Many human diseases are caused by defective genes.

All of these diseases are caused by a defect at a single gene locus. (The inheritance is recessive so both the maternal and paternal copies of the gene must be defective.) Is there any hope of introducing functioning genes into these patients to correct their disorder? Probably.

Other diseases also have a genetic basis, but it appears that several genes must act in concert to produce the disease phenotype. The prospects of gene therapy in these cases seems far more remote.

It is a disease of young children because, until recently, the absence of an immune system left them prey to infections that ultimately killed them.

Once the virus has infected the target cells, this RNA is reverse transcribed into DNA and inserted into the chromosomal DNA of the host.

The first attempts at gene therapy for SCID children (in 1990), used their own T cells (produced following ADA-PEG therapy) as the target cells.

In June of 2002, a team of Italian and Israeli doctors reported on two young SCID patients that were treated with their own blood stem cells that had been transformed in vitro with a retroviral vector carrying the ADA gene. After a year, both children had fully-functioning immune systems (T, B, and NK cells) and were able to live normal lives without any need for treatment with ADA-PEG or immune globulin (IG). The doctors attribute their success to first destroying some of the bone marrow cells of their patients to "make room" for the transformed cells.

Nine years later (August 2011) these two patients are still thriving and have been joined by 28 other successfully-treated children most of whom no longer need to take ADA-PEG.

Gene therapy has also succeeded for 20 baby boys who suffered from another form of severe combined immunodeficiency called X-linked SCID because it is caused by a mutated X-linked gene encoding a subunit called c (gamma-c) of the receptor for several interleukins, including interleukin-7 (IL-7).

IL-7 is essential for converting blood stem cells into the progenitors of T cells. [View]. Boys with X-linked SCID can make normal B cells, but because B cells need T-helper cells to function, these boys could make neither cell-mediated nor antibody-mediated immune responses and had to live in a sterile bubble before their treatment.

Link:

Gene Therapy I - RCN