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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.

References:

[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.

Links:

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


All of Us: Partnering Together for the Future of Precision Medicine

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All of Us Research Program
Credit: All of Us Research Program

Over the past year, it’s been so inspiring to watch tens of thousands of people across the country selflessly step forward for vaccine trials and other research studies to combat COVID-19. And they are not alone. Many generous folks are volunteering to take part in other types of NIH-funded research that will improve health all across the spectrum, including the more than 360,000 who’ve already enrolled in the pioneering All of Us Research Program.

Now in its second year, All of Us is building a research community of 1 million participant partners to help us learn more about how genetics, environment, and lifestyle interact to influence disease and affect health. So far, more than 80 percent of participants who have completed all the initial enrollment steps are Black, Latino, rural, or from other communities historically underrepresented in biomedical research.

This community will build a diverse foundation for precision medicine, in which care is tailored to the individual, not the average patient as is now often the case. What’s also paradigm shifting about All of Us is its core value of sharing information back with participants about themselves. It is all done responsibly through each participant’s personal All of Us online account and with an emphasis on protecting privacy.

All of Us participants share their health information in many ways, such as taking part in surveys, offering access to their electronic health records, and providing biosamples (blood, urine, and/or saliva). In fact, researchers recently began genotyping and sequencing the DNA in some of those biosamples, and then returning results from analyses to participants who’ve indicated they’d like to receive such information. This first phase of genotyping DNA analysis will provide insights into their genetic ancestry and four traits, including bitter taste perception and tolerance for lactose.

Results of a second sequencing phase of DNA analysis will likely be ready in the coming year. These personalized reports will give interested participants information about how their bodies are likely to react to certain medications and about whether they face an increased risk of developing certain health conditions, such as some types of cancer or heart disease. To help participants better understand the results, they can make a phone appointment with a genetic counselor who is affiliated with the program.

This week, I had the pleasure of delivering the keynote address at the All of Us Virtual Face-to-Face. This lively meeting was attended by a consortium of more than 2,000 All of Us senior staff, program leads with participating healthcare provider organizations and federally qualified health centers, All of Us-supported researchers, community partners, and the all-important participant ambassadors.

If you are interested in becoming part of the All of Us community, I welcome you—there’s plenty of time to get involved! To learn more, just go to Join All of Us.

Links:

All of Us Research Program (NIH)

Join All of Us (NIH)


Experts Conclude Heritable Human Genome Editing Not Ready for Clinical Applications

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We stand at a critical juncture in the history of science. CRISPR and other innovative genome editing systems have given researchers the ability to make very precise changes in the sequence, or spelling, of the human DNA instruction book. If these tools are used to make non-heritable edits in only relevant tissues, they hold enormous potential to treat or even cure a wide range of devastating disorders, such as sickle cell disease, inherited neurologic conditions, and muscular dystrophy. But profound safety, ethical, and philosophical concerns surround the use of such technologies to make heritable changes in the human genome—changes that can be passed on to offspring and have consequences for future generations of humankind.

Such concerns are not hypothetical. Two years ago, a researcher in China took it upon himself to cross this ethical red line and conduct heritable genome editing experiments in human embryos with the aim of protecting the resulting babies against HIV infection. The medical justification was indefensible, the safety issues were inadequately considered, and the consent process was woefully inadequate. In response to this epic scientific calamity, NIH supported a call by prominent scientists for an international moratorium on human heritable, or germline, genome editing for clinical purposes.

Following on the heels of this unprecedented ethical breach, the U.S. National Academy of Sciences, U.S. National Academy of Medicine, and the U.K. Royal Society convened an international commission, sponsored by NIH, to conduct a comprehensive review of the clinical use of human germline genome editing. The 18-member panel, which represented 10 nations and four continents, included experts in genome editing technology; human genetics and genomics; psychology; reproductive, pediatric, and adult medicine; regulatory science; bioethics; and international law. Earlier this month, this commission issued its consensus study report, entitled Heritable Human Genome Editing [1].

The commission was designed to bring together thought leaders around the globe to engage in serious discussions about this highly controversial use of genome-editing technology. Among the concerns expressed by many of us was that if heritable genome editing were allowed to proceed without careful deliberation, the enormous potential of non-heritable genome editing for prevention and treatment of disease could become overshadowed by justifiable public outrage, fear, and disgust.

I’m gratified to say that in its new report, the expert panel closely examined the scientific and ethical issues, and concluded that heritable human genome editing is too technologically unreliable and unsafe to risk testing it for any clinical application in humans at the present time. The report cited the potential for unintended off-target DNA edits, which could have harmful health effects, such as cancer, later in life. Also noted was the risk of producing so-called mosaic embryos, in which the edits occur in only a subset of an embryo’s cells. This would make it very difficult for researchers to predict the clinical effects of heritable genome editing in human beings.

Among the many questions that the panel was asked to consider was: should society ever decide that heritable gene editing might be acceptable, what would be a viable framework for scientists, clinicians, and regulatory authorities to assess the potential clinical applications?

