Carl Zimmer has been writing about chimerism in the New York Times: “Having More Than One Set of DNA Carries Legacy of Risk”. As he points out, this condition can cause some interesting consequences in humans:
The results suggest that some people can have serious genetic diseases without any symptoms. That’s because they have the defective version of a gene in only some of their cells, and their other cells compensate for them.
But such people are unknowingly at risk of having children with full-blown versions of their diseases, if the mutation appears in their reproductive cells. Dr. Lupski said that as technology improved, clinical geneticists should test people for this hidden risk.
In humans, somatic mosaicism is fairly rare – although Zimmer reports on a new study that finds it is a bit more common than anyone had previously guessed.
What most people don’t know is that there are some primates where chimerism is nearly universal. The callitrichid monkeys are small anthropoid primates found in South and Central America, and include marmosets and tamarins. Twinning is very rare in most anthropoid primates – in humans, it amounts to between 0.5 and 2 percent of births in different populations. Marmosets have a very high frequency of twin births, and triplet births are more common than singletons, meaning that most marmosets were born as twins or triplets.
In humans, twins sometimes exchange blood through their placentas. With identical twins this exchange is much more common but does not lead to noticeable chimerism, because the twins are genetically identical. Occasionally, identical twins who share a placenta, called monochorionic twins, will have a significant difference in their blood supply, leading to twin-twin transfusion syndrome. This can cause significant differences in development between the twins, and in extreme cases one twin’s development is severely compromised.
Even fraternal twins can exchange blood through their placentas. This kind of exchange can remain evident in the blood, with a small fraction of an individual’s hemapoietic stem cells actually borrowed from her twin – a condition called hemapoietic chimerism. Surveys of blood in fraternal twin children have found that up to 8 percent of such twins have a very small fraction of blood cells (as low as 0.01 percent) expressing the blood types of their twin (van Dijk et al. 1996).
Primatologists have long known that marmosets and tamarins have hemapoietic mosaicism. Twins and triplets have placentas that fuse early in development, developing anastomoses that allow blood to move from one developing embryo to the other (or others). Corinna Ross and colleagues (2007) investigated this phenomenon, reviewing what was then known about it:
Genetic chimerism, the mingling of two or more genomic lineages within an individual (1), is rare in mammals, but chimerism is prevalent in the hematopoietic tissues of marmosets and other callitrichid primates (2, 3). In these species, fraternal twins exchange cell lines through chorionic fusion during early development (2, 4, 5). On the basis of karyotypic evidence from Callithrix jacchus (2, 3), estimates are that 95% of pregnancies result in the birth of hematopoietic chimeric twins. Chorionic fusion of the twins' placentas begins on day 19 and is complete by day 29, forming a single chorion with anastomoses connecting the embryos, which are still at a presomite stage in development (4–7). The fusion of the chorions and a delay in embryonic development at this stage allows the exchange of embryonic stem cells via blood flow between the twins (2, 8). As a result, the infants are genetic chimeras with tissues derived from self and sibling embryonic cell lineages (2, 3, 8).
Ross and colleagues found that chimerism in marmosets was not limited to the blood but occurred in every tissue type that they examined, from kidneys to hair. Two out of twenty-one deceased animals they examined had chimerism in the gonads, four out of seven live animals had chimeric sperm. Those findings show that marmoset males may commonly be fathering the genetic offspring of their twins.
Also, there is this:
We determined that individuals in 5 of the 15 families passed on alleles to their offspring that represented gene lineages inherited horizontally from the sibling (see examples in Fig. 1 and SI Fig. 4). One breeding female, whose uterine twin was a male, produced offspring that inherited her sibling's alleles. This documents the possibility that an XY primordial germ cell is capable of maturing and producing viable eggs in a female, a phenomenon that has not been documented for primates. Although we are not currently able to document the fate of the Y chromosome during development of the female's oocytes, our data suggest the intriguing possibility that a female may pass on a Y chromosome to her offspring.
A genetically male stem cell producing eggs in a chimeric female.
