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64 Protection of Photoreceptors in a Mouse Model of RP10

 

563

ratio of 1:5 (i.e. 1 μl of

rAAV mutant IMPDH1 at 1.4

×

1012vp/ml: 2 μl of

10

12

 

 

rAAV shImp1 at 3.3 ×

 

vp/ml) in eight WT mice, and control eyes were

injected with non-targeting shRNAs. Four weeks post-injection, all eyes receiving a simultaneous delivery of mutant IMPDH1 and shImp1, as compared to control eyes (Fig. 64.2b), showed significant protection of photoreceptor structure (Fig. 64.2c). Furthermore, immunostaining with preand postsynaptic markers such as synaptophysin (Fig. 64.3b) and bassoon (Fig. 64.3c) suggested intact synaptic integrity along the outer plexiform layer in rescued retinas as compared to controls (Fig. 64.3a).

64.3 Discussion

For autosomal dominant retinitis pigmentosa, the ability to attain long-term and stable suppression of mutant protein is essential to achieve therapeutic benefits. In this study, we used rAAV2/5 vectors as the delivery vehicle for shRNA in vivo. The choice for this AAV serotype combination was established by the fact that IMPDH1 expression was shown to be predominantly localised in murine outer segments (Aherne et al. 2004; Bowne et al. 2006), and other studies have demonstrated the effectiveness of this rAAV combination in transducing photoreceptor cells in mouse retinas (Auricchio et al. 2001; O’Reilly et al. 2007). Histological analysis carried out in this study clearly demonstrated the persistence of rAAV expression in the targeted retinal layers four weeks post-injection, and also showed effective knockdown of IMPDH1 at both the mRNA and protein levels. Previous preclinical studies in the canine and primate retina have even reported rAAV-mediated expression of transgenes for up to two years and beyond (Narfstrom et al. 2003; Stieger et al. 2008). Another important observation made in this study was that rAAV-mediated expression of human mutant IMPDH1, but not WT IMPDH1 in WT mouse retinas, induced rapid photoreceptor degeneration within four weeks of injection. Although the underlying pathological mechanism of this effect has not yet been fully deciphered, Aherne et al. (2004) have shown that mutant IMPDH1 has a high tendency to form protein aggregates that may have a negative pathological effect in photoreceptor cells. This hypothesis is supported by observations in other retinal diseases caused by protein aggregation, such as the Pro23His mutation in rhodopsin that causes adRP (Illing et al. 2002), and the R14W mutation in carbonic anhydrase IV that causes the RP17 form of RP (Rebello et al. 2004). Regardless of the diseasecausing mechanism, simultaneous delivery of rAAV human mutant IMPDH1 with rAAV shRNA targeting IMPDH1 by subretinal injection in WT mice provided significant protection of photoreceptor structure, as compared to controls. Furthermore, immunostaining with pre-synaptic markers such as bassoon, which labels photoreceptor ribbons in both cone pedicles and rod spherules, and synaptophysin which labels pre-synaptic elements of cone pedicles and rod spherules, illustrated the protection of synaptic connectivity between neuronal sensory cell layers in treated eyes, as compared to control eyes. These observations provided proof-of-principle of the

564

L.C.S. Tam et al.

potential for rAAV-mediated downregulation of mutant IMPDH1 in vivo as a therapeutic strategy to combat RP10. Nonetheless, a number of issues regarding the safety of this approach, such as toxic effects, if any, as regards long-term rAAV delivery of shRNA in vivo, will need to be addressed. Furthermore it remains a considerable challenge to translate the results from murine models to large animals, and ultimately to human patients.

Acknowledgments This work was supported by grants from Science Foundation Ireland (G20026); The Health Research Board of Ireland (PRO262001); European Union-RETNET (MRTN-CT-2003-504003); European Union-EviGenoRET (LSHG-CT-2005-512036); The British RP Society and Fighting Blindness Ireland. The authors thank Dr. Sara Bowne (University of Texas, Houston) for providing the IMPDH1 antibody, and Dr. Arpad Palfi (Trinity College Dublin, Ireland) for providing the EGFP Dual-Expression plasmid vector.

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