INTRODUCTION

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Hepatitis B virus (HBV) is a major cause of human viral hepatitis, and exposure to the virus often leads to persistent viral infection of the liver, cirrhosis, and hepatocellular carcinoma (HCC). HBV is a DNA virus which replicates asymmetrically through reverse transcription of an RNA intermediate. HBV has a partially double-stranded 3.2-kb DNA genome from which four major classes of transcripts are synthesized. The 3.5-kb pregenomic RNA serves as a template for reverse transcription and also encodes the nucleocapsid protein and the reverse transcriptase. A subclass of this transcript with a 5'-end extension codes for the precore protein which, after processing, is secreted as HBV e antigen (HBeAg). The 2.4-kb RNA encompasses the pre-S1 open reading frame (ORF), which encodes the large surface protein. The 2.1-kb RNA encompasses the pre-S2 and S ORFs, which encode the middle and small surface proteins, respectively. The smallest transcript (approximately 0.8 kb) codes for the X protein.

Ribozymes are naturally occurring enzymes comprised of RNA that catalyze RNA cleavage and splicing reactions (10). Several different ribozyme motifs with RNA cleavage activity have been discovered (27). Of these, the hammerhead ribozyme (29) is one of the smallest and has the simplest minimal target sequence requirements. Hammerhead and hairpin ribozymes have been studied extensively as experimental tools for trans suppression of gene expression and possible therapeutic applications (2). Ribozyme modifications designed to enhance catalytic activity, nuclease resistance, and intracellular efficiency may be useful for increasing intracellular and therapeutic activity (4, 13, 16-18, 24). Engineered hairpin ribozymes have been shown to inhibit replication of human immunodeficiency virus type 1 in cell culture (23, 32, 33).

An essential prerequisite for antiviral approaches involving ribozymes is the need to identify accessible target sites on substrate RNA. While substrate consensus sequences for cleavage by hammerhead and hairpin ribozymes have been identified, the secondary and tertiary structures of the substrate and potential interactions with cellular factors in vivo may reduce the accessibility of these sequences for the ribozyme. Therefore, the selection of potential target sites on substrate RNA (7) solely on the basis of the primary sequence (sequence selection) often yields poor results with respect to the intracellular activity of corresponding ribozymes. In contrast, activity selection of ribozymes from a large ribozyme library with randomized substrate-binding sequences has the potential to identify ribozyme species with in vitro and in vivo cleavage activity against any given substrate RNA (19, 20, 35). With respect to ribozyme-mediated inhibition of gene expression in intact cells, we believe that activity-selected ribozymes are likely to be more effective than sequence-selected ribozymes.

Since previous attempts to use hammerhead ribozymes for the intracellular inhibition of HBV have been largely unsuccessful (3, 30), we wished to investigate activity-selected hairpin ribozymes for the potential to intracellularly inhibit HBV replication. In the present study, a library of modified hairpin ribozymes with randomized substrate-binding domains was incubated with full-length in vitro-transcribed HBV pregenomic RNA. Primer extension analysis was used to identify several ribozyme target sites within the HBV pregenomic RNA. Ribozymes targeting the identified cleavage sites were then tested for intracellular activity by cotransfection of eukaryotic expression vectors carrying ribozyme expression cassettes together with a DNA construct that encodes a replication-competent genome of HBV. Several ribozymes were identified that markedly inhibit HBV replication, demonstrating that activity-based selection is a useful approach for the identification of hairpin ribozymes that are capable of mediating intracellular antiviral effects on HBV.

