28 September 2007. For many neurodegenerative diseases, using RNA interference and antisense approaches to knock down the expression of pathogenic proteins is considered a promising approach (see ARF related news story). But last year, in an alarming development for the blossoming field, researchers from Stanford University showed that one type of RNAi therapy, involving the administration of viral vectors that drive the production of precursors to small interfering RNAs (siRNAs), was often fatal to mice (Grimm et al., 2006). By overwhelming the cellular machinery that produces endogenous microRNAs, the introduced RNAs perturbed gene expression generally, which led to the animals’ death.
However, not all RNAi therapies are created equal. In a response to Grimm et al., researchers from Alnylam Pharmaceuticals in Cambridge, Massachusetts, have produced new data indicating that short synthetic RNAs can selectively silence genes in the livers of mice and hamsters, without the toxicity seen with expression of longer hairpin constructs.
Because synthetic RNAs do not have to be processed to be active, they do not muck up the cellular machinery that produces physiologically important microRNAs. In the paper, David Bumcrot and colleagues provide evidence that the siRNAs do not disrupt processing of endogenous microRNAs or messenger RNA levels, except for the specific genes they target. The work appears in the September 26 Nature.
The results are a positive development for the field, and should help to assuage concerns raised by the dramatic toxicity of shRNA. “The take-home message from our work is that the concerns that were raised by the Stanford researchers, and the toxicity they observed, were a result of the method they used. Our approach using siRNA doesn’t have that concern,” Bumcrot told ARF.
Besides safety, delivery remains a major hurdle separating the promise of siRNA and its use in humans. If they can get to cells, synthetic siRNAs, which are 21 nucleotides in length, can penetrate. Alnylam’s lead product, now in phase 2 clinical trials, is a treatment delivered as an inhaled saline solution for respiratory syncytial virus. The new work uses liposomes, developed by coauthor Daniel Anderson at MIT and injected intravenously, for systemic delivery to liver.
For neurodegenerative diseases, the problem of getting the oligos to neurons or other cells in the CNS adds another level of difficulty. Alnylam is working with Medtronic Corporation to develop an implantable pump delivery system for their discovery-stage therapy for Huntington disease. A different siRNA targeted to pain is intended for intrathecal delivery. Bumcrot said that the company is also working on approaches to deliver siRNAs across the blood-brain barrier from the circulation; that work is at the experimental stage in animals.
Alnylam recently lost a powerful and wealthy backer when it broke its lucrative deal with Merck Pharmaceuticals. At first blush, this might signal concern about how big pharma regards RNAi in general, but Alnylam spokesperson Cynthia Clayton insisted the divorce was initiated by Alnylam after Merck bought their competitor, Sirna. For its part, Merck demonstrated confidence in RNAi by paying a hefty premium of double the going stock price when it acquired Sirna at a final cost of $1.1 billion.—Pat McCaffrey.
John M, Constien R, Akinc A, Goldberg M, Moon YA, Spranger M, Hadwiger P, Soutschek J, Vornlocher HP, Manoharan M, Stoffel M, Langer R, Anderson DG, Horton JD, Koteliansky V, Bumcrot D. Effective RNAi-mediated gene silencing without interruption of the endogenous microRNA pathway. Nature. 2007 Sep 26; [Epub ahead of print] Abstract