Effects of Diminished Editing at the AmberW Site

The requirement of L-HDAg for production of HDV particles is clear (Chang et al. 1991; Ryu et al. 1992). Not surprisingly, inhibition of editing, either by site-directed mutagenesis (Casey 2002; Jayan and Casey 2005) or by siRNA-

mediated knockdown of ADAR1 expression (Jayan and Casey 2002b), inhibited viral particle production in transfected cells. Early studies on the effect of L-HDAg indicated that L-HDAg expression strongly inhibited HDV RNA replication (Chao et al. 1990; Glenn and White 1991). However, editing site mutations that prevent L-HDAg production do not result in increased levels of HDV RNA replication, at least for genotype I constructs (Macnaughton and Lai 2002; Sato et al. 2004; Wu et al. 1995). Partly based on this result, ithas been suggested that L-HDAg might not actually inhibit HDV RNA accumulation, at least under normal circumstances in Huh-7 cells (Macnaughton and Lai 2002). However, the results discussed in Sect. 4.1 indicate that inhibition of replication does occur when L-HDAg is overproduced by excessive editing during the course of HDV RNA replication. In contrast to the observations with genotype I, genotype III RNA replication is increased at least fivefold by mutations that abolish editing (Casey 2002). One interpretation of these results is that, at least in Huh-7 cells, maximum levels of HDV RNA replication are limited by factors other than L-HDAg production; thus, decreased L-HDAg production does not increase replication. However, when L-HDAg is overproduced, replication becomes sensitive. Genotype III RNA replication may either be less sensitive to these as yet undefined limitations, or more sensitive to L-HDAg.

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