	MATERIALS AND METHODS

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Preparation of the randomized ribozyme pool and HBV pregenomic RNA. The pool of ribozymes containing 10 randomized positions in the substrate-binding domain was transcribed from a synthetic DNA template pool using bacteriophage T7 RNA polymerase (pol) as previously described (12, 21). The sequence of the DNA template pool is 5'-TACCAGGTAATGTACCACGACTTACGTC GTGTGTTTCTCTGGTNNRCTTCNNNNNNNCCCTATAGTGAGTCGTAT TA-3', where N is any base, R is A or G, and the underlined sequence represents the bottom strand of the T7 promoter. All DNA oligodeoxynucleotides were synthesized by using standard solid-phase phosphoramidite chemistry on an Applied Biosystems model 392 oligonucleotide synthesizer. Randomized sites were generated by mixing equimolar amounts of all four nucleotides during synthesis. After annealing of the primer T7-Top (5'-TAATACGACTCACTATA-3') to the DNA template pool, RNA transcripts were produced by transcription of the DNA templates with T7 RNA polymerase and purified as previously described (8). The HBV adw2 pregenomic RNA (3.3 kb) was transcribed from linearized plasmid pSP6-HBV (unpublished data; kind gift of Stefan Wieland) by using SP6 RNA polymerase (MEGAscript SP6 Kit; Ambion Inc., Austin, Tex.).

Cleavage reactions using the randomized ribozyme pool. The 3.3-kb HBV pregenomic RNA substrate and the randomized pool of ribozymes were preincubated separately at 37°C for 10 min either in reaction buffer (12 mM MgCl₂, 50 mM Tris HCl, pH 8.0) to permit folding (14) or in control buffer (1 mM EDTA, 50 mM Tris HCl, pH 8.0) in which ribozymes are inactive. Substrate RNA (0.1 µM) was then incubated with the randomized ribozyme pool (20 µM) in reaction buffer or control buffer. Under the assumption of truly random DNA synthesis, the ribozyme pool contained each specific ribozyme molecule at a concentration of 0.04 nM. Cleavage and control reaction mixtures were incubated for 2 h at 37°C (final volume, 5 µl), subsequently desalted with a CentriSep column (Princeton Separations, Adelphia, N.J.), and dried in preparation for primer extension reactions.

Primer extension mapping of accessible cleavage sites. Cleavage sites were identified by primer extension of cleavage products alongside sequencing ladders generated with avian myeloblastosis virus reverse transcriptase and 5'-end-labeled DNA primers as previously described (9). Each primer generated a readable sequence 200 to 250 nucleotides in length. Therefore, 14 different primers were needed to accurately map all of the cleavage sites within the 3.3-kb HBV pregenomic RNA. The nucleotide positions of the complementary primers with respect to the in vitro-transcribed pregenomic RNA sequence were P1 (229 to 238), P2 (451 to 468), P3 (680 to 700), P4 (923 to 945), P5 (1179 to 1196), P6 (1415 to 1434), P7 (1661 to 1680), P8 (1891 to 1910), P9 (2133 to 2154), P10 (2374 to 2395), P11 (2610 to 2627), P12 (2845 to 2862), P13 (3083 to 3103), and P14 (3318 to 3340). The first nucleotide of the in vitro-transcribed HBV pregenomic RNA is nucleotide 1816, taking into account the fact that nucleotide 1 is the first T of the unique EcoRI site.

