The Emperor of All Maladies

by Siddhartha Mukherjee

  By the mid-1940s, three decades after biologists had coined the word, the molecular nature of the gene had come into focus. Functionally, a gene was a unit of inheritance that carried a biological trait from one cell to another or from one generation to the next. Physically, genes were carried within the cell in the form of chromosomes. Chemically, genes were composed of DNA, deoxyribonucleic acid.

  But a gene only carries information. The functional, physical, and chemical understanding of the gene begged a mechanistic understanding: How did genetic information become manifest inside the cell? What did a gene “do”—and how?

  George Beadle, Thomas Morgan’s student, switched from Morgan’s fruit flies to an even more primitive organism, the slime mold, to answer these questions. Collaborating with the biochemist Edward Tatum at Stanford University in California, Beadle discovered that genes carried instructions to build proteins—complex, multidimensional macromolecules that were the workhorses of the cell.

  Proteins, researchers found in the 1940s, carry out the bulk of cellular functions. They form enzymes, catalysts that speed up biochemical reactions vital to the life of the cell. Proteins are receptors for other proteins or molecules, responsible for transmitting signals from one cell to the next. They can create structural components of the cell, such as the molecular scaffolding that allows a cell to exist in a particular configuration in space. They can regulate other proteins, thus creating minuscule circuits inside the cell responsible for coordinating the life cycle of the cell.

  Beadle and Tatum found that a gene “works” by providing the blueprint to build a protein. A protein is a gene realized—the machine built from a gene’s instructions. But proteins are not created directly out of genes. In the late 1950s, Jacques Monod and François Jacob, working in Paris, Sydney Brenner and Matthew Meselson at Caltech, and Francis Crick in Cambridge, discovered that the genesis of proteins from genes requires an intermediary step—a molecule called ribonucleic acid, or RNA.

  RNA is the working copy of the genetic blueprint. It is through RNA that a gene is translated into a protein. This intermediary RNA copy of a gene is called a gene’s “message.” Genetic information is transmitted from a cell to its progeny through a series of discrete and coordinated steps. First, genes, located in chromosomes, are duplicated when a cell divides and are transmitted into progeny cells. Next, a gene, in the form of DNA, is converted into its RNA copy. Finally, this RNA message is translated into a protein. The protein, the ultimate product of genetic information, carries out the function encoded by the gene.

  An example, borrowed from Mendel and Morgan, helps illustrate the process of cellular information transfer. Red-eyed flies have glowering, ruby-colored eyes because they possess a gene that bears the information to build a red pigment protein. A copy of this gene is created every time a cell divides and it thus moves from a fly to its egg cells, and then into the cells of the offspring fly. In the eye cells of the progeny fly, this gene is “deciphered”—i.e., converted into an intermediate RNA message. The RNA message, in turn, instructs the eye cells to build the red pigment protein, thus giving rise to red-eyed flies of the next generation. Any interruption in this information flow might disrupt the transmission of the red eye trait—producing flies with colorless eyes.

  This unidirectional flow of genetic information—DNA → RNA → protein—was found to be universal in living organisms, from bacteria to slime molds to fruit flies to humans. In the mid-1950s, biologists termed this the “central dogma” of molecular biology.

  An incandescent century of biological discovery—spanning from Mendel’s discovery of genes in 1860 to Monod’s identification of the RNA copy of genes in the late 1950s—illuminated the inner workings of a normal cell. But it did little to illuminate the workings of a cancer cell or the cause of cancer—except in two tantalizing instances.

