Could this spell the end for mosquitos?
Researchers have used CRISPR/Cas9 gene editing to target a specific gene tied to fertility in male mosquitoes.
The researchers were then able to discern how this mutation can suppress the fertility of female mosquitoes.
Mosquitoes are one of humanity's greatest nemeses, estimated to spread infections to nearly 700 million people per year and cause more than one million deaths.
As reported in the Proceedings of the National Academy of Sciences, the discovery represents a breakthrough in one technique for controlling populations of Aedes aegypti, a mosquito that transmits dengue, yellow fever, Zika, and other viruses.
Craig Montell, professor of molecular, cellular, and developmental biology at the University of California, Santa Barbara, and coauthors were working to improve a vector-control practice called the sterile insect technique (SIT). To manage populations, scientists raise a lot of sterile male insects. They then release these males in numbers that overwhelm their wild counterparts.
The idea is that females that mate with sterile males before finding a fertile one are themselves rendered infertile, thereby decreasing the size of the next generation. Repeating this technique several times has the potential to crash the population. What's more, because each generation is smaller than the last, releasing a similar number of sterile males has a stronger effect over time.
CRISPR IS A BETTER ALTERNATIVE TO CHEMICALS
SIT has proven effective in managing a number of agricultural pests, including the medfly (Mediterranean fruit fly), a major pest in California. It has also been attempted with A. aegypti mosquitoes, which originated in Africa, but have since become invasive across many parts of the world, due in no small part to climate change and global travel.
In the past, scientists used chemicals or radiation to sterilize male A. aegypti.
"There are enough genes that affect fertility that just a random approach of blasting a large number of genes will cause the males to be infertile," says Montell. However, the chemicals or radiation affected the animals' health to such an extent that they were less successful in mating with females, which undercuts the effectiveness of the sterile insect technique.
Montell figured there had to be a more targeted approach with less collateral damage. He and his colleagues, including co-first authors Jieyan Chen and Junjie Luo, set out to mutate a gene in mosquitoes that specifically caused male sterility without otherwise affecting the insects' health. The best candidate they found was b2-tubulin (B2t); mutation of the related B2t gene in fruit flies is known to cause male sterility.
Using CRISPR/Cas9, the researchers knocked out B2t in male A. aegypti. They found that the mutant males produced no sperm, but unlike in previous efforts, the sterile studs were otherwise completely healthy. There was some debate over whether sperm—albeit defective sperm from the sterile males—was needed to render female mosquitoes infertile, or whether transfer of seminal fluid was all it took.
In one experiment, the researchers introduced 15 mutant males into a group of 15 females for 24 hours. Then they swapped the B2t males for 15 wild-type males, and left them there. "Essentially, all of the females remained sterile," Montell says. This confirmed that B2t males could suppress female fertility without producing sperm.
"THERE IS A PANDEMIC EVERY YEAR FROM MOSQUITO-BORNE DISEASES."
Next the team set out to determine how timing played into the effect. They exposed the females to mutant males for different lengths of time. The scientists noticed little difference after 30 minutes, but female fertility quickly dropped after that. Montell notes that females copulated twice on average even during the first 10 minutes. This indicated to him that females have to mate with many sterile males before being rendered infertile themselves.
Combining the females with the B2t males for four hours cut female fertility to 20% of normal levels. After eight hours the numbers began leveling out around 10%.
MOSQUITO MATING BEHAVIORS
With the insights from the time trials, the team sought to approximate SIT under more natural conditions. They added different ratios of B2t and wild-type males at the same time to a population of 15 females for one week, and recorded female fertility. A ratio of about 5 or 6 sterile males to one wild-type male reduced female fertility by half. A ratio of 15 to 1 suppressed fertility to about 20%, where it leveled off.
Now, Aedes aegypti populations could easily bounce back from an 80% drop in fertility, Montell says. The success of SIT comes from subsequent, successive releases of sterile males, where each release will be more effective than the last as sterile males account for an ever-growing proportion of the population.
Montell plans to continue investigating mosquito mating behaviors and fertility. They are devising a way to maintain stocks of B2t males so they are only sterile in the wild and not in the lab. In addition, they are characterizing male mating behavior to uncover new ways to suppress mosquito populations.