Construction of ribozyme expression vectors. Ribozymes targeting 17 of the selected sites were designed in accordance with Watson-Crick base pair rules in helix 1 and helix 2 (see Fig. 1). The double-stranded DNA fragments corresponding to both active and inactive ribozyme sequences were generated by PCR using the Sindbis virus-specific ribozyme construct pTZ-U6-8242 (26) as a template. PCR products were purified, digested with SalI and XbaI, and inserted into the SalI/XbaI-digested pTZ-U6 vector (15). In addition, the PCR products were also inserted into the XhoI/XbaI-digested pCI-neo vector (Promega, Madison, Wis.). pCI-neo contains a chimeric intron composed of the 5' donor site from the first intron of the human -globin gene and the branch and 3' acceptor site from the intron of an immunoglobulin gene heavy-chain variable region. In addition, pCI-neo contains the simian virus 40 late polyadenylation signal. The sequences of DNA primers for the most active ribozyme constructs targeting HBV pregenomic RNA sites 1401, 1626, 1781, and 1976 are as follows: 1401-FP, 5'-ATACTAGTCGACGAATTCTGAAGCATACCAGAGAAACAGATCTC; 1401-ina-FP, 5'-ATACTAGTCGACGAATTCTaAAGCATACCAGAtAAACAGATCTC; 1626-FP, 5'-ATACTAGTCGACATTCTTAGAAACAAACCAGAGAAACAGATCTC; 1626-ina-FP, 5'-ATACTAGTCGACATTCTTAaAAACAAACCAGAtAAACAGATCTC; 1781-FP, 5'-ATACTAGTCGACGACACACGAAGCGAACCAGAGAAACAGATCTC; 1781-ina-FP, 5'-ATACTAGTCGACGACACACaAAGCGAACCAGAtAAACAGATCTC; 1976-FP, 5'-ATACTAGTCGACGTTTTGCGAAGCAAACCAGAGAAACAGATCTC; 1976-ina-FP, 5'-ATACTAGTCGACGTTTTGCaAAGCAAACCAGAtAAACAGATCTC; RP, 5'-CCGCTCTAGACCAGGTAATG. FP indicates a forward primer, RP indicates a reverse primer, and ina indicates an inactive ribozyme. Underlined bases are restriction enzyme sites, boldface bases are the substrate-binding region of the hairpin ribozyme, and lowercase bases are mutations that abolish the catalytic activity of the hairpin ribozyme (6).

Cells and transfections. The human HCC cell line HuH-7 (22) was grown in modified Eagle minimal essential medium (Cellgro Mediatech, Washington, D.C.) supplemented with 10% fetal calf serum, 1% nonessential amino acid solution (Life Technologies, Gaithersburg, Md.), and a 1% penicillin-streptomycin stock solution (5,000 U of penicillin G sodium per ml, 5,000 mg of streptomycin per ml; Cellgro Mediatech). Transfections were performed by using a modified calcium phosphate precipitation protocol (11), routinely using 10 µg each of DNA plasmids plus 1 µg of reporter plasmid pTKGH encoding cDNA for human growth hormone (hGH) (25) per 100-mm-diameter plate.

DNA analysis. For the preparation of core-associated HBV DNA, transfected cells were lysed in 50 mM Tris-HCl (pH 8.0)-1 mM EDTA-1% Nonidet P-40. The lysate was centrifuged at 10,000 × g for 5 min at room temperature. After the addition of CaCl₂ and MgCl₂ to a final concentration of 10 mM each, the supernatant was incubated with 20 U of DNase I (Boehringer Mannheim, Indianapolis, Ind.) per ml and micrococcal nuclease (final concentration, 150 U/ml; Pharmacia Biotech) for 2 h at 37°C. Next, EDTA (final concentration, 20 mM), 10% sodium dodecyl sulfate (SDS; final concentration, 1%), and proteinase K (final concentration, 1 mg/ml; Promega) were added and the mixture was incubated for 12 to 16 h at 37°C. Finally, the sample was extracted once with 1 volume of phenol-chloroform. After addition of 3 M sodium acetate, pH 5.5, the DNA was precipitated with 1 volume of isopropanol. The pellet was washed with 80% ethanol, vacuum dried, and resuspended in 20 µl of agarose gel loading buffer. DNA was fractionated by 1.2% agarose gel electrophoresis in Tris-acetate buffer (1). Nucleic acids were transferred to Hybond-N+ nylon membranes (Amersham Life Science, Arlington Heights, Ill.). After UV cross-linking of nucleic acids, membranes were prehybridized for 4 h at 42°C in 50% (vol/vol) formamide-5× SSPE (1× SSPE is 0.15 M NaCl, 0.01 M sodium dihydrogen phosphate, and 1 mM EDTA)-2.5× Denhardt's solution (1× Denhardt's solution is 0.02% polyvinylpyrrolidone, 0.02% Ficoll, and 0.02% bovine serum albumin)-0.1% SDS-200 µg of denatured calf thymus DNA per ml. Hybridization with ³²P-labeled recombinant full-length HBV DNA was performed by incubation in the above-described buffer for 16 h at 42°C. After hybridization, membranes were washed once in 5× SSC-(1× SSC is 0.15 M NaCl plpus 0.015 M sodium citrate)-0.1% SDS for 5 min at 42°C and once in 1× SSC-0.1% SDS for 20 min at 65°C. Membranes were exposed to X-ray film at 80°C.

