When Carol W. Greider, Ph.D. (1990 Scholar), a molecular biologist and geneticist at Johns Hopkins University, gives talks, she likes to help the audience understand her subject by finding analogies. As a graduate student, she once found herself presenting a journal article on the cloning of yeast telomeres. To illustrate the unlikely probability of two pieces of DNA coming together, Greider introduced marbles, saying, “Given a bag of marbles, if you reach into the bag ...”
The professor heading the seminar was Elizabeth H. Blackburn, Ph.D., a biochemist/biophysicist at the University of California at San Francisco. She had assigned the paper, which was her own publication, co-written with Jack W. Szostak, Ph.D., a geneticist at Harvard Medical School. Blackburn praised Greider’s presentation and said she was pleased that Greider would be doing a rotation with her soon.
Greider went on to study telomeres— the physical ends of chromosomes that protect DNA molecules after they divide so that the genes they carry remain intact (the ends have been compared to the plastic tips that preserve the ends of shoelaces). While still a graduate student, she made a major advance by identifying the enzyme telomerase, which extends those ends.
Last fall, Blackburn, Szostak and Greider shared the 2006 Albert Lasker Award for Basic Medical Research— the “American Nobel Prize”—for the prediction and discovery of telomerase. Their work “stimulated hundreds of scientists to enter the field in the 1990s,” said Joseph L. Goldstein, M.D., the Nobel laureate who chaired the Lasker jury, at the awards ceremony. “This explosion of research,” he continued, “has led to a new model of how the life span of normal cells is regulated and how this regulation goes astray in cancer.”
Greider herself is one of those leading the way, and she explained her current work at the anniversary meeting. When, she asks in her ongoing research, is it is better that telomeres shorten and when it is better that they lengthen? As she notes, as cells get older, their telomeres shorten, and when they stop functioning properly, the cell dies. But the telomerase enzyme of cancerous cells keeps these killers alive indefinitely—so shutting them off might prove to be a cancer therapy. (She has demonstrated this in mice.)
But other telomeres ought to be maintained—for instance, in blood stem cells, which replenish the blood supply. So, in the case of some diseases of bone-marrow failure, therapy would involve increased telomerase to protect the chromosome ends.
If one were to seek an analogy to describe Greider, her own history provides the best material. As a youngster in northern California, she did handstands on the back of a galloping horse, a sport called “vaulting,” which she performed competitively; and as a teenager, she balanced, arms and legs outstretched, on the shoulders of friends who were standing on a cantering horse. She still competes in triathlons and running races, continuing to dare herself.
As Blackburn recalls, that spirit drew Greider to the search for the telomere enzyme. “For a student to want to get involved in this project was almost unheard of,” she says. “Students want to do safe things, and this was not safe. It could have completely crashed and burned. But Carol had a sense of adventure.”
Greider remembers the arduous process. “The thing about looking for telomerase was that it wasn’t clear how to look,” she says. She tried assay after assay. She used the fresh-water protozoan Tetrahymena—“just a weird organism” found in “pond scum,” she notes, indicating its grubby reputation.
After nine months of looking, her discovery came on Christmas Day, 1984, when she finally saw a repetitive pattern on the gel “that looked like what we thought we should be looking for.” Even then, she stayed balanced: “I didn’t want to kind of get my hopes up. I mean, I tend to say, ‘Okay, we’ll just see.’”
Spring arrived before she completed the control experiments that convinced her telomerase was real, and the results were then published in the journal Cell. One of the reviewers, understanding the difficulty of the purification work, called it a “heroic effort.” Says Greider: “And I thought, ‘Right on!’”
Craig C. Mello, Ph.D. (1995 Scholar), professor of molecular medicine at the University of Massachusetts Medical School, started his talk to the alumni Scholars with a caution. “Scientists can get overconfident that they understand the complexity of life,” he said.
“No matter how the answers seem to be complete”—and he alluded to the work of giants: Charles Darwin, Gregory Mendel, the gene discoverer who has been called the father of genetics, and James Watson and Francis Crick, who deciphered the structure of the DNA molecule—“there’s more to learn.”
His own work is a case in point. For decades, the genome of any organism seemed to center on DNA (deoxyribonucleic acid), which contains the genetic code. RNA (ribonucleic acid) was thought to have less glamorous functions—one as a messenger (mRNA), ferrying information from the nucleus of the cell out to the cytoplasm, where it is translated into protein, the body’s building blocks. That seemed to explain how any life form operated. But there was, as biomedical science found out, more to learn.
