Retroviral Vector Design for Gene Therapy of Cancer: Specific Inhibition and Tagging of BCR-ABLp190 Cells

Cancer development induced by graded expression of Snail in mice

The zinc-finger transcription factor Snail is believed to trigger epithelial-mesenchymal transitions (EMTs) during cancer progression. This idea is supported by analysis of Snail knockout mice, which uncovered crucial role of Snail in gastrulation, and of individuals with cancer, in whom Snail expression is frequently upregulated. However, these results have not shown a direct link between Snail and the pathogenesis of cancer. Here we show that mice carrying hypomorphic tetracycline-repressible Snail transgenes, that increase Snail expression to 20% above normal levels, exhibit no morphological alterations and develop both epithelial and mesenchymal tumours (leukaemias). Suppression of the Snail transgene did not rescue the malignant phenotype, indicating that alterations induced by Snail are irreversible. CombitTA-Snail murine embryonic fibroblasts show similar migratory ability to that of control mouse embryonic fibroblasts (MEFs). However, CombitTA-Snail-MEFs induce tumour formation in nude mice. CombitTA-Snail expression results in increased radioprotection in vivo, although it does not affect p53 regulation in response to DNA damage. In concert with these results, Snail expression is repressed following DNA damage. This regulation of Snail by DNA damage is p53-independent. Our results connect DNA damage with the requirement of a critical level of an EMT regulator and provide genetic evidence that Snail plays essential roles in cancer development in mammals and thereby influences cell fate in the genotoxic stress response.

Philadelphia-positive B-cell acute lymphoblastic leukemia is initiated in an uncommitted progenitor cell

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the c-ABLgene on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, BCR-ABLp190 and BCR-ABLp210, are produced that are characteristic of chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (Ph1-ALL) respectively. In CML, it is evident that the transformation occurs at the level of pluripotent stem cells. However, Ph1-ALL has been thought to affect progenitor cells with lymphoid differentiation. Recently, it has been demonstrated that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation in Ph1-ALL. In this review, we discuss what is known about the relationship between the specific BCR-ABLp190 oncogene, the target cell and the characteristics of the subsequent disease process it causes. We also discuss how this information may be applied to the establishment of new directions in therapy.

A primitive hematopoietic cell is the target for the leukemic transformation in human Philadelphia-positive acute lymphoblastic leukemia

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the chromosomal counterpart (c-ABLgene) on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, BCR-ABLp190 and BCR-ABLp210, are produced that are characteristic of chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph1-ALL). In CML, the transformation occurs at the level of pluripotent stem cells. However, Ph1-ALL is thought to affect progenitor cells with lymphoid differentiation. Here we demonstrate that the cell capable of initiating human Ph1-ALL in non-obese diabetic mice with severe combined immunodeficiency disease (NOD/SCID), termed SCID leukemia–initiating cell (SL-IC), possesses the differentiative and proliferative capacities and the potential for self-renewal expected of a leukemic stem cell. The SL-ICs from all Ph1-ALL analyzed, regardless of the heterogeneity in maturation characteristics of the leukemic blasts, were exclusively CD34+CD38−, which is similar to the cell-surface phenotype of normal SCID-repopulating cells. This indicates that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation in Ph1-ALL.

In vivo inhibition by a site-specific catalytic RNA subunit of RNase P designed against the BCR-ABL oncogenic products: a novel approach for cancer treatment

One major obstacle to the effective treatment of cancer is to distinguish between tumor cells and normal cells. The chimeric molecules created by cancer-associated chromosomal abnormalities are ideal therapeutic targets because they are unique to the disease. We describe the use of a novel approach based on the catalytic RNA subunit of RNase P to destroy specifically the tumor-specific fusion genes created as a result of chromosome abnormalities. Using as a target model the abnormal BCR-ABL p190 and p210 products, we constructed M1-RNA with guide sequences that recognized the oncogenic messengers at the fusion point (M1-p190-GS and M1-p210-GS). To test the effectiveness and the specificity of M1-p190-GS and M1-p210-GS, we studied in vitro and in vivo effects of these RNA enzymes againstBCR-ABLp190 andBCR-ABLp210, bearing in mind that both fusion genes share the ABL sequence but differ in the sequence coming from the BCR gene. We showed that M1-p190-GS and M1-p210-GS can act as sequence-specific endonucleases and can exclusively cleave target RNA that forms a base pair with the guide sequence (GS). We also demonstrated that when M1-p190-GS and M1-p210-GS were expressed in proper mammalian cell models, they abolished the effect of BCR-ABL by specifically decreasing the amount of the target BCR-ABL mRNA and preventing the function of theBCR-ABL oncogenes. These data clearly demonstrate the usefulness of the catalytic activity of M1-GS RNA to cleave specifically the chimeric molecules created by chromosomal abnormalities in human cancer and to represent a novel approach to cancer treatment.

Consequences of chromosomal abnormalities in tumor development

This article highlights recent advances in the molecular structure and function of proteins that are activated or created by chromosomal abnormalities and discusses their possible role in tumor development. The molecular characterization of these proteins has revealed that tumor-specific fusion proteins are the consequence of most chromosome translocations associated with leukemias and solid tumors. An emerging common theme is that creation of these proteins disrupts the normal development of tumor-specific target cells by blocking apoptosis. These insights identify these chromosomal translocation-associated genes as potential targets for improved cancer therapies.

Murine hematopoietic reconstitution after tagging and selection of retrovirally transduced bone marrow cells

A major problem facing the effective treatment of patients with cancer is how to get the specific antitumor agent into every tumor cell. In this report we describe the use of a strategy that, by using retroviral vectors encoding a truncated human CD5 cDNA, allows the selection of only the infected cells, and we show the ability to obtain, before bone marrow transplantation, a population of 5-fluouracil-treated murine bone marrow cells that are 100% marked. This marked population of bone marrow cells is able to reconstitute the hematopoietic system in lethally irradiated mice, indicating that the surface marker lacks deleterious effects on the functionality of bone marrow cells. No gross abnormalities in hematopoiesis were detected in mice repopulated with CD5-expressing cells. Nevertheless, a significant proportion of the hematopoietic cells no longer expresses the surface marker CD5 in the 9-month-old recipient mice. This transcriptional inactivity of the proviral long terminal repeat (LTR) was accompanied by de novo methylation of the proviral sequences. Our results show that the use of the CD5 as a retrovirally encoded marker enables the rapid, efficient, and nontoxic selection in vitro of infected primary cells, which can entirely reconstitute the hematopoietic system in mice. These results should now greatly enhance the power of studies aimed at addressing questions such as generation of cancer-negative hematopoiesis.