One of the great revolutions in modern science rests on the elongated backside of a grotesque, mutant worm. Inexpensive and easy to manipulate in the lab, Caenorhabditis elegans develops from egg to adult in three days and produces a few hundred offspring three days after that. Virtually all of the worms are hermaphrodites, containing both male and female sex organs and capable of making sperm and eggs, so each creature can fertilize itself. And because the worm is transparent and the adult has only 959 cells, development of every stage from egg to adult can be observed under the microscope and documented with near perfect detail while the worm is alive, an achievement accomplished in the 1970s by Sidney Brenner, a University of Cambridge researcher and legend in the field.
C. elegans has been a favorite in biology labs for years due to its transparency, speedy reproductive cycle, and ability to mutate on cue. Just irradiate it or add chemical mutagens to its petri dish, then wait a few days to see what kind of freak worms appear in the progeny. In the late 1970s and 1980s, “worm talks” (as C. elegans lectures were called) inevitably began with a description of development in the normal worm and segued to whatever mutants the lecturer found intriguing.
The “bag of worms” was one such mutant. This version of C. elegans has the singular misfortune of being unable to lay the eggs that it fertilizes. It gets stuck in the earliest stage of wormy development, making the same larval cells repeatedly while failing to form the organs and body parts needed for later life—including the vulva required to get the eggs out of its body. The result is an oversize larva filled with dozens of offspring, hence its nickname. But it does not remain in that state for long. “The eggs hatch inside the worm, and the larvae consume the mother and crawl away,” Victor Ambros, a biologist at the University of Massachusetts, explains.
Ambros first heard about the bag of worms at a 1979 lecture by Robert Horvitz, who had studied C. elegans in Brenner’s Cambridge lab. At the time, Ambros was finishing up a doctoral degree at MIT, studying with Nobel laureate David Baltimore. A few months later he began a postdoctoral fellowship with Horvitz, who suggested he try to identify the defective gene responsible for the grotesquery that was the bag of worms. What followed in fits and starts over a quarter century has evolved into what Baltimore now describes as “a whole new biology.”
It took Ambros 13 years to identify and sequence the defective gene responsible for generating the bag of worms mutant from a normal C. elegans mother; located on chromosome 2 within the worm’s genome, the mutant gene was named lin-4. As it turned out, the gene coded not for a protein—as all genes were then thought to do—but for a tiny snippet of RNA, the simpler molecular cousin of DNA. The RNA molecule was one-hundredth the size of a gene encoding a typical protein, so small it was hard to imagine its having any function whatsoever, let alone producing a mutant as dramatic as the bag of worms.
Ambros’s work on that bizarre mutant provided one of the first signs that RNA might be much more important than anyone had suspected, but not until 2001 did the full story start to unfold. That is when studies finally convinced scientists that the minuscule RNA snippets they had taken to calling “microRNA” were regulating cellular and genetic processes throughout the human body and were critical factors in the determination of health and disease.
The conventional wisdom of what a gene does and how it does it celebrated its 50th birthday just last year. It was in 1958 that Francis Crick of double-helix fame set down the “central dogma of molecular biology,” which could be summed up in six words: DNA makes RNA; RNA makes proteins.
Genes are encoded in the DNA of our chromosomes. They appear as discrete segments of the 3 billion or so pairs of nucleotides, the “letters” of the genetic code that make up the rungs of the double helix. A fertilized human egg begins life with the DNA in its genome, half from the mother and half from the father. From that an entire human being of some 10 trillion cells is programmed. According to the central dogma, this happens as genes are transcribed into RNA, and RNA into proteins. The proteins in turn are the workhorses of biology, spurring chemical reactions inside cells and controlling the expression, transcription, and replication of the genes themselves.
In this picture, RNA—a single-stranded molecule, in contrast to the twin strands of DNA—was perceived as a secondary player, “sort of a slave molecule, copied from DNA in a pretty uninteresting way,” says Philip Sharp, an MIT biologist and Nobel laureate. This belief never wavered, even when geneticists realized that only about 2 percent of the DNA in human cells actually contains genes that make proteins. Most of the remainder was dismissed as “junk DNA,” a term coined in 1972 by the Japanese geneticist Susumu Ohno to capture the notion that most of our DNA is effectively useless, the remains of ancient viruses or now-defunct genes.
With Ambros’s discovery of microRNA came a startling realization: Part of what was considered mysterious junk DNA (and almost 90 percent of all DNA had been so classified) is actually transcribed by the machinery in our cells into bits of RNA that are fundamental controllers of life. Those microRNA molecules have been linked to heart disease and diabetes; to Alzheimer’s, Parkinson’s, and other neurodegenerative diseases; to longevity (at least in worms); and to the entire spectrum of human cancers, including lung, breast, stomach, prostate, colon, pancreatic, and brain.
“People ask me which diseases these RNAs are going to be involved in,” says Thomas Cech, a biochemist at the University of Colorado at Boulder and a Nobel laureate for his work on RNA’s role as an enzyme. “The answer is 100 percent—100 percent of everything that goes on in the body.” The insight demands a fundamental revision in what we thought we knew about how genes work.
Biologists and geneticists now find themselves wondering how they could have missed such a basic aspect of living organisms for so many years. After all, the technology necessary to find microRNA genes has existed since the 1960s. One answer is that they had no particular reason to look. “We bought the paradigm that proteins were the most subtle molecular players in biological systems,” Baltimore says. “We didn’t have a strong feeling that there was something missing, and usually you need a sense that something’s missing to go out and look for it.” Indeed, when Ambros found the first microRNA, he was not looking for it. He had a problem to solve—finding the gene responsible for the lin-4 bag of worms—and the persistence to keep thinking about it when other researchers would have moved on.
Another explanation is that, as with any remarkable scientific discovery, finding microRNA required just the right combination of talent, circumstance, and luck. Ambros found a perfect collaborator in his wife, Candy Lee, who was a lab technician. As Baltimore describes them (having worked with both), they follow the data rather than the scientific fashions; they are both technically adept in the laboratory; and “they have never been ambitious to the point of its getting in the way of reality.” This is not to say that they lack the drive to do good science, but that “they’re not worrying about the trappings of science,” Baltimore says.
Gary Ruvkun, a molecular biologist at Massachusetts General Hospital, has also worked with Ambros and Lee. “Lots of times when you run into people at Harvard and MIT, they’re sort of clenched,” Ruvkun says. “They say, ‘This is my little thing, and I don’t want anybody else to work on it.’ Victor is ‘Hey, let’s crack this thing together!’ Kind of wide-eyed.”
Ambros and Lee met in an organic chemistry class at MIT in 1972, when Ambros was a sophomore and Lee was a freshman. They started dating a year later and were married in 1976, the year Lee graduated. They had both gone to MIT expecting to study physics, but Ambros switched to biology and genetics because he believed they were a better fit for his intellectual talents. There was a narrative about those fields, he says, “and no math that you have to be really good at.” Soon Lee was pursuing biology as well. Because she would receive more pay as a laboratory employee than as a graduate student, she never pursued an advanced degree, instead taking a succession of technician jobs in academic laboratories (including Baltimore’s) and the biotech industry. Eventually she went to work in Ambros’s lab—“the family business,” she says.