Do we dare edit the human genome?
Open questions in the age of CRISPR
Ready or not, we are moving toward a future where we can intelligently design the genetic makeup of our children. At the conclusion of the Second International Summit on Human Genome Editing, the US Academies of Sciences and Medicine issued a statement.
“The organizing committee concludes that the scientific understanding and technical requirements for clinical practice remain too uncertain and the risks too great to permit clinical trials of germline editing at this time. Progress over the last three years and the discussions at the current summit, however, suggest that it is time to define a rigorous, responsible translational pathway toward such trials.” November 29, 2018. Click to view statement in full.
There you have it. A panel of experts are coming up with standards to minimize risk as we edge toward clinical trials of gene editing in the human germline (eggs, sperm, and embryos). It is just a matter of time until edited germ cells are routinely allowed to develop into living human beings who can pass on those genetic changes to their own children. It has already happened once, against the consensus of the scientific community.
Why would we want to do this? Gene editing has the potential to eradicate genetic disease. Our genes produce the proteins that make our cells work, and by extension our tissues and bodies. Since the advent of automated genome sequencing methods less than thirty years ago, scientists have learned an incredible amount about what gene sequences produce functional proteins that make healthy bodies, and have identified thousands of mutations that cause a protein to fail, resulting in disease and impairment in humans (see OMIM). Gene editing can correct those mutations.
The gene editing tool of choice is CRISPR-Cas9. Jennifer Doudna and Emmanuelle Charpentier are the scientists credited with identifying the essential molecular players of the CRISPR-Cas9 system and refining it for use as a biotechnology to precisely cut both strands of a gene’s DNA at a desired location. They published their discovery in 2012, including the following closing line:
“We propose an alternative methodology based on RNA-programmed Cas9 that could offer considerable potential for gene-targeting and genome-editing applications.” Published in Science, Aug 17, 2012. Click to view article in full.
In this one sentence, Doudna and Charpentier recognize the potential for scientists and doctors to target any gene they want, in any species.
How does CRISPR-Cas9 work?
The discovery of CRISPR is fascinating but I will not get into it here because it complicates things and is not important for the biotechnology application of CRISPR-Cas9. If interested, a good starting point is to watch the above talk by Doudna.
Here is all you need to know. If this is still confusing please ask questions in the comments and I will explain further.
Cas9 is a DNA endonuclease protein that cuts both strands of DNA. The location of the cut is determined by two RNAs (called crRNA and tracrRNA) that guide Cas9 to a specific sequence in the DNA. Doudna and Charpentier simplified nature’s approach by combining the two guide-RNAs into a single guide-RNA (a crRNA-tracrRNA chimera).
To make the guide-RNA target a desired sequence of DNA, all one has to do is look at the DNA sequence of the gene of interest and determine a short complementary sequence of RNA. That short RNA sequence is then added to the guide-RNA. Once introduced into a cell, the guide-RNA will lead the Cas9 protein to the complementary DNA sequence where it will create a double-strand break.
Cells repair double-strand breaks in the DNA using a process called homologous recombination. If left alone, DNA cut by CRISPR-Cas9 will use homologous recombination to reconnect the loose ends. Alternatively, a donor DNA sequence of your choosing can be added to the cell. During homologous recombination, the donor DNA will be incorporated at the site of the cut.
Using just three molecular ingredients — Cas9, a guide-RNA designed to target your gene of interest, and donor DNA designed to produce a functional, healthy protein — CRISPR-Cas9 can be used to correct disease-related mutations.
Why wouldn’t we use CRISPR-Cas9 to cure disease?
The idea of gene therapy, where you introduce a healthy version of a gene into a diseased individual, is not new, but CRISPR-Cas9 is the first biotechnology that is easy, adaptable, and cheap enough to make it feasible. Already being used in research labs across the globe to study both fundamental and disease related biology in many species, CRISPR-Cas9 is making its way into our crops, into the clinic, and into human embryos.
Since 2012, Doudna has become a leader in the field and has used her influence to initiate discussions about using this technology responsibly. She has organized multiple meetings for this purpose, inviting scientists, policy makers, and ethicists to weigh in. Among the concerns identified, the biggest open question is this:
Should we allow heritable gene editing in humans?
A major challenge of using the CRISPR-Cas9 system to cure disease in children and adults is getting those three essential ingredients into all of the diseased cells. Initial clinical trials will likely focus on CRISPR-Cas9 therapies to treat diseases of the blood or other cell types and tissues that are easy to access.
