She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (50 page)

A trained expert can spot aneuploid cells with a microscope. Finding smaller mutations—such as short deletions, duplications, or single-base changes—has required far more sophisticated technology. In 2017, for example, researchers at the Wellcome Trust Sanger Institute in England sequenced the entire genomes of immune cells they got from 247 women. In each volunteer, the scientists found around 160 somatic mutations, each present in a sizable fraction of her cells.

Because these somatic mutations were so common, the researchers suspected they arose early in development. To test the idea, they sequenced the genomes of cells from other tissues in the women. They could find most of the somatic mutations in a fraction of those other cells, too. Based on their research, the Sanger scientists estimated that an embryo gains two or three new mutations every time its cells double. As those new mutations arise, embryonic cells pass them
all down to their descendants as a mosaic legacy.

Christopher Walsh, a geneticist at Harvard who studies mosaicism in the brain, wondered how extensive mosaicism is in our neurons. To find out, he and his colleagues got hold of tissue samples from three people who underwent brain surgery. From each sample, they isolated around a dozen neurons and then sequenced the genomes of each one. They then looked for somatic mutations that set each neuron apart from other cells in the brain, as well as the rest of the body.

Every neuron, Walsh found, was a mosaic. It carried around 1,500 single-nucleotide variants, a unique genetic signature that set each neuron apart from the cells in other parts of the body. These mutations accumulated gradually, through many generations of dividing neurons. Recent mutations were shared by only a few neurons, while older ones were shared by many.

It occurred to Walsh that he could use the mutations to
reconstruct the cell lineages of the brain—not to watch the lineages grow forward as Conklin had, but to work his way back up their branches like a genealogist, back to the womb.

To make this trip, Walsh and his colleagues studied a seventeen-year-old boy who had died in a car accident. The boy's family donated his body for scientific research. Walsh got hold of frozen pieces of the boy's brain, and his team plucked 136 neurons from the tissue. They then sequenced the
entire genome in each cell. As a point of comparison, they also sequenced DNA from other organs in the boy's body, such as his heart, his liver, and his lungs.

Scanning the trillions of bases they sequenced, the researchers spotted hundreds of somatic mutations in each neuron. Many of the mutations were shared by some of the neurons, but not all of them. Some were found in only a few of the neurons, and some were unique to a single cell. The researchers used this pattern to draw a genealogy of the brain, linking each neuron to its close cousins and its more distant relatives. Walsh and his colleagues found that the cells belonged to five distinct lineages, the cells in each one inheriting the same distinctive mosaic signature.

The shared mutations must have all arisen when the boy was still an embryo, when the neurons in his brain were still multiplying quickly. But Walsh got even deeper insights into the development of the boy's brain when he compared the neurons to cells from his other organs. One lineage of neurons also included cells from the boy's heart. Other lineages included cells from other organs.

Based on these results, Walsh and his colleagues pieced together the biography of the boy's brain. When he was just an embryonic ball, five lineages of cells emerged, each with a distinct set of somatic mutations. Cells from those lineages migrated in different directions, becoming different organs—including the brain.

The cells that joined to become the brain were transformed into neurons. And these new neurons wandered throughout the brain before settling down and dividing a few more times. That's why Walsh and his colleagues could find neurons belonging to different lineages sitting near each other. The boy's brain ended up divided into millions of patches of tiny cellular cousins.

Mosaics were once the stuff of superstitions, of freak shows. Then they gained recognition as diseases, both rare and common. Now we can see them everywhere. A single genome can no longer define us, because our inner heredity toys with DNA, altering just about every piece of genetic material we inherit. Even in our skulls, we grow a witches'-broom.

I
N 1779
, John Hunter, the British anatomist, sent a letter to the Royal Society. He wanted to describe to them a peculiar sort of cow. If a mother gives birth to twins of the opposite sex, Hunter wrote, “
the bull-calf becomes a very proper bull.” The cow-calf, however, turns out very improperly. “They are known not to breed: they do not even show the least inclination for the bull, nor does the bull ever take the least notice of them,” Hunter explained.

“This cow-calf is called in this country a
free martin
,” he wrote, “and this singularity is just as well known among the farmers as either cow or bull.”

By 1779, freemartins already had a long history. The Romans had called them
taura.
Farmers knew that a freemartin couldn't provide them money by producing calves or milk. But that didn't mean it was worthless. A freemartin could work almost as hard as an ox, and it brought a good price for its meat. “
The flesh of a fatted free martin will fetch a halfpenny a pound more than any cow beef,” according to the 1776 book
A Treatise on Cattle.

A few years before Hunter became famous for dissecting Charles Byrne, the Irish Giant, he had studied freemartins. When he autopsied a freemartin calf, it looked to him like a normal female cow. But when Hunter got the opportunity to inspect a freshly slaughtered adult freemartin, he saw that it had undergone a bizarre change. On the outside, it still resembled an
ordinary female cow. But it now lacked ovaries. In their place, the freemartin grew what looked to Hunter like testicles. He concluded that freemartins were “unnatural hermaphrodites.”