"We've become very interested in studying many aspects of behavior in Aedes aegypti because these mosquitoes impact the health of so many people," says Montell, who has conducted a lot of research using fruit flies in the past. "There is a pandemic every year from mosquito-borne diseases."
"When CRISPR/Cas9 came out several years ago it just offered new opportunities to do things that you couldn't do before. So, the time seemed right to for us to start working on Aedes aegypti."
In the near-term, gene editing is not likely to be useful. Even in the long-term, it may not be very practical.
- Once perfected, gene editing is likely to be useful only under limited conditions.
- Multigenic diseases like schizophrenia and cardiovascular disease are probably too complicated to be fixed by gene editing.
- Embryo screening is a far more effective way to achieve the same objective.
The following is an adapted excerpt from the new book CRISPR People, reprinted with permission of the author.
I see no inherent or unmanageable ethical barriers to human germline genome editing. On the other hand, I see very few good uses for it. That is mainly because other technologies can attain almost all the important hoped-for benefits of human germline genome editing, often with lower risk. Two such technologies are particularly noteworthy: embryo selection and somatic genome editing.
Gene editing vs. embryo selection
The most obvious potential benefit would be to edit embryos, or the eggs and sperm used to make embryos, to avoid the births of children whose genetic variations would give them a certainty or high risk of a specific genetic disease. And here it is time to explain the ways genetic diseases or other traits get inherited. If the disease or trait depends on just one gene, we call it a Mendelian condition or trait, named after Gregor Mendel, the Austrian monk who first discovered this kind of inheritance. If more than one gene is involved, we cleverly call them non-Mendelian conditions or traits. Most of the discussion below is about Mendelian conditions for the simple reason that there is more to say about them.
Mendelian conditions can largely be put into five main categories, depending on where the relevant DNA is found and how many copies of the disease-causing variant are needed to lead to the disease: autosomal dominant, autosomal recessive, X-linked, Y-linked, or mitochondrial. Autosomal dominant diseases require only one copy of the disease-causing genetic variation; autosomal recessive diseases require two copies, one from each parent. X-linked diseases typically require two copies in women (one from each parent) but only one in men (who have only one X chromosome, always inherited from the mother). Y-linked diseases, which are unusual, are found only in men and require only one copy — because only men have a Y chromosome and normally they have only one copy of it. Mitochondrial diseases are inherited only from the mother and any mother with the disease will necessarily pass it on to all her children.
Why take the new, riskier — and, to many people, disconcerting — path of gene editing rather than just selecting embryos?
So, if an embryo has 47 CAG repeats in the relevant region of its huntingtin gene, it is doomed (if born) to have autosomal dominant Huntington's disease. One might use germline editing to reduce those 47 repeats to a safe number, of under 37, and thus prevent the disease. Or if an embryo has two copies of the genetic variation for the autosomal recessive Tay-Sachs disease, it could be edited so that the embryo had one or no copies and would be safe. The same is true of X-linked, Y-linked, or mitochondrial diseases.
If this is safe and effective, it may make sense. But another technology that has been in clinical practice for about 30 years is known to be (relatively) safe and effective and can do the same thing — PGD [preimplantation genetic diagnosis]. PGD involves taking one or a few cells from an ex vivo embryo, testing the DNA in those cells, and using the results to determine whether or not to transfer that particular embryo to a woman's uterus for possible implantation, pregnancy, and birth. The first PGD baby was born in 1990. In 2016, the last year for which data are available, the U.S. Centers for Disease Control and Prevention (CDC) reported that about 22 percent of the roughly 260,000 IVF cycles performed that year in the United States involved PGD (or a version called preimplantation genetic screening, or PGS). That was up from about 5 percent the year before. Anecdotally, from conversations with people working in IVF clinics, it sounds as though PGD or PGS usage in 2019 may well be above 50 percent, at least in some areas of the United States.
If a couple wants to avoid having a child with a nasty Mendelian genetic disease or condition, they could, in a decade or more, use CRISPR or other gene-editing tools to change an embryo's variants into a safer form or, today, they could use PGD to find out which embryos carry, or do not carry, the dangerous variants. For an autosomal recessive condition, on average 25 percent of the embryos will be affected; for an autosomal dominant one, 50 percent will be. Even for dominant conditions, if one looks at 10 embryos, the chance that all 10 will have the "bad" version is one in 1,024. If you have 20 embryos to examine, it becomes one in 1,048,576.
