Researchers at Children’s Hospital of Philadelphia (CHOP) announced the results of a new study offering insights into the development of next-generation gene therapies to treat hemophilia A, a rare bleeding disorder that is characterized by a deficiency of the clotting protein factor VIII or FVIII. The findings were published in the journal Nature Communications.

Hemophilia A is a hereditary condition impacting all races and ethnic groups, in which the FVIII clotting protein in plasma is reduced or absent. Proteins in the body work together in a complex system to form a clot. When one factor is deficient, blood clotting is delayed, which results in poor clotting and excessive bleeding. Approximately 20,000 people with hemophilia live in the United States, including children and adolescents. Although hemophilia A is genetic, about one-third of babies with hemophilia have no known family history, which researchers attribute to a new mutation in the gene that causes hemophilia.  

In 2023, the U.S. Food and Drug Administration (FDA), approved the first adeno-associated virus vector-based gene therapy for the treatment of adults with severe hemophilia A. The one-time gene therapy product administered as a single dose by intravenous infusion consists of a viral vector carrying a gene for clotting FVIII. The gene is expressed in the liver to increase blood levels of FVIII and reduce the risk of uncontrolled bleeding. However, multi-year available follow up data observed that FVIII levels are not consistently durable over time.

This study, led by Lindsey A. George, Director of Clinical In Vivo Gene Therapy and an attending physician in the Division of Hematology at CHOP,  marks the first time that researchers have begun to understand via a mouse model why the FDA-approved treatment isn’t sustainable. The research also evaluated how an enhanced variant of factor VIII protein, FVIII-QQ, which was developed in George’s lab, resolved these issues in mice and could potentially be applied to people, including those under 18, under the rationale that an improved function FVIII may normalize blood clotting at low levels of expression to permit long-lasting treatment.

“This work informs the mechanism of factor VIII durability after gene transfer and supports that factor VIII-QQ may safely overcome current hemophilia A gene therapy limitations,” said George. “We envision these insights will provide a path forward for future research, along with avenues for clinical translation to help hemophilia A patients.”

In the study, researchers used a mouse model and uncovered that the clotting was not sustainable because copies of the AAV vector decreased in quantity over time in the liver, which led to a reduction in FVIII protein levels. Since a sustained level of vector is necessary to continue long-term clotting function, the drop in vector copies signaled a lack of therapeutic durability.

George and her team subsequently showed that expressing the enhanced function FVIII variant, FVIII-QQ, enabled a balanced clotting process at low levels of expression that could be sustained over time. This result is significant because it has the potential to improve the longevity of gene therapy therapeutic applicability, while reducing side effects from high AAV doses. The study also investigated possible side effects of FVIII-QQ in animal models, including excess clotting and an immune response to the novel protein, without evidence of potential risk, though data were limited to mice only.

George, et. Al. “Pre-clinical evaluation of an enhanced-function Factor VIII variant for durable hemophilia A gene therapy in male mice.” Nature Communications. Online August 21, 2024. DOI: https://doi.org/10.1038/s41467-024-51296-8.