The document discusses transgenesis, which is introducing an exogenous gene into an organism so it exhibits a new property transmittable to offspring. Methods described include retrovirus-mediated gene transfer, microinjection of DNA into fertilized eggs, and embryonic stem cell-mediated gene transfer. Transgenesis has advantages like being more specific and faster than traditional breeding. However, it also carries risks of unpredictability if defense mechanisms silence or inactivate foreign genes.

1. A PRESENTATION BY
M.APHURVIKAA
14BBT0161
VIT UNIVERSITY

2. WHAT IS TRANSGENESIS?
• Transgenesis is the process of introducing an exogenous gene –
called a transgene – into a living organism so that the organism will
exhibit a new property and transmit that property to its offspring.
• Transgene: It is a gene or genetic material that has been transferred
naturally or by any of a number of genetic engineering techniques
from one organism to another.
• Transgenic Plants or animals : The plants or animals which
expresses the characters coded by the transgene are called Transgenic
plants or transgenic animals.

3. ADVANTAGES OF TRANSGENESIS
 More specific — scientists can choose with greater
accuracy the trait they want to establish. The number of
additional unwanted traits can be kept to a minimum.
 Faster — establishing the trait takes only one generation
compared with the many generations often needed for
traditional selective breeding, where much is left to
chance.
 More flexible — traits that would otherwise be
unavailable in some animals or plants may be achievable
using transgenic methods.
 Less costly — much of the cost and labour involved in
administering feed supplements and chemical
treatments to animals and crops could be avoided.

4. Embryonic stem cell-mediated gene
transfer

5.  This method involves prior insertion of the desired DNA
sequence by homologous recombination into an in vitro
culture of embryonic stem (ES) cells. Stem cells are
undifferentiated cells that have the potential to differentiate
into any type of cell (somatic and germ cells) and therefore
to give rise to a complete organism. These cells are then
incorporated into an embryo at the blastocyst stage of
development. The result is a chimeric animal. ES cell-
mediated gene transfer is the method of choice for gene
inactivation, the so-called knock-out method.
 This technique is of particular importance for the study of
the genetic control of developmental processes. This
technique works particularly well in mice. It has the
advantage of allowing precise targeting of defined
mutations in the gene via homologous recombination.
Embryonic stem cell-mediated gene
transfer

6. GENE THERAPY
 Gene therapy is a novel therapeutic branch of modern medicine. Its emergence
is a direct consequence of the revolution heralded by the introduction of
recombinant DNA methodology in the 1970s.
 Originally, monogenic inherited diseases (those caused by inherited single gene
defects), such as cystic fibrosis, were considered primary targets for gene
therapy.
 For instance, in pioneering studies on the correction of adenosine deaminase
deficiency, a lymphocyte-associated severe combined immunodeficiency
(SCID), was attempted.
 Although no modulation of immune function was observed, data from this
study, together with other early clinical trials, demonstrated the potential
feasibility of gene transfer approaches as effective therapeutic strategies.
 The first successful clinical trials using gene therapy to treat a monogenic
disorder involved a different type of SCID, caused by mutation of an X
chromosome-linked lymphocyte growth factor receptor

7. TWO PATHS TO GENE TRANFER
1.DIRECT TRANSFER
 Direct gene transfer is particularly attractive because of its relative
simplicity. In this scenario, genes are delivered directly into a patient's
tissues or bloodstream by packaging into liposomes (spherical vessels
composed of the molecules that form the membranes of cells) or other
biological microparticles.
 Alternately, the genes are packaged into genetically-engineered viruses,
such as retroviruses or adenoviruses. Because of biosafety concerns, the
viruses are typically altered so that they are not toxic or infectious (that is,
they are replication incompetent). These basic tools of gene therapists have
been extensively optimized over the past 10 years.
 However, their biggest strength—simplicity—is simultaneously their biggest
weakness. In many cases, direct gene transfer does not allow very
sophisticated control over the therapeutic gene. This is because the
transferred gene either randomly integrates into the patient's chromosomes
or persists unintegrated for a relatively short period of time in the targeted
tissue. Additionally, the targeted organ or tissue is not always easily
accessible for direct application of the therapeutic gene.

8. 2.INDIRECT TRANSFER
 Therapeutic genes can be delivered using living cells. This
procedure is relatively complex in comparison to direct gene
transfer, and can be divided into three major steps.
 1.In the first step, cells from the patient or other sources are
isolated and propagated in the laboratory.
 2. the therapeutic gene is introduced into these cells, applying
methods similar to those used in direct gene transfer.
 3. the genetically-modified cells are returned to the patient.
The use of cells as gene transfer vehicles has certain
advantages. In the laboratory dish (in vitro), cells can be
manipulated much more precisely than in the body (in vivo).
Some of the cell types that continue to divide under laboratory
conditions may be expanded significantly before
reintroduction into the patient.