Later generations of anatomists didn't know what to make of freemartins either. Some argued that they developed from the same fertilized eggs as their brothers. Others thought freemartins and their brothers were fraternal twins, growing from two eggs instead of one. Some experts argued the freemartin was a cow that became bullish, or a bull that became cowish.

The true nature of freemartins was far stranger than they could imagine, but it would not emerge until the twentieth century. The discovery would ultimately challenge how we trace heredity from children to parents.

The first step toward deciphering freemartins came in the early 1900s, when a University of Chicago embryologist named
Frank Lillie started dissecting cow fetuses supplied to him from the Union Stock Yards a few miles away. As part of his research, Lillie examined fraternal calf twins and discovered a strange feature of their development. The calves grew from two fertilized eggs that implanted themselves at different spots on the wall of their mother's uterus. Each then developed its own placenta, which pushed finger-like growths into their mother's blood vessels. But Lillie also noticed that some of the placental blood vessels linked the calves together. Blood could flow from the mother into one calf, then out into its placenta, and then into the other calf. When Lillie injected ink into the umbilical cord of one twin, the placentas of both calves ended up dark.

In 1916, Lillie speculated that these hidden networks of vessels were responsible for freemartins. A fetal bull produced male hormones. If its fraternal twin was female, she could receive those hormones through their joined placentas. The chemicals would then bathe her sex organs and masculinize them. “Nature has performed an experiment of surpassing interest,” Lillie concluded.

Lillie was right to see blood vessels as part of the solution to the freemartin mystery. But it was not hormones that transformed the female calves. The freemartins actually inherited cells from their brothers, which
took hold in their bodies and grew, making them a combination of two different animals in one.

This insight would have to wait for another three decades. It would strike another midwestern biologist, by the name of Ray David Owen. He would look again at freemartins, and he would realize freemartins are cellular mergers.

Cows were Owen's life. His father came to the United States from Wales on a cattle boat transporting purebred Guernseys, and in Wisconsin he established a dairy farm. Owen grew up working long hours on the farm, witnessing the births and deaths of cows on a regular basis. School was an afterthought. Owen went to a two-room schoolhouse with a pair of teachers for eight grades. To pass the time while the older children did recitations, he would practice sewing.

When Owen began traveling to the nearest town for high school, his teachers generally assumed that he would go back to his family's farm afterward and tend to his cows. Only his English teacher, Miss Grubb, recognized his potential for something else. When she suggested he take French, Owen's vocational agriculture teacher snapped, “What the hell do you expect him to do, swear at the cows in French?”

A full scholarship from a small college nearby allowed Owen to continue his education, although he still came home from classes each day to do his farm chores. His family expected he'd become a schoolteacher. But as his graduation from college approached, Owen decided to become a biologist instead.

He went to the University of Wisconsin, where he would fill bushel baskets with chicken heads so that he could examine their irises. He inseminated naked pigeons to trace the genes that robbed them of their feathers. He studied how the germ cells of birds burrowed their way into the depths of embryos to find their proper anatomical place. This work ensured that Owen never forgot that development is more than just the multiplication of cells; it is a time of migrations as well.

After getting his PhD, Owen started working in 1941 for a genetics lab that supported itself by performing paternity tests on cows. “
It was a kind of bio-business venture,” he later said. Farmers from around the country were starting to get their cows inseminated with the sperm from champion bulls. They wanted to make sure their calves were inheriting the expensive pedigree they had paid for, rather than being fathered by some bull that randomly crossed paths with their cows one day.

Not only did the lab make money off the arrangement, but it ended up awash in cow blood. “They'd bleed the whole herd,” Owen said.

For Owen and his fellow biologists, the blood was a scientific godsend. Each sample was accompanied by a wealth of information about the animal it came from, along with its relatives. They could measure different kinds of proteins in the blood—not just the proteins that produced the ABO blood type groups, but many others—and learn about how the cows passed down genes to their offspring. They could ask fundamental questions, such as whether complicated traits could be encoded by lots of simple genes, or genes that were linked together somehow. It all worked out very well. Careers were made.

Only one problem got in the way. “There was something funny about twin calves,” Owen said.

To be more specific, there was something funny about freemartins. Owen would compare the blood proteins of freemartins to their twin brothers. Since they were fraternal twins, he expected that their proteins would be as different as those of any pair of siblings. Instead, the proteins from the freemartins and their brothers were identical. Despite being the opposite sex, they looked biochemically like identical twins.

Owen didn't know how to account for this result. As he puzzled over freemartins,
a Maryland cattle farmer got in touch with Owen for some help. One morning, the farmer had bred a Guernsey cow with a purebred Guernsey bull. Later that day, a white-faced Hereford bull broke through a fence and bred with the Guernsey, too. Nine months later, the cow gave birth to twins.

“They were a remarkable pair,” Owen later recalled, “because, while one was a female and looked as a Guernsey should, the other was a bull and had
the dominant white-faced marking of the Hereford. It seemed evident, just from looking at this pair, that they were twins from different fathers.”

The farmer asked if Owen could sort out the paternity. He sent Owen the blood of the calves, their mother, and both bulls. When Owen looked closely at the proteins in their blood, he discovered something no one had seen before. Both calves
carried proteins matching both bulls.

