We need to talk about CRISPR

By Gesa Junge, PhD

You’ve probably heard of CRISPR, the magic new gene editing technique that will either ruin the world or save it, depending on what you read and whom you talk to? Or the Three Parent Baby, which scientists in the UK have created?

CRISPR is a technology based on a bacterial immune defense system which uses Cas9, a nuclease, to cut up foreign genetic material (e.g., viral RNA). Scientists have developed a method by which they can modify the recognition part of the system, the guide RNA, and make it specific to a site in the genome that Cas9 then cuts. This is often described as “gene editing” which allows disease-causing genes to be swapped out for healthy ones.

CRISPR is now so well known that Google finally stopped suggesting I may be looking for “crisps” instead, but the real-world applications are not so well worked out yet, and there are various issues around CRISPR, including off-target effects, and also the fact that deleting genes is much easier than replacing them with something else. But, after researchers at Oregon Health and Science University managed to change the mutated version of the MYBPC3 gene to the unmutated version in a viable human embryo last month, the predictable bioethical debate was reignited, and terms such as “Designer Babies” got thrown around a lot.

A similar thing happened with the “Three Parent Baby,” an unfortunate term coined to describe mitochondrial replacement therapy (MRT). Mitochondria, the cells’ organelles for providing energy, have their own DNA (making up about 0.2% of the total genome) which is separate from the genomic DNA in the nucleus, which is the body’s blueprint. Mitochondrial DNA can mutate just like genomic DNA, potentially leading to mitochondrial disease, which affects 1 in 5000-10000 children. Mitochondrial disease can manifest in various ways, ranging from growth defects to heart or kidney to disease to neuropsychological symptoms. Symptoms can range from very mild to very severe or fatal, and the disease is incurable.

MRT replaces the mutated mitochondrial DNA in a fertilized egg or in an embryo with the healthy version provided by a third donor, which allows the mitochondria to develop normally. The UK was the first country to allow the “cautious adaption” of this technique.

While headlines need to draw attention and engage the reader for obvious reasons, oversimplifications like “gene editing” and dramatic phrases like “three parent babies” can really get in the way of broadening the understanding of science, which is difficult enough as it is. Research is a slow and inefficient process that easily gets lost in a 24-hour news cycle, and often the context is complex and not easily summed up in 140 characters. And even when the audience can be engaged and interested, the relevant papers are probably hiding behind a paywall, making fact checking difficult.

Aside from difficulties communicating the technicalities and results of studies, there is also often a lack of context in presenting scientific studies – think for example of chocolate and red wine which may or may not protect from heart attacks. What is lost in many headlines is that scientific studies usually express their results as a change in risk of developing a disease, not a direct causation, and very few diseases are caused by one chemical or one food additive. On this topic, WNYC’s “On The Media”-team have an issue of their Breaking News Consumer Handbook that is very useful to evaluate health news.

The causation vs. correlation issue is perhaps a little easier to discuss than big ethical questions that involve changing the germline DNA of human beings because ethical questions do not usually have a scientific answer, let alone a right answer. This is a problem, not just for scientists, but for everyone, because innovation often moves out of the realm of established ethics, forcing us to re-evaluate it.

Both CRISPR and MRT are very powerful techniques that can alter a person’s DNA, and potentially the DNA of their children, which makes them both promising and scary. We are not ready to use CRISPR to cure all cancers yet, and “Three Parent Babies” are not designed by anyone, but unfortunately, it can be hard to look past Designer Babies, Killer Mutations and DNA Scissors, and have a constructive discussion about the real issues, which needs to happen! These technologies exist; they will improve and eventually, and inevitably, play a role in medicine. The question is, would we rather have this development happen in reasonably well-regulated environments where authorities are at least somewhat accountable to the public, or are we happy to let countries with more questionable human rights records and even more opaque power structures take the lead?

Scientists have a responsibility to make sure their work is used for the benefit of humanity, and part of that is taking the time to talk about what we do in terms that anyone can understand, and to clarify all potential implications (both positive and negative), so that there can be an informed public discussion, and hopefully a solution everyone can live with.

 

Further Reading:

CRISPR:

National Geographic

Washington Post

 

Mitochondrial Replacement Therapy:

A paper on clinical and ethical implications

New York Times (Op-Ed)

 

One ring to rule them all: The cohesin complex

By Johannes Buheitel, PhD

In my blog post about mitosis (http://www.myscizzle.com/phases-of-mitosis/), I explained some of the challenges a human cell faces when it tries to disentangle its previously replicated chromosomes (for an overview of the cell cycle, see also http://www.myscizzle.com/cell-cycle-introduction/) and segregate them in a highly ordered fashion into the newly forming daughter cells. I also mentioned a protein complex, which is integral for this chromosomal ballet, the cohesin complex. To recap, cohesin is a multimeric ring complex, which holds the two chromatids of a chromosome together from the time the second sister chromatid is generated in S phase until their separation in M phase. This decreases complexity, and thereby increases the fidelity of chromosome segregation, and thus, mitosis/cell division. And while this feat should already be enough to warrant devoting a whole blog post to cohesin, you will shortly realize that this complex also performs a myriad of other functions during the cell cycle, which really makes it “one ring to rule them all”.

Figure 1: The cohesin complex. The core complex consists of three subunits: Scc1/Rad21, Smc1, and Smc3. They interact to form a ring structure, which embraces ("coheses") sister chromatids.
Figure 1: The cohesin complex. The core complex consists of three subunits: Scc1/Rad21, Smc1, and Smc3. They interact to form a ring structure, which embraces (“coheses”) sister chromatids.

But let’s back up a little first. Cohesin’s integral ring structure is composed of three proteins: Smc1, Smc3 (Structural maintenance of chromosomes), and Scc1/Rad21 (Sister chromatid cohesin/radiation sensitive). These three proteins attach to each other in a more or less end-to-end manner, thereby forming a circular structure (see Figure 1; ONLY for the nerds: Smc1 and -3 form from long intramolecular coiled-coils by folding back onto themselves, bringing together their N- and C-termini at the same end. This means that these two proteins actually interact with their middle parts, forming the so-called “hinge”, as opposed to really “end-to-end”). Cohesin obviously gets its name from the fact that it causes “cohesion” between sister chromatids, which has been first described 20 years ago in budding yeast. The theory that the protein complex does so by embracing DNA inside the ring’s lumen was properly formulated in 2002 by the Nasmyth group, and much evidence supporting this “ring embrace model” has been brought forth over last decades, making it widely (but not absolutely) accepted in the field. According to our current understanding, cohesin is already loaded onto DNA (along the entire length of the decondensed one-chromatid chromosome) in telophase, i.e. only minutes after chromosome segregation, by opening/closing its Smc1-Smc3 interaction site (or “entry gate”). When the second sister chromatid is synthesized in S phase, cohesin establishes sister chromatid cohesion in a co-replicative manner (only after you have the second sister chromatid, you can actually start talking about “cohesion”). Early in the following mitosis, in prophase to be exact, the bulk of cohesin is removed from chromosome arms in a non-proteolytic manner by opening up the Smc3-Scc1/Rad21 interface (or “exit gate”; this mechanism is also called “prophase pathway”). However, a small but very important fraction of cohesin molecules, which is located at the chromosomes’ centromere regions, remains protected from this removal mechanism in prophase. This not only ensures that sister chromatids remain cohesed until the metaphase-to-anaphase transition, but also provides us with the stereotypical image of an X-shaped chromosome. The last stage in the life of a cohesin ring is its removal from centromeres, a tightly regulated process, which involves proteolytic cleavage of cohesin’s Scc1/Rad21 subunit (see Figure 2).

