Population Replacement Strategies for Controlling Vector Populations and the Use of Wolbachia pipientis for Genetic Drive

So my name's Dr.Jason Rascon. I'm an assistant professor in the Johns Hopkins Bloomberg School of Public Health in the Department of Molecular Microbiology and Immunology, and also in the Johns Hopkins Malaria Research Institute. And I'm broadly interested in the relationship between the genetics of vector populations and the epidemiology of vector-borne Pathogens.

So malaria Is the most important vector-borne disease in the world today. It's a huge killer. It kills, the stats vary somewhere between one and 3 million people per year.

Most of them are children under five in Africa. So it's an enormous problem. And just in, that's just in terms of the mortality, but it also is a huge burden in terms of morbidity and public health and burdens on the economy of these, these areas where, where it's afflicted, it really is a problem.

You know, in South America and Africa and Asia, it's, it's, it's, it's a sort of a worldwide problem and it is Reemerging. So Gene drive system is what makes these strategies in theory, economically possible. So the idea is that you can essentially see the population with a very small number of modified insects.

And the mechanism, the gene drive mechanism will spread that trait through the population to a high frequency. And the way to think about it is that in a normal, let's say you've made a transgenic insect that has one copy of the gene, because these insects normally are diploid. They have two copies of every chromosome.

And if they meet with a wild type just based on random chance, you would expect 50%of the, of their offspring to have that trait. And then 50%of those offspring would have it. And so it would just get diluted out over time.

A gene drive mechanism says that by whatev, by a variety of different strat mechanisms or strategies, greater than 50%of those offspring will carry the trait. And so it actually will increase in frequency over time instead of being diluted out or staying at the Same frequency. First, We need a gene drive system.

There really is no viable gene drive system that exists outside of the laboratory. There are a few that that can work under certain conditions in very controlled laboratory environments, but we really don't have anything that's at the point where we can actually deploy this into natural field populations. And all of, there's a, a variety of different gene drive mechanisms that are being considered right now.

They all have advantages, they all have disadvantages, and so there's really no one that seems to be inherently superior to any other One. So Akia Pipi is an obligate maternal inherited bacteria. It's closely related to Rickettsia and it's exclusively transmitted for at least over evolu, over ecological time from mother to offspring.

So the mother is infected, she gives birth to infected offspring, the male's not infected and akia by a variety of different mechanisms just in nature. It's probably the most widespread endos symbiotic, endos, symbiotic bacterium on earth. It's estimated to occur in up to 75%of all known insects.

And it, it manipulates a host reproduction in such a way to maximize its transmission into the next generation. And so since males are dead end hosts for this bacterium, it does things to maximize the number of infected females. So in certain systems, it'll turn genetic males into functional females, or it'll turn females genic where they just never have to mate.

They can just have infected female offspring that have infected female offspring. And mosquitoes. Akia causes a phenomenon called cytoplasmic incompatibility.

And this is where the males, because they're dead end hosts for the bacterium, they're used as Trojan horses to decrease the fitness of uninfected females in the population. So if an infected male mates with an uninfected female, he sterilizes her. So, but infected females are fine, whether they mate with an infected or an uninfected male.

So infected females have more offspring because they're, none of their matings are sterile. Those offspring are infected, which also have more offspring. And so this can allow this bacterium to spread rapidly through populations to high frequency.

So the idea is to use this ability of akia to drive through populations as a way to push genes of interest into populations. So as the Akia spreads your gene of interest, let's say a gene for malaria, raciness would spread By genetic hitchhiking. So Akia is widespread in insects.

If you look at mosquitoes all across mosquito genera, pretty much every genera is infected. Not every species within a genera is infected, but there are some species within every genera except for an olie. There are no known natural anomolie infections.

So not anis, gambier, not any anomolie. And this is actually a major area of interest for my research group. We're trying to infect Anis and we've been able, at least to show that wbaa can infect anis gambier cells.

So in cell culture. And we're trying not to transfer from, from this in vitro cell culture system into an in vivo mosquito system, but nobody's actually done it yet. So This is relatively simple.

You have a cage of mosquitoes that don't have your gene, and you have some strain of mosquito that does, let's say it's attached to Akia or some other gene drive mechanism. You basically seed your unin, your, your naive cage population with mosquitoes from your experimental population, you at a variety of different frequencies, and you rear them up, you sample randomly every generation, and you just keep going every generation to see how the frequency of that gene changes over time. And so then you can just carry it out to a certain number of generations.

You can carry it out to your gene, reaches a, some predetermined frequency or until your gene is lost. And they're really very straightforward kinds of experiments in the laboratory to do. It's really just, you know, burying up mosquitoes every generation the way you normally Would.

So there are a lot of ethical Considerations. I think number one, we have to be sure that, you know, is it actually going to work? Is it going to be better than the strategies that are actually available right now, a lot of people are inherently mistrustful of genetically modified organisms in general.

And so we need to be able to address that. When you release genetically modified mosquitoes into, let's say country A that wants them and country B is right next door and they don't want them, those mosquitoes are not gonna respect that border. And so they're going to, if, if these gene drive mechanisms work the way we want them to work, that gene is going to spread, those mosquitoes are gonna carry that gene basically everywhere throughout the species range.

And so these sort of political, ethical, social legal issues really need to be addressed quite thoroughly. We need to make sure that we're not gonna make things worse instead of better. So we really wanna understand everything that's going on in the environment before We ever actually release them.

So again, There are, are two different categories. There are the scientific obstacles which are severe, but we're working on them. And then again, there are the social, ethical and legal obstacles.

And I, I think that over time we will be able to address the scientific issues. I mean, there is really no gene drive mechanism that that works in mosquitoes that would be applicable. There are some very recent studies that look very promising in Drosophila, but they haven't been transferred over to mosquitoes yet.

But I think these, these are the kind of things that we work on and you know, we've made a lot of progress in the, in the last 10 years, 10, 15 years. We're gonna make a lot of progress in the next 10 years. And I really think that the scientific issues that ultimately are addressable, once we have these mosquito strains that are suitable for release, people need to actually let us release them.

And I think those sorts of obstacles are going to be much more, much more difficult to overcome than the scientific issues. And it's something that's gonna take a lot of time. And it may be at the end that people just don't want to accept them.

And really the, you know, there's not much that we can do about it if that's the, The case at that point. So in general, there is quite a Lot of funding. I mean, we can always use more funding, but you know, national Institutes of Health, the WHO Gates Foundation are really putting a lot of money into vector-borne diseases.

Malaria and dengue in particular. I think that, you know, in terms of the money that's out there, there's a lot of it. It shouldn't all go into one particular, you know, it shouldn't all go into genetically modified mosquitoes.

It shouldn't all go into insecticides. But we really need to sort of, you know, diversify our, our strategies and really try to attack this problem from all angles. But there is funding there.

I mean, funding is difficult for everybody right Now, but.