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In 1976, researchers from the Lister Institute of Preventive Medicine in London published the puzzling case of a woman who had two different blood types: 93 percent of her red cells were type O, while the remaining 7 percent were type A1, the most common type A subgroup.1 A few years later, Winifred Watkins of the MRC Clinical Research Centre and colleagues came across another blood donor with two distinct red blood cell types, and further investigation led to an even more astonishing finding: a phenotypically normal man, with presumably XY cells in his testes and most of his body, was found to carry XX cells in his skin and other tissues.2
These case studies heralded a new appreciation for the phenomenon of genetic chimerism—when an individual carries two or more genetically distinct cell lines in different parts of her body. Until the advent of techniques for blood typing and karyotyping cells, genetic chimeras where thought to be very rare. They only came to light when the phenotypes associated with the two distinct genomes were so discordant that the resulting individual was clearly exceptional, with patches of distinct skin coloration throughout the body, for example, or hermaphroditic genitals. In reality, genetic chimeras may be quite common, disguised in perfectly normal bodies harboring genetically distinct cell lineages.
While most fictional works portray chimeras as an amalgam of two individuals, the truth is that the individuality of the distinct cell lines is lost as the two combine.
In all likelihood, most chimeras are not even aware of their condition. Boston resident Karen Keegan, for example, would have never discovered her mixed genetic makeup if in 1998, at age 52, she hadn’t needed a kidney transplant. When doctors tested the human leukocyte antigen (HLA) type of her three children to see if any of them could donate a kidney, they were surprised to find that two of the children could not have been hers at all: while all three carried one HLA copy that matched their father’s, only one child carried a second copy that matched Karen’s. Of course, having given birth to these children, she knew they were hers. Sure enough, while Karen had one cell line only in her blood, the doctors eventually found the “missing” HLA type in a second cell line in her skin, hair, bladder, mouth mucosa, and thyroid.3
For years the concept of a genetic chimera has sparked the imagination of writers, from Stephen King to Michael Crichton, presenting endless fodder for science fiction, horror, and even murder mysteries. But while most fictional works portray chimeras as an amalgam of two individuals, the truth is that the individuality of the distinct cell lines is lost as the two combine. Indeed, genetic chimerism is subtle, and may still often be overlooked if the second cell line exists at concentrations too low for our current technology to detect.
Since the first cases were published in the 1970s and ’80s, genetic chimeras have been the focus of much research, leading to the advent of new blood testing techniques to identify distinct cells in donors that may cause complications in transfusion recipients. Chimerism in donors could have consequences for organ transplants, too. Furthermore, some types of chimerism have been associated with an increased risk of certain autoimmune diseases, while the stem-like qualities of cells transferred from fetus to mother appear to have implications for tissue repair. Understanding how chimeras form and, in particular, what mechanisms allow the immune system to accept genetically distinct lineages within one body could lead to new therapies for cancer and autoimmune disorders.
Types of chimeras
The best-known form of chimerism happens at conception, when two fertilized eggs join to form a single morula, resulting in one organism with two genetically distinct cell populations—a dispermic chimera. Because each fertilized egg carries its own genome, the resulting chimera has distinct DNA in different tissues—a liver arising from one cell line and kidneys from the other, for example. If the two genomes result in markedly different phenotypes, the individual can be a hermaphrodite or present different hair or skin coloration in different parts of the body. (See photograph below.)
But not all chimeras result from the aggregating of two distinct zygotes. Rather, during embryonic development, mutational events can result in the emergence of two or more genetically distinct cell lines within an individual. Errors in chromosome segregation during cell division, for example, can lead to daughter cells with different chromosome numbers. When this type of error, called chromosome nondisjunction, happens early in fetal development, it gives rise to two different cell lines that become randomly distributed throughout the body as the embryo develops—a type of chimerism called mosaicism.
Meiotic nondisjunction usually causes an individual conceived with an abnormal sperm or egg to have the chromosome error in every somatic cell of the body: for example, a female child with only one X chromosome (Turner syndrome) or a male child with an extra X chromosome, XXY (Klinefelter syndrome). But there are also less-common mosaic forms of these syndromes in which the error occurs after fertilization and affects only some of an individual’s cells. Pallister-Killian syndrome is a rare form of mosaicism where meiotic nondisjunction results in an abnormal copy of chromosome 12, which persists in some cells but is eliminated from others during development.
Somatic mosaicism can also happen later in life. Recent studies have found evidence of mosaicism and aneuploidy—an abnormal number of chromosomes—in healthy brain tissue, introduced by errors in chromosomal segregation during cell division or by the activity of mobile genetic elements called retrotransposons, bits of DNA that can replicate and reinsert themselves in different regions of the DNA.4 This is likely to confer upon the brain the plasticity it needs to constantly adapt to the environment, but it may also impact the development of many diseases.
