|Figure 1: Clotting Cascade in|
Normal and Haemophilic Individuals
by Georgia Sheedy
Haemophilia B is a genetic disorder which prevents the blood from clotting, meaning just a bruise or small injury in a Haemophilic patient can have dire consequences such as arthropathy (joint disease), or in severe cases, even death. It arises when there is a mutation in the F9 gene, which codes for the production of Coagulation Factor IX. Factor IX is a protein which is a vital part of the clotting cascade which forms the blood-clotting fibrin network. The absence of this protein is analogous to a missing domino – without it, the cascade cannot progress (see Figure 1). The inheritance of this gene is X-linked and recessive, which means males are more likely to have the disease due to their single X chromosome, in contrast to females whose 2 X chromosomes equal a greater chance of possessing a normal allele which can mask the defective one (see Figure 2). Current treatment for the disease is intravenous infusion of a Factor IX concentrate, but is very costly and can lead to inhibitor formation, a condition in which patients no longer respond to Factor IX infusions. Thankfully, scientists are now investigating a new method of treatment – somatic gene therapy – which has the demonstrated its capacity to engender long-term endogenous (produced by the person) production of Factor IX.
|Figure 2: Inheritance of Haemophlia B|
A recent clinical trial was conducted by Dr. Nathwani and his colleagues investigating the effectiveness and safety of this treatment. The trial used gene targeting, where a section of viral DNA is removed and replaced with a target gene, in this case a normal F9 allele. The virus used was an adeno-virus associated virus 8 (AAV-8), which is a nonpathogenic virus commonly used as a vector to introduce new genes into an organism due to its' low immunogenicity. Because viruses don't have organelles and are merely a capsule of DNA, they cannot produce proteins on their own, so inject their genetic material into the nucleus of cells (in this case liver cells). The virus then uses the cell's organelles to produce the FIX protein (see Figure 3). In this study, a vector which has a high tropism for the liver was used, allowing it to be injected through a peripheral vein. This non-invasive approach was very important given the nature of this disease.
|Figure 3: Invasion of Hepatocyte by Viral Vector|
In the trial, just 1 administration of the vector was given to six patients, each sufferers of severe Haemophilia B who ceased their bi/tri-weekly infusions of Factor IX just prior to partaking in the study. Severe Haemophilia is defined as having <1% of normal Factor IX levels. After receiving the treatment, all subjects showed an increase in endogenous Factor IX production, however patients who received a high dose exhibited the highest levels (8% and 12%) (see figure 4 for summary of results). An increase to >1% results in the severity being reduced to moderate, whilst an increase to >5% reduces severity to mild. Accordingly, patients who received a medium-high dose were able to permanently stop intravenous FIX infusions, while those administered a low dose required treatment less frequently.
|Figure 4: Trial Results|
These are promising results - Dr Katherine Ponder from the Washington University School of Medicine said that, "This is truly a landmark study, since it is the first to achieve long-term expression of a blood protein at therapeutically relevant levels". Of course, the treatment is not yet perfect – improvements in safety and efficacy are necessary, but the work of Dr. Nathwani et al. has showed what a valuable tool gene therapy can be, and is a step forward in the treatment of not only Haemophilia B, but potentially of other blood protein disorders.
- AAV Vectors and Gene Therapy (online) http://aavvectors.tripod.com/id3.html [accessed 19th March 2012]
- Gallagher, J (2011) Haemophilia Gene Therapy Shows Early Success (online),http://www.bbc.co.uk/news/health-16107411 [accessed 16th March 2012]
- Konkle, B, et.al., (2011) Hemophilia B (online) http://www.ncbi.nlm.nih.gov/books/NBK1495/ [accessed 19th March 2012]
- Nathwani, C,. et. al (2011) 'Adenovirus-Associated Virus Vector–Mediated Gene Transfer in Hemophilia B' The New England Journal of Medicine, Vol. 365, pp 2357-2365.
- Nishida, K et.al., (2008) 'Gene Therapy Approach for Disc Degeneration and Associated Spinal Disorders' European Spine Journal, Vol. 17, pp 459-456
- Ponder, K (2011) 'Merry Christmas for Patients with Haemophilia B' The New England Journal of Medicine, Vol. 365, pp 2424 – 2425
- Seppa, N (2012) 'Gene Therapy Helps Haemophiliacs' Science News, Vol. 181, pp 9.
- Reece, J., et al., (2011) Campbell Biology. 9th Ed. (Australian Version), Pearson Australia Group Pty Ltd.
- U.S. National Library of Medicine (2012) Hemophilia (online) http://ghr.nlm.nih.gov/condition/hemophilia [accessed 19th March 2012]
- U.S. National Library of Medicine (2012) F9 (online) http://ghr.nlm.nih.gov/gene/F9[accessed 19th March 2012]
- Wood, C (2003), Coagulation Factor IX (online),http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2003/WoodC/coag.html[accessed 18th March 2011]
Images Sourced From:
- Nathwani, C,. et. al (2011) 'Adenovirus-Associated Virus Vector–Mediated Gene Transfer in Hemophilia B'
New England Journal of Medicine,
Vol. 365, pp 2357-2365.
- Noonan, K (2011) Successful Gene Therapy for Haemophilia B Reported (online) http://www.patentdocs.org/2011/12/successful-gene-therapy-for-hemophilia-b-reported.html [accessed 17th March 2012]
- World Federation of Haemophilia (2010) Haemophilia in Pictures (online) http://www.wfh.org/index.asp?lang=EN [accessed 20th March 2012]