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genetic variants

Largest-Ever Genetic Study of Autism Yields New Insights

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Baby and DNA Strands

Anyone who’s spent time with people affected by autism spectrum disorder (ASD) can tell you that it’s a very complex puzzle. The wide variability seen among individuals with this group of developmental brain disorders, which can disrupt communication, behavior control, and social skills, has also posed a huge challenge for researchers trying to identify underlying genetic and environmental factors. So, it’s no surprise that there’s been considerable interest in the recent findings of the largest-ever genetic study of ASD.

In a landmark study that analyzed the DNA of more than 35,000 people from around the world, the NIH-funded international Autism Sequencing Consortium (ASC) identified variants in 102 genes associated with increased risk of developing ASD, up from 65 identified previously. Of the 102 genes, 60 had not been previously linked to ASD and 53 appeared to be primarily connected to ASD as opposed to other types of intellectual disability or developmental delay. It is expected that this newfound genetic knowledge will serve to improve understanding of the complex biological mechanisms involved in ASD, ultimately paving the way for new approaches to diagnosis and treatment.

The study reported in the journal Cell was led by Joseph Buxbaum, Icahn School of Medicine at Mount Sinai, New York; Stephan Sanders, University of California, San Francisco; Kathryn Roeder, Carnegie Mellon University, Pittsburgh, PA; and Mark Daly, Massachusetts General Hospital, Boston, MA and the Broad Institute of MIT and Harvard, Cambridge, MA. These researchers and their teams faced what might seem like a rather daunting task.

While common genetic variants collectively are known to contribute substantially to ASD, rare variants have been recognized individually as more major contributors to a person’s risk of developing ASD. The challenge was how to find such rare variants—whether inherited or newly arising.

To do so, the researchers needed to analyze a enormous amount of DNA data. Fortunately, they and their ASC colleagues already had assembled a vast trove of data. Over the last decade, the ASC had collected DNA samples with full consent from thousands of people with and without ASD, including unaffected siblings and parents. All were aggregated with other studies, and, at the time of this investigation, they had gathered 35,584 unique samples. Those included more than 21,000 family-based samples and almost 12,000 samples from people diagnosed with ASD.

In search of rare genetic alterations, they sequenced whole exomes, the approximately 1.5 percent of the genome that codes for proteins. Their search produced a list of 102 ASD-associated genes, including 30 that had never been implicated in any developmental brain disorder previously.

But that was just the beginning. Next, the ASC team dug deeper into this list. The researchers knew from previous work that up to half of people with ASD also have an intellectual disability or developmental delay. Many of the associated genes overlap, meaning they play roles in both outcomes. So, in one set of analyses, the team compared the list to the results of another genetic study of people diagnosed with developmental delays, including problems with learning or gross motor skills such as delayed walking.

The detailed comparison allowed them to discern genes that are more associated with features of ASD, as opposed to those that are more specific to these developmental delays. It turns out that 49 of the 102 autism-associated genes were altered more often in people with developmental delay than in those diagnosed with ASD. The other 53 were altered more often in ASD, suggesting that they may be more closely linked to this condition’s unique features.

Further study also showed that people who carried alterations in genes found predominantly in ASD also had better intellectual function. They also were more likely to have learned to walk without a developmental delay.

The 102 new genes fell primarily into one of two categories. Many play a role in the brain’s neural connections. The rest are involved primarily in switching other genes on and off in brain development. Interestingly, they are expressed both in excitatory neurons, which are active in sending signals in the brain, and in inhibitory neurons that squelch such activity. Many of these genes are also commonly expressed in the brain’s cerebral cortex, the outermost part of the brain that is responsible for many complex behaviors.

Overall, these findings underscore that ASD truly does exist on a spectrum. Indeed, there are many molecular paths to this disorder. The ASC researchers continue to collect samples, so we can expect this list of 102 genes will continue to expand in the future.

With these gene discoveries in hand, the researchers will now also turn their attention to unravelling additional details about how these genes function in the brain. The hope is that this growing list of genes will converge on a smaller number of important molecular pathways, pointing the way to new and more precise ways of treating ASD in all its complexity.

