Posted on by Dr. Francis Collins
Amid all the headlines and uncertainty surrounding the current COVID-19 pandemic, it’s easy to overlook the important progress that biomedical research is making against other diseases. So, today, I’m pleased to share word of what promises to be the first effective treatment to help young people suffering from the consequences of a painful, often debilitating genetic disorder called neurofibromatosis type 1 (NF1).
This news is particularly meaningful to me because, 30 years ago, I led a team that discovered the gene that underlies NF1. About 1 in 3,000 babies are born with NF1. In about half of those affected, a type of tumor called a plexiform neurofibroma arises along nerves in the skin, face, and other parts of the body. While plexiform neurofibromas are not cancerous, they grow steadily and can lead to severe pain and a range of other health problems, including vision and hearing loss, hypertension, and mobility issues.
The good news is the results of a phase II clinical trial involving NF1, just published in the New England Journal of Medicine. The trial was led by Brigitte Widemann and Andrea Gross, researchers in the Center for Cancer Research at NIH’s National Cancer Institute.
The trial’s results confirm that a drug originally developed to treat cancer, called selumetinib, can shrink inoperable tumors in many children with NF1. They also establish that the drug can help affected kids make significant improvements in strength, range of motion, and quality of life. While selumetinib is not a cure, and further studies are still needed to see how well the treatment works in the long term, these results suggest that the first effective treatment for NF1 is at last within our reach.
Selumetinib blocks a protein in human cells called MEK. This protein is involved in a major cellular pathway known as RAS that can become dysregulated and give rise to various cancers. By blocking the MEK protein in animal studies and putting the brakes on the RAS pathway when it malfunctions, selumetinib showed great initial promise as a cancer drug.
Selumetinib was first tested several years ago in people with a variety of other cancers, including ovarian and non-small cell lung cancers. The clinical research looked good at first but eventually stalled, and so did much of the initial enthusiasm for selumetinib.
But the enthusiasm picked up when researchers considered repurposing the drug to treat NF1. The neurofibromas associated with the condition were known to arise from a RAS-activating loss of the NF1 gene. It made sense that blocking the MEK protein might blunt the overactive RAS signal and help to shrink these often-inoperable tumors.
An earlier phase 1 safety trial looked promising, showing for the first time that the drug could, in some cases, shrink large NF1 tumors . This fueled further research, and the latest study now adds significantly to that evidence.
In the study, Widemann and colleagues enrolled 50 children with NF1, ranging in age from 3 to 17. Their tumor-related symptoms greatly affected their wellbeing and ability to thrive, including disfigurement, limited strength and motion, and pain. Children received selumetinib alone orally twice a day and went in for assessments at least every four months.
As of March 2019, 35 of the 50 children in the ongoing study had a confirmed partial response, meaning that their tumors had shrunk by more than 20 percent. Most had maintained that response for a year or more. More importantly, the kids also felt less pain and were more able to enjoy life.
It’s important to note that the treatment didn’t work for everyone. Five children stopped taking the drug due to side effects. Six others progressed while on the drug, though five of them had to reduce their dose because of side effects before progressing. Nevertheless, for kids with NF1 and their families, this is a big step forward.
Drug developer AstraZeneca, working together with the researchers, has submitted a New Drug Application to the Food and Drug Administration (FDA). While they’re eagerly awaiting the FDA’s decision, the work continues.
The researchers want to learn much more about how the drug affects the health and wellbeing of kids who take it over the long term. They’re also curious whether it could help to prevent the growth of large tumors in kids who begin taking it earlier in the course of the disease, and whether it might benefit other features of the disorder. They will continue to look ahead to other potentially promising treatments or treatment combinations that may further help, and perhaps one day even cure, kids with NF1. So, even while we cope with the COVID-19 pandemic, there are reasons to feel encouraged and grateful for continued progress made throughout biomedical research.
 Selumitinib in children with inoperable plexiform neurofibromas. New England Journal of Medicine. Gross AM, Wolters PL, Dombi E, Baldwin A, Whitcomb P, Fisher MJ, Weiss B, Kim A, Bornhorst M, Shah AC, Martin S, Roderick MC, Pichard DC, Carbonell A, Paul SM, Therrien J, Kapustina O, Heisey K, Clapp DW, Zhang C, Peer CJ, Figg WD, Smith M, Glod J, Blakeley JO, Steinberg SM, Venzon DJ, Doyle LA, Widemann BC. 18 March 2020. N Engl J Med. 2020 Mar 18. [Epub ahead of publication.]