Indeed, while somatic mosaicism has been observed in healthy brain tissue, there is evidence that it could also play a role in diseases such as Alzheimer’s. A group of researchers from the Scripps Research Institute analyzed the nuclei of neurons harvested from the prefrontal cortex and cerebellum of 134 postmortem brains, 47 of which came from subjects with Alzheimer’s, and found that the gene APP, which encodes for an amyloid precursor protein, was mosaically amplified in brains affected by the disease. Some neurons had up to 12 copies of the gene.5
Each one of us is made of trillions of cells, and some of those cells could very well belong to our siblings, our mother, or our child.
Sometimes the second cell line is present in very low concentrations, a form of chimerism called microchimerism, which may be much more widespread than realized. Not surprisingly, this type of chimerism has been detected in recipients of organ transplants. However, a study conducted by researchers at the Leiden University Medical Center in the Netherlands found that microchimerism unrelated to transplants is also quite common. Among autopsy samples from 75 women, the researchers found male cells in 13 kidneys, 10 livers, and 4 hearts of 23 of the women. Notably, none of these women showed signs of organ failure, thus demonstrating that perfectly healthy organs can harbor small numbers of chimeric cells.6
One common cause of such microchimerism is maternal-fetal trafficking of cells during pregnancy. The placenta is not an unbreachable barrier. Evidence of two-way cell transport across the placenta was reported as early as the 1950s and ’60s. While the mother’s immune system gets rid of most of her baby’s cells shortly after delivery, small numbers of fetal cells have been observed in mothers decades after they have given birth. In fact, because even spontaneous abortions cause fetal cells to be released into the mother’s body, women who become pregnant but never give birth can also display this form of microchimerism.7 Conversely, maternal cells have been found in the liver, lung, heart, thymus, spleen, adrenal, kidney, pancreas, brain, and gonads of healthy adults.8 Microchimerism could also originate from siblings’ cells, transferred from the mother during successive pregnancies. Regardless of the direction of transfer, the exogenous cells can migrate to a certain tissue, where they differentiate and proliferate, acting as if they were engrafted.
Likely critical to this integration of fetal cells in a mother’s tissues are the immune changes a woman undergoes during pregnancy to allow for tolerance of the fetus, which is genetically distinct from the mother and therefore potentially subject to immune attack. These changes could also explain why numerous autoimmune disorders seem to be associated with pregnancy. For example, the symptoms of rheumatoid arthritis sometimes improve during gestation but tend to return within three months of delivery. But could microchimerism also play a role in the etiology of these and other disorders?
Microchimerism and disease
© MARINA MUUNBecause it’s easier to test for a male’s DNA in a mother by looking for evidence of the Y chromosome, many studies have focused on detecting microchimerism in mothers of boys. In a 2012 study, William Chan of the Fred Hutchinson Cancer Research Center in Seattle and colleagues looked at the autopsied brains of 59 women, 33 of whom had Alzheimer’s, while 26 exhibited no evidence of neurological disease. They found male DNA—detected by the Y-chromosome marker DYS14—in diverse brain regions of 63 percent of the women, and, notably, the prevalence and concentration of microchimerism were significantly lower in the women affected by Alzheimer’s.9
Conversely, a number of studies have found significantly higher levels of fetal microchimerisms in mothers affected by systemic sclerosis, an autoimmune disorder characterized by the hardening and scarring of the skin. Similarly, both animal and human studies have found higher concentrations of microchimerism in females with some form of thyroiditis,6 an autoimmune disorder in which the immune system perceives the thyroid and its hormones as threats and starts producing antibodies against the gland. Interestingly, thyroid disorders of this kind are much more common in women than men, with females being at particularly higher risk shortly after giving birth.
Microchimerism has also been reported in several cancer studies. For example, Valentina Cirello of Fondazione Policlinico IRCCS in Milan, Italy, and colleagues found lower circulating levels of fetal DNA in the blood of female patients with thyroid cancer than in the blood of healthy controls, though they pinpointed higher concentrations near or around tumor tissue.10 These findings have fueled speculation that fetal cells could be recruited from the blood to the tumor site in an attempt to repair the tissue.11 Researchers have reported similar findings for breast cancer, where fetal microchimerism also occurs at lower concentrations in the blood of cancer patients, compared with controls.12,13 It is well known that multiparous women have a lower risk of developing breast cancer compared with women who have no children, yet why this happens is not entirely known. These latest findings on fetal chimerism could perhaps explain part of the puzzle.
