In vitro selection supports the view of a kinetic control of antisense RNA-mediated inhibition of gene expression in mammalian cells

RNA Structure Design Improves Activity and Specificity of trans-Splicing-Triggered Cell Death in a Suicide Gene Therapy Approach

Spliceosome-mediated RNA trans-splicing enables correction or labeling of pre-mRNA, but therapeutic applications are hampered by issues related to the activity and target specificity of trans-splicing RNA (tsRNA). We employed computational RNA structure design to improve both on-target activity and specificity of tsRNA in a herpes simplex virus thymidine kinase/ganciclovir suicide gene therapy approach targeting alpha fetoprotein (AFP), a marker of hepatocellular carcinoma (HCC) or human papillomavirus type 16 (HPV-16) pre-mRNA. While unstructured, mismatched target binding domains significantly improved 3′ exon replacement (3’ER), 5′ exon replacement (5’ER) correlated with the thermodynamic stability of the tsRNA 3′ end. Alternative on-target trans-splicing was found to be a prevalent event. The specificity of trans-splicing with the intended target splice site was improved 10-fold by designing tsRNA that harbors secondary target binding domains shielding alternative on-target and blinding off-target splicing events. Such rationally designed suicide RNAs efficiently triggered death of HPV-16-transduced or hepatoblastoma-derived human tissue culture cells without evidence for off-target cell killing. Highest cell death activities were observed with novel dual-targeting tsRNAs programmed for trans-splicing toward AFP and a second HCC pre-mRNA biomarker. Our observations suggest trans-splicing represents a promising approach to suicide gene therapy.

Coronary heart disease-associated variation in TCF21 disrupts a miR-224 binding site and miRNA-mediated regulation

Genome-wide association studies (GWAS) have identified chromosomal loci that affect risk of coronary heart disease (CHD) independent of classical risk factors. One such association signal has been identified at 6q23.2 in both Caucasians and East Asians. The lead CHD-associated polymorphism in this region, rs12190287, resides in the 3′ untranslated region (3′-UTR) of TCF21, a basic-helix-loop-helix transcription factor, and is predicted to alter the seed binding sequence for miR-224. Allelic imbalance studies in circulating leukocytes and human coronary artery smooth muscle cells (HCASMC) showed significant imbalance of the TCF21 transcript that correlated with genotype at rs12190287, consistent with this variant contributing to allele-specific expression differences. 3′ UTR reporter gene transfection studies in HCASMC showed that the disease-associated C allele has reduced expression compared to the protective G allele. Kinetic analyses in vitro revealed faster RNA-RNA complex formation and greater binding of miR-224 with the TCF21 C allelic transcript. In addition, in vitro probing with Pb²⁺ and RNase T1 revealed structural differences between the TCF21 variants in proximity of the rs12190287 variant, which are predicted to provide greater access to the C allele for miR-224 binding. miR-224 and TCF21 expression levels were anti-correlated in HCASMC, and miR-224 modulates the transcriptional response of TCF21 to transforming growth factor-β (TGF-β) and platelet derived growth factor (PDGF) signaling in an allele-specific manner. Lastly, miR-224 and TCF21 were localized in human coronary artery lesions and anti-correlated during atherosclerosis. Together, these data suggest that miR-224 interaction with the TCF21 transcript contributes to allelic imbalance of this gene, thus partly explaining the genetic risk for coronary heart disease associated at 6q23.2. These studies implicating rs12190287 in the miRNA-dependent regulation of TCF21, in conjunction with previous studies showing that this variant modulates transcriptional regulation through activator protein 1 (AP-1), suggests a unique bimodal level of complexity previously unreported for disease-associated variants.

Both genetic and environmental factors cumulatively contribute to coronary heart disease risk in human populations. Large-scale meta-analyses of genome-wide association studies have now leveraged common genetic variation to identify multiple sites of disease susceptibility; however, the causal mechanisms for these associations largely remain elusive. One of these disease-associated variants, rs12190287, resides in the 3′untranslated region of the vascular developmental transcription factor, TCF21. Intriguingly, this variant is shown to disrupt the seed binding sequence for microRNA-224, and through altered RNA secondary structure and binding kinetics, leads to dysregulated TCF21 gene expression in response to disease-relevant stimuli. Importantly TCF21 and miR-224 expression levels were perturbed in human atherosclerotic lesions. Along with our previous reports on the transcriptional regulatory mechanisms altered by this variant, these studies shed new light on the complex heritable mechanisms of coronary heart disease risk that are amenable to therapeutic intervention.

