The document discusses the use of mice as models for studying human diseases due to their genetic similarities and the feasibility of genetic experiments. It details techniques such as gene targeting, knockout, and transgenic methods, which are essential for creating genetically modified mice to study specific diseases and gene functions. Additionally, it highlights the challenges and potential of gene therapy in treating genetic disorders in humans.

1. By:- Annapurna Kumari
MSc Medical Microbiology

2. Mice - Models of Human Diseases
Although the human is the mammal we are generally most interested in learning more
about, it is also the one animal we cannot use for genetic experiments for obvious
ethical reasons.
Mice naturally develop conditions that mice human disease, such as cardiovascular
disease, cancer and diabetes.
Mouse are a favorite model for human disease because it has a relatively low cost of
maintenance and a generation time that measures only nine weeks.
Developments in molecular biology and stem cell biology have allowed researchers to
create custom-made mice through gene targeting in mouse embryonic stem (ES) cells.
Certain diseases that afflict only humans, such as cystic fibrosis and Alzheimer's can
also be induced by manipulating the mouse genome and environment

3. ES Cells and Chimeric Mice
 Embryonic stem (ES) cells are pluripotent cell
lines with the capacity of self-renewal and a
broad differentiation plasticity
 They are derived from pre-implantation
embryos and can be propagated as a
homogeneous, uncommitted cell population for
an almost unlimited period of time
 Even after extensive genetic manipulation,
mouse ES cells are able to reintegrate fully into
viable embryos when injected into a host
blastocyst
 .After these pre-implantation embryos are
implanted into a surrogate mother, they develop
into mosaic offspring known as chimeras. The
tissues of chimeric mice are comprised of a
mixture of cells that originated from both the
host embryo and the ES cells.

4. Transgenics :-
 This technique permits the introduction of foreign genes or altered forms of an
endogenous gene into an organism
 Mostly, this technique does not result in replacement of the endogenous gene, but
rather the integration of additional copies of it
 The introduced gene is called transgene and the organism carrying it is referred to as
transgenic

5. Knockout Mice:
• A knockout mouse is a genetically engineered mouse in which one or more genes
have been turned off through a gene knockout.
• Important animal models for studying the role of genes which have been sequenced,
but have unknown functions.
• By causing a specific gene to be inactive in the mouse, and observing any differences
from normal behaviour or condition, researchers can infer its probable function.

8. Transferring DNA into eukaryotic cells
Production of both knockout and transgenic animals requires the transfer of DNA into
eukaryotic cells.
• Calcium precipitation
• Liposome delivery
• Microinjection
• Electroporation
DNA incorporation in the cell
 Once the foreign DNA is inside the host cell, enzymes that function normally in DNA
repair and recombination join the fragments of foreign DNA into the host cells
chromosome
 The new fragment can either replace an endogenous gene- homologous recombination
or it can remain as an independent extrachromosomal DNA molecule referred to as an
episome

11. Identification of cells
• Since only a relatively small fraction of cells take up DNA, a selective technique must
be available to identify the transgenic cells
• In most cases the exogenous DNA includes two additional genes
• The small fraction of cells in which homologous recombination takes place can be
identified by a combination of positive and negative selection
Positive Selection- One of the additional genes (neoR) confers neomycin
resistance; it permits positive selection of cells in which either
homologous (specific) or non-homologous (random) recombination has
occurred
Positive Selection-

12. Negative selection
 Negative selection- The second gene, thymidine kinase gene from Herpes Simplex
Virus (tkHSV) confers sensitivity to gancyclovir (a cytotoxic nucleotide analog). This
gene permits negative selection of ES cells in which non-homologous recombination
has occurred
 Only ES cells that undergo homologous recombination (i.e. gene-targeted specific
insertion of the DNA construct) can survive this selection scheme

14. Positive and Negative selection of recombinant ES cells
Recombinats with
random insertion Nonrecombinat cell
Recombinats with gene-
targeted insertion
Treat with neomycin
(positive selection)
Treat with gancyclovir
(negative selection)

15. Knockout Mice
• Gene knockout is a technique for selectively inactivating a gene by replacing it with a
mutant allele in an otherwise normal organism (mice)
• Knockout mice are a useful model system for studying certain human genetic diseases.

16. Making knockout mice
 Mutant alleles are introduced by
homologous recombination into Embryonic
Stem cells
 ES cells containing the knockout mutation
are introduced into early mouse embryos.
The resultant mice will be chimeras
containing tissues derived from both the
transplanted ES cells and host cells. These
cells can contribute to both germ cell and
somatic cell population
 Chimeric mice are mated to assess whether
the mutation is incorporated into the
germline
 Chimeric mice each heterozygous for the
knockout mutation are mated to produce
homozygous knockout mice

18. Knockout Mice to study genetic diseases
• Knockout mice make good model systems for investigating the nature of genetic
diseases and the efficacy of different types of treatment and for developing effective
gene therapies to cure these often devastating diseases
• For instance, the knockout mice for CFTR gene show symptoms similar to those of
humans with cystic fibrosis

20. Transgenic animals
• Transgenic animals carry cloned genes that have integrated randomly into the host genome
• Transgenic technology has numerous experimental application and potential therapeutic value.
• The frequency of random integration of exogenous DNA into mouse genome at non-homologous
sites is very high, therefore, the production of transgenic mice is a highly efficient and
straightforward process.
•Foreign DNA containing a gene of
interest is injected into one of the two
pronuclei (the male and female nuclei
contributed by the parent) of a fertilized
mouse egg before they fuse.
•The injected DNA has a good likelihood
of being randomly integrated into the
chromosome of the diploid zygote.

21. Injected eggs are then transferred to foster mothers in which normal cell growth and
differentiation occurs.
About 10-30% of progeny will contain the foreign DNA in equal amounts in all tissues,
including the germ cells.
Immediate breeding and backcrossing of these mice can produce pure transgenic strains
homozygous for the transgene.

22. Transgenics and gene therapy
Once a gene mutation is identified to be the cause of a disease, the next step is to cure
the disease by introducing normal genes( transgene) into affected individual.
In experimental animals, some genetic disorders have been cured by gene therapy, but
in humans, numerous technical issues need to be resolved before it can be widely used
a) how to reliably and safely introduce various genes into human cells
b) tissue/ cell specific introduction of genes
c) large size of genes
d)how to address the ethical issues
ref. – Books and net site
S. chand