by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes frequently to Skeptic as well.
The CRISPR Complex at work.
Should biologists use new gene-editing technology to modify or “correct” the human germline? Will our methods soon prove sufficiently safe and efficient and, if so, for what purposes? Much-celebrated CRISPR pioneer, Jennifer Doudna, recently recalled her initial trepidations over that very prospect:
Humans had never before had a tool like CRISPR, and it had the potential to turn not only living people’s genomes but also all future genomes into a collective palimpsest upon which any bit of genetic code could be erased and overwritten depending on the whims of the generation doing the writing…. Somebody was inevitably going to use CRISPR in a human embryo … and it might well change the course of our species’ history in the long run, in ways that were impossible to foretell.
(Doudna and Sternberg 2017). And it didn’t take long. Just one month after Doudna and others called for a moratorium on human germline editing in the clinical setting, scientists in Junjiu Huang’s lab at Sun Yat-sen University in Guangzhou, China published a paper describing their exclusively in vitro use of CRISPR on eighty-six human embryos (Liang et al. 2015). Huang’s goal was to edit mutated beta-globin genes that would otherwise trigger a debilitating blood disorder called beta-thalassemia.
But the outcomes were mixed, at best. After injecting each embryo with a CRISPR complex composed of a guide RNA molecule, a gene-slicing Cas-9 enzyme, a synthetic repair DNA template, and a “glow-in-the-dark” jellyfish gene that allows investigators to track their results as cells continue to divide, Huang’s team delivered a paltry five percent efficiency rate. Some embryos displayed unintended, “off-target” editing. In others, cells ignored the repair template and used the related delta-globin gene as a model instead. A third group of embryos turned mosaic, containing cells with an untidy jumble of editions. Part of the problem was that CRISPR had initiated only after the fertilized egg had begun to divide.
By using non-viable triploid embryos containing three sets of chromosomes, instead of the usual two, Huang avoided objections that he had destroyed potential human lives. Nevertheless, both Science and Nature rejected his manuscript based in part on ethical concerns. Several scientific agencies also promptly reemphasized their stances against human germline modification in viable embryos, and, in the US, the Obama Administration announced its position that the human germline should not be altered at that time for clinical purposes. Francis Collins, director of the National Institutes of Health, emphasized that the US government would not fund any experiments involving the editing of human embryos. And finally, earlier this year, a committee of the US National Academies of Sciences and Medicine decreed that clinical use of germline editing would be allowed only when prospective parents had no other opportunities to birth healthy children.
Meanwhile, experimentation continued in China, with similarly grim results. But this past August, an international team based in the US—this time led by embryologist Shoukhrat Mitalipov at the Oregon Health and Science University in Portland—demonstrated that, under certain circumstances, genetic defects in human embryos can, in fact, be efficiently and safely repaired (Ma et al. 2017).
Embyologist Shoukhrat Mitalipov.
Mitalipov’s group attempted to correct an autosomal dominant mutation—where a single copy of a mutated gene results in disease symptoms—of the MYBPC3 gene. Crucially, such mutations are responsible for an estimated forty percent of all genetic defects causing hypertrophic cardiomyopathy (HCM), along with ample portions of other inherited cardiomyopathies. Afflicting one in every 500 adults, HCM cannot be cured, and remains the most common cause of heart failure and sudden death among otherwise healthy young athletes. These mutations have escaped the pressures of natural selection, unfortunately, due to the disorder’s typically late onset—that is, following reproductive maturity.
Prospective parents can, however, prevent HCM in their children during the in vitro fertilization/preimplantation genetic diagnosis (IVF/PGD) process. Where only one parent carries a heterozygous mutation, fifty percent of the resulting embryos can be diagnosed as healthy contenders for implantation. The remaining unhealthy fifty percent will be discarded. As such, correction of mutated MYBPC3 alleles would not only rescue the latter group of embryos, but improve pregnancy rates and save prospective mothers—especially older women with high aneuploidy rates and fewer viable eggs—from risks associated with increasing numbers of IVF/PGD cycles as well
With these critical facts in mind, Mitalipov and colleagues employed a CRISPR complex generally similar to that used by Huang. It included a guide RNA sequence, a Cas-9 endonuclease, and a synthetic repair template. In one phase of their investigation, the team fertilized fifty-four human oocytes (from twelve healthy donors) with unhealthy sperm carrying the MYBPC3 mutation (from a single donor), and injected the resulting embryos eighteen hours later with the CRISPR complex. The result? Thirteen treated embryos became jumbled mosaics.
Mitalipov changed things up considerably, however, in the study’s second phase by delivering the complex much earlier than he and others had done in previous experiments—indeed, at the very brink fertilization. More precisely, his colleagues injected the CRISPR components along with the mutated sperm cells into fifty-eight healthy, “wild-type” oocytes during metaphase of the second meiotic division. Here, the results were impressive, to say the least. Forty-two treated embryos were normalized, carrying two copies of the healthy MYBPC3 allele—a seventy-two percent rate of efficiency, no “off-target effects” were detected, and only one embryo turned mosaic.
Mosaicism and Off-target Effects.
