Into the World of Chimeras

By Yang Lin, Chemical Engineering 2018

Genetically engineering humans remains one of the most controversial topics today. Altering the germline, which is defined as the DNA that is passed on from generation to generation, is often seen among the scientific community as taboo, unethical, and “playing God.” Critics of human germline modification argue that it is changing the very genetic makeup of the human species.

However, the possibilities are tantalizing for some of the scientific community, whom argue that germline modification potentially prevents debilitating genetic disorders like sickle-cell disease, mitochondrial diseases, and Down syndrome. This is the very argument that researchers use to support further study into human chimeras.

A chimera, in terms of genetics, is any organism that contains genetically distinct cells. By that definition, a chimera can be created in multiple ways. A blood marrow transplant induces chimerism, as the patient then produces blood cells with genetic material other than that of their own.

Another form of chimerism, called tetragametic chimerism, sometimes occurs when non-identical twins share a blood supply before birth. Fascinatingly, these twins would then have two different blood types; to clarify, they are not simply blood type AB, but rather have a mix of A, B, O, or AB blood.

It has been reported by Bob A. van Dijk from the American Journal of Medical Genetics that as many as 8% of non-identical (fraternal) twins could have chimeric blood.

Another method of human chimera creation, called cytoplasm transfer, was discovered in the 1990s by the pioneering work of Dr. Jacques Cohen. In this method, the cytoplasm of a donor egg was injected directly into a host egg, and then fertilized with sperm.

Because the mitochondria, located in the cytoplasm, has its own DNA called mitochondrial DNA (mtDNA), the resulting combination produced a zygote that had mtDNA from the donor, and nuclear DNA from the host parents. In essence, this means that an organism produced by cytoplasmic transfer has three parents.

So, what benefits could cytoplasmic transfer bring? Dr. Jacques Cohen claimed that this procedure would allow parents with mitochondrial disease, a debilitating genetic disorder, to have healthy, normal children. The evidence to support the viability of cytoplasm transfer chimeras remains sparse.

Dr. Jacques Cohen reported in his book, Human Preimplantation Embryo Selection, the creation of seventeen babies by this procedure, of which one was miscarried, and another was aborted.

When the FDA shut down the practice of cytoplasm transfer in 2001, labeling it “a biological product” and therefore within their domain, the clinic of Dr. Cohen lost the funding and support to pursue follow-up studies on the fifteen other chimeras.

Human chimeras remain largely unstudied, but they may be a door to a new realm of possibilities in medicine when certain ethical questions are answered, and risks assessed.

Yet, chimeras might be more common than you think; for example, marmosets almost always give birth to fraternal twins, and studies have shown that up to 95% of marmosets have chimeric blood.

Genetics remains elusive to our understanding, and chimeras serve as a prime example of the mysteries and possibilities that they bring.