Ribozymes and the anti-gene therapy: how a catalytic RNA can be used to inhibit gene function

Ribozyme: A clinical tool

Catalytic RNAs (ribozymes) are capable of specifically cleaving RNA molecules, a property that enables them to act as potential antiviral and anti-cancer agents, as well as powerful tools for functional genomic studies. Recently, ribozymes have been used successfully to inhibit gene expression in a variety of biological systems in vitro and in vivo. Phase I clinical trials using ribozyme gene therapy to treat AIDS patients have been conducted. Despite initial success, there are many areas that require further investigation. These include stability of ribozymes in cells and designing highly active ribozymes in vivo, identification of target sequence sites and co-localization of ribozymes and substrates, and their delivery to specific tissues and maintenance of its stable long-term expression. This review gives a brief introduction to ribozyme structure, catalysis and its potential applications in biological systems as therapeutic agents.

Hairpin ribozymes in combination with siRNAs against highly conserved hepatitis C virus sequence inhibit RNA replication and protein translation from hepatitis C virus subgenomic replicons

Chronic hepatitis C virus (HCV) infection is a clinically important liver disease with limited therapeutic options in a significant proportion of patients. Therefore, novel efficient therapeutic agents are needed. Because the 5'- and 3'-untranslated regions (UTRs) of HCV are highly conserved and functionally important for HCV replication, they are attractive targets for RNA-cleaving ribozymes or small interfering RNAs (siRNAs). In this study hairpin ribozymes (Rz) targeting HCV 5'- and 3'-UTR sequences were expressed from a retroviral vector transcript under control of two different RNA polIII promoters (tRNA(Val), U6). Ribozymes were evaluated in monocistronic, subgenomic I389/hyg-ubi/NS3-3'/5.1 HCV replicon cells as single agents or in combination with siRNAs against HCV 5'- or 3'-UTR recently demonstrated to inhibit HCV replicons. Additionally, ribozyme constructs were generated with the 3'-terminus of the ribozyme flanked by constitutive transport element (CTE) sequences, an RNA motif that has previously been shown to enhance cleavage activity of hammerhead ribozymes. In our study, tRNA(Val) as well as U6 promoter-driven Rzs markedly reduced HCV replicon RNA expression and HCV internal ribosome entry site (IRES)-mediated HCV NS5B protein translation from monocistronic subgenomic replicons. However, attachment of CTE sequences to the 3'-terminus did not significantly enhance activity of Rzs tested in this study. Interestingly, we detected additive HCV inhibitory effects for combinations of tRNA(Val)-driven Rzs and U6-derived siRNAs both directed against highly conserved 5'- and 3'-UTR sequence, suggesting that a dual strategy of ribozymes and siRNAs might become a powerful molecular tool to specifically silence HCV RNA replication.

Ribozymes against TGFbeta1 reverse character of activated hepatic stellate cells in vitro and inhibit liver fibrosis in rats

BACKGROUND/AIMS

Transforming growth factor beta (TGFbeta1) is considered the key mediator in the process of liver fibrosis. The purpose of this investigation was to evaluate the activity of ribozymes against TGFbeta1 in a cell-free system and activated hepatic stellate cells (HSCs), and antifibrotic effect in activated HSCs in vitro and in rats.

METHODS

Three ribozymes targeting against TGFbeta1 mRNA were designed, and then cloned into the U1 snRNA expression cassette. The chimeric ribozymes were selected for the analysis of their performances in activated HSCs through the detection of their cleavage activities in a cell-free system. After ribozyme-encoding plasmids had been transfected into HSC-T6 cells, the effects of ribozymes on activated HSCs were evaluated through the analysis of proliferation, activation and collagen deposition of HSC-T6. The adenoviral vector expressing the ribozymes was constructed, and then delivered into rat models of hepatic fibrosis induced by carbon tetrachloride.