RNA analysis. Total cellular RNA from cells cotransfected with the HBV head-to-tail dimer (HTD) and various ribozyme expression constructs was extracted with the Ultraspec RNA isolation system (Biotecx Laboratories, Houston, Tex.). Transcripts corresponding to U6 and cytomegalovirus (CMV) ribozymes were detected by primer extension with primers pA-U6 (5'-ACCAGGT AATGTACGATC) and pB-CMV (5'-GTCAGAAGCACTGACTGCG), respectively. HBV pregenomic RNA was detected by primer extension with the primer PREG-PE (5'-AAACGAGAGTAACTCCACAG). Reactions also contained the primer pC-U6 (5'-GGCCATGCTAATCTTCTCTG) that annealed to positions 42 to 61 of the endogenous U6 small nuclear snRNA and yielded signals corresponding to the U6 snRNA used as an internal control. HBV and ribozyme transcript copy numbers were calculated by radiodensitometry (Bio-Rad) and use of endogenous U6-specific signals as an internal reference.

Enzymatic and immunological assays. For the determination of transfection efficiencies, a commercially available assay for secreted hGH (Tandem-R HGH; Hybritech, San Diego, Calif.) was used. HBV surface antigen (HBsAg) was determined by a commercially available radioimmunoassay (AUSRIA II; Abbott Laboratories, North Chicago, Ill.). HBV e antigen (HBeAg) was determined by a commercially available radioimmunoassay (Incstar Corp., Stillwater, Minn.) which does not exhibit cross-reactivity with HBV core antigen.

	RESULTS

Top Abstract Introduction Materials and Methods Results Discussion References

Generation of a hairpin ribozyme library with randomized substrate-binding domains. A hairpin ribozyme library was generated by in vitro transcription of synthetic DNA templates. Nine positions within the substrate-binding domain were randomized, and one was fixed as a pyrimidine (Fig. 1A), while the bases which are important for catalytic activity (G8, A9, A10, and G11) remained fixed (Fig. 1A). The resulting library of hairpin ribozymes was expected to contain 5 × 10⁵ different members. For cleavage assays, the quantity of ribozymes used (100 pmol; 6 × 10¹³ molecules) greatly exceeded the sequence complexity of the pool. Therefore, all potential sequences within the randomized pool were likely to be present in multiple copies in the assay.

View larger version (29K): [in this window] [in a new window]   FIG. 1.   Structure of the hairpin ribozyme (A) and in vitro site screening (B). (A) The modified hairpin ribozyme used in the present study consists of a 57-nucleotide RNA molecule which binds to and cleaves an RNA substrate (top strand). The catalytic RNA folds into a two-dimensional structure that resembles a hairpin consisting of helices 3 and 4 and internal loop B. In addition, helices 1 and 2 and internal loop A form between the ribozyme and its substrate. Recognition of the substrate by the ribozyme utilizes Watson-Crick base pairing within helices 1 and 2 (N, any nucleotide; R, A or G; Y, T or C). Cleavage occurs immediately 5' of the substrate G in loop A, as indicated by an arrow. The catalytic activity, but not substrate-binding activity, of the hairpin ribozyme is abolished by mutating G8 and G21 to A and U, respectively (6). (B) In vitro combinatorial activity-based screening. A large library (nine ribozyme positions are totally randomized [N], and one position is randomized between T and C [Y], yielding a complexity of approximately 5 × 105 members) of in vitro-generated hairpin ribozymes (Rz) was incubated with HBV pregenomic RNA as the substrate. Control reactions were performed in the absence of the ribozyme library and MgCl2. After cleavage occurred, primer extension assays using various primers (P1, P2, and P3) were performed to map the 5' ends of HBV pregenomic RNA cleavage products. Extension products were separated on sequencing gels together with RNA sequencing reactions. Criteria for the identification of extension products corresponding to authentic ribozyme cleavage sites were that the product be detectable only in the presence of both the ribozyme pool and MgCl2 and have a guanosine residue at its 5' end.

