Jenny Nicholls explores some bizarre quirks of heredity.
TO ANCIENT GREEKS, the chimera was a spitting, snarling, fire-breathing female patchwork of scary animals, with a goat thrown in for good measure. “A thing of immortal make,” wrote Homer, “not human, lion-fronted and snake behind, a goat in the middle, and snorting out the breath of the terrible flame of bright fire.” She apparently lived in Lycia ( part of modern- day Turkey).
This might seem a long way from the Department of Social Services office in Washington State, USA, where a solo mother was summoned after applying for welfare in 2003. The incredible story of Lydia Fairchild – an American chimera and a living testament to the tricks heredity plays on us – is recounted in a new book by US science writer Carl Zimmer, She Has Her Mother’s Laugh: the Powers, Perversions and Potential of Heredity (Macmillan, $40).
On the surface, Fairchild, at 27, seemed about as far from a fire-breathing snakegoat hybrid as you could get. Pregnant with her fourth child, she found herself in need of state support after she broke up with her children’s father. State law in Washington required a DNA test to prove her children were hers. A formality, she thought.
When the results came back, they astounded everyone who knew her. Fairchild’s ex, Jamie, was confirmed as the father of her three children – but Fairchild herself had no genetic link to them. Despite the evidence of the obstetrician and Fairchild’s mother, who’d seen her grandchildren being born, two further tests replicated the findings.
To Fairchild’s horror, the Department of Social Services threatened to foster out her children, and charge her with welfare fraud.
Her story became nothing short of surreal after she gave birth to a fourth child under the beady gaze of a court official, who watched as nurses drew blood from the infant for a DNA test. The results came back two weeks later: her DNA did not match the baby a court official had seen being born.
Unfortunately for Fairchild, the court gave more weight to DNA evidence than to the testimony of those present at the birth. Her pregnancy was not enough to prove motherhood. Social Services prepared to charge her with fraud.
WHEN WE take a cheek swab of cells and send it to a Dna-testing company, we make the same assumption the US Social Services made in Fairchild’s case: that every cell in our body contains the same genome. Our DNA is the same, whether it comes from cells in our blood, our brain or our kidneys, right?
Fairchild’s lawyer heard of a case in Boston which echoed the experience of his client. When 52-year- old renal patient Karen Keegan needed a second kidney transplant, her family was tested for compatibility. To everyone’s astonishment, she was told the test proved she could not be the biological mother of two of her three sons. The hospital even suggested she might have stolen the two boys as babies.
Lynne Uhl, a transfusion medicine specialist at Beth Israel Deaconess Medical Centre in Boston, wasn’t buying it. “We knew this woman very well and knew without any doubt that she was the mother of these kids. That prompted us to look more carefully for an explanation for this finding.”
Uhl and her team began testing other kinds of tissue belonging to Keegan. They checked her hair follicles, cheek swabs and old stored tissue samples from earlier minor surgical procedures.
Bingo. Keegan, it transpired, was made up of two different kinds of cells, each with their own DNA, as if two people had fused. This is, in fact,
precisely what happened.
In science- speak, Keegan was a “tetragametic chimera”: the result of two female non-identical twins who became one early in their development. One twin gave rise to all her blood, but only some of her eggs. The son credited to her developed from one of them. Her other boys began as eggs from the other twin’s cell lineage, containing the other twin’s DNA.
UHL AND HER team agreed to test Fairchild’s tissues. The first tests were unpromising. DNA from Fairchild’s hair, saliva and skin didn’t match her children’s. And then Uhl tested cells from a cervical smear. These revealed the bizarre truth: Fairchild was also a chimera. Her twin may have only ever existed as a bunch of cells – but, fertilised by a different sperm to the sperm that led to Fairchild, those gave rise to the lineage that produced her eggs. The transient twin was the biological mother of her children.
“The stories of Lydia Fairchild and Karen Keegan both ended happily,” writes Zimmer. “But they left the women with haunting questions not only about their families but about themselves. Fairchild’s eggs, cervix and perhaps some other tissues in her body all had a direct genetic link to her children. But what of the rest of her body. Was she partly their aunt, too? As for Keegan, were her sons half-brothers to each other, with two sisters for their mothers?”