RESULTS

TGFbeta1 expression was efficiently down-regulated in activated HSCs by U1 snRNA chimeric ribozymes which possessed perfect cleavage activity in a cell-free system. Further studies demonstrated that U1 snRNA chimeric ribozymes inhibited the synthesis of collagen I, reduced deposition of collagen I, suppressed BrdU incorporation, but had no effect on desmin and alpha-SMA expression in transfected HSC-T6 cells. Histological analysis demonstrated that the adenoviral vector expressing ribozyme (Rz803) could alleviate fibrotic pathology in rats treated with carbon tetrachloride.

CONCLUSIONS

The anti-TGFbeta1 ribozymes could reverse the character of activated HSCs in vitro and improve fibrotic pathology in vivo. It indicated that TGFbeta1 could be considered as a novel candidate for a therapeutic agent against hepatic fibrosis.

Efficacy and mode of action of hammerhead and hairpin ribozymes against various HIV-1 target sites

Ribozymes have been proposed as gene therapy agents against HIV-1, although many fundamental questions about their mechanism of action remain unclear. Few studies have compared directly the potential of different modified ribozyme species against a particular target. Here we compare the relative abilities of hammerhead (HhU5) and hairpin (HpU5) ribozymes directed against a well-studied target RNA that has therapeutic potential, located in the untranslated 5' region (U5), to inhibit HIV-1 replication. The two types of ribozymes showed similar antiviral efficacy after being stably transfected into HUT78 cells and subsequently challenged with HIV-1(SF2), but the HhU5 ribozyme showed faster cleavage kinetics when tested in a cell-free system. In the second part of this study, we examined whether different ribozymes were able to inhibit the integration of proviral DNA in infected HUT78 cells. We found that cell pools stably expressing HpU5 could limit the appearance of integrated provirus, indicating that they could inhibit the infecting viral RNA before reverse transcription. A preintegration effect was also found for cell pools expressing a ribozyme targeting the nef gene (HhNef) or a ribozyme targeting the LTR (HhLTR). However, no discernible preintegration effects were seen for the HhU5 ribozyme or an active ribozyme directed against an RNA target site in the pol gene (HhPol). Thus, the results suggest that the mode of ribozyme action varied between sites and is not dependent solely on inhibiting the infecting viral RNA. Evidence for a preintegration effect is extremely encouraging and indicates that "resistant" cells have some chance to repopulate the immune system through such a selective advantage. We also studied the ability of the different ribozymes to down regulate viral RNA postintegration.

Catalytic Beacons for the Detection of DNA and Telomerase Activity

DNA and telomerase activity are detected by a DNAzyme generated upon hybridization and opening of a functional catalytic beacon.

Ribozymes: a modern tool in medicine

Since the discovery of ribozymes and self-splicing introns, it has been estimated that this biological property of RNA combined with other recombinant DNA technologies would become a tool to combat viral diseases and control oncogenes. These goals seem like a distinct possibility now. However, there is still a lot to be learned about the mobility of RNA inside the cells and the cellular factors that can impede ribozyme action in order to capitalize fully on the targeted RNA inactivation property of ribozymes. The most effective approach to maximize ribozyme function in a complex intracellular environment is to understand as much as possible about the intracellular fate of the RNA that is being targeted. As new techniques in cell biology become available, such understanding will be less problematic. Fundamental studies of ribozyme structure and mechanism of catalysis are flourishing both at the academic and industrial level and it can be expected that many new developments will continue to take place in these areas in the near future. Here, we review the design, stability and therapeutic application of these technologies illustrating relevant gene targets and applications in molecular medicine. Relevant problems in implementation of the technology, group I and II introns and the differences in applications, ribozyme structure and the application of this technology to virus attack and oncogene downregulation are discussed. Also some of the latest RNA-based technologies such as siRNA, RNA/DNA duplexes and RNA decoys have been introduced.