Cleavage of HBV pregenomic RNA by the ribozyme library in vitro. In vitro-transcribed HBV pregenomic RNA (0.1 µM) was incubated at 37°C for 2 h with the randomized pool of hairpin ribozymes (20 µM), either in the presence or in the absence of MgCl₂. Because the hairpin ribozyme library contained 5 × 10⁵ different members, the concentration of each individual ribozyme species was calculated to be 0.04 nM, assuming truly random DNA synthesis. Therefore, the HBV substrate RNA was calculated to be present in approximately 2,500-fold molar excess over each individual ribozyme species. Because most substrate RNA molecules remained uncleaved at the end of the digestion, most products resulted from a single cleavage event within target RNA molecules, not from multiple cleavage events within the same target.

Mapping of hairpin ribozyme cleavage sites on HBV pregenomic RNA. Primer extension assays (Fig. 1B; see also Materials and Methods) were carried out to map 5' ends of HBV pregenomic RNA cleavage products that occurred after incubation with the hairpin ribozyme library. Extension products were separated on sequencing gels together with sequencing reaction mixtures (Fig. 2). With this method, sites on the HBV pregenomic RNA that were accessible for hairpin ribozymes and subsequent cleavage could be accurately mapped. As an example, Fig. 2, lane 3, illustrates in vitro cleavage of HBV pregenomic RNA target sites 1401, 1626, 1781, and 1976 by the randomized pool of hairpin ribozymes (the first nucleotide of the in vitro-transcribed HBV pregenomic RNA is nucleotide 1816, taking into account the fact that nucleotide 1 is the first T of the unique EcoRI site).

View larger version (65K): [in this window] [in a new window]   FIG. 2.   In vitro cleavage of HBV pregenomic RNA target sites 1401, 1626, 1781, and 1976 by the randomized pool of hairpin ribozymes. Sites are numbered according to the position of the G located immediately 3' to the cleavage site in the in vitro-transcribed HBV pregenomic RNA sequence. Control reactions were performed in the absence of ribozyme (Rz) (lane 1) or MgCl2 (Mg++) (lane 2). Specific cleavage products (compare lane 3 with lanes 1 and 2, respectively) were readily detectable (arrows) and had a guanosine at the 5' terminus. Note that the folded structure of the RNA template caused nonspecific terminations of the extension reaction that appeared as background bands in lanes 1, 2, and 3. Lanes 4 to 7 contained parallel RNA sequencing reactions with the same primers used for primer extension that enabled the accurate mapping of cleavage sites.

Design and construction of hairpin ribozymes for selected targets. Of the 40 cleavage sites on the HBV pregenomic RNA that were initially identified, 17 sites whose sequence was conserved among all HBV subtypes were chosen for the construction of corresponding hairpin ribozymes and viral inhibition assays. The locations of these 17 sites on the pregenomic RNA and their positions with respect to HBV subgenomic transcripts and ORFs are illustrated in Fig. 3. Cleavage sites were clustered between nucleotides 520 and 600, 1400 and 2100, and 3000 and 3200. Two single sites were localized at nucleotide positions 122 and 2595. Interestingly, no sites were identified between nucleotides 612 and 1256, suggesting that this region of the pregenomic RNA was not accessible for cleavage by hairpin ribozymes in vitro.

View larger version (13K): [in this window] [in a new window]   FIG. 3.   Locations of conserved cleavage sites for hairpin ribozymes within the HBV pregenomic (3.5-kb) RNA. The first nucleotide of the in vitro-transcribed HBV pregenomic RNA is located 5 nucleotides upstream from the naturally occurring transcriptional start point. The positions of the encapsidation signal () and direct repeats DR1 and DR2 are shown. HBV subgenomic transcripts 2.4, 2.1, and 0.9 kb length are depicted by lines below the pregenomic RNA. The core (C); pol; large (preS1), middle (preS2), and small (S) surface antigen; and X ORFs are depicted as open arrows. The identified hairpin ribozyme cleavage sites were clustered between nucleotides 520 and 600, 1400 and 2100, and 3000 and 3200. Two single sites were localized at nucleotide positions 122 and 2595. pA, polyadenylation site.