Figure 2: The cohesin cycle. Cohesin is topologically loaded onto DNA in telophase by opening up the Smc1-Smc3 interphase ("entry gate"). Sister chromatid cohesion is established during S phase, coinciding with the synthesis of the second sister. In prophase of early mitosis, the bulk of cohesin molecules are removed from chromosome arms (also called "prophase pathway") by opening up the interphase between Scc1/Rad21 and Smc3 ("exit gate"). Centromeric cohesin is ultimately proteolytically removed at the metaphase-to-anaphase transition.
Figure 2: The cohesin cycle. Cohesin is topologically loaded onto DNA in telophase by opening up the Smc1-Smc3 interphase (“entry gate”). Sister chromatid cohesion is established during S phase, coinciding with the synthesis of the second sister. In prophase of early mitosis, the bulk of cohesin molecules are removed from chromosome arms (also called “prophase pathway”) by opening up the interphase between Scc1/Rad21 and Smc3 (“exit gate”). Centromeric cohesin is ultimately proteolytically removed at the metaphase-to-anaphase transition.

As you can see, during the 24 hours of a typical mammalian cell cycle, cohesin is pretty much always directly associated with the entire genome (the exceptions being chromosomes arms during most of mitosis, i.e. 20-40 minutes and entire chromatids during anaphase, i.e. ~10 minutes). This means that cohesin has at least the potential to influence a whole bunch of other chromosomal events, like DNA replication, gene expression and DNA topology. And you know what? Turns out it does!

Soon after cohesin was described as this guardian of sister chromatid cohesion, it also became clear that there is just more to it. Take DNA replication for example. There is good evidence that initial cohesin loading is already topological (meaning, the ring closes around the single chromatid). That poses an obvious problem during S phase: While DNA replication machineries (“replisomes”) zip along the chromosomes trying to faithfully duplicate the entire genome in a matter of just a couple of hours, they encounter – on average – multiple cohesin rings that are already wrapped around DNA. Simultaneously, cohesin’s job is to take those newly generated sister chromatids and hold them tightly to the old one. Currently, we don’t really know how this works, whether the replisome can pass through closed cohesin rings, or whether cohesin gets knocked off and reloaded after synthesis. What we do know, however, is that cohesion establishment and DNA replication are strongly interdependent, with defects in cohesion metabolism causing replication phenotypes and vice versa.

Cohesin has also been shown to have functions in transcriptional regulation. It was observed quite early that cohesin can act as an insulation factor, blocking long-range promoter-enhancer association. Today we have good evidence showing that cohesin binds to chromosomal insulator elements that are usually associated with the CTCF (CCCTC-binding factor) transcriptional regulator. Here, the ring complex is thought to help CTCF’s agenda by creating internal loops, i.e. inside the same sister chromatid!

Studying cohesin has, of course, not only academic value. Because of its pleiotropic functions, defects in human cohesin biology can cause a number of clinically relevant issues. Since actual cohesion defects will cause mitotic failure (which most surely results in cell death), most of cohesin-associated diseases are believed to be caused by misregulation of the complex’s non-canonical functions in replication/transcription. These so-called cohesinopathies (e.g. Roberts syndrome and Cornelia de Lange syndrome) are congenital birth defects with widely ranging symptoms, which usually include craniofacial/upper limb deformities as well as mental retardation.

It is important to mention that cohesin also has a very unique role in meiosis where it not only coheses sister chromatids but also chromosomal homologs (the two maternal/paternal versions of a chromosome, each consisting of two sisters, which themselves are cohesed). As a reminder, the lifetime supply of all oocytes of a human female is produced before puberty. These oocytes are arrested in prophase I (prophase of the first meiotic division) with fully cohesed homologs and sisters, and resume meiosis one by one each menstrual cycle. This means that some oocytes might need to keep up their cohesion (between sisters AND homologs) over decades, which, considering the half-life of your average protein, can be challenging. This has important medical relevance as cohesion failure is believed to be the main cause behind missegregation of homologs, and thus, age-related aneuploidies, like e.g. trisomy 21.

After twenty years of research, the cohesin complex still manages to surprise us regularly, as new functions in new areas of cell cycle regulation come to light. Currently, extensive research is conducted to better understand the role of certain cohesin mutations in cancers such as glioblastoma, or Ewing’s sarcoma. And while we’re still far away from completely understanding this complex complex, we already know enough to say that cohesin really is “one ring to rule them all”.

 

HeLa, the VIP of cell lines

By  Gesa Junge, PhD

A month ago, The Immortal Life of Henrietta Lacks was released on HBO, an adaptation of Rebecca Skloot’s 2010 book of the same title. The book, and the movie, tell the story of Henrietta Lacks, the woman behind the first cell line ever generated, the famous HeLa cell line. From a biologist’s standpoint, this is a really unique thing, as we don’t usually know who is behind the cell lines we grow in the lab. Which, incidentally, is at the centre of the controversy around HeLa cells. HeLa was the first cell line ever made over 60 years ago and today a PubMed search for “HeLa” return 93274 search results.

Cell lines are an integral part to research in many fields, and these days there are probably thousands of cell lines. Usually, they are generated from patient samples which are immortalised and then can be grown in dishes, put under the microscope, frozen down, thawed and revived, have their DNA sequenced, their protein levels measured, be genetically modified, treated with drugs, and generally make biomedical research possible. As a general rule, work with cancer cell lines is an easy and cheap way to investigate biological concepts, test drugs and validate methods, mainly because cell lines are cheap compared to animal research, readily available, easy to grow, and there are few concerns around ethics and informed consent. This is because although they originate from patients, the cell lines are not considered living beings in the sense that they have feelings and lives and rights; they are for the most part considered research tools. This is an easy argument to make, as almost all cell lines are immortalised and therefore different from the original tissues patients donated, and most importantly they are anonymous, so that any data generated cannot be related back to the person.

But this is exactly what did not happen with HeLa cells. Henrietta Lack’s cells were taken without her knowledge nor consent after she was treated for cervical cancer at Johns Hopkins in 1951. At this point, nobody had managed to grow cells outside the human body, so when Henrietta Lack’s cells started to divide and grow, the researchers were excited, and yet nobody ever told her, or her family. Henrietta Lacks died of her cancer later that year, but her cells survived. For more on this, there is a great Radiolab episode that features interviews with the scientists, as well as Rebecca Skloot and Henrietta Lack’s youngest daughter Deborah Lacks Pullum.

In the 1970s, some researchers did reach out to the Lacks family, not because of ethical concerns or gratitude, but to request blood samples. This naturally led to confusion amongst family members around how Henrietta Lack’s cells could be alive, and be used in labs everywhere, even go to space, while Henrietta herself had been dead for twenty years. Nobody had told them, let alone explained the concept of cell lines to them.

The lack of consent and information are one side, but in addition to being an invaluable research tool, cell lines are also big business: The global market for cell lines development (which includes cell lines and the media they grow in, and other reagents) is worth around 3 billion dollars, and it’s growing fast. There are companies that specialise in making cell lines of certain genotypes that are sold for hundreds of dollars, and different cell types need different growth media and additives in order to grow. This adds a dimension of financial interest, and whether the family should share in the profit derived from research involving HeLa cells.

We have a lot to be grateful for to HeLa cells, and not just biomedical advances. The history of HeLa brought up a plethora of ethical issues around privacy, information, communication and consent that arguably were overdue for discussion. Innovation usually outruns ethics, but while nowadays informed consent is standard for all research involving humans, and patient data is anonymised (or at least pseudonomised and kept confidential), there were no such rules in 1951. There was also apparently no attempt to explain scientific concept and research to non-scientists.

