2. Outlines of the presentation
Introduction
Genetic basis of human disease
Development of gene therapy
Classification of gene therapy
Gene delivery
Benefits and limitation of gene therapy
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3. Objectives
After the presentation you will be able to:
Understand definition and mechanism of gene
therapy.
Explain different classification of gene therapy
Discuss potential advantage and disadvantage
of gene therapy.
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4. Introduction
Gene therapy is the mechanism of using gene for
treatment or prevention of different genetic disease.
Genes are segments of DNA, that code for specific
protein.
The most common form of gene therapy include:
Inserting normal gene to replace abnormal gene
Inactivating, or “knocking out,” a mutated
gene that is functioning improperly.
Introducing a new gene into the body to help
fight a disease.
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5. cont’d…
Enhances the effect of a normally functioning
gene.
Activates the gene that was shut down during
fetal grownth.
Some of the disease treated by gene therapy includes:-
Cystic fibrosis
Severe combined immunodeficiency syndrome
Haemophilia
Many cancers
Sickle cell disease
HIV etc.
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6. Genetic Basis of Human Diseases
Genetic disease is a disease that are inherited from ancestors
Any defects in human genome including
chromosome,DNA,gene or allele will results in genetic
disease.
Defects in human genome will occur by:-
Mutation, Polymorphism
Mutation can be caused by different disease, chemical damage
or mechanical damage to genetic material.
Based on their genetic contribution, human diseases can be
classified as:
1.Monogenic,
2.Chromosomal,
3.Multifactorial
6
7. Cont’d…
1.Monogenic disease
Are caused by alterations in a single gene,
Example:Huntingtons disease, tuberous sclerosis,cystic
fibrosis,Haemophilia,Sickle cell disease,Lebers heriditary
optic neuropathy etc.
2.Chromosomal disorder: As their name implies, are caused
by alterations in chromosomes. For instance, within an
individual's genome,
some chromosomes may be missing ,extra chromosome
copies may be present, o certain portions of chromosomes
may be deleted or duplicated.
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8. cont’d…
Chromosomal disorder may results in developmental
delay,intellectual disablity,infertility etc.
3.Multifactorial disorders
The vast majority of human diseases can be categorized as
multifactorial.
They are also referred to as complex diseases,
Resulted by multiple gene defect, life style and environmental
factors.
Example:Cardiac disease,Diabetes,cancer etc.
Those genetic disorders can be managed by:
Enzyme replacement therapy
Organ transplant
Surgery
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9. Development of Gene Therapy
The goal of gene therapy was to identify the genes responsible
for disease and its exact location
In addition to that gene therapy is used to know mechanisms
of disease so that new treatment approaches and preventative
measures can be proposed.
After the completion of Human Genome Project(HGP) in 2003
providing complete human DNA sequencing ,many genetic
researches were conducted including gene therapy.
But the idea of gene therapy was started before the completion
of HGP in the early 1970’s.
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10. Cont’d…
The first approved gene therapy took place in US on 14
September 1990, at the National Institutes of Health
(NIH), under the direction of William French Anderson.
Four-year-old Ashanti DaSilva received treatment for a
genetic defect that left her with SCID(a severe combined
immuno deficiency caused by adenosine deaminase
deficiency).
Medical community accepted that it can cure HIV as in
2008, Gero Hutter has cured a man from HIV using gene
therapy.
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11. Classification of gene Therapy
Based on the mechanism, gene therapy can be
divided as:
1) Gene replacement therapy
2) Gene deactivation therapy
3) Transgenesis
4)Gene Enhancement or activation therapy
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12. Cont’d…
1.Gene replacement therapy: is a gene therapy in
which defective gene is replaced by normal gene.
2. Gene deactivation therapy: associated with gene
silencing and reducing gene expression.e.g. deactivation
of the gene of cancerous cell.
3. Transgenesis:process of introducing a gene (referred
to as a transgene) from one organism into the genome of
another organism.
.The aim is that the resulting transgenic organism will
express the gene and exhibit some new characteristic.
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13. Cont’d…
4.Gene Enhancement or activation therapy:is the process
of activating gene by transferring gene activating molecule
called transcription factors.
Based on site of gene introduction gene therapy can be
divided as:
A) Somatic cell therapy
B) Germ line therapy
A) Somatic cell therapy
Therapeutic genes transferred into the somatic cells.
Will not be inherited to later generations.
E.g.. Introduction of genes into bone marrow cells, blood
cells,skin cells etc
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14. Cont’d…
B)Germ line therapy
Therapeutic genes transferred into the germ cells
It is heritable and passed on to later generations.
E.g.. Genes introduced into eggs and sperms
There also another mechanisms in which gene therapy is
delivered:
Suicide gene therapy:Provide ‘suicide’ genes to target cancer
cells for destruction.
Antisense gene therapy
Provides a single stranded gene in an ’antisense’
(backward) orientation to block the production of harmful
proteins by turning off that genes.e.g.AIDS/HIV
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15. Gene Delivery
Gene delivery is the process by which gene is
transferred to the target cell or tissue.
The carriers used to transduce genes are called
vector.
Vectors are needed since the genetic material has to
be transferred across the cell membrane and
preferably in to the cell nucleus.
Generally gene delivery can occur in two ways:
1.Viral vector
2.Non viral vector
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16. Cont’d…
1.Viral vector
Are recombinant viruses
Virus bind to their hosts and introduce their genetic
material into the host cell.
Plausible strategy for gene therapy, by removing the
viral DNA and using the virus as a vehicle to deliver
the therapeutic DNA.
