Varicella-zoster virus ORF 58 gene is dispensable for viral replication in cell culture
© Yoshii et al; licensee BioMed Central Ltd. 2008
Received: 04 January 2008
Accepted: 30 April 2008
Published: 30 April 2008
Open reading frame 58 (ORF58) of varicella-zoster virus (VZV) lies at the 3'end of the Unique long (UL) region and its functional is unknown. In order to clarify whether ORF58 is essential for the growth of VZV, we constructed a deletion mutant of ORF58 (pOka-BACΔ58) from the Oka parental genome cloned into a bacterial artificial chromosome (pOka-BAC).
The ORF58-deleted virus (rpOkaΔ58) was reconstituted from the pOka-BACΔ58 genome in MRC-5 cells, indicating that the ORF58 gene is non-essential for virus growth. Comparison of the growth rate of rpOkaΔ58 and recombinant wild-type virus by assessing plaque sizes revealed no significant differences between them both in MRC-5 cells and malignant melanoma cells.
This study shows that the ORF58 gene is dispensable for viral replication and does not affect the virus' ability to form plaques in vitro.
Varicella-zoster virus (VZV) is a member of the herpesviridae family, and its primary infection causes varicella in children. VZV often persistently infects dorsal root ganglia (DRG) and is sometimes activated from a latent to lytic state, causing zoster in aged and immunosuppressed individuals . The double-stranded VZV genome contains approximately 125 kbp with at least 71 open reading flames (ORFs) . Although understanding VZV virulence and attenuation mechanisms requires study of the VZV-encoded genes, little has been reported on VZV genes compared with those of herpes simplex virus (HSV).
The ORF58 of VZV lies at the 3'end of the Unique long (UL) region and its function is unknown. Although ORF57, its neighboring gene, is dispensable in cell culture , there has been no report yet on ORF58. Therefore, to investigate the functional roles of this gene in VZV infection, we constructed an ORF58-deletion mutant of VZV, and analyzed its susceptibility in both MRC-5 cells and malignant melanoma cells.
Results and Discussion
Before performing the remaining experiments, in order to exclude the possibility to affect the packaging of viral genome, we deleted the BAC sequences from the recombinant viruses derived from pOka-BAC and pOka-BACΔ58 using the Cre-loxP system  (data not shown), and the resulting viruses were named rpOka and rpOkaΔ58, respectively.
We have succeeded in deleting the entire ORF58 gene from the VZV genome using the BAC system. Infectious viruses could be reconstituted from the ORF58 deletion mutant, and the reconstituted viruses had similar growth kinetics to wild-type VZV in cell culture.
In this study, the N terminus of ORF57 was deleted in the process of deleting ORF58, because the C terminus of ORF58 overlaps with the N terminus of ORF57. The ORF57 gene product has been shown to be expressed in the cytosol and to be dispensable for viral growth in cell culture . Therefore, we were not concerned that any observed effects would reflect the loss of the ORF57 N-terminus.
In HSV-1 and HSV-2, UL3 is the positional homologue of ORF58. The UL3 gene product is a phosphoprotein that is localized to the cytoplasmic and nuclear portions of HSV-1-infected cells . In HSV-2-infected cells, the UL3 gene product localizes to the nucleus at the early stage of infection . Whether the ORF58 gene product possesses similar characteristics to UL3 remains unknown. Further study will be required to demonstrate the localization and possible role of the ORF58 gene product in VZV infection.
Here we show that the ORF58 gene is dispensable for viral replication and that the deletion mutant, rpOkaΔ58, grows as same as wild-type VZV in both MRC-5 cells and malignant melanoma cells. Construction and investigation of deletion mutants utilizing BAC system will make it easier to understand the virulence and attenuation mechanisms of VZV.
Cells and viruses
MRC-5 cells were cultured with modified minimum essential medium (MEM) supplemented with 10% fetal bovine serum (FBS). MeWo cells were cultured with Dulbecco's modified eagle medium (DMEM) supplemented with 10% FBS. pOka possessing BAC sequence were obtained previously. Recombinant VZV was propagated by inoculation of MRC-5 cells with virus-infected cells.