And clearly we still have not fully grasped the issues at hand, as in 2013 researchers sequenced the HeLa cell genome – and published it. Again, without the family’s consent. The main argument in defence of publishing the HeLa genome was that the cell line was too different from the original cells to provide any information on Henrietta Lack’s living relatives. There may some truth in that; cell lines change a lot over time, but even after all these years there will still be information about Henrietta Lack’s and her family in there, and genetic information is still personal and should be kept private.

HeLa cells have gotten around to research labs around the world and even gone to space and on deep sea dives. And they are now even contaminating other cell lines (which could perhaps be interpreted as just karma). Sadly, the spotlight on Henrietta Lack’s life has sparked arguments amongst the family members around the use and distribution of profits and benefits from the book and movie, and the portrayal of Henrietta Lack’s in the story. Johns Hopkins say they have no rights to the cell line, and have not profited from them, and they have established symposiums, scholarships and awards in Henrietta Lack’s honour.

The NIH has established the HeLa Genome Data Access Working Group, which includes members of Henrietta Lack’s family. Any researcher wanting to use the HeLa cell genome in their research has to request the data from this committee, and explain their research plans, and any potential commercialisation. The data may only be used in biomedical research, not ancestry research, and no researcher is allowed to contact the Lacks family directly.

End Crisis, Bridges and Scattered Genes: Chromatin Bridges and their Role in Genomic Stability

By Gesa Junge, PhD

Each of our cells contains about two meters of DNA which needs to be stored in cells that are often less than 100uM in diameter, and to make this possible, the DNA is tightly packed into chromosomes. As the human cell prepares to divide, the 23 pairs of chromosomes neatly line up and attach to the spindle apparatus via their middle point, the centrosome. The spindle apparatus is part of the cell’s scaffolding and it pulls the chromosomes to opposite ends of the cell as the cell divides, so that every new daughter cell ends up with exactly one copy of each chromosome. This is important; cells with more or less than one copy of a chromosome are called aneuploid cells, and aneuploidy can lead to genetic disorders such as Down Syndrome (three copies of chromosome 21).

In some cancer cells, chromosomes with two centromeres (dicentric chromosomes) can be detected, which can happen when the ends of two chromosomes fuse in a process called telomere crisis. Telomeres are a sort of buffer zone at the ends of the chromosome which consist of repeats of non-coding DNA sequences, meaning there are no genes located here. As one of the DNA strands is not replicated continuously but in fragments, the telomeres get shorter over the lifespan of a cell, and short telomeres can trigger cell cycle arrest before the chromosomes get so short that genetic information is lost. But occasionally, and especially in cancer cells, chromosome ends fuse and a chromosome becomes dicentric. Then it can attach to the spindle apparatus in two points and may end up being pulled apart as the two daughter cells separate, sort of like a rope tied to two cars that drive in opposite directions. This string of chromosome is referred to as a chromatin bridge.

Researchers at Rockefeller University are studying these chromatin bridges and what their relevance is for the health of the cell. A paper by John Maciejowski and colleagues found that the chromatin bridges actually stay intact for quite a long time. Chromosomes are pretty stable, and so the chromatin bridges lasted for an average of about 9 hours (3-20h) before snapping and quickly being pulled back into the original cell (see video). Also, the nucleus of the cell was often heart-shaped as opposed to the usual round shape, which suggests that the chromatin bridge physically pulls on the membrane surrounding the nucleus, the nuclear envelope. Indeed, proteins that make up the nuclear envelope (e.g. LAP2) were seen on the chromatin bridge, suggesting that they take part of the nuclear envelope with them as they divide.  Also, cells with chromatin bridges had temporary disruptions to their nuclear envelope at some point after the bridge was resolved, more so than cells without chromatin bridges.

The chromatin bridges also stained positive for replication protein A (RPA), which binds single stranded DNA. DNA usually exists as two complementary strands bound together, and the two strands really only separate to allow for DNA to be copied or transcribed to make protein. Single-stranded DNA is very quickly bound by RPA, which stabilises it so it does not loop back on itself and get tangled up in secondary structures. The Rockefeller study showed that a nuclease, a DNA-eating enzyme, called TREX1 is responsible for generating the single-stranded DNA on chromatin bridges. And this TREX1 enzyme seems to be really important in resolving the chromatin bridges: cells that do not have TREX1 resolve their chromatin bridges later than cells that do have TREX1.

So how are chromatin bridges important for cells, the tissue and the organism (i.e. us)? The authors of this study suggest that chromatin bridges can lead to a phenomenon called chromothripsis. In chromothripsis, a region of a chromosome is shattered and then put back together in a fairly random order and with some genes facing the wrong direction. Think of a new, neatly color-sorted box of crayons that falls on the floor, and then someone hastily shoves all the crayons back in the box with no consideration for color coordination or orientation. Chromothripsis occurs in several types of cancers, but it is still not really clear how often, in what context and exactly how the genes on a chromosome end up in such a mess.

According to this study, chromothripsis may be a consequence of telomere crisis, and chromatin bridges could be part of the mechanism: A chromosome fuses ends with another chromosome and develops two centromeres. The dicentric chromosome attaches to two opposite spindles and is pulled apart during cell division, generating a chromatin bridge which is attacked by TREX that turns it into single-stranded DNA, the bridge snaps and in the process the DNA scatters, and returns to the parent cell where it is haphazardly reassembled, leaving a chromothripsis region.

The exact mechanisms of this still need to be studied and the paper mentions a few important discussion points. For example, all the experiments were performed in cell culture, and the picture may look very different in a tumor in a human being. And what exactly causes the bridge to break? Also, there are probably more than one potentially mechanism linking telomere crisis to chromothripsis. But it is a very interesting study that shines some light on the somewhat bizarre phenomenon of chromothripsis, and the importance of telomere crisis.

Reference: Maciejowski et al, Cell. 2015 Dec 17; 163(7): 1641–1654.

 

 

In the Life of a Cell

An introduction to the cell cycle

 

By Johannes Buheitel, PhD

Omnis cellula e cellula”. We all heard or read this sentence probably sometime during college or grad school and no, it’s not NYU’s university motto. This short Latin phrase, popularized by the German physician/biologist Rudolf Virchow, states a simple fact, which, however, represents a fundamental truth of biology: “All cells come from cells”. It’s so fundamental that we often take it for granted that the basis for all of those really interesting little pathways and mechanisms that we study is life itself; and, moreover, that life is not simply “created” from thin air but can actually only derive from other life. Macroscopically, you (and Elton John) might call this “the circle of life” but microscopically, we’re talking about nothing less than the cell cycle. But what is the cell cycle exactly? What has to happen when and how does the cell maintain this order of events?

The cell cycle’s main purpose is to generate two identical daughter cells from one mother cell by first, duplicating all its genetic content in order to get two copies of each chromosome (DNA replication), and then carefully distributing those two copies into the newly forming daughter cells (mitosis and cytokinesis). These two major chromosomal events take place during S phase (DNA replication) and M phase (mitosis), which during consecutive cycles alternate, separated by two “gap” phases (G1 between M and S phase and G2 between S and M phase; FYI: everything outside M phase is also sometimes also called interphase). It goes without saying that the temporal order of events, G1 to S to G2 to M phase, must be maintained at all times; just imagine trying to divide without previously having replicated your DNA! And not only the order is important, but each phase must also be given enough time to faithfully fulfill its purpose. How is this achieved?

If you want to boil it down, there are two main principles that drive the cell cycle: timely expression and degradation of key proteins and irreversible switch-like transitions, called checkpoints. So let’s try and get an overview over each of these principles.

Cell-cycle
Overview over the different phases of the cell cycle: G1 (“gap”) phase, S (“synthesis”) phase, G2 phase and M (“mitosis”) phase.

