Yet for some scientists attending the conference, Temin's work, pushed to its logical extreme, suggested a powerful mechanistic explanation for cancer, and thus a well-defined path toward a cure. Sol Spiegelman, a Columbia University virologist known for his incendiary enthusiasm and relentless energy, heard Temin's talk and instantly built a monumental theory out of it--a theory so fiercely logical that Spiegelman could almost conjure it into reality. Temin had suggested that an RNA virus could enter a cell, make a DNA copy of its genes, and attach itself to a cell's genome. Spiegelman was convinced that this process, through a yet unknown mechanism, could activate a viral gene. That activated viral gene must induce the infected cell to proliferate--unleashing pathological mitosis, cancer.
It was a tantalizingly attractive explanation. Rous's viral theory of the origin of cancer would fuse with Boveri's internal genetic theory. The virus, Temin had shown, could become an endogenous element attached to a cell's genes, and thus both an internal aberration and an exogenous infection would be responsible for cancer. "Spiegelman's conversion to the new religion [of cancer viruses] took only minutes," Robert Weinberg, the MIT cancer biologist recalled. "The next day [after Temin's conference] he was back in his lab at Columbia University in New York City, setting up a repeat of the work."
Spiegelman raced off to prove that retroviruses caused human cancers. "It became his single-minded preoccupation," Weinberg recalled. The obsession bore fruit quickly. For Spiegelman's schema to work, he would need to prove that human cancers had retrovirus genes hidden inside them. Working fast and hard, Spiegelman found traces of retroviruses in human leukemia, in breast cancer, lymphomas, sarcomas, brain tumors, melanomas--in nearly every human cancer that he examined. The Special Virus Cancer Program, launched in the 1950s to hunt for human cancer viruses, and moribund for two decades, was swiftly resuscitated: here, at long last, were the thousands of cancer viruses that it had so long waited to discover. Money poured into Spiegelman's lab from the SVCP's coffers. It was a perfect folie a deux--endless funds fueling limitless enthusiasm and vice versa. The more Spiegelman looked for retroviruses in cancer cells, the more he found, and the more funds were sent his way.
In the end, though, Spiegelman's effort turned out to be systematically flawed. In his frenzied hunt for human cancer retroviruses, Spiegelman had pushed the virus-detection test so hard that he saw viruses or traces of viruses that did not exist. When other labs around the nation tried to replicate the work in the mid-1970s, Spiegelman's viruses were nowhere to be found. Only one human cancer, it turned out, was caused by a human retrovirus--a rare leukemia endemic in some parts of the Caribbean. "The hoped-for human virus slipped quietly away into the night," Weinberg wrote. "The hundreds of millions of dollars spent by the SVCP . . . could not make it happen. The rocket never left its launching pad."
Spiegelman's conjecture about human retroviruses was half-right and half-wrong: he was looking for the right kind of virus but in the wrong kind of cell. Retroviruses would turn out to be the cause of a different disease--not cancer. Spiegelman died in 1983 of pancreatic cancer, having heard of a strange illness erupting among gay men and blood-transfusion recipients in New York and San Francisco. One year after Sol Spiegelman's death in New York, the cause of that disease was finally identified. It was a human retrovirus called HIV.
* Other cancer-causing viruses, such as SV40 and human papillomavirus (HPV), would eventually be discovered in 1960 and 1983, respectively.
* Temin's statement was speculative, but it bore his unerring biological instinct. Formal proof of the structural attachment of RSV genes into the cellular genome would only come years later.
* The term retrovirus was coined later by virologists.
"The hunting of the sarc"
For the Snark was a Boojum, you see.
--Lewis Carroll
Sol Spiegelman had got hopelessly lost hunting for cancer-causing retroviruses in humans. His predicament was symptomatic: cancer biology, the NCI, and the targeted Special Virus Cancer Program had all banked so ardently on the existence of human cancer retroviruses in the early 1970s that when the viruses failed to materialize, it was as if some essential part of their identity or imagination had been amputated. If human cancer retroviruses did not exist, then human cancers must be caused by some other mysterious mechanism. The pendulum, having swung sharply toward an infectious viral cause of cancer, swung just as sharply away.
