Clinical laboratories involved in genetic testing may find this welcomed news, after a pair of studies conducted in 2019 raised concerns about CRISPR base editing. The researchers of those studies observed that it “causes a high number of unpredictable mutations in mouse embryos and rice,” Chemical and Engineering News (C&EN) reported, adding, “Other groups have raised concerns about off-target mutations caused when the traditional CRISPR-Cas9 form of gene editing cuts DNA at a location that it wasn’t supposed to touch. The results of the new studies are surprising, however, because scientists have lauded base editors as one of the most precise forms of gene editing yet.”
Nevertheless, UC Berkeley’s latest breakthrough is expected to drive development of new and more accurate CRISPR-Cas genome-editing tools, which consist of base editors as well as nucleases, transposases, recombinases, and prime editors.
Understanding CRISPR Base Editors At a ‘Deeper Level’
Harvard University Chemistry and Chemical Biology Professor David Liu, PhD, who co-authored the study, explained the significance of this latest discovery.
“While base editors are now widely used to introduce precise changes in organisms ranging from bacteria to plants to primates, no one has previously observed the three-dimensional molecular structure of a base editor,” he said in a UC Berkeley news release. “This collaborative project reveals the beautiful molecular structure of a state-of-the-art highly-active base editor—ABE8e—caught in the act of engaging a target DNA site.”
UC Berkeley Professor Jennifer Doudna, PhD (above), who served as senior author of the study, says scientists may now have the information necessary to refine base editors and improve their precision and genome-targeting ability. “This structure helps us understand base editors at a much deeper level,” she said in the UC Berkeley statement. “Now that we can see what we’re working with, we can develop informed strategies to improve the system.” (Photo copyright: UC Berkeley.)
Jennifer Doudna, PhD, UC Berkeley Professor, Howard Hughes Medical Institute Investigator, and senior author of the study, has been a leading figure in the development of CRISPR-Cas9 gene editing. In 2012, Doudna and Emmanuelle Charpentier, PhD, Founding, Scientific and Managing Director at Max Planck Unit for the Science of Pathogens in Berlin, led a team of researchers who “determined how a bacterial immune system known as CRISPR-Cas9 is able to cut DNA, and then engineered CRISPR-Cas9 to be used as a powerful gene editing technology.” In a 2017 news release, UC Berkeley noted that the work has been described as the “scientific breakthrough of the century.”
Viewing the Base Editor’s 3D Shape
CRISPR-Cas9 gene editing allows scientists to permanently edit the genetic information of any organism, including human cells, and has been used in agriculture as well as medicine. A base editor is a tool that manipulates a gene by binding to DNA and replacing one nucleotide with another.
According to the recent UC Berkeley news release, the research team used a “high-powered imaging technique called cryo-electron microscopy” to reveal the base editor’s 3D shape.
Genetic Engineering and Biotechnology News notes that, “The high-resolution structure is of ABE8e bound to DNA, in which the target adenine is replaced with an analog designed to trap the catalytic conformation. The structure, together with kinetic data comparing ABE8e to earlier ABEs [adenine base editors], explains how ABE8e edits DNA bases and could inform future base-editor design.”
The graphic above, taken from the UC Berkeley news release, shows the “3D structure of a base editor, comprised of the Cas9 protein (white and gray), which binds to a DNA target (teal and blue helix) complementary to the RNA guide (purple), and the deaminase proteins (red and pink), which switch out one nucleotide for another.” (Image and caption copyright: UC Berkeley.)
Knowing the Cas9 fusion protein’s 3D structure may help eliminate unintended off-target effects on RNA, extending beyond the targeted DNA. However, until now, scientists have been hampered by their inability to understand the base editor’s structure.
“If you really want to design truly specific fusion protein, you have to find a way to make the catalytic domain more a part of Cas9, so that it would sense when Cas9 is on the correct target and only then get activated, instead of being active all the time,” study co-first author Audrone Lapinaite, PhD, said in the news release. At the time of the study, Lapinaite was a postdoctoral fellow at UC Berkeley. She is now an assistant professor at Arizona State University.
“As a structural biologist, I really want to look at a molecule and think about ways to rationally improve it. This structure and accompanying biochemistry really give us that power,” added UC Berkeley postdoctoral fellow Gavin Knott, PhD, another study co-author. “We can now make rational predications for how this system will behave in a cell, because we can see it and predict how it’s going to break or predict ways to make it better.”
