Myb targeted therapeutics for the treatment of human malignancies

c-MYB and DMTF1 in Cancer

The c-Myb gene encodes a transcription factor that regulates cell proliferation, differentiation, and apoptosis through protein-protein interaction and transcriptional regulation of signaling pathways. The protein is frequently overexpressed in human leukemias, breast cancers, and other solid tumors suggesting that it is a bona fide oncogene. c-MYB is often overexpressed by translocation in human tumors with t(6;7)(q23;q34) resulting in c-MYB-TCRβ in T cell ALL, t(X;6)(p11;q23) with c-MYB-GATA1 in acute basophilic leukemia, and t(6;9)(q22-23;p23-24) with c-MYB-NF1B in adenoid cystic carcinoma. Antisense oligonucleotides to c-MYB were developed to purge bone marrow cells to eliminate tumor cells in leukemias. Recently, small molecules that inhibit c-MYB activity have been developed to disrupt its interaction with p300. The Dmp1 (cyclin D binding myb-like protein 1; Dmtf1) gene was isolated through its virtue for binding to cyclin D2. It is a transcription factor that has a Myb-like repeat for DNA binding. The Dmtf1 protein directly binds to the Arf promoter for transactivation and physically interacts with p53 to activate the p53 pathway. The gene is hemizygously deleted in 35-42% of human cancers and is associated with longer survival. The significances of aberrant expression of c-MYB and DMTF1 proteins in human cancers and their clinical significances are discussed.

Reassessing the Potential of Myb-targeted Anti-cancer Therapy

Transcription factor MYB is essential for the tumorigenesis of multiple cancers, especially leukemia, breast cancer, colon cancer, adenoid cystic carcinoma and brain cancer. Thus, MYB has been regarded as an attractive target for tumor therapy. However, pioneer studies of antisense oligodeoxynucleotides against MYB, which were launched three decades ago in leukemia therapy, were discontinued because of their unsatisfactory clinical outcomes. In recent years, the roles of MYB in tumor transformation have become increasingly clear. Moreover, the regulatory mechanisms of MYB, such as the vital effects of MYB co-regulators on MYB activity and of transcriptional elongation on MYB expression, have been unveiled. These observations have underpinned novel approaches in inhibiting MYB. This review discusses the structure, function and regulation of MYB, focusing on recent insights into MYB-associated oncogenesis and how MYB-targeted therapeutics can be explored. Additionally, the main MYB-targeted therapies, including novel genetic therapy, RNA interference, microRNAs and low-molecular-weight compounds, which are especially promising inhibitors that target MYB co-regulators and transcriptional elongation, are described, and their prospects are assessed.

G-Quadruplex formation using fluorescent oligonucleotides as a detection method for discriminating AGG trinucleotide repeats

We have developed a simple and sensitive system for detecting AGG trinucleotide repeats through the formation of intermolecular G-quadruplexes using a fluorescent oligonucleotide. The fluorescence signal increased rapidly and dramatically by 44.7-fold with respect to the low background signal in the presence of RNA agg repeats and by 35.0-fold in the presence of DNA AGG repeats.

The formation and characteristics of the i-motif structure within the promoter of the c-myb proto-oncogene

C-myb proto-oncogene is a potential therapeutic target for some human solid tumors and leukemias. A long cytosine-rich sequence, which locates the downstream of the transcription initiation site, is demonstrated to fold into an intramolecular i-motif DNA using electrospray ionization mass spectrometry (ESI-MS) and circular dichroism (CD) spectroscopy. Effects of factors, including the pH value, the number of C:C(+) dimers, the concentration of buffer, the molecular crowding condition, and the coexistence of the complementary DNA, on the formation and the structural stability of the i-motif DNA are systematically studied. We have demonstrated that the i-motif folding in the c-myb promoter could be accelerated upon synergistic physiological stimuli including intracellular molecular crowding and low pH values, as well as the large number of the i-motif C:C(+) dimers. Meanwhile, various inputs, such as acids/bases and metal ions, have exhibited their abilities in controlling the conformational switch of the c-myb GC-rich DNA. Acidic pH values and the presence of K(+) ions can induce the dissociation of the double helix. Our present strategy can greatly extend the potential usages of i-motif DNA molecules with specific sequences as conformational switch-controlled devices. Moreover, this work demonstrates the superiority of CD spectroscopy associated with ESI-MS as a rapid, more cost-effective and sensitive structural change responsive method in the research of DNA conformational switching.

