Download Lecture 21 Retroviral Vector Method To increase the probability of

Lecture 21
Retroviral Vector Method
To increase the probability of expression, gene transfer is mediated by means of a
carrier or vector, generally a virus or a plasmid. Retroviruses are commonly used as
vectors to transfer genetic material into the cell, taking advantage of their ability to
infect host cells in this way. Offspring derived from this method are chimeric, i.e.,
not all cells carry the retrovirus. Transmission of the transgene is possible only if the
retrovirus integrates into some of the germ cells.
For any of these techniques the success rate in terms of live birth of animals
containing the transgene is extremely low. Providing that the genetic manipulation
does not lead to abortion, the result is a first generation (F1) of animals that need to
be tested for the expression of the transgene. Depending on the technique used, the
F1 generation may result in chimeras. When the transgene has integrated into the
germ cells, the so-called germ line chimeras are then inbred for 10 to 20 generations
until homozygous transgenic animals are obtained and the transgene is present in
every cell. At this stage embryo carrying the transgene can be frozen and stored for
subsequent implantation.
Of the various gene transfer methods, the use of retroviral vectors (Fig 1) has
the advantage of being an effective means of integrating the transgene into the
genome of a recipient cell. However, these vectors can transfer only small pieces
(approximately 8 kilo bases) of DNA, which, because of the size constraint, may lack
essential adjacent sequences for regulating the expression of the transgene.
There is a further major drawback to the use of retroviral vectors. Although
these vectors are designed to be replication defective, the genome of the retroviral
strain (helper virus) that is needed to create large quantities of the vector DNA can be
integrated into the same nucleus as the transgene. Despite special precautions, helper
strain retroviruses could be produced by the transgenic organism. Consequently, for
applications in which either a commercial product is to be synthesized by the
transgenic organism or the transgenic organism is to be used as food, it is absolutely
necessary that there be no retroviral contamination. Therefore, because alternative
methods are available, retroviral vectors are rarely used for creating transgenic
animals that have a commercial end use.
Figure.1 Establishing transgenic mice with retroviral vectors. Cleavage-Stage
embryos, usually at the eight-cell stage, are infected with a defective retrovirus
carrying a transgene. Implanted females (foster mothers) give birth to transgenic
pups. Matings are carried out to determine which pups have the transgene in their
germ line cells. Transgenic lines can be established from these founder transgenic
animals.