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Making viruses the natural way

The Loom
By Carl Zimmer
Dec 2, 2011 11:57 AMNov 20, 2019 1:33 AM


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When it comes to viruses, we humans like to pretend we know much more than we really do. It's understandable. The influenza virus, for example, has only ten genes. It is just a shell that delivers genes and proteins into a host cell, where it hacks the biochemistry to manufacture more viruses. It seems like such an easy biological problem to solve. Yet the flu and other viruses hide a complexity which virologists have only partly uncovered. The idea that someone could intentionally design a super-lethal virus from scatch--as plausible as it may seem--is, for now, a delusion. If you've been following the news this past week, you may think I've just been proven wrong. Reports have surfaced about two teams of scientists producing flu viruses that could potentially kill millions if they escaped from the labs. The scientists have the viruses locked up tight for now, and government officials are debating whether they can publish their results. (New Scientist and Science have excellent reports.) So is this evidence that scientists have become viral Frankensteins, who can engineer pathogens at will? Hardly. The new research is part of a long-running struggle to understand how new flu strains arise. It's clear that all flu viruses that infect humans ultimately evolved from viruses that infect birds. From time to time, people can pick up these viruses, which infect their airway. Depending on the strain, bird flu may be harmless or lethal to humans. But for the most part, it can't get from one human to another. It's too well adapted for life in birds. On rare occasion, a bird flu does manage to adapt to humans. It may experience natural selection, it may pick up some genes from human flu viruses, or both. Scientists are still trying to figure out what it takes for a flu virus to make this transition. It's an important question, not just as a matter of fundamental biology but as a matter of global health. When new bird flus jump to humans, we lack immune defenses against them, and they can thus cause worldwide pandemics. Flu experts have had their eye on one strain of bird flu in particular for a while now: H5N1. It's proven extraordinarily lethal, and yet, since it first came to light in 1997, it hasn't managed to make the big leap and start spreading from person to person. If you get H5N1, you're in big trouble. But not many people get it. Yet. Does this mean that H5N1 just doesn't have what it takes to become the next great pandemic? Or does it mean the virus simply hasn't evolved the right recipe yet? Scientists have tried to answer this question by tinkering with the virus. Instead of trying to make a virus that spreads among people, they infected ferrets, which turn out to have much the same experience with the flu as we humans do. In April, CDC scientists published the latest of these studies. They focused their attention on a protein called hemagglutinin, which flu viruses use to get into host cells. Based on earlier experiments, the CDC scientists reasoned that the right tweak to the structure of hemagglutinin in H5N1 could switch it from binding strongly to bird cells to mammal cells. But their rational tweaks failed. They concluded that there was a lot more to becoming a human flu that we don't yet understand. The studies that have now hit the news have succeeded where other experiments have failed. The difference is that instead of trying rational tweaks, the scientists sat back and let evolution do the tweaking. According to the news reports, the scientists used a tried-and-true method known as serial passage. You infect an animal. It gets sick. You wait for the virus to replicate inside its animal host--as new mutants arise and natural selection favors some mutants over others--and then take some viruses from the sick animal and infect a healthy one. You repeat this, moving the virus from host to host. Interesting things can happen when you let viruses evolve under these conditions. Natural selection can produce viruses with many new mutations, which together let them reproduce faster in the lab than their ancestors. And those viruses, in some cases, can be a lot more dangerous than their ancestors. Back in 2007, for example, a virologist named Kanta Subbarao and her colleagues transformed the SARS virus this way. SARS evolved from a bat virus, crossing over into humans in 2003. It killed over 900 people before it mysteriously disappeared. Subbarao wanted to find a way to study SARS in lab animals, such as mice. Mice normally don't get sick from human SARS viruses, though, even though the virus can replicate at a low rate inside them. Even when mice are genetically engineered so that they can't develop an immune system, SARS can't harm them. So Subbarao and her colleagues that instead of changing the mice, they'd change the virus. They inoculated mice with the SARS virus, gave it a chance to replicate inside them, and then isolated the new viruses to infect new mice. Over the course of just 15 passages, it changed from a harmless virus into a fatal one. One sniff of SARS was now enough to kill a mouse. As Martin Enserink reports in Science, the new experiments on bird flu were similarly effective. They turned H5N1 into a ferret flu in just 10 generations. By the time the scientists were done, they no longer had to ferry the flu from one ferret to the next. A healthy ferret just had to be placed near a sick one; the virus could travel through the air. When they examined the new strain, they discovered five mutations in two genes. All five mutations have been found in natural H5N1 viruses--just not all in one virus. A mammal-ready flu virus was beyond human reason, in other words, but it was fairly easy for evolution to find, given the right condtions. That suggests that H5N1 may not have far to evolve to make us its host. Of course, a serial passage experiment is not identical to the flu's natural world, where it circulates among millions of birds and sometimes encounters people. But it's disturbingly close. And if it's so easy for mutations to turn H5N1 into a human flu, the experimental viruses have a lot to tell us about what we may be facing in the future. There's no point in condemning the scientists for tampering with nature. They were watching nature do what it does disturbingly well. [Update: The excellent podcast This Week in Virology discusses the new research. They think the hype to reality ratio is very high.] [Image: Virology Blog]

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