MicroRNA-193b-3p acts as a tumor suppressor by targeting the MYB oncogene in T-cell acute lymphoblastic leukemia

The MYB oncogene is a leucine zipper transcription factor essential for normal and malignant hematopoiesis. In T-cell acute lymphoblastic leukemia (T-ALL), elevated MYB levels can arise directly through T-cell receptor-mediated MYB translocations, genomic MYB duplications or enhanced TAL1 complex binding at the MYB locus or indirectly through the TAL1/miR-223/FBXW7 regulatory axis. In this study, we used an unbiased MYB 3'untranslated region-microRNA (miRNA) library screen and identified 33 putative MYB-targeting miRNAs. Subsequently, transcriptome data from two independent T-ALL cohorts and different subsets of normal T-cells were used to select miRNAs with relevance in the context of normal and malignant T-cell transformation. Hereby, miR-193b-3p was identified as a novel bona fide tumor-suppressor miRNA that targets MYB during malignant T-cell transformation thereby offering an entry point for efficient MYB targeting-oriented therapies for human T-ALL.

ESI mass spectrometric exploration of selective recognition of G-quadruplex in c-myb oncogene promoter using a novel flexible cyclic polyamide

In this research, electrospray ionization mass spectrometry (ESI-MS) was used to probe the binding selectivity of a flexible cyclic polyamide (cβ) to G-quadruplexes from the long G-rich sequences in the c-myb oncogene promoter. The results show that three G-rich sequences, including d[(GGA)3GGTCAC(GGA)4], d[(GGA)4GAA(GGA)4], and d[(GGA)3GGTCAC(GGA)4GAA(GGA)4] species in the c-myb promoter can form parallel G-quadruplexes, and cβ selectively binds towards these G-quadruplexes over both several other G-quadruplexes and the duplex DNA. These properties of cβ have profound implications on future studies of the regulation of c-myb oncogene expression.

Formation and recognition of G-quadruplex in promoter of c-myb oncogene by electrospray ionization mass spectrometry

In this study, electrospray ionization mass spectrometry (ESI-MS) is used to study the formation of G-quadruplex by d(GGAGGAGGAGGA) which locates at the promoter region of c-myb gene. In addition, a natural small molecule, dehydrocorydaline from a Chinese herb, is found to have the highest binding affinity with the G-quadruplex in nine natural small molecules studied, and the binding selectivity of this natural molecule toward the c-myb G-quadruplex with respect to corresponding duplex DNA is significantly higher than that of the broad-spectrum G-quadruplex-ligand TMPyP4. The result from ESI-MS indicates that the gas-phase kinetic stability of the G-quadruplex can be enhanced by binding of dehydrocorydaline. To further investigate the binding properties of dehydrocorydaline to the G-quadruplex, Autodock3 is used to calculate the docked sites and docked energies of small molecules binding to the G-quadruplex and the result shows that the docked energy of dehydrocorydaline is the biggest in the nine small molecules used, consistent with the result from ESI-MS.

New tricks from an old oncogene: gene fusion and copy number alterations of MYB in human cancer

MYB is a leucine zipper transcription factor that is essential for hematopoesis and for renewal of colonic crypts. There is also ample evidence showing that MYB is leukemogenic in several animal species. However, it was not until recently that clear evidence was presented showing that MYB actually is an oncogene rearranged in human cancer. In a recent study, a novel mechanism of activation of MYB involving gene fusion was identified in carcinomas of the breast and head and neck. A t(6;9) translocation was shown to generate fusions between MYB and the transcription factor gene NFIB. The fusions consistently result in loss of the 3'-end of MYB, including several highly conserved target sites for microRNAs that negatively regulate MYB expression. Deletion of these target sites may disrupt the repression of MYB, leading to overexpression of MYB-NFIB transcripts and protein and to transcriptional activation of critical MYB target genes associated with apoptosis, cell cycle control, cell growth/angiogenesis and cell adhesion. This study, together with previous and recent data showing rearrangements and copy number alterations of the MYB locus in T-cell leukemia and certain solid tumors, will be the main focus of this review.

