Recombinant AAV vectors are generated by replacing the endogenous viral genome with a therapeutic or marker gene expression cassette. The prototype AAV vector is based on AAV serotype 2 (AAV-2). In the past few years, several new AAV serotypes have been developed. Due to differences in cell transduction pathways, these new serotypes open up more avenues for tailoring AAV gene therapy for specific clinical applications.
AAV serotype 9 (AAV-9) was discovered from human tissues several years ago. Due to its unique serological properties, it is classified as the F clade, which is different from all known AAVs. AAV-9 has been shown to effectively transduce a variety of tissues, including heart, liver, lung, kidney, pancreas, and skeletal muscle. In addition, it has recently been reported that AAV-9 is able to bypass the blood-brain barrier and effectively target cells of the central nervous system. This unique property may help in the development of gene therapies for a range of neurodegenerative diseases.
To identify host factors that may influence other critical steps in adeno-associated virus (AAV) infection, the researchers screened an siRNA library and identified several candidate genes, including PHD finger domain protein 5A (PHF5A), a U2 snRNP-related proteins. Disruption of PHF5A expression selectively enhances transgene expression from AAV by increasing transcript levels and appears to affect steps following second-strand synthesis in a serotype- and cell-type-independent manner. Genetic disruption of U2 snRNP and associated proteins such as SF3B1 and U2AF1 also increases AAV vector expression, suggesting that the U2 snRNP spliceosome complex plays a critical role in this host-mediated restriction. Notably, adenovirus coinfection and U2 snRNP inhibition appear to target common pathways that increase AAV vector expression. Furthermore, pharmacological inhibition of U2 snRNP by the potent SF3B1 inhibitor meayamycin B significantly enhanced transduction of clinically relevant cell types by AAV vectors. Further analysis revealed that the U2 snRNP protein inhibits AAV vector transgene expression by directly recognizing intact AAV capsids. In conclusion, the researchers identified U2 snRNP and related splicing factors, known to be affected during adenovirus infection, as novel host restriction factors that effectively limit AAV transgene expression.
Finally, the researchers tested the ability of meayamycin B to promote AAV transduction in various cell types, which is relevant for gene therapy applications. When primary pancreatic islets were transduced with AAV8 CMV-GFP and treated with 2 nM meayamycin B 3 hours after infection, the number of cells expressing GFP increased in drug-treated mouse islets (Figure 1A). When primary human islets were infected with AAV2 or AAV9 CMV-Luc vectors and treated with 0, 2, 5, or 20 nM meayamycin B 7 hours after infection, the researchers observed a dose-dependent increase in luciferase expression in AAV2- and AAV9-infected cells (Figure 1B). Similarly, meayamycin B treatment increased AAV2 and AAV9 transduction of primary neonatal rat cardiomyocytes as well as porcine hepatocytes (Figures 1C and 1D). These results suggest that meayamycin B can enhance transduction of multiple cell types from different host species by AAV vectors.
Figure 1. Meayamycin B increases AAV vector transduction of clinically relevant cell types. (Schreiber C A, et al., 2015)
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