Ribozymes. Their functions and strategies for their use

Two tales of Annexin A2 knock-down: One of compensatory effects by antisense RNA and another of a highly active hairpin ribozyme

Besides altering its own expression during cell transformation, Annexin A2 is upregulated during the progression of many cancer types and also plays key roles during viral infection and multiplication. Consequently, there has been great interest in Annexin A2 as a potential drug target. The successful design of efficient in vivo delivery systems constitutes an obstacle in full exploitation of antisense and RNA-cleaving technologies for the knock-down of specific targets. Efficiency is dependent on the method of delivery and accessibility of the target. Here, hairpin ribozymes and an antisense RNA against rat annexin A2 mRNA were tested for their efficiencies in a T7-driven coupled transcription/translation system. The most efficient ribozyme and antisense RNA were subsequently inserted into a retroviral vector under the control of a tRNA promoter, in a cassette inserted between retroviral Long Terminal Repeats for stable insertion into host DNA. The Phoenix package system based on defective retroviruses was used for virus-mediated gene transfer into PC12 cells. Cells infected with the ribozyme-containing particles died shortly after infection. However, the same ribozyme showed a very high catalytic effect in vitro in cell lysates, explained by its loose hinge helix 2 region. This principle can be transferred to other ribozymes, such as those designed to cleave the guide RNA in the CRISPR/Cas9 technology, as well as to target specific viral RNAs. Interestingly, efficient down-regulation of the expression of Annexin A2 by the antisense RNA resulted in up-regulation of Annexin A7 as a compensatory effect after several cell passages. Indeed, compensatory effects have previously been observed during gene knock-out, but not during knock-down of protein expression. This highlights the problems in interpreting the phenotypic effects of knocking down the expression of a protein. In addition, these data are highly relevant when considering the effects of the CRISPR/Cas9 approach.

Preparation and identification of anti-transforming growth factor β1 U1 small nuclear RNA chimeric ribozyme in vitro

AIM: To study the preparation and cleavage activity of anti-transforming growth factor (TGF)β1 U1 small nuclear (sn) RNA chimeric hammerhead ribozymes in vitro.

METHODS: TGFβ1 partial gene fragment was cloned into T-vector at the downstream of T7 promoter. ³²p-labeled TGFβ1 partial transcripts as target RNA were transcribed in vitro and purified by denaturing polyacrylamide gel electrophoresis (PAGE). Anti-TGFβ1 ribozymes were designed by computer, then synthetic ribozyme fragments were cloned into the U1 ribozyme vector pZeoU1EcoSpe containing U1 snRNA promoter/enhancer and terminator. ³²p-labeled U1 snRNA chimeric ribozyme transcripts were gel-purified, incubated with target-RNAs at different conditions and autoradiographed after running denaturing PAGE.

RESULTS: Active U1snRNA chimeric ribozyme (U1Rz803) had the best cleavage activity at 50 °C; at 37 °C, it was active, K_m = 34.48 nmol/L, K_cat = 0.14 min^-1; while the point mutant ribozyme U1Rz803_m had no cleavage activity, so these indicated the design of U1Rz803 was correct.

CONCLUSION: U1Rz803 prepared in this study possessed the perfect specific catalytic cleavage activity. These results indicate U1 snRNA chimeric ribozyme U1Rz803 may suppress the expression of TGFβ1 in vivo, therefore it may provide a new avenue for the treatment of liver fibrosis in the future.

Genetic targeting of the renin-angiotensin system for long-term control of hypertension

Although traditional approaches are effective for the treatment and control of hypertension, they have not succeeded in curing the disease, and have therefore reached a plateau. As a result of the completion of the Human Genome Project and the continuous advancement in gene delivery systems, it is now possible to investigate genetic means for the treatment and possible cure for hypertension. In this review we discuss the potential of genetic targeting of the renin-angiotensin system for the treatment of hypertension. We provide examples of various approaches that have used antisense technology with a high degree of success. We focus on our own research, which targets the use of antisense of the angiotensin type I receptor in various models of hypertension. Finally, we discuss the future of antisense technology in the treatment of human hypertension.