Temin, too, had dismissed retroviruses as the causal agents for human cancer by the mid-1970s. His discovery of reverse transcription had certainly overturned the dogma of cellular biology, but it had not pushed the understanding of human carcinogenesis far. Viral genes could attach themselves to cellular genes, Temin knew, but this could not explain how viruses caused cancer.
Faced with yet another discrepancy between theory and data, Temin proposed another bold conjecture--again, standing on the thinnest foundation of evidence. Spiegelman and the retrovirus hunters, Temin argued, had conflated analogy with fact, confused messenger with message. Rous sarcoma virus could cause cancer by inserting a viral gene into cells. This proved that genetic alterations could cause cancer. But the genetic alteration, Temin proposed, need not originate in a virus. The virus had merely brought a message into a cell. To understand the genesis of cancer, it was that culprit message--not the messenger--that needed to be identified. Cancer virus hunters needed to return to their lamplit virus again, but this time with new questions: What was the viral gene that had unleashed pathological mitosis in cells? And how was that gene related to an internal mutation in the cell?
In the 1970s, several laboratories began to home in on that gene. Fortuitously, RSV possesses only four genes in its genome. In California, by then the hotbed of cancer virus research, the virologists Steve Martin, Peter Vogt, and Peter Duesberg made mutants of the Rous virus that replicated normally, but could no longer create tumors--suggesting that the tumor-causing gene had been disrupted. By analyzing the genes altered in these mutant viruses, these groups finally pinpointed RSV's cancer-causing ability to a single gene in the virus. The gene was called src (pronounced "sarc"), a diminutive of sarcoma.
Src, then, was the answer to Temin's puzzle, the cancer-causing "message" borne by Rous sarcoma virus. Vogt and Duesberg removed or inactivated src from the virus and demonstrated that the src-less virus could neither induce cell proliferation nor cause transformation. Src, they speculated, was some sort of malformed gene acquired by RSV during its evolution and introduced into normal cells. It was termed an oncogene,* a gene capable of causing cancer.
A chance discovery in Ray Erikson's laboratory at the University of Colorado further elucidated src's function. Erikson had been a graduate student in Madison in the early 1960s when Temin had found retroviruses. Erikson had followed the discovery of the src gene in California and had been haunted by the function of src ever since. In 1977, working with Mark Collett and Joan Brugge, Erikson set out to decipher the function of src. Src, Erikson discovered, was an unusual gene. It encoded a protein whose most prominent function was to modify other proteins by attaching a small chemical, a phosphate group, to these proteins--in essence, playing an elaborate game of molecular tag.+ Indeed, scientists had found a number of similar proteins in normal cells--enzymes that attached phosphate groups to other proteins. These enzymes were called the "kinases," and they were soon found to behave as molecular master switches within a cell. The attachment of the phosphate group to a protein acted like an "on" switch--activating the protein's function. Often, a kinase turned "on" another kinase, which turned "on" another kinase, and so forth. The signal was amplified at each step of the chain reaction, until many such molecular switches were thrown into their "on" positions. The confluence of many such activated switches produced a powerful internal signal to a cell to change its "state"--moving, for instance, from a nondividing to a dividing state.
Src was a prototypical kinase--although a kinase on hyperdrive. The protein made by the viral src gene
was so potent and hyperactive that it phosphorylated anything and everything around it, including many crucial proteins in the cell. Src worked by unleashing an indiscriminate volley of phosphorylation--throwing "on" dozens of molecular switches. In src's case, the activated series of proteins eventually impinged on proteins that controlled cell division. Src thus forcibly induced a cell to change its state from nondividing to dividing, ultimately inducing accelerated mitosis, the hallmark of cancer.
By the late 1970s, the combined efforts of biochemists and tumor virologists had produced a relatively simple view of src's ability to transform cells. Rous sarcoma virus caused cancer in chickens by introducing into cells a gene, src, that encoded a hyperactive overexuberant kinase. This kinase turned "on" a cascade of cellular signals to divide relentlessly. All of this represented beautiful, careful, meticulously crafted work. But with no human cancer retroviruses in the study, none of this research seemed relevant immediately to human cancers.