Substrate specificity and reaction kinetics of an X-motif ribozyme

The X-motif is an in vitro-selected ribozyme that catalyzes RNA cleavage by an internal phosphoester transfer reaction. This ribozyme class is distinguished by the fact that it emerged as the dominant clone among at least 12 different classes of ribozymes when in vitro selection was conducted to favor the isolation of high-speed catalysts. We have examined the structural and kinetic properties of the X-motif in order to provide a framework for its application as an RNA-cleaving agent and to explore how this ribozyme catalyzes phosphoester transfer with a predicted rate constant that is similar to those exhibited by the four natural self-cleaving ribozymes. The secondary structure of the X-motif includes four stem elements that form a central unpaired junction. In a bimolecular format, two of these base-paired arms define the substrate specificity of the ribozyme and can be changed to target different RNAs for cleavage. The requirements for nucleotide identity at the cleavage site are GD, where D = G, A, or U and cleavage occurs between the two nucleotides. The ribozyme has an absolute requirement for a divalent cation cofactor and exhibits kinetic behavior that is consistent with the obligate binding of at least two metal ions.

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.

Ribozyme-mediated suppression of G protein gamma subunits

Efforts to determine the sequence of the human genome have resulted in sequence information on thousand of genes. Now, the challenge is to determine the functions of this myriad of genes, including those encoding the G protein subunit families. In this chapter, we describe the successful use of ribozymes to inactivate mRNAs expressed from the G protein gamma subunit genes. Ribozymes are unique in that they can inactivate specific gene expression, and thereby can be used to help identify the function of a protein or the role of a gene in a functional cascade. Compared to other means of identifying the role of a gene (i.e., transgenic or knockout animals), ribozymes are specific and relatively easy to use. Moreover, ribozymes are able to discriminate closely related, or even mutated, sequences within gene families. Thus, in addition to elucidating functions, ribozymes have the potential to be used in treating genetic disorders associated with mutations of G protein subunits.

Anti-HBV hairpin ribozyme-mediated cleavage of target RNA in vitro

AIM: To study the preparation and cleavage activity of HpRz directed against the transcript of HBV core gene in vitro.

METHODS: HpRz gene designed by computer targeting the transcript of HBV core gene was cloned into the vector p1.5 between 5’-cis-Rz and 3’-cis-Rz. ³²p-labeled HpRz transcript proved whether the vector fit for the preparation of hairpin ribozyme in vitro. ³²p-labeled pKC transcript containing HBV core region as target-RNA was transcribed using T₇ RNA polymerase and purified by denaturing PAGE. Cold HpRz transcript was incubated with ³²p-labeled target-RNAs under different conditions and radio autographed after denaturing polyacrylamide gel electrophoresis.

RESULTS: HpRz has the specific ability of cleavage of target RNA at 37 °C and 12 mM MgCl₂. K_m = 26.31 nmol/L, K_cat = 0.18/min. These results revealed that the design of HpRz was correct.

CONCLUSION: HpRz prepared in this study possesses specific catalytic activity from the identification of cleavage activity. These results indicate that hairpin ribozyme may intracellularly inhibit the replication of HBV, therefore it may become a novel potent weapon for the treatment of hepatitis B.

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.

Coronary in-stent restenosis: current status and future strategies

In-stent restenosis (ISR) is a novel pathobiologic process, histologically distinct from restenosis after balloon angioplasty and comprised largely of neointima formation. As percutaneous coronary intervention increasingly involves the use of stents, ISR is also becoming correspondingly more frequent. In this review, we examine the available studies of the histology and pathogenesis of ISR, with particular reference to porcine and other animal models. An overview of mechanical treatments is then provided, which includes PTCA, directional coronary atherectomy and high speed rotational atherectomy. Radiation-based therapies are discussed, including a summary of current problems associated with this modality of treatment. Finally, novel strategies for the prevention of ISR are addressed, including novel developments in stents and stent coatings, conventional drugs, nucleic acid-based drugs and gene transfer. Until recently, limited pharmacologic and mechanical treatment options have been available for both treatment and prevention of ISR. However, recent advances in gene modification and gene transfer therapies and, more particularly, in local stent-based drug delivery systems make it conceivable that the incidence of ISR will now be seriously challenged.