The viruses used are altered to make them safe,
although some risks still exist with gene therapy
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17. Viral vector gene deliver can be conducted:
1. Ex vivo/invitro
2. Invivo
Ex vivo:cells are modified outside the body
and then transplanted back in again
E.g. SCID therapy
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18. Cont’d…
called ex vivo because the cells are treated outside the
organism.
In vivo
genes are changed in cells when the cells are still in
the body.
Called in vivo because the gene is transferred to cells
Is used when individual cells cannot be cultured in
vitro in sufficient numbers (e.g. brain cells)
E.g. Cystic fibrosis therapy
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22. Cont’d…
A)Adenovirus
• medium-sized (90–100 nm),
• Non-enveloped (naked),
• Composed of a nucleocapsid and a double-stranded
linear DNA genome.
• There are over 51 different serotypes in humans,
which are responsible for 5–10% of upper respiratory
infections in children, and many infections in adults
as well.
• Adenoviral DNA does not integrate into the genome
and is not replicated during cell division.
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23. Cont’d…
• The genes of the virus are removed so they lose
the ability to divide.
• The human genes are inserted and the vector is
transfected in the culture containing the
sequences for replication.
• The virus thus replicates in the cell culture.
• The packed viruses are then introduced in to
the patient.
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24. Cont’d…
B)Adeno associated virus-
• It is also DNA virus.
• It has no known pathogenic effect and has wide
tissue affinity.
• AAV enters host cell, becomes double stranded
and gets integrated into chromosome
• AAV is not currently known to cause disease and
consequently the virus causes a very mild immune
response.
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25. Cont’d…
C) Retroviruses
• The recombinant retroviruses have the ability to
integrate into the host genome.
• Can carry a DNA of small siz
D) Lentiviruses
• Subclass of Retroviruses.
• Are slowly dividing virus
• The viral genome in the form of RNA is reverse-
transcribed when the virus enters the cell to produce
DNA,which is then inserted into the genome at a
random position via viral integrase enzyme.
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26. Cont’d…
E)Herpes simplex virus
• Viruses which have natural tendency to infect a
particular type of cell.
• The Herpes simplex virus is a human neurotropic
virus.
• This is mostly examined for gene transfer
in the nervous system.
Viral gene delivery has some limitations like
• Immune reaction
• Infection of normal cell
• Harmful mutation due to insertion of DNA to wrong
site etc.
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27. Non Viral Vector System
1.Spontaneous uptake by endocytosis complexed
with liposome.
2.Gene gun methods
3.Electroporation
4.Naked DNA injection
5.Polyplexes
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29. Cont’d…
2.Gene gun methods
originally designed for plant transformation,
Is a device for injecting cells with genetic
material.
The payload is an elemental particle of a heavy
metal coated with plasmid DNA.
Gene guns have also been used to deliver DNA
vaccines.
DNA is introduced by accelerating the DNA-
Particle complex in partial vacuum.
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31. 3.Electroporation
Electroporation designates the use of short
high-voltage pulses to overcome the barrier of
the cell membrane.
can be used to load cells with a variety of
different molecules,like DNA transfer.
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32. 4. Naked DNA
• Direct introduction of pure DNA construct into target cell or
tissue by microinjection .
5. polyplexes
polyplexes, have the ability to protect the DNA
from undesirable degradation during the transfection process.
• Plasmid DNA can be covered with lipids in an organized
structure like a micelle or a liposome.
• Complexes of polymers with DNA are called polyplexes.
Limitation of non viral vector
Non viral vectors have poor chromosomal integration
Less effective invivo
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33. Benefits and Limitation of Gene Therapy
Some advantages of gene therapy include:
Used to manage different inherited immune
deficiency disease like SCID,Sickle cell
disease,albinism etc
Is used for discovery and development of new drugs.
Like anti virals
Effective for prevention of disease
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34. Cont’d…
• Limitations of gene therapy includes:
It is Hard to rapidly integrate therapeutic DNA into genome.
Some vectors may not accommodate full length of genes.
Immune hyper responsiveness(in vector or in target cells).
Viral vectors may cause infection.
If integration is not controlled oncogenes might be inserted
and cause cancer.
Difficult to treat multigene disease e.g. cardiac disease,
diabetes etc.
Ethical consideration for germ line gene therapy.
Unaffordable
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Mutation-is change in single DNA Sequence can be deletion ,insertion,inversion….in less than 1% of population
Polymorphism-is variation in DNA sequnce,chromosme structure or phenotype in more than 1% of population
One well-known example is familial breast cancer related to mutations in the BRCA1 and BRCA2 genes. Management may include more frequent cancer screening or preventive (prophylactic) surgery to remove the tissues at highest risk of becoming cancerous.
Enzyme replacement therapy can be conducted by relacing the defective enzyme with normal enzyme.
ADA-It is autosomal recessive metabolic disorder that causes immunodeficiency.sign diarrhoea,pneumonia,grownth delay.This enzyme is encoded by a gene found on chromosome 20.Reduce DNA synthesis which in turn affects mitotically dividing T and B cells thus impairing immune system.Intervention bone marrow transplant.
The reason why viruses are selected as a vector is that virus has the ablity to encapsulate and deliver their genes to human cells in a pathogenic manner hence scientists remove the disease causing gene and insert therapeutic ones.
In patients with cystic fibrosis, a protein called cystic fibrosis trans-membrane regulator (CFTR) is absent due to a gene defect.
In the absence of CFTR chloride ions concentrate within the cells and it draws water from surrounding.
This leads to the accumulation of sticky mucous in respiratory tract and lungs.
Treated by in vivo replacement of defective gene by adenovirus vector
DNA Vaccine-
Non viral vectors are –cheaper,no immune reaction,less chance of infection
It takes time for a gene to be integrated into the target cell