Читать книгу The Fevers of Reason - Gerald Weissmann - Страница 12

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Arrowsmith and CRISPR at the Marine Biological Laboratory

As the audience flows out of the auditorium [after] the Friday Evening Lecture, the MBL’s weekly grand occasion when the guest lecturers from around the world turn up to present their most stunning pieces of science, there is the same jubilant descant . . . half shout, half song made by confluent, simultaneously raised human voices explaining things to each other.

—Lewis Thomas, The MBL (1972)

ON ONE OF THOSE GRAND OCCASIONS, Jennifer Doudna of Berkeley presented the latest news of CRISPR to a packed auditorium at the Marine Biological Laboratory. Her audience was the usual mixed crowd one finds at the Friday Evening Lectures in Woods Hole. I spotted working scientists, grad students, lab assistants, undergraduates, a score of academicians, a Nobel laureate or two, attentive families and friends, hailing from all corners of the globe. The dress code ranged from country jeans to khakis, bike gear to saris, backpacks to bow ties.

After the smartphones were turned off and the tablets stowed, Doudna proceeded to hold the audience in thrall for an hour with bulletins from the front lines of the war against error—in the gene.

Doudna began by spelling out the acronym CRISPR, explaining that it stands for “clustered regularly interspaced short palindromic repeats” in the DNA of bacteria. When viruses called bacteriophages infect a bacterium, the CRISPR system filches DNA from the phage virus and inserts it into its own DNA. That genetic memory of the encounter will be passed to its progeny. In 2005, French scientists (Pourcel et al.) studying genes of a bacterium (Yersinia pestis) that caused plague in 1960s Indochina, found remnants of bacteriophage DNA at CRISPR spacers in the plague bacilli. They proposed that the locus “may represent a memory of past genetic aggressions.” With its neighboring cas (CRISPR-associated system) genes, CRISPR works in bacteria like a smallpox vaccination in humans, providing adaptive immunity to a virus.

CRISPR generates a unique set of RNAs that guide Cas proteins directly to the DNA of any future phage aggressor. The neatest of the Cas proteins is Cas9, from a streptococcus; it’s a DNA-cleaving enzyme. But progress with the Cas9 system for gene editing had been hampered because it required two different guide RNAs. Doudna detailed the remarkable contribution she and Emanuelle Charpentier made to the field in 2012, as described in Jinek et al. They engineered single, specific RNAs that could guide the Cas protein to cleave DNA of any species at any given site, permitting normal DNA repair by the cell’s built-in machinery.

By excising an unwanted gene and replacing it with a desired substitute, they’d waved the magic wand of genetic engineering. Doudna went on to show dazzling animations in which twists in RNA, tweaks of DNA, and acrobatics of the Cas proteins accomplished the task. She gave examples of how the method had already been applied to modify the color of mushrooms and to repair the faulty genes in models of muscular dystrophy.

Doudna closed by warning that perhaps “the science is going too fast.” While CRISPR technology can erase crippling misprints in our genes, there remain ethical roadblocks to extending the method to cells of the human germ line. At present, she argued, CRISPR should be kept away from human sperm and eggs until there is a general consensus as to how, when—and if. The audience was clearly in accord and showed their agreement by waves of applause at the end of the talk.

AS THE AUDIENCE MOVED OUT over the steps of the auditorium, one heard—as Thomas put it—the customary descant of “confluent, simultaneously raised human voices explaining things to each other.”

A senior scientist enthused, “Great stuff! No wonder her paper went viral! But, on the other hand . . .”

A course instructor interrupted, “You know, that egg slide of Doudna’s looked like the old MBL physiology experiment: you get sea urchin eggs to divide by changing salts rather than adding sperm.”

A grad student chimed in, “That was Jacques Loeb, the guy who got famous for parthenogenesis!”

“Wasn’t he the old prof with the accent in Arrowsmith?” a jean-clad fan of oldies asked.

That one clicked: “Yeah sure, Loeb was Dr. Gottlieb in Arrowsmith, the book and the movie.” . . . “I think the young doc tried to use phage to cure the plague.” . . . “Great movie, but the wife died.”

As the group broke up and traipsed along to the evening reception, it seemed to me I had heard that song before. Sure enough, guided by the rich archives of the MBL library and my tattered copies of Loeb’s The Mechanistic Conception of Life (1912) and Sinclair Lewis’s Arrowmith (1925), I came to the conclusion that Doudna’s lecture couldn’t have been given at a location more appropriate—nor on a topic more likely to go viral.

The Friday Evening Lectures at the MBL in Wood’s Holl, as it was then called, descend from a series of public lectures that began in 1889, the year after the lab was founded. And over the next decade, Jacques Loeb (1859–1924) gave several of these, published in Biological Lectures at MBL Wood’s Holl. Based on his summer experiments at the MBL on sea urchin sperm and eggs, in 1909 he issued a challenge:

Whoever claims to have succeeded in making living matter from inanimate will have to prove that he has succeeded in producing nuclear material which acts as a ferment for its own synthesis and thus reproduces itself.

Doudna and the other “heroes of CRISPR,” as geneticist Eric Lander called them, met that challenge just over a century later. The CRISPR-Cas method uses “inanimate RNA” and protein to snip out the old “nuclear material” (DNA) to add the new which can “act as ferment” for its own reproduction. Going viral, we might say.

