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Editors Selection IGR 19-2
Basic Research: Adeno-associated viral vector transduction
Comment by Nils Loewen on:
25151 Self-complementary AAV virus (scAAV) safe and long-term gene transfer in the trabecular meshwork of living rats and monkeys, Buie LK; Rasmussen CA; Porterfield EC et al., Investigative Ophthalmology and Visual Science, 2010; 51: 236-248
Buie et al.'s (405) work impresses in three ways: it 1) confirms that the lack of adeno-associated viral (AAV) vector transduction of the trabecular meshwork (TM) can be overcome in vivo; 2) provides an unusually comprehensive analysis that includes all steps from vector production to immune response and microarray analysis; this was made possible by 3) extensive inter-institutional and inter-departmental collaborations.
The authors use modified AAV vectors that are 'self-complementary' (scAAV) to bypass the second-strand DNA synthesis block in TM cells and transduce the anterior segment by intracameral bolus injection. Expression is followed with a standard marker (GFP) in rats for 3.5 months and in monkeys for 2.4 years. Expression could be seen throughout the anterior segment and was not limited to the conventional outflow system as, e.g., in non-targeted lentiviral vectors (due to flow and transduction kinetics). Borras et al. had published an in-vitro proof-of-concept with scAAV in TM cells in 2006, but were unable to identify the exact cause of the second strand block in TM cells. ScAAV vectors had been first described by co-author Samulski's group in 2001. AAV vectors have shortcomings: as parvoviridae the packaging capacity is very limited by the 4.8 kilobase (kb) pair size of the genome which is already too small for many transgenes. The scAAV strategy further reduces this to a mere 2.2 kb . While the wild-type integrates preferentially into a specific site on the human chromosome 19 this is lost in AAV vectors. Although long-term expression can be achieved, AAV differs from lenti- and retro-viral vectors that the majority of transgene expression does not occur from genomically integrated proviruses, but rather from large AAV concatomers that form in the cell. As such, cell division typically causes dilutional attrition. Luckily, TM cells do not normally divide and lack of integration suggests absence of mutagenesis. The naturally low immunogenicity and absence of AAV disease is desirable in the eye.
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