Innovative approaches to gene therapy

Retroviral vectors for human gene delivery

The potential for gene therapy to cure a wide range of diseases has lead to high expectations and a great increase in research efforts in this area. At present, viral vectors are the most efficient means of delivering a corrective gene into human cells. While a number of different viral vectors are under development, retroviral vectors are currently the most common type used in clinical trials today. However, the production of retroviral vectors for gene therapy applications faces a number of challenges. Of primary concern is the low titre of vector stocks produced by packaging cells in culture and the inherent instability of retroviral vector activity. The problems facing large-scale retroviral vector production are outlined in this review and the research efforts by a number of groups who have attempted to optimise production methods are presented.

Conditional site-specific integration into human chromosome 19 by using a ligand-dependent chimeric adeno-associated virus/Rep protein

It is of great interest for gene therapy to develop vectors that drive the insertion of a therapeutic gene into a chosen specific site on the cellular genome. Adeno-associated virus (AAV) is unique among mammalian viruses in that it integrates into a distinct region of human chromosome 19 (integration site AAVS1). The inverted terminal repeats (ITRs) flanking the AAV genome and the AAV-encoded nonstructural proteins Rep78 and/or Rep68 are the only viral elements necessary and sufficient for site-specific integration. However, it is also known that unrestrained Rep activity may cause nonspecific genomic rearrangements at AAVS1 and/or have detrimental effects on cell physiology. In this paper we describe the generation of a ligand-dependent form of Rep, obtained by fusing a C-terminally deleted Rep68 with a truncated form of the hormone binding domain of the human progesterone receptor, which does not bind progesterone but binds only its synthetic antagonist RU486. The activity of this chimeric protein, named Rep1-491/P, is highly dependent on RU486 in various assays: in particular, it triggers site-specific integration at AAVS1 of an ITR-flanked cassette in a ligand-dependent manner, as efficiently as wild-type Rep68 but without generating unwanted genomic rearrangement at AAVS1.

Effects of altering dosing on cationic liposome-mediated gene transfer to the respiratory epithelium

Liposome-mediated gene transfer is currently sub-optimal with respect to both the extent and duration of transgene expression. We investigated whether simple changes in DNA dosing could enhance either of these outcomes. Increasing DNA doses produced highest transgene expression at an intermediate dose with toxicity observed at higher doses, thereby likely limiting expression. Adminis- tering an equivalent DNA dose in aliquots over a 1-3 day period resulted in significantly lower gene expression and did not increase the duration of expression. Administration at different times of the day (and hence wake/sleep cycles of the animals) did not alter gene expression. We conclude that such simple changes in dosing regimes are unlikely to contribute to improvements in gene transfer efficiency.

Lipitoids--novel cationic lipids for cellular delivery of plasmid DNA in vitro

BACKGROUND

Although synthetic nonviral vectors hold promise for the delivery of plasmid DNA, their gene-transfer efficiencies are far from matching those of viruses. To systematically investigate the structure-activity relationship of cationic lipids, a small library of cationic lipid-peptoid conjugates (lipitoids) was synthesized. The compounds were evaluated for their ability to form complexes with plasmid DNA and to mediate DNA transfer in vitro.

RESULTS

Lipid-peptoid conjugates were conveniently prepared in high yield using solid-phase synthesis. Several lipitoids condensed plasmid DNA into 100 nm spherical particles and protected the DNA and DNase digestion. A subset of lipitoids with a repeated (aminoethyl, neutral, neutral) sidechain trimer motif conjugated with dimyristoyl phosphatidyl-ethanolamine (DMPE) mediated DNA transfer with high efficiency.

CONCLUSIONS

Automated solid-phase synthesis of cationic lipids allowed the rapid synthesis of a diverse set of transfection reagents. The most active compound DMPE-(Nae-Nmpe-Nmpe)3 (Nae, N-aminoethyl glycine; Nmpe, N-p-methoxyphenethyl-glycine) is more efficient than lipofectin or DMRIE-C (two commercial cationic lipid transfection reagents) and is active in the presence and absence of serum. The activity in the presence of serum suggests potential for applications in vivo.