9. EMBRYONIC STEM CELLS IN GENE THERAPY
 Human embryonic stem cells (hESCs) are cell lines derived from the inner
cell mass of human blastocysts.
 More than 120 hESCs lines have been derived throughout the world that all
share the main characteristics of mouse embryonic stem cells (mESCs): self-
renewal, clonogenicity and pluripotentiality.
 This last trait may provide a unique and unlimited source of cells for cell
replacement therapy, and the two former make these cells particularly
amenable to genetic engineering.
 Many different methods of directing the differentiation of ESCs toward
particular lineages have been studied. Most rely on the combined use of
soluble molecules, defined conditioned media and coculture with feeder cell
lines.
 Such conditions have been described to drive the differentiation of hESC to
neurons,definitive endoderm,cardiomyocytes,hematopoietic
cells,hepatocytes,osteogenic cells and germ cells.
 The traditional approach of gene therapy has mostly been aimed at inserting
a new copy of a gene under the control of an exogenous promoter. This
approach, also known as gene augmentation
 The principal barriers to gene augmentation are (1) the transduction rates of
target cells; (2) the risks associated with random integration; and (3) gene
silencing which may occur during the life of the cell. The development of
replication-deficient lentiviral vectors may address some of these issues.

12. RETRO VIRUS MEDIATED GENE TRANSFER
• A retrovirus is a single-stranded RNA virus
• It has a cap at the 5' end and a poly(A) tail at the 3' end.
Elements needed to generate Replication-defective virus
1. Transfer Vector: Transgene+ Packaging signal
sequence
2. Helper virus (packaging of transfer vector): deleted
Packaging signal sequence

13. Retrovirus-mediated gene transfer
 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.
 The traditional approach of gene therapy has mostly been aimed at
inserting a new copy of a gene under the control of an exogenous
promoter. This approach, also known as gene augmentation, has
been used with success in preclinical settings, but has met many
obstacles when used to treat patients with an integrating retrovirus.

14. RETRO VIRUS MEDIATED GENE TRANSFER
 Viral-mediated infection provides a rapid and efficient means to transfer
genetic material to ESCs; however, transgenes are subject to progressive
transcriptional gene silencing resulting in variable to complete loss of
expression.
 Certain modified retroviruses, such as murine stem cell virus-based vectors
can resist transgene silencing to a certain extent. The development of
replication-deficient, self-inactivating, pseudotyped lentiviruses used in
gene therapy provided a way to introduce a gene in the hES genome with
higher efficiency.
 Lentiviral vectors have also been described to minimize the occurrence of
silencing compared to retroviruses, but it is not clear whether this results
from overall higher copy numbers in the target cells or from inherent
features of lentivirus-based vectors, such as the distribution of the
integration sites in the genome.
 Further modification of the lentiviral constructs with novel insulator
elements that block chromatin silencing, to ensure a stabilized expression
of transgenes throughout ESC differentiation has been particularly
encouraging

15. MICROINJECTION

16. MICROINJECTION
 This method involves the direct microinjection of a chosen gene construct
(a single gene or a combination of genes) from another member of the same
species or from a different species, into the pronucleus of a fertilized ovum.
 The introduced DNA may lead to the over- or under-expression of certain
genes or to the expression of genes entirely new to the animal species. The
DNA construct (usually about 100 to 200 copies in 2 pl of buffer) is
introduced by microinjection through a fine glass needle into the male
pronucleus - the nucleus provided by the sperm before fusion with the
nucleus of the egg.
 The diameter of the egg is 70 µm and that of the glass needle is 0.75 µm;
the experimenter performs the manipulations with a binocular microscope
at a magnification of 200 x.
 The insertion of DNA is, however, a random process, and there is a high
probability that the introduced gene will not insert itself into a site on the
host DNA that will permit its expression.
 The manipulated fertilized ovum is transferred into the oviduct of a
recipient female, or foster mother that has been induced to act as a
recipient by mating with a vasectomized male.

17. Positive Implications of Transgenesis:
Animals
 The main aim in using transgenic technology in animal
agriculture is to improve livestock by altering their
biochemistry, their hormonal balance or their important
protein products. Scientists hope to produce animals that
are larger and leaner, grow faster and are more efficient
at using feed, more productive, or more resistant to
disease. Examples of transgenic breeding programs
include:
 producing faster-growing and leaner pigs that use food
more efficiently and resist common diseases
 breeding transgenic sheep that grow better wool without
needing dietary supplements of sulphur-containing
amino acids.

18. Negative implications of transgenesis
 GE technology carries certain inherent
unpredictability
 Some facts
 Isolation of a gene from its natural environment
and integration into entirely different organism
 Possible transgenic instability due to triggering of
the inbuilt defense mechanisms of the host
organism leading to inactivation or silencing of
foreign genes.