In November last year (Gene Therapy: 03/11/14) I discussed the topic of gene therapy and noted that this novel form of treatment was on the cusp of moving from the laboratory into the clinic. At that time, gene therapy involved “putting a message into an envelope” – inserting a new gene into a virus – and then getting the virus to “post” the new gene into the cells which make our blood. The concern with this technique was always that the messenger virus might have unintended effects and that if the new gene was inserted into the wrong place in the patient’s DNA this might also have serious consequences. Last month, in the journal Blood, scientists from California described successful experiments with a different form of gene therapy which cleverly avoids these two problems.
The new technique, called “gene editing”, does not replace the patient’s faulty gene with a new one, but rather gets the patients’ own cells to repair, or edit, the faulty gene, removing the mutation which causes it to malfunction in the first place and replacing it with a normal DNA sequence. Every day our DNA suffers breaks and other forms of damage, which the cells of our body have developed sophisticated mechanisms to repair and put right. The scientists have co-opted these naturally occurring mechanisms to correct the defect in the sickle haemoglobin gene.
First, they designed a specific enzyme, called a zinc finger nuclease (ZFN), which recognises the gene making the beta chain of haemoglobin and they then used this enzyme to deliberately cause a break in the beta globin gene. Next, they used the cells own biochemical repair mechanisms to repair the break. But rather than let the cells restore the gene with the sickle mutation intact, they provided the cells with a new template, so that when the repair mechanisms swung into action the gene was repaired as a normal haemoglobin gene without the harmful mutation. Reassuringly, they found that the ZFN’s were very specific and did not cause breaks in any other genes and that the “homology directed repair” (HDR), or use of the new template to correct the DNA sequence and repair the damage, was very precise.
Having undertaken these initial experiments they then had to prove that the repaired genes could work and produce significant amounts of normal haemoglobin. They collected blood stem cells (haemopoietic stem cells – HSC’s), from patients with sickle cell anaemia and treated these cells in a test tube with the two step gene editing therapy. The treated HSC’s were then injected into mice and the scientists waited to see what the cells would do. Remarkably, about 20% of the HSC’s were successfully modified and were able to start making normal haemoglobin (haemoglobin A), instead of sickle haemoglobin, for the very first time.
This was an extraordinary achievement and means that for the first time we are able to correct an inherited defect in a single gene, using the body’s own repair mechanisms and, what is more, the repaired gene is then able to work normally. This advance has obvious implications for the treatment of people with sickle cell disease but also for patients with many other types of inherited disease. There are now at least two ways of correcting gene function, it remains to be seen whether one or both of these can be successfully used in patients.
Correction of the sickle cell disease mutation in haemopoietic stem / progenitor cells. Hoban MD, Cost GJ, Mendel MC, et al. Blood (2015), volume 125, pages 2597-2604