Sail For Epilepsy, Part 3: How Would Gene Therapy Work to Address Monogenic Epilepsies?

The underlying strategy for treatment of monogenic epilepsies is to provide a functioning version of a defective gene in order to restore normal activity of that gene’s product inside the neurons in the brain.  For instance, in the case of a lysosomal storage disease, cells lack specific enzymes needed to clear substrate build-up in the lysosomes inside the neurons. By expressing a corrected enzyme in the neurons, one could reestablish those cellular processes and thereby “de-gum the works” to allow cells to function properly. And, if the enzyme in question is secreted outside of the cell, then corrected cells may even be able to clear the accumulated substrate from neighboring cells as well. Ultimately, the goal is to find a way to sustainably restore the gene’s function in as many neurons of the brain as possible.   

A common approach to achieve this is to deliver a functioning copy of the faulty gene via an adeno-associated virus (or AAV) vector to the nervous system. There are three possible routes of administration to get the AAV to the target cells in the brain.  

The first route of administration is an injection into a vein (e.g. intravenous) of an AAV vector that can cross the blood-brain barrier. Importantly, due to the inefficient uptake of viruses (and many other kinds of therapies) across the blood-brain barrier, the efficiency of correcting neurons via intravenous injection is currently low. Thus, there are continued efforts to discover novel AAV capsids which can overcome this challenge. Today, the intravenous approach is still pursued in cases where there is a need for correction in the peripheral nervous system and/or other organs outside the brain as well.  A second, more directed administration approach is intrathecal injection through a lumbar injection to the spinal cord or to the cisterna magna6. The third, even more focused strategy is intracerebroventricular injection (e.g. directly to the brain’s cerebroventrical space). Each of these approaches has pros and cons and all administration approaches depend on the biodistribution and “tropism” or homing of the virus to the cells that need to be corrected in the brain.   

Once the AAV vector arrives at the target cells in the brain, the virus will transduce. or infect the cell and set up shop in the nucleus. Unlike some viruses (e.g. lentiviruses) which have a high rate of integration into the chromosomes of the patient’s cells, AAV viruses generally function by establishing an episome. which is a somewhat stable genomic structure inside the nucleus but apart from the patient’s chromosomes. The challenge with these episomes is that, unlike real chromosomes, they don’t faithfully segregate when cells divide. This means that in rapidly dividing cells, the functional copy of the gene can be lost over time. However, in non-dividing or slowly dividing cells like neurons in the brain, there is a higher likelihood that the delivered gene will durably persist in the cells over time, ideally for the lifetime of the patient.   

Once a cell is transduced by the virus and the episome established, then the corrected gene contained within the viral DNA can be expressed and replace the faulty gene’s function. However, that of course is not the end of the story. Because the gene was delivered by a virus, the cells that are transduced can become targets of the patient’s immune system. This can result in local inflammation which can cause issues for the brain, but it can also result in the loss of the cells that have been corrected by the virus. Therefore, often prior to administration of AAV gene therapies, patients will be prophylactically dosed with immunosuppressant drugs prior to receiving the viral vector. The use of immunosuppressants is intended to reduce the likelihood of an adverse reaction to the virus and increase immune system tolerance for the cells which have been successfully transduced by the virus. These immunosuppressive regimens vary between clinical trials and are sometimes not fully effective against the immune reactions. Because of this, improving immunosuppressive regimens is an area of active focus by the industry. 

Importantly, you may have heard people refer to AAV vectors as “one and done” therapies. While this sounds attractive, it is actually a double-edged sword. On the positive side, this means that a single administration may be able to provide meaningful, durable clinical benefit for that patient for life. On the negative side, if additional doses are needed, we don’t yet know how to re-dose these viruses effectively. That is because after a first exposure, patients typically develop neutralizing antibodies and other immune responses to the capsid of the vector which prevents that virus from efficiently transducing the patient in the future. Consequently, if a patient doesn’t receive enough correction of the target cells on the first pass, with today’s technology, we don’t have a second opportunity with the same vector to try again to boost treatment. The industry is looking into ways to prevent this with immune suppression, virus cloaking, serotype-switching, and novel capsid approaches. However, these technologies are still in the research phase.    

As a result, there is a strong focus among regulators, therapy developers, investigators, providers, and patients, on understanding the best starting dose for each therapy in the clinic. The overarching goal is to select doses that have an opportunity to give a meaningful benefit for each patient in the trial without resulting in undue risk from immune reaction or other effects of therapy.   

In the next blog, we’ll dive a bit deeper into how the doses are selected for clinical trials and endeavor to explore a number of questions: 

• What are the goals of the nonclinical studies that are performed prior to entering the clinic? 

• What are the challenges and limitations of those studies? 

• How does one draw conclusions about the benefit/risk of a therapy based on these studies? 

Emily Walsh Martin is a volunteer crew member for Sail For Epilepsy’s Atlantic crossing on the vessel Ingwe. When she’s not sailing, she is a consultant for gene and cell therapy companies and investors who are seeking to advance novel therapies in the clinic.

Read the Series

Part 2: An Overview of Syndromic and Monogenic Non-syndromic Epilepsies

Part 1: A Worthy Excuse for Missing the ASGCT Annual Meeting