 Activity of selumetinib in neurofibromatosis type 1-related plexiform neurofibromas. Dombi E, Baldwin A, Marcus LJ, Fisher MJ, Weiss B, Kim A, Whitcomb P, Martin S, Aschbacher-Smith LE, Rizvi TA, Wu J, Ershler R, Wolters P1, Therrien J, Glod J, Belasco JB, Schorry E, Brofferio A, Starosta AJ, Gillespie A, Doyle AL, Ratner N, Widemann BC. N Engl J Med. 2016 Dec 29;375(26):2550-2560.
Neurofibromatosis Fact Sheet (National Institute of Neurological Disorders and Stroke/NIH)
Brigitte Widemann (National Cancer Institute/NIH)
Andrea Gross (National Cancer Institute/NIH)
NIH Support: National Cancer Institute
Posted on by Dr. Francis Collins
Tomorrow is Rare Disease Day at NIH, marking the 12th year that this annual event has been held on the NIH campus. Similar gatherings have been organized independently around the world this week, all to raise awareness for the nearly 7,000 rare diseases, some affecting just a few dozen people. But, collectively, rare diseases are hardly rare. One in 10 Americans has a rare disease (defined as affecting 200,000 or fewer individuals in the US), and about half are children. Without needed treatments, about 30 percent of these children will die by age 5.
To join everyone in raising awareness, I wanted to feature on my blog a unique perspective about rare diseases, and David Fajgenbaum certainly has one. Fajgenbaum is an immunologist and NIH grantee at the Perelman School of Medicine, University of Pennsylvania, Philadelphia. When Fajgenbaum isn’t running studies or clinical trials, he must remain vigilant of his own health. Fajgenbaum has a rare disease called idiopathic multicentric Castleman disease (iMCD), and this devastating condition, which emerged while he was in medical school, nearly claimed his life several times.
Now 34 years old and in a long remission, Fajgenbaum can discuss rare diseases as a doctor, as a patient, as a researcher, and as an advocate. His personal journey, published in his recent book Chasing My Cure, is a gripping read. Fajgenbaum was kind enough to answer a few of my questions on rare diseases and share some of his lessons learned.
The last time that I saw you, David, you looked great. How long have you been in remission?
I have been in remission for 73.83 months. I say 73.83, because I know that I can’t round up—I may relapse tomorrow. But I also refuse to round down because so many colleagues and I have worked so hard for every day of remission for me and other patients with my disease.
For me, every day is particularly special, because I never thought that I would be alive this long. As you know, I became deathly ill during medical school in 2010 and even had my last rites read to me when my doctors didn’t think I would survive. I was eventually diagnosed with idiopathic multicentric Castleman disease (iMCD), which is like a deadly cross between cancer and autoimmunity. Chemotherapy saved my life, but I would go on to have four near-death relapses.
After one of those relapses, I got out of the hospital and dedicated my life to conducting iMCD research and co-founded the Castleman Disease Collaborative Network (CDCN). Later, I identified a particular cellular pathway called mTOR that was highly active in my samples. I began testing on myself an mTOR inhibitor [sirolimus]—developed 30 years before and approved for kidney transplantation but never considered for iMCD. It’s this drug that has kept me in remission for the last 73.83 months and helped other people. During this time, I’ve been able to marry my wife, have a daughter, help launch a new center at Penn specializing in rare diseases, and write a book to share my personal journey with others.
As a physician-scientist and as a person with a rare disease, what have you learned about the biomedical research process?
I’ve learned so much, but I’d like to highlight three lessons in particular. First, we must leverage all perspectives to prioritize research and give us the best chance of translating research into meaningful breakthroughs. The traditional approach to rare disease research involves a subset of researchers within a rare disease field submitting their best ideas for funding and a panel selecting the best applicant.
Through the CDCN, we’ve spearheaded a new approach called the Collaborative Network Approach, where we crowdsource research questions from the entire community of patients, physicians, and researchers (not just a subset of researchers) and then recruit the best researchers in the world (not just from within the Castleman disease field) to perform the prioritized studies. We’re now working to improve and spread this approach to other diseases.