Reference:

[1] Large-scale exome sequencing study implicates both developmental and functional changes in the neurobiology of autism. Satterstrom FK, Kosmicki JA, Wang J, Breen MS, De Rubeis S, An JY, Peng M, Collins R, Grove J, Klei L, Stevens C, Reichert J, Mulhern MS, Artomov M, Gerges S, Sheppard B, Xu X, Bhaduri A, Norman U, Brand H, Schwartz G, Nguyen R, Guerrero EE, Dias C; Autism Sequencing Consortium; iPSYCH-Broad Consortium, Betancur C, Cook EH, Gallagher L, Gill M, Sutcliffe JS, Thurm A, Zwick ME, Børglum AD, State MW, Cicek AE, Talkowski ME, Cutler DJ, Devlin B, Sanders SJ, Roeder K, Daly MJ, Buxbaum JD.Cell. 2020 Jan 23. {Epub ahead of print]

Links:

Autism Spectrum Disorder (NIH/National Institute of Mental Health)

Joseph Buxbaum (Icahn School of Medicine at Mount Sinai, New York)

Sanders Lab (University of California, San Francisco)

Kathryn Roeder (Carnegie Mellon University, Pittsburgh, PA)

Mark Daly (Broad Institute of MIT and Harvard, Cambridge, MA)

Autism Sequencing Consortium (Emory University, Atlanta)

NIH Support: National Institute Mental Health; National Human Genome Research Institute


Gene-Editing Advance Puts More Gene-Based Cures Within Reach

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Prime Editing
Caption: The prime editing system (left) contains three parts: two enzymes, Cas9 and reverse transcriptase, and an engineered guide RNA, pegRNA. Unlike regular CRISPR gene editing, prime editing nicks just one strand of the DNA molecule (right) and then uses RNA and reverse transcriptase to direct highly targeted changes to a cell’s DNA. Credit: Broad Institute of MIT and Harvard, Cambridge, MA.

There’s been tremendous excitement recently about the potential of CRISPR and related gene-editing technologies for treating or even curing sickle cell disease (SCD), muscular dystrophy, HIV, and a wide range of other devastating conditions. Now comes word of another remarkable advance—called “prime editing”—that may bring us even closer to reaching that goal.

As groundbreaking as CRISPR/Cas9 has been for editing specific genes, the system has its limitations. The initial version is best suited for making a double-stranded break in DNA, followed by error-prone repair. The outcome is generally to knock out the target. That’s great if eliminating the target is the desired goal. But what if the goal is to fix a mutation by editing it back to the normal sequence?

The new prime editing system, which was described recently by NIH-funded researchers in the journal Nature, is revolutionary because it offers much greater control for making a wide range of precisely targeted edits to the DNA code, which consists of the four “letters” (actually chemical bases) A, C, G, and T [1].

Already, in tests involving human cells grown in the lab, the researchers have used prime editing to correct genetic mutations that cause two inherited diseases: SCD, a painful, life-threatening blood disorder, and Tay-Sachs disease, a fatal neurological disorder. What’s more, they say the versatility of their new gene-editing system means it can, in principle, correct about 89 percent of the more than 75,000 known genetic variants associated with human diseases.

In standard CRISPR, a scissor-like enzyme called Cas9 is used to cut all the way through both strands of the DNA molecule’s double helix. That usually results in the cell’s DNA repair apparatus inserting or deleting DNA letters at the site. As a result, CRISPR is extremely useful for disrupting genes and inserting or removing large DNA segments. However, it is difficult to use this system to make more subtle corrections to DNA, such as swapping a letter T for an A.

To expand the gene-editing toolbox, a research team led by David R. Liu, Broad Institute of MIT and Harvard, Cambridge, MA, previously developed a class of editing agents called base editors [2,3]. Instead of cutting DNA, base editors directly convert one DNA letter to another. However, base editing has limitations, too. It works well for correcting four of the most common single letter mutations in DNA. But at least so far, base editors haven’t been able to make eight other single letter changes, or fix extra or missing DNA letters.

In contrast, the new prime editing system can precisely and efficiently swap any single letter of DNA for any other, and can make both deletions and insertions, at least up to a certain size. The system consists of a modified version of the Cas9 enzyme fused with another enzyme, called reverse transcriptase, and a specially engineered guide RNA, called pegRNA. The latter contains the desired gene edit and steers the needed editing apparatus to a specific site in a cell’s DNA.

Once at the site, the Cas9 nicks one strand of the double helix. Then, reverse transcriptase uses one DNA strand to “prime,” or initiate, the letter-by-letter transfer of new genetic information encoded in the pegRNA into the nicked spot, much like the search-and-replace function of word processing software. The process is then wrapped up when the prime editing system prompts the cell to remake the other DNA strand to match the new genetic information.

So far, in tests involving human cells grown in a lab dish, Liu and his colleagues have used prime editing to correct the most common mutation that causes SCD, converting a T to an A. They were also able to remove four DNA letters to correct the most common mutation underlying Tay-Sachs disease, a devastating condition that typically produces symptoms in children within the first year and leads to death by age four. The researchers also used their new system to insert new DNA segments up to 44 letters long and to remove segments at least 80 letters long.

Prime editing does have certain limitations. For example, 11 percent of known disease-causing variants result from changes in the number of gene copies, and it’s unclear if prime editing can insert or remove DNA that’s the size of full-length genes—which may contain up to 2.4 million letters.

