Anti-HIV ribozymes

Phase I/II Clinical Trials Using Gene-Modified Adult Hematopoietic Stem Cells for HIV: Lessons Learnt

Gene therapy for individuals infected with HIV has the potential to provide a once-only treatment that will act to reduce viral load, preserve the immune system, and mitigate cumulative toxicities associated with highly active antiretroviral therapy (HAART). The authors have been involved in two clinical trials (phase I and phase II) using gene-modified adult hematopoietic stem cells (HSCs), and these are discussed as prototypic trials within the general field of HSC gene therapy trials for HIV. Taken as a group these trials have shown (i) the safety of both the procedure and the anti-HIV agents themselves and (ii) the feasibility of the approach. They point to the requirement for (i) the ability to transduce and infuse as many as possible gene-containing HSC and/or (ii) high engraftment and in vivo expansion of these cells, (iii) potentially increased efficacy of the anti-HIV agent(s) and (iv) automation of the cell processing procedure.

Phase 2 gene therapy trial of an anti-HIV ribozyme in autologous CD34+ cells

Gene transfer has potential as a once-only treatment that reduces viral load, preserves the immune system, and avoids lifetime highly active antiretroviral therapy. This study, the first randomized, double-blind, placebo-controlled, phase II cell-delivered gene transfer clinical trial, was conducted in 74 HIV-1 infected adults who received a tat/vpr specific anti-HIV ribozyme (OZ1) or placebo delivered in autologous CD34+ hematopoietic progenitor cells. There were no OZ1-related adverse events. There was no statistical difference in viral load between the OZ1 and placebo group at the primary end-point (average at weeks 47 and 48) but time weighted areas under the curve from weeks 40-48 and 40-100 were significantly lower in the OZ1 group. Throughout the 100 weeks, CD4+ lymphocyte counts were higher in the OZ1 group. This study provides the first indication that cell-delivered gene transfer is safe and biologically active in HIV patients and can be developed as a conventional therapeutic product.

A new modified DNA enzyme that targets influenza virus A mRNA inhibits viral infection in cultured cells

DNA enzymes are RNA-cleaving single-stranded DNA molecules. We designed DNA enzymes targeting the PB2 mRNA translation initiation (AUG) region of the influenza A virus (A/PR/8/34). The modified DNA enzymes have one or two N3'-P5' phosphoramidate bonds at both the 3'- and 5'-termini of the oligonucleotides, which significantly enhanced their nuclease resistance. These modified DNA enzymes had the same cleavage activity as the unmodified DNA enzymes, determined by kinetic analyses, and reduced influenza A virus replication by more than 99%, determined by plaque formation. These DNA enzymes are highly specific; their protective effect was not observed in influenza B virus (B/Ibaraki)-infected Madin-Darby canine kidney cells.

A novel single-stranded DNA enzyme expression system using HIV-1 reverse transcriptase

In this study, we exploited a DNA enzyme expression system using the mechanism of HIV-1 reverse transcription in vitro. HIV-1 reverse transcription is initiated when its cognate primer tRNA (Lys-3) binds to the primer binding site (PBS) of the viral RNA template. Therefore, this RNA contains the HIV-1 PBS, the DNA enzyme, and a tRNA (Lys-3) at the 3(')-end of its RNA transcript, such that a single-stranded DNA (ssDNA) is synthesized by the HIV-1 reverse transcriptase. We constructed RNA expression vectors including the HIV-1 PBS, the DNA enzyme, and either a native tRNA (Lys-3) or one of two truncated tRNAs (Lys-3), Delta tRNA (Lys-3) and Delta Delta tRNA (Lys-3). The reactions of the pVAX1-Dz-tRNA (Lys-3), pVAX1-Dz-Delta tRNA (Lys-3), and pVAX1-Dz-Delta Delta tRNA (Lys-3) vectors with T7 RNA polymerase in vitro gave the corresponding RNAs. The liberated RNAs were treated with HIV-1 reverse transcriptase (HIV-1 RT) in vitro, which yielded the corresponding ssDNA. The cleavage assay results demonstrated that the expressed DNA enzyme has cleavage ability against the target sequence. Thus, we have found a new DNA enzyme oligonucleotide expression system using the HIV-1 reverse transcriptase in vitro.