Thinking back to Lillie's research, Owen speculated that each calf had been fathered by a different bull, but then their blood mixed together through their united placentas. He wondered how much they had blended. After all, red blood cells last for only a few months, replaced by cells from bone marrow. Owen decided to keep track of the Maryland calves as they grew older to see if they developed into normal animals.

Owen arranged to get more blood from the calves when they were six months old. Their blood still remained a mixture. Even on their first birthday, Owen was surprised to find, they still had blood proteins of both bulls. Owen realized that it wasn't a blood transfusion that the calves had given each other. They had transplanted their stem cells into each other's bone marrow.

With this discovery, Owen proved just how fragile our notion of heredity really is. We think of ourselves as having inherited our genes from our parents, brought together by a single egg and a single sperm into a single zygote, defined by a single genome. Now Owen had discovered cows whose bodies were made up of cells belonging to different lineages.

You could trace some of the Guernsey purebred calf cells back to its own original cell. But you could also trace some of its stem cells back into its Hereford twin. If embryologists were to draw the pedigree of their cells, they would have to draw two trees, with separate bases and intermingled branches. And if they were to trace the genes in those cells to the previous generation, some would go back to the Guernsey bull, and the rest to the Hereford. Despite breaking the rules of heredity, however, the calves were perfectly healthy. An amalgam of different cells from different parents, a divergent heredity, worked just fine.

Owen wondered if he had merely discovered a rare fluke. He inspected
the blood of hundreds of pairs of twin calves. In 90 percent of the cases, he found, their blood was a mix. What made Owen's discovery all the more remarkable was that the immune systems of the twins didn't seem bothered by the blending. By the 1940s, blood transfusions had become standard medical practice, but only because doctors could very carefully avoid giving patients the wrong blood type and triggering deadly immune responses. Perhaps, Owen thought, an early exposure to foreign cells taught the immune system tolerance.

Owen published the story of the freemartins in October 1945, and on the strength of those findings Caltech offered him a job. He and his wife left behind the Wisconsin winters for Southern California, where Owen gave up his research on freemartins. Settling into a conventional lab, he turned his attention to rats instead, stitching together blood vessels from one rodent to another to see if they could also trade stem cells through their shared circulation.

His work on freemartins might have slipped into obscurity if it hadn't caught the attention of a British physician a few years later.
Peter Medawar was, at the time, running pioneering experiments on transplantation. He fell into the line of research in World War II, hoping to find a way to treat Royal Air Force pilots covered in burns. Medawar found that if he took healthy skin from a patient's own body and cultured it, he could then graft it successfully to the wound. But if he transplanted tissue from other people, the skin usually died.

Sometimes Medawar would try to graft a second patch of skin from the same donor to the same patient. Now the patient rejected the skin in even less time. Medawar realized that the patient's immune system was attacking the transplant like an invading enemy and launching its assault faster as it grew familiar with the foreign tissue.

That discovery led Medawar to wonder how immune cells could tell the difference between self and nonself. He suspected that in the developing embryo, the immune system learned to recognize genetically encoded proteins on cells as an identity badge. If they later encountered cells without that right badge, they turned hostile. If that were true, then Medawar
realized there was a simple way to find out. Identical twins, having the same genes, should accept transplants from each other. Fraternal twins and other siblings would be more likely to reject them.

Medawar and his colleagues traveled to a research farm in Staffordshire to run the test on cows. They
punched out bits of skin from the cows' ears and inserted them into the withers of other cows. The experiment proved both a success and a failure. Siblings typically didn't accept transplants, while identical twins did. So far so good. But Medawar was surprised to find that fraternal twins—including freemartins—accepted transplants as well.

The results confused Medawar at first because, unlike Owen, he didn't understand cows very well. That confusion disappeared when he discovered Owen's research. As embryos, Owen had shown, fraternal twin calves trade cells with each other through their joined bloodstream. Medawar realized that their developing immune systems accept both kinds of cells as their own self. When Medawar gave an adult freemartin a skin punch from her brother, her immune system gave a molecular yawn.

Building on Owen's insights, Medawar went on to gain an even deeper understanding of the immune system. Ultimately, his research would open the way to the modern practice of transplanting organs. In 1960, Medawar won the Nobel Prize, but later he sent a letter to Owen complaining they should have shared the honor.

The skin-punch experiments told Medawar important things not only about the immune system but about heredity as well. Freemartins and other fraternal cow twins represented a kind of heredity not documented before, one in which the cells in their bodies belonged to more than one lineage. Medawar thought they deserved a name of their own. He called them chimeras.

The name echoed back thousands of years, to Greek myths that were probably inspired by strange births. Chimera was the name of a monster that had the front of a lion, the back of a snake, and a goat for its middle. For Medawar, the word also had a more recent resonance. Horticulturalists like Luther Burbank would sometimes graft the top of one plant onto the stem of another, creating so-called graft-hybrids. In 1903, a German botanist
named Hans Winkler produced an exceptional graft: a tomato plant on one side and nightshade on the other. He chose to call his creation a chimera. Winkler's new name became familiar among botanists, but only for their handiwork.