In the early eighties, a scientist called Tim Hunt performed a series of experiments, unknowing that these will turn into a body of work, which will ultimately win him a Nobel prize. For these experiments, he radioactively labeled proteins in sea urchin embryos (yes, you read correctly!) and stumbled across one that exhibited an interesting pattern of abundance over time in that it appeared and vanished in a fashion that was not only cyclic but also seemed to be in sync with the embryos’ division cycles. Dr. Hunt had just found the first member of a protein family, which later turned out to be one of the main drivers of the cell cycle: the cyclins. What do cyclins do? Cyclins are co-activators of cyclin-dependent kinases or Cdks, whose job it is to phosphorylate certain target proteins in order to regulate their function in a cell cycle-dependent fashion. Since Cdks are pretty much around all the time, they need the cyclins to tell them, when to be active and when not to be. There are a variety of different Cdks, which interact very specifically with various cyclins. For example, cyclin D interacts with Cdk4 and 6 to drive the transition from G1 to S phase, while a complex between cyclin B and Cdk1 is required for mitotic entry. This system allows for enough complexity to explain how the proper length of each phase is assured (slow accumulation of a specific cyclin until the respective Cdk can be fully activated), but also how the correct order of events is maintained; because it turns out that the expression of, say, the cyclin assigned to start replication (cyclin E) is dependent on the activity of the Cdk/cyclin complex of the previous phase (in this example: Cdk4/6-cyclin D) via phosphorylation-dependent regulation of transcription factors.

The second principle I was talking about, are checkpoints. A checkpoint is a way for the cell to take a short breath and check if things are running smoothly so far, and if they are not, to halt the cell cycle in order to give itself some time to either resolve the issue or, if that’s not working out, throw in the towel (i.e. apoptosis). Researchers describe more and more checkpoint-like pathways that react to different stimuli all over the cell cycle, but canonically, we distinguish three main ones: the restriction checkpoint at the G1 to S phase transition, the DNA damage checkpoint at the G2 to M phase transition and the spindle assembly checkpoint (SAC) during mitosis at the transition from metaphase to anaphase. What do these checkpoints look for, or in more technical words, what requirements have to be met in order for a checkpoint to become satisfied? The restriction checkpoint integrates a variety of internal and external signals, but is ultimately satisfied by proper activation of S phase Cdk complexes (see above). The DNA damage checkpoint’s main function is to give the cell time to correct DNA damage, which naturally occurs during genome replication but can also be introduced chemically or by ionizing radiation. Therefore, it remains unsatisfied as long as the DNA damage kinases ATM and ATR are active. Finally, the SAC governs one of the most intricate processes of the cell cycle: the formation of the mitotic spindle including proper attachment of each and every chromosome to its microtubules. After a checkpoint becomes satisfied, one or more positive feedback loops spring into action and effectively jump re-start the cell cycle.

As one can imagine, all of these processes must be exquisitely controlled to ensure the mission’s overall success. In future posts, we will explore those mechanisms in more detail and will furthermore discuss, how a handful of biochemical fallacies can have the potential to turn this wonderful circle of life into a wicked cycle of death.

Engineering Babies One Crispr at a Time

 

By Sophie Balmer, PhD

Over the past few weeks, the scientific community has been overwhelmed with major advances in human embryonic research. Whether researchers report for the second time the use of Crispr to edit the human germline or extend the conditions of in vitro culture of human embryos (also here), these issues have been all over the news. However, as all topics can not be raised in only one post, therefore, I will focus on genome editing studies.

 

About a year ago, one research group in China reported the first genome editing of human embryos using Crispr technology. Although these embryos were not viable due to one additional copy of each chromosome, this study quickly became highly controversial and raised strong concerns. The public and scientific communities questioned whether editing the human germline for therapeutic benefits was legitimate, leading to numerous ethical discussions. A few of weeks ago, a second study reported genome editing of embryos reinforcing the debate around this issue. Additionally, several research proposal involving genomic modification of healthy human embryos’ DNA have been validated recently in other countries. In this post, I want to address several questions. What are the possible advances or consequences of such work? What is the current legislation on human genome editing worldwide? Are these studies as alarming as what is written in some newspaper articles?

 

The emergence of the Crispr technology a few years ago has revolutionized the way scientists work since this method greatly improves the efficiency of DNA alteration of model organisms. However, this powerful tool has also raised many concerns, notably on the possibility to easily tweak the human genome and generate modified embryos.

In the eyes of the general public, this kind of experiment resonates with science fiction books or movies. Because of the high potential of this technique, it is crucial to inform everyone correctly to avoid clichés. Recently, one of my favorite comedian and television host John Oliver depicted in a very bright and amusing way how small scientific advances are sometimes presented in the media. Although the examples he uses are dramatic, every scientific breakthrough gets its share of overselling to the public. In the case of gene-editing of human embryos, pretending we are about to use eugenics principles to engineer babies and their descendants with beneficial genes is pure fiction. However, to prevent any potential malpractice from happening, clear ethical discussions and regulations need to be established and then explained to the public to prevent misunderstanding of these issues.

Within the scientific community, last year’s results triggered the need for new discussions and regulations on human cloning. Modifying the genome of human embryos involves modifying the germline as well, leading eventually to the transmission of the genetic alteration to future generations. However, the consequences of such transmission are unknown. Potentially, this could resolve a number of congenital genetic diseases for the individual him/herself and be used for gene therapy but would result in generations of genetically modified humans.

 

Because of cultural and ethical differences between countries, the legislation (if there is any) around working with human embryos or cells derived from human embryos (hESC for human embryonic stem cells) is variable. International ethical committees have only been able to establish guidelines as instituting international laws on human cloning is impossible. Ultimately, each country is responsible for enforcing these rules. Most countries and international ethics committees agree on a ban on reproductive and therapeutic human cloning. Moreover, following last year published experiments, a summit held in December 2015 gathered experts from all around the world. The consortium concluded that gene-editing of embryos used to establish pregnancy should not be performed (for now) and to follow up on all-related issues, new sets of guidelines are coming out imminently.

 

Still, it seems difficult to get an idea of the consensus depending on the countries in which scientists perform experiments. There is range of possibilities when working with human samples: some countries completely prohibit any manipulation of human embryos or hESC while others authorize genetic modification of the embryo for research purposes only under specific conditions. In between several nations authorize research exclusively on already derived lines of hESC and others authorize derivation of hESC but no manipulation of the embryos themselves.

Besides these general rules and as of today, three countries have approved proposals for gene-editing of human embryos: China, the UK and Sweden. Research proposals in both European countries have authorized Crispr targeting of specific genes in healthy human embryos to assess the function of these genes during early human development. However, these embryos can not be used for in vitro fertilization (IVF) and have to be destroyed at the end of the study. The purpose of these studies would be to confirm what has been described in hESC and in mammalian model systems and contribute to our knowledge of human development.

 

On the other hand, both published studies from China focused on Crispr targeting towards clinical therapies of an incurable blood disease or HIV. The overall purpose of such projects is to test the use of the Crispr technology for gene therapy. Although rendering embryos immune to several diseases using Crispr is an attractive possibility, it seems more urgent to probe the validity of the technique in humans and assess whether the mechanisms of human embryonic development are similar to what has been hypothesized. Gene therapies have already been successfully attempted in humans using other techniques to modify the genome. Yet, the modifications were targeted towards specific cells in already-born individuals. Again, modifying the genome of embryos implies that the mutation will be inherited in future generations and is in a large part the reason of this debate. Moreover, Crispr targeting still leads to unspecific modification of the genome, although very promising results show that newly engineered cas9 could lead to very specific targeting. The consequences of such off-target modification are unknown and could be disastrous for the following generations.