In response to that question, the experts replied: heritable gene editing, if ever permitted, should be limited initially to serious diseases that result from the mutation of one or both copies of a single gene. The first uses of these technologies should proceed incrementally and with extreme caution. Their potential medical benefits and harms should also be carefully evaluated before proceeding.

The commission went on to stress that before such an option could be on the table, all other viable reproductive possibilities to produce an embryo without a disease-causing alteration must be exhausted. That would essentially limit heritable gene editing to the exceedingly rare instance in which both parents have two copies of a recessive, disease-causing gene variant. Or another quite rare instance in which one parent has two copies of an altered gene for a dominant genetic disorder, such as Huntington’s disease.

Recognizing how unusual both scenarios would be, the commission held out the possibility that some would-be parents with less serious conditions might qualify if 25 percent or less of their embryos are free of the disease-causing gene variant. A possible example is familial hypercholesterolemia (FH), in which people carrying a mutation in the LDL receptor gene have unusually high levels of cholesterol in their blood. If both members of a couple are affected, only 25 percent of their biological children would be unaffected. FH can lead to early heart disease and death, but drug treatment is available and improving all the time, which makes this a less compelling example. Also, the commission again indicated that such individuals would need to have already traveled down all other possible reproductive avenues before considering heritable gene editing.

A thorny ethical question that was only briefly addressed in the commission’s report is the overall value to be attached to a couple’s desire to have a biological child. That desire is certainly understandable, although other options, such an adoption or in vitro fertilization with donor sperm, are available. This seems like a classic example of the tension between individual desires and societal concerns. Is the drive for a biological child in very high-risk situations such a compelling circumstance that it justifies asking society to start down a path towards modifying human germline DNA?

The commission recommended establishing an international scientific advisory board to monitor the rapidly evolving state of genome editing technologies. The board would serve as an access point for scientists, legislators, and the public to access credible information to weigh the latest progress against the concerns associated with clinical use of heritable human genome editing.

The National Academies/Royal Society report has been sent along to the World Health Organization (WHO), where it will serve as a resource for its expert advisory committee on human genome editing. The WHO committee is currently developing recommendations for appropriate governance mechanisms for both heritable and non-heritable human genome editing research and their clinical uses. That panel could issue its guidance later this year, which is sure to continue this very important conversation.

Reference:

[1] Heritable Human Genome Editing, Report Summary, National Academy of Sciences, September 2020.

Links:

Heritable Genome Editing Not Yet Ready to Be Tried Safely and Effectively in Humans,” National Academies of Sciences, Engineering, and Medicine news release, Sep. 3, 2020.

International Commission on the Clinical Use of Human Germline Genome Editing (National Academies of Sciences, Engineering, and Medicine/Washington, D.C.)

Video: Report Release Webinar , International Commission on the Clinical Use of Human Germline Genome Editing (National Academies of Sciences, Engineering, and Medicine)

National Academy of Sciences (Washington, D.C.)

National Academy of Medicine (Washington, D.C.)

The Royal Society (London)


A New View of the 3D Genome

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Caption: 3D model of a chromatin “forest.” Each sphere represents a tree-shaped domain of about 10 nucleosomes, the basic structural unit of DNA packaging. Larger domains are green; smaller ones are red. Credit: Northwestern University, Evanston, IL

 

This lush panoply of color might stir up daydreams of getting away to explore a tropical rain forest. But what you see here is a new model that’s enabling researchers to explore something equally amazing: how a string of DNA that measures 6 feet long can be packed into the microscopic nucleus of a human cell. Fitting that much DNA in a nucleus is like fitting a thread the length of the Empire State building underneath your fingernail!

Scientists have known for a while that that the answer lies in how DNA is folded onto spool-like complexes called chromatin, but many details of the process still remain to be worked out. Recently, an NIH-funded team, led by Vadim Backman and Igal Szleifer, Northwestern University, Evanston, IL, developed this new model of chromatin folding by pairing sophisticated mathematical modeling and optical imaging.In a study published in the journal Science Advances [1], the team found that chromatin is folded into a variety of tree-like domains along a chromatin backbone, which they liken to an aggregation of trees growing from the forest floor. The colorful spheres you see above represent trees of varying sizes.

Earlier models of chromatin folding had suggested that DNA folds into regular and orderly fibers. In the new study, the Northwestern researchers used their own specially designed Partial Wave Spectroscopic microscope. This high-powered system, coupled with electron imaging, allowed them to peer deep inside living cells to “sense” real-time alterations in chromatin packing. What makes their new view on chromatin so interesting is it suggests our DNA is packaged in a way that’s much more disorderly and unpredictable than initially thought.

Chromatin Forest
Caption: Schematic shows the interplay between transcription and chromatin packing. Inactive high DNA density (blue) regions and active low DNA density (red). The horizontal chromatin backbone includes RNA polymerase (green), activating factors (yellow), and repressing factors (purple). Credit: Huang et al., Sci. Adv. 2020

As Backman notes, it is reasonable to assume that a forest would be filled with trees of varying sizes and shapes. But you couldn’t predict the exact location of each tree or its particular size and configuration. The same appears to be true of these tree-like structures within chromatin. Their precise location and size vary, seemingly unpredictably, from cell to cell.