Second, collaboration between all players is critical. Patient advocacy groups are uniquely positioned to serve as the glue between all stakeholders. Researchers and physicians need to share ideas, data, and samples with one another. Patients need to be actively involved in research question prioritization and study design. Biopharma and the Food and Drug Administration (FDA) need to be engaged early in the process of research discoveries and drug development.
Third, we must leverage all 1,500-plus, existing FDA-approved drugs to help as many patients without any options as quickly as possible. As you know, less than 5 percent of the nearly 7,000 rare diseases have an FDA-approved therapy, but many diseases share similar cellular and genetic defects that could make them susceptible to the same drugs. I’m literally alive today thanks to a drug developed for another disease. How many of the drugs approved for one disease may be effective for many of the 7,000 diseases without any? I don’t know the answer, but I hope we can begin to address this important question and incentivize repurposing.
In your experience, how can people with rare diseases help to advance progress for their conditions?
There is so much work to be done for so many rare diseases. Sometimes it can feel so overwhelming and like “what can I really do?”
But I’ve learned that there are so many ways that we can each contribute and so many incredible examples of advocates who have made a difference for themselves and those that they love. Cystic fibrosis and chordoma are just two of many examples where patient-advocates have been critical partners in transforming their diseases.
People with rare diseases can raise funds for research. Every dollar truly counts. We can work with existing organizations for our disease to ensure that those funds are distributed as efficiently and effectively as possible. If there are major gaps within our rare disease fields that aren’t being addressed by existing organizations, we can start new rare disease organizations (but we should try to avoid this whenever possible). We can contribute samples and data towards research, participate in clinical trials, and share with other patients about our experiences. We can advocate for new drug development and repurposing already-FDA approved drugs for our diseases.
What would you tell other researchers who are studying rare diseases?
I would tell other rare disease researchers that you are doing such important work. You give us hope that a treatment can be identified that will change our lives. It’s an incredible responsibility and incredibly stressful. There are unfortunately far too many scientific questions and diseases with major unmet need for any of us to compete over the use of samples and data. We have to share these within our fields. And we must also work together across rare diseases. We can’t continue to reinvent the wheel; we must share learnings with one another
I enjoyed doing the CastleMan Warrior Flex with you. Tell us more about what it represents?
Doing the CastleMan Warrior Flex with you is one of my favorite pictures. In fact, it’s hanging up in my office.
Castleman disease was named after Dr. Benjamin Castleman, who first described our disease in 1954. We have repurposed the “Castleman” name to be a “CastleMan Warrior” (below is our cartoon mascot). We do the “CastleMan Warrior” Flex to raise awareness for Castleman disease and rare diseases generally—we’re all warriors in the rare disease space.
What are your future plans as a rare disease advocate and as a researcher?
We’ve made a lot of progress for Castleman disease: we’ve advanced our findings about mTOR towards a clinical trial, gained approval for the treatment siltuximab for iMCD, developed diagnostic criteria and treatment guidelines, and invested about $1.5 million into Castleman disease research, which has led to over $7 million in additional funding from other sources.
But we still have important work ahead of us. The treatments sirolimus and siltuximab work for only a portion of all iMCD patients. We need to identify more effective treatments for all forms of Castleman disease.
I will continue to study Castleman disease and other diseases at the intersection of autoimmunity and oncology to gain insights into how the immune system works in myriad diseases. In parallel, I will continue to advocate for the adoption of the “Collaborative Network Approach” to crowdsource all stakeholder perspectives as well as for new models for drug repurposing.
Any other issues that you’d like to address?
I feel a responsibility to share with the world the lessons that I’ve learned about life from nearly dying five times. This is a major reason that I wrote my book.
One lesson that I think about a lot is related to my growing up playing football. Some of my games were extended into an overtime period to decide the outcome. In overtime, every second counts and you’re totally focused on what’s important. I’ve lived with that exact same feeling ever since I had my last rites read to me.
I’ve also learned that humor can be incredibly powerful. You may think that a good laugh may be the last thing that you’d want to do when you’re dying in the ICU. But laughing with the people that I love actually helped me feel like I could transcend my illness, and it helped to connect us.
