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Somatic Cell Genome Editing

Nano-Sized Solution for Efficient and Versatile CRISPR Gene Editing

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Nanocapsule for Gene Editing
Caption: Nanocapsule containing its gene-editing payload.
Credit: Guojun Chen and Amr Abdeen, University of Wisconsin-Madison

If used to make non-heritable genetic changes, CRISPR gene-editing technology holds tremendous promise for treating or curing a wide range of devastating disorders, including sickle cell disease, vision loss, and muscular dystrophy. Early efforts to deliver CRISPR-based therapies to affected tissues in a patient’s body typically have involved packing the gene-editing tools into viral vectors, which may cause unwanted immune reactions and other adverse effects.

Now, NIH-supported researchers have developed an alternative CRISPR delivery system: nanocapsules. Not only do these tiny, synthetic capsules appear to pose a lower risk of side effects, they can be precisely customized to deliver their gene-editing payloads to many different types of cells or tissues in the body, which can be extremely tough to do with a virus. Another advantage of these gene-editing nanocapsules is that they can be freeze-dried into a powder that’s easier than viral systems to transport, store, and administer at different doses.

In findings published in Nature Nanotechnology [1], researchers, led by Shaoqin Gong and Krishanu Saha, University of Wisconsin-Madison, developed the nanocapsules with specific design criteria in mind. They would need to be extremely small, about the size of a small virus, for easy entry into cells. Their surface would need to be adaptable for targeting different cell types. They also had to be highly stable in the bloodstream and yet easily degraded to release their contents once inside a cell.

After much hard work in the lab, they created their prototype. It features a thin polymer shell that’s easily decorated with peptides or other ingredients to target the nanocapsule to a predetermined cell type.

At just 25 nanometers in diameter, each nanocapsule still has room to carry cargo. That cargo includes a single CRISPR/Cas9 scissor-like enzyme for snipping DNA and a guide RNA that directs it to the right spot in the genome for editing.

In the bloodstream, the nanocapsules remain fully intact. But, once inside a cell, their polymer shells quickly disintegrate and release the gene-editing payload. How is this possible? The crosslinking molecules that hold the polymer together immediately degrade in the presence of another molecule, called glutathione, which is found at high levels inside cells.

The studies showed that human cells grown in the lab readily engulf and take the gene-editing nanocapsules into bubble-like endosomes. Their gene-editing contents are then released into the cytoplasm where they can begin making their way to a cell’s nucleus within a few hours.

Further study in lab dishes showed that nanocapsule delivery of CRISPR led to precise gene editing of up to about 80 percent of human cells with little sign of toxicity. The gene-editing nanocapsules also retained their potency even after they were freeze-dried and reconstituted.

But would the nanocapsules work in a living system? To find out, the researchers turned to mice, targeting their nanocapsules to skeletal muscle and tissue in the retina at the back of eye. Their studies showed that nanocapsules injected into muscle or the tight subretinal space led to efficient gene editing. In the eye, the nanocapsules worked especially well in editing retinal cells when they were decorated with a chemical ingredient known to bind an important retinal protein.  

Based on their initial results, the researchers anticipate that their delivery system could reach most cells and tissues for virtually any gene-editing application. In fact, they are now exploring the potential of their nanocapsules for editing genes within brain tissue.

I’m also pleased to note that Gong and Saha’s team is part of a nationwide consortium on genome editing supported by NIH’s recently launched Somatic Cell Genome Editing program. This program is dedicated to translating breakthroughs in gene editing into treatments for as many genetic diseases as possible. So, we can all look forward to many more advances like this one.

Reference:

[1] A biodegradable nanocapsule delivers a Cas9 ribonucleoprotein complex for in vivo genome editing. Chen G, Abdeen AA, Wang Y, Shahi PK, Robertson S, Xie R, Suzuki M, Pattnaik BR, Saha K, Gong S. Nat Nanotechnol. 2019 Sep 9.

Links:

Somatic Cell Genome Editing (NIH)

Saha Lab (University of Wisconsin-Madison)

Shaoqin (Sarah) Gong (University of Wisconsin-Madison)

NIH Support: National Eye Institute; National Institute of General Medical Sciences; National Institute of Neurological Disorders and Stroke; National Heart, Lung, and Blood Institute; Common Fund


More Progress Toward Gene Editing for Kids with Muscular Dystrophy

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Normal and treated muscles with DMD
Caption: Muscles of untreated mouse model of Duchenne muscular dystrophy (left) compared to muscles of similar mice one year after gene-editing treatment (right). Dystrophin production (green) is restored in treated animals, despite therapy-related immune response to the Cas9 editing enzyme (dark spots in white inset). Credit: Charles Gersbach, Duke University, Durham, NC

Thanks to CRISPR and other gene editing technologies, hopes have never been greater for treating or even curing Duchenne muscular dystrophy (DMD) and many other rare, genetic diseases that once seemed tragically out of reach. The latest encouraging news comes from a study in which a single infusion of a CRISPR editing system produced lasting benefits in a mouse model of DMD.

There currently is no way to cure DMD, an ultimately fatal disease that mainly affects boys. Caused by mutations in a gene that codes for a critical protein called dystrophin, DMD progressively weakens the skeletal and heart muscles. People with DMD are usually in wheelchairs by the age of 10, with most dying before the age of 30.

