Wednesday, 19 September 2012

Gene Therapy for Duchenne Muscular Dystrophy


Duchenne Muscular Dystrophy (DMD) is a recessive X-linked lethal childhood disorder (1/3300 boys) characterised by progressive muscle wasting and degeneration (Eppie & Kornberg, 2008).  It is caused by mutation in the gene within the X chromosome that provides instructions for the formation of the structural dystrophin protein (Figure 1). The dystrophin gene is the largest gene in the human genome, consisting of 75 exons and 14,000bp and this large size makes it prone to recombination events (Van Deutekom & Van Ommen, 2003). In DMD patients, the majority of mutations result in the deletion of exons (typically exons 45-47) causing a premature termination and producing a dysfunctional dystrophin protein (Eppie & Kornberg, 2008).

 
Dystrophin binds to the skeletal muscle membrane, anchoring contractile proteins in the cytoskeleton to those in the fibre membrane and acting to maintain the structure of muscle cells (Van Deutekom & Van Ommen, 2003). Absence of functional dystrophin causes disruption of the cytoskeleton, membrane instability and increased susceptibility to mechanical stress during muscle contraction, manifesting in degeneration and necrosis of muscle fibres.  This progressive muscle loss leads to skeletal deformities and impaired motor skills where patients are confined to a wheelchair by the age of 12 and die by their early twenties due to respiratory or cardiac failure (Gregorevic et al, 2006).


 
   
 

 
In a study by Gregorevic et al (2006), gene therapy using an adeno-associated virus (AAV) vector (which is a nonvirulent, single stranded DNA virus) is currently being explored as curative treatment to DMD. Gene therapy (Figure 2) to correct cellular deficiency of dystrophin involves exploiting the virus lifecycle where viral DNA inserts into the hosts chromosomes, causing the host cell to express proteins coded by the viral DNA (Reece et al, 2012). Due to the large size of the dystrophin gene, prior studies identified certain domains of the dystrophin gene which were non-essential for proper functioning of dystophin so could be shortened or removed (Gregorevic et al, 2006). This enabled the engineering of a micro-dystrophin gene (3, 800 bp) which still produced functional dystrophin, yet was small enough to be packaged into and delivered into the host cell via an AAV vector.


 In the Gregorevic et al study, mico-dystrophins were cloned into recombinant AAV vectors and injected into dystrophic mice. It was found that dystrophic mice treated with the recombinant AAV had increased muscle mass by more than 90%, a considerably extended life-span and an overall reduced DMD phenotype (Figure 3) due to the ability of the micro-dystrophin gene to successfully integrate into the host’s chromosome 19 and the consequent production of dystrophin via transcription and translation (Gregorevic et al, 2006). These findings assert that the administration of AAV vectors carrying the micro-dystrophin gene restore expression of dystrophin, thereby improving muscle function and extending lifespan without serious pathogenic effects in dystrophic mice. Although this evidence suggests that gene therapy has potential benefits for treatment of DMD in humans, gene therapy for DMD is still in the early stages and more studies and assessment of side-effects are required before this technique can be implemented in a clinical setting.

 
      
Reference List

Eppie, Y & Kornberg, A 2008, ‘Duchenne muscular dystrophy’, Neurology India, vol. 3, no. 56, pp. 236-237

Gregorevic, P, Allen, J, Minami, E, Blankinship, M, Haraguchi, M, Meuse, L, Finn, E, Adams, M, Froehner, S, Murry, C & Chamberlain, J 2006, ‘rAAV6-microdystrophin preserves muscle function and extends lifespan in severely dystrophic mice’, Nature Medicine, vol. 12, no. 7, pp 787-789

Reece, JB, Meyers, N, Urry, LA, Cain, ML, Wasserman, SA, Minorsky, PV, Jackson, RB & Cooke BN 2011, Campbell Biology, 9th edn (Australian version), Pearson Education, Australia.

Van Deutekom, J & Van Ommen, G  2003, ‘Advances in Duchenne muscular dystrophy gene therapy’, Nature Reviews Genetics, vol. 4, pp. 774-783

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