This apparently random DNA packing structure might seem surprising given chromatin’s importance in influencing the expression and function of our genes. But the researchers think such variability likely has its advantages.

Here’s the idea: If all of our cells responded to stressful conditions (such as heat or a toxic exposure) in exactly the same way and that way happened to be suboptimal, the whole tissue or organ might fail. But if differences in chromatin structure lead each cell to respond somewhat differently to the same stimulus, then some cells might be more likely to survive or even thrive under the stress. It’s a built-in way for cells to hedge their bets.

These new findings offer a fundamentally new three-dimensional view of the human genome. They might also inspire innovative strategies to understand and fight cancer, as well as other diseases. And, while most of us probably won’t be venturing off into the rain forest anytime soon, this work does give us all something to think about next time we’re enjoying the great outdoors in our own neck of the woods. 

Reference:

[1] Physical and data structure of 3D genome. Huang K, Li Y, Shim AR, Virk RKA, Agrawal V, Eshein A, Nap RJ, Almassalha LM, Backman V, Szleifer I. Sci Adv. 2020 Jan 10;6(2):eaay4055.

Links:

Deoxyribonucleic Acid (DNA) (National Human Genome Research Institute/NIH)

4D Nucleome (Common Fund/NIH)

Vadim Backman (Northwestern University, Evanston, IL)

Igal Szleifer (Northwestern University, Evanston, IL)

NIH Support: National Cancer Institute


The Perfect Cytoskeletal Storm

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Ever thought about giving cell biology a whirl? If so, I suggest you sit down and take a look at this full-blown cytoskeletal “storm,” which provides a spectacular dynamic view of the choreography of life.

Before a cell divides, it undergoes a process called mitosis that copies its chromosomes and produces two identical nuclei. As part of this process, microtubules, which are structural proteins that help make up the cell’s cytoskeleton, reorganize the newly copied chromosomes into a dense, football-shaped spindle. The position of this mitotic spindle tells the cell where to divide, allowing each daughter cell to contain its own identical set of DNA.

To gain a more detailed view of microtubules in action, researchers designed an experimental system that utilizes an extract of cells from the African clawed frog (Xenopus laevis). As the video begins, a star-like array of microtubules (red) radiate outward in an apparent effort to prepare for cell division. In this configuration, the microtubules continually adjust their lengths with the help of the protein EB-1 (green) at their tips. As the microtubules grow and bump into the walls of a lab-generated, jelly-textured enclosure (dark outline), they buckle—and the whole array then whirls around the center.

Abdullah Bashar Sami, a Ph.D. student in the NIH-supported lab of Jesse “Jay” Gatlin, University of Wyoming, Laramie, shot this movie as a part his basic research to explore the still poorly understood physical forces generated by microtubules. The movie won first place in the 2019 Green Fluorescent Protein Image and Video Contest sponsored by the American Society for Cell Biology. The contest honors the 25th anniversary of the discovery of green fluorescent protein (GFP), which transformed cell biology and earned the 2008 Nobel Prize in Chemistry for three scientists who had been supported by NIH.

Like many movies, the setting was key to this video’s success. The video was shot inside a microfluidic chamber, designed in the Gatlin lab, to study the physics of microtubule assembly just before cells divide. The tiny chamber holds a liquid droplet filled with the cell extract.

When the liquid is exposed to an ultra-thin beam of light, it forms a jelly-textured wall, which traps the molecular contents inside [1]. Then, using time-lapse microscopy, the researchers watch the mechanical behavior of GFP-labeled microtubules [2] to see how they work to position the mitotic spindle. To do this, microtubules act like shapeshifters—scaling to adjust to differences in cell size and geometry.

The Gatlin lab is continuing to use their X. laevis system to ask fundamental questions about microtubule assembly. For many decades, both GFP and this amphibian model have provided cell biologists with important insights into the choreography of life, and, as this work shows, we can expect much more to come!

References:

[1] Microtubule growth rates are sensitive to global and local changes in microtubule plus-end density. Geisterfer ZM, Zhu D, Mitchison T, Oakey J, Gatlin JC. November 20, 2019.

[2] Tau-based fluorescent protein fusions to visualize microtubules. Mooney P, Sulerud T, Pelletier JF, Dilsaver MR, et al. Cytoskeleton (Hoboken). 2017 Jun;74(6):221-232.

Links:

Mitosis (National Human Genome Research Institute/NIH)

Gatlin Lab (University of Wyoming, Laramie)

Green Fluorescent Protein Image and Video Contest (American Society for Cell Biology, Bethesda, MD)

2008 Nobel Prize in Chemistry (Nobel Foundation, Stockholm, Sweden)

NIH Support: National Institute of General Medical Sciences


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