Involvement of proteasome alpha-subunit PSMA7 in hepatitis C virus internal ribosome entry site-mediated translation

Ribozymes are small catalytic RNA molecules that can be engineered to enzymatically cleave RNA transcripts in a sequence-specific fashion and thereby inhibit expression and function of the corresponding gene product. With their simple structures and site-specific cleavage activity, they have been exploited as potential therapeutic agents in a variety of human disorders, including hepatitis C virus (HCV) infection. We have designed a hairpin ribozyme (Rz3'X) targeting the HCV minus-strand replication intermediate at position 40 within the 3'X tail. Surprisingly, Rz3'X was found to induce ganciclovir (GCV)-resistant colonies in a bicistronic cellular reporter system with HCV internal ribosome entry site (IRES)-dependent translation of herpes simplex virus thymidine kinase (TK). Rz3'X-transduced GCV-resistant HeLa reporter cells showed substantially reduced IRES-mediated HCV core protein translation compared with control vector-transduced cells. Since these reporter systems do not contain the HCV 3'X tail sequences, the results indicate that Rz3'X probably exerted an inhibitory effect on HCV IRES activity fortuitously through another gene target. A novel technique of ribozyme cleavage-based target gene identification (cleavage-specific amplification of cDNA ends) (M. Krüger, C. Beger, P. J. Welch, J. R. Barber, and F. Wong-Staal, Nucleic Acids Res. 29:e94, 2001) revealed that human 20S proteasome alpha-subunit PSMA7 mRNA was a target RNA recognized and cleaved by Rz3'X. We then showed that additional ribozymes directed against PSMA7 RNA inhibited HCV IRES activity in two assay systems: GCV resistance in the HeLa IRES TK reporter cell system and a transient transfection assay performed with a bicistronic Renilla-HCV IRES-firefly luciferase reporter in Huh7 cells. In contrast, ribozymes were inactive against IRES of encephalomyocarditis virus and human rhinovirus. Additionally, proteasome inhibitor MG132 exerted a dose-dependent inhibitory effect on HCV IRES-mediated translation but not on cap-dependent translation. These data suggest a principal role for PSMA7 in regulating HCV IRES activity, a function essential for HCV replication.

Delivery of novel macromolecular drugs against HIV-1

The development of new low molecular weight drugs against human immunodeficiency virus Type 1 (HIV-1) targets other than reverse transcriptase (RT) and protease, such as the integrase and the envelope glycoprotein, is likely to take many years. Macromolecular drugs, including antisense oligonucleotides, ribozymes, RNA decoys and transdominant mutant proteins, may be able to interfere with a relatively large number of viral targets, thereby decreasing the likelihood of the emergence of drug-resistant strains. It may also be relatively easy to alter the sequence of some of the macromolecular drugs to counter emerging drug-resistant viruses. The delivery of antisense oligonucleotides and ribozymes to HIV-1 infected or potentially infectable cells by antibody-targeted liposomes, certain cationic lipid formulations and pH-sensitive liposomes results in significant anti-HIV-1 activity. These carriers not only facilitate cytoplasmic delivery but also protect the drugs from nuclease digestion. Delivery of therapeutic genes (another form of macromolecular drug) to target cells is an important challenge of gene therapy. Following delivery by a viral vector, sufficient levels of gene expression must be maintained over an extended period of time to have therapeutic activity. Robust expression of therapeutically useful ribozymes, antisense, decoys and aptamers can be achieved by the use of Pol III expression systems. Moloney murine leukaemia virus- (MoMuLV), adeno-associated virus (AAV)-, or HIV-derived vectors expressing a variety of therapeutic genes have been used successfully to inhibit HIV-1 replication in cultured cells.

