The term chimera is derived from classical Greek
mythology in which there was a beast containing a body made up of parts of
three different animals. The term
chimera has come to be used to refer to any animal that consists of parts from
more than one animal. This seemingly
bizarre occurrence turns out to be rather common in nature with we humans being
no exception. This recently discovered
fact is only one of the many revelations provided by Carl Zimmer in his fascinating
book She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity.
Zimmer tells the story of a woman
from Washington state, Lydia Fairchild, who in 2003 filed for welfare
benefits. She had three children and was
pregnant with a fourth. She was required
to have a DNA test to verify that she was the mother of her children. The test results indicated that her husband
was the father of her children, but she was not the mother. The state officials suspected that she was
guilty of at least welfare fraud if not of having kidnapped the children. She was being threatened with the loss of her
children and jail time. None of her
records or personal references helped because the DNA test was deemed
infallible. When she delivered her
fourth child the state sent a witness to observe the event and to get a DNA
sample from the child. Again, the DNA
results indicated that Lydia could not be the child’s mother. Even though a state witness observed the birth,
the DNA tests were still treated as being incontrovertible.
Fortunately for Lydia, her legal representation heard
about a similar case in Boston where a woman named Karen Keegan needed a kidney
transplant and had her three sons and husband tested for acceptability as
donors. The DNA studies indicated that
her husband was the father of the sons, but she couldn’t possibly be the mother
of two of them. Again, she was suspected
of foul play, but this time the doctors involved decided to look further for an
explanation. They acquired as many
tissue examples from Karen as they could and discovered that her organs
contained two separate pedigrees, as if she had been the product of two
separate fertilized eggs instead of one.
Both Karen and Lydia are what would come to be known as a tetragametic
chimera.
In pregnancy, more than one egg can be fertilized at a
time. If two are fertilized they would
normally develop into fraternal twins.
In some cases, the two fertilized eggs can merge very early in
development. If the embryos are of the
same sex, the merger is difficult to recognize, if of different genders, the
sexual development can be abnormal.
“Tetragametic twins start out as
two embryos with separate genomes, and then merge entirely. Only one child is born, and there is no other
human being to point to. All we can do
is trace their intimately mingled cell lineages to their separate sources.”
Consider Karen Keegan’s development.
“The cells of one twin gave rise
to all her blood. They also helped to
give rise to other tissues, as well as some of her eggs. One of her sons developed from an egg that
belonged to the same cell lineage as her blood.
Her other two children developed from eggs belonging to the lineage that
arose from the other twin.”
This form of chimerism is rare, but common enough that
physicians now know to consider it when genetic anomalies appear. It is rather astonishing that a single person
can essentially be constructed from two different people and continue to
function normally. Isn’t our immune
system designed to attack cells that are not our own? How does it deal with two classes of cells?
We now know that there is much more common form of
chimerism that can occur during pregnancy that does not require the merging of
embryos. Embryos develop in the
placenta. There is a barrier that can
transmit nutrients and other molecules from the mother’s blood to support the
embryo’s development. But there is
supposed to be no transmission of cells.
Over a century ago it was discovered that transmission of cells could
occur, but it was not until the 1960s that it became clear that this
transmission was a common occurrence during pregnancy. It would take another thirty years before it
was realized that these cells, transmitted in both directions, mother to embryo
and embryo to mother, could live for a very long time and could become an
active participant in bodily functions.
This form of chimerism came to be called microchimerism.
“Their research has revealed
that all pregnant women have fetal cells in their bloodstream at thirty-six
weeks. After birth, the fraction drops,
but up to half of mothers still carry fetal cells in their blood decades after
carrying their children.”
“Very often, a mother’s cells
will infiltrate their children’s bodies, where they can endure and grow long
after her death. According to one
estimate, 42 percent of children end up with cells from their mothers.”
It gets even more complicated when a mother has multiple
pregnancies. Cells from a son who was
born can then be transferred from the mother to a subsequent daughter. One study detected this occurrence, and it
was not particularly rare. Males have an
X chromosome and a Y chromosome. Females
have two X chromosomes instead. Women
should not have any Y chromosomes in their bodies. If they do, it is evidence of a transfer that
occurred from another human entity.
“At the University of
Copenhagen, scientists got blood samples from 154 girls ranging in age from ten
to fifteen. They cracked open the cells
in the blood and searched them for Y chromosomes. In 2016 they reported that twenty-one girls—more
than thirteen percent—had them.”
Searching for Y chromosomes in women is a simple way to
determine how long these cells persist.
“Lee Nelson, a rheumatologist at
the Fred Hutchinson Cancer Research Center, and her colleagues examined the
cadavers of fifty-nine women who died, on average, in their seventies. In 63 percent of the women, the scientists
found Y chromosomes in their brains.”
Zimmer also notes that given this propensity for
microchimerism, the practice of utilizing a surrogate mother to carry a couple’s
child is no longer as simple as it once seemed.
The surrogate can inherit cells from the fetus and the fetus may no
longer become the genetically simple child that the parents assume.
Given that this form of chimerism is so common and that
it can persist indefinitely, what might its effects be? It is known that women are more susceptible to
autoimmune diseases. Could this be because
females, as mothers, will generally be subjected to higher concentrations of foreign
cells than a male who will only encounter them once? Once again, scientists looked for evidence of
Y chromosomes in females suffering from the autoimmune skin disease scleroderma.
“Nelson and Bianchi found that
women with scleroderma had far more fetal cells from their sons than did the
healthy women. Other scientists who
carried out similar studies got the same results for a number of other
diseases. These findings aren’t definitive
proof that microchimerism made these women sick, however. It was also possible that the diseases came
first, and the fetal cells only later flocked to the diseased tissues where
they could multiply.”
And then there are these observations which suggest that
having these fetal cells acquired in pregnancy can be beneficial.
“In another woman, Bianchi
discovered that an entire lob of her liver was made up of Y-chromosome bearing
cells. Bianchi was even able to trace
the paternity of the cells to the woman’s boyfriend. She had had an abortion years before, but
some of the cells from the fetus still remained inside her. When her liver was damaged later by hepatitis
C, Bianchi’s research suggested, her son’s cells rebuilt it.”
“It is also possible that fetal
cells help mothers fight cancer. In
2013, Peter Geck of Tufts University and his colleagues looked for cells with Y
chromosomes in the breast tissue from 114 women who died of breast cancer and
68 women who died of other causes.
Fifty-six percent of the healthy samples had male fetal cells in
them. Only twenty percent of the
cancerous tissue had them. Geck
speculated that fetal cells swooped into the niches of breast tissue that are
good for proliferating cells. Those may
be the same niches cancer cells need to find in order to grow into tumors.”
What is startling about this topic is that this notion of
humans as chimera is so new, and the knowledge about the consequences of this
phenomenon is so inadequate. Who knows
what else we might yet learn about ourselves?
Occasionally, we hear of “miracles” occurring where no medical
explanation is possible. Perhaps they
are not so much miracles as indications that the human body is still too
complicated for us to understand.
The interested reader may find informative the following
articles based on information found in Zimmer’s book:
No comments:
Post a Comment