Some 24 per cent of all gene therapy clinical trials undertaken to date have employed retroviral vectors as gene delivery systems. Retroviruses are enveloped viruses. Their genome consists of ssRNA of approximately 5 - 8 kb. Upon entry into sensitive cells, the viral RNA is reverse transcribed and eventually yields double-stranded DNA. This subsequently integrates into the host cell genome. The basic retroviral genome contains a minimum of three structural genes : gag (codes for core viral protein), pol (codes for reverse transcriptase) and env (codes for the viral envelope proteins).
At either end of the viral genome are the long terminal repeats (LTRs), which harbour powerful promoter and enhancer regions and sequences required to promote integration into the host DNA. Also present, immediately adjacent to the 5' LTR, is the packing sequence (ψ). This is required to promote viral RNA packaging.
The ability of such retroviruses to (a) effectively enter various cell types and (b) integrate their genome into the host cell genome in a stable, long-term fashion, made them obvious potential vectors for gene therapy.
The construction of retroviruses to function as gene vectors entails replacing the endogenous viral genes, required for normal viral replication, with the exogenous gene of interest. Removal of the viral structural genes means that the resulting vector cannot itself replicate.
Retroviruses display a number of properties/characteristics that influence their potential as vectors in gene therapy protocols. These may be summarized as follows:
Retroviral life cycle
At either end of the viral genome are the long terminal repeats (LTRs), which harbour powerful promoter and enhancer regions and sequences required to promote integration into the host DNA. Also present, immediately adjacent to the 5' LTR, is the packing sequence (ψ). This is required to promote viral RNA packaging.
The ability of such retroviruses to (a) effectively enter various cell types and (b) integrate their genome into the host cell genome in a stable, long-term fashion, made them obvious potential vectors for gene therapy.
The construction of retroviruses to function as gene vectors entails replacing the endogenous viral genes, required for normal viral replication, with the exogenous gene of interest. Removal of the viral structural genes means that the resulting vector cannot itself replicate.
Retroviruses display a number of properties/characteristics that influence their potential as vectors in gene therapy protocols. These may be summarized as follows:
- Retroviruses as a group have been studied in detail and their biochemistry and molecular biology are well understood.
- Most retroviruses can integrate their proviral DNA only into actively replicating cells.
- The efficiency of gene transfer to most sensitive cell types is very high, often approaching 100 percent.
- Integrated DNA can be subject to long-term, relatively high-level expression.
- Proviral DNA integrates randomly into the host chromosomes.
- Retroviruses are promiscuous, in that they infect a variety of dividing cell types.
- Complete copies of the proviral DNA are passed on to daughter cells if the original recipient cell divides.
- Good, high-level, titre stocks of replication-incompetent retroviral particles can be produced.
- Safety studies using retroviral vectors have already been carried out on various animal species.
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