Anti-CRISPRs Could Fine-Tune Genome Editing

Everything needs an off switch. I would have been bankrupt a long, long time ago if I could not turn off the lights in my apartment and C-3PO would have quickly worn out his welcome if he could not shut himself down like he did in Ben Kenobi’s hut. The important thing to remember here is that these things are useful most of the time: light helps me to see but it would not do me any good in the daytime, and C-3PO is like a sassy Google Translate…sometimes too sassy though. And it turns out that even the genome editor CRISPR-Cas9 has an off switch.

Maybe this is the first biology piece you have read in the last three years. If so, you may not know about CRISPR-Cas9 and the genome editing revolution. Commonly referred to as simply “CRISPR” in the popular press, CRISPR-Cas9 is a laboratory method for editing the DNA sequence in a living organism. Throughout the last several years, CRISPR-Cas9 has shown itself time and time again to be a simple and effective way of changing the genome of many different organisms. One group even pursued a controversial study that edited non-viable human embryos, showing that the method can likely be used to edit viable human embryos–as well as setting off a firestorm in the popular press and a lot of ethical hand-wringing within the biomedical community.

The CRISPR-Cas9 system was originally discovered in bacteria, and it functions a kind of anti-viral immune system in bacteria. As I have written before, viruses do their job by injecting a genetic material–DNA in some cases–into a host cell. Some viruses specifically target bacteria. Much like our bodies have evolved defenses against pathogens, bacteria have evolved defenses against viral invaders. This is where CRISPR-Cas9 comes in. Scientists–at a yogurt company of all places–discovered pieces of viral DNA in the genome of a bacterial species that is normally used in yogurt-making. Interestingly, bacteria with these viral signatures were also immune to the corresponding virus. Later work showed that these stretches of viral DNA were actually added to the bacteria’s genome after a viral infection. After that initial infection, the new viral DNA pieces in the bacterium could be made into RNA and loaded onto the protein Cas9. The RNA-Cas9 complex is then free to go bind to DNA that is specified by the RNA, which would be viral DNA in this example. After seeking out complementary DNA from an invading virus, Cas9 performs its molecular function: cutting that DNA into pieces that cannot take over the host cell.

Research on CRISPR-Cas9 has been moving forward at a rapid pace, so I could write exclusively about it and never run out of things to talk about. But a recent published result showed that some bacterial viruses have evolved special proteins to inactivate Cas9, effectively shutting down the CRISPR-Cas9 immune system. It has been known since the middle of the 20th century that protein activity can be controlled by the binding of another molecule. The phenomenon is broadly known as protein regulation, and it is useful because a cell often needs to fine-tune the activity of certain proteins in order to survive. For example, Escherichia coli bacteria prefer to use glucose sugar for energy, but they also can also produce an enzyme to utilize another sugar, lactose, for energy. Interestingly, a lactose molecule can bind to the protein that prevents the production of the lactose-digesting enzyme and allow for the utilization of lactose. Similarly to how lactose can control the protein that shuts down lactose metabolism, scientists recently discovered that a group of viral proteins can shut down Cas9. Importantly, they showed that the “anti-CRISPRs,” as they dubbed the molecules, can bind to the RNA-Cas9 complex and strongly inhibit the DNA-cutting activity of Cas9 in a test tube.

However, the real appeal of CRISPR-Cas9 is not that we can mix it with DNA in a test tube and see DNA cleavage. Instead, we can do all of this in a living cell and cause DNA mutations that can be useful for research or maybe even therapy. If we are going to continue using CRISPR-Cas9 in living cells–perhaps someday therapeutically–we are going to want to fine-tune its activity. Luckily, these same researchers showed that anti-CRISPRs can block CRISPR-Cas9 genome editing in human cells. This result could someday help to avoid “off-target effects” that CRISPR-Cas9 sometimes causes, which are basically just unintended editing effects that could cause more harm than good.

A Voyage of Viral Discovery

Richard Dawkins’ Selfish Gene came out 40 years ago, so it is only fitting that I get to write about the most selfish genes of all: viruses. Basically, viruses are pieces of genetic material–either DNA or RNA–surrounded by a protein shell and maybe some lipid membrane. Viruses are not living cells, and they do not fulfill most of the hallmarks of life that many of us learned in middle school: viruses do not catalyze their own chemical
reactions, they are not made up of cells, and they do not reproduce on their own. In order to do the chemical reactions necessary to reproduce and make more copies of themselves, viruses must find a way to put that genetic material that they carry into a living host cell and trick the host into using the code as it would use its own genome. This is how the virus manages to make the host into a veritable virus factory.

