Gene therapy may be classified into two types
1) Germ line gene therapy
2) Somatic cell gene therapy
a) Incase of germ line gene therapy germ cells that is sperms or eggs are modified by the introduction of functional genes, which are ordinarily integrated into their genomes.
Therefore the change due to therapy is heritable and passed onto the later generations. This approach, heretically, is highly effective in counteracting the genetic disorders. However this option is not consider, at least for the present for application in human beings for a variety of technical and ethical reasons.
b) In the case of somatic cell gene therapy the gene is introduced only in somatic cells, especially of those tissues in which expression of the concerned gene is critical for health. Expression of the introduced gene relieves symptoms of the disorder, but this effect is not heritable, as it does not involve the germ line. It is the only feasible option, and clinical trials have already started mostly for the treatment of cancer and blood disorders.
GENERAL GENE THERAPY STRATEGIES
1) Gene augmentation therapy (GAT): -
It is done by simple addition of functional alleles has been used to treat several inherited disorders caused by genetic deficiency of a gene product. It is also involved in transfer to cells of genes encoding toxic compounds (suicide genes) or prodrugs (reagents which confer sensitivity to subsequent treatment with a drug). It has been particularly applied to autosomal recessive disorders where even modest expression levels of an introduced gene may make a substantial difference.
2) Targeted killing of specific cells: -
Artificial cell killing and immune system assisted cell killing have been popular in the treatment of cancers. It can be done by two ways.
a) Direct cell killing: - it is possible if the inserted genes are expressed to produce a lethal toxin (suicide genes), or a gene encoding a prodrug is inserted, conferring susceptibility to killing by a subsequently administered drug. Alternatively selectively lytic viruses can be used.
b) Indirect cell killing: - It uses immunostimulatory genes to provoke or enhance an immune response against the target cell.
3) Targeted mutation correction: -
The repair of a genetic defect to restore a functional allele, is the exception, technical difficulties have meant that it is not sufficiently reliable to warrant clinical trails.
4) Targeted inhibition of gene expression: -
It is suitable for treating infectious diseases and some cancers. If disease cells display a novel gene product or inappropriate expression of a gene a variety of different systems can be used specifically to block the expression of a single gene at the DNA, RNA or Protein levels.
1) Tom strachan and Andrew P. Read, Human Molecular Genetics, Second edition.
2) T.A. Brown, Gene Cloning an introduction, Third Edition.
3) S.N. Jogdand, Gene Biotechnology.
4) B.D Singh, Biotechnology.