Regulators of G protein signaling (RGS proteins): novel central nervous system drug targets

Many drugs of abuse signal through receptors that couple to G proteins (GPCRs), so the factors that control GPCR signaling are likely to be important to the understanding of drug abuse. Contributions by the recently identified protein family, regulators of G protein signaling (RGS) to the control of GPCR function are just beginning to be understood. RGS proteins can accelerate the deactivation of G proteins by 1000-fold and in cell systems they profoundly inhibit signaling by many receptors, including mu-opioid receptors. Coupled with the known dynamic regulation of RGS protein expression and function, they are of obvious interest in understanding tolerance and dependence mechanisms. Furthermore, drugs that could inhibit their activity could be useful in preventing the development of or in treating drug dependence.

Gene therapy of HIV-1 infection using lentiviral vectors expressing anti-HIV-1 genes

The use of vectors based on primate lentiviruses for gene therapy of human immunodeficiency virus type 1 (HIV-1) infection has many potential advantages over the previous murine retroviral vectors used for delivery of genes that inhibit replication of HIV-1. First, lentiviral vectors have the ability to transduce dividing and nondividing cells that constitute the targets of HIV-1 infection such as resting T cells, dendritic cells, and macrophages. Lentiviral vectors can also transfer genes to hematopoietic stem cells with a superior gene transfer efficiency and without affecting the repopulating capacity of these cells. Second, these vectors could be potentially mobilized in vivo by the wild-type virus to secondary target cells, thus expanding the protection to previously untransduced cells. And finally, lentiviral vector backbones have the ability to block HIV-1 replication by several mechanisms that include sequestration of the regulatory proteins Tat and Rev, competition for packaging into virions, and by inhibition of reverse transcription in heterodimeric virions with possible generation of nonfunctional recombinants between the vector and viral genomes. The inhibitory ability of lentiviral vectors can be further increased by expression of anti-HIV-1 genes. In this case, the lentiviral vector packaging system has to be modified to become resistant to the anti-HIV-1 genes expressed by the vector in order to avoid self-inhibition of the vector packaging system during vector production. This review focuses on the use of lentiviral vectors as the main agents to mediate inhibition of HIV-1 replication and discusses the different genetic intervention strategies for gene therapy of HIV-1 infection.

Dose-response resistance to HIV-1/MuLV pseudotype virus ex vivo in a hairpin ribozyme transgenic mouse model

We have investigated the efficacy of a hairpin ribozyme targeting the 5' leader sequence of HIV-1 RNA in a transgenic model system. Primary spleen cells derived from transgenic or control mice were infected with HIV-1/MuLV pseudotype virus. A significantly reduced susceptibility to infection in ribozyme-expressing transgenic spleen cells (P = 0.01) was shown. Variation of transgene-expression levels between littermates revealed a dose response between ribozyme expression and viral resistance, with an estimated cut off value below 0.2 copies of hairpin ribozyme per cell. These findings open up possibilities for studies on ribozyme efficacy and anti-HIV-1 gene therapy.

Generation and application of asymmetric hammerhead ribozymes

Most researchers who intend to suppress a particular gene are interested primarily in the application of ribozyme technology rather than its mechanistic details. This article provides some background information and describes a straightforward strategy to generate and test a special design of a ribozyme: the asymmetric hammerhead ribozyme. This version of a hammerhead ribozyme carries at its 5' end the catalytic domain and at its 3' end a relatively long antisense flank that is complementary to the target RNA. Asymmetric hammerhead ribozymes can be constructed via polymerase chain reaction amplification, and rules are provided on how to select the DNA oligonucleotides required for this reaction. In addition to details on construction, we describe how to test asymmetric hammerhead ribozymes for association with the target RNA in vitro, so that RNA constructs can be selected and optimized for fast hybridization with their target RNA. This test can allow one to minimize association problems caused by the secondary structure of the target RNA. Additionally, we describe the in vitro cleavage assay and the determination of the cleavage rate constant. Testing for efficient cleavage is also a prerequisite for reliable and successful application of the technology. A carefully selected RNA will be more promising when eventually used for target suppression in living cells.

