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DNA Base Editing May Treat Progeria, Study in Mice Shows

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Sam Berns with personalized snare drum carrier
Credit: Progeria Research Foundation

My good friend Sam Berns was born with a rare genetic condition that causes rapid premature aging. Though Sam passed away in his teens from complications of this condition, called Hutchinson-Gilford progeria syndrome, he’s remembered today for his truly positive outlook on life. Sam expressed it, in part, by his willingness to make adjustments that allowed him, in his words, to put things that he always wanted to do in the “can do” category.

In this same spirit on behalf of the several hundred kids worldwide with progeria and their families, a research collaboration, including my NIH lab, has now achieved a key technical advance to move non-heritable gene editing another step closer to the “can do” category to treat progeria. As published in the journal Nature, our team took advantage of new gene-editing tools to correct for the first time a single genetic misspelling responsible for progeria in a mouse model, with dramatically beneficial effects [1, 2]. This work also has implications for correcting similar single-base typos that cause other inherited genetic disorders.

The outcome of this work is incredibly gratifying for me. In 2003, my NIH lab discovered the DNA mutation that causes progeria. One seemingly small glitch—swapping a “T” in place of a “C” in a gene called lamin A (LMNA)—leads to the production of a toxic protein now known as progerin. Without treatment, children with progeria develop normally intellectually but age at an exceedingly rapid pace, usually dying prematurely from heart attacks or strokes in their early teens.

The discovery raised the possibility that correcting this single-letter typo might one day help or even cure children with progeria. But back then, we lacked the needed tools to edit DNA safely and precisely. To be honest, I didn’t think that would be possible in my lifetime. Now, thanks to advances in basic genomic research, including work that led to the 2020 Nobel Prize in Chemistry, that’s changed. In fact, there’s been substantial progress toward using gene-editing technologies, such as the CRISPR editing system, for treating or even curing a wide range of devastating genetic conditions, such as sickle cell disease and muscular dystrophy

It turns out that the original CRISPR system, as powerful as it is, works better at knocking out genes than correcting them. That’s what makes some more recently developed DNA editing agents and approaches so important. One of them, which was developed by David R. Liu, Broad Institute of MIT and Harvard, Cambridge, MA, and his lab members, is key to these latest findings on progeria, reported by a team including my lab in NIH’s National Human Genome Research Institute and Jonathan Brown, Vanderbilt University Medical Center, Nashville, TN.

The relatively new gene-editing system moves beyond knock-outs to knock-ins [3,4]. Here’s how it works: Instead of cutting DNA as CRISPR does, base editors directly convert one DNA letter to another by enzymatically changing one DNA base to become a different base. The result is much like the find-and-replace function used to fix a typo in a word processor. What’s more, the gene editor does this without cutting the DNA.

Our three labs (Liu, Brown, and Collins) first teamed up with the Progeria Research Foundation, Peabody, MA, to obtain skin cells from kids with progeria. In lab studies, we found that base editors, targeted by an appropriate RNA guide, could successfully correct the LMNA gene in those connective tissue cells. The treatment converted the mutation back to the normal gene sequence in an impressive 90 percent of the cells.

But would it work in a living animal? To get the answer, we delivered a single injection of the DNA-editing apparatus into nearly a dozen mice either three or 14 days after birth, which corresponds in maturation level roughly to a 1-year-old or 5-year-old human. To ensure the findings in mice would be as relevant as possible to a future treatment for use in humans, we took advantage of a mouse model of progeria developed in my NIH lab in which the mice carry two copies of the human LMNA gene variant that causes the condition. Those mice develop nearly all of the features of the human illness

In the live mice, the base-editing treatment successfully edited in the gene’s healthy DNA sequence in 20 to 60 percent of cells across many organs. Many cell types maintained the corrected DNA sequence for at least six months—in fact, the most vulnerable cells in large arteries actually showed an almost 100 percent correction at 6 months, apparently because the corrected cells had compensated for the uncorrected cells that had died out. What’s more, the lifespan of the treated animals increased from seven to almost 18 months. In healthy mice, that’s approximately the beginning of old age.

This is the second notable advance in therapeutics for progeria in just three months. Last November, based on preclinical work from my lab and clinical trials conducted by the Progeria Research Foundation in Boston, the Food and Drug Administration (FDA) approved the first treatment for the condition. It is a drug called Zokinvy, and works by reducing the accumulation of progerin [5]. With long-term treatment, the drug is capable of extending the life of kids with progeria by 2.5 years and sometimes more. But it is not a cure.

We are hopeful this gene editing work might eventually lead to a cure for progeria. But mice certainly aren’t humans, and there are still important steps that need to be completed before such a gene-editing treatment could be tried safely in people. In the meantime, base editors and other gene editing approaches keep getting better—with potential application to thousands of genetic diseases where we know the exact gene misspelling. As we look ahead to 2021, the dream envisioned all those years ago about fixing the tiny DNA typo responsible for progeria is now within our grasp and getting closer to landing in the “can do” category.


[1] In vivo base editing rescues Hutchinson-Gilford Progeria Syndrome in mice. Koblan LW et al. Nature. 2021 Jan 6.

[2] Base editor repairs mutation found in the premature-ageing syndrome progeria. Vermeij WP, Hoeijmakers JHJ. Nature. 6 Jan 2021.

[3] 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.

[4] 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.

[5] FDA approves first treatment for Hutchinson-Gilford progeria syndrome and some progeroid laminopathies. Food and Drug Administration. 2020 Nov 20.


Progeria (Genetic and Rare Diseases Information Center/NIH)

What are Genome Editing and CRISPR-Cas9? (National Library of Medicine/NIH)

Somatic Cell Genome Editing Program (Common Fund/NIH)

David R. Liu (Harvard University, Cambridge, MA)

Collins Group (National Human Genome Research Institute/NIH)

Jonathan Brown (Vanderbilt University Medical Center, Nashville, TN)

NIH Support: National Human Genome Research Institute; National Center for Advancing Translational Sciences; National Institute of Biomedical Imaging and Bioengineering; National Institute of Allergy and Infectious Diseases; National Institute of General Medical Sciences; Common Fund

Celebrating 2019 Biomedical Breakthroughs

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Science 2019 Biomedical Breakthroughs and a Breakdown

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 [1]. 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!


[1] 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.

One Little Girl’s Story Highlights the Promise of Precision Medicine

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Photo of Dr. Yu taking a selfie with Mila and her mom
Caption: Mila with researcher Timothy Yu and her mother Julia Vitarello. Mila’s head is covered in gauze because she’s undergoing EEG monitoring to determine if her seizures are responding to treatment. Credit: Boston Children’s Hospital

Starting about the age of 3, Mila Makovec’s parents noticed that their young daughter was having a little trouble with words and one of her feet started turning inward. Much more alarmingly, she then began to lose vision and have frequent seizures. Doctors in Colorado diagnosed Mila with a form of Batten disease, a group of rare, rapidly progressive neurological disorders that are often fatal in childhood or the teenage years. Further testing in Boston revealed that Mila’s disease was caused by a genetic mutation that appears to be unique to her.

No treatment existed for Mila’s condition. So, in an effort to meet that urgent need, Timothy Yu and his colleagues at Boston Children’s Hospital set forth on a bold and unprecedented course of action. In less than a year, they designed a drug that targeted Mila’s unique mutation, started testing the tailor-made drug for efficacy and safety on cells derived from her skin, and then began giving Mila the drug in her own personal clinical trial.

The experimental drug, which has produced no adverse side effects to date, hasn’t proved to be a cure for Mila’s disease [1]. But it’s helped to reduce Mila’s seizures and also help her stand and walk with assistance, though she still has difficulty communicating. Still, the implications of this story extend far beyond one little girl: this work demonstrates the promise of precision medicine research for addressing the unique medical challenges faced by individuals with extremely rare diseases.

Mila’s form of Batten disease usually occurs when a child inherits a faulty copy of a gene called CLN7 from each parent. What surprised doctors is Mila seemed to have inherited just one bad copy of CLN7. Her mother reached out online in search of a lab willing to look deeper into her genome, and Yu’s lab answered the call.

Yu suspected Mila’s second mutation might lie buried in a noncoding portion of her DNA. The lab’s careful analysis determined that was indeed the case. The second mutation occurred in a stretch of the gene that normally doesn’t code for the CLN7 protein at all. Even more unusual, it consisted of a rogue snippet of DNA that had inserted itself into an intron (a spacer segment) of Mila’s CLN7 gene. As a result, her cells couldn’t properly process an RNA transcript that would produce the essential CLN7 protein.

What might have been the end of the story a few years ago was now just the beginning. In 2016, the Food and Drug Administration (FDA) approved a novel drug called nusinersen for a hereditary neurodegenerative disease called spinal muscular atrophy (SMA), caused by another faulty protein. As I’ve highlighted before, nusinersen isn’t a typical drug. It’s made up of a small, single-stranded snippet of synthetic RNA, also called an oligonucleotide. This drug is designed to bind to faulty RNA transcripts in just the right spot, “tricking” cells into producing a working version of the protein that’s missing in kids with SMA.

Yu’s team thought the same strategy might work to correct the error in Mila’s cells. They reasoned that an appropriately designed oligonucleotide could block the effect of the rogue snippet in her CLN7 gene, allowing her cells to restore production of working protein.

The team produced candidate oligonucleotides and tested them on Mila’s cells growing in a lab dish. They found three candidates that worked. The best, which they named milasen after Mila, was just 22-nucleotides long. They designed it to have some of the same structural attributes as nusinersen, given its established safety and efficacy in kids with SMA.

Further study suggested that milasen corrected abnormalities in Mila’s cells in a lab dish. The researchers then tested the drug in rats and found that it appeared to be safe.

A month later, with FDA approval, they delivered the drug to Mila, administered through a spinal tap (just like nusinersen). That’s because the blood-brain barrier would otherwise prevent the drug from reaching Mila’s brain. Beginning in January 2018, she received gradually escalating doses of milasen every two weeks for about three months. After that, she received a dose every two to three months to maintain the drug in her system.

When Mila received the first dose, her condition was rapidly deteriorating. But it has since stabilized. The number of seizures she suffers each day has declined from about 30 to 10 or less. Their duration has also declined from 1 or 2 minutes to just seconds.

Milasen remains an investigational drug. Because it was designed specifically for Mila’s unique mutation, it’s not a candidate for use in others with Batten disease. But the findings do show that it’s now possible to design, test, and deploy a novel therapeutic agent for an individual patient with an exceedingly rare condition on the basis of a thorough understanding of the underlying genetic cause. This is a sufficiently significant moment for the development of “n = 1 therapeutics” that senior leaders of the Food and Drug Administration (FDA) published an editorial to appear along with the clinical report [2].

Yu’s team suspects that a similar strategy might work in other cases of people with rare conditions. That tantalizing possibility raises many questions about how such individualized therapies should be developed, evaluated, and tested in the months and years ahead.

My own lab is engaged in testing a similar treatment strategy for kids with the very rare form of premature aging called Hutchinson-Gilford progeria, and we were heartened by this report. As we grapple with those challenges, we can all find hope and inspiration in Mila’s smile, her remarkable story, and what it portends for the future of precision medicine.


[1] Patient-customized oligonucleotide therapy for a rare genetic disease. Kim J, Hu C, Moufawad El Achkar C, Black LE, Douville J, Larson A, Pendergast MK, Goldkind SF, Lee EA, Kuniholm A, Soucy A, Vaze J, Belur NR, Fredriksen K, Stojkovska I, Tsytsykova A, Armant M, DiDonato RL, Choi J, Cornelissen L, Pereira LM, Augustine EF, Genetti CA, Dies K, Barton B, Williams L, Goodlett BD, Riley BL, Pasternak A, Berry ER, Pflock KA, Chu S, Reed C, Tyndall K, Agrawal PB, Beggs AH, Grant PE, Urion DK, Snyder RO, Waisbren SE, Poduri A, Park PJ, Patterson A, Biffi A, Mazzulli JR, Bodamer O, Berde CB, Yu TW. N Engl J Med. 2019 Oct 9 [Epub ahead of print]

[2] Drug regulation in the era of individualized therapies. Woodcock J, Marks P. N Engl J Med. 2019 Oct 9 {Epub ahead of print]


Batten Disease Fact Sheet (National Institute of Neurological Disorders and Stroke/NIH)

Mila’s Miracle Foundation (Boulder, CO)

Timothy Yu (Boston Children’s Hospital, MA)

NIH Support: National Center for Advancing Translational Sciences

Whole-Genome Sequencing Plus AI Yields Same-Day Genetic Diagnoses

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Caption: Rapid whole-genome sequencing helped doctors diagnose Sebastiana Manuel with Ohtahara syndrome, a neurological condition that causes seizures. Her data are now being used as part of an effort to speed the diagnosis of other children born with unexplained illnesses. Credits: Getty Images (left); Jenny Siegwart (right).

Back in April 2003, when the international Human Genome Project successfully completed the first reference sequence of the human DNA blueprint, we were thrilled to have achieved that feat in just 13 years. Sure, the U.S. contribution to that first human reference sequence cost an estimated $400 million, but we knew (or at least we hoped) that the costs would come down quickly, and the speed would accelerate. How far we’ve come since then! A new study shows that whole genome sequencing—combined with artificial intelligence (AI)—can now be used to diagnose genetic diseases in seriously ill babies in less than 24 hours.

Take a moment to absorb this. I would submit that there is no other technology in the history of planet Earth that has experienced this degree of progress in speed and affordability. And, at the same time, DNA sequence technology has achieved spectacularly high levels of accuracy. The time-honored adage that you can only get two out of three for “faster, better, and cheaper” has been broken—all three have been dramatically enhanced by the advances of the last 16 years.

Rapid diagnosis is critical for infants born with mysterious conditions because it enables them to receive potentially life-saving interventions as soon as possible after birth. In a study in Science Translational Medicine, NIH-funded researchers describe development of a highly automated, genome-sequencing pipeline that’s capable of routinely delivering a diagnosis to anxious parents and health-care professionals dramatically earlier than typically has been possible [1].

While the cost of rapid DNA sequencing continues to fall, challenges remain in utilizing this valuable tool to make quick diagnostic decisions. In most clinical settings, the wait for whole-genome sequencing results still runs more than two weeks. Attempts to obtain faster results also have been labor intensive, requiring dedicated teams of experts to sift through the data, one sample at a time.

In the new study, a research team led by Stephen Kingsmore, Rady Children’s Institute for Genomic Medicine, San Diego, CA, describes a streamlined approach that accelerates every step in the process, making it possible to obtain whole-genome test results in a median time of about 20 hours and with much less manual labor. They propose that the system could deliver answers for 30 patients per week using a single genome sequencing instrument.

Here’s how it works: Instead of manually preparing blood samples, his team used special microbeads to isolate DNA much more rapidly with very little labor. The approach reduced the time for sample preparation from 10 hours to less than three. Then, using a state-of-the-art DNA sequencer, they sequence those samples to obtain good quality whole genome data in just 15.5 hours.

The next potentially time-consuming challenge is making sense of all that data. To speed up the analysis, Kingsmore’s team took advantage of a machine-learning system called MOON. The automated platform sifts through all the data using artificial intelligence to search for potentially disease-causing variants.

The researchers paired MOON with a clinical language processing system, which allowed them to extract relevant information from the child’s electronic health records within seconds. Teaming that patient-specific information with data on more than 13,000 known genetic diseases in the scientific literature, the machine-learning system could pick out a likely disease-causing mutation out of 4.5 million potential variants in an impressive 5 minutes or less!

To put the system to the test, the researchers first evaluated its ability to reach a correct diagnosis in a sample of 101 children with 105 previously diagnosed genetic diseases. In nearly every case, the automated diagnosis matched the opinions reached previously via the more lengthy and laborious manual interpretation of experts.

Next, the researchers tested the automated system in assisting diagnosis of seven seriously ill infants in the intensive care unit, and three previously diagnosed infants. They showed that their automated system could reach a diagnosis in less than 20 hours. That’s compared to the fastest manual approach, which typically took about 48 hours. The automated system also required about 90 percent less manpower.

The system nailed a rapid diagnosis for 3 of 7 infants without returning any false-positive results. Those diagnoses were made with an average time savings of more than 22 hours. In each case, the early diagnosis immediately influenced the treatment those children received. That’s key given that, for young children suffering from serious and unexplained symptoms such as seizures, metabolic abnormalities, or immunodeficiencies, time is of the essence.

Of course, artificial intelligence may never replace doctors and other healthcare providers. Kingsmore notes that 106 years after the invention of the autopilot, two pilots are still required to fly a commercial aircraft. Likewise, health care decisions based on genome interpretation also will continue to require the expertise of skilled physicians.

Still, such a rapid automated system will prove incredibly useful. For instance, this system can provide immediate provisional diagnosis, allowing the experts to focus their attention on more difficult unsolved cases or other needs. It may also prove useful in re-evaluating the evidence in the many cases in which manual interpretation by experts fails to provide an answer.

The automated system may also be useful for periodically reanalyzing data in the many cases that remain unsolved. Keeping up with such reanalysis is a particular challenge considering that researchers continue to discover hundreds of disease-associated genes and thousands of variants each and every year. The hope is that in the years ahead, the combination of whole genome sequencing, artificial intelligence, and expert care will make all the difference in the lives of many more seriously ill babies and their families.


[1] Diagnosis of genetic diseases in seriously ill children by rapid whole-genome sequencing and automated phenotyping and interpretation. Clark MM, Hildreth A, Batalov S, Ding Y, Chowdhury S, Watkins K, Ellsworth K, Camp B, Kint CI, Yacoubian C, Farnaes L, Bainbridge MN, Beebe C, Braun JJA, Bray M, Carroll J, Cakici JA, Caylor SA, Clarke C, Creed MP, Friedman J, Frith A, Gain R, Gaughran M, George S, Gilmer S, Gleeson J, Gore J, Grunenwald H, Hovey RL, Janes ML, Lin K, McDonagh PD, McBride K, Mulrooney P, Nahas S, Oh D, Oriol A, Puckett L, Rady Z, Reese MG, Ryu J, Salz L, Sanford E, Stewart L, Sweeney N, Tokita M, Van Der Kraan L, White S, Wigby K, Williams B, Wong T, Wright MS, Yamada C, Schols P, Reynders J, Hall K, Dimmock D, Veeraraghavan N, Defay T, Kingsmore SF. Sci Transl Med. 2019 Apr 24;11(489).


DNA Sequencing Fact Sheet (National Human Genome Research Institute/NIH)

Genomics and Medicine (NHGRI/NIH)

Genetic and Rare Disease Information Center (National Center for Advancing Translational Sciences/NIH)

Stephen Kingsmore (Rady Children’s Institute for Genomic Medicine, San Diego, CA)

NIH Support: National Institute of Child Health and Human Development; National Human Genome Research Institute; National Center for Advancing Translational Sciences

A CRISPR Approach to Treating Sickle Cell

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Unedited and edited sickle cells
Caption: Red blood cells from patient with sickle cell disease. The cells were differentiated from bone marrow with unedited and edited hematopoietic stem cells, and the red arrows show the sickled cells. Credit: Wu et al. Nature Medicine. March 25, 2019

Recently, CBS’s “60 Minutes” highlighted the story of Jennelle Stephenson, a brave young woman with sickle cell disease (SCD). Jennelle now appears potentially cured of this devastating condition, thanks to an experimental gene therapy being tested at the NIH Clinical Center in Bethesda, MD. As groundbreaking as this research may be, it’s among a variety of innovative strategies now being tried to cure SCD and other genetic diseases that have long seemed out of reach.

One particularly exciting approach involves using gene editing to increase levels of fetal hemoglobin (HbF) in the red blood cells of people with SCD. Shortly after birth, babies usually stop producing HbF, and switch over to the adult form of hemoglobin. But rare individuals continue to make high levels of HbF throughout their lives. This is referred to as hereditary persistence of fetal hemoglobin (HPFH). (My own postdoctoral research in the early 1980s discovered some of the naturally occurring DNA mutations that lead to this condition.)

Individuals with HPFH are entirely healthy. Strikingly, rare individuals with SCD who also have HPFH have an extremely mild version of sickle cell disease—essentially the presence of significant quantities of HbF provides protection against sickling. So, researchers have been exploring ways to boost HbF in everyone with SCD—and gene editing may provide an effective, long-lasting way to do this.

Clinical trials of this approach are already underway. And new findings reported in Nature Medicine show it may be possible to make the desired edits even more efficiently, raising the possibility that a single infusion of gene-edited cells might be able to cure SCD [1].

Sickle cell disease is caused by a specific point mutation in a gene that codes for the beta chain of hemoglobin. People with just one copy of this mutation have sickle cell trait and are generally healthy. But those who inherit two mutant copies of this gene suffer lifelong consequences of the presence of this abnormal protein. Their red blood cells—normally flexible and donut-shaped—assume the sickled shape that gives SCD its name. The sickled cells clump together and stick in small blood vessels, resulting in severe pain, anemia, stroke, pulmonary hypertension, organ failure, and far too often, early death.

Eleven years ago, a team led by Vijay Sankaran and Stuart Orkin at Boston Children’s Hospital and the Dana-Farber Cancer Institute discovered that a protein called BCL11A seemed to determine HbF levels [2]. Subsequent work showed the protein actually works as a master mediator of the switch from fetal to adult hemoglobin, which normally occurs shortly after birth.

Five years ago, Orkin and Daniel Bauer identified a specific enhancer of BCL11A expression that could be an attractive target for gene editing [3]. They could knock out the enhancer in the bone marrow, and BCL11A would not be produced, allowing HbF to stay switched on.

Because the BCL11A protein is required to turn off production of HbF in red cells. the researchers had another idea. They thought it might be possible to keep HbF on permanently by disrupting BCL11A in blood-forming hematopoietic stem cells (HSCs). The hope was that such a treatment might offer people with SCD a permanent supply of healthy red blood cells.

Fast-forward to the present, and researchers are now testing the ability of gene editing tools to cure the disease. A favorite editing system is CRISPR, which I’ve highlighted on my blog.

CRISPR is a highly precise gene-editing tool that relies on guide RNA molecules to direct a scissor-like Cas9 enzyme to just the right spot in the genome to correct the misspelling. The gene-editing treatment involves removing bone marrow from a patient, modifying the HSCs outside the body using CRISPR gene-editing tools, and then returning them back to the patient. Preclinical studies had shown that CRISPR can be effective in editing BCL11A to boost HbF production.

But questions lingered about the editing efficiency in HSCs versus more common, shorter-lived progenitor cells found in bone marrow samples. The efficiency greatly influences how long the edited cells might benefit patients. Bauer’s team saw room for improvement and, as the new study shows, they were right.

To produce lasting HbF production, it’s important to edit as many HSCs as possible. But it turns out that HSCs are more resistant to editing than other types of cells in bone marrow. With a series of adjustments to the gene-editing protocol, including use of an optimized version of the Cas9 protein, the researchers showed they could push the number of edited genes from about 80 percent to about 95 percent.

Their studies show that the most frequent Cas9 edits in HSCs are tiny insertions of a single DNA “letter.” With that slight edit to the BCL11A gene, HSCs reprogram themselves in a way that ensures long-term HbF production.

As a first test of their CRISPR-edited human HSCs, the researchers carried out the editing on HSCs derived from patients with SCD. Then they transferred the editing cells into immune-compromised mice. Four months later, the mice continued to produce red blood cells that produced high levels of HbF and resisted sickling. Bauer says they’re already taking steps to begin testing cells edited with their optimized protocol in a clinical trial.

What’s truly exciting is that the first U.S. human clinical trials of such a gene-editing approach for SCD are already underway, led by CRISPR Therapeutics/Vertex Pharmaceuticals and Sangamo Therapeutics/Sanofi. In January, CRISPR Therapeutics/Vertex Pharmaceuticals announced that the U.S. Food and Drug Administration (FDA) had granted Fast Track Designation for their CRISPR-based treatment called CTX001 [4].

In that recent “60 Minutes” segment, I dared to suggest that we now have what looks like a cure for SCD. As shown by this new work and the clinical trials underway, we in fact may soon have multiple different strategies to provide cures for this devastating disease. And if this can work for sickle cell, a similar strategy might work for other genetic conditions that currently lack any effective treatment.


[1] Highly efficient therapeutic gene editing of human hematopoietic stem cells. Wu Y, Zeng J, Roscoe BP, Liu P, Yao Q, Lazzarotto CR, Clement K, Cole MA, Luk K, Baricordi C, Shen AH, Ren C, Esrick EB, Manis JP, Dorfman DM, Williams DA, Biffi A, Brugnara C, Biasco L, Brendel C, Pinello L, Tsai SQ, Wolfe SA, Bauer DE. Nat Med. 2019 Mar 25.

[2] Human fetal hemoglobin expression is regulated by the developmental stage-specific repressor BCL11A. Sankaran VG, Menne TF, Xu J, Akie TE, Lettre G, Van Handel B, Mikkola HK, Hirschhorn JN, Cantor AB, Orkin SH.Science. 2008 Dec 19;322(5909):1839-1842.

[3] An erythroid enhancer of BCL11A subject to genetic variation determines fetal hemoglobin level. Bauer DE, Kamran SC, Lessard S, Xu J, Fujiwara Y, Lin C, Shao Z, Canver MC, Smith EC, Pinello L, Sabo PJ, Vierstra J, Voit RA, Yuan GC, Porteus MH, Stamatoyannopoulos JA, Lettre G, Orkin SH. Science. 2013 Oct 11;342(6155):253-257.

[4] CRISPR Therapeutics and Vertex Announce FDA Fast Track Designation for CTX001 for the Treatment of Sickle Cell Disease, CRISPR Therapeutics News Release, Jan. 4, 2019.


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)

Could Gene Therapy Cure Sickle Cell Anemia? (CBS News)

Daniel Bauer (Dana-Farber Cancer Institute, Boston)

Somatic Cell Genome Editing Program (Common Fund/NIH)

NIH Support: National Heart, Lung, and Blood Institute; National Institute of General Medical Sciences; National Institute of Allergy and Infectious Diseases; National Institute of Diabetes and Digestive and Kidney Diseases

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