  The first came from human studies. Nineteenth-century physicians had noted that some forms of cancer, such as breast and ovarian cancer, tended to run in families. This in itself could not prove a hereditary cause: families share not just genes, but also habits, viruses, foods, exposures to chemicals, and neurotic behaviors—all factors, at some time or another, implicated as causes of cancer. But occasionally, a family history was so striking that a hereditary cause (and, by extension, a genetic cause) could not be ignored. In 1872, Hilário de Gouvêa, a Brazilian ophthalmologist practicing in Rio, treated a young boy with a rare cancer of the eye called a retinoblastoma by removing the eye surgically. The boy had survived, grown up, and married a woman with no family history of cancer. The couple had several children, and two of the daughters developed their father’s retinoblastoma in both eyes—and died. De Gouvêa reported this case as a puzzling enigma. He did not possess the language of genetics, but to later observers, the case suggested an inherited factor that “lived” in genes and caused cancer. But such cases were so rare that it was hard to test this hypothesis experimentally, and de Gouvêa’s report was largely ignored.

  The second time scientists circled around the cause of cancer—almost hitting the nerve spot of carcinogenesis—came several decades after the strange Brazilian case. In the 1910s, Thomas Hunt Morgan, the fruit fly geneticist at Columbia, noticed that mutant flies occasionally appeared within his flock of flies. In biology, mutants are defined as organisms that differ from the normal. Morgan noticed that an enormous flock of flies with normal wings might occasionally give birth to a “monster” with rough or scalloped wings. These mutations, Morgan discovered, were the results of alterations in genes and the mutations could be carried from one generation to the next.

  But what caused mutations? In 1928, Hermann Joseph Muller, one of Morgan’s students, discovered that X-rays could vastly increase the rate of mutation in fruit flies. At Columbia, Morgan had produced mutant flies spontaneously. (When DNA is copied during cell division, a copying error occasionally generates an accidental change in genes, thus causing mutations.) Muller found that he could accelerate the incidence of these accidents. Using X-rays to bombard flies, he found that he could produce hundreds of mutant flies over a few months—more than Morgan and his colleagues had produced using their vast breeding program over nearly two decades.

  The link between X-rays and mutations nearly led Morgan and Muller to the brink of a crucial realization about cancer. Radiation was known to cause cancer. (Recall Marie Curie’s leukemia, and the tongue cancers of the radium-watch makers.) Since X-rays also caused mutations in fruit fly genes, could cancer be a disease of mutations? And since mutations were changes in genes, could genetic alterations be the “unitary cause” of cancer?

  Had Muller and Morgan, student and mentor, pitched their formidable scientific skills together, they might have answered this question and uncovered this essential link between mutations and malignancy. But once close colleagues, they became pitted and embittered rivals. Cantankerous and rigid with old age, Morgan refused to give Muller full recognition for his theory of mutagenesis, which he regarded as a largely derivative observation. Muller, in turn, was sensitive and paranoid; he felt that Morgan had stolen his ideas and taken an undue share of credit. In 1932, having moved his lab to Texas, Muller walked into the nearby woods and swallowed a roll of sleeping pills in an attempted suicide. He survived, but haunted by anxiety and depression, his scientific productivity lapsed in his later years.

  Morgan, in turn, remained doggedly pessimistic about the relevance of the fruit fly work in understanding human diseases. In 1933, Morgan received the Nobel Prize in Physiology or Medicine for his far-reaching work on fruit fly genetics. (Muller would receive the Nobel Prize independently in 1946.) But Morgan wrote self-deprecatingly about the medical relevance of his work, “The most important contribution to medicine that genetics has made is, in my opinion, intellectual.” At some point far in the future, he imagined a convergence between medicine and genetics. “Possibly,” he speculated, “the doctor may then want to call in his geneticist friends for consultation!”


  But to oncologists in the 1940s, such a “consultation” seemed far-fetched. The hunt for an internal, genetic cause of cancer had stalled since Boveri. Pathological mitosis was visible in cancerous tissue. But both geneticists and embryologists failed to answer the key question: what caused mitosis to turn so abruptly from such an exquisitely regulated process to chaos?

