From gene to drug
The gene is a portion of deoxyribonucleic acid (DNA) which carries our genetic information. Genes control the production of proteins.
The principle of gene therapy is based on the fact that a human cell can turn any gene into protein, even if the gene is not part of the repertoire of human genes, and on the possibility of inserting this gene into the cell nucleus using various types of vectors.
Gene therapy assumes that you know either the genetic anomaly which caused the disease, so as to provide the cell with a functional copy of the absent or defective gene, or the function of the gene being inserted and that this function can repair the pathological mechanism which is the source of the disease.
In practice, it also assumes that you are an expert in genetic engineering! Indeed, the gene-drug must be isolated. Then, if you want only certain types of cells to express the protein they encode, you must add a "promoter", i.e. a small DNA sequence which regulates expression, to this gene. Then find a means to transport the gene into the nucleus, i.e. usually a viral vector.
From rare genetic diseases to all diseases!
The gene therapy concept grew from the desire to treat genetic diseases at their source. The idea was simple: abnormal copies of a gene are responsible for a disease, so let's provide the cells with normal copies!
The first diseases which benefited from gene therapy were rare genetic diseases in which the gene responsible had been identified, in particular severe immune deficiencies
However, proofs of concept that have been provided for diseases such as Leber's amaurosis or Huntington's disease are also applicable to more common diseases such as age-related macular degeneration (ARMD) and Parkinson's disease.
Above all, the idea gradually grew that it was not enough to replace a defective gene, but the applications of gene therapy could be significantly expanded by inhibiting or stimulating the expression of genes not necessarily defective. So that almost all diseases could be treated by gene therapy eventually. An example? Researchers propose to insert a suicide-gene into cancer cells to force them to self-destruct and are also thinking about vaccination using gene therapy.