Monday, 14 May 2012

A Cure is in Sight for Retinitis Pigmentosa


Earlier this year scientists from the University of Florida published exciting news regarding a gene augmentation therapy used to treat a disease that causes a debilitating blindness called Retinitis Pigmentosa [2]. Retinitis Pigmentosa is an X-linked disorder, meaning it is caused by a malfunctioning gene on the X chromosome. This malfunctioning gene isn’t able to produce the proteins necessary to maintain eyesight, leading to a gradual loss of vision [5].

The gene that can lead to X-linked Retinitis Pigmentosa is called the Retinitis Pigmentosa GTPase regulator (RPGR) gene [1]. This gene encodes the proteins necessary to maintain healthy eye function. The absence of the RPGR gene leads to a gradual breakdown of photoreceptor cells, namely the rods and cones that line the retina [5]. Rods are much more numerous than cones, and have a greater sensitivity to light [4]. They are the first cells to start disintegrating, which leads the first manifestation of Retinitis Pigmentosa, the loss of night vision [3]. Over time, the colour sensitive cones also start to break down, leading to further vision loss. Most people with the disorder are legally blind by about 40 years of age [2].

As the RPGR gene is carried on the X chromosome and is a recessive trait, it is very rare for women to have this disorder, as women have a pair of X chromosomes, and the RPGR gene on the other chromosome can usually compensate for the malfunctioning one [1]. However, much like colour blindness, if men inherit the faulty RPGR gene, then they will develop the disorder, as their Y chromosome doesn’t have the corresponding RPGR gene to compensate. Women can still be carriers of this gene though, and any sons born to a female carrier have a 50% chance of inheriting the disorder. This is a common disorder that was known to affect over 100 000 Americans in 2011[6].

The recent breakthrough in the search for a cure for Retinitis Pigmentosa has come from a successful attempt to treat the disorder in canines using gene augmentation therapy. This therapy works by sending a small, non-pathogenic virus (called an adeno-associated virus) that carries a functional copy of the RPGR gene to the affected area [5]. The gene is embedded with a cloned genetic switch that ensures the gene is activated in the right cells [5]. Once activated, the RPGR gene then begins producing the protein necessary to restore proper eye function and repair the damaged cells [5]. The treated animals experienced a drastic improvement in both rod and cone photoreceptor function [1]. Researchers are now trying to determine which genetic switch would be optimal in the activation of the gene in human cells [2].

Despite these encouraging results, there is still a long way to go before the augmentation therapy can be trialled on humans. However once it reaches this stage, the effect the treatment would have on the lives of sufferers of X-linked Retinitis Pigmentosa is staggering. The results indicate that vision would be almost fully restored in the majority of cases. In addition to this, the process is non-invasive and one treatment is predicted to last for years [2]. There is no doubt that this successful trial is a major step forward in the fight against Retinitis Pigmentosa.


Reference List
[1]          Beltran, WA, Cideciyan, AV, Lewin, AS, Iwabe, S, Khanna, H, Sumaroka, A, Chiodo, VA, Fajardo, DS, Román, AJ, Deng, WT, Swider, M, Alemán, TS, Boye, SL, Genini, S, Swaroop, A, Hauswirth, WW, Jacobson, SG & Aguirre, GD 2012, Gene therapy rescues photoreceptor blindness in dogs and paves the way for treating human X-linked retinitis pigmentosa, viewed 14 March 2012, <http://www.pnas.org/gca?allch=&submit=Go&gca=pnas%3B109%2F6%2F2132>.

[2]          Foundation Fighting Blindness 2012, Success in a Canine Study Moves Gene Therapy for X-linked RP Closer to a Clinical Trial, viewed 14 March 2012, <http://www.blindness.org/index.php?view=article&catid=65%3Aretinitis-pigmentosa&id=3063%3Asuccess-in-a-canine-study-moves-gene-therapy-for-x-linked-rp-closer-to-a-clinical-trial&option=com_content&Itemid=121>.
                Gene Therapy Net 2012, Adeno-Associated Viral Vectors, viewed 17 March 2012, <http://www.genetherapynet.com/viral-vectors/adeno-associated-viruses.html>.
[3]          National Human Genome Research Institute 2010, Learning About Retinitis Pigmentosa, viewed 17 March 2012, <http://www.genome.gov/1351.4348>.

Reece, JB, Meyers, N, Urry, LA, Cain, ML, Wasserman, SA, Minorsky, PV, Jackson, RB & Cooke, BN 2012, Campbell Biology, 9th edn (Australian version), Pearson Australia Group Pty Ltd., NSW.

[4]          Rods and Cones n.d., viewed 17 March 2012,
<http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html>.

[5]          Science Daily 2012, Researchers Develop Gene Therapy That Could Correct a Common Form of Blindness, viewed 14 March 2012, <http://www.sciencedaily.com/releases/2012/01/120123163412.htm>.
Twyman, R 2002, Gene Therapy, viewed 17 March 2012, <http://genome.wellcome.ac.uk/doc_WTD020911.html>.


[6]          University of Illinios 2011, Retinitis Pigmentosa, viewed 17 March 2012, <http://www.uic.edu/com/eye/LearningAboutVision/EyeFacts/RetinitisPigmentosa.shtml>.

Images
Genetic Engineering n.d., viewed 17 March 2012,
< http://www.daviddarling.info/childrens_encyclopedia/Genetic_Engineering_Chapter3.html>.
Hise, L 2012, Man’s best friend could be best hope for XLRP cure, viewed 18 March 2012, <http://news.medill.northwestern.edu/chicago/news.aspx?id=199212>.
St Luke’s Cataract & Laser Institute 2010, Retinitis Pigmentosa, viewed 18 March 2012, <http://www.stlukeseye.com/conditions/RetinitisPigmentosa.html>.


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