So, why take the new, riskier — and, to many people, disconcerting — path of gene editing rather than just selecting embryos?
Gene editing in somatic cells vs. germline cells
Somatic cell therapy does not change the germline, and it comprises a technology much closer to being shown safe and effective than human germline genome editing. Arguably, the fact that the change is only being made in one or a few of the many tissues of the body would improve its safety over a change that exists in every cell, including cells where a particular off-target change has harmful effects.
On the other hand, genome editing of an egg, a sperm, or a zygote needs to change only one cell. This might prove more effective than changing, say, 100 million blood-forming stem cells or several billion lung cells. Furthermore, somatic cell editing would not necessarily work for all conditions. For some, too many different cells or tissues may have to be targeted. For others, the damage may begin before birth, or even before the stage of fetal development where in utero somatic editing becomes plausible. For diseases with very early consequential effects, somatic cell therapy may be inferior to embryo editing or embryo selection.
Even when somatic editing is possible, human germline genome editing retains one advantage: the process would not have to be repeated in the next generation. If somatic editing is used, that person would still have eggs or sperm that could pass on the disease. If she or he wanted to avoid a sick child, PGD or somatic cell gene therapy might be necessary. If germline editing is used, that child's children will be free from the risk of inheriting the disease from their edited parents. But is this a bug or a feature? It adds a choice — not a choice for the embryo that is, or isn't, edited but for the parents of that embryo. Somatic cell editing continues the possibility of a disease in the next generation — but allows that generation's parents to make the decision. One might — or might not — see that as a benefit.
Gene editing in multigenic diseases
In non-Mendelian (sometimes called multigenic) diseases, no one variant plays a powerful role in causing the disease. Variations in two, or twenty, or two hundred genes may influence the condition. Collectively, those influences might be 100 percent, though the cases we know now add up to much lower certainties. We do not yet know of many good examples, though at least one paper claims to have found strong evidence of that variations of different genes, working together, increase the risk for some cases of autism. And, more generally, we know of many combinations of shared genomic regions that (slightly) increase or lower the risk for various diseases or traits in particular, studied populations. (These have led to the hot area of "polygenic risk scores," whose ultimate significance remains to be seen.)
The biggest problem with human germline genome editing for non-Mendelian conditions is that we do not know nearly enough about the conditions. We believe that many conditions are non-Mendelian, but how many genes are involved? Which genomic variations add or subtract risk? How do the effects of variations from different genes combine to create risks? In a simple world, they would be additive: if having a particular variation of one gene increases a person's risk of a disease by 10 percentage points and having a particular variation of a different gene increases that person's risk by 5 percentage points, then having both would increase the risk by 15 percent. But there is no inherent reason nature has to work that way; the combined effects may be greater or less than their sum. It is even conceivable that having two variations that each, individually, raise a person's risk might somehow lower the overall risk. We know almost nothing about the structure of these non-Mendelian, or multigenic, risks.
It is clear, though, that, in general, PGD would be much less useful for non-Mendelian diseases than for Mendelian ones. The chances of finding an embryo with "the right" set of genetic variations at five different spots along the genome will be much smaller than of finding an embryo with just one "right" variation. If the odds for any one variation are 50/50, the overall odds for any five variations in one embryo are one in 32. If gene editing could safely and effectively edit five places in an embryo's genome (or in two gametes' genomes), it could deliver the preferred outcome. On the other hand, if we can use genome editing to do that in an embryo or gamete, we may well be able to do the same in a fetus, a baby, a child, or an adult through somatic cell gene therapy — unless the condition begins to cause harm early in development, or broadly enough in the body that it needs to be delivered to all the body's cells.
Is gene editing practical?
Right now, there is no non-Mendelian condition for which we are confident we know the exact set of genes involved. Neither do we know the negative and positive effects of different combinations of genetic variants. Until these uncertainties are adequately resolved, human germline genome editing, though in theory better than PGD, will not be safe or effective enough for use. Once they are resolved, in many situations it will be no better than somatic cell genome editing, except for the possible absence of needing to hit targets in multiple tissues or cell types and the absence of a need to repeat the editing for the next generation.
