Pushing The Limits Of Gene Therapy By Manipulating How Proteins Are Glued Together

Gene therapy is likely to be an integral part of the future of medicine. By delivering genetic code to specific cells, we can instruct them to produce vital proteins, to stop producing harmful proteins, or correct diseased proteins that don’t function properly. In this way, gene therapy has the potential to correct genetic diseases and even some non-genetic diseases. However, many hurdles still need to be overcome, many of them involving the methods used to deliver the gene therapy to the target tissue.

Take for example Stargardt disease, a rare genetic condition in which fatty material accumulates on the macula (the area at the centre of the retina that is responsible for sharp, detailed vision), eventually damaging it and resulting in a loss of central vision. Stargardt diseases is caused by mutations in a single gene called ABCA4, which encodes the protein that usually prevents fatty deposits from forming on the macula.

Stargardt disease could be fixed relatively easily with gene therapy if delivered early enough during the disease. Introducing a healthy copy of the ABCA4 gene into retinal cells should allow them to clean up fatty deposits and prevent permanent damage to the macula. Unfortunately, there’s a problem with this approach: the ABCA4 gene is simply too big to deliver via the current gold-standard gene therapy delivery method, the adeno-associated virus (AAV). Genes larger than around 5000 base pairs (the ‘genetic letters’ that make up the DNA code) cannot fit inside AAVs, but the ABCA4 gene is about 6800 base pairs long. While there are other delivery methods that can carry larger cargoes, these bring their own set of problems and limitations.

Enter SpliceBio, a company aiming to overcome some of the limitations of gene therapy using split inteins. What are split inteins? In some single celled organisms, multiple DNA sequences each code for a separate piece of a protein, which are then joined together in a process called protein splicing. This process is carried out by inteins, which are segments of the precursor proteins that are removed as those proteins are joined together.

Rather than trying to introduce the entire ABCA4 gene at once, SpliceBio’s strategy is to introduce the gene in two halves using two separate AAVs, each also containing DNA coding for an intein. Once each half of the gene has been translated into a protein, those proteins will be spliced together to form the full healthy version. If successful, this approach could widen the range of diseases targetable with gene therapy.