Gene therapy involves the introduction of one or more foreign genes into an organism to treat hereditary or acquired genetic defects. In gene therapy, DNA encoding a therapeutic protein is packaged within a 'vector', which transports the DNA inside cells within the body.
Medical conditions for which gene therapy is being studied 
ADA deficiency    Hemophilia
AIDS                    Liver cancer
Asthma                Lung cancer
Brain tumour        Melanoma
Colo cancer      Neurodegenerative conditions
Diabetes              Ovarian Cancer
Heart disease       Prostate Cancer
26.1.    Types of gene therapy - two types.

26.2.    Gene therapy Approach - two types of Gene therapy approaches are utilized.
26.2.1.    Classical gene therapy: Involves therapeutic gene delivery & their optimum expression once inside the cell.    
26.2.2.    Non-Classical gene therapy: Involves the inhibition of expression of genes associated with the pathogenesis or to correct a genetic defect and restore the normal gene expression
The foreign genes carry out the following functions:-
Produce a product (protein) that the patient lacks.
Produces toxin so that diseased cell is killed.
Activate cells of the immune system so as to help in the killing of diseased cells.
26.3.    Gene therapy strategies:-
26.3.1.    Gene Augmentation Therapy (GAT)
The simple addition of functional allele is used to treat inherited disorders. It has been applied to treat autosomal recessive disorders.
26.3.2.    Targeted killing of specific cells.
It involves utilizing genes encoding toxic compounds (suicide genes) or prodrugs (reagents which confer sensitivity to subsequent treatment with a drug) to kill the transformed cells. This approach is popular in cancer gene therapies. 

26.3.3.    Targeted Inhibition of gene expression:- (Antisense Technology)
It is to block the expression of any diseased gene is a new gene expressing which is harmful to a cell.
Particularly suitable for treating infectious diseases and some cancers.

26.4.    Adenoviruses:-
Adenoviruses are DNA viruses with a linear, double-stranded genome of approximately 36kbp. When these viruses infect a host cell, they introduce their DNA molecule into the host. The genetic material of the adenoviruses is not incorporated into the host cells genetic material. The linear dsDNA genome remains non-integrated as an episome within the cell nucleus and the instructions in this extra DNA molecule are transcribed just like any other gene. These extra genes also do not replicate when the cell is about to undergo cell division so, the descendants of that all will not have the extra gene. This means that the treatment with the adenovirus will require regular doses to add the missing gene every time.
26.5.    Adeno-associated viruses:-
Adeno-associated viruses (AAV), from the parvovirus us family, are non-pathogenic ssDNA viruses. They rely on co-infection by an adeno or herpes helper virus to replicate.
Potential Advantages and Disadvantages of various gene delivery systems:-

26.6.    Non-Viral Vectors
It involves chemical and physical methods such as direct injection of naked plasmid DNA (particle bombardment), receptor-mediated endocytosis and gene transfer through liposomes, polymers, nanoparticles etc.
26.6.1.    Direct injection/particle bombardment 
DNA can be injected parenterally which can be considered for Duchenne muscular dystrophy (DMD). An alternative approach was particle born baronet (gene gun) technique, in which DNA is coated on to metal microparticles and firval from a biolistic gun into cells. It can cross the physical barriers like skin, muscle layer for which it is used for vaccination. This method was initially named as biolistic. These are other names like particle gun, gene gun, bio-blaster. 
Factor Affecting Bombardment:-
i.    Nature of Microparticles
ii.    Nature of tissue
iii.    Amount of DNA
Advantage \rightarrow Simple and comparatively safe.
Disadvantage \rightarrow Poor efficiency of gene transfers.
A low level of stable integration of the injected DNA.
26.6.2.    Liposome-Mediated 
They are artificially created lipid vesicles containing phospholipid membrane. The DNA to be transferred is packaged in vitro with liposome and used directly for gene transfer to a target tissue in vivo. Liposomes may be cationic as well as anionic. Anionic liposomes have a negative surface charge and bind DNA on the inside. The lipid coating allows the DNA to survive in vivo binds to cells and be endocytosed into the cells.

Advantages:-No viral components, non-pathogenic, no immunity Problems, no limit of line of the foreign gene.
Disadvantages:-Low transfection efficiency, short term gene expressions.

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