There have been concerns about the idea of “designer babies” for almost as long as in vitro fertilization and technology to screen embryos for inherited disorders have existed.
While the recent live births resulting from human embryonic CRISPR editing have heightened global attention to these issues, currently, the most practical approach to genetic “enhancement” of embryos is preimplantation genetic screening of IVF embryos.
But according to a study publishing November 21 in the journal Cell, the ability to select for traits that are brought about by multiple genes–rather than genetic diseases caused by a single mutation–is more far off and complicated than most people probably realize.
“The ability to do genomic sequencing of embryos is much easier than it was even five years ago, and we know many more gene variants linked to certain traits,” says co-corresponding author Shai Carmi, of the Hebrew University of Jerusalem.
“But selecting embryos for particular traits is very controversial except when it relates to a serious disease like cystic fibrosis. It raises many issues related to eugenics and unequal opportunities.”
Carmi’s team looked at the feasibility of selecting embryos based on each of two traits caused by multiple genes–IQ and height–as a kind of thought experiment. While there are many traits determined by multiple genes that the researchers could have investigated, they chose to focus on IQ because it is frequently brought up in concerns regarding eugenics and on height because it is objectively measurable and a lot is known about the complex genetics influencing height.
Their findings suggest that our current knowledge of the genetics of these types of traits may not be enough to generate a substantial increase in the desired traits in an IVF embryo selection scenario.
In the study, the researchers ran computer simulations using genomic sequences from real people to model genomic profiles of hypothetical embryos that would result from pairs of those people–some actual couples and some artificially paired. In the simulations, they assumed that each couple would have ten embryos to choose from.
They then predicted the IQ or adult height for each of the offspring based on the gene variants present in the genomes of the simulated embryos.
Their experiments were based on the assumption that the embryo with the top score could then be selected for implantation.
They found that expected advantages to these theoretical offspring would be relatively small. For IQ, the most it increased above the average of the embryos was three points. For height, the most it increased above the average was three centimeters.
And even if some people might believe that those increases were great enough to warrant using the technology, they are not guaranteed.
“There is much about these traits that is unpredictable,” Carmi says.
“If someone selected an embryo that was predicted to have an IQ that was two points higher than the average, this is no guarantee it would actually result in that increase. There is a lot of variability that is not accounted for in the known gene variants.”
There are several other limitations, Carmi notes, that would make it challenging to accurately select embryos for desired traits.
For one, the researchers conducted their simulations using ten embryos from each couple, but in reality, many couples get far fewer viable embryos when they do in vitro fertilization. For example, with five embryos, the gain would be reduced to 2.5 IQ points or 2.5 cm.
When they based the simulation on 50 or 100 embryos, they found that the benefit per embryo decreased as the number of embryos increased, indicating diminishing returns even with large numbers of hypothetical embryos to choose from.
Carmi’s team looked at the feasibility of selecting embryos based on each of two traits caused by multiple genes–IQ and height–as a kind of thought experiment. The image is in the public domain.
In addition, what is known about the gene variants linked to traits like height and IQ–as well as other health-related traits like blood pressure and cholesterol–applies mainly to people of European descent.
They would be much less applicable for people from other parts of the world. Finally, attempting to maximize more than one trait, a potential future scenario, would make embryo selection far more complicated: an embryo that ranked highest for IQ may rank lowest for height, for example.
Furthermore, the researchers used real-world data to confirm that predictions about traits made using what’s currently known about gene variants are not always accurate.
They reported on an analysis of 28 families with up to 20 children who have grown to adulthood–and found that the offspring they would have selected for having the greatest height based on gene variants was not always the tallest one in adulthood.
Funding: This research was supported by the Abisch-Frenkel Foundation, the National Institutes of Health, and the James S. McDonnell Centennial Fellowship in Human Genetics.
On 29 November 2018, at the Second International Summit on Human Genome Editing in Hong Kong, the scientist Jiankui He, of Southern University of Science and Technology of China, claimed he has created the world’s first genetically altered babies. This announcement sparked controversy and criticism and was almost universally denounced.
