Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 423

Nucleus

Vector

Gene to be
transferred

(a)

Target cell
DNA

Intracellular
effect

or

(b)

(c)

Extracellular
effect

Figure 14.1 Simplifi ed schematic representation of the basis of gene therapy. The genetic material to be
transferred is fi rst usually packaged into some form of vector that serves to deliver the nucleic acid to the
target cell. (a) Entry of the therapeutic nucleic acid, often still associated with its vector, into the cell cyto-
plasm. (b) Transfer of the nucleic acid into the nucleus of the recipient cell. This is often, though not always,
followed by integration of the foreign genetic material into the cellular DNA. (c) The foreign gene (whether
integrated or not) is expressed, resulting in the synthesis of the desired protein product. Regulatory elements
of the nucleic acid transferred may be designed to ensure the protein product is retained within the cell, or
is exported from the cell, as is necessary. Refer to the text for further details

Once assimilated by the cell, the exogenous nucleic acid must now travel/be delivered to the
nucleus. In some cases, the mechanism by which this transfer occurs is understood, at least in part
(e.g. in the case of retroviral vectors). In other cases (e.g. use of liposome vectors or naked DNA),
this process is less well understood. At a practical level, gene therapy protocols may entail one of
three different strategies (Figure 14.3).

Table 14.2 Vector systems used to deliver genes into mammalian cells a

Viral-based vector systems Non-viral-based vector systems

Retroviruses Nucleic-acid-containing liposomes

Adenoviruses Molecular conjugates

Adeno-associated virus Direct injection of naked DNA

Herpes virus CaPO 4 precipitation

Polio virus Electroporation

Vaccinia virus Particle acceleration

a Prior to 2003 the majority of clinical trials undertaken utilized retroviral vector systems, although
adenoviral-based systems have now come to the fore. Non-viral systems have generally been employed
least often, although some, e.g. nucleic-acid-containing liposomes, may be used more extensively in
the future. Some of the methods tested, e.g. calcium phosphate precipitation, electroporation and parti-
cle acceleration, are unlikely to be employed to any great extent in gene therapy protocols.