In primary effusion lymphoma cells, MYB transcriptional repression is associated with v-FLIP expression during latent KSHV infection while both v-FLIP and v-GPCR become involved during the lytic cycle

Primary effusion lymphoma (PEL) is a rare, distinct subtype of non-Hodgkin lymphoma, which is associated with Kaposi sarcoma-associated herpesvirus (KSHV) infection. Although MYB levels are high in most neoplastic B cells, we found that, unexpectedly, both PEL cells and uncultured PEL patients' samples contained very low levels of MYB mRNA when compared to B-cell leukaemia samples obtained from KSHV(-) patients. These results were further confirmed at the protein level. Both latent viral FLICE inhibitory protein (v-FLIP) and early lytic viral G protein coupled receptor (v-GPCR) KSHV proteins were found to activate nuclear factor (NF)-kappaB and transrepress a MYB promoter reporter construct. In contrast, a dominant negative inhibitor of NF-kappaB (IkappaB-alpha) mutant prevented v-FLIP and v-GPCR from inhibiting MYB functions while a v-GPCR mutant that was impaired for NF-kappaB activation could not repress the MYB construct. Transduction of a v-FLIP expressing vector or stable transfection of v-GPCR both resulted in a marked downregulation of the endogenous MYB protein expression. However, MYB expression transactivated the lytic switch Replication and Transcription Activator (RTA) promoter in transient transfection assays. Taken together, our results demonstrate that, contrary to a number of other haematological malignancies, MYB expression is not required for PEL cell proliferation. Repressing MYB expression also helps in maintaining the virus in latency.

The C-MYB locus is involved in chromosomal translocation and genomic duplications in human T-cell acute leukemia (T-ALL), the translocation defining a new T-ALL subtype in very young children

The C-Myb transcription factor is essential for hematopoiesis, including in the T-cell lineage. The C-Myb locus is a common site of retroviral insertional mutagenesis, however no recurrent genomic involvement has been reported in human malignancies. Here, we identified 2 types of genomic alterations involving the C-MYB locus at 6q23 in human T-cell acute leukemia (T-ALL). First, we found a reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n = 6 cases). Second, a genome-wide copy-number analysis by array-based comparative genomic hybridization (array-CGH) identified short somatic duplications that include C-MYB (MYB(dup), n = 13 cases of 84 T-ALL, 15%). Expression analysis, including allele-specific approaches, showed stronger C-MYB expression in the MYB-rearranged cases compared with other T-ALLs, and a dramatically skewed C-MYB allele expression in the TCRB-MYB cases, which suggests that a translocation-driven deregulated expression may overcome a cellular attempt to down-regulate C-MYB. Strikingly, profiling of the T-ALLs by clinical, genomic, and large-scale gene expression analyses shows that the TCRB-MYB translocation defines a new T-ALL subtype associated with a very young age for T-cell leukemia (median, 2.2 years) and with a proliferation/mitosis expression signature. By contrast, the MYB(dup) alteration was associated with the previously defined T-ALL subtypes.

Nucleic acid therapeutics for hematologic malignancies--theoretical considerations

Our work is motivated by the belief that RNA targeted gene silencing agents can be developed into effective drugs for treating hematologic malignancies. In many experimental systems, antisense nucleic acids of various composition, including antisense oligodeoxynucleotides (AS ODNs) and short interfering RNA (siRNA), have been shown to perturb gene expression in a sequence specific manner. Nevertheless, our clinical experience, and those of others, have led us to conclude that the antisense nucleic acids (ASNAs) we, and others, employ need to be optimized with regard to intracellular delivery, targeting, chemical composition, and efficiency of mRNA destruction. We have hypothesized that addressing these critical issues will lead to the development of practical and effective nucleic acid drugs. An overview of our recent work which seeks to addresses these core issues is contained within this review.

RNA targeted therapeutics for hematologic malignancies

In a variety of experimental systems, antisense nucleic acids (ASNA) of various composition, including antisense oligodeoxynucleotides (ODN) and siRNA, have been shown to have the ability to variably perturb gene expression in a sequence specific manner. Pilot clinical studies from our group, and others, have demonstrated that gene silencing is a therapeutic strategy that is starting to make a real contribution to the treatment of various diseases. The development of this field, with specific reference to hematologic malignancies, is reviewed very briefly below.

