Redesigning Animal Agriculture

124 T. Doran and L. Lambeth
is estimated to be ten- to 100-fold higher than
previous methods (McGrew et al., 2004).
The use of lentiviral vectors does have
some limitations, such as the possible integration
of retroviruses in close proximity
to potential oncogenes, followed by the
activation of these oncogenes to convert a
normal cell into a tumour cell (although the
statistical chances of such an insertion are
estimated to be in the range of 10 −5 )(Baum
et al., 2004).
Sperm-mediated gene transfer
Sperm-mediated gene transfer (SMGT) is a
very appealing transgenic technology as it
is far simpler than the other methods. However,
until recent developments, SMGT
has not been reliable enough to attract the
more general interest of the other methods.
SMGT was first suggested as early as
1971 (Brackett et al., 1971) and since then
numerous reports have been made showing
successful sperm-mediated transfer of foreign
DNA into both non-mammalian and
mammalian animals, but with inefficient
germ-line transmission and unreliable
transgene expression (Smith, 1999). This
has now been greatly improved upon by
ways of increasing DNA binding to sperm
without interfering with fertilization. Chang
et al. (2002) reported the development of a
sperm-reactive monoclonal antibody that
can be used as a cross linker to facilitate
the binding of exogenous DNA to sperm
(linker-based sperm-mediated gene transfer
– LB-SMGT). This antibody is a basic
protein that binds to DNA through ionic
interaction, allowing exogenous DNA to be
linked specifically to sperm. After fertilization
of an egg with cross-linked sperm, the
DNA is shown to be successfully integrated
into the genome of viable pig offspring
with germ-line transfer to the F1 generation
at the highly efficient rate of 37.5%.
This linker antibody is reactive to a surface
antigen on sperm of not only pigs but other
livestock species including chicken, cow,
goat and sheep. Preliminary data using LB-
SMGT through artificial insemination in
chickens indicated the presence of a transgene
in 49% (44/90) of chicken embryos by
PCR analysis. Further to this, expression of
the transgene was detected in 53% (18/34)
of the chicks in the F0 generation.
Resistance mechanisms of livestock
to viruses – genetic engineering strategies
A number of approaches have been developed
with the potential that insertion of a
transgene into the genome of an animal may
confer resistance to specific viruses. The
goal of developing resistance to viral infection
is to engineer animals that express molecules
that block productive infection and
therefore reduce the risk of transfer from
animal to animal or animal to human. The
extent of the research effort to genetically
engineer new resistance mechanisms in
animals is much smaller than that in plants.
The major focus in animals has been to
block viral attachment and penetration into
a host cell by developing transgenes that:
(i) produce viral antireceptor proteins to
block cellular receptors; or (ii) replace host
receptor genes with a modified form that is
able to perform the receptor’s physiological
function but does not allow the attachment
of the virus (Gavora, 1996).
In a recent review by Hunter et al.
(2005), alternative strategies were suggested
specifically for engineering resistance to
avian influenza in poultry and these are
described in the following sections.
The Mx gene
The Mx genes of vertebrates were first discovered
in mice through their ability to
confer a potent antiviral state in response
to influenza virus (Staeheli et al., 1984).
The mechanism of action is not fully understood,
but some evidence indicates that Mx
interacts with the viral polymerase during
influenza virus infection. Mx genes have
been characterized in pigs and cattle and the
use of transgenesis to introduce functional
Mx genes to increase resistance against viral
diseases that are susceptible to Mx function
has been investigated in pigs (Muller et al.,
1992). An Mx gene has been identified in
the chicken but the allele present in most