Genetic Chimeras: From Myth to Real Life

 Figure 1: Depiction of a mythical Chimera (Greek Mythology)

Creation

The Scientist recently reported that scientists are creating human-animal chimeras in various experiments. Humans have been fascinated by such creations since ancient times. In general, a chimera is an organism composed of cells that originate from at least two different species. Typically they're made by injecting stem cells from one species into a developing fetus or embryo of a different species. Each chimera cell contains the genetic material of either one parent species or the other, but not both.

Chimeras aren't the same as hybrids, which are bred from two parents of different species (e.g., a donkey and a horse can mate to make a mule), and therefore contain both species' genes in all cells. Hybrids can be multivariate and may contain a varying percentage of genetic material from both species in their cells. Hybrids can often come about without any human intervention, whereas chimeras require it.

Chimeras are also different from the condition of chimerism that is found in humans and other animals. Chimerism refers to an individual with two complete genomes from the same species/parents. This occurs spontaneously, most often when a person absorbs their twin while in utero. Chimerism is incredibly rare, as only around 100 cases have been recorded in all of modern medical literature.

Figure 2: Diagram of the creation of a chimeric organism using induced pluripotent stem cells, or iPSCs. Source

Although people have talked about chimeras for a long time, the technologies required are still in their infancy. In the 1960s, researchers tried unsuccessfully to create a quail-chicken chimera. This research eventually found success in 1973 with the creating of a mouse-rat chimera, although many of those created did not live to adulthood. The first chimera that fully survived to adulthood was a goat-sheep chimera created in 1984 (see Figure 3). Despite the strides that chimera creation methods have made, we have still only scratched the surface of the potential uses of chimeras for humans.

Figure 3:  A picture of one of the earliest successful chimeras, a goat-sheep chimera

Uses for Chimeras

Many if not all of the projected uses for chimeras are found in medical fields. For example, in various current studies, scientists are investigating the transplant of functioning human organs from chimeras.

Other chimera-focused medical studies include pig-human, sheep-human, rabbit-human, and monkey-human chimeras, and their applications in organ transplant, stem cell research, and the study of neurological diseases.  

Figure 4: Diagram of how organ transplant between humans and livestock may be undertaken. Source

Opposition to Creating Chimeras 

With all of the scientific efforts involving human chimeras, it was inevitable that moral dilemmas involving animal rights and concerns about the health and well-being of animals involved would come forward. The Guidelines of Human Embryonic Stem Cell Research, published by the National Academies, outlawed the breeding of chimeras in the United States, principally because it could create a human zygote if two chimeras with human gametes were to mate. Currently, there is no international consensus over these or any guidelines.

Many people also object to the use of human embryonic cells to help create chimeras. Multiple studies have specifically involved the growth of human brain cells inside of a chimera, for research or organ transplant. But if those animals eventually have a completely human brain, can they be counted as human? (Source)

My Takeaways

Just like any new and experimental technology, chimera creation and testing have many potential positives and pitfalls. It could be used to save many lives, but it could also establish dangerous moral precedents that we as a species may regret pursuing. Chimeras are in the realm of what scientists considered impossible 75 years ago, and show the massive progression of human science.

In my opinion, chimera creation is a technology that we as a species must be incredibly careful with. It is not enough that there are guidelines set in the United States. While many other countries also have guidelines, not all do, and some other countries' restrictions may not be as strict as those here. Until there are unified guidelines adopted by all countries I will remain concerned.

I became interested in this topic from reading some of the numerous articles about chimera studies and research published by news organizations such as The Scientist, the BBC, and CNN in recent years about chimera studies and research. I will continue to track this topic.