Adapted from CRISPR PEOPLE: The Science and Ethics of Editing Humans by Henry Greely. Copyright 2021. Reprinted with Permission from The MIT PRESS.
The potential of CRISPR technology is incredible, but the threats are too serious to ignore.
- CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a revolutionary technology that gives scientists the ability to alter DNA. On the one hand, this tool could mean the elimination of certain diseases. On the other, there are concerns (both ethical and practical) about its misuse and the yet-unknown consequences of such experimentation.
- "The technique could be misused in horrible ways," says counter-terrorism expert Richard A. Clarke. Clarke lists biological weapons as one of the potential threats, "Threats for which we don't have any known antidote." CRISPR co-inventor, biochemist Jennifer Doudna, echos the concern, recounting a nightmare involving the technology, eugenics, and a meeting with Adolf Hitler.
- Should humanity even have access to this type of tool? Do the positives outweigh the potential dangers? How could something like this ever be regulated, and should it be? These questions and more are considered by Doudna, Clarke, evolutionary biologist Richard Dawkins, psychologist Steven Pinker, and physician Siddhartha Mukherjee.
How would the ability to genetically customize children change society? Sci-fi author Eugene Clark explores the future on our horizon in Volume I of the "Genetic Pressure" series.
- A new sci-fi book series called "Genetic Pressure" explores the scientific and moral implications of a world with a burgeoning designer baby industry.
- It's currently illegal to implant genetically edited human embryos in most nations, but designer babies may someday become widespread.
- While gene-editing technology could help humans eliminate genetic diseases, some in the scientific community fear it may also usher in a new era of eugenics.
Imagine it's 2045. You start hearing rumors from your well-heeled friends about a mysterious corporation based on an undisclosed island that's offering an unprecedented service: the ability to genetically design your baby.
The baby will have some of your genetics, and some genetics from a sperm or egg donor, selected by you. But the rest of your child's genetic profile will be engineered by science. These changes will make it impossible for your child to develop genetic diseases. They'll also allow you to customize your child for dozens of traits, including intelligence level, emotional disposition, sexual orientation, height, skin tone, hair color, and eye color, to name a few.
This raises unsettling philosophical questions for some customers. "When does my child stop being my child?" they ask the corporate representatives. These wary customers are reminded of how risky it is to reproduce the old-fashioned way. The Better Genetics Corporation's motto sums it up: "Only God plays dice—humans don't have to."
This is the world described in a new science-fiction series by Eugene Clark titled "Genetic Pressure", which explores the moral and scientific implications of a future in which designer babies are becoming a major industry. The first book begins with the story of Rachel, a renowned horse breeder who befriends a billionaire client, and soon gets the funding to visit the tropical island on which the Better Genetics Corporation is headquartered.
There, corporate executives walk her through the process of designing a baby—an experience that feels like an uncanny mix between visiting a doctor and designing a luxury car. The series is told from multiple perspectives, serving as a deep dive into a complex moral web that today's scientists may already be weaving.
[T]he introduction of designer babies would create a labyrinth of philosophical dilemmas that society is only beginning to explore.
Case in point: In 2018, Chinese scientist He Jiankui announced that he had helped create the world's first genetically engineered babies. Using the gene-editing tool CRISPR on embryos, He Jiankui modified a gene called CCR5, which enables HIV to enter and infect immune system cells. His goal was to engineer children that were immune to the virus.
It's unclear whether he succeeded. But what's certain is that the experiment shocked the international scientific community, which generally agreed that it's unethical to conduct gene-editing procedures on humans, given that scientists don't yet fully understand the consequences.
"This experiment is monstrous," Julian Savulescu, a professor of practical ethics at the University of Oxford, told The Guardian. "The embryos were healthy. No known diseases. Gene editing itself is experimental and is still associated with off-target mutations, capable of causing genetic problems early and later in life, including the development of cancer."
Importantly, He Jiankui wasn't treating a disease, but rather genetically engineering babies to prevent the future contraction of a virus. These kinds of changes are heritable, meaning the experiment could have major downstream effects on future generations. So, too, would a designer-baby industry, even if scientists can do it safely.
With major implications on inequality, discrimination, sexuality, and our conceptions of life, the introduction of designer babies would create a labyrinth of philosophical dilemmas that society is only beginning to explore.
Tribalism and discrimination
One question the "Genetic Pressure" series explores: What would tribalism and discrimination look like in a world with designer babies? As designer babies grow up, they could be noticeably different from other people, potentially being smarter, more attractive and healthier. This could breed resentment between the groups—as it does in the series.
"[Designer babies] slowly find that 'everyone else,' and even their own parents, becomes less and less tolerable," author Eugene Clark told Big Think. "Meanwhile, everyone else slowly feels threatened by the designer babies."
For example, one character in the series who was born a designer baby faces discrimination and harassment from "normal people"—they call her "soulless" and say she was "made in a factory," a "consumer product."
Would such divisions emerge in the real world? The answer may depend on who's able to afford designer baby services. If it's only the ultra-wealthy, then it's easy to imagine how being a designer baby could be seen by society as a kind of hyper-privilege, which designer babies would have to reckon with.
Even if people from all socioeconomic backgrounds can someday afford designer babies, people born designer babies may struggle with tough existential questions: Can they ever take full credit for things they achieve, or were they born with an unfair advantage? To what extent should they spend their lives helping the less fortunate?
Sexuality presents another set of thorny questions. If a designer baby industry someday allows people to optimize humans for attractiveness, designer babies could grow up to find themselves surrounded by ultra-attractive people. That may not sound like a big problem.
But consider that, if designer babies someday become the standard way to have children, there'd necessarily be a years-long gap in which only some people are having designer babies. Meanwhile, the rest of society would be having children the old-fashioned way. So, in terms of attractiveness, society could see increasingly apparent disparities in physical appearances between the two groups. "Normal people" could begin to seem increasingly ugly.
But ultra-attractive people who were born designer babies could face problems, too. One could be the loss of body image.
When designer babies grow up in the "Genetic Pressure" series, men look like all the other men, and women look like all the other women. This homogeneity of physical appearance occurs because parents of designer babies start following trends, all choosing similar traits for their children: tall, athletic build, olive skin, etc.
Sure, facial traits remain relatively unique, but everyone's more or less equally attractive. And this causes strange changes to sexual preferences.
"In a society of sexual equals, they start looking for other differentiators," he said, noting that violet-colored eyes become a rare trait that genetically engineered humans find especially attractive in the series.
But what about sexual relationships between genetically engineered humans and "normal" people? In the "Genetic Pressure" series, many "normal" people want to have kids with (or at least have sex with) genetically engineered humans. But a minority of engineered humans oppose breeding with "normal" people, and this leads to an ideology that considers engineered humans to be racially supreme.
Regulating designer babies
On a policy level, there are many open questions about how governments might legislate a world with designer babies. But it's not totally new territory, considering the West's dark history of eugenics experiments.
In the 20th century, the U.S. conducted multiple eugenics programs, including immigration restrictions based on genetic inferiority and forced sterilizations. In 1927, for example, the Supreme Court ruled that forcibly sterilizing the mentally handicapped didn't violate the Constitution. Supreme Court Justice Oliver Wendall Holmes wrote, "… three generations of imbeciles are enough."
After the Holocaust, eugenics programs became increasingly taboo and regulated in the U.S. (though some states continued forced sterilizations into the 1970s). In recent years, some policymakers and scientists have expressed concerns about how gene-editing technologies could reanimate the eugenics nightmares of the 20th century.
Currently, the U.S. doesn't explicitly ban human germline genetic editing on the federal level, but a combination of laws effectively render it illegal to implant a genetically modified embryo. Part of the reason is that scientists still aren't sure of the unintended consequences of new gene-editing technologies.
But there are also concerns that these technologies could usher in a new era of eugenics. After all, the function of a designer baby industry, like the one in the "Genetic Pressure" series, wouldn't necessarily be limited to eliminating genetic diseases; it could also work to increase the occurrence of "desirable" traits.