Suppression of smooth muscle cell proliferation by a c-myc RNA-cleaving deoxyribozyme

A small catalytic DNA molecule targeting c-myc RNA was found to be a potent inhibitor of smooth muscle cell (SMC) proliferation. The catalytic domain of this molecule was based on that previously derived by in vitro selection (Santoro, S. W., and Joyce, G. F. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 4262-4266) and is known as the "10-23" general purpose RNA-cleaving deoxyribozyme. In addition to inhibiting SMC proliferation at low concentration, this molecule (targeting the translation initiation region of c-myc RNA) was found to efficiently cleave its full-length substrate in vitro and down-regulate c-myc gene expression in smooth muscle cells. The serum nuclease stability of this molecule was enhanced without substantial loss of kinetic efficiency by inclusion of a 3'-3'-internucleotide inversion at the 3'-terminal. The extent of SMC suppression was found to be influenced by the length of the substrate binding arms. This correlated to some extent with catalytic activity in both the short substrate under multiple turnover conditions and the full-length substrate under single turnover conditions, with the 9 + 9 base arm molecule producing the greatest activity.

Length variation of helix III in a hammerhead ribozyme and its influence on cleavage activity

The previously described HIV-1 directed hammerhead ribozyme 2as-Rz12 can form with its target RNA 2s helices I and III of 128 and 278 base pairs (bp). A series of derivatives was made in which helix III was truncated to 8, 5, 4, 3, and 2 nucleotides (nt). These asymmetric hammerhead ribozymes were tested for in vitro cleavage and for inhibition of HIV-1 replication in human cells. Truncation of helix III to 8 bp did not affect the in vitro cleavage potential of the parental catalytic antisense RNA 2as-Rz12. Further truncation of helix III led to decreased cleavage rates, with no measurable cleavage activity for the 2 bp construct. All catalytically active constructs showed complex cleavage kinetics. Three kinetic subpopulations of ribozyme-substrate complexes could be discriminated that were cleaved with fast or slow rates or not at all. Gel purification of preformed ribozyme-substrate complexes led to a significant increase in cleavage rates. However, the complex cleavage pattern remained. In mammalian cells, the helix III-truncated constructs showed the same but no increased inhibitory effect of the comparable antisense RNA on HIV-1 replication.

A hammerhead ribozyme targeted to the human chemokine receptor CCR5

The CCR5 chemokine receptor plays a crucial role in the initiation of in vivo HIV infection, acting as a critical coreceptor molecule for primary strains. Individuals with mutations in the CCR5 gene that reduce its level of expression are resistant to HIV-1 infection. Since these mutations are not associated with any known clinical condition, CCR5 may be an ideal target for anti-HIV therapy. We have designed an artificial hammerhead ribozyme, denoted RzR5-76, targeted to exon 2 of the human CCR5 mRNA. When RzR5-76 activity is induced in HEK 293 cells transfected with a CCR5 expression plasmid, the surface levels of this chemokine receptor are reduced up to 60%. The results indicate that this inhibitory effect is mainly due to the catalytic activity of the ribozyme and not to its antisense properties. These preliminary data suggest that intracellular ribozymes could be used in vivo to block HIV-1 entry into human cells.

Effective inhibition of influenza virus production in cultured cells by external guide sequences and ribonuclease P

The polymerase (PB2) and nucleocapsid (NP) genes encoded by the genome of influenza virus are essential for replication of the virus. When synthetic genes that express RNAs for external guide sequences targeted to the mRNAs of the PB2 and NP genes are stably incorporated into mouse cells in tissue culture, infection of these cells with influenza virus is nonproductive. Endogenous RNase P cleaves the targeted influenza virus mRNAs when they are in a complex with the external guide sequences. Targeting two different mRNAs simultaneously inhibits viral particle production more efficiently than does targeting only one mRNA.