Multigene targeting with antisense oligodeoxynucleotides: an exploratory study using primary human leukemia cells

PURPOSE

We previously reported that the c-myb and Vav proto-oncogenes are amenable to silencing with antisense oligodeoxynucleotides and that inhibition of either impairs leukemic cell growth. Because the expression of these genes is not known to be linked, we sought to determine the therapeutic value of silencing both genes simultaneously in K562 and primary patient (n = 9) chronic myelogenous leukemia cells.

EXPERIMENTAL DESIGN

K562 and primary chronic myelogenous leukemia cells were exposed to antisense oligodeoxynucleotides (alone or in combination) for 24 or 72 hours and then cloned in methylcellulose cultures. Effects on K562 cluster, and blast-forming unit-erythroid colonies and granulocyte-macrophage colony-forming units were determined and correlated with the ability to down-regulate the targeted mRNA.

RESULTS

After 24-hour exposure, K562 cell growth was inhibited in a sequence specific, dose-responsive manner with either c-myb or Vav antisense oligodeoxynucleotides. Exposure to both oligodeoxynucleotides simultaneously considerably enhanced growth inhibition and accelerated apoptosis. Primary cell results were more complex. After 24- and 72-hour exposures to either anti-vav or anti-myb antisense oligodeoxynucleotides, equivalent colony-forming unit inhibition was observed. Exposing cells to both antisense oligodeoxynucleotides simultaneously for 24 hours did not result in additional inhibition of colony formation. However, after 72-hour incubation with both oligodeoxynucleotides, colony formation was diminished significantly when compared with either oligodeoxynucleotides alone (from approximately 30% to approximately 78% for granulocyte-macrophage colony-forming unit; approximately 50% to approximately 80% for blast-forming unit-erythroid).

CONCLUSIONS

We hypothesize that exposing primary leukemic cells to antisense oligodeoxynucleotides targeted to two, or possibly more, genes might significantly augment the therapeutic utility of these molecules.

The theoretical basis of transcriptional therapy of cancer: can it be put into practice?

Aberrant gene silencing is a frequent event in cancer and plays a critical role in the molecular pathogenesis of malignant transformation. The two major mechanisms of silencing in cancer include transcriptional repression by mutated or aberrantly expressed transcription factors, and aberrant epigenetic silencing by hypermethylation of tumor suppressor or DNA repair-related genes. Both of these mechanisms require the activities of multiprotein chromatin remodeling and modifying machines, several of which may be mutated in cancer. The end result is genetic reprogramming of cells to express combinations of genes that confer the neoplastic phenotype. Recent discoveries in transcriptional biochemistry and gene regulation indicate that therapeutic agents can be engineered to specifically target these mechanisms. We provide a framework for the clinical or translational scientist to consider how such drugs might be developed and what their impact might be on restoring cells to normal genetic programming.

The effects of benzene and the metabolites phenol and catechol on c-Myb and Pim-1 signaling in HD3 cells

Exposure to the environmental toxicant benzene has been proposed to lead to leukemogenesis. The transcription factor c-Myb plays a role in blood cell differentiation and can be regulated by the serine-threonine kinase Pim-1. Overexpressed versions of c-Myb and Pim-1 are believed to play a key role in the development of a wide variety of leukemias and tumors. In our study, we evaluated the effects of benzene and the metabolites catechol and phenol on c-Myb signaling to investigate our hypothesis that benzene exerts its toxicity by interfering with this pathway. To evaluate this hypothesis, HD3 chicken erythroblast cells were transiently transfected with a c-Myb responsive luciferase reporter plasmid and then exposed to benzene, catechol, or phenol (0-300 microM) for 1-24 h before nonproprietary dual luciferase activities were measured. Our results demonstrated that catechol exposure caused a time- and concentration-dependent increase in c-Myb activity with significance occurring at 100 and 300 microM after 24 h of exposure, which was independent of increased Pim-1 protein, but dependent on increased c-Myb phosphorylation. Benzene and phenol exposure resulted in small but significant decreases in c-Myb activity that were not dose- and time-dependent, nor was increased Pim-1 protein involved. These results are consistent with other studies, which suggest metabolite differences in benzene-mediated toxicity. More importantly, this study supports the hypothesis that benzene may mediate its toxicity through metabolite-mediated alterations in the c-Myb signaling pathway.

