Technology surrounding the human embryo has moved out of the realm of science fiction and into the reality of difficult decisions. Clinical embryologists fertilize human eggs for the purpose of helping couples conceive. The genetic makeup of these embryos are tested on a routine basis. And today, we no longer ask “can we,” but rather, “should we” edit human embryos with the goal of implantation and delivery of a baby?
As a reproductive endocrinologist, I frequently encounter couples grappling with complicated reproductive issues. If one or both parents are affected by single gene disorders, these couples have the opportunity to first test their embryos and then decide whether to transfer an embryo carrying a mutation rather than finding out the genetic risk of their baby while pregnant. In some cases they may decide not to transfer an embryo that carries the mutation as part of the in vitro fertilization procedure.
These issues seem simple, but carry large consequences for patients. “Should we transfer an embryo affected with our genetic disorder?” “What should we do with our affected embryos if we do not transfer them?” Some patients will opt to skip testing altogether.
Clinical trials of GM embryos banned in the US
The Appropriations Committee of the U.S. House of Representatives recently reinstated a ban that prevents the U.S. Food and Drug Administration from approving any clinical trial or research “in which a human embryo is intentionally created or modified to include a heritable genetic modification.” The current gene-editing ban prohibits editing the genes inside the cell’s nucleus, as Chinese scientist He Jiankui did. He used the gene-editing tool CRISPR to modify the CCR5 gene in twin girls to give them immunity from HIV.
The current ban also prohibits so-called mitochondrial replacement therapy, or three-parent babies.
Mitochondria replacement therapy, in which mitochondria carrying defective genes are replaced by healthy mitochondria from a third party is more palatable to some as mitochondrial DNA only carries a handful of genes that provide cellular energy production.
These scenarios of a three-parent baby involve transfer of the nucleus – containing the 23 chromosomes – from the egg of the mother with the defective mitochondria into an egg from which the nucleus has been removed but the healthy mitochondria remain. The actual genetic material is changed because there is DNA from two women. However, the DNA has not been cut, pasted or otherwise modified. Although testing the safety of three-parent babies will be allowed in some countries such as the United Kingdom, the U.S. ban includes this procedure.
What is germline editing?
At the heart of the issue is making genetic changes to cells that could be passed on to the next generation. These are called germline cells, and changing them is called germline editing. This brings these questions to the next level, with little information to support these heartwrenching choices.
Germline editing can happen at different phases of fertilization. If we change the genetic makeup of a human egg or sperm, fertilize it, and transfer the resulting embryo into the womb, the result is a heritable genetic modification. Similarly, genetic changes to the embryo itself within the first few days after fertilization will be inherited by the embryo’s offspring. Both of these actions are currently banned.