Homologous SV40 RNA trans-splicing: a new mechanism for diversification of viral sequences and phenotypes

Simian Virus 40 (SV40) is a polyomavirus found in both monkeys and humans, which causes cancer in some animal models. In humans, SV40 has been reported to be associated with cancers but causality has not been proven yet. The transforming activity of SV40 is mainly due to its 94-kD large T antigen, which binds to the retinoblastoma (pRb) and p53 tumor suppressor proteins, and thereby perturbs their functions. Here we describe a 100 kD super T antigen harboring a duplication of the pRB binding domain that was associated with unusual high cell transformation activity and that was generated by a novel mechanism involving homologous RNA trans-splicing of SV40 early transcripts in transformed rodent cells. Enhanced trans-splice activity was observed in clones carrying a single point mutation in the large T antigen 5' donor splice site (ss). This mutation impaired cis-splicing in favor of an alternative trans-splice reaction via a cryptic 5'ss within a second cis-spliced SV40 pre-mRNA molecule and enabled detectable gene expression. Next to the cryptic 5'ss we identified additional trans-splice helper functions, including putative dimerization domains and a splice enhancer sequence. Our findings suggest RNA trans-splicing as a SV40-intrinsic mechanism that supports the diversification of viral RNA and phenotypes.

Effects of local mRNA structure on posttranscriptional gene silencing

Antisense oligodeoxynucleotides (AONs) and short interfering RNAs (siRNAs) effect posttranscriptional gene silencing (PTGS) by hybridizing to an mRNA and then directing its cleavage. To understand the constraints that mRNA structure imposes on AON- vs. siRNA-mediated PTGS, AON- and siRNA-mediated cleavage of defined mRNA structures was monitored in Drosophila embryo whole-cell lysates. We observed that AON-directed cleavage was approximately 3-fold faster than cleavage with a siRNA directed to the same target site. Furthermore, and unexpectedly, AON-mediated cleavage was found to be much less fastidious with respect to target sequence accessibility, as measured by the presence of unpaired nucleotides, than a corresponding siRNA. Nonetheless, in vivo, siRNAs silenced their mRNA target at least 2-fold more efficiently than the corresponding AON. These seemingly contradictory results suggested that additional, as yet undefined factors play an important role in regulating PTGS efficiency in vivo. We used a well defined RNA-binding protein, alphaCP, and its corresponding high-affinity RNA-binding site to explore this hypothesis. We found that prebound alphaCP effectively blocked AON-mediated cleavage of the RNA-binding site compared with cleavage of the site in the absence of alphaCP. We conclude that higher-order structures formed by RNA and bound proteins play an important role in determining the efficiency of AON-directed PTGS. We hypothesize that strategies aimed at removing RNA-binding proteins might significantly improve AON-mediated PTGS in vivo.

Hitting bacteria at the heart of the central dogma: sequence-specific inhibition

An important objective in developing new drugs is the achievement of high specificity to maximize curing effect and minimize side-effects, and high specificity is an integral part of the antisense approach. The antisense techniques have been extensively developed from the application of simple long, regular antisense RNA (asRNA) molecules to highly modified versions conferring resistance to nucleases, stability of hybrid formation and other beneficial characteristics, though still preserving the specificity of the original nucleic acids. These new and improved second- and third-generation antisense molecules have shown promising results. The first antisense drug has been approved and more are in clinical trials. However, these antisense drugs are mainly designed for the treatment of different human cancers and other human diseases. Applying antisense gene silencing and exploiting RNA interference (RNAi) are highly developed approaches in many eukaryotic systems. But in bacteria RNAi is absent, and gene silencing by antisense compounds is not nearly as well developed, despite its great potential and the intriguing possibility of applying antisense molecules in the fight against multiresistant bacteria. Recent breakthrough and current status on the development of antisense gene silencing in bacteria including especially phosphorothioate oligonucleotides (PS-ODNs), peptide nucleic acids (PNAs) and phosphorodiamidate morpholino oligomers (PMOs) will be presented in this review.