Germline hMLH1 promoter mutation in a Newfoundland HNPCC kindred

Hereditary non-polyposis colon cancer (HNPCC) is an autosomal dominant form of inherited predisposition to colorectal and other malignancies. It is associated with mutations in DNA mismatch-repair genes, especially hMSH2 and hMLH1. Management of HNPCC families is improved if the underlying mutation in each family can be discovered. We describe a Newfoundland kindred, meeting the Amsterdam Criteria for HNPCC, in which a mutation in the promoter region of the hMLH1 gene co-segregates with the disease phenotype. The -42C > T mutation is within a putative Myb proto-oncogene binding site. Using electrophoretic mobility shift assays, we demonstrated that the mutated Myb binding sequence is less effective in binding nuclear proteins than the wild-type promoter sequence. Using in vivo transfection experiments in HeLa cells, we further demonstrated that the mutated promoter has only 37% of the activity of the wild-type promoter in driving the expression of a reporter gene. The average age of onset in six family members affected with colorectal cancer is 62 years, which is substantially later than the typical age of onset in HNPCC families. This is consistent with a substantial decrease, but not total elimination, of mismatch repair function in affected members of this family. This is the first report of a heritable hMLH1 promoter mutation in any HNPCC family.

Specific protein redirection as a transcriptional therapy approach for t(8;21) leukemia

Important progress has been achieved in the knowledge about the pathogenesis of cancer. However, despite these advances, the therapeutic strategies are still limited. Leukemias are often characterized by specific balanced translocations, with the t(8;21) balanced translocation being the most frequent chromosomal aberration in acute myeloid leukemia (AML). This translocation produces the AML1-ETO fusion protein, which binds to AML1 target promoter sequences. Transcriptional repression of AML1-dependent genes by AML1-ETO and associated corepressors represents the pathogenetic mechanisms of t(8;21). Here, we show that targeting of AML1-ETO to essential, MYB-dependent gene promoters induces t(8;21)-restricted cell death. We constructed a chimeric protein that contained the MYB DNA-binding domain and the AML1-binding domain of myeloid Elf-1-like factor (MEF). This protein associated with AML1-ETO and directed the complex to MYB-responsive promoters in vitro and in vivo. In the presence of AML1-ETO, the chimeric protein repressed the activity of MYB-responsive promoters, rapidly induced apoptosis, and specifically inhibited colony growth. All these effects occurred only in AML1-ETO-positive cells, whereas no adverse effects were observed in cells not expressing AML1-ETO. Taken together, this study demonstrates that redirection of oncogenic proteins can be used as a strategy to dramatically influence their cellular effects, with the ultimate goal to design highly specific therapies for cancer.

The quest for an efficacious antiviral for respiratory syncytial virus

Respiratory syncytial virus (RSV) continues as an emerging infectious disease not only among infants and children, but also for the immune-suppressed, hospitalized and the elderly. To date, ribavirin (Virazole) remains the only therapeutic agent approved for the treatment of RSV. The prophylactic administration of palivizumab is problematic and costly. The quest for an efficacious RSV antiviral has produced a greater understanding of the viral fusion process, a new hypothesis for the mechanism of action of ribavirin, and a promising antisense strategy combining the 2'-5' oligoadenylate antisense (2-5A-antisense) approach and RSV genomics.

Artificial neural network prediction of antisense oligodeoxynucleotide activity

An mRNA transcript contains many potential antisense oligodeoxynucleotide target sites. Identification of the most efficacious targets remains an important and challenging problem. Building on separate work that revealed a strong correlation between the inclusion of short sequence motifs and the activity level of an oligo, we have developed a predictive artificial neural network system for mapping tetranucleotide motif content to antisense oligo activity. Trained for high-specificity prediction, the system has been cross-validated against a database of 348 oligos from the literature and a larger proprietary database of 908 oligos. In cross- validation tests the system identified effective oligos (i.e. oligos capable of reducing target mRNA expression to <25% that of the control) with 53% accuracy, in contrast to the <10% success rates commonly reported for trial-and-error oligo selection, suggesting a possible 5-fold reduction in the in vivo screening required to find an active oligo. We have implemented a web interface to a trained neural network. Given an RNA transcript as input, the system identifies the most likely oligo targets and provides estimates of the probabilities that oligos targeted against these sites will be effective.