A new and efficient DNA enzyme for the sequence-specific cleavage of RNA

A new DNA enzyme, the "Bipartite DNAzyme", suitable for the sequence-specific cleavage of RNA, was obtained from a random DNA library by in vitro selection. Only a single family of catalytic molecules emerged from the selection, and a 22 nucleotide consensus sequence common to all clones defined a putative catalytic core. The most abundant clone self-cleaved at a single internal ribonucleotide phosphodiester with a relatively fast k(obs) value of 1.7 min(-1), in 10 mM MgCl(2) at 23 degrees C. This DNAzyme ("Bipartite I") required divalent cations, with magnesium and manganese most optimally supporting cleavage. A reselection from a mutagenized DNAzyme pool for the ability to cleave at extended RNA substrates yielded an unchanged catalytic core sequence. From this re-selection a DNAzyme ("Bipartite II") capable of sequence-specifically cleaving extended stretches of RNA was derived. A rate versus pH analysis of the Bipartite II DNAzyme revealed a two-phase profile, similar to that reported for the hepatitis delta virus (HDV) ribozyme, suggesting that the Bipartite II DNAzyme and the HDV ribozyme may share similar catalytic strategies. Multiple-turnover kinetics, measured in 30 mM MgCl(2), at 37 degrees C, with an HIV-1-derived RNA substrate, yielded a k(cat) value of approximately 1.4 min(-1) and a K(M) value of approximately 230 nM, which were of the same order as k(cat) and K(M )values measured for other ribozymes and DNAzymes in general use for RNA cleavage. The Bipartite DNAzyme therefore represents a new and potentially useful reagent, both for the processing of RNA transcripts in vitro and for mRNA ablation procedures in vivo.

C-SPACE (cleavage-specific amplification of cDNA ends): a novel method of ribozyme-mediated gene identification

A hairpin ribozyme, RzCR2A, directed against position 323 of the hepatitis C virus 5'-untranslated region (HCV 5'-UTR) was used to establish and validate a novel method for the detection of cellular target molecules for hairpin ribozymes, termed C-SPACE (cleavage-specific amplification of cDNA ends). For C-SPACE, HeLa mRNA containing the transcript of interest was subjected to in vitro cleavage by RzCR2A in parallel with a control ribozyme, followed by reverse transcription using a modified SMART cDNA amplification method and cleavage-specific PCR analysis. C-SPACE allowed identification of the RzCR2A target transcript from a mixture containing the entire cellular mRNA while only requiring knowledge of the ribozyme binding sequence for amplification. In a similar approach, C-SPACE was used successfully to identify human 20S proteasome alpha-subunit PSMA7 mRNA as the cellular target RNA of Rz3'X, a ribozyme originally designed to cleave the negative strand HCV 3'-UTR. Rz3'X was found to substantially inhibit HCV internal ribosome entry site (IRES) activity and PSMA7 was subsequently confirmed to be involved in HCV IRES-mediated translation. Thereby, C-SPACE was validated as a powerful tool to rapidly identify unknown target RNAs recognized and cleaved by hairpin ribozymes.

Exploiting genetic alterations to design novel therapies for cancer

In the 3 decades since the signing of the National Cancer Act, there has been tremendous progress in the elucidation of the molecular underpinnings of cancer. Molecular genetic studies have been particularly insightful, revealing genetic rearrangements, such as chromosomal translocations, which may be the seminal event leading to deregulated cell growth for many childhood and adult cancers. These findings have led to new diagnostic and prognostic tools but have been slow to be translated into new therapeutic modalities. This article reviews a variety of methods now under development to exploit genetic changes in cancer to develop specific anticancer agents using gene therapy, viral therapy, and immunotherapy approaches. As many of these strategies inevitably enter the clinic, it will be imperative for health care professionals to be familiar with these novel approaches as they help patients navigate the likely broad array of treatment options.