Computational identification of antisense oligonucleotides that rapidly hybridize to RNA

The ability of a computational model to determine the relative rate of hybridization between anti-sense oligonucleotides and RNA was tested using HIV-1 tat mRNA. The model, which was based on the assumptions that hybridization is a second-order reaction and that early in the hybridization reaction the concentrations of intermediates are approximately constant (steady-state), allows calculation of a rate factor that is proportional to the reaction constant. Formation of oligodeoxynucleotide (ODN)-RNA hybrid, detected by RNase H-dependent cleavage, increased nearly linearly during an initial incubation period, consistent with the steady-state approximation. The initial hybridization rate increased linearly with substrate RNA concentration and with ODN concentration, indicating a second-order reaction. The logarithm of the second-order reaction constant, determined from the initial rate for hybridization between tat mRNA and 16 ODNs targeted to various sites, was linearly related to the logarithm of the calculated rate factor (r = 0.83, p < 0.001). Thus, the rate factor can be used to identify rapidly hybridizing antisense sequences using target nucleotide sequence information.

ROLL: a method of preparation of gene-specific oligonucleotide libraries

The selection of nucleic acid sequences capable of specifically and efficiently hybridizing to target sequences is crucial to the success of many applications, including microarrays, PCR and other amplification procedures, antisense inhibition, ribozyme-mediated cleavage, and RNA interference (RNAi). Methods of selection using nucleotide sequence libraries have several advantages over rational approaches using defined sequences. However, the high complexity of completely random (degenerate) libraries and their high toxicity in cell-based assays make their use in many applications impractical. Gene-specific oligonucleotide libraries, which contain all possible sequences of a certain length occurring within a given gene, have much lower complexity and, thus, can significantly simplify and accelerate sequence screening. Here, we describe a new method for the preparation of gene-specific libraries using the ligation of randomized oligonucleotide probes hybridized adjacently on target polynucleotide templates followed by PCR amplification. We call this method random oligonucleotide ligated libraries (ROLL).

Antisense technology in molecular and cellular bioengineering

Antisense technology is finding increasing application not only in clinical development, but also for cellular engineering. Several types of antisense methods (e.g. antisense oligonucleotides, antisense RNA and small interfering RNA) can be used to inhibit the expression of a target gene. These antisense methods are being used as part of metabolic engineering strategies to downregulate enzymes controlling undesired pathways with regard to product formation. In addition, they are beginning to be utilized to control cell phenotype in tissue engineering constructs. As improved methods for antisense effects that can be externally regulated emerge, these approaches are likely to find increased application in cellular engineering applications.

The possible physiological function of thioltransferase in cells

We sought to study the possible physiological function of thioltransferase (TTase) in combating oxidative damage in the lens epithelial cells. The cells transfected with either TTase-containing plasmid or vector only were compared for their resistance to oxidative stress in the presence of a bolus of H2O2 (0.1 mM) for 3 h. Cells depleted of TTase activity upon cadmium treatment were also examined for the resistance to oxidative stress under the same conditions. TTase activity assay, Western blot, and Northern blot analyses confirmed that hTTase gene was successfully transfected into the HLE B3 cells and was overexpressed. The TTase-transfected cells detoxified H2O2 as efficiently as the control cells but displayed a faster and more complete recovery of oxidatively inactivated glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and glutathione peroxidase (GPx) activities and suppressed protein thiolation (PSSG formation). With TTase activity being inhibited by cadmium, the spontaneous reactivation of GAPDH under bolus H2O2 treatment was not accomplished in cadmium-pretreated cells. These data indicate a new physiological function of TTase, which involves in the reactivation of the oxidatively inactivated enzymes through dethiolation; thus this redox-regulating enzyme can protect the human lens epithelial cells and maybe other cell types by preventing them from permanent oxidative damage.

Modeling hybridization kinetics

Formation of complementary base pairs between nucleic acids over a short region (<or=15 nucleotides) is described by a kinetic model in which the intermediate state is assumed to be locally single stranded. The model enables calculation of a rate factor that is proportional to the rate constant for hybridization under steady-state reaction conditions. Rate factors calculated for various sites in acetylcholinesterase mRNA correlated with sites found previously to be experimentally accessible for hybridization to antisense oligonucleotides. Hybridization rate of longer antisense oligodeoxynucleotides was modeled by calculation of a maximal rate factor for all possible 15-nucleotide segments of a given antisense molecule. Maximal rate factor calculated for a set of antisense oligonucleotides correlated (r=0.95) with initial rate of hybridization reported previously. Two other models proposed for identifying accessible sites for hybridization were less predictive than the rate factor calculation.