Induction of apoptosis in K562 cells by dominant negative c-myb

OBJECTIVE

The aberrant expression of c-myb in leukemic cells suggests that c-myb may play an important role in leukemogenesis. Therefore, disrupting c-myb function might provide a strategy for controlling leukemic cell growth. Use of dominant negative mutants as a strategy for inhibiting oncogene function has attracted considerable attention. The aim of this study was to induce apoptosis in K562 cells by dominant negative c-myb (DN-myb).

MATERIALS AND METHODS

We constructed a DN-myb plasmid containing the DNA-binding domain of c-myb and transfected the dominant negative mutant, like its wild-type (WT) counterpart, into K562 cells. Consequently, cell viability and induction of apoptosis were measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, nuclear condensation, DNA fragmentation, and Western hybridization analysis for expression of poly(ADP-ribose) polymerase. In addition, the effect of DN-myb on bcl-2 promoter activity and expression of bcl-2 and bcr-abl was studied.

RESULTS

We observed that DN-myb, cotransfected with WT c-myb and a chloramphenicol acetyltransferase reporter construct containing the bcl-2 promoter, bound competitively to the bcl-2 promoter and significantly decreased the activation of chloramphenicol acetyltransferase induced by WT c-myb. Moreover, the inactivation of transcription induced by DN-myb reduced not only the expression of bcl-2 but also the expression of bcr-abl. Further functional studies focused on the effect of DN-myb on the induction of apoptosis in K562 cells. Transfection of DN-myb into K562 cells caused a significant reduction in cell proliferation when cells were exposed to low concentrations of DNA-damaging agents (approximately 30% of control) and remarkably increased apoptosis.

CONCLUSIONS

Our data demonstrate that disruption of c-myb function by dominant negative c-myb is an effective strategy to induce apoptosis of leukemic cells. The results of these studies support the thesis that dominant negative c-myb gene therapy may be useful for treatment of leukemia patients.

From molecular biology to nanotechnology and nanomedicine

Great progress in the development of molecular biology techniques has been seen since the discovery of the structure of deoxyribonucleic acid (DNA) and the implementation of a polymerase chain reaction (PCR) method. This started a new era of research on the structure of nucleic acids molecules, the development of new analytical tools, and DNA-based analyses. The latter included not only diagnostic procedures but also, for example, DNA-based computational approaches. On the other hand, people have started to be more interested in mimicking real life, and modeling the structures and organisms that already exist in nature for the further evaluation and insight into their behavior and evolution. These factors, among others, have led to the description of artificial organelles or cells, and the construction of nanoscale devices. These nanomachines and nanoobjects might soon find a practical implementation, especially in the field of medical research and diagnostics. The paper presents some examples, illustrating the progress in multidisciplinary research in the nanoscale area. It is focused especially on immunogenetics-related aspects and the wide usage of DNA molecules in various fields of science. In addition, some proposals for nanoparticles and nanoscale tools and their applications in medicine are reviewed and discussed.

[Contribution of antineoplastic biotherapy in the treatment of leukemia in children]

Improvements in the chemotherapeutic and transplant regimens have had a significant impact in improving survival rates for pediatric leukemia. However, there are still major problems to address including what options are available for patients with chemoresistant disease and what strategies are available to avoid toxicity associated with highly cytotoxic treatment regimens. Gene and immunotherapy protocols hold great promise. Using gene transfer of a marker gene, a number of biologic issues in the therapy of leukemia have been addressed. For example, by gene marking autologous bone marrow grafts it has been possible to demonstrate that infused marrow contributes to relapse in acute and chronic myeloid leukemias. In the allogeneic transplant setting, genetically modified T-cells have proven valuable for the prophylaxis and treatment of viral diseases and may have an important role in preventing or treating disease relapse. Gene transfer is also being used to modify tumor function, enhance immunogenicity, and confer drug-resistance to normal hematopoietic stem cells. With the continued scientific advancements in this field, gene therapy will almost certainly have a major impact on the treatment of pediatric leukemia in the future.