The exquisite targeting ability of CRISPR/Cas9 editing systems rely on a sequence-specific guide RNA to direct a scissor-like, bacterial enzyme (Cas9) to just the right spot in the genome, where it can be used to cut out, replace, or repair disease-causing mutations. In previous studies in mice and dogs, researchers directly infused CRISPR systems directly into the animals bodies. This “in vivo” approach to gene editing successfully restored production of functional dystrophin proteins, strengthening animals’ muscles within weeks of treatment.

But an important question remained: would CRISPR’s benefits persist over the long term? The answer in a mouse model of DMD appears to be “yes,” according to findings published recently in Nature Medicine by Charles Gersbach, Duke University, Durham, NC, and his colleagues [1]. Specifically, the NIH-funded team found that after mice with DMD received one infusion of a specially designed CRISPR/Cas9 system, the abnormal gene was edited in a way that restored dystrophin production in skeletal and heart muscles for more than a year. What’s more, lasting improvements were seen in the structure of the animals’ muscles throughout the same time period.

As exciting as these results may be, much more research is needed to explore both the safety and the efficacy of in vivo gene editing before it can be tried in humans with DMD. For instance, the researchers found that older mice that received the editing system developed an immune response to the bacterially-derived Cas9 protein. However, this response didn’t prevent the CRISPR/Cas9 system from doing its job or appear to cause any adverse effects. Interestingly, younger animals didn’t show such a response.

It’s worth noting that the immune systems of mice and people often respond quite differently. But the findings do highlight some possible challenges of such treatments, as well as approaches to reduce possible side effects. For instance, the latest findings suggest CRISPR/Cas9 treatment might best be done early in life, before an infant’s immune system is fully developed. Also, if it’s necessary to deliver CRISPR/Cas9 to older individuals, it may be beneficial to suppress the immune system temporarily.

Another concern about CRISPR technology is the potential for damaging, “off-target” edits to other parts of the genome. In the new work, the Duke team found that its CRISPR system made very few “off-target” edits. However, the system did make a surprising number of complex edits to the targeted dystrophin gene, including integration of the viral vector used to deliver Cas9. While those editing “errors” might reduce the efficacy of treatment, researchers said they didn’t appear to affect the health of the mice studied.

It’s important to emphasize that this gene editing research aimed at curing DMD is being done in non-reproductive (somatic) cells, primarily muscle tissue. The NIH does not support the use of gene editing technologies in human embryos or human reproductive (germline) cells, which would change the genetic makeup of future offspring.

As such, the Duke researchers’ CRISPR/Cas9 system is designed to work optimally in a range of muscle and muscle-progenitor cells. Still, they were able to detect editing of the dystrophin-producing gene in the liver, kidney, brain, and other tissues. Importantly, there was no evidence of edits in the germline cells of the mice. The researchers note that their CRISPR system can be reconfigured to limit gene editing to mature muscle cells, although that may reduce the treatment’s efficacy.

It’s truly encouraging to see that CRISPR gene editing may confer lasting benefits in an animal model of DMD, but a great many questions remain before trying this new approach in kids with DMD. But that time is coming—so let’s boldly go forth and get answers to those questions on behalf of all who are affected by this heartbreaking disease.

Reference:

[1] Long-term evaluation of AAV-CRISPR genome editing for Duchenne muscular dystrophy. Nelson CE, Wu Y, Gemberling MP, Oliver ML, Waller MA, Bohning JD, Robinson-Hamm JN, Bulaklak K, Castellanos Rivera RM, Collier JH, Asokan A, Gersbach CA. Nat Med. 2019 Feb 18.

Links:

Muscular Dystrophy Information Page (National Institute of Neurological Disorders and Stroke/NIH)

Gersbach Lab (Duke University, Durham, NC)

Somatic Cell Genome Editing (Common Fund/NIH)

NIH Support: National Institute of Arthritis and Musculoskeletal and Skin Diseases; National Institute of Biomedical Imaging and Bioengineering


Gene Editing in Dogs Boosts Hope for Kids with Muscular Dystrophy

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Dystrophin before and after treatment

Caption: A CRISPR/cas9 gene editing-based treatment restored production of dystrophin proteins (green) in the diaphragm muscles of dogs with Duchenne muscular dystrophy.
Credit: UT Southwestern

CRISPR and other gene editing tools hold great promise for curing a wide range of devastating conditions caused by misspellings in DNA. Among the many looking to gene editing with hope are kids with Duchenne muscular dystrophy (DMD), an uncommon and tragically fatal genetic disease in which their muscles—including skeletal muscles, the heart, and the main muscle used for breathing—gradually become too weak to function. Such hopes were recently buoyed by a new study that showed infusion of the CRISPR/Cas9 gene editing system could halt disease progression in a dog model of DMD.

As seen in the micrographs above, NIH-funded researchers were able to use the CRISPR/Cas9 editing system to restore production of a critical protein, called dystrophin, by up to 92 percent in the muscle tissue of affected dogs. While more study is needed before clinical trials could begin in humans, this is very exciting news, especially when one considers that boosting dystrophin levels by as little as 15 percent may be enough to provide significant benefit for kids with DMD.