"The most exciting part of the study was finding one siRNA that clearly is the top candidate for a clinical trial, where the patient population is predicted to be sufficiently large that it merits the development of a drug you could take into trial," Zamore said.
"That takes away the biggest worry we had, which was that the number of siRNAs we would have to test in order to have impact on the disease would be too large, and as a consequence the FDA would never approve any trial," he continued.
By adding an siRNA against one of two other common SNPs, Zamore says gene silencing could be effective in 75 percent of patients with Huntington's disease in the U.S. and Europe. Although the group found other SNPs, targeting more of them failed to increase the number of patients who could be helped, he said.
The next problem became developing siRNAs that could discriminate between target mRNAs and non-target. "That turned out to be frustratingly difficult," Zamore said. In tests of human cells, the siRNA sometimes sliced up the disease RNA, as it should. But sometimes it destroyed the normal Huntingtin RNA as well. To prevent this error, Zamore and his colleagues changed one more nucleotide base on the siRNA. Now, the silencing RNA was different from the healthy mRNA by two nucleotides, making it less likely to grab the good RNA.
Further research in mice will examine the efficacy of the siRNA tool. "The siRNA has to be sufficiently stable, and has to get into the right cells, and has to discriminate between the two (genes). It's incredibly expensive work," he said.
Zamore acknowledges that even with this progress, they're a long way from a treatment for Huntington's. "T
|Contact: Jennifer Michalowski|
Howard Hughes Medical Institute