The Chinese Academy of Medical Sciences responded: “we are opposed to any clinical operation of human embryo genome editing for reproductive purposes in violation of laws, regulations, and ethical norms in the absence of full scientific evaluation”.1 The National Health Commission of China responded: “This illegal behavior will be verified and punished”.2 The genetic alteration of human eggs, sperm, and embryos is prohibited for germ line purposes. The relevant guidelines already exist in China. Jiankui He’s work violated those guidelines.
CRISPR/Cas9 techniques have been applied in many kinds of animals, including human cells. It is very clear that this system can be used to genetically modify the human germ line today. However, many questions remain to be answered before this technique can be used to alter the human genome for reproductive purposes. Although the intent may be to create perfect human beings, the result may be a monster.
WHAT IS CRISPR/CAS9?
Mammalian genomes contain billions of base pairs and are difficult to manipulate. With the development of homologous recombination (HR), we can precisely modify the genome, with expected outcomes. However, precise HR‐mediated alteration occurs at a very low frequency (one in 106‐109 cells).3 A series of programmable nuclease‐based genome editing tools, such as Zinc finger nucleases (ZFNs),4 transcription activator‐like effector nucleases (TALENs)5, 6, 7 and the RNA‐guided DNA endonuclease Cas9 (CRISPR/Cas9),8, 9, 10 have been developed in recent years, which enable efficient genetic modifications of many species. The ZFNs were derived from eukaryotic transcription factors,4 TALENs were derived from Xanthomonas bacteria,5, 6, 7 and CRISPR/Cas9 was derived from the type II CRISPR system.8, 9, 10 Of the current genome editing tools, the RNA‐guided Cas9 system has been developed most rapidly. This system can easily be used to target a genomic locus with a small guide RNA (sgRNA) complementary to the target DNA sequence.11, 12
CRISPRs were first reported in Escherichia coli in 1987 and are present in over 40% of sequenced bacteria and 90% of sequenced archaea.13 Currently, the type II CRISPR system, first identified as part of an adaptive immunity system that protects the hosts against invasion by plasmids and other DNA contaminants, is the most commonly used.14, 15
Since the first report of CRISPR/Cas9 techniques being used for gene targeting in mammalian cells in 2013, these techniques have been applied in many species.8, 10, 16, 17 In theory, they can be used for human germ line modification, but there are still many open questions to be solved before any attempts to apply it should be made.
Targeting difficulties
All programmable nuclease‐based editing tools work via introduction of a site‐specific DNA double strand break (DSB).4, 5, 6, 7, 8, 9, 10, 18 The DSB will stimulate DNA repair through nonhomologous end‐joining (NHEJ) and/or homologous recombination (HR)‐directed repair mechanisms. HR‐mediated repair occurs only in specific phases of the cell cycle (G2 and S), while NHEJ‐mediated repair occurs throughout the cell’s life.
NHEJ‐mediated repair is the primary damage‐mediated repair mechanism. NHEJ‐mediated repair is not an entirely accurate progress and may induce small deletions or insertions at the target sequence. In order to achieve very precise genome modification, various kinds of CRISPR‐based genome editing tools were developed including adenine base editors (ABEs), cytosine base editors3 (BE3), and so on.19, 20, 21, 22, 23 NHEJ‐mediated small deletions or insertions result in: (a) frameshifts causing a stop codon occurrence at or after DSB sites, which results in elimination of the target gene; (b) frameshifts which introduce a new amino acid strand or protein, resulting in a different protein; (c) deletion of several amino acids.
In the second situation, the newly produced amino acid or protein may be toxic and have unexpected consequences. More basic research is necessary to evaluate the safety and validity of these techniques. According to present information, the system which Jiankui He used have resulted in different base insertion and deletions. Using this technique could lead to unintended results for the organism.
Off‐target effects
The most important concern a newly developed gene editing tool must address before any kind of application is attempted is to show that there are no off‐target effects. Based on present data, the CRISPR/Cas9 system does induce off‐target mutations. And further, these mutations can be transmitted to the organism’s descendants.24, 25, 26, 27, 28, 29
While we know a lot about this technique, there are still many unknowns. Further research and development of this technology may uncover more unintended off‐target and other effects, as well as other unexpected consequences. Such off‐target mutations or other effects could lead to cancer or other diseases in the early or later life of genetically modified babies.