The opposite trend has surfaced in colon cancer, however, with higher blood concentrations of fetal DNA detected in cancer patients.14 Rather than having a salutary effect, fetal cells could possibly contribute to cancer pathology by triggering inflammatory responses that lead to tissue damage, similar to what occurs during transplant rejection. Alternatively, the chimeric cells may be completely neutral to the host, and their presence or absence in damaged tissues could be due to side effects of the disease.
The reverse form of mother-fetus chimerism—the presence of maternal chimeric cells in her children’s tissues—has also been studied extensively in both infants and adults. This type of microchimerism has been associated with diseases such as diabetes15 and biliary atresia, a congenital defect in which the bile duct between the liver and the small intestine is blocked or absent.16 But again, results gathered thus far seem to be inconclusive as to whether the presence of the maternal DNA is a cause or result of the disease.
“It is such a new area of work and there is so little knowledge,” fetal microchimerism specialist J. Lee Nelson of the University of Washington told me in an e-mail. “Overall we expect microchimerism to have the potential for both beneficial and/or detrimental effects, whether it is of fetal origin or maternal, or other sources of microchimerism.”
Diverse factors can complicate the influence of microchimerism on the health of host tissues. The compatibility of the HLA genes of the mother with those of the fetus could play an important role in the way they interact, for example. Normally, cells exchanged between mother and fetus are rapidly cleared by the host immune system, but matching HLA types could increase the chance that the exchanged cells behave like grafts and persist in the host long after arrival. Certain HLA combinations may also translate to a higher chance of developing autoimmune diseases.
The origin of the microchimerism could also be a key factor, whether it’s from the mother, a full-term pregnancy, a sibling, or a spontaneous abortion. As miscarriages are often caused by genetic defects, this could lead to defective cells being transferred, which could be potentially harmful.
Understanding how chimeras form and what mechanisms allow the immune system to accept genetically distinct lineages within one body could lead to new therapies for cancer and autoimmune disorders.
Finally, the gestational age of the fetal cells that are transferred to the mother could also influence their effects on her physiology, because the cellular composition of the fetus changes as it develops.17 For example, fetal T cells are formed around week 13 of pregnancy, and couldn’t be transferred prior to that. Transfer of cells from the immune system could be either beneficial or detrimental depending on HLA compatibility. Conversely, the age of the maternal cells transferred to the fetus could affect whether or not maternal microchimerism is beneficial, as older cells may have accumulated DNA damage or other signs of aging. While a transfer of cells from the maternal immune system could bring beneficial acquired immunity, some cells could have also accumulated precancerous mutations. In fact, although cancer during pregnancy is uncommon, there have been reports of maternal tumors that metastasized to the placenta and, in rare instances, to the fetus.
Beyond DNA: Expression chimerism
A. BYGUM ET AL., BMC MEDICAL GENETICS, 12:79, 2011.During embryonic development, the genomes of different tissues acquire signature epigenetic patterns in the form of DNA methylation marks. But within one cell population, the assumption is that the epigenome is largely uniform. When some cells out of the group acquire a different set of methylation marks, creating a chimeric mixture within the tissue, the result is an epigenetic chimera. Changes in epigenetic patterns may happen throughout our lifetime as a result of environmental changes such as stress and trauma, but early in embryonic development, such changes can be fatal. (See “Pushing the Limits,” The Scientist, February 2015.)
One of the genes involved in early epigenetic regulation is TRIM28. This gene mediates transcriptional control in certain regions of the chromatin and is thought to play a fundamental role in resetting the epigenome during embryonic development. Researchers in Singapore, at the Jackson Laboratory in Maine, and at Stanford University created epigenetic chimeras by disrupting the TRIM28 gene in mouse mothers.18 The scientists showed that not all cells in the resulting embryos had their epigenomes reset, as is expected following genomic reprogramming, but instead ranged from normal to completely unmethylated. This effectively created a chimeric mixture—with a disastrous outcome. Mouse embryos produced by TRIM28-deficient mothers did not survive more than five days postfertilization.
Many cancers can also be considered chimeric, resulting from genetic or epigenetic alterations that activate oncogenes capable of promoting cell growth or, conversely, silencing tumor-suppressor genes that inhibit cell growth. Understanding epigenetic chimerism, as well as the mechanisms that disrupt the epigenetic markers and how to fix such disruptions, could help provide new targets for fighting cancer. Indeed, while genetic mutations that lead to cancer are irreversible, the same is not true of epigenetic changes, and restoring normal methylation patterns could reverse the disease phenotypes. When the research team from Singapore and the U.S. transferred wild-type pronuclei into the mutant mouse embryos, for example, 25 percent survived.
Another form of mosaic expression stems from the phenomenon of X inactivation. During early embryonic development of female mammals, one of the two copies of the X chromosome is largely silenced in each cell, and it may be the maternal X that is silenced in some cells and the paternal X in others. In this sense, all female mammals are mosaics, since the X they inherit from the mother is not identical to the one they inherit from the father. Tortoiseshell cats are an example of this type of mosaic, with a heterozygous X-linked gene that regulates coat coloration leading to the patchwork pattern.