It’s also worth noting that now-standard CRISPR editing and base editors have been tested far more thoroughly than prime editing in many different kinds of cells and animal models. These earlier editing technologies also may be more efficient for some purposes, so they will likely continue to play unique and useful roles in biomedicine.

As for prime editing, additional research is needed before we can consider launching human clinical trials. Among the areas that must be explored are this technology’s safety and efficacy in a wide range of cell types, and its potential for precisely and safely editing genes in targeted tissues within living animals and people.

Meanwhile, building on all these bold advances, efforts are already underway to accelerate the development of affordable, accessible gene-based cures for SCD and HIV on a global scale. Just last month, NIH and the Bill & Melinda Gates Foundation announced a collaboration that will invest at least $200 million over the next four years toward this goal. Last week, I had the chance to present this plan and discuss it with global health experts at the Grand Challenges meeting Addis Ababa, Ethiopia. The project is an unprecedented partnership designed to meet an unprecedented opportunity to address health conditions that once seemed out of reach but—as this new work helps to show—may now be within our grasp.

References:

[1] Search-and-replace genome editing without double-strand breaks or donor DNA. Anzalone AV, Randolph PB, Davis JR, Sousa AA, Koblan LW, Levy JM, Chen PJ, Wilson C, Newby GA, Raguram A, Liu DR. Nature. Online 2019 October 21. [Epub ahead of print]

[2] 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.

[3] 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.

Links:

Tay-Sachs Disease (Genetics Home Reference/National Library of Medicine/NIH)

Sickle Cell Disease (National Heart, Lung, and Blood Institute/NIH)

Cure Sickle Cell Initiative (NHLBI)

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

Somatic Cell Genome Editing Program (Common Fund/NIH)

David R. Liu (Harvard, Cambridge, MA)

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


A New Piece of the Alzheimer’s Puzzle

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A couple enjoying a hot drink

Credit: National Institute on Aging, NIH

For the past few decades, researchers have been busy uncovering genetic variants associated with an increased risk of Alzheimer’s disease (AD) [1]. But there’s still a lot to learn about the many biological mechanisms that underlie this devastating neurological condition that affects as many as 5 million Americans [2].

As an example, an NIH-funded research team recently found that AD susceptibility may hinge not only upon which gene variants are present in a person’s DNA, but also how RNA messages encoded by the affected genes are altered to produce proteins [3]. After studying brain tissue from more than 450 deceased older people, the researchers found that samples from those with AD contained many more unusual RNA messages than those without AD.


Big Data Study Reveals Possible Subtypes of Type 2 Diabetes

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Computational model

Caption: Computational model showing study participants with type 2 diabetes grouped into three subtypes, based on similarities in data contained in their electronic health records. Such information included age, gender (red/orange/yellow indicates females; blue/green, males), health history, and a range of routine laboratory and medical tests.
Credit: Dudley Lab, Icahn School of Medicine at Mount Sinai, New York

In recent years, there’s been a lot of talk about how “Big Data” stands to revolutionize biomedical research. Indeed, we’ve already gained many new insights into health and disease thanks to the power of new technologies to generate astonishing amounts of molecular data—DNA sequences, epigenetic marks, and metabolic signatures, to name a few. But what’s often overlooked is the value of combining all that with a more mundane type of Big Data: the vast trove of clinical information contained in electronic health records (EHRs).

In a recent study in Science Translational Medicine  [1], NIH-funded researchers demonstrated the tremendous potential of using EHRs, combined with genome-wide analysis, to learn more about a common, chronic disease—type 2 diabetes. Sifting through the EHR and genomic data of more than 11,000 volunteers, the researchers uncovered what appear to be three distinct subtypes of type 2 diabetes. Not only does this work have implications for efforts to reduce this leading cause of death and disability, it provides a sneak peek at the kind of discoveries that will be made possible by the new Precision Medicine Initiative’s national research cohort, which will enroll 1 million or more volunteers who agree to share their EHRs and genomic information.


Precision Medicine: Who Benefits from Aspirin to Prevent Colorectal Cancer?

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Aspirin and DNA StethoscopeIn recent years, scientific evidence has begun to accumulate that indicates taking aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs) on a daily basis may lower the risk of developing colorectal cancer. Now, a new study provides more precise information on who might benefit from this particular prevention strategy, as well as who might not.

Published in the journal JAMA, the latest work shows that, for the majority of people studied, regular use of aspirin or NSAIDs was associated with about a one-third lower risk of developing colorectal cancer. But the international research team, partly funded by NIH, also found that not all regular users of aspirin/NSAIDs reaped such benefits—about 9 percent experienced no reduction in colorectal cancer risk and 4 percent actually appeared to have an increased risk [1]. Was this just coincidence, or might there be a biological explanation?


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