Novel therapies for chronic myelogenous leukemia

The BCR-ABL oncogene is essential to the pathogenesis of chronic myelogenous leukemia, and immune mechanisms play an important role in control of this disease. Understanding of the molecular pathogenesis of chronic myelogenous leukemia has led to the development of several novel therapies, which can be broadly divided into therapies based on 1) inhibition of the BCR-ABL oncogene expression, 2) inhibition of other genes important to the pathogenesis of chronic myelogenous leukemia, 3) inhibition of BCR-ABL protein function, and 4) immunomodulation. We have systematically reviewed each of these novel therapeutic approaches in this article.

Biological barriers to cellular delivery of lipid-based DNA carriers

Although lipid-based DNA delivery systems are being assessed in gene therapy clinical trials, many investigators in this field are concerned about the inefficiency of lipid-based gene transfer technology, a criticism directed at all formulations used to enhance transfer of plasmid expression vectors. It is important to recognize that many approaches have been taken to improve transfection efficiency, however because of the complex nature of the formulation technology being developed, it has been extremely difficult to define specific carrier attributes that enhance transfection. We believe that these optimization processes are flawed for two reasons. First, a very defined change in formulation components affects the physical and chemical characteristics of the carrier in many ways. As a consequence, it has not been possible to define structure/activity relationships. Second, the primary endpoint used to assess plasmid delivery has been transgene expression, an activity that is under the control of cellular processes that have nothing to do with delivery. Gene expression following administration of a plasmid expression vector involves a number of critical steps: (i) DNA protection, (ii) binding to a specific cell population, (iii) DNA transfer across the cell membrane, (iv) release of DNA into the cytoplasm, (v) transport through the cell and across the nuclear membrane as well as (vi) transcription and translation of the gene. The objective of this review is to describe lipid-based DNA carrier systems and the attributes believed to be important in regulating the transfection activity of these formulations. Although membrane destabilization activity of the lipid-based carriers plays an important role, we suggest here that a critical element required for efficient transfection is dissociation of lipids bound to the plasmid expression vector following internalization.

Hammerhead ribozymes targeted to the FBN1 mRNA can discriminate a single base mismatch between ribozyme and target

Hammerhead ribozymes are catalytic RNA molecules that can act in trans, with ribozyme and substrate being two different oligoribonucleotides with regions of complementarity. Mutations in the gene for fibrillin-1 (FBN1) cause Marfan syndrome. The majority of mutations are single-base changes, many of which exert their effect via a dominant-negative mechanism. Previously we have shown that an antisense hammerhead ribozyme, targeted to the FBN1 mRNA can reduce deposition of fibrillin to the extracellular matrix of cultured fibroblasts, suggesting it may be possible to utilize ribozymes to down regulate the production of mutant protein and thus restore normal fibrillin function. This might be achieved by the mutation creating a ribozyme cleavage site that is not present in the normal allele, however this is likely to limit the number of mutations that could be targeted. Alternatively, it might be possible to target the mutant allele via the ribozyme binding arms. To determine the potential of ribozymes to preferentially target mutant FBN1 alleles via the latter approach, the effect of mismatches in helix I of a hammerhead ribozyme, on the cleavage of fibrillin (FBN1) mRNA was investigated. A single base mismatch significantly reduced ribozyme cleavage efficiency both in vitro and in vivo. The discrimination between fully-matched and mismatched ribozyme varied with the length of helix I, with the discrimination being more pronounced in ribozymes with a shorter helix. These data suggest that it should be possible to design hammerhead ribozymes that can discriminate between closely related (mutant and normal) target RNAs varying in as little as a single nucleotide, even if the mutation does not create a ribozyme cleavage site.