My greatest regrets on my deathbed were not things that I had done or said. I regretted what I didn’t do or didn’t say and that I would no longer be able to do. I now follow the motto: “Think It, Do It.” In other words, we should reflect on what we’re hoping for and then turn our hopes into action.
Finally, I’ve learned that it really takes a strong team to make a difference in the world, especially against diseases. If it was just me on my own, we would have made less than 1 percent of the progress that’s been achieved. I hope that all rare disease warriors will join together into strong teams, armies even, and make a difference in the world.
Multicentric Castleman Disease (Genetic and Rare Diseases Information Center/NIH)
Castleman Disease Collaborative Network (Paso Robles, CA)
His Doctors Were Stumped. Then He Took Over (New York Times, February 4, 2017)
Video: Chasing My Cure: Dr. David Fajgenbaum’s Lessons From His Rare Disease And On Finding Cures For Others (Exponential Medicine, November 4, 2019)
Rare Disease Day at NIH 2020 (National Center for Advancing Translational Sciences/NIH)
Posted on by Dr. Francis Collins
When injury strikes a limb or an organ, our bodies usually heal quickly and correctly. But for some people, the healing process doesn’t shut down properly, leading to excess fibrous tissue, scarring, and potentially life-threatening organ damage.
This permanent scarring, known as fibrosis, can occur in almost every tissue of the body, including the heart and lungs. With support from a 2019 NIH Director’s New Innovator Award, April Kloxin is applying her expertise in materials science and bioengineering to build sophisticated fibrosis-in-a-dish models for unraveling this complex process in her lab at the University of Delaware, Newark.
Though Kloxin is interested in all forms of fibrosis, she’s focusing first on the incurable and often-fatal lung condition called idiopathic pulmonary fibrosis (IPF). This condition, characterized by largely unexplained thickening and stiffening of lung tissue, is diagnosed in about 50,000 people each year in the United States.
IPF remains poorly understood, in part because it often is diagnosed when the disease is already well advanced. Kloxin hopes to turn back the clock and start to understand the disease at an earlier stage, when interventions might be more successful. The key is to develop a model that better recapitulates the complexity and irreversibility of the disease process in people.
Building that better model starts with simulating the meshwork of collagen and other proteins in the extracellular matrix (ECM) that undergird every tissue and organ in the body. The ECM’s interactions with our cells are essential in wound healing and, when things go wrong, also in causing fibrosis.
Kloxin will build three-dimensional hydrogels, crosslinked sponge-like networks of polymers, peptides, and proteins, with structures that more accurately capture the biological complexities of human tissues, including the ECMs within fibrous collagen-rich microenvironments. Her synthetic matrices can be triggered with light to lock in place and stiffen. The matrices also will make it possible to culture the lung’s epithelium, or outermost layer of cells, and connective tissue that surrounds it, to study cellular responses as the model shifts from a healthy and flexible to a stiffened, disease-like state.
Kloxin and her team will also integrate into their model system lung cells that have been engineered to fluoresce or light up under a microscope when the wound-healing program activates. Such fluorescent reporters will allow her team to watch for the first time how different cells and their nearby microenvironment respond as the composition of the ECM changes and stiffens. With this system, she’ll also be able to search for small molecules with the ability to turn off excessive wound healing.
The hope is that what’s learned with her New Innovator Award will lead to fresh insights and ultimately new treatments for this mysterious, hard-to-treat condition. But the benefits could be even more wide-ranging. Kloxin thinks that her findings will have implications for the prevention and treatment of other fibrotic diseases as well.
Idiopathic Pulmonary Fibrosis (National Heart, Lung, and Blood Institute/NIH)
April Kloxin Group (University of Delaware, Newark)
Kloxin Project Information (NIH RePORTER)
NIH Director’s New Innovator Award (Common Fund)
NIH Support: Common Fund; National Heart, Lung, and Blood Institute
Posted on by Dr. Francis Collins
Happy New Year! As we say goodbye to the Teens, let’s take a look back at 2019 and some of the groundbreaking scientific discoveries that closed out this remarkable decade.
Each December, the reporters and editors at the journal Science select their breakthrough of the year, and the choice for 2019 is nothing less than spectacular: An international network of radio astronomers published the first image of a black hole, the long-theorized cosmic singularity where gravity is so strong that even light cannot escape . This one resides in a galaxy 53 million light-years from Earth! (A light-year equals about 6 trillion miles.)
