Anti-oncogene ribozymes for cancer gene therapy

Strategies for reversing drug resistance

Drug resistance, intrinsic or acquired, is a problem for all chemotherapeutic agents. In this review, we examine numerous strategies that have been tested or proposed to reverse drug resistance. Included among these strategies are approaches targeting the apoptosis pathway. Although the process of apoptosis is complex, it provides several potential sites for therapeutic intervention. A variety of targets and approaches are being pursued, including the suppression of proteins inhibiting apoptosis using antisense oligonucleotides (ASOs), and small molecules targeted at proteins that modulate apoptosis. An alternate strategy is based on numerous studies that have documented methylation of critical regions in the genome in human cancers. Consequently, efforts have been directed at re-expressing genes, including genes that affect drug sensitivity, using 5-azacytidine and 2'-deoxy-5-azacytidine (DAC, decitabine) as demethylating agents. While this strategy may be effective as a single modality, success will most likely be achieved if it is used to modulate gene expression in combination with other modalities such as chemotherapy. At a more basic level, attempts have been made to modulate glutathione (GSH) levels. Owing to its reactivity and high intracellular concentrations, GSH has been implicated in resistance to several chemotherapeutic agents. Several approaches designed to deplete intracellular GSH levels have been pursued including the use of buthionine-(S,R)-sulfoxime (BSO), a potent and specific inhibitor of gamma-glutamyl cysteine synthetase (gamma-GCS), the rate-limiting step in the synthesis of GSH, a hammerhead ribozyme against gamma-GCS mRNA to downregulate specifically its levels and targeting cJun expression to reduce GSH levels. Alternate strategies have targeted p53. The frequent occurrence of p53 mutations in human cancer has led to the development of numerous approaches to restore wild-type (wt) p53. The goals of these interventions are to either revert the malignant phenotype or enhance drug sensitivity. The approach most extensively investigated has utilized one of several viral vectors. An alternate approach, the use of small molecules to restore wt function to mutant p53, remains an option. Finally, the conceptually simplest mechanism of resistance is one that reduces intracellular drug accumulation. Such reduction can be effected by a variety of drug efflux pumps, of which the most widely studied is P-glycoprotein (Pgp). The first strategy utilized to inhibit Pgp function relied on the identification of non-chemotherapeutic agents as competitors. Other approaches have included the use of hammerhead ribozymes against the MDR-1 gene and MDR-1-targeted ASOs. Although modulation of drug resistance has not yet been proven to be an effective clinical tool, we have learned an enormous amount about drug resistance. Should we succeed, these pioneering basic and clinical studies will have paved the road for future developments.

Hammerhead ribozymes as therapeutic agents for bladder cancer

Hammerhead ribozymes have been investigated extensively as therapeutic agents against cancer. Aberrant or overexpression of genes related to tumorigenicity or cancer growth might be the appropriate targets for ribozyme strategies. Ribozyme-mediated gene therapy should be applied to those diseases that have no successful conventional therapy such as advanced or treatment-resistant bladder cancer. Many genetic alterations have been identified in bladder cancer related to both tumorigenesis and disease progression. Mutated H-ras, fos, and erb-B2 genes have been chosen as targets for ribozymes in previous studies, and antitumor efficacy has been demonstrated by reversion of the malignant phenotypes and by inhibition of tumor growth both in vitro and in vivo. The efficiency of various delivery systems has also been evaluated. An overview of ribozyme strategies, especially for therapeutic applications against bladder cancer, is described here.

Nucleic-acid therapeutics: basic principles and recent applications

The sequencing of the human genome and the elucidation of many molecular pathways that are important in disease have provided unprecedented opportunities for the development of new therapeutics. The types of molecule in development are increasingly varied, and include antisense oligonucleotides and ribozymes. Antisense technology and catalytic nucleic-acid enzymes are important tools for blocking the expression of abnormal genes. One FDA-approved antisense drug is already in the clinic for the treatment of cytomegalovirus retinitis, and other nucleic-acid therapies are undergoing clinical trials. This article reviews different strategies for modulating gene expression, and discusses the successes and problems that are associated with this type of therapy.

