Abstract
Viral vectors are effective tools in gene therapy, but their limited packaging capacity can be restrictive. Larger clinically-relevant vectors are needed. Foamy viruses have the largest genomes among mammalian retroviruses and their vectors have shown potential for gene therapy in preclinical studies. However, the effect of vector genome size on titre has not been determined. We inserted increasing lengths of the dystrophin open reading frame in a foamy virus vector and quantified packaged vector RNA and integrated DNA. For both measures, a semi-logarithmic reduction in titre was observed as genome size increased. Concentrated titres were reduced 100-fold to approximately 106 transducing units per ml when vector genomes harboured a 12 kb insert, approximately twice that reported for lentivirus vectors in a comparable study. This potential was applied by optimising foamy virus vectors carrying the full-length dystrophin open-reading frame for transduction of human muscle derived cells. Full-length dystrophin protein was expressed and transduced cells remained able to form myotubes in vitro. Foamy virus vectors are well-suited for stable delivery of large transgene cassettes and warrant further investigation for development as a therapy for Duchenne or Becker muscular dystrophy.
Highlights
Two gene therapies, Glybera[1] and Strimvelis[2], have been licensed in Europe for the treatment for rare genetic diseases and a number of clinical trials are showing promise for a range of diseases[3,4,5,6]
An ability of foamy virus vectors to efficiently deliver large transgene cassettes could be of high value to gene therapy
The most common and severe type, Duchenne muscular dystrophy (DMD), affects approximately 1 in 3500 male births[20, 21] and is caused by nonsense or frame-shift mutations in the dystrophin gene located on the X chromosome[22]
Summary
Glybera[1] and Strimvelis[2], have been licensed in Europe for the treatment for rare genetic diseases and a number of clinical trials are showing promise for a range of diseases[3,4,5,6]. Viral vectors are often used to exploit the efficient mechanisms they have evolved to deliver and express their genomes. An ability of foamy virus vectors to efficiently deliver large transgene cassettes could be of high value to gene therapy. Gene therapy has the potential to correct or improve disease in DMD and BMD, but the size of the dystrophin ORF makes this challenging since it exceeds the packaging capacities of favoured viral vectors, such as AAV and www.nature.com/scientificreports/. The delivery of the whole dystrophin ORF is a desirable approach, but clinically-relevant vectors able to deliver larger transgene cassettes efficiently are needed to achieve this. As a clinically-relevant model, the potential for FVVs to deliver and express the full-length dystrophin ORF in human skeletal muscle-derived stem cells was evaluated
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