Mitalipov’s team achieved a genuine breakthrough in terms of both efficacy and safety. Perhaps nearly as interesting—and, in fact, the study’s primary finding, according to the authors—is that, in both experimental phases, the embryos consistently ignored Mitalipov’s synthetic repair template and turned instead to the healthy maternal allele as their model. Such is not the case when CRISPR is used to edit somatic (body) cells, for example. Apparently, the team surmised, human embryos evolved an alternative, germline-specific DNA repair mechanism, perhaps to afford the germline special protection.
The clinical implications of this repair preference are profound and, at least arguably, very unfortunate. First, with present methods, it now appears unlikely that scientists could engineer so-called “designer babies” endowed with trait enhancements. Second, it seems nearly as doubtful that CRISPR can be used to repair homozygous disease mutations where both alleles are mutant. Nevertheless, Mitalipov’s method could be applied to more than 10,000 diseases, including breast and ovarian cancers linked to BRCA gene mutations, Huntington’s, cystic fibrosis, Tay-Sachs, and even some cases of early-onset Alzheimer’s.
At least in theory. As of this writing, Mitalipov’s results have yet to be replicated, and even he warns that, despite the new safety assurances and the remarkable targeting efficiencies furnished by his most recent work, gene-editing techniques must be “further optimized before clinical application of germline correction can be considered.” According to stem-cell biologist George Daley of Boston Children’s Hospital, Mitalipov’s experiments have proven that CRISPR is “likely to be operative,” but “still very premature” (Ledford 2017). And while Doudna characterized the results as “one giant leap for (hu)mankind,” she also expressed discomfort with the new research’s unmistakable inclination toward clinical applications (Belluck 2017).
Indeed, within a single day of Mitalipov’s report, eleven scientific and medical organizations, including the American Society of Human Genetics, published a position statement outlining their recommendations regarding the human germline (Ormond et al. 2017). Therein, the authors appeared to encourage not only in vitro research but public funding as well. Although they advised against any contemporary gene-editing process intended to culminate in human pregnancy, but also suggested that clinical applications might proceed in the future subject to a compelling medical rationale, a sufficient evidence base, an ethical justification, and a transparent and public process to solicit input.
And of course researchers like Mitalipov will be forced to contend with those who claim that, regardless of purpose, the creation and destruction of human embryos is always ethically akin to murder (Mitalipov destroyed his embryos within days of their creation). But others have lately expressed even less forward-thinking and, frankly, even more irrational and dangerous sentiments.
For example, a thoroughly galling article I can describe further only as “pro-disability” (In stark contrast to “pro-disabled”) was recently published, surprisingly to me, in one of the world’s most prestigious science publications (Hayden 2017). It begins by describing a basketball game in which a nine-year-old girl, legally blind due to genetic albinism, scored not only the winning basket, but, evidently—through sheer determination—all of her team’s points. Odd, perhaps, but great! So far.
But the story quickly turns sour, to the girl’s father who apparently had asked the child, first, whether she wished she would have been born with normal sight, and, second (excruciatingly), whether she would ever help her own children achieve normal sight through genetic correction. Unsurprisingly, the nine-year-old is said to have echoed what we then learn to be her father’s heartfelt but nonetheless bizarre conviction: “Changing her disability … would have made us and her different in a way we would have regretted,” which to him, would be “scary.”
To be fair, the article very briefly appends counsel from a man with Huntington’s, for instance, who suggests that “[a]nyone who has to actually face the reality … is not going to have a remote compunction about thinking there is any moral issue at all.” But the narrative quickly circles back to a linguist, for example, who describes deaf parents who deny both their and their children’s disabilities and have even selected for deafness in their children through IVF/PGD, and a literary scholar who believes that disabilities have brought people closer together to create a more inclusive world (much as some claim Western terrorism has). The author then laments the fact that, due to modern reproductive technology, fewer children are being born with Down’s syndrome.
To summarize, according to one disabilities historian, “There are some good things that come from having a genetic illness.” Uh-huh. In other words, disabilities are actually beneficial because they provide people with challenges to overcome—as if relatively healthy people are incapable of voluntarily and thoughtfully designing both mental and physical challenges for themselves and their kids.
I think not. Disabilities, by definition, are bad. And, as even a minimally compassionate people, if we possess a safe and efficient technological means of preventing blindness, deafness, or any other debilitating disease in any child or in any child’s progeny, we also necessarily have an urgent ethical obligation to use them.
References:
Belluck, P. 2017. In breakthrough, scientists edit a dangerous mutation from genes in human embryos. Available online at https://nyti.ms/2hnZ9ey; accessed August 9, 2017.
Doudna, J.A., and S.H. Sternberg. 2017. A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution. Boston: Houghton Mifflin Harcourt.
Hayden, E.C. 2017. Tomorrow’s children. Nature 530:402-05.
Ledford, H. 2017. CRISPR fixes embryo error. Science 548:13-14.
Liang, P., Y. Xu, X. Zhang, et al. 2015. CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes. Protein and Cell 6(5):363-72.
Ma, H., N. Marti-Gutierrez, S. Park, et al. 2017. Correction of a pathogenic gene mutation in human embryos. Nature DOI:10.1038/nature23305.
Ormond, K.E., D.P. Mortlock, D.T. Scholes, et al. 2017. Human germline genome editing. The American Journal of Human Genetics 101:167-76.
The Doting Skeptic 