN.

Oligodeoxyribonucleotide Uptake in Primary Human Hematopoietic Cells Is Enhanced by Cationic Lipids and Depends on the Hematopoietic Cell Subset

The use of antisense oligodeoxyribonucleotides (ODN) is a potential method to switch off gene expression. The poor cellular uptake of ODN in primary cells still is a limiting factor that may contribute to the lack of functional efficacy. Various forms of cationic lipids have been developed for efficient delivery of nucleic acids into different cell types. We examined the two cationic lipids DOTAP and DOSPER to improve uptake of ODN into primary human hematopoietic cells. Using a radiolabeled 23-mer, ODN uptake into blood-derived mononuclear cells could be increased 42- to 93-fold by DOTAP and 440- to 1,025-fold by DOSPER compared with application of ODN alone. DOTAP was also effective for delivery of ODN into leukocytes within whole blood, which may resemble more closely the in vivo conditions. As assessed by fluorescein isothiocyanate–conjugated ODN both cationic lipids enhanced cytoplasmic accumulation of ODN in endosome/lysosome-like structures with a partial shift of fluorescence to the whole cytoplasm and the nucleus following an incubation of 24 hours. ODN uptake by cationic lipids into different hematopoietic cell subsets was examined by dual-color immunofluorescence analysis with subset-specific monoclonal antibodies. We found a cell type–dependent delivery of ODN with greatest uptake in monocytes and smallest uptake in T cells. CD34+ cells, B cells, and granulocytes took up ODN at an intermediate level. Uptake of ODN into isolated CD34+cells could be increased 100- to 240-fold using cationic lipids compared with application of ODN alone. Stimulation of CD34+ cells by interleukin-3 (IL-3), IL-6, and stem cell factor did not significantly improve cationic lipid-mediated ODN delivery. Sequence-specific antisense effects in clonogenic assays could be shown by transfection of bcr-abl oncogene-directed antisense ODN into primary cells of patients with chronic myelogenous leukemia using this established protocol. In conclusion, cationic lipids may be useful tools for delivery of antisense ODN into primary hematopoietic cells. These studies provide a basis for clinical protocols in the treatment of hematopoietic cells in patients with hematologic malignancies and viral diseases by antisense ODN.