Since viruses rely on living cells for almost everything, it has not been easy to study them. In fact, we did not even know that viruses existed until the late 19th century. The first viruses were isolated when scientists studying a pathogen found that they could run infectious material through the smallest available filters without removing the infectious factor. At that point, they just called them “non-filterable agents” and reasoned that they must be extremely small, even smaller than bacteria. Experiments by others in the early and mid-20th century went on to discover that viruses were mostly protein and nucleic acid (RNA or DNA), making them radically different from previously known cellular life.

As biologists, we were pretty late to the virus party–shoot, we pretty much knew what cells were shortly after the first microscopes were built in the 1600s, but it somehow took until the 1800s to know that there was something smaller that could cause disease–so it is no surprise that there is still a lot for us to learn about the tiny “non-filterable agents.” Appropriately, a recent paper in Nature claimed to find over 1000 distinct viruses that are all new to science. To make this discovery, the scientists first had to pick a group of cellular hosts in which to look for viruses. They settled on invertebrates, a diverse group of animals that include everything from insects and squids to sea urchins and earthworms. They also had to decide what type of viruses they would look for, opting to search for RNA viruses, which invade a host using RNA instead of DNA as their genetic material. By collecting and sequencing RNA from over 200 different invertebrate species, they were able to piece together long strands of RNA using the sequencing data and a computer program. However, those long reconstructed strands of RNA did not necessarily come from a virus present within the host. Host cells make their own RNA all of the time using their own DNA as a template. In order to be sure that the piece of RNA they found originated in a virus, they needed a signature that could only be present in a viral RNA. They found that signature in the form of a RNA virus-specific gene called “RNA-dependent RNA polyermase” or RdRp. RNA viruses use RdRp to copy their RNA genome when they invade a host cell, but they have to bring their own as part of their RNA genome; animals just do not have an RdRp. (That is, unless you believe this group that claims to have found a possibly-functional RdRp gene in a bat genome. I hope you will agree with me when I say that living things tend to be amazing because all of the rules we have about them are inevitably broken in some other organism.)

With this handy tool to distinguish viral RNAs from the rest of the pool, the authors had a field day discovering new RNA viruses. In addition to classifying viruses based on the host they were discovered within, they also used a technique known as “phylogenetics” to compare the RNA sequence of all viruses in order to place them on a tree of life relative to each other. Since all life on earth can ultimately trace its root back to one common ancestor that is the evolutionary relative to all of us, from human to bacterium, we can compare the nucleic acid sequences of organisms or viruses in order to infer their evolutionary distance from each other. For example, two viruses with relatively similar RdRp genes would be inferred to be quite closely related compared to a third virus with less sequence in common in the RdRp gene.

These new viruses were not discovered as human pathogens, so it is unlikely that this finding will have any direct medical relevance. This result can instead be useful for ecologists and evolutionary biologists who want to understand the variety of viruses that infect the invertebrates studied. Moreover, since we know quite a lot about the evolutionary relationships between different invertebrates–owing to us having studied them quite intensely for decades or even centuries–we can now use the new phylogenetic information about viral genome relatedness to start to ask questions about how the viruses co-evolved with their hosts. For instance, a group of related beetles may tend to be infected with related RNA viruses. If this is the case, then it is possible that an early ancestor of those RNA viruses made a living infecting an early ancestor of those beetles. Basic studies like that might also help us to someday understand host-virus co-evolution in humans and our viruses. After all, humans are in no danger of hitting an evolutionary brick wall, and neither are our viral foes.

Sacrificing for Science

It is probably easy to forget it with all of the baseball and politics flying around this page, but I am a biologist and that is what currently pays most of my bills. As a biologist, I do try to keep up on the interesting goings-on in my field, which happens to be somewhere between genetics and genomics. To that end, I printed out a research article about a week ago to read on the train, so naturally I just got around to reading it. The title, “A renewed model of pancreatic cancer evolution based on genomic rearrangement patterns,” caught my eye because DNA rearrangement is something of an interest of mine. Little did I know I would nearly cry while reading this paper.

