She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (73 page)

Strange possibilities have a way these days of becoming real. To make sense of them and to make ethical judgments, we need a deep sense of heredity, of the full scope of what the word means. We have to recognize Mendel's Law as one of many ways that genes move naturally from ancestors to their offspring, something that we are learning to manipulate. We have to recognize that the cells in our bodies have ancestors and descendants of their own—ones that can become mosaics or mingle together in chimeras. We have to loosen the boundaries of what we call heredity, to consider other ways in which today correlates with yesterday—be they molecules other than DNA that slip into future generations, or microbes that hitchhike along as well, or the tools and traditions of human culture, or even the environment into which our children are born. Only then will we have the language to talk about the ways in which we can control heredity for our benefit, and the dangers that we leave to the future.

W
HEN
V
ALEN
TINO
G
ANTZ
first heard about CRISPR, it sounded like a godsend. At the time, he was getting his PhD at the University of California, San Diego, studying genes in
Drosophila
and related flies. He tinkered with their genes and observed whether he could change how their embryos developed. But the best tools he could use were clumsy and crude. In 2013, Gantz heard that researchers had figured out how to use CRISPR to alter a gene in
Drosophila
with easy precision.

“It was one of the things I was waiting for,” Gantz told me when I visited him at his laboratory on a eucalyptus-covered hillside by the Pacific. After hearing the news, he had immediately ordered CRISPR molecules of his own and started trying them out. He had no idea he was about to discover a way to use CRISPR to alter heredity on a species-wide scale.

Gantz decided to try out CRISPR by altering a
Drosophila
gene called
yellow.
It was, in a sense, an inherited choice. The
yellow
gene was discovered just over a century earlier in the lab of none other than Thomas Hunt Morgan. One day in 1911, Morgan's team of students were inspecting their grayish flies when they spotted a single golden insect. They bred the fly and determined that it had a recessive mutation in a gene they dubbed
yellow.

The
yellow
gene proved useful to Morgan's team, because they could see with the naked eye which allele of the gene any given fly carried. Morgan's
students bred a line of
yellow
flies, and when they became professors years later, they taught their own students how to breed the flies for experiments. Those students started their own careers, and took the
yellow
flies with them. Over the twentieth century, each new generation inherited this knowledge, just as earlier generations had learned how to make stone tools and how to plant barley. There's even a website, called
FlyTree—The Academic Genealogy of Drosophila Genetics, that chronicles this particular line of cultural inheritance.

The entire tree begins, of course, with Thomas Hunt Morgan. It branches down to his many graduate students. Their ranks included Max Delbrück, a physicist who turned to the mysteries of life, traveling from Germany to Caltech in 1937 to study under Morgan. Delbrück went on to become a professor at Caltech himself, and among his own graduate students he trained another physicist turned biologist, Lily Y. Jan. Jan got a job at the University of California, San Diego. In the 1980s, Ethan Bier joined her lab as a postdoctoral researcher. There he bred countless yellow flies, and became a professor at San Diego as well. Two decades later, Valentino Gantz arrived in Bier's lab and learned the
yellow
craft. He became Morgan's scientific great-great-grandchild.

To try out CRISPR, Gantz fashioned an RNA guide to alter the
yellow
gene in
Drosophila
embryos, introducing a mutation to make them golden. He added the molecules to the fly cells, let them develop into adults, and bred them together, hoping to produce flies with two CRISPR-altered copies of the
yellow
gene. To his delight, among the grayish flies, there were some golden ones. The technology had worked just as advertised. “I was sold,” Gantz said.

Gantz then began playing around with the CRISPR molecules to see if he could use them on another species of fly he was studying for his PhD, called
Megaselia scalaris.
Unlike
Drosophila melanogaster
, it has a distinctively hunched thorax, which has earned it the common name of the humpbacked fly. It behaves differently, too, for which it has earned other names. It's known as the scuttle fly for the way it runs along the ground in bursts. And it's sometimes called the coffin fly for the way its maggots dig
deep into the ground in search of food, sometimes traveling all the way down into buried caskets.

Gantz tailored the
Drosophila
CRISPR molecules so that they would target the
yellow
gene in humpbacked flies. But when he used them on the flies, the experiment failed. “It was very frustrating that we weren't recovering any mutants,” said Gantz.

