Tumor-Specific Antisense Therapy to Inhibit Bcl-2 Expression and Induce Apoptosis for Treatment of Non-Hodgkin's Lymphoma
Topics in This Section
Several hematologic malignancies possess chromosomal translocations which represent targets for novel therapeutic strategies. Examples include chronic myelogenous leukemia with a t(9;22) (the Philadelphia chromosome) chromosomal translocation, acute myelogenous leukemia (t(8;21) translocation), acute promyelocytic leukemia (t(15;17)), acute lymphoblastic - T cell type(t(11;14)), and acute lymphoblastic - B cell type((8;14 or 8;22)). These translocations result in fusion genes that appear to have oncogenic properties. Several laboratories have designed methods utilizing antisense technology to inhibit expression of the oncogenic member of the fusion gene pair. A major drawback, however, is that the gene is also inhibited in normal cells, often resulting in deleterious side-effects. If one can inhibit tumor growth by targeting non-oncogenic sequences rather than just the breakpoints of fusion transcripts, the potential for tumor-specific genetic manipulation would be greatly enhanced. As long as the non-oncogenic fusion partner is itself not critical for growth, its down-regulation is likely to be relatively non-toxic to normal cells.
Low grade non-Hodgkin's lymphoma, which include most follicular lymphomas and are characterized by a t(14;18) chromosomal translocation, cannot be cured with currently available therapies. The translocation results in juxtaposition the 3' untranslated region of the bcl-2 gene on chromosome 18 with the immunoglobulin (Ig) heavy chain J region of chromosome 14 creating a fused messenger RNA encoding a full-length bcl-2 protein, but whose expression is under the control of Ig regulatory sequences. The end result is an aberrant pattern of bcl-2 expression, usually expressed at much higher levels due to the strong immunoglobulin promoter. In an animal model that can be used for studying follicular lymphoma, transgenic mice overexpressing the fusion gene accumulate large numbers of small B lymphocytes that mimic these low-grade lymphomas in many ways. This observation supports the idea that dysregulated bcl-2 expression plays an important role in the pathogenesis of non-Hodgkin's lymphomas.
Due to its role in apoptosis as well as its oncogenic activity, inhibition of bcl-2 has been the focus of considerable research as a means of inhibiting tumor cell proliferation. One approach that has received wide-spread attention is using antisense oligonucleotides. Initial efforts using oligonucleotides directed against the bcl-2 promoter region successfully inhibited expression. However, as expression was also repressed in normal cells, inhibiting the bcl-2 promoter region is not optimal for specifically targeting malignant cells.
Fox Chase researchers have developed a method for treating non-Hodgkin's lymphoma by which the non-oncogenic partner in the t(14;18) translocation is targeted for inhibition of expression by antisense oligodeoxyribonucleotides.
Applications and Advantages:
Utilizing the immunoglobulin sequences in the fused messenger mRNA results in tumor- specific cell death with minimal toxicity to normal cells. In addition, since breakpoints can be variable from patient to patient, antisense oligonucleotides targeted to the Ig sequences abrogates the need for customizing treatment for each patient's particular breakpoint sequence. Further, this technique can be applied to targeting chromosomal translocations associated with other malignancies.
The Fox Chase tumor-specific antisense oligonucleotide will provide the means to purge malignant bone marrow or peripheral stem cells isolated from patients with lymphomas carrying the t(14;18) translocation. Normal stem cells without the translocation can then be reinfused. New, more effective delivery vehicles will lead to an in vivo therapy based on tumor-specific antisense oligonucleotides. As additional tumor-specific fusion transcripts are sequenced and characterized, this approach will be useful against broad array of lymphomas, leukemias, and other cancers.
- Smith MR, "Non-Hodgkin's lymphoma," Current Problems in Cancer (1996); 20(1):6-77
- Smith MR, Abubakr Y, et al., "Antisense oligodeoxyribonucleotide down-regulation of bcl-2 gene expression inhibits growth of the low-grade non-Hodgkin's lymphoma cell line WSU-FSCCL," Cancer Gene Therapy (1995); 2(3):207-212
U.S Patent 5,935,937: Compositions and methods for inducing apoptosis