Human Chimeras

Some more interesting stuff from She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity by Carl Zimmer… It turns out that Biology 101 contains a lot of simplifications (lies!).

Wikipedia: “A genetic chimerism or chimera … is a single organism composed of cells with distinct genotypes. In animals, this means an individual derived from two or more zygotes, …

How can this happen to a human and how does that interact with our “science is settled” attitude regarding DNA tests? Zimmer gives some examples:

In 2001, a thirty-year-old woman in Germany discovered she was a chimera while she was trying to get pregnant. For the previous five years, she and her husband had been trying to have a baby. They were fairly certain the problem didn’t lie with her biology, because she had gotten pregnant when she was seventeen and had had regular menstrual cycles ever since. A fertility test revealed that her husband had a low level of viable sperm, and so they made plans for IVF. As a routine check, the woman’s doctors took blood samples from her and her husband. They looked at the chromosomes in the couple’s cells, to make sure neither would-be parent had an abnormality that would torpedo the IVF procedure. The woman’s chromosomes looked normal—if she were a man. In every white blood cell they inspected, they found a Y chromosome. Given that she had given birth, this was a weird result. And a careful exam revealed that all her reproductive organs were normal. To get a broader picture of the woman’s cellular makeup, her doctors took samples of her muscle, ovaries, and skin. Unlike her immune cells, none of the cells from these other tissues had a Y chromosome in them. The researchers then carried out a DNA fingerprinting test on the different tissues, looking at the women’s microsatellites—the repeating sequences that can distinguish people from one another. They found that her immune cells belonged to a different person than her other tissues. It turned out that the woman had had a twin brother who died only four days after birth. Although he was unable to survive on his own, his cells took over his sister’s blood and lived on within her.

In 2003, a woman in Washington State named Lydia Fairchild had to get a DNA test. Fairchild, who was then twenty-seven, was pregnant with her fourth child, unemployed, and single. To get welfare benefits, state law required that she prove that her children were genetically related both to herself and to their father, Jamie. One day, Fairchild got a call from the Department of Social Services to come in immediately. A DNA test had confirmed that Jamie was the father of the three children. But Fairchild was not their mother.

When Fairchild was rushed to a hospital to deliver her fourth child, a court officer was there to witness the birth. The officer also oversaw a blood draw for a DNA test. The results came back two weeks later. Once again, Fairchild’s DNA didn’t match her child’s. Even though the court officer had witnessed the child’s birth, the court still refused to consider any evidence beyond DNA.

In Boston, a woman named Karen Keegan had developed kidney disease and needed a transplant. To see if her husband or three sons were a match, her doctors drew blood from the whole family in order to examine a set of immune-system genes called HLA. A nurse called Keegan with the results. Not only were her sons not suitable as organ donors, but the HLA genes from two of them didn’t match hers at all. It was impossible for them to be her children. The hospital went so far as to raise the possibility she had stolen her two sons as babies. Since Keegan’s children were now grown men, she didn’t have to face the terrifying prospect of losing her children as Fairchild did. But Keegan’s doctors were determined to figure out what was going on. Tests on her husband confirmed he was the father of the boys. Her doctors took blood samples from Keegan’s mother and brothers, and collected samples from Keegan’s other tissues, including hair and skin. Years earlier, Keegan had had a nodule removed from her thyroid gland, and it turned out that the hospital had saved it ever since. Her doctors also got hold of a bladder biopsy. Examining all these tissues, Keegan’s doctors found that she was made up of two distinct groups of cells. They could trace her body’s origins along a pair of pedigrees—not to a single ancestral cell but to a pair. They realized Keegan was a tetragametic chimera, the product of two female fraternal twins. The cells of one twin gave rise to all her blood. They also helped give rise to other tissues, as well as to 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. When Lydia Fairchild’s lawyer heard about the Keegan case, he immediately demanded that his client get the same test. At first, it looked as if things were going to go against Fairchild yet again. The DNA in her skin, hair, and saliva failed to match her children’s. But then researchers looked at a sample taken from a cervical smear she had gotten years before. It matched, proving she was a chimera after all.

More: Read She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity