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Huntington’s disease

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)


Making Personalized Blood-Brain Barriers in a Dish

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Credit: Vatine et al, Cell Stem Cell, 2019

The blood-brain barrier, or BBB, is a dense sheet of cells that surrounds most of the brain’s blood vessels. The BBB’s tiny gaps let vital small molecules, such as oxygen and water, diffuse from the bloodstream into the brain while helping to keep out larger, impermeable foreign substances that don’t belong there.

But in people with certain neurological disorders—such as amyotrophic lateral sclerosis (ALS) and Huntington’s disease—abnormalities in this barrier may block the entry of biomolecules essential to healthy brain activity. The BBB also makes it difficult for needed therapies to reach their target in the brain.

To help look for solutions to these and other problems, researchers can now grow human blood-brain barriers on a chip like the one pictured above. The high-magnification image reveals some of the BBB’s cellular parts. There are endothelial-like cells (magenta), which are similar to those that line the small vessels surrounding the brain. In close association are supportive brain cells known as astrocytes (green), which help to regulate blood flow.

While similar organ chips have been created before, what sets apart this new BBB chip is its use of induced pluripotent stem cell (iPSC) technology combined with advanced chip engineering. The iPSCs, derived in this case from blood samples, make it possible to produce a living model of anyone’s unique BBB on demand.

The researchers, led by Clive Svendsen, Cedars-Sinai, Los Angeles, first use a biochemical recipe to coax a person’s white blood cells to become iPSCs. At this point, the iPSCs are capable of producing any other cell type. But the Svendsen team follows two different recipes to direct those iPSCs to differentiate into endothelial and neural cells needed to model the BBB.

Also making this BBB platform unique is its use of a sophisticated microfluidic chip, produced by Boston-based Emulate, Inc. The chip mimics conditions inside the human body, allowing the blood-brain barrier to function much as it would in a person.

The channels enable researchers to flow cerebral spinal fluid (CSF) through one side and blood through the other to create the fully functional model tissue. The BBB chips also show electrical resistance and permeability just as would be expected in a person. The model BBBs are even able to block the entry of certain drugs!

As described in Cell Stem Cell, the researchers have already created BBB chips using iPSCs from a person with Huntington’s disease and another from an individual with a rare congenital disorder called Allan-Herndon-Dudley syndrome, an inherited disorder of brain development.

In the near term, his team has plans to model ALS and Parkinson’s disease on the BBB chips. Because these chips hold the promise of modeling the human BBB more precisely than animal models, they may accelerate studies of potentially promising new drugs. Svendsen suggests that individuals with neurological conditions might one day have their own BBB chips made on demand to help in selecting the best-available therapeutic options for them. Now that’s a future we’d all like to see.

Reference:

[1] Human iPSC-Derived Blood-Brain Barrier Chips Enable Disease Modeling and Personalized Medicine Applications. Vatine GD, Barrile R, Workman MJ, Sances S, Barriga BK, Rahnama M, Barthakur S, Kasendra M, Lucchesi C, Kerns J, Wen N, Spivia WR, Chen Z, Van Eyk J, Svendsen CN. Cell Stem Cell. 2019 Jun 6;24(6):995-1005.e6.

Links:

Tissue Chip for Drug Screening (National Center for Advancing Translational Sciences/NIH)

Stem Cell Information (NIH)

Svendsen Lab (Cedars-Sinai, Los Angeles)

NIH Support: National Institute of Neurological Disorders and Stroke; National Center for Advancing Translational Sciences


What a Year It Was! A Look Back at Research Progress in 2017

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I want to wish everyone a Happy New Year! Hope your 2018 is off to a great start.

Over the holidays, the journal Science published its annual, end-of-the-year list of research breakthroughs, from anthropology to zoology. I always look forward to seeing the list and reflecting on some of the stunning advances reported in the past 12 months. Last year was no exception. Science’s 2017 Breakthrough of the Year, as chosen by its editors, was in the field of astrophysics. Scientists were able to witness the effects of the collision of two neutron stars—large stars with collapsed inner cores—smacking into each other 130 million light years away. How cool is that!