Design of antisense RNA constructs for downregulation of the acetone formation pathway of Clostridium acetobutylicum

We investigated the effect of antisense RNA (asRNA) structural properties on the downregulation efficacy of enzymes in the acetone-formation pathway (acetoacetate decarboxylase [AADC] and coenzyme A-transferase [CoAT]) of Clostridium acetobutylicum strain ATCC 824. First, we generated three strains, C. acetobutylicum ATCC 824 (pADC38AS), 824(pADC68AS), and 824(pADC100AS), which contain plasmids that produce asRNAs of various lengths against the AADC (adc) transcript. Western analysis showed that all three strains exhibit low levels of AADC compared to the plasmid control [ATCC 824(pSOS95del)]. By using computational algorithms, the three different asRNAs directed toward AADC, along with previously reported clostridial asRNAs, were examined for structural features (free nucleotides and components). When the normalized metrics of these structural features were plotted against percent downregulation, only the component/nucleotide ratio correlated well with in vivo asRNA effectiveness. Despite the significant downregulation of AADC in these strains, there were no concomitant effects on acetone formation. These findings suggest that AADC does not limit acetone formation and, thus, we targeted next the CoAT. Using the component/nucleotide ratio as a selection parameter, we developed three strains [ATCC 824 (pCTFA2AS), 824(pCTFB1AS), and 824(pCOAT11AS)] which express asRNAs to downregulate either or both of the CoAT subunits. Compared to the plasmid control strain, these strains produced substantially low levels of acetone and butanol and Western blot analyses showed significantly low levels of both CoAT subunits. These results show that CoAT is the rate-limiting enzyme in acetone formation and strengthen the hypothesis that the component/nucleotide ratio is a predictive indicator of asRNA effectiveness.

Elucidation and characterization of oligonucleotide-accessible sites on HIV-2 leader region RNA

The retroviruses, including the human pathogens HIV-1 and HIV-2, are diploid inasmuch as they encapsidate two copies of their RNA genome. Prior to or during encapsidation, two copies of full-length genomic RNA recognize and stably bind each other in a process called dimerization. RNA structures within the viral genome promote dimerization in both HIV-1 and HIV-2 and are located in the 5'-untranslated leader region. Inhibition of dimerization by mutation of these RNA signals has been demonstrated to drastically reduce viral infectivity and replication kinetics and, thus, represents a potential target for antiretroviral therapy. In this study, we identified sites in HIV-2 leader region RNA that are functionally accessible to hybridization with oligonucleotides (ODNs) by reverse transcription with random ODN libraries (RT-ROL). We then tested specific ODNs directed against these regions for their efficacy in inhibiting RNA dimerization in vitro. We determined that of several hybridization-competent ODNs, only two were very effective in inhibiting RNA dimerization. Both of these ODNs were complementary to viral RNA at the primer binding site (PBS). These results identify regions with high accessibility to ODN binding on HIV-2 RNA and help to map the region(s) essential for dimerization within the viral RNA.

Modulation of RNA function by oligonucleotides recognizing RNA structure

Numerous RNA structures are responsible for regulatory processes either because they constitute a signal, like the hairpins or pseudoknots involved in ribosomal frameshifting, or because they are binding sites for proteins such as the trans-activating responsive RNA element of the human immunodeficiency virus whose binding to the viral protein Tat and cellular proteins allows full-length transcription of the retroviral genome. Selective ligands able to bind with high affinity to such RNA motifs may serve as tools for dissecting the molecular mechanisms in which they are involved. Such ligands might also constitute prototypes of therapeutic agents when RNA structures play a role in the expression of dysfunctional genes or in the multiplication of pathogens. Different classes of ligands (aminoglycosides, interacalating agents, peptides) are of interest to this aim. However, oligonucleotides deserve particular consideration. They have been extensively used in the frame of the antisense strategy. The apparent simplicity of this rational approach is, at first sight, very attractive. Indeed, numerous successful studies have been published describing the efficient inhibition of translation, splicing, or reverse transcription in cell-free systems, in cultured cells, or in vivo by oligomers complementary to an RNA region. However, RNA structures restrict the access of the target site to the antisense sequence: The competition between the intramolecular association of RNA regions weakens or even abolishes the antisense effect. Various possibilities have been developed to circumvent this limitation. This includes both rational and combinatorial strategies. High-affinity oligomers were designed to invade the RNA structure. Alternatively, triplex-forming oligonucleotides (TFO) and aptamers may recognize the folded RNA motif. Whereas the use of TFOs is rather limited owing to the strong sequence constraints for triple-helix formation, in vitro selection offers a way to explore vast oligoribo or oligodeoxyribo libraries to identify strong, selective oligonucleotide binders. The candidates (aptamers) selected against the TAR RNA element of HIV-1, which form stable loop-loop (kissing) complexes with the target, provide interesting examples of oligonucleotides recognizing a functional RNA structure through an important contribution of tertiary interactions.