It took 16 months for the case against Fairchild to be dismissed. Without the help of Keegan’s doctors, she would have gone to jail and, even worse – lost her children. Her attorney, Alan Tindell, had his own views about a system which put such implacable faith in DNA. “People go to death row because of DNA tests,” he said. “People are released from death row because of DNA tests.”
WE HAVE KNOWN about human chimera since 1953, when a young woman recorded in medical history as “Mrs MCK” donated a pint of blood in England. It turned out to be a mix of type O and type A.
When Mrs MCK told scientists about a twin brother who died of pneumonia at three months, they (eventually) realised his stem cells had entered her body in the womb and set up shop in her bone marrow. There, they produced enough type A blood to make her bloodstream a cocktail of two parts O to one part A. If Mrs MCK and her twin had shared a blood type, her chimerism would not have been discovered.
We now know that chimeras are hardly rare. In fact, you might well be one yourself – although probably in a less dramatic sense than Mrs MCK, Fairchild or Keegan.
The placenta, it turns out, is leakier than we once thought. An unborn child’s cells often cross into their mother, where they may produce a lineage of cells with their DNA that lasts for days, weeks or even years. If the woman gets pregnant again, these cells, as well as her own, can cross the placenta into her new fetus.
To find out how common this is, US researcher Diana Bianchi tested women with sons to find out how many had tissue with their son’s Y chromosomes. She excluded women who had ever received a blood transfusion or had an organ transplant. Bianchi found fetal cells with Y chromosomes in six of the eight mothers she tested. “One of the women with Y chromosomes had a 27-year-old son,” writes Zimmer, “meaning his cells had remained established in her body for more than a quarter- century.”
In the years since Bianchi’s paper was published in the Proceedings of the National Academy of Sciences journal in 1996, researchers have found that more than half of mothers carry fetal cells decades after their pregnancies. And according to a recent estimate, says Zimmer, 42% of us carry cells containing our mother’s DNA.
But surely a few alien cells wouldn’t affect our health? “Fetal cells don’t simply migrate around their mother’s bodies,” says Zimmer. “They sense the tissue around them and develop into the same types of cells.” One US scientist, Lee Nelson, has speculated this might explain why women are more prone to autoimmune diseases. Foreign fetal cells, he hypothesised, might trigger a woman’s immune system into attacking her own tissues. Although research appears to bear this idea out, Zimmer remains cautious about the link. One of the strengths of his book is his reluctance to over-egg research, and his familiarity with the peer-reviewed papers he writes about – every study has its footnote.
And, anyway, being a chimera might also be a good thing. Bianchi found a woman with goitre, which had destroyed her own thyroid cells. Oddly, the gland was still secreting healthy amounts of thyroid hormone. The gland, it turned out, was stuffed with her son’s cells. The evidence, writes Zimmer, pointed to an amazing conclusion.
“A fetal cell from her son had wended its way through her body to her diseased thyroid gland. It had sensed the damage there and responded by multiplying into new thyroid cells, regenerating the gland.”
In another case, a woman’s liver had been ravaged by hepatitis C. Years before, she’d had a pregnancy terminated. Incredibly, her aborted son’s cells, still bearing the Y chromosome of the father, came to the rescue by rebuilding an entire lobe of her liver.
And what does this all mean for surrogate mothers? They may well end up connected to the child they bear, in ways they didn’t imagine.
BUT WE DON’T need to be a mother to have, in some bodily nook or cranny, traces from a ghostly twin who bequeathed us their stem cells.
Charles Boklage, a developmental biologist who has studied chimerism for more than two decades, explains: “About one- eighth of all conceptions and about one- eighth of live births are ‘twins’ – the majority of whom are born alone without a live twin. About one in eight of everybody walking around is a twin who was born single.”
Heredity, says Zimmer – and this is the overarching theme of his book – can surge and commingle in strange ways, blowing backwards and sideways like a strange eddy of wind, or a river current flowing the wrong way. +
We have known about human chimera since 1953, when a young woman known to medical history as “Mrs MCK” donated a pint of blood in England.