Malignancy: Gene Therapy Vaccines in Acute Myeloid Leukemia : A Need for Clinical Evaluation

In the last decade our understanding of the processes that govern cell growth and differentiation, malignant transformation, and metastasis has become quite sophisticated. These new insights have revolutionized our ability to diagnose and to formulate prognoses for patients with cancer, and have inspired the design and development of novel therapeutic strategies that are based on modern gene-transfer technologies and act at the gene level. Gene therapy, broadly defined as the introduction of genetic material (transgenes) into a patient's cells with an intent to confer a therapeutic benefit, represents the most direct application of recombinant DNA technology in the clinical setting. The challenging concept of modifying the genetic properties of human cells captivated very quickly the interest of clinical and molecular oncologists, and currently, numerous gene therapy clinical trials in cancer patients are under investigation worldwide. Most of these studies involve manipulating the patient's immune response to tumors. The identification of tumor-specific antigens stimulating humoral and cellular responses in cancer patients, together with a better understanding of the molecular mechanisms controlling T cell activation have dramatically accelerated the search for potent cancer vaccines. In this review, we highlight important principles of cancer immunity and cancer vaccines, we discuss critical features of genetic manipulation of tumor cells, and particularly focus on preclinical studies on gene therapy vaccines in acute myeloid leukemia (AML).

Interleukin-12 gene therapy vaccines: directing the immune system against minimal residual leukemia

Current overall survival rates for patients with AML remain poor and there is need for novel therapeutic approaches. One such approach is to use the patient's own immune system to eliminate minimal residual disease. Recent advances in genetic manipulation of tumor cells, together with a better understanding of the immune mechanisms controlling the host-tumor relationship have led to a flurry of preclinical and clinical studies on tumor cell vaccines. Here we present a brief overview of genetic manipulation of tumor cells, and highlight important principles of cancer immunity and cancer vaccines. Special emphasis is given on recent work on the role of interleukin-12 (IL-12) based vaccines in murine AML. These studies have shown that vaccines with AML cells, genetically modified to secrete IL-12, are potent stimulators of the immune system and lead to the development of both prophylactic and therapeutic anti-leukemia immunity.

Inhibition of telomerase activity by a hammerhead ribozyme targeting the RNA component of telomerase in human melanoma cells

Reactivation of telomerase, an RNA-dependent DNA polymerase that synthesizes new telomeric repeats at the end of chromosomes, is a very common feature in human cancers. Telomerase is thought to be essential in maintaining the proliferative capacity of tumor cells and, as a consequence, it could represent an attractive target for new anti-cancer therapies. In this study, we generated a hammerhead ribozyme composed of a catalytic domain with flanking sequences complementary to the RNA component of human telomerase and designed to cleave specifically a site located at the end of the telomerase template sequence. In vitro the ribozyme induced cleavage of a synthetic RNA substrate obtained by cloning a portion of the RNA component of human telomerase. The extent of cleavage was dependent on the ribozyme/substrate ratio as well as the Mg2+ concentration. Moreover, when added to cell extracts from two human melanoma cell lines (JR8 and M14), or three melanoma surgical specimens, the ribozyme inhibited telomerase activity in a concentration- and time-dependent manner. When the ribozyme was delivered to growing JR8 melanoma cells by (N-(1-(2,3 dioleoxyloxy)propil)-N,N,N trimethylammonium methylsulfate-mediated transfer, a marked inhibition of telomerase activity was observed. Next, the ribozyme sequence was cloned in an expression vector and JR8 cells were transfected with it. The cell clones obtained showed a reduced telomerase activity and telomerase RNA levels and expressed the ribozyme. Moreover, ribozyme transfectants had significantly longer doubling times than control cells and showed a dendritic appearance in monolayer culture. No telomere shortening, however, was observed in these clones. Overall, our results indicate that the hammerhead ribozyme is a potentially useful tool for the inactivation of telomerase in human tumors.