Generation of virus deletion mutants
VZV ORF58 was deleted within Escherichia coli (E. coli) by homologous recombination using ET recombinase. pOka-BAC clone which containing pOka full genome was generating as described previously .
E. coli harboring pOka-BAC DNA was transformed by pGETrec plasmid which express ET recombinase(kindly provided by Dr. Panayiotis A Ioannou, Murdoch Children's Research Institute, Department of Pediatrics, The University of Melbourne, Royal Children's Hospital) using MicroPulser electroporator(BIO-RAD).
To introduce homologous recombination, PCR reaction was performed in order to generate [flanking-kanamycinR-flanking] fragment using pCRII-TOPO plasmid (Invitrogen) as template. Primer pairs were designed as follows ; Forward primer(ACAAATTTCTGATGTTCCCCCGGCGTGGCAACGCTGGCATTTCCAAACACAGAAGTTCCTATTCTCTAGAAAGTATAGGAACTTCAGCAAGCGAACCGGA ATTGC) contains homologous sequence of the upstream of ORF58(ACAAATTTCTGATGTTCCCCCGGCGTGGCAACGCTGGCATTTCCAAACACA) as flanking sequence, FRT sequence(GAAGTTCCTATTCTCTAGAAAGTATAGGAACTTC, single under line), homologous sequence of kanamycin resistant gene within pCRII-TOPO plasmid(AGCAAGCGAACCGGAATTGC, double under line). Reverse primer (GATCGATTGGAGTGTTATATAACACTCCAATCGACCCTCTCGCGTACCATGAAGTTCCTATACTTTCTAGAGAATAGGAACTTCCTTTTTCAATTCAGAAGAACTC) contains homologous sequence of the downstream of ORF58 (GATCGATTGGAGTGTTATATAACACTCCAATCGACCCTCTCGCGTACCAT) as flanking sequence, FRT sequence (GAAGTTCCTATACTTTCTAGAGAATAGGAACTTC, single under line), homologous sequence of kanamycin resistant gene within pCRII-TOPO plasmid (CTTTTTCAATTCAGAAGAACTC, double under line).
PCR products were transformed into E. coli harboring pOka-BAC DNA and pGETrec plasmid. The recombined clones were selected by chloramphenicol/kanamycin on LB plates and the correct recombination was confirmed by PCR (data not shown).
Preparation of pOka-BAC and pOka-BACΔ58 genome
pOka-BAC and pOka-BACΔ58 genome was isolated using a NucleoBond BAC 100 kit (Macherey-Nagel) following the manufacturer's protocol.
Reconstitution of infectious virus
One μg of pOka-BAC or pOka-BACΔ58 genome was transfected into MRC-5 cells by electroporation using a Nucleofection unit (Amaxa biosystems). The cells were then cultured with MEM supplement with 10% FBS for 4 days. To remove BAC sequence, MRC-5 cells were first infected with a recombinant adenovirus, AxCANCre, which expresses the Cre recombinase (kindly provided by Dr. Yasushi Kawaguchi, University of Tokyo). Twenty-four hrs later, the cells were super-infected with the recombinant viruses obtained from pOka-BAC genome(rpOka-BAC) or pOka-BACΔ58 genome(rpOka-BACΔ58), and cultured until plaques without GFP appeared. The plaques without GFP were isolated using glass isolation cups and transferred onto newly prepared MRC-5 cells to obtain BAC-deleted viruses, rpOka or rpOkaΔ58. After several rounds of isolation, cell-free viruses were obtained by sonicating the rpOka or rpOkaΔ58-infected cells and stored at -80°C.
