Gene therapy for Cystic Fibrosis
Funded by The Cystic Fibrosis Trust
Cystic fibrosis (CF) is a common disease in the UK with over 7000 individuals affected and over 700 in Scotland alone. Although patients can now expect to live into their thirties most will still die from CF lung disease. Gene therapy is particularly suited to the treatment of CF lung disease, because the disease arises from mutations in a single gene and is recessive – meaning that only modest levels of the correct gene may be needed to make a large difference.
With identification of the CF gene in 1989, expectations were high that aerosol gene delivery to the “accessible” cells lining the airways would result in clinical benefit to patients. We now believe that the formidable array of barriers that the lungs have evolved to minimise the risk from inhaled particulates are the major hurdles that have limited progress in lung-directed gene therapy. In the 1990s, Phase 1 clinical trials were instigated on the basis of encouraging data from small animal, particularly mouse, model systems. The conclusion from those trials would have to be that these models were poorly predictive of outcome in patients.
This resulted in collaboration between scientists led by Dr Gerry McLachlan in the CF gene therapy group at the MRC Human Genetics Unit (Now Medical Genetics Section – see Related links page) and Dr David Collie (R(D)SVS), funded initially by the CF Trust and subsequently an MRC Programme Grant, to develop the sheep as an improved model system.
Why sheep? Sheep lungs are anatomically and functionally similar to human lungs. Further, similar mechanisms are initiated in response to inflammation in the lungs and follow a familiar pattern of repair and regeneration. For these and other reasons several different groups throughout the world interested in human lung disease benefit from comparative studies in this species. Whilst we considered using other animal species, our opinion was such that the benefits accrued from studying sheep outweighed any potential advantages of other model systems.
Further information can be found by following this link: www.cfgenetherapy.org.uk
Shared goals and complementary expertise amongst three major UK groups led to the establishment of the UK Cystic Fibrosis Gene Therapy Consortium in 2001 (Related links). Currently the consortium consists of over 60 clinicians, scientists and technical staff based in London, Oxford and Edinburgh. The prerogative of this consortium, is to develop, and bring forward to clinical trial a novel gene therapy for cystic fibrosis.
The sheep model system has been adopted as a core facility and a key component of the evaluation strategy for candidate gene transfer agents. The main objectives at the outset were the following:
- To ensure that a product with proven efficacy can be delivered to the appropriate cells.
- Explore ways of improving the mechanical delivery system, or modifying the biological barriers such as mucus, ASL etc.
- To obtain dose-response information for the product.
- To estimate the duration of expression of the product.
- To evaluate safety of the product
A senior scientist from the Medical Genetics Section Dr Gerry McLachlan is now based at The Roslin Institute with Dr David Collie and a team of researchers and technicians.
To date we have demonstrated that:
- The sheep is a valid intermediate model system for assessing the efficacy of gene transfer. In addition the model has been utilised to generate toxicity data for our candidate gene transfer agent. This safety data forms a significant part of the investigators brochure submitted to GTAC and MHRA.
- Sendai virus is a highly efficient viral vector in sheep with good expression @ 48h and minimal toxicity
- Glycopyrrolate promotes the efficacy of gene transfer in sheep
- Gene transfer agents (GTAs) can be efficiently delivered to anaesthetised sheep using the same nebuliser technology that is used to treat patients in the clinic.
- Electroporation in vivo could be used to increase gene expression (100x) following naked DNA delivery in the sheep lung.
- There are marked species differences in the expression profiles obtained when plasmids driven by certain promoters are delivered to the airways of sheep or mice (current 4th generation promoters share similar characteristics in both model systems). It may be possible to predict the expression profile in sheep from studies in primary airway cells cultured at an air/liquid interface.
- CpG content, which plays a crucial role in dictating the extent of inflammation following gene delivery in mice, may not influence inflammation in sheep to the same degree.