  More deeply, what had failed was a kind of biological imagination. Boveri’s mind had so acrobatically leapt from sea urchins to carcinomas, or Morgan’s from pea plants to fruit flies, in part because biology itself was leaping from organism to organism, finding systematic cellular blueprints that ran deeply through all the living world. But extending that same blueprint to human diseases had turned out to be a much more challenging task. At Columbia, Morgan had assembled a fair collection of fruit fly monsters, but none that even remotely resembled a real human affliction. The notion that the cancer doctor might call in a “genetic friend” to help understand the pathophysiology of cancer seemed laughable.

  Cancer researchers would return to the language of genes and mutations again in the 1970s. But the journey back to this language—and to the true “unitary” cause of cancer—would take a bewildering detour through the terrain of new biology, and a further fifty years.

  Under the Lamps of Viruses

  Unidentified flying objects, abominable snowmen, the Loch Ness monster and human cancer viruses.

  —Medical World News, 1974,

  on four “mysteries” widely reported

  and publicized but never seen

  The biochemist Arthur Kornberg once joked that the discipline of modern biology in its early days often operated like the man in the proverbial story who is frantically searching for his keys under a streetlamp. When a passerby asks the man whether he lost his keys at that spot, the man says that he actually lost them at home—but he is looking for the keys under the lamp because “the light there is the brightest.”

  In the predawn of modern biology, experiments were so difficult to perform on biological organisms, and the results of manipulations so unpredictable, that scientists were severely constrained in their experimental choices. Experiments were conducted on the simplest model organisms—fruit flies, sea urchins, bacteria, slime molds—because the “light” there was the brightest.

  In cancer biology, Rous’s sarcoma virus represented the only such lamplit spot. Admittedly, it was a rare virus that produced a rare cancer in a species of chicken.* But it was the most reliable way to produce a real cancer in a living organism. Cancer researchers knew that X-rays, soot, cigarette smoke, and asbestos represented vastly more common risk factors for human cancers. They had heard of the odd Brazilian case of a family that seemed to carry retinoblastoma cancer in its genes. But the capacity to manipulate cancer in an experimental environment was unique to the Rous virus, and so it stood center stage, occupying all the limelight.

  The appeal of studying Rous virus was further compounded by the formidable force of Peyton Rous’s personality. Bulldogish, persuasive, and inflexible, Rous had acquired a near paternal attachment to his virus, and he was unwilling to capitulate to any other theory of cause. He acknowledged that epidemiologists had shown that exogenous carcinogens were correlated with cancer (Doll and Hill’s study, published in 1950, had clearly shown that smoking was associated with an increase in lung cancer), but this had not offered any mechanistic explanation of cancer causation. Viruses, Rous felt, were the only answer.

  By the early 1950s, cancer researchers had thus split into three feuding camps. The virologists, led by Rous, claimed that viruses caused cancer, although no such virus had been found in human studies. Epidemiologists, such as Doll and Hill, argued that exogenous chemicals caused cancer, although they could not offer a mechanistic explanation for their theory or results. The third camp, of Theodor Boveri’s successors, stood at the farthest periphery. They possessed weak, circumstantial evidence that genes internal to the cell might cause cancer, but had neither the powerful human data of the epidemiologists nor the exquisite experimental insights of the chicken virologists. Great science emerges out of great contradiction, and here was a gaping rift slicing its way through the center of cancer biology. Was human cancer caused by an infectious agent? Was it caused by an exogenous chemical? Was it caused by an internal gene? How could the three groups of scientists have examined the same elephant and returned with such radically variant opinions about its essential anatomy?

  In 1951, a young virologist named Howard Temin, then a postdoctoral researcher, arrived at the California Institute of Technology in Pasadena, California, to study the genetics of fruit flies. Restless and imaginative, Temin soon grew bored with fruit flies. Switching fields, he chose to study Rous sarcoma virus in Renato Dulbecco’s laboratory. Dulbecco, a suave, exquisitely mannered Calabrian aristocrat, ran his lab at Caltech with a distant and faintly patrician air. Temin was a perfect fit: if Dulbecco wanted distance, Temin wanted independence. Temin found a house in Pasadena with several other young scientists (including John Cairns, the future author of the Scientific American article on the War on Cancer) and spent his time cooking up unusual meals in heavy communal pots and talking volubly about biological riddles late into the night.

