Download DMD Reviews 85 - Action Duchenne

Research Review No. 85.
Exciting new Developments with Viral Vectors.
A recent review.
Introduction:
It has been known for some time that Adeno-associated viruses (AAV) can
be used as a means to transport genetic material into cell, i.e. act as genetic
vectors. In the three recent studies, which I am discussing today these vectors
are being developed further with promising results for the Duchenne community.
As the authors (2) of one of the studies being discussed state in their
introduction: “Wild-type AAV is nonpathogenic and recombinant AAV vector is
weakly immunogenic Further, AAV is the only gene transfer vector that can
effectively transduce all striated muscles in the body in small and large
mammals”. Nevertheless, despite all these advantages, they note that there is a
major hurdle for the creation of AAV-vector based therapy for the treatment of
Duchenne Muscular Dystrophy (DMD). This is the great size of the Dystrophin
gene, whose coding sequence is about 11.2kb (kilobases), while the packaging
capacity of the AAV is only 5kb. While there have been some studies trying to
reduce the size of the Dystrophin to create a so-called mini-dystrophin, which will
be able to carry out some of the functions of the a full-length gene, this Holy Grail
of gene therapy with regard to replacing the complete functional gene may well
be a significant stage nearer.
Until full length dystrophin can be produced, we should stil look at the
possibility of treating DMD with increasing (up-regulating) the production of
Utrophin, as a substitute for dystrophin. As this study (3) also uses AAV-vectors,
I thought it useful to add it to the review.
The Studies:
A study from the U.K., reported earlier this year (1), on similar
investigations. They based their study on the fact that recombinant AAV (rAAV)
are very attractive potential vectors for DMD gene therapy. They based their
studies on the principle of trans-splicing or recombination based overlapping of
AAV vectors. The basis of the method is that two independent AAV vectors each
of which carries parts of a gene to be incorporated are supplied with appropriate
splice signals or with overlapping sequence elements such that they link in only
the desired way like are independently packaged in separate AAV vectors. Single
appropriate cells are simultaneously infected and thereby one full-length
messenger RNA (mRNA) is produced by head-to-tail intermolecular
recombination between two independent genomes. This is transcribed in the coinfected cells. However, the problem with DMD is that due to the size of the
dystrophin gene, three parts have to be successfully incorporated, hence the
phrase ‘Triple Trans-Splicing’ has been coined for this procedure.
In their paper (1), which fully described the details of the complex
technology, the authors demonstrate the principle that ‘triple trans-splicing
vectors have the potential to overcome the size limitation of AAV vector vehicles
for the delivery of large genes’. They note that no other data on this concept have
been published and they note that they are currently aiming to optimize the
design of these triple trans-splicing vectors further so that the ‘splicing between
the three vectors is more efficient and expression of the dystrophin protein is
enhanced’.
In the study, from U.S.A., which has just been published (2) their results
suggest that triple trans-splicing AAV vectors can between them expand the total
carrying capacity of the AAV vectors to 15 kb, which is more than is essential for
the transcription of the dysptophin gene. They consider this as a significant
advance in human gene therapy. In their final sentence of their summary they
state ‘Further optimization of the trivector strategy may expand the utility of AAV
for human gene therapy’, which is a positive sign for the future treatment of DMD.
The study (3), from Italy, the authors developed an artificial transcription
factor, which goes under the name: “Jazz”. It up-regulates both the human and
the mouse utrophin promoter. They had observed a significant recovery of
muscle strength in dystrophic Jazz-transgenic mdx mice, a well-known model for
human DMD. In their study, they demonstrate the efficacy of an experimental
gene therapy based on the systemic delivery of Jazz gene in mdx mice by AAV
and they consider that this combination of the techniques “can open a new
avenue to obtain a therapeutic strategy for treatment of DMD.
References:
1.
Koo, T., Popplewell, L., Athanasopoulos, T. & Dickson, G. (2014) Triple
Trans-Splicing Adeno-Associated Virus Vectors Capable of Transferring the
Coding Sequence for Full-Length Dystrophin Protein into Dystrophic Mice.
Human Gene Therapy. 25(2):98-108.
2.
Lostal, W., Kodippili, K., Yue, Y.P. & Duan, D.S. (2014) Full-Length
Dystrophin Reconstitution with Adeno-Associated Viral Vectors. Human Gene
Therapy. 25(6):552-562.
3.
Strimpakos, G., Corbi, N., Pisani, C., Di Certo, M.G., Onori, A., Luvisetto,
S., Severini, C., Gabanella, F., Monaco, L., Mattei, E. & Passananti, C. (2014)
Novel Adeno-Associated Viral Vector Delivering the Utrophin Gene Regulator
Jazz Counteracts Dystrophic Pathology in mdx Mice. Journal of Cellular
Physiology. 229(9):1283-1291.
Karl A. Bettelheim
21.7.2014