Though the competition was certainly stiff in 2019, the biomedical sciences were well represented among Science’s “runner-up” breakthroughs. They include three breakthroughs that have received NIH support. Let’s take a look at them:
In a first, drug treats most cases of cystic fibrosis: Last October, two international research teams reported the results from phase 3 clinical trials of the triple drug therapy Trikafta to treat cystic fibrosis (CF). Their data showed Trikafta effectively compensates for the effects of a mutation carried by about 90 percent of people born with CF. Upon reviewing these impressive data, the Food and Drug Administration (FDA) approved Trikafta, developed by Vertex Pharmaceuticals.
The approval of Trikafta was a wonderful day for me personally, having co-led the team that isolated the CF gene 30 years ago. A few years later, I wrote a song called “Dare to Dream” imagining that wonderful day when “the story of CF is history.” Though we’ve still got more work to do, we’re getting a lot closer to making that dream come true. Indeed, with the approval of Trikafta, most people with CF have for the first time ever a real chance at managing this genetic disease as a chronic condition over the course of their lives. That’s a tremendous accomplishment considering that few with CF lived beyond their teens as recently as the 1980s.
Such progress has been made possible by decades of work involving a vast number of researchers, many funded by NIH, as well as by more than two decades of visionary and collaborative efforts between the Cystic Fibrosis Foundation and Aurora Biosciences (now, Vertex) that built upon that fundamental knowledge of the responsible gene and its protein product. Not only did this innovative approach serve to accelerate the development of therapies for CF, it established a model that may inform efforts to develop therapies for other rare genetic diseases.
Hope for Ebola patients, at last: It was just six years ago that news of a major Ebola outbreak in West Africa sounded a global health emergency of the highest order. Ebola virus disease was then recognized as an untreatable, rapidly fatal illness for the majority of those who contracted it. Though international control efforts ultimately contained the spread of the virus in West Africa within about two years, over 28,600 cases had been confirmed leading to more than 11,000 deaths—marking the largest known Ebola outbreak in human history. Most recently, another major outbreak continues to wreak havoc in northeastern Democratic Republic of Congo (DRC), where violent civil unrest is greatly challenging public health control efforts.
As troubling as this news remains, 2019 brought a needed breakthrough for the millions of people living in areas susceptible to Ebola outbreaks. A randomized clinical trial in the DRC evaluated four different drugs for treating acutely infected individuals, including an antibody against the virus called mAb114, and a cocktail of anti-Ebola antibodies referred to as REGN-EB3. The trial’s preliminary data showed that about 70 percent of the patients who received either mAb114 or the REGN-EB3 antibody cocktail survived, compared with about half of those given either of the other two medicines.
So compelling were these preliminary results that the trial, co-sponsored by NIH’s National Institute of Allergy and Infectious Diseases (NIAID) and the DRC’s National Institute for Biomedical Research, was halted last August. The results were also promptly made public to help save lives and stem the latest outbreak. All Ebola patients in the DRC treatment centers now are treated with one or the other of these two options. The trial results were recently published.
The NIH-developed mAb114 antibody and the REGN-EB3 cocktail are the first therapeutics to be shown in a scientifically rigorous study to be effective at treating Ebola. This work also demonstrates that ethically sound clinical research can be conducted under difficult conditions in the midst of a disease outbreak. In fact, the halted study was named Pamoja Tulinde Maisha (PALM), which means “together save lives” in Kiswahili.
To top off the life-saving progress in 2019, the FDA just approved the first vaccine for Ebola. Called Ervebo (earlier rVSV-ZEBOV), this single-dose injectable vaccine is a non-infectious version of an animal virus that has been genetically engineered to carry a segment of a gene from the Zaire species of the Ebola virus—the virus responsible for the current DRC outbreak and the West Africa outbreak. Because the vaccine does not contain the whole Zaire virus, it can’t cause Ebola. Results from a large study in Guinea conducted by the WHO indicated that the vaccine offered substantial protection against Ebola virus disease. Ervebo, produced by Merck, has already been given to over 259,000 individuals as part of the response to the DRC outbreak. The NIH has supported numerous clinical trials of the vaccine, including an ongoing study in West Africa.
Microbes combat malnourishment: Researchers discovered a few years ago that abnormal microbial communities, or microbiomes, in the intestine appear to contribute to childhood malnutrition. An NIH-supported research team followed up on this lead with a study of kids in Bangladesh, and it published last July its groundbreaking finding: that foods formulated to repair the “gut microbiome” helped malnourished kids rebuild their health. The researchers were able to identify a network of 15 bacterial species that consistently interact in the gut microbiomes of Bangladeshi children. In this month-long study, this bacterial network helped the researchers characterize a child’s microbiome and/or its relative state of repair.
But a month isn’t long enough to determine how the new foods would help children grow and recover. The researchers are conducting a similar study that is much longer and larger. Globally, malnutrition affects an estimated 238 million children under the age 5, stunting their normal growth, compromising their health, and limiting their mental development. The hope is that these new foods and others adapted for use around the world soon will help many more kids grow up to be healthy adults.
Measles Resurgent: The staff at Science also listed their less-encouraging 2019 Breakdowns of the Year, and unfortunately the biomedical sciences made the cut with the return of measles in the U.S. Prior to 1963, when the measles vaccine was developed, 3 to 4 million Americans were sickened by measles each year. Each year about 500 children would die from measles, and many more would suffer lifelong complications. As more people were vaccinated, the incidence of measles plummeted. By the year 2000, the disease was even declared eliminated from the U.S.
But, as more parents have chosen not to vaccinate their children, driven by the now debunked claim that vaccines are connected to autism, measles has made a very preventable comeback. Last October, the Centers for Disease Control and Prevention (CDC) reported an estimated 1,250 measles cases in the United States at that point in 2019, surpassing the total number of cases reported annually in each of the past 25 years.
The good news is those numbers can be reduced if more people get the vaccine, which has been shown repeatedly in many large and rigorous studies to be safe and effective. The CDC recommends that children should receive their first dose by 12 to 15 months of age and a second dose between the ages of 4 and 6. Older people who’ve been vaccinated or have had the measles previously should consider being re-vaccinated, especially if they live in places with low vaccination rates or will be traveling to countries where measles are endemic.
Despite this public health breakdown, 2019 closed out a memorable decade of scientific discovery. The Twenties will build on discoveries made during the Teens and bring us even closer to an era of precision medicine to improve the lives of millions of Americans. So, onward to 2020—and happy New Year!
 2019 Breakthrough of the Year. Science, December 19, 2019.
NIH Support: These breakthroughs represent the culmination of years of research involving many investigators and the support of multiple NIH institutes.
Posted on by Dr. Francis Collins
There’s been tremendous excitement recently about the potential of CRISPR and related gene-editing technologies for treating or even curing sickle cell disease (SCD), muscular dystrophy, HIV, and a wide range of other devastating conditions. Now comes word of another remarkable advance—called “prime editing”—that may bring us even closer to reaching that goal.
As groundbreaking as CRISPR/Cas9 has been for editing specific genes, the system has its limitations. The initial version is best suited for making a double-stranded break in DNA, followed by error-prone repair. The outcome is generally to knock out the target. That’s great if eliminating the target is the desired goal. But what if the goal is to fix a mutation by editing it back to the normal sequence?
The new prime editing system, which was described recently by NIH-funded researchers in the journal Nature, is revolutionary because it offers much greater control for making a wide range of precisely targeted edits to the DNA code, which consists of the four “letters” (actually chemical bases) A, C, G, and T .
Already, in tests involving human cells grown in the lab, the researchers have used prime editing to correct genetic mutations that cause two inherited diseases: SCD, a painful, life-threatening blood disorder, and Tay-Sachs disease, a fatal neurological disorder. What’s more, they say the versatility of their new gene-editing system means it can, in principle, correct about 89 percent of the more than 75,000 known genetic variants associated with human diseases.
In standard CRISPR, a scissor-like enzyme called Cas9 is used to cut all the way through both strands of the DNA molecule’s double helix. That usually results in the cell’s DNA repair apparatus inserting or deleting DNA letters at the site. As a result, CRISPR is extremely useful for disrupting genes and inserting or removing large DNA segments. However, it is difficult to use this system to make more subtle corrections to DNA, such as swapping a letter T for an A.
