making muscle wasting history

Research Review No. 47.

Microdystophin in a virus, a canine study.
A recent review.

Introduction:

I discussed the use of viruses as vectors (means of transport) of genes into cells in earlier research reviews No. 3 and No. 9. The current study has taken this approach using Messenger RNA (mRNA) instead of DNA. To explain the differences between RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), it is important to realize that the chromosomes, which carry the inherited genetic information, are made of DNA, while the message that is transcribed from the chromosomal DNA is in the form of RNA. Following removal of the intron RNA, the parts coding for the exonal RNA become the mRNA and are used by the protein synthesizing mechanisms to make the appropriate protein. This process goes on continually in all living cells from bacteria to whales and algae to oak trees.

The Studies:

In this joint study, the authors from Royal Holloway University, near London and Tokyo, Japan (1) used previous studies, which had shown that the AAV (adeno-associated viruses) (see research reviews No. 3 and No. 9) were suitable vectors for the transmission of genetic material into cells. However, there are problem to overcome and as the authors note ‘to overcome the first hurdle’, which is the large size of the dystrophin gene, partially deleted but still highly functional so-called mini-dystrophin genes ‘can be packaged successfully inside AAV vectors, which can transduce skeletal muscle at high efficiency to rescue the dystrophin-deficient phenotype in animal models of DMD’.
The second hurdle to be overcome is that the AAV vectors may elicit an immune response. The authors refer to an earlier study (2) of a clinical trial ‘on the delivery of an AAV2.5 vector encoding a functional microdystrophin transgene to skeletal muscle in six patients with DMD’ in which only a limited transgene expression was noted and the clinical benefit was minimal. Immunity was suggested as one of the problems in that study.
So, based on earlier studies (3), the authors in this study hoped to investigate whether ‘the application of a canine-specific and mRNA sequence optimized microdystrophin could improve gene transfer efficiency, leading to extensive muscle transgene expression, by using relatively low titers of AAV vectors in the CXMDj dog model’. The results of these studies, which include excellent photomicrographs, show for the first time, according to the authors, ‘that the delivery of AAV2/8 carrying an mRNA sequence-optimized canine specific microdystrophin under the control of muscle-specific promoter into the CXMDj muscles can lead to high expression of functional microdystrophin, without the need for an immunomodulation/suppression regimen’. They consider that these results could provide the basis for successful gene therapy clinical trials for Duchenne Muscular Dystrophy (DMD).

References:

1. Koo, T., Okada, T., Athanasopoulos, T., Foster, H., Takeda, S. & Dickson, G. (2011) Long-term functional adeno-associated virus-microdystrophin expression in the dystrophic CXMDj dog. Journal of Gene Medicine. 13(9):497-506.

2. Mendell, J.R., Katherine Campbell, K., Rodino-Klapac, L.-R., Sahenk, Z., Shilling, C., Lewis, S., Bowles, D., Gray, S., Li, C., Galloway, G., Malik, V., Coley, B., Clark, K.R., Li, J., Xiao, X., Samulski, J., McPhee, S.W., Samulski, R.J. & Walker, C.M. (2010) Dystrophin Immunity in Duchenne’s Muscular Dystrophy. New England Journa of Medicine. 363(15):1429-1437.


3. Koo, T., Malerba, A., Athanasopoulos, T., Trollet, C., Boldrin, L., Ferry, A., Popplewell, L., Foster, H., Foster, K. & Dickson, G. (2011) Delivery of AAV2/9-Microdystrophin Genes Incorporating Helix 1 of the Coiled-Coil Motif in the C-Terminal Domain of Dystrophin Improves Muscle Pathology and Restores the Level of α1-Syntrophin and α-Dystrobrevin in Skeletal Muscles of mdx Mice. Human Gene Therapy. 22(11):1379-1388.
Karl A. Bettelheim
12.2.2012