The hammerhead ribozyme

The hammerhead ribozyme is an intriguing RNA molecule with the ability to serve as a catalyst to cleave sequence-specifically RNA molecules in an intermolecular reaction. Preferentially Mg(2+) is required for optimal activity by inducing the catalytically competent conformation and by possibly acting as an acid-base catalyst. Even though the three-dimensional structure has been elucidated details of the structure-function relationship and of the mechanism remain unanswered. The hammerhead ribozyme has stimulated the concept of the sequence-specific cleavage of mRNAs intracellularly and thus to inhibit gene expression by preventing translation. This represents an area of considerable interest as it has the potential for the development of drugs.

HIV-1 LTR as a target for synthetic ribozyme-mediated inhibition of gene expression: site selection and inhibition in cell culture

A library of three synthetic ribozymes with randomized arms, targeting NUX, GUX and NXG triplets, respectively, were used to identify ribozyme-accessible sites on the HIV-1 LTR transcript comprising positions -533 to 386. Three cleavable sites were identified at positions 109, 115 and 161. Ribozymes were designed against these sites, either unmodified or with 2'-modifications and phosphorothioate groups, and their cleavage activities of the transcript were determined. Their biological activities were assessed in cell culture, using a HIV-1 model assay system where the LTR is a promoter for the expression of the reporter gene luciferase in a transient expression system. Intracellular efficiency of the ribozymes were determined by cotransfection of ribozyme and plasmid DNA, expressing the target RNA. Modified ribozymes, directed against positions 115 and 161, lowered the level of LTR mRNA in the cell resulting in inhibition of expression of the LTR-driven reporter gene luciferase of 87 and 61%, respectively. In the presence of Tat the inhibitions were 43 and 25%. The inactive variants of these ribozymes exhibited a similar inhibitory effect. RNase protection revealed a reduction of RNA which was somewhat stronger for the active than the inactive ribozymes, particularly for ribozyme 115. Unmodified ribozymes showed no inhibition in the cell. The third ribozyme, targeting a GUG-triplet at position 109, possessed only low cleavage activity in vitro and no inhibitory effect in cell culture.

Rev-binding aptamer and CMV promoter act as decoys to inhibit HIV replication

We examined whether the antiviral effect of an HIV-1 Rev-binding aptamer [RBE(apt)] could be enhanced by a ribozyme directed against the HIV-1 env gene, and whether the antiviral activity was affected by different promoters. The efficacy of the aptamer and ribozyme DNAs was tested in HeLa cells co-transfected with the HIV-1 proviral clones, HXBDeltaBgl or pNL4-3, using transferrin-lipoplexes. The RBE(apt) and anti-env ribozyme genes were inserted into the pTZU6+27 plasmid, or constructed under the control of the human cytomegalovirus (CMV) or Rous sarcoma virus (RSV) promoters. The parental vector plasmids were used as controls. Co-transfection of the pTZU6+27 RBE(apt) plasmid with HXBDeltaBgl, or pNL4-3, at a weight ratio of 5:1, inhibited p24 production by 70 and 45%, respectively. The RSV RBE(apt) plasmid co-transfected with either HIV clone, at the same weight ratio, reduced viral production by 88%. The addition of the anti-env ribozyme to the RSV RBE(apt) did not enhance its antiviral activity. When the constructs were under the control of the CMV promoter, the expression of the HIV plasmids was very low and was independent of the presence of the RBE(apt). Thus, the effect of the RBE(apt) was strongly dependent on the promoter of the tested construct. The anti-HIV activity of the CMV RBE(apt) construct was non-specific, because co-transfection with either pCMV. SPORT-betagal or pCMVlacZ significantly suppressed HIV production from the HIV proviral clones. The reduction in p24 could not be attributed to the non-specific toxicity of the transfection procedure. Transfection of acutely HIV-infected HeLa-CD4 cells with pCMV.SPORT-betagal reduced the p24 level by 35%, while the expression of the U6 RBE(apt) did not affect p24 production. The suppression of HIV production from the HIV proviral clones by the CMV promoter constructs in the co-transfection assays may be explained by competition for transcription factors (TFs) between HIV and CMV promoters. This observation points to the potential for misleading results in co-transfections involving CMV constructs and HIV.