To expand the gene-editing toolbox, a research team led by David R. Liu, Broad Institute of MIT and Harvard, Cambridge, MA, previously developed a class of editing agents called base editors [2,3]. Instead of cutting DNA, base editors directly convert one DNA letter to another. However, base editing has limitations, too. It works well for correcting four of the most common single letter mutations in DNA. But at least so far, base editors haven’t been able to make eight other single letter changes, or fix extra or missing DNA letters.
In contrast, the new prime editing system can precisely and efficiently swap any single letter of DNA for any other, and can make both deletions and insertions, at least up to a certain size. The system consists of a modified version of the Cas9 enzyme fused with another enzyme, called reverse transcriptase, and a specially engineered guide RNA, called pegRNA. The latter contains the desired gene edit and steers the needed editing apparatus to a specific site in a cell’s DNA.
Once at the site, the Cas9 nicks one strand of the double helix. Then, reverse transcriptase uses one DNA strand to “prime,” or initiate, the letter-by-letter transfer of new genetic information encoded in the pegRNA into the nicked spot, much like the search-and-replace function of word processing software. The process is then wrapped up when the prime editing system prompts the cell to remake the other DNA strand to match the new genetic information.
So far, in tests involving human cells grown in a lab dish, Liu and his colleagues have used prime editing to correct the most common mutation that causes SCD, converting a T to an A. They were also able to remove four DNA letters to correct the most common mutation underlying Tay-Sachs disease, a devastating condition that typically produces symptoms in children within the first year and leads to death by age four. The researchers also used their new system to insert new DNA segments up to 44 letters long and to remove segments at least 80 letters long.
Prime editing does have certain limitations. For example, 11 percent of known disease-causing variants result from changes in the number of gene copies, and it’s unclear if prime editing can insert or remove DNA that’s the size of full-length genes—which may contain up to 2.4 million letters.
It’s also worth noting that now-standard CRISPR editing and base editors have been tested far more thoroughly than prime editing in many different kinds of cells and animal models. These earlier editing technologies also may be more efficient for some purposes, so they will likely continue to play unique and useful roles in biomedicine.
As for prime editing, additional research is needed before we can consider launching human clinical trials. Among the areas that must be explored are this technology’s safety and efficacy in a wide range of cell types, and its potential for precisely and safely editing genes in targeted tissues within living animals and people.
Meanwhile, building on all these bold advances, efforts are already underway to accelerate the development of affordable, accessible gene-based cures for SCD and HIV on a global scale. Just last month, NIH and the Bill & Melinda Gates Foundation announced a collaboration that will invest at least $200 million over the next four years toward this goal. Last week, I had the chance to present this plan and discuss it with global health experts at the Grand Challenges meeting Addis Ababa, Ethiopia. The project is an unprecedented partnership designed to meet an unprecedented opportunity to address health conditions that once seemed out of reach but—as this new work helps to show—may now be within our grasp.
 Search-and-replace genome editing without double-strand breaks or donor DNA. Anzalone AV, Randolph PB, Davis JR, Sousa AA, Koblan LW, Levy JM, Chen PJ, Wilson C, Newby GA, Raguram A, Liu DR. Nature. Online 2019 October 21. [Epub ahead of print]
 Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Komor AC, Kim YB, Packer MS, Zuris JA, Liu DR. Nature. 2016 May 19;533(7603):420-424.
 Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage. Gaudelli NM, Komor AC, Rees HA, Packer MS, Badran AH, Bryson DI, Liu DR. Nature. 2017 Nov 23;551(7681):464-471.
Tay-Sachs Disease (Genetics Home Reference/National Library of Medicine/NIH)
Sickle Cell Disease (National Heart, Lung, and Blood Institute/NIH)
Cure Sickle Cell Initiative (NHLBI)
What are Genome Editing and CRISPR-Cas9? (National Library of Medicine/NIH)
Somatic Cell Genome Editing Program (Common Fund/NIH)
David R. Liu (Harvard, Cambridge, MA)
NIH Support: National Institute of Allergy and Infectious Diseases; National Human Genome Research Institute; National Institute for General Medical Sciences; National Institute of Biomedical Imaging and Bioengineering; National Center for Advancing Translational Sciences