In the roughly 10 years since lin-4 became the first microRNA discovered, hundreds more have been identified in mammals, invertebrates, and plants. And yet some of the most fundamental questions about these potentially therapeutically significant miRNAs remain unanswered. With the help of a three-year grant from the National Science Foundation worth $420,000, Yale researcher Frank Slack aims to help remedy this situation.
Slack says that the project — which was set to begin on Jan. 15 this year and end on Jan. 31, 2007 — has two goals: The first is to “understand how … these microRNAs are expressed only at particular times during development. What are the transcriptional inputs to these microRNAs that turn them on only at a particular time?” The second is to figure out just what these so-called temporal miRNAs are doing.
“My lab’s main interest is in the timing of development, and through that interest we stumbled upon … miRNAs, because two of them turn out to be very important for the timing of C. elegans development,” Slack says. “They come on only at particular times during development — one’s called lin-4 and one’s called let-7, and they’re sort of the founding members of the microRNA family.”
Studying these miRNAs “has become more relevant recently because lin-4 and let-7 have both been discovered to have human and fly and mouse … homologues,” Slack says. “What we’re hoping is that what we discover in C. elegans will also be true in those other organisms.”
He notes that since the miRNA families appear to be regulating “important developmental steps, the timing of when a cell will differentiate, for example, we predict that these RNAs are going to be doing a similar thing in humans.”
If true, that means “these genes are going to turn out to be oncogenes or tumor-suppressor genes, because they’re timing when cells stop dividing,” he says. “There is an increasing number of papers where people have implicated microRNAs in cancer.”
While in the future this kind of miRNA research may yield treatments for cancer — by delivering a specific miRNA to an oncogene, for example — Slack notes that “the delivery process is just not there yet.” It will be a complicated path, he contends: “You’ve heard about the problems of getting siRNAs into animals — it’s even more difficult to get a microRNA in.”
First Things First
The miRNA project has roots in work conducted by the Whitehead Institute’s Dave Bartel, who showed that a number of other miRNAs in C. elegans besides lin-4 and let-7 “also come on at particular times during development,” Slack says. “[Bartel] found about 20 that are very robustly turned on at any one particular time during development.”
While he initially proposed that his lab study all of the microRNAs under the project, “the reviewers thought that might be a little too ambitious, so we cut the number down to just the microRNAs that are in the let-7 and lin-4 families.” He notes that there are four miRNAs in C. elegans that are “very similar to let-7, and there are two that are similar to lin-4.”
According to the grant’s abstract, Slack and his colleagues will determine the expression pattern of the lin-4 and let-7 family miRNAs by developmental Northern blot and GFP fusion technology. At the same time, the abstract states, the project will also genetically test the developmental role of the miRNAs by examining the phenotype resulting from overexpression of let-7 and lin-4 homologs in a wild-type background.
“It’s not exceedingly novel, but what we discover in terms of targets might be novel,” Slack says.
An expanded version of this story appeared in the January 9, 2004, edition of RNAi News.
Doug Macron is the editor of RNAi News, a weekly newsletter from GenomeWeb at www. rnainews.com. He can be reached at dmacron @genomeweb.com.