CRISPR-Cas HAS ALSO GONE VIRAL in the scientific literature. According to the website Web of Science, the topic of CRISPR-Cas went from a few dozen citations in 2005 to over ten thousand on the eve of Doudna’s lecture. Her seminal 2012 paper with Charpentier has itself been cited over fifteen hundred times. (To put this into current pop perspective: one notes that one tweet by Kim Kardashian has reproduced itself 137,369 times.)

As for Jacques Loeb, he went as far as one could in the pre-viral days. His papers were cited fifteen hundred times in his lifetime, thrice the number of his contemporary, Paul Ehrlich. He was also a figure in the press: “Loeb Tells of Artificial Life,” proclaimed the Chicago Daily Tribune in 1900. In1907, under the headline “Believes Germ of Life Will Be Discovered,” the San Francisco Call reported that “Professor Jacques Loeb . . . in a bulletin issued today from the office of the president, makes the statement that he believes the germ of life can be discovered, provided the chemical reactions surrounding the process of fertilization, are investigated.”

Loeb’s public persona also went viral in his role as the fictional Dr. Gottlieb, the mentor in Arrowsmith. The novel by Sinclair Lewis won a Pulitzer Prize, and the film was nominated for an Oscar. Lewis described how Gottlieb and Martin Arrowsmith (Ronald Colman on screen) not only discovered bacteriophage, but used it to stem an outbreak of plague (Y. pestis) on a tropical island.

BACTERIOPHAGE AND THE PLAGUE may have been brought to modern attention in 2005 when French scientists found phage-like DNA sequences at the CRISPR locus of plague bacilli, but Martin Arrowsmith got there first! His story was patched together from Paul de Kruif’s memories of work at Rockefeller Institute (now Rockefeller University) and Sinclair Lewis’s progressive politics. De Kruif and Lewis made a joint field trip to the West Indies, which gave Lewis the setting for an epidemic of plague in a colonial context; it also set the stage for an exercise in bioethics.

Plucked from an earnest career as a Midwest clinician, Martin Arrowsmith is taken up by the McGurk (read Rockefeller) Institute, where he stumbles on filterable “Factor X,” which seems to kill bacteria, staph at first. But then, Gottlieb (Loeb) steps in:

“Why have you not planned to propagate it on dead staph? That is most important of all.”

“Why?” Gottlieb flew instantly to the heart of the jungle in which Martin had struggled for many days: “Because that will show whether you are dealing with a living virus.”

Martin continues the work and finds not only that this virus kills several kinds of bacteria but also that he can make batches of the purified material to test in infected animals. He’s ready to publish his work, when Gottlieb brings bad news—in a German accent:

“Vell. Anyvay. D’Hérelle of the Pasteur Institute has just now published in the Comptes Rendus, Académie des Sciences, a report—it is your X Principle, absolute. Only he calls it ‘bacteriophage.’ . . . Bacteriophage, the Frenchman calls it. Too long. Better just call it phage.”

Martin goes on to further basic, laborious studies on phage and, lo and behold, he discovers a century before Doudna’s work that his phage “can cause mutations in bacterial species.” He plugs on but is soon confronted by the director of the McGurk Institute, Dr. Tubbs (read Simon Flexner, the first director of the Rockefeller Institute), who faults Martin for not making his discoveries go viral and makes a suggestion:

“D’Hérelle’s discovery hasn’t aroused the popular interest I thought it would. If he’d only been here with us, I’d have seen to it that he got the proper attention. Practically no newspaper comment at all. . . . I think it may now be time for you to use phage in practical healing. I want you to experiment with phage in pneumonia, plague, perhaps typhoid.”

As expected, Martin gets phage to cure plague in experimental animals—and people come next. There’s plague in the Caribbean, on the quarantined island of “St. Hubert,” a colony beset by rats and shiftless administrators. Arrowsmith heads there with a team of microbe hunters and volunteers, including his faithful wife, Leora (Helen Hayes in the film). They’ll try to stem the epidemic with a batch of Yersinia phage that’s cured plague in rats.

The team faces personal tragedy and ethical conflict. For any new treatment, science demands controls, but can an investigator willfully deprive anyone of lifesaving measures? Does protocol trump empathy? Arrowsmith, book and film, throws a curveball: Martin’s beloved Leora dies from the plague and a depressed Martin stands down from his task. Others prevail, and Yersinia phage is finally given to one and all. Since this is fiction, the effort succeeds and the plague is quenched.

A tear-filled Martin returns to New York, where the McGurk honchos are in bliss: the newspapers have gone viral with news of the phage and the plague. The director congratulates Martin:

“You have done what few other men living could do, both established the value of bacteriophage in plague by tests on a large scale, and saved most of the unfortunate population.”

In Martin Arrowsmith’s fictional experiments with phage, empathy ran ahead of clinical science. In the case of CRISPR-Cas9, bacterial immunity to phage, the opposite may be true. Doudna and others, David Baltimore among them, have argued that since CRISPR can be used to hand-pick the DNA of our gonads, human evolution could be played out at the lab bench. If the science has moved too fast for the ethics, perhaps the time has come to stop and think about Lionel Trilling’s warning:

The apparatus of the researcher’s bench is not nature itself, but an artificial . . . contrivance much like the novelist’s plot, which is devised to foster or force a fact into being.

The plot of CRISPR is before us, but we’re only into the first chapter of this book—and we’re not talking fiction.