 

Overall, no research proposal dares to consider genetically modified embryos to establish pregnancy but as research moves faster, increasing demand for ethical discussion and regulations are brought forward. As more studies come out, it will be interesting to follow the evolution of this debate. Additionally, informing clearly the population of the possibilities and outcomes of ongoing projects should be a priority so that they can give an informed consent towards such research. In any case, a clear boundary needs to be established between selecting the fittest embryo by pre-implantation genetic diagnosis, which is routinely performed for IVF and playing the sorcerer’s apprentice with human embryo’s

How Low Can You Go? Designing a Minimal Genome

By Elizabeth Ohneck, PhD

How many genes are necessary for life? We humans have 19,000 – 20,000 genes, while the water flea Daphnia pulex has over 30,000 and the microbe Mycoplasma genitalium has only 525. But how many of these genes are absolutely required for life? Is there a minimum number of genes needed for a cell to survive independently? What are the functions of these essential genes? Researchers from the J. Craig Venter Institute and Synthetic Genomics, Inc., set out to explore these questions by designing the smallest cellular genome that can maintain an independently replicating cell. Their findings were published in the March 25th version of Science.

The researchers started with a modified version of the Mycoplasma mycoides genome, which contains over 900 genes. Mycoplasmas are simplest cells capable of autonomous growth, and their small genome size provides a good starting point for building minimal cells. To identify genes unnecessary for cell growth, the team used Tn5 transposon mutagenesis, in which a piece of mobile DNA is introduced to the cells and randomly “jumps” into the bacterial chromosome, thereby disrupting gene function. If many cells were found to have the transposon inserted into the same gene at any position in the gene sequence, and these cells were able to grow normally, the gene was considered non-essential, since its function was not required for growth; such genes were candidates for deletion in a minimal genome. In some genes, the transposon was only found to insert at the ends of the genes, and cells with these insertions grew slowly; such genes were considered quasi-essential, since they were needed for robust growth but were not necessary for cell survival. Genes which were never found to contain the transposon in any cells were considered essential, since cells that had transposon insertions in these genes did not survive; these essential genes were required in the minimal genome.

The researchers then constructed genomes with various combinations of non-essential and quasi-essential gene deletions using in vitro DNA synthesis and yeast cells. The synthetic chromosomes were transplanted into Mycoplasma capricolum, replacing its normal chromosome with the minimized genome. If the M. capricolum survived and grew in culture, the genome was considered viable. Some viable genomes, however, caused the cells to grow too slowly to be practical for further experiments. The team therefore had to find a compromise between small genome size and workable growth rate.

The final bacterial strain containing the optimized minimal genome, JCVI-syn3.0, had 473 genes, a genome smaller than any autonomously replicating cell found in nature. Its doubling time was 3 hours, which, while slower than the 1 hour doubling time of the M. mycoides parent strain, was not prohibitive of further experiments.

What genes were indispensable for an independently replicating cell? The 473 genes in the minimal genome could be categorized into 5 functional groups: cytosolic metabolism (17%), cell membrane structure and function (18%), preservation of genomic information (7%), expression of genomic information (41%), and unassigned or unknown function (17%). Because the cells were grown in rich medium, with almost all necessary nutrients provided, many metabolic genes were dispensable, aside from those necessary to effectively use the provided nutrients (cytosolic metabolism) or transport nutrients into the cell (cell membrane function). In contrast, a large proportion of genes involved in reading, expressing, replicating, and repairing DNA were maintained (after all, the presence of genes is of little use if there is no way to accurately read and maintain them). As the cell membrane is critical for a defined, intact cell, it’s unsurprising that the minimal genome also required many genes for cell membrane structure.

Of the 79 genes that could not be assigned to a functional category, 19 were essential and 36 were quasi-essential (necessary for rapid growth). Thirteen of the essential genes had completely unknown functions. Some were similar to genes of unknown function in other bacteria or even eukaryotes, suggesting these genes may encode proteins of novel but universal function. Those essential genes that were not similar to genes in any other organisms might encode novel, unique proteins or unusual sequences of genes with known function. Studying and identifying these genes could provide important insight into the core molecular functions of life.

One of the major advancements resulting from this study was the optimization of a semi-automated method for rapidly generating large, error-free DNA constructs. The technique used to generate the genome of JCVI-syn3.0 allows any small genome to be designed and built in yeast and then tested for viability under standard laboratory conditions in a process that takes about 3 weeks. This technique could be used in research to study the function of single genes or gene sets in a well-defined background. Additionally, genomes could be built to include pathways for the production of drugs or chemicals, or to enable cells to carry out industrially or environmentally important processes. The small, well-defined genome of a minimal cell that can be easily grown in laboratory culture would allow accurate modeling of the consequences of adding genes to the genome and lead to greater efficiency in the development of bacteria useful for research and industry.

Leaving Your Mark on the World

By Danielle Gerhard

 

The idea that transgenerational inheritance of salient life experiences exists has only recently entered the world of experimental research. French scientist Jean-Baptiste Lamarck proposed the idea that acquired traits throughout an organism’s life could be passed along to offspring. This theory of inheritance was originally disregarded in favor of Mendelian genetics, or the inheritance of phenotypic traits isn’t a blending of the traits but instead a specific combination of alleles to form a unique gene encoding the phenotypic trait. However, inheritance is much more complicated than either theory allows for. While Lamarckian inheritance has largely been negated by modern genetics, recent findings in the field of genetics have caused some to revisit l’influence des circonstances, or, the influence of circumstances.

 

Over the past decade, efforts have shifted towards understanding the mechanisms underlying the non-Mendelian inheritance of experience-dependent information. While still conserving most of the rules of Mendelian inheritance, new discoveries like epigenetics and prions challenge the central dogma of molecular biology. Epigenetics is the study of heritable changes in gene activity as a result of environmental factors. These changes do not affect DNA sequences directly but instead impact processes that regulate gene activity such as DNA methylation and histone acetylation.

 

Epigenetics has transformed how psychologists approach understanding the development of psychological disorders. The first study to report epigenetic effects on behavior came from the lab of Michael Meany and Moshe Szyf at McGill University in the early 2000s. In a 2004 Nature Neuroscience paper they report differential DNA methylation in pups raised by high licking and grooming mothers compared to pups raised by low licking and grooming mother. Following these initial findings, neuroscientists have begun using epigenetic techniques to better understand how parental life experiences, such as stress and depression, can shape the epigenome of their offspring.

 

Recent research coming out from the lab of Tracy Bale of the University of Pennsylvania has investigated the heritability of behavioral phenotypes. A 2013 Journal of Neuroscience paper found that stressed males went on to produce offspring with blunted hypothalamic pituitary (HPA) axis responsivity. In simpler terms, when the offspring were presented with a brief, stressful event they had a reduction in the production of the stress hormone corticosterone (cortisol in humans), symptomatic of a predisposition to psychopathology. In contrast, an adaptive response to acute stressors is a transient increase in corticosterone that signals a negative feedback loops to subsequently silence the stress response.

 

The other key finding from this prior study is the identification of nine small non-coding RNA sperm microRNAs (miRs) increased in stressed sires. These findings begin to delve into how paternal experience can influence germ cell transmission but does not explain how selective increases in these sperm miRs might effect oocyte development in order to cause the observed phenotypic and hormonal deficits seen in adult offspring.

 

A recent study from the lab published in PNAS builds off of these initial findings to further investigate the mechanisms underlying transgenerational effects of paternal stress. Using the previously identified nine sperm miRs, the researchers performed a multi-miR injection into single-cell mouse zygotes that were introduced into healthy surrogate females. To confirm that all nine of the sperm miRs were required to recapitulate the stress phenotype, another set of single-cell mouse zygotes were microinjected with a single sperm miR. Furthermore, a final set of zygotes received none of the sperm miRs. Following a normal rearing schedule, the adult offspring were briefly exposed to an acute stressor and blood was collected to analyze changes in stress hormones. As hypothesized, male and female adult offspring from the multi-miR group had a blunted stress response relative to both controls.