Oligodeoxynucleotide-mediated inhibition of c-myb gene expression in autografted bone marrow: a pilot study

Antisense oligodeoxynucleotide (ODN) drugs might be more effective if their delivery was optimized and they were targeted to short-lived proteins encoded by messenger RNA (mRNA) species with equally short half-lives. To test this hypothesis, an ODN targeted to the c-myb proto-oncogene was developed and used to purge marrow autografts administered to allograft-ineligible chronic myelogenous leukemia patients. CD34(+) marrow cells were purged with ODN for either 24 (n = 19) or 72 (n = 5) hours. After purging, Myb mRNA levels declined substantially in approximately 50% of patients. Analysis of bcr/abl expression in long-term culture-initiating cells suggested that purging had been accomplished at a primitive cell level in more than 50% of patients and was ODN dependent. Day-100 cytogenetics were evaluated in surviving patients who engrafted without infusion of unmanipulated "backup" marrow (n = 14). Whereas all patients were approximately 100% Philadelphia chromosome-positive (Ph(+)) before transplantation, 2 patients had complete cytogenetic remissions; 3 patients had fewer than 33% Ph(+) metaphases; and 8 remained 100% Ph(+). One patient's marrow yielded no metaphases, but fluorescent in situ hybridization evaluation approximately 18 months after transplantation revealed approximately 45% bcr/abl(+) cells, suggesting that 6 of 14 patients had originally obtained a major cytogenetic response. Conclusions regarding clinical efficacy of ODN marrow purging cannot be drawn from this small pilot study. Nevertheless, these results lead to the speculation that enhanced delivery of ODN, targeted to critical proteins of short half-life, might lead to the development of more effective nucleic acid drugs and the enhanced clinical utility of these compounds in the future.

Human T-cell lymphotropic virus type 1 Tax represses c-Myb-dependent transcription through activation of the NF-kappaB pathway and modulation of coactivator usage

The proto-oncogene c-myb is essential for a controlled balance between cell growth and differentiation. Aberrant c-Myb activity has been reported for numerous human cancers, and enforced c-Myb transcription can transform cells of lymphoid origin by stimulating cellular proliferation and inhibiting apoptotic pathways. Here we demonstrate that activation of the NF-kappaB pathway by the HTLV-1 Tax protein leads to transcriptional inactivation of c-Myb. This conclusion was supported by the fact that Tax mutants unable to stimulate the NF-kappaB pathway could not inhibit c-Myb transactivating functions. In addition, inhibition of Tax-mediated NF-kappaB activation by coexpression of IkappaBalpha restored c-Myb transcription, and Tax was unable to block c-Myb transcription in a NEMO knockout cell line. Importantly, physiological stimuli, such as signaling with the cellular cytokines tumor necrosis factor alpha, interleukin 1 beta (IL-1beta), and lipopolysaccharide, also inhibited c-Myb transcription. These results uncover a new link between extracellular signaling and c-Myb-dependent transcription. The mechanism underlying NF-kappaB-mediated repression was identified as sequestration of the coactivators CBP/p300 by RelA. Interestingly, an amino-terminal deletion form of p300 lacking the C/H1 and KIX domains and unable to bind RelA retained the ability to stimulate c-Myb transcription and prevented NF-kappaB-mediated repression.

Psoralen-modified clamp-forming antisense oligonucleotides reduce cellular c-Myc protein expression and B16-F0 proliferation

The c-myc protooncogene plays an important role in the abnormal growth pattern of melanoma cells. In an attempt to inhibit c-Myc expression and the growth of an established murine melanoma cell line, we targeted homopurine sequences within the mouse myc mRNA with modified antisense oligonucleotides (AS ODNs). Psoralen was conjugated to the 5'-end of these clamp-forming oligonucleotides (clamp ODNs). Gel mobility shift analysis demonstrated a sequence-specific interaction between the active clamp ODNs (Myc-E2C and Myc-E3C) and the 1.4 kb c-myc mRNA, but no interaction with the control clamp ODN (SCR**). This association was further confirmed by thermal denaturation studies. In vitro translation assays demonstrated that both Myc-E2C and Myc-E3C at 5 microM inhibited c-Myc expression >99% after UV activation at 366 nm. Immunostaining of B16-F0 cells with a c-Myc monoclonal antibody revealed a significant reduction in c-Myc after clamp ODN treatment compared with the untreated or SCR** control-treated cells. This result was corroborated by western blot analysis. Utilizing the MTT assay to determine the effects of ODN-mediated c-Myc reduction on B16-F0 growth, we observed 60 and 64% reductions in growth after treatment with 5 microM Myc-E3C and Myc-E2C, respectively. We attribute the enhanced effectiveness of the clamp ODNs to psoralen activation. Our preliminary data suggest that inhibiting c-Myc overexpression results in a significant reduction in abnormal proliferation of B16-F0 melanoma cells and that the increased efficiency of clamp ODNs may provide an important advantage for their use in antisense therapies.