Yet the indefatigable Temin still felt that viral src would solve the mystery of human cancers. In Temin's mind, there was one riddle yet to be solved: the evolutionary origin of the src gene. How might a virus have "acquired" a gene with such potent, disturbing qualities? Was src a viral kinase gone berserk? Or was it a kinase that the virus had constructed out of bits of other genes like a cobbled-together bomb? Evolution, Temin knew, could build new genes out of old genes. But where had Rous sarcoma virus found the necessary components of a gene to make a chicken cell cancerous?
At the University of California in San Francisco (UCSF), in a building perched high on one of the city's hills, a virologist named J. Michael Bishop became preoccupied with the evolutionary origin of viral src. Born in rural Pennsylvania, where his father had been a Lutheran minister, Bishop had studied history at Gettysburg College, then drastically altered his trajectory to attend Harvard Medical School. After a residency at Massachusetts General Hospital, he had trained as a virologist. In the 1960s, Bishop had moved to UCSF to set up a lab to explore viruses.
UCSF was then a little-known, backwater medical school. Bishop's shared office occupied a sliver of space at the edge of the building, a room so cramped and narrow that his office-mate had to stand up to let him through to his desk. In the summer of 1969, when a lanky, self-assured researcher from the NIH, Harold Varmus, then on a hiking trip in California, knocked on Bishop's office door to ask if he might join the lab to study retroviruses, there was hardly any standing room at all.
Varmus had come to California seeking adventure. A former graduate student in literature, he had become enthralled by medicine, obtained his M.D. at Columbia University in New York, then learned virology at the NIH. Like Bishop, he was also an academic itinerant--wandering from medieval literature to medicine to virology. Lewis Carroll's Hunting of the Snark tells the story of a motley crew of hunters that launch an agonizing journey to trap a deranged, invisible creature called the Snark. That hunt goes awfully wrong. Unpromisingly, as Varmus and Bishop set off to understand the origins of the src gene in the early 1970s, other scientists nicknamed the project "the hunting of the sarc."
Varmus and Bishop launched their hunt using a simple technique--a method invented, in part, by Sol Spiegelman in the 1960s. Their goal was to find cellular genes that were distantly similar to the viral src gene--and thus find src's evolutionary precursors. DNA molecules typically exist as paired, complementary strands, like yin and yang, that are "stuck" together by powerful molecular forces. Each strand, if separated, can thus stick to another strand that is complementary in structure. If one molecule of DNA is tagged with radioactivity, it will seek out its complementary molecule in a mixture and stick to it, thereby imparting radioactivity to the second molecule. The sticking ability can be measured by the amount of radioactivity.
In the mid-1970s, Bishop and Varmus began to use the viral src gene to hunt for its homologues, using this "sticking" reaction. Src was a viral gene, and they expected to find only fragments or pieces of src in normal cells--ancestors and distant relatives of the cancer-causing src gene. But the hunt soon took a mystifying turn. When Varmus and Bishop looked in normal cells, they did not find a genetic third or fifth cousin of src. They found a nearly identical version of viral src lodged firmly in the normal cell's genome.
Varmus and Bishop, working with Deborah Spector and Dominique Stehelin, probed more cells, and again the src gene appeared in them: in duck cells, quail cells, and geese cells. Closely related homologues of the src gene were strewn all over the bird kingdom; each time Varmus's team looked up or down an evolutionary branch, they found some variant of src staring back. Soon, the UCSF group was racing through multiple species to look for homologues of src. They found src in the cells of pheasants, turkeys, mice, rabbits, and fish. Cells from a newborn emu at the Sacramento zoo had src. So did sheep and cows. Most important, so did human cells. "Src," Varmus wrote in a letter in 1976, ". . . is everywhere."