 

To further investigate potential effects on neural development, the researchers dissected out the paraventricular nucleus (PVN) of the hypothalamus, a region of the brain that has been previously identified by the group to be involved in regulation of the stress response. Using RNA sequencing and gene set enrichment analysis (GSEA) techniques they found a decrease in genes involved in collagen formation and extracellular matrix organization which the authors go on to hypothesize could be modifying cerebral circulation and blood brain barrier integrity.

 

The final experiment in the study examined the postfertilization effects of multi-miR injected zygotes. Specifically, the investigators were interested in the direct, combined effect of the nine identified sperm miRs on stored maternal mRNA. Using a similar design as the initial experiment, the zygote mRNA was collected and amplified 24 hours after miR injection in order to examine differential gene expression. The researchers found that microinjection of the nine sperm miRs reduced stored maternal mRNA of candidate genes.

 

This study is significant as it has never been shown that paternally derived miRs play a regulatory role in zygote miR degradation. In simpler terms, these findings contradict the conventional belief that zygote development is solely maternally driven. Paternal models of transgenerational inheritance of salient life experiences are useful as they avoid confounding maternal influences in development. Studies investigating the effects of paternal drug use, malnutrition, and psychopathology are ongoing.

 

Not only do early life experiences influence the epigenome passed down to offspring but recent work out of the University of Copenhagen suggests that our diet may also have long-lasting, transgenerational effects. A study that will be published in Cell Metabolism next year examined the effects of obesity on the epigenome. They report differential small non-coding RNA expression and DNA methylation of genes involved in central nervous system development in the spermatozoa of obese men compared to lean controls. Before you start feeling guilty about the 15 jelly donuts you ate this morning, there is hope that epigenetics can also work in our favor. The authors present data on obese men who have undergone bariatric surgery-induced weight loss and they show a remodeling of DNA methylation in spermatozoa.

 

Although still a nascent field, epigenetics has promise for better understanding intergenerational transmission of risk to developing a psychopathology or disease. The ultimate goal of treatment is to identify patterns of epigenetic alternations across susceptible or diagnosed individuals and develop agents that aim to modify epigenetic processes responsible for regulating genes of interest. I would argue that it will one day be necessary for epigenetics and pharmacogenetics, another burgeoning field, to come into cahoots with one another to not only identify the epigenetic markers of a disease but to identify the markers on an person by person basis. However, because the fields of epigenetics and pharmacogenetics are still in the early stages, the tools and techniques currently available limit them. As a result, researchers are able to extract correlations in many of their studies but unable to determine potential causality. Therefore, longitudinal, transgenerational studies like those from the labs of Tracy Bale and others are necessary to provide insight into the lability of our epigenome in response to lifelong experiences.

CRISPR/Cas9: More Than a Genome Editor

By Rebecca Delker, PhD

 

The bacterial defense system, CRISPR/Cas9, made huge waves in the biomedical community when the seemingly simple protein-RNA complex of Type II CRISPR systems was engineered to target DNA in vitro and in complex eukaryotic genomes. The introduction of double-strand breaks using CRISPR/Cas9 in a targeted fashion opened the portal to highly affordable and efficient site-specific genomic editing in cells derived from yeast to man.

 

To get a sense of the impact CRISPR technology has had on biological research, one simply needs to run a search of the number of publications containing CRISPR in the title or abstract over the past handful of years; the results practically scream in your face. From 2012, the year of the proof-of-principle experiment demonstrating the utility of engineered Cas9, to 2015, CRISPR publications rose steadily from a mere 138 (in 2012) to >1000 (at the time of this post). Publications more than doubled between the years of 2012 and 2013, as well as between 2013 and 2014. Prior to the use of CRISPR as a technology, when researchers studied the system for the (very cool) role it plays in bacterial defense, publications-per-year consistently fell below 100. In other words, it’s a big deal.

 

In fact, during my 10 years at the bench I have never witnessed a discovery as transformative as CRISPR/Cas9. Overnight, reverse genetics on organisms whose genomes were not amenable to classical editing techniques became possible. And with the increasing affordability of high-throughput sequencing, manipulation of the genomes of non-model organisms is now feasible. Of course there are imperfections with the technology that require greater understanding to circumvent (specificity, e.g.), but the development of CRISPR as a tool for genomic engineering jolted biological research, fostering advances more accurately measured in leaps rather than steps. These leaps – and those expected to occur in the future – landed the discoverers of CRISPR/Cas9 at the top of the list of predicted recipients of the Nobel Prize in Chemistry; though they didn’t win this year (the award went to researchers of the not-totally-unrelated field of DNA repair), I anticipate that a win lies ahead. The rapid success of CRISPR genome editing has also sparked patent battles and incited public debate over the ethics of applying the technology to human genomes. With all of the media attention, it’s hard not to know about CRISPR.

 

The transformative nature of CRISPR/Cas9 does not, however, end with genome editing; in fact, an even larger realm of innovation appears when you kill the enzymatic activity of Cas9. No longer able to cut DNA, dead Cas9 (dCas9) becomes an incredibly good DNA-binding protein guided to its target by a programmable RNA molecule (guide RNA, gRNA). If we think of active Cas9 as a way to better understand genes (through deletions and mutations), then dCas9 is the route to get to know the genome a bit better – a particularly enticing mission for those, including myself, invested in the field of Genomics. From high-throughput targeted gene activation and repression screens to epigenome editing, dCas9 is helping scientists probe the genome in ways that weren’t possible before. Here, I put forth some of the best (in my humble opinion) applications, actual and potential, of CRISPR technology that go beyond genome editing.

 

Cas9 and Functional (Epi)Genomics

 

For many years the genome was considered as the totality of all genes in a cell; the additional junk DNA found was merely filler between the necessary gene units, stitching together chromosomes. We’ve come a long way since this naiveté, especially in recent years. We understand that the so-called junk DNA contains necessary regulatory information to get the timing and position of gene expression correct; and now, more than ever, we have a greater appreciation for the genome as a complex macromolecule in its own right, participating in gene regulation rather than acting as a passive reservoir of genetic material. The genome, it has been shown, is much more than just its sequence.

 

The epigenome, consisting of a slew of modifications to the DNA and the histones around which the DNA is wrapped, as well as the 3D organization of the genome in the nucleus, collaborates with DNA binding proteins to accurately interpret sequence information to form a healthy, functional cell. While mutations and/or deletions can be made – more easily, now, with Cas9 – to genomic sequences to test functionality, it is much harder to conduct comparable experiments on the epigenome, especially in a targeted manner. Because of the inability to easily perturb features of the epigenome and observe the consequences, our understanding of it is limited to correlative associations. Distinct histone modifications are associated with active versus inactive genes, for example; but, how these modifications affect or are affected by gene expression changes remains unknown.

 

Taking advantage of the tight binding properties of dCas9, researchers have begun to use the CRISPR protein as a platform to recruit a variety of functionalities to a genomic region of interest. Thus far, this logic has most commonly been employed to activate and/or repress gene expression through recruitment of dCas9 fused to known transcriptional activator or repressor proteins. Using this technique, scientists have conducted high-throughput screens to study the role of individual – or groups of – genes in specific cellular phenotypes by manipulating the endogenous gene locus. And, through a clever extension of the gRNA to include a hairpin bound by known RNA-binding proteins, the targeted functionality has been successfully transferred from dCas9 to the gRNA, allowing for simultaneous activation and repression of independent genes in the same cell with a single dCas9 master regulator – the beginnings of a simple, yet powerful, synthetic gene circuit.