Numbered prominently among the nine other breakthroughs were five from biomedicine: gene therapy, gene editing, cancer immunotherapy, cryo-EM, and biology preprints. All involved varying degrees of NIH support, and all drew great interest from readers. In fact, three of the top four vote-getters in the “People’s Choice” category came from biomedicine. That includes the People’s 2017 Breakthrough of the Year: gene therapy success. And so, in what has become a Director’s Blog tradition, I’ll kick off our new year of posts by taking a closer look at these biomedical breakthroughs—starting with the little girl in the collage above, and moving clockwise around the images:


Huntington’s Disease: Gene Editing Shows Promise in Mouse Studies

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Cas9 clipping the Huntington's repeatsMy father was a folk song collector, and I grew up listening to the music of Woody Guthrie. On July 14th, folk music enthusiasts will be celebrating the 105th anniversary of Guthrie’s birth in his hometown of Okemah, OK. Besides being renowned for writing “This Land is Your Land” and other folk classics, Guthrie has another more tragic claim to fame: he provided the world with a glimpse at the devastation caused by a rare, inherited neurological disorder called Huntington’s disease.

When Guthrie died from complications of Huntington’s a half-century ago, the disease was untreatable. Sadly, it still is. But years of basic science advances, combined with the promise of innovative gene editing systems such as CRISPR/Cas9, are providing renewed hope that we will someday be able to treat or even cure Huntington’s disease, along with many other inherited disorders.


Creative Minds: Can Diseased Cells Help to Make Their Own Drugs?

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Matthew Disney

Matthew Disney

Matthew Disney grew up in a large family in Baltimore in the 1980s. While his mother worked nights, Disney and his younger brother often tagged along with their father in these pre-Internet days on calls to fix the microfilm machines used to view important records at hospitals, banks, and other places of business. Watching his father take apart the machines made Disney want to work with his hands one day. Seeing his father work tirelessly for the sake of his family also made him want to help others.

Disney found a profession that satisfied both requirements when he fell in love with chemistry as an undergraduate at the University of Maryland, College Park. Now a chemistry professor at The Scripps Research Institute, Jupiter, FL, Disney is applying his hands and brains to develop a treatment strategy that aims to control the progression of a long list of devastating disorders that includes Huntington’s disease, amyotrophic lateral sclerosis (ALS), and various forms of muscular dystrophy.

The 30 or so health conditions on Disney’s list have something in common. They are caused by genetic glitches in which repetitive DNA letters (CAGCAGCAG, for example) in transcribed regions of the genome cause some of the body’s cells and tissues to produce unwieldy messenger RNA molecules that interfere with normal cellular activities, either by binding other intracellular components or serving as templates for the production of toxic proteins.

The diseases on Disney’s list also have often been considered “undruggable,” in part because the compounds capable of disabling the lengthy, disease-causing RNA molecules are generally too large to cross cell membranes. Disney has found an ingenious way around that problem [1]. Instead of delivering the finished drug, he delivers smaller building blocks. He then uses the cell and its own machinery, including the very aberrant RNA molecules he aims to target, as his drug factory to produce those larger compounds.

Disney has received an NIH Director’s 2015 Pioneer Award to develop this innovative drug-delivery strategy further. He will apply his investigational approach initially to treat a common form of muscular dystrophy, first using human cells in culture and then in animal models. Once he gets that working well, he’ll move on to other conditions including ALS.

What’s appealing about Disney’s approach is that it makes it possible to treat disease-affected cells without affecting healthy cells. That’s because his drugs can only be assembled into their active forms in cells after they are templated by those aberrant RNA molecules.

Interestingly, Disney never intended to study human diseases. His lab was set up to study the structure and function of RNA molecules and their interactions with other small molecules. In the process, he stumbled across a small molecule that targets an RNA implicated in a rare form of muscular dystrophy. His niece also has a rare incurable disease, and Disney saw a chance to make a difference for others like her. It’s a healthy reminder that the pursuit of basic scientific questions often can lead to new and unexpectedly important medical discoveries that have the potential to touch the lives of many.

Reference:

[1] A toxic RNA catalyzes the in cellulo synthesis of its own inhibitor. Rzuczek SG, Park H, Disney MD. Angew Chem Int Ed Engl. 2014 Oct 6;53(41):10956-10959.

Links:

Disney Lab (The Scripps Research Institute, Jupiter, FL)

Disney NIH Project Information (NIH RePORTER)

NIH Director’s Pioneer Award Program

NIH Support: Common Fund; National Institute of Neurological Disorders and Stroke


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