The authors discuss some of the consequences of chimerism from the point of view of the unique social system of callitrichids. Their system of twin births depends on extensive help from males and from other females besides the mother. Help, in an evolutionary perspective, depends on relatedness, and the extent of chimerism affects the relatedness between twins and between individuals in a group:
Based on the prevalence of chimerism, the proportion of cells within a tissue that carry sibling alleles, and the probability of the direction of exchange obtained from our data, we estimate that male twins are on average related by r = 0.574 (see SI Text for calculations). More specifically, in a case of unidirectional exchange in which the soma of the donating twin is nonchimeric, he is related to the sperm of the recipient twin by an average r of 0.625 (see SI Text ). The relatedness calculations suggest that chimeric callitrichid siblings are more closely related to each other than typical nonchimeric mammalian siblings.
Callitrichids may be, in the limited sense of relatedness, nearly as comparable to social insects as to other primates. Genes that promote cooperativeness should have a greater payoff considering that they will sometimes be expressed in siblings’ bodies and behavior. Yet germline chimerism might give rise to some very interesting genomic conflict between twins in utero. Callitrichids do have a fairly high rate of fetal loss and perinatal mortality.
Chimerism conceivably may have bad side effects upon immunity. The immune system may attack chimeric cells, or alternatively the presence of chimeric cells may prevent the immune system from raising up antibodies to certain pathogenic antigens. A marmoset genome sequencing project has been underway for some time, and has this month reported a draft of 90% of the common marmoset genome (Marmoset Genome Sequencing and Analysis Consortium 2014). They investigated whether genes related to immunity might have changed under selection during marmoset evolution. They drew some hits:
Hematopoietic chimerism of marmosets was expected to correlate with marked changes in immune system function. We found positively selected genes related to the immune response significantly enriched in marmoset (threshold of P < 0.05; Table 1). NAIP and NLRC4 homologs, conserved in mammals, were absent in marmoset (Supplementary Table 38). These proteins form the NAIP inflammasome in macrophages, a cytoplasmic complex that triggers macrophage inflammatory death through activation of caspase-1 (refs. 29,30) and could affect reproduction, as human NAIP is expressed in the placenta.
Other positively selected genes potentially involved in circumventing unwanted chimerism-associated responses included CD48, encoding a ligand for CD244 (2B4), which is found on the surface of hematopoietic cells and regulates natural killer cells and the levels of interleukins IL-5 and IL-12B, involved in T cell development and in allergic responses32. Finally, in contrast to the extensive family of KIR genes that are integral to immune system function in humans and other catarrhine primates, the marmoset genome contained only two KIR genes, one of which was partial.
All told, it is impossible to tell whether those instances of selection really are connected to chimerism; similar evidence of positive selection occurs for some immune system genes in every species of primate so far subjected to sequencing. Immunity evolves.
The system of genes related to inflammation is most interesting, as anything that may affect placental health would be under stronger constraints in callitrichids than in any other anthropoid. Likewise, the limited number of KIR genes is provocative, but it is not clear whether that is callitrichid-specific, or whether other New World primates may have a similar system.
UPDATE (2014-08-05): Sweeney and collegues (2012) re-examined the issue of chimerism across cell types in a small sample of marmosets and tamarins. They found much higher levels of chimerism in blood cell lineages than in other tissue types, and suggested that the mechanism of the chimerism is infiltration of the other tissues by hematopoietic tissue.
Taken together, the levels of chimerism in tissues of different origins coupled with other lines of evidence suggest that indeed only hematopoietic cell lineages are chimeric in callitrichids. The chimerism detected in other tissues is likely the result of blood or lymphocytic infiltration. Using molecular methods to detect chimerism in a tissue sample seems to have allowed a substantial increase in the ability to detect these minor cell populations.
They discussed earlier results that found chimerism in sperm, and note that this result is hard to reconcile with purely hematopoietic infiltration.
van Dijk, B. A., Boomsma, D. I., & de Man, A. J. (1996). Blood group chimerism in human multiple births is not rare. American journal of medical genetics, 61(3), 264-268.
Ross, C. N., French, J. A., & Ortí, G. (2007). Germ-line chimerism and paternal care in marmosets (Callithrix kuhlii). Proceedings of the National Academy of Sciences, 104(15), 6278-6282. doi:10.1073/pnas.0607426104
The Marmoset Genome Sequencing and Analysis Consortium. (2014). The common marmoset genome provides insight into primate biology and evolution. Nature Genetics. doi:10.1038/ng.3042
Sweeney, C. G., Curran, E., Westmoreland, S. V., Mansfield, K. G., & Vallender, E. J. (2012). Quantitative molecular assessment of chimerism across tissues in marmosets and tamarins. BMC genomics, 13(1), 98 doi:10.1186/1471-2164-13-98