Gene Therapy Overview DMD is an inherited, genetic disease. A gene is a very large molecule, and the gene for dystrophin is the longest known human gene. To treat DMD, we need to repair or deliver a new copy of this gene to every cell in the body where it is needed. In the last few years, huge progress has been made in gene therapy to treat DMD. Gene therapy is delivering new genetic material to cells, to overcome the errors/deletions in the dystrophin gene. Using a virus, called Adeno-associated virus (AAV), researchers are able to insert a shortened, man-made version of the dystrophin gene, called microdystrophin, into a patient's cells and replace the faulty gene. However, gene therapy poses some big technical challenges, including the body's natural and developing immunity to the AAV itself. We, at Duchenne UK, are committed to overcoming these challenges. How is DMD treated currently? Most of the current treatments for DMD only treat the symptoms of DMD - they do not address the underlying cause i.e. the lack of dystrophin. Treatments prescribed include corticosteroids, ACE inhibitors, and bisphosphonates. You can find out more about these on our page about Current Treatments. Other treatments being explored include steroid alternatives and exon skipping but these are still in clinical trials. Currently the only drugs approved that target the disease itself are Exondys51 (previously known as eteplirsen), in the US, and Ataluren, in the UK. However, such treatments can only correct specific exon mutations on the DMD gene and so are only suitable for small groups of patients with DMD. How does gene therapy work? Gene therapy is the delivery of a working copies of a desirable gene to the cells that need it. However, our cells have evolved mechanisms to prevent such intrusion of foreign molecules. To overcome this barrier we use vectors to carry the new gene into the cell, whilst shielding it from the mechanisms designed to prevent this happening. Currently, the most promising approach is based on the use of a relatively harmless virus called Adeno-associated virus (AAV) as the carrier or vector. Viruses have evolved to get inside cells by recognising specific proteins on the surface – allowing them to pass through the cell membrane. Once inside the cell, they deliver their genetic material, forcing the cell to make many copies of the virus – this often makes us feel unwell. However, if scientists remove the unwanted, disease-causing viral genes and replace them with appropriate beneficial genes, it could restore desirable gene expression. This is the basis of the ‘gene therapy’ that is causing great excitement at the moment. One challenge of DMD gene therapy is that the dystrophin gene is the longest known gene. Because of its size, it is impossible to insert the entire gene into the AAV vector (we call such viral vectors ‘capsids’). So, researchers have created microdystrophin - a shortened version of the dystrophin gene which can fit into the AAV capsid. This shortened but still functional gene has been shown to induce the production of a shortened but still functional dystrophin protein. Gene therapy using microdystrophin in this way has resulted in improved muscle function in relevant animal models of the disease. What trials and research are being done using gene therapy? Company Trial Phase Location Update Pfizer PF-06939926 2 US June 2019 Sarepta Therapeutics EMBARK - SRP-9001 1/2a US June 2020 Solid Biosciences IGNITE DMD - SGT-01 1/2 US July 2020 Updates from these companies on these trials will be shared on our Facebook page. If you have any questions, please get in touch. What are the challenges around gene therapy? As described above, it is hard to deliver new genes into cells. The next challenge is to ensure they work and keep working. Once the gene reaches the correct destination, it must be activated or turned on, in order to produce the protein. Once the gene is turned on, it should stay turned on. Cells tend to shut down genes which are too active or are exhibiting unusual behaviours, this reduces the risk of developing such problems as cancer. As described above, a viral vector may carry the replacement gene into the cell body, but this technique presents the risks. Such risks can be classified under three headings and are discussed below. Unwanted immune system reaction. The body's immune system has evolved to recognise and remove unwanted viruses, to stop us getting ill. The microdystrophin-carrying viruses are seen by the immune system as intruders. If the immune system recognises the virus as an intruder it will try to attack and remove the virus. This often results in inflammation, and in severe cases can cause organ failure. Targeting the wrong cells. Because viruses may be capable of ‘infecting’ more than one type of cell, it's possible that they may deliver their therapeutic genetic material to cells other than those they were designed for. If this happens, these healthy cells may be damaged, perhaps causing other illness or diseases such as cancer/tumours. Infection caused by the virus. It is not impossible that viruses, when delivered into the body, may after a while revert to their ‘wild type’ and cause the illnesses they were originally responsible for. In thinking about the unwanted immune responses to gene therapy using viruses we can imagine two scenarios: Gene therapy may be a one-time treatment. In theory, a single administration of gene therapy treatment should be enough to induce dystrophin production. If, however, the effect of the gene therapy does not last, we may need to re-administer the drug. However, there are challenges with this. The first time your body ‘sees’ this virus the immune system will take a while to respond, allowing time for the desired effect to take place. The second time your body ‘sees’ this virus, the response will be much more rapid and lead to neutralisation and removal of the virus before it has had a chance to deliver its genetic payload. This means that re-administration of beneficial virus cannot be carried out. Some patients may have developed pre-existing antibodies to AAV, meaning they would be ineligible to receive AAV-mediated gene transfer even on one occasion. This, we believe, is the case for a significant percentage of DMD patients. Further to this, regulators, such as the FDA in the US and EMA in Europe, have strict scientific guidelines for clinical trials to ensure the drug is as safe as it can be before being tested on patients and subsequent authorisation for use. Any new treatment can take many years to develop and it is important that we are diligent about the drug development process. To find out about how Duchenne UK is tackling this particular issue, read more about our Project Hercules. Are there any alternative delivery methods for gene therapy which avoid immune responses? Duchenne UK wants to find a way of delivering the dystrophin gene without the serious immune responses. In 2019, we made one of our largest investments in a technology which could potentially overcome this problem. This project, with Evox Therapeutics, will investigate exosomes as a novel delivery method. In 2020, we also held a Grant Call with PPMD looking at how to improve delivery and overcome the immune response. We are now funding a project, run by Prof Kanneboyinga Nagarju, which is investigating how to manage the body's response to AAV.