Using the roundworm C. elegans, Mello and his science partner Andrew Fire, Ph.D., now of Stanford University, discovered that mRNA from a specific gene could be turned off when new RNA molecules were introduced. The key: The new molecules must be double-stranded (they resemble the helical spiral-staircase structure of the DNA) and carry a genetic code identical to that of the gene to be silenced. When cells see RNA in that doublestranded form, their silencing mechanism is activated. It is called RNAi (i for “interference”). They also found that RNAi could spread from cell to cell and even be inherited.
They published their work in 1998, and, last year, it won them the Nobel Prize in Medicine or Physiology.
Previously, scientists had learned how to block gene expression with single-stranded RNA, but the method was difficult and unreliable. As it happens, several researchers produced results with the silencing process that Mello and Fire discovered, but without understanding what was occurring. In 1990, plant biologists in California, trying to make a purple petunia deeper in color, induced an RNA gene juiced to intensify the color; they produced flowers that were either mottled or entirely white. Recognizing that something spectacular had happened, they published their findings. And geneticists at Cornell University in 1995 succeeded in silencing a gene with the older method but were puzzled when the experimental control, which should have done nothing, also disabled the gene.
Mello and Fire’s discovery, the Nobel Assembly stated, “clarified many confusing and contradictory experimental observations and revealed a natural mechanism for controlling the flow of genetic information. This heralded the start of a new research field.”
In their paper, the scientists also seemed to offer a dare. “The mechanisms underlying RNA interference probably exist for a biological purpose,” they observed. This was not taken as mere commentary. “Great papers give rise to whole fields when they not only report a discovery but also pose a challenge to the scientific community,” wrote Phillip D. Zamore, Ph.D. (2000 Scholar), in a “Leading Edge” essay in the journal Cell last December.
The pull was certainly irresistible to Zamore, a biochemist and molecular pharmacologist at the University of Massachusetts Medical School (where, coincidentally, Mello is located). “I was bit late in discovering the paper, reading it only in March 1999; by May, RNAi was my consuming passion,” he wrote. He was part of a team that, in an in-vitro study of RNA, determined how small RNAs mediate RNA interference (“one of the major discoveries,” says Mello). His lab has also discovered that Dicer, maker of silencer RNA, also makes microRNA, molecules that regulate gene expression.
RNAi has become a consuming passion for many others, too. As of last December, according to the Essential Science Indicators database, there were 8,382 articles on gene silencing in the previous decade—5,468 of them in the past two years—involving 27,892 authors from 3,127 institutions worldwide.
For biomedical research, the technique created a paradigm shift because of its value as a tool in experiments. Scientists can shut off genes one at a time, isolating them for investigation in a matter of weeks rather than months or longer. And they can use plants, flies, fungi, worms, humans—apparently any organism.
How potent and reliable it can be for human therapies remains to be seen, but there is no shortage of effort to find out. In mice, scientists have already reversed heart disease by turning off their bad-cholesterol gene, and they have blocked herpes infections in mice. They are hunting for ways to silence the genes that cause (among other diseases) cancer, macular degeneration, HIV and neurological disorders like Huntingdon’s chorea and Lou Gehrig’s disease. There may be yet further uses. RNAi protects the genome against “jumping genes”—DNA sequences that move around in the genome and can cause damage if they end up in the wrong place. And the Nobel citation mentioned agricultural applications.
“It’s so important that people almost take it for granted already, even though it was discovered fairly recently,” said Thomas R. Cech, Ph.D., who won the Nobel Prize in 1989 for his discoveries about the catalytic properties of RNA.
In addition to RNAi research, Mello’s lab is exploring the mechanisms of cells in differentiating and communicating when the embryo is formed and first begins to develop. His work has shown that a cell’s position in the embryo can determine the type of tissue it will become, and he has identified genes involved in cell fate in C. elegans embryos.
“We have to rethink theories of inheritance and evolution,” he told the Scholars at the anniversary meeting, “because it’s possible that, in this kind of epigenetic regulation, inheritance of RNA plays a big role in controlling the level of gene expression— a very exciting idea.”
Mello’s restlessness recalls an anecdote of him told in the Bulletin of the Howard Hughes Medical Institute. When the Mello family took hikes in the Blue Ridge Mountains, young Craig typically walked faster than the others. “I just want to see what’s past that next ridge,” he would say to them.
And his mother would tell him, “You go look, and then come back and tell us.”
Marshall Ledger is the editor of Trust.