A safer approach to RNA therapy

HD Lighthouse Contributing Editor's Comment: A safe, effective RNA interference therapy could be a virtual cure for Huntington's Disease. Success with the HD mice has led to much interest in the HD community. (see HDL: Davidson)

There are a number of technical difficulties to work out, including how to silence the HD gene while a normal huntingtin's gene remains functional. Delivery is another problem, getting RNAi to the entire area of the brain which is affected by the disease.

Safety of course is paramount. It is important that any treatment, including pharmacological, be specific, addressing the disease target without affecting other structures and functions in a toxic way. Last year, Stanford University researchers (Grimm 2006) reported that short hairpin RNA interference can result in the cell's microRNA pathways being overwhelmed. In trials of 49 different adenoviral vector delivered short hairpin RNAs directed against a variety of targets in the livers of mice, 36 resulted in toxicity with death resulting from nearly half of the shRNAs.

Fortunately, there are other ways to achieve RNA interference. Daniel Anderson and colleagues report on a similar rodent study which used a synthetic small interfering RNA instead of the short hairpin method. They achieved safety and efficiency. The cells own microRNA pathways were unaffected and 80 percent of the target genes were silenced.

Reference

Dirk Grimm, et al., "Fatality in mice due to oversaturation of cellular microRNA/short hairpin RNA pathways." Nature 441, 537-541 (25 May 2006)

-- Marsha L. Miller, Ph.D.
Posted to the HDL: 10-01-2007

Effective RNAi-mediated gene silencing without interruption of the endogenous microRNA pathway

M John, R Constein, A Akinc, M Goldberg, YA Moon, M Spranger, P Hadwiger, J Soutschek, HP Vornlocker, M Manoharan, M Stoffel, R Langer, DG Anderson, HD Horton, V Koteliansky, D Bumcrot

The Press Release

Research could lead to treatments for cancer, many other diseases

CAMBRIDGE, MA -- Researchers from MIT, Alnylam Pharmaceuticals and other institutions have demonstrated the safety of a promising type of genetic therapy that could lead to treatments for a wide range of diseases such as cancer.

The work, which will be published in the Sept. 27 issue of Nature, describes a new approach to conducting the therapy. A paper in Nature last year reported that another commonly used approach caused fatalities in mice.

The research focuses on RNA interference, or RNAi, a key part of the body's genetic machinery. RNAi works by using short strands of RNA to block the expression of specific genes.

“RNAi has huge potential as a therapeutic agent,” said Daniel Anderson, a research associate at MIT's Center for Cancer Research and one of the authors of the new paper.

However, a paper published in Nature last year by a different team showed that large doses of one type of RNA used for RNAi, short hairpin RNA, disrupted another key RNA pathway, the microRNA pathway, and caused the mice in the study to die. That result worried some RNAi researchers, said Anderson.

“That first paper demonstrated that short hairpin RNA could lead to mouse fatality,” he said. “Researchers were concerned that a second type of RNA, small interfering RNA (siRNA), would induce the same toxicity.”

In the current study, the researchers demonstrated that siRNA did not have the same toxic effects as large doses of shRNA because it does not interfere with the microRNA pathway. Further, they achieved 80 percent silencing of target genes in mice and hamster liver cells.

“Using chemically synthesized siRNA, you can deliver sufficient siRNA to achieve therapeutically valuable gene silencing, without interfering with the cell's endogenous microRNA,” said David Bumcrot, a director of research at Alnylam (an MIT startup) and one of the authors of the paper.

The research team used a new RNA delivery system developed at MIT, the details of which will be published in another upcoming paper, to perform the RNA interference.

In many RNAi studies, including the one that the MIT/Alnylam team was following up on, researchers use retroviruses to deliver genes that code for short hairpin RNA, which is a precursor to siRNA. Once the gene is incorporated into the cell's DNA, short hairpin RNA is synthesized and transported from the cell nucleus to the cytoplasm for further processing.

The earlier study showed that large amounts of short hairpin RNA blocked the cell's ability to export microRNA, which uses the same export pathway. Without normally functioning microRNA, the mice died. Low doses of short hairpin RNA were not toxic, but the dosage is difficult to control because once the shRNA gene is incorporated into the DNA of the host cells, it is expressed for long periods of time, said Bumcrot.

In the current MIT/Alnylam study, siRNA was delivered directly to the cell cytoplasm, so it did not compete with the export of microRNA.

“We wanted to demonstrate that if you go downstream of that (export) step in the pathway, you don't get interference with the microRNA pathway,” said Bumcrot. “With synthetic siRNAs, we deliver a defined dose and we know how long the effect lasts. If toxicity issues arise, dosing can be stopped at any time. It's much easier to control and, therefore, safer.”

Other MIT authors on the paper are Institute Professor Robert Langer and Michael Goldberg, a graduate student in chemistry. Researchers from the University of Texas Southwestern Medical Center and the Swiss Federal Institute of Technology are also authors on the paper.

The work at MIT was funded by the National Institutes of Health.

The abstract

Systemic administration of synthetic small interfering RNAs (siRNAs) effectively silences hepatocyte gene expression in rodents and primates. Whether or not in vivo gene silencing by synthetic siRNA can disrupt the endogenous microRNA (miRNA) pathway remains to be addressed. Here we show that effective target-gene silencing in the mouse and hamster liver can be achieved by systemic administration of synthetic siRNA without any demonstrable effect on miRNA levels or activity. Indeed, siRNA targeting two hepatocyte-specific genes (apolipoprotein B and factor VII) that achieved efficient ( approximately 80%) silencing of messenger RNA transcripts and a third irrelevant siRNA control were administered to mice without significant changes in the levels of three hepatocyte-expressed miRNAs (miR-122, miR-16 and let-7a) or an effect on miRNA activity. Moreover, multiple administrations of an siRNA targeting the hepatocyte-expressed gene Scap in hamsters achieved long-term mRNA silencing without significant changes in miR-122 levels. This study advances the use of siRNAs as safe and effective tools to silence gene transcripts in animal studies, and supports the continued advancement of RNA interference therapeutics using synthetic siRNA.

Source: Nature 2007 Sep 26; [Epub ahead of print]