 

Though powerful in its ability to decipher gene networks, dCas9-based activation and repression screens are still gene-centric; can this recruitment technique help us better understand the epigenome? The first attempts at addressing this question used dCas9 to target histone acetyltransferase, p300, to catalyze the acetylation of lysine 27 on histone 3 (H3K27) at specific loci. The presence of H3K27 at gene regulatory regions has been known to be strongly associated with active gene expression at the corresponding gene(s), but the direction of the histone modification-gene expression relationship remained in question. Here, Hilton et al. demonstrate that acetylation of regulatory regions distal to gene promoters strongly activates gene expression, demonstrating causality of the modification.

 

More recently, recruitment of a dCas9-KRAB repressor fusion to known regulatory regions catalyzed trimethylation of lysine 9 on histone 3 (H3K9) at the enhancer and associated promoters, effectively silencing enhancer activity. Though there have only been a few examples published, it will likely not be long until researchers employ this technique for the targeted analysis of additional epigenome modifiers. Already, targeted methylation, demethylation and genomic looping have been accomplished using the DNA-binders, Zinc Finger Nucleases and TALEs. With the increased simplicity in design of gRNAs, dCas9 is predicted to surpass these other proteins in its utility to link epigenome modifications with gene expression data.

 

Visualization of Genomic Loci

 

When you treat dCas9 as a bridge between DNA and an accessory protein, just as in the recruitment of activators, repressors and epigenome modifiers, there are few limits to what can be targeted to the genome. Drawing inspiration from the art of observation that serves as the foundation of scientific pursuit, researchers have begun to test whether dCas9 can be used to visualize genomic loci and observe their position, movements, and interactions simply by recruiting a fluorescent molecule to the locus of interest.

 

This idea, of course, is not entirely new. In situ hybridization techniques (ISH, and its fluorescent counterpart, FISH) have been successfully used to label locus position in fixed cells but cannot offer any information about the movement of chromosomes in living cells. Initial studies to conquer this much harder feat made use of long tracts of repetitive DNA sequence bound by its protein binding partner fused to fluorescing GFP; though surely an advance, this technique is limited because of the requirement to engineer the repetitive DNA motifs prior to imaging.

 

To circumvent this need, researchers have recently made use of TALEs and dCas9 (and here) carrying fluorescent tags to image unperturbed genomic loci in a variety of live cell cultures. The catch is that both TALEs and dCas9 perform much better when targeting repetitive regions, such that multiple copies of the fluorescent molecule are recruited, enhancing the intensity of the signal. Tiling of fluorescent dCas9 across a non-repetitive region using 30-70 neighboring gRNAs (a task made much more feasible with CRISPR versus TALEs) can similarly pinpoint targeted loci, albeit with much higher background. As is, the technique lacks the resolution desired for live imaging, but current advances in super-resolution microscopy and single-molecule tracking, as well as improvements in the brightness of fluorescent molecules available, will likely spur improvements in dCas9 imaging in the coming years.

 

Finally, dCas9 is not only useful in live cells. CASFISH, an updated Cas9-mediated FISH protocol, has been successfully used to label genomic loci in fixed cells and tissue. This updated version holds many benefits over traditional FISH including a streamlined protocol; but, most notably, CASFISH does not require the denaturation of genomic DNA, a necessary step for the hybridization of FISH probes, eliminating positional artifacts due to harsh treatment of the cells. Unfortunately, as of now, CASFISH also suffers from a need for repetitive sequences or tiling of gRNAs to increase signal intensity at the locus of interest.

 

Targeting RNA with Cas9

 

From cutting to tagging to modifying, it is clear that Cas9 has superstar potential when teamed up with double-stranded DNA (dsDNA); however, recent data suggests that this potential may not be limited to DNA. Mitchell O’Connell and colleagues at Berkeley found that Cas9 could bind and cleave single-stranded RNA (ssRNA) when annealed to a short DNA oligonucleotide containing the necessary NGG sequence. In addition, the authors made use of dCas9 and biotin-tagged gRNA to capture and immobilize targeted messenger RNA from cell extract. Though it remains to be shown, this proof-of-principle binding of dCas9 suggests that it is plausible to recruit a variety of functionalities to RNA as has been done for dsDNA. Recruitment of RNA processing factors through Cas9 could potentially enhance translation, generate known RNA editing events (deamination, e.g.), regulate alternative splicing events, or even allow visualization of RNA localization with conjugated fluorescent molecules. Again, each of these processes requires no modification to the RNA sequence or fixation, both of which can disrupt normal cell physiology.

 

Improving CRISPR Technology

 

The development of CRISPR technology, particularly the applications discussed here, is still in its infancy. It will likely take years of research for Cas9 and dCas9 to reach their full potential, but advances are underway. These developments pertain not only to the applications discussed here, but also genome engineering.

 

Specificity of Cas9

 

Cas9’s biggest flaw is its inability to stay focused. Off-target (OT) binding (and here) of Cas9 and DNA cutting have been reported and both present problems. With particular relevance to dCas9-based applications, promiscuous binding of Cas9 to regions of the genome that contain substantial mismatches to the gRNA sequence raises concerns of non-specific activity of the targeted functionality. Efforts to reduce OT binding are needed to alleviate these concerns, but progress has been made with the finding that truncated gRNA sequences are less tolerant of mismatches, reducing off-target Cas9 activity, if not also binding.

 

Temporal Precision of Cas9

 

One of the most exciting developments in dCas9 genome targeting is the potential to manipulate the genome and epigenome in select cell populations within a whole animal to gain spatial resolution in our understanding of genome regulation; however, as we have learned over the years, gene expression patterns don’t only change with space, but also time. A single cell, for example, will alter its transcriptome at different points during development or in response to external stimulus. The development of split versions of Cas9 (and dCas9), which require two-halves of the protein to be expressed simultaneously for function, will not only improve spatial specificity of Cas9 activity but holds the potential to restrict its activity temporally. Drug-inducible and photoactivatable (!) versions of split Cas9 restrict function to time windows of drug treatment or light activation, respectively. In addition, a ligand-sensitive intein has been shown to temporally control Cas9 activity by releasing functional Cas9 through protein splicing only in the presence of ligand.

 

Expanding the CRISPR Protein Repertoire

 

Finally, CRISPR technology will likely benefit from taking all of the weight off of the shoulders of Cas9. Progress toward designing Cas9 molecules with altered PAM specificity, as well as the isolation of Cas9 from different species of bacteria, has helped expand the collection of genomic sites that can be targeted. It has also enabled multiplexing of orthogonal CRISPR proteins in a single cell to effect multiple functions simultaneously. More recently, the Zhang lab isolated an alternative type II CRISPR protein, Cpf1, purified from Francisella novicida. Cas9’s new BFF is also able to cut genomic DNA (as shown in human cells), but in a slightly different fashion than Cas9, generating sticky overhangs rather than blunt ends. Cpf1 also naturally harbors an alternate PAM specificity; rather than targeting sequences upstream of NGG, it prefers T-rich signatures (TTN), further expanding the genomes and genomic sites that can be targeted.

 

CRISPR/Cas9 has already proven to be one of the most versatile tools in the biologist’s toolbox to manipulate the genomes of a variety of species, but its utility continues to grow beyond these applications. Targeting Cas9 to the mitochondria rather than the nucleus can specifically edit the mitochondrial genome, with implications for disease treatment. Cas9 has been used for in vitro cloning experiments when traditional restriction enzymes just won’t do. And, by directly borrowing the concept of Cas9 immunity from bacteria, researchers have enabled enhanced resistance to viruses in plants engineered with Cas9 and gRNAs. While we ponder what innovative technique will come next, it’s important to think about how this cutting-edge technology that promises to bolster both basic and clinical research came to be: this particular avenue of research was paved entirely by machinery provided by the not-so-lowly bacteria. That’s pretty amazing, if you ask me.