Gantz wasn't sure what went wrong. It was still possible that he had managed to edit one copy of the
yellow
gene in a few flies. But the CRISPR molecules were succeeding so rarely that Gantz could never bring together two flies that both carried an altered copy of the
yellow
gene.

When Gantz told Bier the news, his advisor was disappointed but not surprised. Failure is common in science. Bier headed off on a much-needed vacation in Italy and didn't give the disappointment any more thought.

But as soon as Bier stepped back into his office in San Diego, Gantz bounded in. He immediately started describing an idea for how to get CRISPR to work. The idea was simple. Gantz would get the flies to edit their own DNA.

First, Gantz would use CRISPR molecules to chop out a gene, which he would replace with a segment of DNA. That segment would include not only an altered gene but genes for CRISPR molecules, too. The fly's own cells would then make CRISPR molecules that would seek out the matching chromosome and edit the second copy of the gene. Gantz would then have humpbacked flies with two mutated copies of the same gene. He could then use them to start a line of mutants. Gantz dubbed the process a “mutagenic chain reaction.”

The idea seemed far-fetched to Bier. He doubted it would work reliably enough to deliver the genetic changes Gantz hoped for. But if it did work, Bier could tell, it might become a powerful tool for studying genetics not just in humpbacked flies but in many other species as well. And as Bier and Gantz talked about the mutagenic chain reaction more, it occurred to them that it might be more powerful than Gantz had initially thought.

“We realized, ‘Whoa, this could go through the germ line,'” Bier told me.

If Gantz mated a CRISPR-carrying fly to an ordinary fly, it could break
Mendel's Law. It would pass down one of its chromosomes that carried its genes for CRISPR, along with the gene that the CRISPR molecules were designed to copy. In the embryos of the second-generation flies, the CRISPR molecules would rewrite its partner chromosome. The result would be that the second generation would not be hybrids, each with a single copy of the CRISPR genes. They would all carry two copies. And the result would be the same when they bred with ordinary flies, too.

“Imagine that a blond person married a dark-haired person, and all their kids were blond,” Bier told me. “All their grandkids were blond. All their great-grandkids were blond, and that went on forever. Imagine something like that.”

Bier told Gantz to hold off on testing the mutagenic chain reaction for a while. He wanted Gantz to reflect on the risks and benefits first. In his mandatory reflection, it occurred to Gantz that if a single altered animal got into the wild—either intentionally released or accidentally allowed to escape—it could mate with other members of its species. Its CRISPR genes could drive themselves further into a population with every new generation.

Under the right conditions, that might be a good thing. Instead of targeting the
yellow
gene in
Drosophila
, scientists could target genes in insects that destroy crops or spread disease. Other researchers had been searching for years for gene drives that might fight pests, and Gantz may have found one at last. But if an animal slipped out of a lab while Gantz was still doing basic experiments on the mutagenic chain reaction, he had no idea what sort of unplanned changes he might unleash.

Gantz devised a way to safely test the mutagenic chain reaction. He would attempt to edit the
yellow
gene in
Drosophila
. But he would prevent the flies from sneaking out of his lab by working in a secure room, housing the insects in shatterproof plastic vials sealed with tight plugs, which would go inside larger tubes, which would be sealed in turn inside plastic boxes.

Gantz and Bier invited a group of senior geneticists at the university to hear them out. They described their concept of the mutagenic chain reaction, and their plan for a safe experiment. They wanted to know if it sounded crazy to someone else.

“‘Yeah, do it'” was the consensus, Bier recalled. “‘Be careful, but do it.'”

In October 2014, Gantz used CRISPR to modify some
Drosophila
larvae. If the molecules worked as he hoped, they would replace one copy of the
yellow
gene with a different stretch of DNA. That new stretch carried an altered version of the gene, along with genes for CRISPR molecules. Gantz hoped that the fly's own cells would then use its genes to alter its other copy of
yellow
. But the only way to know for sure if the procedure worked would be to breed the flies.

Gantz let the flies mature and then bred them with ordinary mates. The female flies laid their eggs, which grew into larvae and then built cases called pupae around themselves. Inside the pupae, they matured into adults. And when the mature flies broke out at last, some of them—both males and females—were golden.