  In the laboratory, too, Temin was cooking up an unusual experiment that was virtually guaranteed to fail. Until the late fifties, Rous sarcoma virus had been shown to cause tumors only in live chickens. Temin, working closely with Harry Rubin, wanted to study how the virus converted normal cells into cancer cells. To do this, they needed a vastly simplified system—a system free of chickens and tumors, and analogous to bacteria in a petri dish. And so Temin imagined creating cancer in a petri dish. In 1958, in his seventh year in Dulbecco’s lab, Temin succeeded. He added Rous sarcoma virus to a layer of normal cells in a petri dish. The infection of the cells incited them to grow uncontrollably, forcing them to form tiny distorted heaps containing hundreds of cells that Temin called foci (the plural of focus). The foci, Temin reasoned, represented cancer distilled into its essential, elemental form: cells growing uncontrollably, unstoppably—pathological mitosis. It was the sheer, driving power of Temin’s imagination that allowed him to look at a tiny heap of cells and reimagine that heap as the essence of the diffuse systemic disease that kills humans. But Temin believed that the cell, and its interaction with the virus, had all the biological components necessary to drive the malignant process. The ghost was out of the organism.

  Temin could now use his cancer-in-a-dish to perform experiments that would have been nearly impossible using whole animals. One of his first experiments with this system, performed in 1959, produced an unexpected result. Normally, viruses infect cells, produce more viruses, and infect more cells, but they do not directly affect the genetic makeup, the DNA, of the cell. Influenza virus, for instance, infects lung cells and produces more influenza virus, but it does not leave a permanent fingerprint in our genes; when the virus goes away, our DNA is left untouched. But Rous’s virus behaved differently. Rous sarcoma virus, having infected the cells, had physically attached itself to the cell’s DNA and thereby altered the cell’s genetic makeup, its genome. “The virus, in some structural as well as functional sense, becomes part of the genome of the cell,” Temin wrote.*

  This observation—that a DNA copy of a virus’s genes could structurally attach itself to a cell’s genes—intrigued Temin and Dulbecco. But it raised an even more intriguing conceptual problem. In viruses, genes are sometimes carried in their intermediary RNA form. Certain viruses have dispensed with the original DNA copy of genes and keep their genome in the RNA form, which is directly translated into viral proteins once the virus infects a cell.

  Temin knew from work performed by other researchers that Rous sarcoma virus is one such RNA virus. But if the virus genes started as RNA, then how could a copy of its genes convert into DNA? The central dogma of molecular biology forbade such a transition. Biological information, the dogma proposed, only travels down a one-way street from D
NA to RNA to proteins. How on earth, Temin wondered, could RNA turn around acrobatically and make a DNA copy of itself, driving the wrong way down the one-way street of biological information?

  Temin made a leap of faith; if the data did not fit the dogma, then the dogma—not the data—needed to be changed. He postulated that Rous sarcoma virus carried a special property, a property unprecedented in any other living organism: it could convert RNA back into DNA. In normal cells, the conversion of DNA into RNA is called transcription. The virus (or the infected cell) therefore had to possess the reverse capacity: reverse transcription. “Temin had an inkling, but his proof was so circumstantial—so frail—that he could barely convince anyone,” the virologist Michael Bishop recalled twenty-five years later. “The hypothesis had earned him little but ridicule and grief.”

  At first, Temin could barely even convince himself. He had made a bold proposition, but he needed proof. In 1960, determined to find experimental proof, Temin moved his lab to the McArdle laboratory in Wisconsin. Madison, unlike Caltech, was a frozen, faraway place, isolated both physically and intellectually, but this suited Temin. Standing unknowingly at the edge of a molecular revolution, he wanted silence. On his daily walk along Lakeshore path, often blanketed in dense snow, Temin planned experiments to find evidence for this reverse flow of information.