Potent knockdown of the X RNA of hepatitis B by a novel chimeric siRNA-ribozyme construct and modulation of intracellular target RNA by selectively disabled mutants.

A multitarget approach is needed for effective gene silencing that combines more than one antiviral strategy. With this in mind, we designed a wild-type (wt) and selectively disabled chimeric mutant (mt) constructs that consisted of small hairpin siRNA joined by a short intracellular cleavable linker to a known hammerhead ribozyme, both targeted against the full-length X RNA of hepatitis B. These chimeric RNAs possessed the ability to cleave the target RNA under in vitro conditions and were efficiently processed at the cleavable site. When this wt chimeric RNA construct was introduced into a liver-specific mammalian cell line, HepG2, along with the HBx substrate encoding DNA, very significant (approximately 70%) intracellular downregulation in the levels of target RNA was observed. When the siRNA portion of this chimeric construct was mutated, keeping the ribozyme (Rz) region unchanged, it caused only approximately 25% intracellular reduction. On the contrary, when only the Rz was made catalytically inactive, about 55% reduction in the target RNA was observed. Construct possessing mt Rz and mt siRNA caused only 10% reduction. This wt chimeric construct also resulted in almost complete knockdown of intracellular HBx protein production, and the mt versions were less effective. The intracellular reduction of target RNA with either wt or mt constructs also interfered with the known functions of HBx protein with varying efficiencies. Thus, in this proof of concept study we show that the levels of the target RNA were reduced potently by the wt chimeric siRNA-Rz construct, which could be modulated with mt versions of the same.