If the industry did that, it'd effectively signal that the opposites of those traits are undesirable. As the International Bioethics Committee wrote, this would "jeopardize the inherent and therefore equal dignity of all human beings and renew eugenics, disguised as the fulfillment of the wish for a better, improved life."
"Genetic Pressure Volume I: Baby Steps" by Eugene Clark is available now.
Researchers from the University of Toronto published a new map of cancer cells' genetic defenses against treatment.
There's a great diversity among cancer cells, though many of them share one unfortunate trait: They're often quite adept at resisting the body's immune system. The immune system's T killer cells can theoretically take out cancer cells, and immunotherapies enhance existing T cells' potency. Still, cancer cells are often impervious to them, can mutate to evade them, and worst of all, can acquire "cancer resistance mutations" that cause the disease to worsen in response to T cells. This is especially true of the cells in solid tumors.
A study from researchers at University of Toronto catalogs the genes in cancer tumors that allow the disease to so effectively resist immunotherapy. Its authors hope that their findings will eventually lead to the development of more successful cancer treatments.
The study is published in the journal Nature.
A moving target
Credit: Marcelo Leal/Unsplash
Speaking to U of T News, lead author of the study molecular geneticist Jason Moffat of the university's Donnelly Centre for Cellular and Biomolecular Research says, "Over the last decade, different forms of immunotherapy have emerged as really potent cancer treatments, but the reality is that they only generate durable responses in a fraction of patients and not for all tumor types."
There can be a significant degree of heterogeneity between cancer cells from human to human, and even within the same person, making the development of therapies maddeningly difficult. Attempting to address potential cancer-cell vulnerabilities across these variations is a life-or-death game of whack-a-mole.
"It's an ongoing battle between the immune system and cancer, where the immune system is trying to find and kill the cancer whereas the cancer's job is to evade that killing," says Moffat.
Mapping the mechanisms
Illustration: genes (red, green, and blue spots within the nuclei of HeLa cells) artificially superimposed on images of multi-well plates.
Credit: National Cancer Institute/Unsplash
Moffat and his colleagues decided to investigate and identify genes within cancer cells that help them defeat treatment. Co-lead author Keith Lawson of Moffat's lab explains that "it's important to not just find genes that can regulate immune evasion in one model of cancer, but what you really want are to find those genes that you can manipulate in cancer cells across many models because those are going to make the best therapeutic targets."
To accomplish this, the researchers, working with scientists at Agios Pharmaceuticals in Cambridge, Massachusetts, first exposed cells from breast, colon, kidney and skin cancer tumors to T cells in lab dishes. This established a baseline of their responses to treatment. Next, using CRISPR, the scientists went through the cells, exhaustively turning off one gene at a time to determine its role in immunotherapy resistance by comparing the cells' response to the T cells compared to their original baseline response.
The team identified 182 "core cancer intrinsic immune evasion genes" that affected the cells' response to T cells. The fact that some of the identified genes were already known to be involved in resistance provided the researchers with some confidence that they were on the right track.
Still, many of the genes they ID'ed had not been previously implicated. "That was really exciting to see because it means that our dataset was very rich in new biological information," says Lawson.
Unfortunately, Moffat's research also makes clear that defeating cancer-cell resistance is not as simple as removing certain genes. It's true that when the team switched off some of the genes they'd identified, the cancer cells became more vulnerable to T cells, but on the other hand, removal of some other genes made the cancer cells more resistant.
There also appear to be relationships between multiple genes that complicate matters.
The team explored the manipulation of the genes that allow cancer cells to engage in autophagy, the process by which cells clear out no-longer useful materials to facilitate speedy recovery from damage. Surprisingly, when the researchers deleted certain genes responsible for cancer cells' autophagy, they found the cells' resistance to T cells increased. Apparently, removing one autophagy gene strengthened another mutated autophagy gene.
"We found this complete inversion of gene dependency," said Moffat. "We did not anticipate this at all. What it shows us is that genetic context — what mutations are present — very much dictates whether the introduction of the second mutation will cause no effect, resistance or sensitivity to therapy."
There remains a long road ahead when it comes to unraveling cancer cells' resistance to immunotherapy. However, this new study presents a new map that can help scientists navigate what comes next.