The delivery challenge is not an issue when editing the germline. One simply microinjects the CRISPR-Cas9 system into a single cell where it will edit the gene of interest. That gene-edited cell will then replicate and divide to make every cell in the body. The ease of introducing genetic changes to every cell makes editing the germline tempting. Why wait to develop better approaches for CRISPR-Cas9 delivery into diseased individuals when we could simply repair the disease-creating gene before disease manifests? Even more incredible, editing the germline makes genetic changes heritable, freeing all future generations of the gene edited individual’s offspring from disease.
So should we use CRISPR-Cas9 to edit genes in the human germline, where any genetic changes would be passed along to subsequent generations? As mentioned, heritable changes could eradicate many genetic diseases. Why wouldn’t we use CRISPR-Cas9 to reduce human suffering?
For starters, CRISPR-Cas9 and our understanding of gene function are imperfect, so heritable gene editing will likely have off-target effects and unintended consequences. There have been some alarming results in gene edited farm animals. There are also many ethical considerations (see some of the open questions below). All of these issues require further research and discussion before we can make a decision about how to proceed.
Beyond organizing meetings of experts, Doudna wants to educate the public and encourage our input, since this decision affects everyone. Her book, “A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution,” written with co-author and former PhD student Samuel Sternberg, is one of her efforts to bring a wider population into this urgent discussion.
In her book Doudna is not naïve. She knows it is just a matter of time before people start using this technology in human beings and ultimately in a heritable way. She also knows that gene editing will not be limited to disease prevention. Before long people will try to mutate genes in order to engineer desirable traits or enhance human abilities. Doudna’s goal is to at least try to get people thinking and talking about the consequences that may come of this, so that we may make better decisions for our collective future.
So let’s discuss, shall we? I will start by listing some of the potential problems and solutions of human gene editing as presented in the book.
Problem: We don’t want another Hitler.
Potential Solution: Gene editing must be the choice of the individual. Governments and insurance companies must not regulate or coerce us into changing our genes in the way they have decided is best.
Sub-problem: Even if we maintain our individual rights to choose our own genetics, we may fragment into groups that judge others’ choices, creating tensions and possibly discrimination and violence.
Potential Solution: You’ve got me. Tribalism is one of our worst qualities. Any ideas?
Problem: If we edit the genes of our children, we are taking away their right to choose.
Potential Solution: Gene editing must not be heritable.
Sub-problem: But what if our child has a disease and will die before they are old enough to choose otherwise?
Potential Solution: Gene editing of a minor (but not germ cells) can be allowed in this case only.
Problem: Without equal access to gene editing technology, only some of us will be able to change our genes, further dividing humanity along both economic and genetic lines.
Potential Solution: If the edits are not allowed to be heritable, the divide will be restricted to one generation. Doudna believes that with time, this technology will become accessible to all, similar to genome sequencing. Universal health care and covering the costs of disease-related genetic corrections would be one way to smooth things out in the interim, but I can’t currently imagine that happening in the US.
Problem: It is arrogant to think we understand biology well enough to intelligently design ourselves. There will certainly be unintended consequences.
Potential Solution: Further study is needed before this technology is used in human beings. That said, we will never know enough and will ultimately be taking a gamble. If we initially limit ourselves to using this technology as a therapy for disease in a way that is not heritable, we will limit our risks.
Only fools rush in
Based on my potential solutions above, it appears my personal instinct is that heritable germline editing should remain off limits for a lot longer. I would be much more comfortable with gene editing of targeted cell types in diseased individuals. Better gene therapy delivery methods are needed whether or not CRISPR turns out to be the fix all, so I think that is the most important area to focus on for good clinical outcomes.
In the meantime, researchers should continue using germline editing as a tool in the non-human plant and animal kingdom and observing its effects through many generations. The results so far reveal how little we understand gene function, genetic interactions, and compensation in complex systems. That is precisely why this research is important, and, in my mind, necessary before proceeding with human germline editing.
I am surprised by my caution against rushing to edit our genes. I am as excited as anyone by a future where human beings can live longer, suffer less, and have enhanced abilities. That future could be ours, but we must be patient.
I have no doubt that human germline editing will happen eventually, “like a river flows surely to the sea.” If we are wise and practice caution, we may learn to wield this technology in a way that benefits us all.
Wise men say only fools rush in
but I can’t help falling in love with you
Shall I stay would it be a sin
if I can’t help falling in love with you
Like a river flows surely to the sea
darling so it goes some things are meant to be
(Can’t Help Falling in Love, by Elvis Presley)
That is my view, but what do you think? Please comment with any thoughts or references you believe would aid the discussion. What are your biggest concerns about heritable gene editing in humans?
Thanks for reading
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