NON-VIRAL
Non-viral gene transfer is arguably the safest way in which to introduce exogenous nucleic acid into living cells. However, in general this is also a very inefficient means of introducing DNA or RNA. In order for a gene to be expressed it must find its way into the cell’s nucleus, this means it has to pass two lipid barriers; the extracellular membrane and the nuclear membrane. Given that nucleic acid and the lipids that comprise a cell’s membrane are both negatively charged, electrostatic forces would result in the repulsion of any nucleic acid that approached a cell. To circumvent this, it has proved effective to neutralize the negative charge of the nucleic acid with a positively charged substance. The most effective chemicals employed to date are those based on cationic lipids. The positive charge on the lipid neutralizes the charge and transfer across the membrane is achieved as a result of the lipid’s ability to fuse with the lipid-based membrane structure that surrounds the cell.
VIRAL
Viruses are parasitic entities that have successfully evolved over millions of years due to their ability to introduce their own genetic code into the cells of their target hosts. They are capable of entering a host, passing the physical barriers that comprise tissue, infecting cells by active mechanisms and delivering their genetic cargo direct to the nucleus of a cell. As such they provide the perfect vehicle from which to deliver therapeutic DNA. One of the biggest hurdles to the widespread application of viruses in patients is the design of a safe attenuated virus that maintains its ability to efficiently transfer genetic material. Given that most viruses are associated with induction of significant pathology in humans, this is no easy task. Also, after infection, most viruses elicit a strong immune response that effectively removes virally infected cells. Despite of these hurdles significant progress has been made in the design of gene transfer vehicles based on attenuated viruses.