Ribozyme as an approach for growth suppression of human pancreatic cancer

Ribozymes (catalytic RNAs, RNA enzymes) are effective modulators of gene expression because of their simple structure, site-specific cleavage activity, and catalytic potential, and have potentially important implications for cancer gene therapy. Point mutations in the K-ras oncogene are found in approx 90% of human pancreatic carcinomas, and can be used as potential targets for specific ribozyme-mediated reversal of the malignant phenotype. In this study, we focused on in vitro manipulation of ribozyme targeting of the mutated K-ras oncogene in a human pancreatic carcinoma cell line. We evaluated the efficacy of an anti-K-ras hammerhead ribozyme targeted against GUU-mutated codon 12 of the K-ras gene in cultured pancreatic carcinoma cell lines. The anti-K-ras ribozyme significantly reduced cellular K-ras mRNA level (GUU-mutated codon 12) when the ribozyme was transfected into the Capan-1 pancreatic carcinoma cells. The ribozyme inhibited proliferation of the transfected Capan-1 cells. These results suggested that this ribozyme is capable of reversing the malignant phenotype in human pancreatic carcinoma cells.

Hammerhead ribozyme specifically inhibits mutant K-ras mRNA of human pancreatic cancer cells

We have evaluated the efficacy of an anti-K-ras hammerhead ribozyme targeted against GUU-mutated codon 12 of the K-ras gene in a cell-free system as well as in cultured pancreatic carcinoma cell lines. In the cell-free system, the anti-K-ras ribozyme specifically cleaved K-ras RNA with GUU-mutation at codon 12, but not other triplet sequences at codon 12 of K-ras RNA. In the cell culture system, the anti-K-ras ribozyme significantly reduced K-ras mRNA level (GUU-mutated codon 12) in Capan-1 pancreatic carcinoma cells, but less significantly suppressed K-ras mRNA in Capan-2 (GUU/GGU heterozygous-mutation at codon 12) or MIA PaCa-2 (UGU-mutated codon 12) pancreatic carcinoma cells. The ribozyme inhibited proliferation of transfected Capan-1 cells. These results suggest that this ribozyme selectively recognizes single-base mutation of K-ras mRNA and is able to reverse the malignant phenotype in human pancreatic carcinoma cells.

Hammerhead ribozymes against gamma-glutamylcysteine synthetase mRNA down-regulate intracellular glutathione concentration of mouse islet cells

gamma-Glutamylcysteine synthetase (gamma-GCS) is a key enzyme in glutathione synthesis and is thought to play a significant role in intracellular detoxification systems. To specifically suppress gamma-GCS gene expression, we constructed two different hammerhead ribozymes against gamma-GCS mRNA transcripts. Two cleavage sites were targeted as follows: site 1 for anti-gamma-GCS ribozyme (H), a GUU triplet located from +348 to +350 of the gamma-GCS heavy chain, and site 2 for anti-gamma-GCS ribozyme (L), a GUU triplet located from +235 to +237 of the gamma-GCS light chain. The anti-gamma-GCS ribozymes effectively cleaved gamma-GCS mRNA in a cell-free system. When transfected into a Min-6 mouse islet cell line, these anti-gamma-GCS ribozymes not only suppressed gamma-GCS gene expression, but also reduced intracellular glutathione concentration. These results suggest that the ribozyme-mediated down-regulation of gamma-GCS gene expression may be useful for analyzing the glutathione-associated cellular defense systems of pancreatic islet cells.

Gene therapy for leukemia and lymphoma

Gene therapy of malignant diseases can be divided into four basic approaches: gene interference, gene insertion, immunopotentiation, and suicide gene approaches. This article reviews the application of these approaches in the therapy of leukemias and lymphomas.