Gene therapy for paediatric leukaemia

Improvements in the chemotherapeutic and transplant regimens have had a significant impact in improving survival rates for paediatric leukaemia. However, there are still important problems to address including what options are available for patients with chemoresistant disease and what strategies are available to avoid the concerns regarding the toxicity associated with highly cytotoxic treatment regimens. Gene therapy and immunotherapy protocols hold great promise. Using gene transfer of a marker gene, a number of biological issues in the therapy of leukaemia have been addressed. For example, by gene marking autologous bone marrow grafts it has been possible to demonstrate that infused marrow contributes to relapse in acute and chronic myeloid leukaemias. In the allogeneic transplant setting, genetically modified T-cells have proven valuable for the prophylaxis and treatment of viral diseases and may have an important role in preventing or treating disease relapse. Gene transfer is also being used to modify tumour function, enhance immunogenicity, and confer drug-resistance to normal haematopoietic stem cells. With the continued scientific advancements in this field, gene therapy will almost certainly have a major impact on the treatment of paediatric leukaemia in the future.

Gene transfer and the treatment of haematological malignancy

Gene therapy offers an additional therapeutic modality for treating haematological malignancy. Because gene therapies could be truly specific for the malignancy, they should ultimately prove both safe and effective. We have far to go before this full potential is realized, but gene transfer strategies are already showing therapeutic promise. Gene transfer may be used to correct the genetic defect in the tumour, to render it more susceptible to conventional therapies, or the normal host cells more resistant, to induce or amplify an antitumour immune response, or simply as a means of tracking the tumour or cells used for treatment. This article describes examples of each approach and discusses future prospects for the field.

Characterization of direct readout contacts of the Myb DNA-binding domain

The Myb protein contacts its recognition sequence by means of direct protein-DNA interactions. We used site-directed mutagenesis in order to substitute amino acids crucial for these contacts and probed the mutant proteins for their DNA-binding and transactiving activities. We could show that amino acids involved in direct readout contacts do not contribute equivalently in the recognition process.

Intra- and intermolecular triplex DNA formation in the murine c-myb proto-oncogene promoter are inhibited by mithramycin

Mithramycin inhibits transcription by binding to G/C-rich sequences, thereby preventing regulatory protein binding. However, it is also possible that mithramycin inhibits gene expression by preventing intramolecular triplex DNA assembly. We tested this hypothesis using the DNA triplex adopted by the murine c-myb proto-oncogene. The 5'-regulatory region of c-myb contains two polypurine:polypyrimidine tracts with imperfect mirror symmetry, which are highly conserved in the murine and human c-myb sequences. The DNA binding drugs mithramycin and distamycin bind to one of these regions as determined by DNase I protection assay. Gel mobility shift assays, nuclease and chemical hypersensitivity and 2D-gel topological analyses as well as triplex-specific antibody binding studies confirmed the formation of purine*purine:pyrimidine inter- and pyrimidine*purine:pyrimidine intra-molecular triplex structures in this sequence. Mithramycin binding within the triplex target site displaces the major groove-bound oligonucleotide, and also abrogates the supercoil-dependent H-DNA formation, whereas distamycin binding had no such effects. Molecular modeling studies further support these observations. Triplex-specific antibody staining of cells pretreated with mithramycin demonstrates a reversal of chromosomal triplex structures compared to the non-treated and distamycin-treated cells. These observations suggest that DNA minor groove-binding drugs interfere with gene expression by precluding intramolecular triplex formation, as well as by physically preventing regulatory protein binding.