Gene Therapy III

• retroviruses as vectors (Gene Therapy I)
• adeno-associated virus as the vector (Gene Therapy II)

One problem with these approaches is that foreign DNA is inserted into the host genome. It is possible — and has been demonstrated — that the foreign DNA may be inserted into a chromosomal position that disturbs normal gene function there. In fact, three boys treated with vectors containing a gene to cure their X-linked severe combined immunodeficiency (X-linked SCID) developed cancer because of this. [Link]

But now Urnov, F. D. et al., report (in Nature, 6 June 2005) their success — with cultured cells — in correcting the molecular deficiency in X-linked SCID without the need for any vector.

X-linked SCID is caused by a mutated X-linked gene encoding a subunit — called γc (gamma-c) — of the receptor for several interleukins.

• a zinc-finger transcription factor. This can be engineered to recognize and bind to any desired DNA sequence in the genome. It is coupled to
• a restriction enzyme that cuts through both strands of DNA near that location.
• a separate plasmid containing the correct version of the γc gene.

• its own machinery for the repair of DSBs by homologous recombination and
• the plasmid as a template

• not only avoids the danger of introducing DNA into random sites in the genome but also
• avoids the need for introducing foreign genes to aid in selection of successfully-repaired cells [Link to discussion].

It's a long way from something that works in cultured cells to something that works in human patients, but here at least is a promising procedure. Instead of adding a functioning gene anywhere in the genome, both copies of the cell's own defective genes are repaired.

Humans with single-gene disorders like

• sickle-cell disease
• hemophila
• severe-combined immunodeficiency (SCID)

• removed
• treated in vitro by this method and then
• returned to them.