Gene transfer using recombinant rabbit hemorrhagic disease virus capsids with genetically modified DNA encapsidation capacity by addition of packaging sequences from the L1 or L2 protein of human papillomavirus type 16

Abstract

The aim of this study was to produce gene transfer vectors consisting of plasmid DNA packaged into virus-like particles (VLPs) with different cell tropisms. For this purpose, we have fused the N-terminally truncated VP60 capsid protein of the rabbit hemorrhagic disease virus (RHDV) with sequences which are expected to be sufficient to confer DNA packaging and gene transfer properties to the chimeric VLPs. Each of the two putative DNA-binding sequences of major L1 and minor L2 capsid proteins of human papillomavirus type 16 (HPV-16) were fused at the N terminus of the truncated VP60 protein. The two recombinant chimeric proteins expressed in insect cells self-assembled into VLPs similar in size and appearance to authentic RHDV virions. The chimeric proteins had acquired the ability to bind DNA. The two chimeric VLPs were therefore able to package plasmid DNA. However, only the chimeric VLPs containing the DNA packaging signal of the L1 protein were able efficiently to transfer genes into Cos-7 cells at a rate similar to that observed with papillomavirus L1 VLPs. It was possible to transfect only a very limited number of RK13 rabbit cells with the chimeric RHDV capsids containing the L2-binding sequence. The chimeric RHDV capsids containing the L1-binding sequence transfer genes into rabbit and hare cells at a higher rate than do HPV-16 L1 VLPs. However, no gene transfer was observed in human cell lines. The findings of this study demonstrate that the insertion of a DNA packaging sequence into a VLP which is not able to encapsidate DNA transforms this capsid into an artificial virus that could be used as a gene transfer vector. This possibility opens the way to designing new vectors with different cell tropisms by inserting such DNA packaging sequences into the major capsid proteins of other viruses.

FIG. 1

Schematic maps of VP60-L1BS and VP60-L2BS chimeric proteins.

FIG. 2

Western blot analysis of VP60Δ-L1BS and VP60Δ-L2BS protein expression. Recombinant capsid proteins were detected in the nuclear fraction of infected sf21 cells with an anti-RHDV monoclonal antibody. Lanes 1 and 2, VP60Δ-L1BS; lanes 4 and 5, VP60Δ-L2BS; lane 3, HPV-16 VLPs.

FIG. 3

Detection of DNA binding to VP60Δ-L1BS and VP60Δ-L2BS by Southwestern blotting using a digoxigenin-labeled plasmid probe. Lane 1, VP60Δ-L1BS; lane 2, VP60Δ; lane 3, VP60Δ-L2BS.

FIG. 4

Subcellular localization of VP60Δ (a and b) and VP60Δ-L1BS (c and d) VLPs by electron microscopy (bars, 200 nm [a and c] and 100 nm [b and d]).

FIG. 5

Identification of chimeric RHDV and HPV-16 VLPs by transmission electron microscopy. (a) VP60; (b) VP60Δ; (c) VP60Δ-L1BS; (d) VP60Δ-L2BS; (e) HPV-16 L1; (f) HPV-16 L1+L2. Bar, 100 nm.

FIG. 6

Purified VP60Δ-L2BS VLPs (a), capsomer-like structures obtained after treatment of VLPs with EGTA and dithiothreitol (b), VP60Δ-L2BS VLPs refolded after reassembly of the capsomers in the presence of CaCl₂, DMSO and β-galactosidase plasmid (c). Bar, 100 nm.

FIG. 7

DNA protection assay using VP60Δ VLPs (lane 1), VP60Δ-L1BS VLPs (lane 2), VP60Δ-L2BS (lanes 3 and 4), and HPV-16 L1 VLPs (lane 5). M, molecular weight markers (λ phage digested with EcoRI and HindIII).

FIG. 8

Demonstration of gene expression of β-galactosidase in Cos-7 (a and b), R17 (c), HuH-7 (d), and CaCo2 (e and f) cells after gene transfer using VP60Δ-L1BS (a, c, d, and e) or HPV-16 L1 (b and f) capsids. Transfected cells expressing β-galactosidase were stained with X-Gal.