Clinical laboratory leaders and pathologists will want to monitor these new advances in CRISPR technology. Each breakthrough has the power to fuel development of cost-effective, rapid point-of-care diagnostics.
CRISPR-Cas9 connection to cancer prompts research to investigate different approaches to gene editing
Dark Daily has covered CRISPR-Cas9 many times in previous e-briefings. Since its discovery, CRISPR, or Clustered Regularly Interspaced Short Palindromic Repeats, has been at the root of astonishing breakthroughs in genetic research. It appears to fulfill precision medicine goals for patients with conditions caused by genetic mutations and has anatomic pathologists, along with the entire scientific world, abuzz with the possibilities such a tool could bring to diagnostic medicine.
All of this research has contributed to a deeper understanding of how cells function. However, as is often the case with new technologies, unforeseen and problematic questions also have arisen.
“Here we report significant on-target mutagenesis, such as large deletions and more complex genomic rearrangements at the targeted sites in mouse embryonic stem cells, mouse hematopoietic progenitors, and a human differentiated cell line,” wrote the authors in their introduction.
Another study, this one conducted by biomedical researches at Cambridge, Mass., and published in Nature, describes possible toxicity caused by Cas9.
“Our results indicate that Cas9 toxicity creates an obstacle to the high-throughput use of CRISPR-Cas9 for genome engineering and screening in hPSCs [human pluripotent stem cells]. Moreover, as hPSCs can acquire P53 mutations, cell replacement therapies using CRISPR-Cas9-enginereed hPSCs should proceed with caution, and such engineered hPSCs should be monitored for P53 function.”
Essentially what both groups of researchers found is that CRISPR-Cas9 cuts through the double helix of DNA, which the cell responds to as it would any injury. A gene called p53 then directs a cellular “first-aid kit” to the “injury” site that either initiates self-destruction of the cell or repairs the DNA.
Therefore, in some instances, CRISPR-Cas9 is inefficient because the repaired cells continue to function. And, the repair process involves the p53 gene. P53 mutations have been implicated in ovarian, colorectal, lung, pancreatic, stomach, liver, and breast cancers.
Though important, some experts are downplaying the significance of the findings.
Erik Sontheimer, PhD (above), Professor, RNA Therapeutics Institute, at the University of Massachusetts Medical School, told Scientific American that the two studies are important, but not show-stoppers. “This is something that bears paying attention to, but I don’t think it’s a deal-breaker,” he said. (Photo copyright: University of Massachusetts.)
“It’s something we need to pay attention to, especially as CRISPR expands to more diseases. We need to do the work and make sure edited cells returned to patients don’t become cancerous,” Sam Kulkarni, PhD, CEO of CRISPR Therapeutics, told Scientific American.
Both studies are preliminary. The implications, however, is in how genes that have become corrupted are used.
A team from the Salk Institute may have found a solution. They are investigating a different enzyme—Cas13d—which, in conjunction with CRISPR would target RNA rather than DNA. “DNA is constant, but what’s always changing are the RNA messages that are copied from the DNA. Being able to modulate those messages by directly controlling the RNA has important implications for influencing a cell’s fate,” Silvana Konermann, PhD, a Howard Hughes Medical Institute (HHMI) Hanna Gray Fellow and member of the research team at Salk, said in a news release.
The Salk team published their findings in the journal Cell. The paper describes how “scientists from the Salk Institute are reporting for the first time the detailed molecular structure of CRISPR-Cas13d, a promising enzyme for emerging RNA-editing technology. They were able to visualize the enzyme thanks to cryo-electron microscopy (cryo-EM), a cutting-edge technology that enables researchers to capture the structure of complex molecules in unprecedented detail.”
The researchers think that CRISPR-Cas13d may be a way to make the process of gene editing more effective and allow for new strategies to emerge. Much like how CRISPR-Cas9 led to research into recording a cell’s history and to tools like SHERLOCK (Specific High-sensitivity Enzymatic Reporter unLOCKing), a new diagnostic tool that works with CRISPR and changed clinical laboratory diagnostics in a foundational way.
Each discovery will lead to more branches of inquiry and, hopefully, someday it will be possible to cure conditions like sickle cell anemia, dementia, and cystic fibrosis. Given the high expectations that CRISPR and related technologies can eventually be used to treat patients, pathologists and medical laboratory professionals will want to stay informed about future developments.