Expression of ribozymes in cells. Ribozyme expression vectors were cotransfected into HuH-7 HCC cells together with a replication-competent HBV HTD. Two days after transfection, total cellular RNA was prepared. Endogenously synthesized U6 snRNA and ribozymes transcribed from U6 snRNA and CMV promoters, as well as HBV pregenomic RNA (data not shown), were detected by primer extension (Fig. 4). As demonstrated in Fig. 4, lanes 1 to 6, specific extension products corresponding to U6 ribozymes 1401, 1781, and 1976 and their catalytically inactive variants were detectable in transfected cells. The CMV ribozyme 1626 and its inactive variant were also readily detectable (Fig. 4, lanes 9 and 10).

View larger version (39K): [in this window] [in a new window]   FIG. 4.   Expression of ribozymes and catalytically inactive variants in HuH-7 cells. Primer extension analysis of total cellular RNA from cells cotransfected with an HBV HTD and ribozyme expression constructs U6-1401, U6-1781, U6-1976, and CMV-1626, as well as catalytically inactive variants. HBV pregenomic RNA was detected by primer extension (data not shown). Transcripts corresponding to U6 and CMV ribozymes were detected by using primers pA-U6 (5'-ACCAGGTAATGTACGATC) and pB-CMV (5'-GTCAGAAGCACTGACTGCG), respectively. Reaction mixtures also contained primer pC-U6 (5'-GGCCATGCTAATCTTCTCTG), which yielded signals corresponding to endogenous U6 snRNA. Note that all of the ribozymes and their catalytically inactive variants were expressed (lanes 1 to 6 and 9 to 10). NC, negative control.

Inhibition of HBV replication in cells. Table 1 summarizes results from transient-cotransfection experiments with an HBV HTD and all 17 ribozyme expression constructs or the vector (pTZ-U6 or pCI-neo) in HuH-7 HCC cells. All cotransfections were performed at least three times, and plate-to-plate variations in transfection efficiency were determined by analyzing hGH levels in cell culture supernatants. Nucleocapsid-associated HBV replication products were detected on Southern blots with an HBV-specific probe and quantified by radiodensitometry. The inhibition percentage was calculated as follows: 1  ([Signal_Rz/Signal_wt]/[hGH_Rz/hGH_wt]) × 100. No inhibition of HBV replication was observed when a series of ribozymes directed against an unrelated virus (Sindbis virus) were expressed from a U6 snRNA or CMV promoter (data not shown). Most of the U6 and CMV ribozymes inhibited HBV replication to some degree. Figure 5 summarizes the results obtained with ribozymes U6-1401, U6-1781, U6-1976, and CMV-1626 and their catalytically inactive controls (designated ina). Ribozyme U6-1401 inhibited HBV replication by 69% (lane 1), whereas U6-1401-ina affected HBV replication by 28% (lane 2). Inhibition values for the other ribozymes were as follows: U6-1781, 49% (lane 4); U6-1781-ina, 0% (lane 5); U6-1976, 80% (lane 7); U6-1976-ina, 36% (lane 8); CMV-1626, 66% (lane 10); CMV-1626-ina, 0% (lane 11). Thus, the ribozyme with the highest inhibitory potential against HBV replication was U6-1976, with 80% inhibition. To assess the specificity of the inhibitory effect of the four best, they were also tested for the ability to inhibit duck HBV in LMH chicken hepatoma cells. None of the four ribozymes affected duck HBV replication (data not shown). In conclusion, several of the in vitro-selected ribozymes had inhibitory effects on HBV replication in transfected cells, and three ribozymes inhibited HBV replication by more than 50%.