But the src gene that existed in normal cells was not identical to the viral src. When Hidesaburo Hanafusa, a Japanese virologist at Rockefeller University in New York, compared the viral src gene to the normal cellular src gene, he found a crucial difference in the genetic code between the two forms of src. Viral src carried mutations that dramatically affected its function. Viral src protein, as Erikson had found in Colorado, was a disturbed, hyperactive kinase that relentlessly tagged proteins with phosphate groups and thus provided a perpetually blaring "on" signal for cell division. Cellular src protein possessed the same kinase activity, but it was far less hyperactive; in contrast to viral src, it was tightly regulated--turned "on" and turned "off"--during cell division. The viral src protein, in contrast, was a permanently activated switch--"an automaton," as Erikson described it--that had turned the cell into a dividing machine. Viral src--the cancer-causing gene--was cellular src on overdrive.
A theory began to convulse out of these results, a theory so magnificent and powerful that it would explain decades of disparate observations in a single swoop: perhaps src, the precursor to the cancer-causing gene, was endogenous to the cell. Perhaps viral src had evolved out of cellular src. Retrovirologists had long believed that the virus had introduced an activated src into normal cells to transform them into malignant cells. But the src gene had not originated in the virus. It had originated from a precursor gene that existed in a cell--in all cells. Cancer biology's decades-long hunt had started with a chicken and ended, metaphorically, in the egg--in a progenitor gene present in all human cells.
Rous's sarcoma virus, then, was the product of an incredible evolutionary accident. Retroviruses, Temin had shown, shuttle constantly out of the cell's genome: RNA to DNA to RNA. During this cycling, they can pick up pieces of the cell's genes and carry them, like barnacles, from one cell to another. Rous's sarcoma virus had likely picked up an activated src gene from a cancer cell and carried it in the viral genome, creating more cancer. The virus, in effect, was no more than an accidental courier for a gene that had originated in a cancer cell--a parasite parasitized by cancer. Rous had been wrong--but spectacularly wrong. Viruses did cause cancer, but they did so, typically, by tampering with genes that originate in cells.
Science is often described as an iterative and cumulative process, a puzzle solved piece by piece, with each piece contributing a few hazy pixels of a much larger picture. But the arrival of a truly powerful new theory in science often feels far from iterative. Rather than explain one observation or phenomenon in a single, pixelated step, an entire field of observations suddenly seems to crystallize into a perfect whole. The effect is almost like watching a puzzle solve itself.
Varmus and Bishop's experiments had precisely such a crystallizing, zippering effect on cancer genetics. The crucial implication of the Varmus and Bishop experiment was that a precursor of a cancer-causing gene--the "proto-oncogene," as Bishop and Varmus called it--was a normal cellular gene. Mutations induced by chemicals or X-rays caused cancer not by "inserting" foreign genes into cells, but by activa
ting such endogenous proto-oncogenes.
"Nature," Rous wrote in 1966, "sometimes seems possessed of a sardonic humor." And the final lesson of Rous sarcoma virus had been its most sardonic by far. For nearly six decades, the Rous virus had seduced biologists--Spiegelman most sadly among them--down a false path. Yet the false path had ultimately circled back to the right destination--from viral src toward cellular src and to the notion of internal proto-oncogenes sitting omnipresently in the normal cell's genome.
In Lewis Carroll's poem, when the hunters finally capture the deceptive Snark, it reveals itself not to be a foreign beast, but one of the human hunters sent to trap it. And so it had turned out with cancer. Cancer genes came from within the human genome. Indeed the Greeks had been peculiarly prescient yet again in their use of the term oncos. Cancer was intrinsically "loaded" in our genome, awaiting activation. We were destined to carry this fatal burden in our genes--our own genetic "oncos."
Varmus and Bishop were awarded the Nobel Prize for their discovery of the cellular origin of retroviral oncogenes in 1989. At the banquet in Stockholm, Varmus, recalling his former life as a student of literature, read lines from the epic poem Beowulf, recapitulating the slaying of the dragon in that story: "We have not slain our enemy, the cancer cell, or figuratively torn the limbs from his body," Varmus said. "In our adventures, we have only seen our monster more clearly and described his scales and fangs in new ways--ways that reveal a cancer cell to be, like Grendel, a distorted version of our normal selves."
Siddhartha Mukherjee - The Emperor of All Maladies: A Biography of Cancer Page 43
