have been derived from a range of viruses including adenovirus, adeno-associated
virus (AAV), herpesvirus and retrovirus including lentivirus. These vectors can be
produced in cell lines in which the viral genes required for replication are provided
in a form that cannot be packaged into the subsequent virus. The room provided by
deletion of viral genes is used to accommodate the therapeutic gene(s). Delivery of
therapeutic genes, either to substitute for dysfunctional genes, or to produce
products that help fight acquired disease, is termed gene therapy. Non-viral gene
delivery uses purified circular plasmid DNA that is prepared in bacteria. It can be
delivered in two versions, either as a complex with lipids and/or peptides or as a
naked molecule with a physical driving force. Examples of the latter include high
pressure/volume injections (hydrodynamic), the application of a series of electrical
pulses (electroporation) and the use of ultrasound combined with microbubbles.
All these methods cause transient pores in the target cell membrane and so aid the
entry of the plasmid into the cell (Wells 2004 ). Viral vectors tend to be more
efficient but the presence of viral surface proteins often leads to an acquired
immune response that limits the possibility of repeated administrations.
A wide range of animals have been used as model systems for developing gene
therapy treatments for human disease. As in pharmacological research, the majority
of these have been rodents but large animal models, commonly dogs and non-
human primates, have also been used, often as a final test before taking the
treatment to human clinical trial. In a number of these cases natural spontaneous
mutants have been used such as the golden retriever with muscular dystrophy
(Cooper et al. 1988 ; Valentine et al. 1988 ), the German shepherd with haemophilia
A (Parry et al. 1988 ) and the Plott Hound with Hurler disease (Spellacy et al. 1983 ).
An alternative approach to testing gene therapies that might be used in man is
exemplified by treatments for cancers. Studies in laboratory rodents mostly use
inducible tumour models in inbred experimental animals. These are poor models for
human disease and thus there has been an increasing interest, both for experimental
and veterinary reasons, in testing gene therapy treatments in spontaneous animal
tumours. This was first highlighted by Vail and MacEwen ( 2000 ) and a number of
reports have subsequently been published examining treatments in dogs (e.g. Dow
et al. 1998 , 2005 ; Bergman et al. 2003 ; Kamstock et al. 2006 ; Cutrera et al. 2008 ;
Finocchiaro and Glikin 2008 ) and cats (Jahnke et al. 2007 ;Hu ̈ttinger et al. 2008 ).
It is interesting to note that most of the above have used non-viral gene transfer
methods. The majority of studies have reported a modest improvement with some
partial and some complete remissions.
Other gene therapies for spontaneous disease that have been tested in pets
include treatments for weight loss and anaemia associated with cancer and kidney
disease. Electroporation of growth hormone-releasing hormone (GHRH) plasmid
has been used to treat cats and dogs with chronic renal failure (Brown et al. 2009 ).
Treated animals showed an increase in body weight, improved appetite and activity,
maintained kidney function and survived longer than the untreated controls. This
group have also used the same treatment for cancer associated anaemia in dogs
(Bodles-Brakhop et al. 2008 ). They noted improvements in red blood cell numbers,
Genetically Modified Animals and Pharmacological Research 221