By Reagan Smith, part of the Helyx Newport Chapter
Hemophilia is a disease that affects the ability of the blood to clot. There are two types: hemophilia A and hemophilia B, both of which result in a person bleeding for longer than normal or healthy. Type A is caused by a mutation on the F8 gene, while type B is due to an F9 mutation; both of the genes are located on the X chromosome, meaning hemophilia is more common in males than females. A person needs one working copy of the F8 and F9 genes in order for enough working proteins, called coagulation factor VIII protein and coagulation factor IX protein, to be synthesized and diffused into the bloodstream.
When an injury occurs in a healthy person without hemophilia, the body initiates what researchers have named a waterfall sequence, where protein coagulation factors function to form a clot at the site of the wound. These factors are circulating in the bloodstream as inactive enzymes, then get converted into active enzymes through limited proteolysis (protease enzymes break the peptide bonds between key amino acid loci in the coagulation protein). The body is able to convert these inactive enzymes into large amounts of fibrin by activating a small amount of Hageman factor XII, the starting factor in the waterfall sequence. However, the signal to begin converting the enzymes must first be relayed, and this is done through a protein complex made of Factor IX and Factor VIII. The signal passes through this junction of molecules and initiates the waterfall cascade to form the blood clot, made of fibrin and platelets.
Platelets provide a surface for coagulation factors to interact on and signal for the production of fibrin, which can then form the mesh that blocks blood from leaving the body or pooling around joints when an injury occurs.
With hemophilia, however, the person does not synthesize enough of the fully functioning factor IX and VIII protein. This means that the protein complex is unable to transfer the signal, and no blood clot at the wound site forms.
While there is no cure for hemophilia, there are lifestyle changes that alleviate the negative effects of the disease. These include maintaining a healthy weight and active lifestyle, to reduce stress on joints and blood vessels, regular and effective management of symptoms and side effects, and accessibility to a hemophilia treatment center. However, more impactful, effective solutions– the most common being gene therapy– to stop the problem at the source show encouraging results.
Gene therapy to treat hemophilia is a current area of study for clinical trials. Scientists are looking into using genes, genetic material, and CRISPR/cas9 technology to promote the production of clotting factors by the somatic (body) cells.
A current experimental gene therapy for treating Hemophilia B specifically is AMT-060. Hemophilia B is found generally in males; it is caused by a mutation in the allele that codes for coagulation factor IX that results in none of the coagulation protein being made.
AMT delivers a functional IX gene to liver cells via an Adeno-associated Virus (AAV), a small virus without a protein envelope covering the surface. AAVs contain linear, single stranded DNA. When these viruses infect a cell, they do not provoke an immediate response or begin to multiply, instead becoming latent and multiplying only when the host cell is under stress.
Because of these qualities, an AAV is ideal for gene therapy as there is minimal risk of negatively impacting the patient while delivering a new allele to be integrated into the host cell. AMT-060 treatment has been deemed a breakthrough therapy by the FDA, and current experiments have been extremely successful. Patients have tolerated the virus with no activated T-cell response, meaning the body has not reacted to the foreign particle and attempted to kill it.
Another treatment utilizing the AAV is AAV5-hFVIII-SQ gene therapy for hemophilia A. To break down that complicated name, AAV5 refers to the type of adeno-associated virus that can infect humans; this treatment uses serotype 5. HFVIII stands for human factor VII– the mutated gene that codes for coagulation factor VII found in humans with hemophilia A. Finally, the SQ dictates that the gene is found in humans.
A post-3 year follow up shows that administration of AAV5 for hemophilia A was effective, in the right concentration. The cohort of patients that received 6x10¹³ vector genomes, meaning segments of the viral allele coding for factor VIII, per kilogram of body weight had the most success with the experimental treatment. The 7 patients expressed a significant amount of VIII protein, more so than any other cohort, had a median of 0 annual treated bleeding events, reduced their annual number of factor VIII infusions 138.5 to 0, and resolved issues with bleeding at key joint areas.
The viral-derived VIII factor showed the same functionality and ability as human-derived VIII factor, and there were no significant negative side effects of the treatment, including impact on liver-function or death. The patients had no inhibitor development, meaning resistance to the treatment, nor did they experience thrombosis– a harmful blood clot occurring in a blood vessel. Overall, AAV5-hFVIII-SQ gene therapy for hemophilia A has thus far shown to be effective, and the applicability of AAV to other treatments for hemophilia is promising.
Educational Content
Why does a person with hemophilia bleed excessively?
They don’t have a working copy of the F8 or F9 allele, so they produce a mutated version of the Factor IX or Factor VIII. This means that the signal to tell the body to form a blood clot is either weak or not transmuted, so the blood clot doesn’t form, causing injury to the person.
What are some potential treatments for hemophilia?
Current experimental gene therapies include AMT-060 for hemophilia B and AAV5 for hemophilia A. Both therapies utilize an adeno-associated virus to “infect” the host liver cells with a working copy of the corresponding allele. The studies show promising results, with the FDA endorsing the former as a “breakthrough.”
Sources
Malcolm, Emily. “Gene Therapy.” Hemophilia News Today, Https://Hemophilianewstoday.com/Gene-Therapy/, 14 Feb. 2020.
Berns K.I., Giraud C. (1996) Biology of Adeno-associated Virus. In: Berns K.I., Giraud C. (eds) Adeno-Associated Virus (AAV) Vectors in Gene Therapy. Current Topics in Microbiology and Immunology, vol 218. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-80207-2_1
Vilar, Rui, et al. “Fibrin(Ogen) in Human Disease: Both Friend and Foe.” Haematologica, vol. 105, no. 2, Feb. 2020, doi:10.3324.
Kresge, Nicole, et al. “The Waterfall Sequence for Blood Clotting: the Work of Earl W. Davie.” The Journal of Biological Chemistry, vol. 281, no. 48, 1 Dec. 2006, pp. 39–41. Classics, doi:10.5270/esa-nu1d6p4.
Hemophilia [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2019 [cited 2021 Feb 12] Available from https://learn.genetics.utah.edu/content/genetics/hemophilia/
Pasi, K. John, et al. “Multiyear Follow-up of AAV5-HFVIII-SQ Gene Therapy for Hemophilia A.” New England Journal of Medicine, vol. 382, no. 1, 2020, pp. 29–40., doi:10.1056/nejmoa1908490.
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