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Study Finds Genetic Mutations in Healthy Human Tissues

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General mutations throughout the body

The standard view of biology is that every normal cell copies its DNA instruction book with complete accuracy every time it divides. And thus, with a few exceptions like the immune system, cells in normal, healthy tissue continue to contain exactly the same genome sequence as was present in the initial single-cell embryo that gave rise to that individual. But new evidence suggests it may be time to revise that view.

By analyzing genetic information collected throughout the bodies of nearly 500 different individuals, researchers discovered that almost all had some seemingly healthy tissue that contained pockets of cells bearing particular genetic mutations. Some even harbored mutations in genes linked to cancer. The findings suggest that nearly all of us are walking around with genetic mutations within various parts of our bodies that, under certain circumstances, may have the potential to give rise to cancer or other health conditions.

Efforts such as NIH’s The Cancer Genome Atlas (TCGA) have extensively characterized the many molecular and genomic alterations underlying various types of cancer. But it has remained difficult to pinpoint the precise sequence of events that lead to cancer, and there are hints that so-called normal tissues, including blood and skin, might contain a surprising number of mutations —perhaps starting down a path that would eventually lead to trouble.

In the study published in Science, a team from the Broad Institute at MIT and Harvard, led by Gad Getz and postdoctoral fellow Keren Yizhak, along with colleagues from Massachusetts General Hospital, decided to take a closer look. They turned their attention to the NIH’s Genotype-Tissue Expression (GTEx) project.

The GTEx is a comprehensive public resource that shows how genes are expressed and controlled differently in various tissues throughout the body. To capture those important differences, GTEx researchers analyzed messenger RNA sequences within thousands of healthy tissue samples collected from people who died of causes other than cancer.

Getz, Yizhak, and colleagues wanted to use that extensive RNA data in another way: to detect mutations that had arisen in the DNA genomes of cells within those tissues. To do it, they devised a method for comparing those tissue-derived RNA samples to the matched normal DNA. They call the new method RNA-MuTect.

All told, the researchers analyzed RNA sequences from 29 tissues, including heart, stomach, pancreas, and fat, and matched DNA from 488 individuals in the GTEx database. Those analyses showed that the vast majority of people—a whopping 95 percent—had one or more tissues with pockets of cells carrying new genetic mutations.

While many of those genetic mutations are most likely harmless, some have known links to cancer. The data show that genetic mutations arise most often in the skin, esophagus, and lung tissues. This suggests that exposure to environmental elements—such as air pollution in the lung, carcinogenic dietary substances in the esophagus, or the ultraviolet radiation in sunlight that hits the skin—may play important roles in causing genetic mutations in different parts of the body.

The findings clearly show that, even within normal tissues, the DNA in the cells of our bodies isn’t perfectly identical. Rather, mutations constantly arise, and that makes our cells more of a mosaic of different mutational events. Sometimes those altered cells may have a subtle growth advantage, and thus continue dividing to form larger groups of cells with slightly changed genomic profiles. In other cases, those altered cells may remain in small numbers or perhaps even disappear.

It’s not yet clear to what extent such pockets of altered cells may put people at greater risk for developing cancer down the road. But the presence of these genetic mutations does have potentially important implications for early cancer detection. For instance, it may be difficult to distinguish mutations that are truly red flags for cancer from those that are harmless and part of a new idea of what’s “normal.”

To further explore such questions, it will be useful to study the evolution of normal mutations in healthy human tissues over time. It’s worth noting that so far, the researchers have only detected these mutations in large populations of cells. As the technology advances, it will be interesting to explore such questions at the higher resolution of single cells.

Getz’s team will continue to pursue such questions, in part via participation in the recently launched NIH Pre-Cancer Atlas. It is designed to explore and characterize pre-malignant human tumors comprehensively. While considerable progress has been made in studying cancer and other chronic diseases, it’s clear we still have much to learn about the origins and development of illness to build better tools for early detection and control.

Reference:

[1] RNA sequence analysis reveals macroscopic somatic clonal expansion across normal tissues. Yizhak K, Aguet F, Kim J, Hess JM, Kübler K, Grimsby J, Frazer R, Zhang H, Haradhvala NJ, Rosebrock D, Livitz D, Li X, Arich-Landkof E, Shoresh N, Stewart C, Segrè AV, Branton PA, Polak P, Ardlie KG, Getz G. Science. 2019 Jun 7;364(6444).

