Box 1. Synthetic miRNAs, a new attack on disease

They don't call them targets for nothing, and target screening and identification is the bait that small startups use to raise money. The fact is most drug therapies seek to inhibit some pathway to bring a cascade of events to a standstill in order to change a disease phenotype. But with miRNAs clinicians in some cases might be able to go the other way by introducing an miRNA molecule (or mimicking one) into a system instead of shooting it down.

"Of course, we're thinking about synthetic microRNAs," says Dalia Cohen, head of R&D at Rosetta Genomics (Rehovot, Israel). "There are several tumor-suppressor miRs that have been identified, and increasing their expression should have a benefit through inhibition of cancer cell proliferation."

Some very interesting situations have come to light. For example, miR-15 and miR-16 are underexpressed in CLL. And now the miRNA LET7, which is underexpressed in certain situations, such as benign mesenchymal tumors and lung cancers, has been found to be a classic tumor suppressor through its degradation of the HMGA2 oncogene mRNA. Recently researchers Yong Sun Lee and Anindya Dutta, both of the University of Virginia in Charlottesville, found that expression of LET7 diminished both HMGA2 levels and lung cancer cell growth⁶. And in 2004 Takashi Takahashi of the Aichi Cancer Center Research Institute in Nagoya, Japan showed that human lung cancer patients with lower than normal LET7 expression survived for a significantly shorter time (n = 143; P = 0.003) in⁷.

Most researchers who consider the possibilities of adding miRNAs to living systems are thinking in terms of creating molecules that preserve the desired activity but add new dimensions, such as better stability, greater potency or both. Because of its long experience and expertise in RNA chemistry, Isis Pharmaceuticals sounds like a natural entry into this race. Spokesman William Craumer says that although the company is "involved in synthetic RNA research," it's too early to discuss results. And Alnylam Pharmaceuticals cofounder and molecular biologist Phillip Zamore of the University of Massachusetts Medical School in Worcester would say only that he thinks about synthetic miRNAs "all the time."

Medical oncologist and translational investigator David Chang and cancer cell biologist Constantin Ioannides, both of the University of Texas M.D. Anderson Cancer Center (Houston), have an idea they want to advance. They propose to synthesize miRNAs they believe will be more powerful than naturally occurring forms. In a first, Chang, Ioannides and colleagues have designed several miRNAs targeting glioma-associated antigen-1 (Gli-1) mRNA, which is necessary for the progressive development and metastasis of NSCLCs, ovarian and pancreatic cancers, which are defiantly resistant to cytotoxic and radiation therapies, as well as RNAi with siRNAs⁸. "We searched but did not find strong binding endogenous microRNAs," says Chang. "So instead of finding which of those [weak binding miRNAs] were missing and trying to reconstruct them, we designed new ones." He adds, "We wanted to use cancer cell lines with a very aggressive phenotype."

A number of miRNAs, single stranded and double stranded, were created to hybridize with the GLI1 mRNA. Chang says mitotic and proliferative activity of in vitro pancreatic and ovarian cancer cells were "significantly inhibited" by the synthetic miRNAs. "The double strand is really stronger," he says, "but the single strand also inhibited very well." Chang says his molecules are made from two complementary single-stranded miRNAs. "I guess our designed double-stranded microRNA can be considered siRNA," he says. "It depends on how you want to name it." The bottom line, explains Chang, is that one strand will be the active agent and the other will add stability.

No venture capitalist or drug developer has beaten a path to their door yet, but Chang and his team have taken their invention to the university's IP department to apply for a patent. Their next step is an animal study, which is planned for this year using a xenograft murine model—research they hope will ultimately lead to human clinical trials for pancreatic cancer.