“This was the first indication that things were going the right way,” Gantz recalled.

But now came the real test of the mutagenic chain reaction: Could it carry over to the next generation? Gantz picked out golden females, which carried the altered
yellow
gene and its CRISPR package on both of their X chromosomes. He put them in tubes with ordinary male flies and waited for them to mate. In a conventional experiment with golden females and gray males, the results would reliably follow Mendel's Law. Their sons would all be golden, because they always inherited their one X chromosome from their mother. Their daughters, on the other hand, would inherit an X chromosome from each parent. And since
yellow
is recessive, the daughters would all be gray.

If the mutagenic chain reaction worked, Gantz would see a very different result. The next generation of flies would also make CRISPR molecules. The molecules would alter the second X chromosome in the daughters, and they would turn out golden instead.

Once the flies laid their eggs, Gantz and Bier could only wait for them to mature into adults. “For the whole two weeks I drove my wife crazy, saying ‘
yellow-yellow-yellow
' and knocking on wood everywhere,” said Bier. His colleagues prepared him for failure. “Don't get too excited,” one of
Bier's colleagues told him. “I'll bet you almost everything that in the next generation it'll just be Mendelian.”

Gantz knew that when the eggs of
yellow
mutants hatch, the larvae sometimes take on a faint golden cast. He squinted through his boxes and tubes, hoping to glimpse it on some of the maggots. But as best as he could tell, none of them looked yellow at all.

“I told everyone, ‘Okay, this is not working,'” said Gantz. “I'm a very pessimistic person.”

Just to be thorough, though, Gantz let the larvae pupate and unfurl their wings. He gassed his newly adult flies with carbon dioxide to put them to sleep. Then he sat down at a lab bench and dumped them out onto a pad. He would inspect their bodies before declaring the experiment an official failure.

But when Gantz looked down at the pad, all he saw was gold. The females had converted their own genes as he had hoped. When he stepped into Bier's office to deliver the news, Bier screamed and jumped in the air. Bier had bred thousands of
yellow
mutants, watching Mendel's Law work every time. Now suddenly the rules had changed.

“You walk into a room, you're normally used to walking on the floor, and all of a sudden you're walking on the ceiling,” said Bier. “I mean, that's how weird it was to me.”

One of the first people Bier called about the experiment was a biologist named Anthony James. “Holy mackerel,” James replied.

He had been searching for twenty years for what Bier and Gantz had just found. In the 1980s, James had set out to fight diseases carried by mosquitoes, such as malaria and dengue fever. He would wage his personal war by studying mosquito genes. James set up an insectarium not far from Bier and Gantz, at the University of California campus in Irvine. There he raised mosquitoes on warm blood and experimented on their DNA.

James started by mapping their genes, since the mosquito genome at the time was terra incognita. As James and his colleagues pinned down the
location of individual genes, he would be able to experiment with them. Perhaps there were genes that controlled exactly which pathogens can survive inside mosquitoes and use the insects to get to a new victim. Over 200 million people each year develop malaria because they're bitten by mosquitoes that carry the single-celled parasite
Plasmodium.
But no one has ever gotten the flu from a mosquito bite. James wondered if there might be genetic variants in certain mosquitoes that made them resistant to malaria.

“If we could just figure out how to get those genes out there in the populations at high enough frequencies, then—you know, game over,” James told me when I visited him in Irvine.

James and his colleagues succeeded in mapping some mosquito genes. But the work was so slow—thanks in part to the challenge of raising blood-sucking mosquitoes—that he began to despair of ever finding a way to fight mosquito-borne diseases. He thought about how he could use what he had learned, to fight them in a different way.

“I thought, ‘Well, we'll just
make
genes,'” James said.

He had an idea of the right gene to make. In the late 1960s, Ruth Nussenzweig, a biologist at New York University, discovered that mice don't get sick with human malaria. She found that the immune cells in the mice produce an antibody that clamps onto the parasite, essentially suffocating it. In later experiments, scientists fed Nussenzweig's antibody to mosquitoes, mixed in their meals of blood. Somehow, the antibody escaped being digested inside the mosquitoes and attacked the parasites inside the insects. After this treatment, the mosquitoes couldn't transmit malaria.