View larger version (48K): [in this window] [in a new window]   FIG. 5.   Intracellular inhibition of HBV replication by four hairpin ribozymes. Southern blot analysis of nucleocapsid-associated HBV DNA isolated from HuH-7 cells cotransfected with an HTD of HBV and ribozyme expression constructs U6-1401, U6-1781, U6-1976, and CMV-1626, as well as their catalytically inactive variants (designated ina). hGH levels for the assessment of plate-to-plate variations in transfection efficiency and inhibition percentages calculated from radiodensitometry are depicted below each panel. Ribozyme U6-1401 inhibited HBV replication by 69% (lane 1), whereas U6-1401-ina affected HBV replication by 28% (lane 2). Inhibition values for the other ribozymes were as follows: U6-1781, 49% (lane 4); U6-1781-ina, 0% (lane 5); U6-1976, 80% (lane 7); U6-1976-ina, 36% (lane 8); CMV-1626, 66% (lane 10); CMV-1626-ina, 0% (lane 11). Results are representative of at least three independent experiments. DS, position of HBV double-stranded linear DNA.

Effects of ribozymes on HBV antigen levels in cell culture supernatants. To assess the effects of the most effective ribozymes on HBsAg and HBeAg synthesis and secretion, cell culture supernatants from cotransfections with an HBV HTD and ribozymes U6-1401, U6-1781, U6-1976, and CMV-1626 were analyzed for HBsAg and HBeAg levels (Fig. 6). After correction for variations in transfection efficiency by the hGH assay, the normalized antigen (Ag_n) levels were used to monitor the decrease in antigen expression relative to the control (1  [ribozyme Ag_n/control Ag_n] × 100). Thus, variations in antigen levels reflecting antigen secretion were correlated to variations in transfection efficiency, and inhibition values are expressed in percent. As demonstrated in Fig. 6, all four ribozymes reduced extracellular HBeAg levels. The reductions were as follows: U6-1401, 62%; U6-1781, 38%; U6-1976, 64%; CMV-1626, 42%. Since all four ribozymes targeted sequences that were also present on HBV subgenomic transcripts coding for HBsAgs, extracellular HBsAg levels were determined. As demonstrated in Fig. 6, the effects of ribozymes on HBsAg levels were generally weaker. The reductions were as follows: U6-1401, 38%; U6-1781, 34%; U6-1976, 5%; CMV-1626, 12%. In conclusion, the ribozymes with the greatest potential to inhibit HBV replication also had significant effects on extracellular HBeAg levels. Extracellular HBsAg levels were affected to a lesser degree. This antigen is translated from the subgenomic 2.1-kb transcript, which was not used as target in the in vitro selection assay. It is therefore possible that the selected ribozymes did not efficiently anneal to this transcript, resulting in smaller reductions of HBsAg versus HBeAg levels.

View larger version (38K): [in this window] [in a new window]   FIG. 6.   Effects of four ribozymes on HBsAg and HBeAg levels in cell culture supernatants. Cell culture supernatants from cotransfections with an HBV HTD and ribozymes U6-1401, U6-1781, U6-1976, and CMV-1626 were analyzed for HBsAg and HBeAg levels. Variations in transfection efficiency were assessed by hGH assay. After correction for variations in transfection efficiency by the hGH assay, normalized antigen levels (Agn) were used to monitor the decrease of antigen expression relative to the control (1  [ribozyme Agn/control Agn] × 100). Results were expressed as percent inhibition compared with the wild type (cotransfection of vector). All four ribozymes reduced extracellular HBeAg levels. The reductions were as follows: U6-1401, 62%; U6-1781, 38%; U6-1976, 64%; CMV-1626, 42%. Since all four ribozymes also targeted sequences that were present on HBV subgenomic transcripts coding for HBsAgs, extracellular HBsAg levels were determined. The effects of the four ribozymes on HBsAg levels were weaker. The reductions were as follows: U6-1401, 38%; U6-1781, 34%; U6-1976, 5%; CMV-1626, 12%.