Pancreatic cancer is interesting among cancers because it tends to go undiagnosed until a relatively late stage. By that time, it is often hard to stop metastasis of the primary tumor to other organs, leading to a pretty short five year survival rate. Due to the loss of life that pancreatic cancer causes, it represents an active field of research. Specifically, a group in Toronto looked at the genetic changes that have to happen in a precancerous lump of cells in the pancreas in order to allow those cells to break from the main tumor and metastasize to other organs, which is how cancer causes the most damage. A popular model to explain the progression of pancreatic cancer dictates that a specific series of gene mutations has to occur in order to allow a precancerous cell to get to the point where metastasis can occur. Interestingly, this group showed that precancerous cells often did not exhibit the step-by-step accumulation of mutations that we thought would lead to metastasis. Instead, many tumors show signatures of simultaneous mutations that could only occur in the event of whole chromosome rearrangements. Which takes us right back to the DNA rearrangement that I am interested in.

Now let me address how I ended that first paragraph. I do not usually get emotional while I am reading papers, but I do not usually read papers that study case-by-case examples of patients who died of pancreatic cancer. It is incredible to think that there could be something growing inside you right now that will kill you in mere months or years. Tragedies like this are enough to make me not want to get out of bed in the morning, because what’s the point? Anything can happen tomorrow: “Pcsi_0410” was a real person with real friends and a family and ideas and a personality, but now they are just Figure 2 in a Nature paper. But I want to thank Pcsi_0410 for teaching us a little more about a terrifying disease that we all want to learn to treat more effectively. Maybe this is the only way we can make the best of pancreatic cancer. People are going to be unlucky and die from it, but we humans have been trying to make sense of shit like this for millennia. We will continue to try to solve the pancreatic cancer mystery, but we need people like Pcsi_0410, who were dealt a shit hand but use it to help others. But as scientists, let’s never forget the important sacrifice that some people have to make in order for us to get some cool sequencing data.

If you like to read the primary literature, check out the paper I talked about at Nature.

Another Zika Structure…and a Mea Culpa

From time to time, I make a mistake by failing to keep up with the primary scientific literature as closely as I should. If I had been on my grind, I would have noticed that another Zika structure was published in Science at around the same time as the Nature structure that I blogged about earlier. The group that put together this structure also compared the Zika particle to related viruses, this time choosing to focus on a region of the viral protein coat that is especially dissimilar to related viruses. The authors go on to suggest that this region of the coat may be involved in attaching to host cells, which could explain how transmissible Zika is compared to its relatives.

Scientists Publish Zika Snapshot

(Image credit: Kostyuchenko et al. (2016) Nature.)

Update (4/27/2016): Science also published a Zika structure, drawing complementary conclusions from it. I thought it would be a good idea to post a small blurb about it here.

A group in Singapore published a structure of the Zika virus particle in Nature on Wednesday. Zika, which the Centers for Disease Control recently concluded is responsible for birth defects in children of infected mothers, has become a growing public health concern.

Victor A. Kostyuchenko and his colleagues at the Duke-National University of Singapore Medical School used cryo-electron microscopy to see the structure of Zika particles incubated at different temperatures. Importantly, the scientists found that the Zika particle is stable over a broader range of temperatures than other related viruses. On a practical level, this could mean that the virus is more transmissible than related viruses, and may be more challenging to control.

Virus particles are simply genetic material–either DNA or RNA–surrounded by a protein coat that protects and transports the genetic material. When the protein coat comes into contact with a susceptible cell, the virus can inject its genetic material into the host. The virus then uses its own genetic material to take over the cell’s own protein-producing machinery in order to produce more viruses. Eventually, those new viruses will be released and go on to infect other cells.

The authors note that their structural model can allow others to find drugs that may destabilize the virus. The hardiness of the Zika particle is almost certainly due to a tough protein coat, but certain drugs may make that protein coat more susceptible to degradation at higher temperatures or other harsh environments. All of this can be used to help stem the transmission of the virus.

For more information, check out the article at Nature: http://www.nature.com/nature/journal/vnfv/ncurrent/full/nature17994.html