This kind of mosaicism gives females an advantage when it comes to X-linked genetic diseases: cells that express the “healthy” X gene will often compensate for those expressing the mutant variety. In heterozygous women carrying the allele for the X-linked lysosomal disorder called Fabry disease, for example, cells that produce the healthy enzyme can make enough to at least partially compensate for the lack of production from those carrying the mutated gene. Another X-linked disorder, called Lesch-Nyhan, is characterized by the lack of production of another important enzyme, leading to a build-up of uric acid in the body. While boys born with this defect face severe mental and physical problems, in heterozygous females, cells expressing the defective gene usually receive the missing enzyme from normal cells in at least some body tissues and are gradually eliminated from tissues where they do not. In the latter case, the cells that express the normal allele have a growth advantage and outgrow those expressing the mutant gene.19
One or many?
We consider ourselves to be single individuals, but when you stop and think about it, who are we, really? Each one of us is made of trillions of cells, and some of those cells could very well belong to our siblings, our mother, or our child. Add the fact that we harbor millions of bacteria and other microorganisms that can indeed impact our phenotype depending on what genes they express, and you get the picture of an organism made of many entities rather than a single individual. A new school of thought in biology claims that we are not individuals, but rather communities of symbionts—distinct organisms living with one another.
In this light, chimeras are no longer a genetic paradox—rather, just another path in an incredibly variegated evolutionary landscape.
Elena E. Giorgi is a computational biologist at the Los Alamos National Laboratory. Although she normally works on HIV research, she learned a great deal about chimeras while writing her debut novel Chimeras.
- G.W. Bird et al., “Further observations on the Birmingham chimaera,” J Med Genet, 13:70-71, 1976.
- W.M. Watkins et al., “A human dispermic chimaera first suspected from analyses of the blood group gene-specified glycosyltransferases,” J Immunogenet, 8:113-28, 1981.
- N. Yu et al., “Disputed maternity leading to identification of tetragametic chimerism,” N Engl J Med, 346:1545-52, 2002.
- D.M. Bushman, J. Chun, “The genomically mosaic brain: Aneuploidy and more in neural diversity and disease,” Semin Cell Dev Biol, 24:357-69, 2013.
- D.M. Bushman et al., “Genomic mosaicism with increased amyloid precursor protein (APP) gene copy number in single neurons from sporadic Alzheimer’s disease brains,” eLife, 4:e05116, 2015.
- M. Koopmans et al., “Chimerism in kidneys, livers and hearts of normal women: Implications for transplantation studies,” Am J Transplant, 5:1495-502, 2005.
- H.S. Gammill, J.L. Nelson, “Naturally acquired microchimerism,” Int J Dev Biol, 54: 531-43, 2010.
- L.S. Loubière et al., “Maternal microchimerism in healthy adults in lymphocytes, monocyte/macrophages and NK cells,” Lab Invest, 86:1185-92, 2006.
- W.F. Chan et al., “Male microchimerism in the human female brain,” PLOS ONE, 7:e45592, 2012.
- V. Cirello et al., “Fetal cell microchimerism in papillary thyroid cancer: studies in peripheral blood and tissues,” Int J Cancer, 126:2874-78, 2010.
- V. Cirello et al., “Fetal cell microchimerism in papillary thyroid cancer: a possible role in tumor damage and tissue repair,” Cancer Res, 68:8482-88, 2008.
- V.K. Gadi et al., “Case-control study of fetal microchimerism and breast cancer,” PLOS ONE, 3:e1706, 2008.
- V.K. Gadi, J.L. Nelson, “Fetal microchimerism in women with breast cancer,” Cancer Res, 67:9035-38, 2007.
- M. Kamper-Jørgensen et al., “Opposite effects of microchimerism on breast and colon cancer,” Eur J Cancer, 48:2227-35, 2012.
- J. Ye et al., “Maternal microchimerism: Friend or foe in type 1 diabetes?” Chimerism, 5:21-23, 2014.
- T. Muraji, “Maternal microchimerism in biliary atresia: are maternal cells effector cells, targets, or just bystanders?” Chimerism, 5:1-5, 2014.
- J.L. Nelson, “Naturally acquired microchimerism: For better or for worse,” Arthritis Rheum, 60:5-7, 2009.
- C. Lorthongpanich et al., “Single-cell DNA-methylation analysis reveals epigenetic chimerism in preimplantation embryos,” Science, 341:1110-12, 2013.
- B.R. Migeon, “Why females are mosaics, X-chromosome inactivation, and sex differences in disease,” Gend Med, 4:97-105, 2007.