Mosaic issue
The CRISPR/Cas9 system may continue to work beyond one‐cell fertilized eggs and result in a mosaic genotype.10, 29, 30, 31 This means that different tissues or organs will have different genetic modifications, even within the same organism. We are still uncertain what the effects of the gene editing would be in the genome of babies.Go to:
WHICH IS THE PERFECT TARGET GENE?
In order to select the perfect target gene and an efficient target site, we need to understand that gene’s function well, and the target sgRNA should have very few or no off‐target effects. This requires a significant accumulation of knowledge, which is so far lacking. Currently, most knowledge about gene function comes from basic research, which often uses mice missing a gene of interest (called knockout mice) to understand the effects of the gene. However, whether genes function in the same way in mice and humans is still unclear, and gene function studies in humans are still in their infancy.
There is currently very little known about how gene knockout in humans will affect a person’s behavior, health, and lifespan and how it could be transmitted to their descendants. There is no effective method to evaluate those effects in human beings.
Jiankui He selected the Ccr5 gene as the target gene, with the stated purpose of preventing HIV infection. However, is this the perfect target for HIV prevention? The C‐C chemokine receptor 5 (CCR5) is a seven‐transmembrane G protein‐coupled receptor (GPCR) and is highly expressed in bone marrow‐derived cells including T cells and macrophages.32 In other tissues, CCR5 is expressed on epithelium, endothelium, vascular smooth muscle, and fibroblasts.33, 34, 35 Many studies have demonstrated that CCR5 has an important role in HIV virus infection.36, 37 CCR5 is therefore a potential target for HIV infection protection.
In another study, however, Ccr5 gene deletion also showed lupus nephritis susceptibility.38 In the central nervous system, Ccr5 is expressed on neurons, astrocytes, and microglia and functions as a suppressor for cortical plasticity and hippocampal learning and memory.35 The Ccr5 gene function in many tissues is still unclear. Deletion of this gene may result in unexpected disease.38 It is therefore a very risky target for gene editing.
ETHICAL PROBLEMS
The scientific community has already developed a broad social consensus about the application of these techniques. It strongly encourages basic research and manipulation in laboratories, but does not condone use of the technique for genetically altering human babies. We agree with the major recommendations:
Research: “Intensive” research is encouraged and should proceed, including in human germ line cells, subject to appropriate legal and ethical oversight.
Clinical use (somatic): Treating adults with gene editing therapies should proceed within existing regulatory frameworks and guidelines.
Clinical use (germ line): Gene editing for human reproductive purposes is in principle prohibited until all safety and ethical issues can be addressed.
Ongoing forum: The international community should establish “norms” and every country makes its own laws for human gene editing.
Many countries already have principles and guidelines regulating human embryo experiments. In the United States of America, use of federal funds to finance genetic modification experiments in gametes and embryos is prohibited.
In China there are already guidelines for genetic manipulation for human reproductive purposes. The guidelines including: Guiding Principles of Ethics for Human Embryonic Stem Cell Research (2003), Ethics Principles for Human Assisted Reproductive Technology and Human Sperm Bank (2003), Ethical Review Measures for Biomedical Research Involving Human Beings (2016), and Safety Management Measures for Biotechnology Research and Development (2017).1
While regulations and guidelines already exist and regulate government‐funded studies, but there are few restrictions for privately funded research. The CRISPR technique is still in the initial stages of evaluation and it is premature to consider it for clinical use, especially for reproductive purposes. In order to avoid the birth of “a second CRISPR baby,” we strongly recommend that the government should regulate clinical experiments using this technique for human reproductive purpose.
Source:
Cell Pressr
Media Contacts:
Carly Britton – Cell Press
Image Source:
The image is in the public domain.
Original Research: Closed access
“Screening human embryos for polygenic traits has limited utility”. Cell, Karavani et al.
Cell doi:10.1016/j.cell.2019.10.033.
