Making Gene Therapy Safer

If you want to cause a long lasting alteration in DNA one of the hazards of gene therapy is a risk of cancer - caused by accidental insertion of genes into sequences prone to cause cancer. Using new virus types could reduce these events

The SCID-X1 (x-linked severe combined immune deficiency) trial in 2000 was a great success in many respects, providing beneficial in 17 of 20 boys that participated, unfortunately 5 of the 20 also developed leukaemia as a result of the therapy. This confounded scientists at the time and set the field back considerably when one of these patients sadly died. It eventually emerged that the retrovirus and retroviral genes used alongside the therapeutic gene were risky, with a tendency to integrate nearby genes called 'proto-oncogenes'. These are genes with a capacity to induce cancer if altered in some way (usually being involved with cell division and growth). The sequences in the therapeutic DNA strands injected into infected cells contained elements that enhanced gene activity, and therefore boosted expression of these dangerous genes - triggering leukaemia in 5 of the patients. 

An evolving story

We have learned a great many lessons since the early days of gene therapy and now utilise self inactivating vectors or even viruses that don't integrate DNA at all; leaving a circular strand of DNA that isn't replicated when a cell divides. This is much safer, but is also impermanent and so offers a temporary fix for a condition. In the case of monogenic diseases we really need to develop techniques that permanently fix a faulty gene by integrating into the genome itself. A team from Washington State University has been working on that problem by testing an alternative type of virus called a 'foamy' retrovirus. 

A safer alternative

The type of retrovirus (which is a class of virus type) used in initial studies was a class designed to infect humans, but it also had an inclination to integrate DNA into undesirable areas of the human genome. Ideally integration should occur in 'safe' areas of the genome, away from other genes so that no normal activity is disrupted. Foamy retroviruses don't usually infect humans and also have less tendency to integrate into danger zones, so they represent a safer alternative for more permanent, integrative gene therapy. 

A Redesign

The Washington team were able to alter the foamy virus by modifying specific viral proteins called Gag and Pol which are involved with the structure of the virus, and integration of its DNA sequences into the host's genome. By changing these sequences they increased the preference to integrate in particular areas away from these danger genes, in satellite regions of the genome. When they tested the modified virus on human cells they found that it performed as hoped - integrating in undesirable regions at a very low frequency. This is a great improvement over previous systems. 

While there is some risk involved even with this improved system, it demonstrates that altering of viral proteins can improve integration profiles and safety. With further development we could arrive at an extremely safe integrating gene therapy system; allowing us to permanently alter the genome with less fear of cancer events. The researchers also suggest that in the future new protein fusions with the integrase protein including specific targeting elements could enable even safer and more accurate gene integration at desired regions. 

Read the paper here: Retargeted Foamy Virus Vectors Integrate Less Frequently Near Proto-oncogenes