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In human beings, about 5,000 genetic diseases due to single gene mutation have been known, e.g., sickle cell anemia, cancer due to mutation in oncogenes or tumour suppressor genes, and at present, most genetic diseases have no effective treatment. Through gene therapy, normal functional gene is transferred in the patient’s specific target cells that have defective genes. Its expression (through transcription and translation) produces therapeutic product (protein) that helps in the treatment of such genetic disease by restoring normal cellular function. Live adeno viruses or microinjection are used to introduce the functional genes. Human genome mapping has helped in locating the relative and actual positions of about 1,00,000 genes in 3 billion nucleotides in a haploid set of chromosomes (23) and has also helped in determining the sequences of many of these genes. There are two ways of gene therapy:

(i)                 Germ cell gene therapy, in which genetic changes are directed into the germ cells (sperms or eggs) or zygotes which is heritable and can be passed on to the next generation. This type of therapy is presently banned and not permitted in any country.

(ii)               Somatic cell gene therapy, in which the functional genes are introduced only into those somatic cells of the body which have defective genes. It is not heritable and its effect would be confined to the person undergoing treatment and would not be passed to its offspring. Clinical trials are going on to correct single-gene defects (mutations), e.g., cystic fibrosis (1 in 500 individuals) where the target tissue is liver, haemophilia (l in 6,000 males) where the target tissue is liver or fibroblasts, thalassaemia where the target tissue is bone marrow, etc.