Genome DNA of pOka-BAC and rpOkaΔ58 were extracted from E. coli. One μg of both DNAs were digested with Bam HI and loaded onto 0.5% agarose gel and electrophoresis was performed in 0.5 × TBE (44.5 mM Tris, 44.5 mM Borate, 1 mM EDTA). At 72 hour later, DNA fragments were transferred to Hybond N+ nylon membrane(GE healthcare) with 0.4 N NaOH followed by washing with 2 × SSC (300 mM NaCl, 30 mM Na3-citrate). Hybridization and detection were performed using ECL direct labeling and detection system (GE healthcare). Probes used to detect ORF58, ORF62 and kanamycin resistant gene were labeled using the system following manufacture's protocol. The primer pairs used to create probes were: ORF58, VZ58F (aggacacgatctaaagccgt) and VZ58R (tccgtaccgacggcattgct); ORF62/71, G62N4 (gatcaaagcttagcgcag) and G62R4 (cctatagcatggctccag); kanamycin-resistance gene, KMF (atgattgaacaagatggattg) and KMR (aagaaggcgatagaaggcgatg). The transferred membrane was treated with hybridization buffer for 2 hours at 42°C followed by hybridization with the labeled probes for 18 hours at 42°C following manufacture's protocol, then was washed with primary wash buffer (6 M urea, 0.4% SDS and 0.5 × SSC) for 4 times at 42°C followed by washing with secondary wash buffer (2 × SSC), and the signals were detected with ECL detection reagents (GE healthcare) followed by exposing to X-ray film.
rpOka or rpOkaΔ58-infected cells were harvested at 24 hours p.i. Cells were extracted with 1 mL of TRIzol Reagent (Invitrogen) and 200 μL of chloroform. Cell extract was centrifuge and supernatant was added with 500 μL of isopropanol. Nucleic acid containing total RNA was obtained by centrifuge and resolved with 20 μL of DEPC-treated water. Seven μL of solution was added with 2 μL of 10× DNase buffer and 1 μL of DNaseI and incubated for 20 minutes followed by added with 1 μL of 25 mM EDTA and incubated at 60°C for 20 minutes thereafter transferred on ice. Eight micro litters of solution was added with 1 μL of oligo(dT)15 and 4 μL of dNTP(2.5 mM each) and incubated at 65°C for 5 minutes thereafter incubated on ice for 5 minutes. Solution was then added with 4 μL of 5× buffer, 1 μL of 0.1 M DTT, 1 μL of RNase inhibitor and 1 μL of SuperScriptIII reverse transcriptase(Invitrogen) and incubated at 50°C for 60 minutes followed by incubated at 70°C for 15 minutes. Single stranded RNA was digested from DNA/RNA hybrid by adding 0.5 μL of RNaseH and incubated at 37°C for 20 minutes.
Expression of mRNAs were confirmed using primers set anneal to ORF58 (forward primer: VZ58F (aggacacgatctaaagccgt), reverse primer: VZ58R (tccgtaccgacggcattgct)), ORF62 (forward primer: G62N4 (gatcaaagcttagcgcag), reverse primer: G62R4 (cctatagcatggctccag)) and GAPDH (forward primer: G3PDHF (accacagtccatgccatcac), reverse primer: G3PDHR (tccaccaccctgttgctgta)). pOka-BAC DNA was used as positive control.
Comparison of Plaque sizes
VZV recombinants were assessed for the property of cell-to-cell spread by comparison of plaque sizes. Briefly, MRC-5 cells or MeWo cells were infected with approximately 50 PFU of cell-free viruses of rpOka or rpOkaΔ58, which was produced from pOka-BAC or pOka-BACΔ58 genome. The cells were cultured for 7 days at 37°C followed by stained with 1% crystal violet/70% ethanol. Plaque sizes were calculated with ImageJ software (NIH, USA).
We thank Dr. Ulrich Koszinowski (Max von Pettenkofer institute, Germany) for providing the plasmid, pHA-2, Dr. Yasushi Kawaguchi (University of Tokyo, Japan) for providing the AxCANCre, and Dr. Panayiotis A Ioannou (Murdoch Children's Research Institute, Department of Pediatrics, The University of Melbourne, Royal Children's Hospital, Australia) for providing the pGETrec plasmid. This work was supported in part by a grant for Research Promotion of Emerging and Re-emerging Infectious Diseases (H18-Shinko-004) from the Ministry of Health, Labour and Welfare of Japan.
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