Double Strand Breaks For The Win

 

By Rebecca Delker, PhD

The blueprint of an organism is its genome, the most fundamental code necessary for life. The carefully ordered – and structured – composition of As, Ts, Cs and Gs provides the manual that each cell uses to carry out its diverse function. As such, unintended alterations to this code often produce devastating consequences, manifesting themselves in disease phenotypes. From mutations to insertions and deletions, changes in the sequence of nucleotides alter the cell’s interpretation of the genome, like changing the order of words in a sentence. However, arguably one of the most threatening alterations is the double-strand break (DSB), a fracture in the backbone of the helical structure, splitting a linear piece of DNA in two, as if cut by molecular scissors. While the cell has a complex set of machinery designed to repair the damage, this process can be erroneous generating deletions, or even worse, translocations – permanently reordering the pages of the manual and ultimately transforming the cell. Given the central role translocations can play in oncogenic transformation, DSBs have understandably received a bad rap; but, as can be expected, not all is black and white and it’s worth asking whether there is an upside to DSBs.

 

One such commendable pursuit of the DSB serves to expand the capabilities of our genome. While it is true that the genome is the most basic code necessary for life, many of the processes within a cell actually require changes to the code. These can occur at all levels of the Central Dogma – modifications of proteins, RNA, and even DNA. B- and T-lymphocytes, cells that provide a good amount of heft to our immune system, are notable for their DNA editing skills. Tasked with protecting an organism from billions of potential pathogens, B- and T-cells must generate receptors specific for each unique attack. Rather than encoding each of these receptors in the genome – an impossibility due to size restrictions – B- and T-lymphocytes use DSBs to cut and paste small gene fragments to build a myriad of different receptor genes, each with a unique sequence and specificity (reviewed here). For immune cells, and for the survival of the organism, these DSBs are essential. Although tightly controlled, DNA rearrangements in immune cells are mechanistically similar to the sinister DSB-induced translocations that promote cancer formation; however, rather than causing disease, they help prevent it.

 

New research published this summer points to exciting, and even more unusual uses of DSBs in the regulation of gene expression. In a quest to understand the molecular effects of DSBs that are causally linked to a variety of neurological disorders, Ram Madabhushi, Li-Huei Tsai and colleagues instead discovered a necessary role for DSBs in the response of neurons to external stimulus. To adapt to the environment and generate long-term memories, changes in the “morphology and connectivity of neural circuits” occur in response to neuron-activation. This synaptic plasticity relies on a rapid increase in gene expression of a select set of early-response genes responsible for initiating the cascade of cellular changes needed for synaptogenic processes. In their paper published in Cell this summer, the authors reveal that the formation of DSBs in the promoter of early-response genes induces gene expression in response to neuron stimulation.

 

By treating neuronal cells with etoposide, an inhibitor of type-II topoisomerase enzymes (TopoII) that causes DSB formation, the researchers expected to find that DSBs interfere with transcription. In fact, most genes found to be differentially expressed in cells treated with the drug showed a decrease in expression; however, a small subset of genes, including the early-response genes, actually increased. Through a series of in vivo and ex vivo experiments, the researchers showed that even in the absence of drug treatment, DSB formation in the promoters of early-response genes is critical for gene expression – beautifully weaving a connection between neuronal activation, DSB formation and the rapid initiation of transcription in this subset of genes.

 

The serendipitous discovery of the positive effect of etoposide on gene expression lead the researchers to focus in on the role of topoisomerases, the guardians of DNA torsion, in DSB formation. As a helical structure composed of intertwined strands, nuclear processes like replication and transcription cause the over- or under-twisting of the DNA helix, leading the DNA molecule to twist around itself to relieve the torsional stress and form a supercoiled structure. Topoisomerases return DNA to its relaxed state by generating breaks in the DNA backbone – single-strand breaks by type I enzymes and DSBs by type II – untwisting the DNA and religating the ends. While etoposide can artificially force sustained DSBs, physiological TopoII-induced breaks are typically too transient to allow recognition by DNA repair proteins. The finding that TopoIIb-induced DSBs at the promoters of neuronal early-response genes are persistent and recognized by DNA repair machinery suggests a non-traditional role for TopoII enzymes, and DSBs, in transcription initiation and regulation.

 

In fact, the contribution of TopoII and DSBs in the regulation of neuronal genes may not be so niche. Another study published recently found a similar relationship between transcriptional activation and Topo-mediated DSB formation. Using the primordial cells of the germline in C. elegans as a model system, Melina Butuči, W. Matthew Michael and colleagues found that the abrupt increase in transcription as embryonic cells switch from a dependence on maternally provided RNA and protein to activation of its own genome induced widespread DSB formation. Amazingly, TOP-2, the C. elegans ortholog of TopoII is required for break formation; but, in contrast to neuronal activation, these DSBs occur in response to transcription rather than as a causative agent.

 

These recent studies build upon a growing recognition of a potentially intimate relationship between DSBs, torsion and transcription. DNA repair proteins, as well as topoisomerase enzymes have been shown to physically interact with transcription factors and gene regulatory elements; topoisomerase I and II facilitate the transcription of long genes; and, as in neuronal cells, studies of hormone-induced gene expression in cell culture reveal an activation mechanism by which TopoIIb induces DSBs selectively in the promoters of hormone-sensitive genes. Thus, DSBs may constitute a much broader mechanism for the regulation of gene-specific transcription than previously thought.

 

Given the grave danger associated with creating breaks in the genome, it is curious that the use of DSBs evolved to be an integral component of the regulation of transcription – an inescapable and ubiquitously employed process; however, as we expand our understanding of transcription to include the contribution of the higher-order structure of DNA, the utility of this particular evolutionary oddity comes into focus. Genomic DNA is not naked, but rather wrapped around histone proteins and packaged in the 3D space of the nucleus such that genomic interactions influence gene expression. Changes in the torsion and supercoiling of DNA have been associated with histone exchange, as well as changes in the affinity of DNA-binding proteins for DNA. In addition, the necessity of topoisomerase for the transcription of long genes occurs early as RNA polymerase transitions from initiation to elongation, suggesting that the role of TopoI and II is not to relieve transcription-induced torsion, but rather to resolve an inhibitory, likely 3D, genomic structure that is specific to genes of longer length. A similar mechanism may be involved at the neuronal early-response genes. In these cells, genomic sites of TopoIIb-binding and DSB-formation significantly overlap binding sites of CTCF – a crucial protein involved in genomic looping and higher-order chromatin structure – and, again, DNA breaks may function to collapse a structure constraining gene activation. Whatever the exact mechanisms at play here, these results inspire further inquiry into the relationship between DSBs, genome topology and transcription.

 

A cell’s unique interpretation of the genome via distinct gene expression programs is what generates cell diversity in multicellular organisms. Immune cells, like B- and T-lymphocytes, are different from neurons, which are different from skin cells, despite working from the same genomic manual. In B- and T-cells, DSBs are essential to piece together DNA fragments in a choose-your-own-adventure fashion to produce a reorganization of the manual necessary for cell function. And, as is emphasized in this growing body of research, DSBs function along with a variety of other molecular mechanisms to highlight, underline, dog-ear, and otherwise mark-up the genome in a cell-specific manner to facilitate the activation and repression of the correct genes at the correct time. Here, DSBs may not reorder the manual, but, nevertheless, play an equally important role in promoting proper cell function.