	DISCUSSION

Top Abstract Introduction Materials and Methods Results Discussion References

In this study, we have used a combinatorial screening method, employing a complex library of hairpin ribozymes with all possible specificities, to identify conserved sites within the full-length HBV pregenomic RNA that could be effectively cleaved by corresponding hairpin ribozymes. A total of 40 cleavage sites were identified. Seventeen sites were conserved among all HBV subtypes and were subjected to further analysis in transfected HuH-7 HCC cells. Interestingly, most of the conserved cleavage sites were positioned in three distinct clusters on the HBV pregenomic RNA. However, a large segment of the pregenomic RNA, from nucleotide position 612 to position 1256, was entirely devoid of cleavage sites. It is possible that the in vitro secondary and tertiary structures of the HBV pregenomic RNA in this region prevent ribozymes from binding to their substrate RNA and subsequent cleavage. Hairpin ribozymes corresponding to the 17 conserved cleavage sites were designed and expressed in cells together with a replication-competent genome of HBV. While several ribozymes showed only modest inhibition of HBV replication, four ribozymes (U6-1401, U6-1781, U6-1976, and CMV-1626) markedly inhibited HBV replication and decreased extracellular HBeAg levels, whereas HBsAg levels were less affected. Catalytically inactive variants of these ribozymes had much weaker effects, suggesting that the majority of the inhibitory effects on HBV replication were mediated by the catalytic activity of the ribozymes rather than by an antisense mechanism. Therefore, we have identified hairpin ribozymes by in vitro selection that effectively inhibit HBV replication, presumably by cleavage of HBV pregenomic RNA under physiological conditions within the complex cellular environment.

Another study has described hairpin ribozyme genes that were cotransfected into HuH-7 HCC cells together with small amounts of an HTD of HBV (31). Under these conditions, the virus particle-associated HBV levels, as determined by the endogenous pol assay, were reduced by up to 83%. However, this study did not correlate inhibition levels with plate-to-plate variations in transfection efficiency, and it is therefore unclear to which extent HBV replication was truly inhibited. Interestingly, the two hairpin ribozymes used in this study targeted cleavage sites on HBV pregenomic RNA at nucleotide positions 1703 and 2938 that were not identified in our experiments. However, in vitro cleavage was demonstrated by using short (ca. 40 nucleotides) substrates, not full-length HBV pregenomic RNA.

Two studies have demonstrated that hammerhead ribozymes can specifically cleave HBV RNA in a cell-free system (30) and cell extracts (3). The first study used three hammerhead ribozymes transcribed from a single DNA template that were directed against three adjacent sites within the HBV pregenomic RNA (30). It was demonstrated that all three ribozymes cleaved HBV substrate RNA and that cleavage efficiency was similar to that of single-ribozyme constructs. The second study demonstrated that efficient hammerhead ribozyme-mediated cleavage of the viral encapsidation signal could be achieved in vitro and in cell extracts but not in intact cells (3). Cleavage was most efficient after treatment of cotransfected cells with proteinase K, extraction with phenol, and supplementation of the extract with Mg²⁺ indicating that, in this case, cellular proteins and low Mg²⁺ concentrations limited intracellular ribozyme activity. However, cellular proteins may also have positive effects on hammerhead ribozyme-mediated catalysis (5, 28). Because of its complex structure and its nature as a binding site for cellular and/or viral proteins, the viral encapsidation signal may not be the optimal target for ribozyme attack, and more accessible sites on the HBV pregenomic RNA might be identified that are susceptible to hammerhead ribozyme-mediated cleavage in intact cells.

Our study demonstrates that in vitro selection yields hairpin ribozymes that can effectively reduce HBV replication in transfected cells. One of the major challenges of gene therapy for HBV is the delivery of therapeutic genes to infected cells. In the future, recombinant retroviruses, adenoviruses, or adeno-associated viruses containing therapeutic ribozyme genes will have to be generated to allow efficient delivery of the therapeutic transgenes into cells. Thus, the full potential of a molecular therapeutic approach involving hairpin ribozymes directed against HBV may be realized.