Date
Time
Speakers: Ben Samelson-Jones, MD, PhD, Children's Hospital of Philadelphia; Halli Benasutti, PhD, NBDF; Bill Maurits
Current gene therapy approaches for hemophilia A have been limited by suboptimal durability and inability to sustain factor (F) VIII activity in the therapeutic range. These limitations are thought to result, in part, from cellular toxicity associated with high-level FVIII expression. To overcome this barrier, we developed gain-of-function FVIII variants with increased specific activity, enabling higher functional activity at lower protein expression levels. These variants enhance FVIIIa–FIXa interactions without altering activation or inactivation kinetics and demonstrate improved thrombin generation and hemostatic efficacy in vitro and in hemophilia A mice. In AAV-mediated gene therapy studies, vectors achieved significantly higher FVIII activity levels than wild-type FVIII, supporting their potential to improve therapeutic durability while mitigating dose-related toxicity.
Collectively, these findings support high-specific-activity FVIII variants as a promising next-generation strategy to achieve sustained, clinically meaningful FVIII activity levels without increasing thrombogenicity or immunogenicity.
Plain Language Summary:
Next-generation factor VIII variants to improve gene therapy for hemophilia A
In this webinar pediatric hematologist Dr Ben Samelson-Jones, MD, PhD presented research his team is doing to improve gene therapy for hemophilia A. NBDF Director of Research Grants, Halli Benasutti, PhD introduced his talk with a quick explanation of an NBDF grant that supported Dr Samelson-Jones and his team in this work.
Why this matters
Funding research is an important way to improve options and outcomes for people with inheritable bleeding disorders. Dr Samelson-Jones is doing research, funded in part by NBDF, to improve gene therapy for hemophilia A.
A limitation of currently available hemophilia A gene therapy is that it cannot predictably produce high enough levels of factor VIII for long enough. It seems possible to either produce factor VIII levels that are high enough for a good therapeutic effect, but are not stable over several years. Or to produce factor VIII at a level that stays the same for several years, but that level is lower than would be really attractive compared to current treatments. Dr Samelson-Jones and his team have developed some slightly modified versions of factor VIII that are extra active in blood clotting. Their results suggest that a gene therapy with these new versions of factor VIII might be able to produce low enough factor VIII levels to be maintained for several years and still be effective enough in blood clotting to be therapeutic. More research is needed to test this in people with hemophilia A.
The NBDF-Sanofi Career Development Award
NBDF provides grants, awards, and fellowships to further research to find cures for inheritable blood and bleeding disorders, to address and prevent complications of these disorders, and to enable people and families to thrive. Established investigators, researchers who are at least assistant professors, can apply for the NBDF-Sanofi Career Development Award if they have demonstrated a commitment to bleeding disorders. The award is worth $70,000/year for up to 3 years. Innovative projects that develop novel technologies and/or therapies in bleeding disorders research are especially encouraged. Dr Ben Samelson-Jones, MD, PhD received this award in 2021.
Learn more about the NBDF-Sanofi Career Development Award for Bleeding Disorders
Hemophilia A gene therapy
The goal of gene therapy for hemophilia is to provide safe, durable (long-lasting) production of the missing factor at a high enough level to prevent bleeding in all recipients.
Hemophilia B gene therapy has been developed that is able to provide factor IX at a therapeutic level for at least 5 years. Not everyone with hemophilia B is eligible or can access this gene therapy, and it does not always work perfectly for everyone who receives it. In addition, a slightly different version, or variant, of factor IX was found in some people with too much blood clotting (thrombosis). Called the Padua variant, this form of factor IX is extra good at blood clotting. For the same amount or level of regular factor IX and Padua factor IX, the Padua factor IX has higher blood clotting activity. This means you can get adequate protection from bleeding with less of it, if it is used in gene therapy. Gene therapies using the Padua factor IX have been very effective at preventing bleeds in people with hemophilia B.
In hemophilia A there seems to be a problem. If the gene therapy produces high levels of factor VIII, it usually drops off within a year or so. It seems that long-lasting production of factor VIII is only possible if that level is not very high, which does not protect against bleeds very well. If the gene therapy is successful in getting a cell to make lots of factor VIII, then that cell often gets targeted for destruction! Something about the high levels of factor VIII in a cell triggers a mechanism to eliminate that cell. So, the very cells that make lots of factor VIII that can help prevent bleeding are being killed off. Research is ongoing into why this is and how it might be avoided. Dr Samelson-Jones and his team decided to try and make a variant of factor VIII that, like the Padua factor IX, has higher blood clotting activity than regular factor VIII. They hope it could be used in a gene therapy that produces low enough levels not to trigger the elimination of the cells.
Designing extra active factor VIII variants
Proteins are made up of amino acids. There are 22 different amino acids that can be used to build proteins by stringing them together in a chain with a specific order and a specific length. The chain then folds up on itself to make a unique three-dimensional structure. In Padua factor IX one of those amino acid building blocks has been swapped for a different one. That small change is enough to make Padua factor IX much better at blood clotting than the regular version.
Dr Samelson-Jones and his team studied the chain of amino acids that make up factor VIII in lots of different animals. They chose found several spots where all the animals have exactly the same amino acid in their factor VIII, and tried swapping in different amino acids at this position in the protein. They found that four of the new versions they made were extra active in laboratory tests for blood clotting, similar to Padua factor IX. They did further experiments to understand why their new versions have this higher activity than regular factor VIII. They showed that their variants are better at binding to activated factor IX. Binding to factor IXa and bringing it close to factor X so it gets activated too, is a key function of factor VIII in blood clotting.
From the four extra active variants, they chose two variants to study further in mice. New medicines are often tested in mice first to see how well they work and if they can be used safely. The ones that do work well can then be tested in other animals and eventually in humans to make sure they are safe and effective. Testing them in mice first allows researchers to eliminate the ones that do not work well without exposing humans, and to learn more about how they work. Dr Samelson-Jones and his team gave mice with hemophilia A gene therapy with the new variants, or regular factor VIII. The mice made a similar amount of the new factor VIII variants as they did regular factor VIII, but the new variants were better at making blood clot. This makes the team hopeful these variants could be used in gene therapy in humans without triggering the mechanism that eliminates cells that make too much factor VIII. They did not see any signs of the new factor VIII variants causing too much blood clotting, or thrombosis in the mice. It will be important to test carefully for this if they are used in humans.
To learn more about this exciting research
• Watch the full recording of this webinar to hear all the details of the research Dr Samelson-Jones and his team performed, the interpretation of their results, and how they hope they might contribute to the future of hemophilia gene therapy
• Contact Dr Samelson-Jones via email or social media
@DrSamelsonJones
@samelsonjones.bsky.social
samelsonjonesb@chop.edu