Links:

Genotype-Tissue Expression Program

The Cancer Genome Atlas (National Cancer Institute/NIH)

Pre-Cancer Atlas (National Cancer Institute/NIH)

Getz Lab (Broad Institute, Cambridge, MA)

NIH Support: Common Fund; National Heart, Lung, and Blood Institute; National Human Genome Research Institute; National Institute of Mental Health; National Cancer Institute; National Library of Medicine; National Institute on Drug Abuse; National Institute of Neurological Diseases and Stroke


Taking In Some Science-Inspired Art

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Dr. Francis Collins poses with the artist Nana Bagdavadzeand her work depicting DNA
Enjoyed visiting Artists & Makers Studio 2 in Rockville, MD, where Nana Bagdavadze has an exhibit of DNA art that opened May 3, 2019. The artist’s work was inspired by her experience as a bone marrow donor for her sister, who’s doing great! Credit: NIH

Looking for Answers to Epilepsy in a Blood Test

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Gemma Carvill and lab members
Gemma Carvill (second from right) with members of her lab. Courtesy of Gemma Carvill

Millions of people take medications each day for epilepsy, a diverse group of disorders characterized by seizures. But, for about a third of people with epilepsy, current drug treatments don’t work very well. What’s more, the medications are designed to treat symptoms of these disorders, basically by suppressing seizure activity. The medications don’t really change the underlying causes, which are wired deep within the brain.

Gemma Carvill, a researcher at Northwestern University Feinberg School of Medicine, Chicago, wants to help change that in the years ahead. She’s dedicated her research career to discovering the genetic causes of epilepsy in hopes of one day designing treatments that can control or even cure some forms of the disorder [1].

It certainly won’t be easy. A recent paper put the number of known genes associated with epilepsy at close to 1,000 [2]. However, because some disease-causing genetic variants may arise during development, and therefore occur only within the brain, it’s possible that additional genetic causes of epilepsy are still waiting to be discovered within the billions of cells and their trillions of interconnections.

To find these new leads, Carvill won’t have to rely only on biopsies of brain tissue. She’s received a 2018 NIH Director’s New Innovator Award in search of answers hidden within “liquid biopsies”—tiny fragments of DNA that research in other forms of brain injury and neurological disease [3] suggests may spill into the bloodstream and cerebrospinal fluid (CSF) from dying neurons or other brain cells following a seizure.

Carvill and team will start with mouse models of epilepsy to test whether it’s possible to detect DNA fragments from the brain in bodily fluids after a seizure. They’ll also attempt to show DNA fragments carry telltale signatures indicating from which cells and tissues in the brain those molecules originate. The hope is these initial studies will also tell them the best time after a seizure to collect blood samples.

In people, Carvill’s team will collect the DNA fragments and begin searching for genetic alterations to explain the seizures, capitalizing on Carvill’s considerable expertise in the use of next generation DNA sequencing technology for ferreting out disease-causing variants. Importantly, if this innovative work in epilepsy pans out, it also can be applied to any other neurological condition in which DNA spills from dying brain cells, including Alzheimer’s disease and Parkinson’s disease.

References:

[1] Unravelling the genetic architecture of autosomal recessive epilepsy in the genomic era. Calhoun JD, Carvill GL. J Neurogenet. 2018 Sep 24:1-18.

[2] Epilepsy-associated genes. Wang J, Lin ZJ, Liu L, Xu HQ, Shi YW, Yi YH, He N, Liao WP. Seizure. 2017 Jan;44:11-20.

[3] Identification of tissue-specific cell death using methylation patterns of circulating DNA. Lehmann-Werman R, Neiman D, Zemmour H, Moss J, Magenheim J, Vaknin-Dembinsky A, Rubertsson S, Nellgård B, Blennow K, Zetterberg H, Spalding K, Haller MJ, Wasserfall CH, Schatz DA, Greenbaum CJ, Dorrell C, Grompe M, Zick A, Hubert A, Maoz M, Fendrich V, Bartsch DK, Golan T, Ben Sasson SA, Zamir G, Razin A, Cedar H, Shapiro AM, Glaser B, Shemer R, Dor Y. Proc Natl Acad Sci U S A. 2016 Mar 29;113(13):E1826-34.

Links:

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

Gemma Carvill Lab (Northwestern University Feinberg School of Medicine, Chicago)

Carvill Project Information (NIH RePORTER)

NIH Director’s New Innovator Award (Common Fund)

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


World’s Smallest Tic-Tac-Toe Game Built from DNA

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Check out the world’s smallest board game, a nanoscale match of tic-tac-toe being played out in a test tube with X’s and O’s made of DNA. But the innovative approach you see demonstrated in this video is much more than fun and games. Ultimately, researchers hope to use this technology to build tiny DNA machines for a wide variety of biomedical applications.

Here’s how it works. By combining two relatively recent technologies, an NIH-funded team led by Lulu Qian, California Institute of Technology, Pasadena, CA, created a “swapping mechanism” that programs dynamic interactions between complex DNA nanostructures [1]. The approach takes advantage of DNA’s modular structure, along with its tendency to self-assemble, based on the ability of the four letters of DNA’s chemical alphabet to pair up in an orderly fashion, A to T and C to G.

To make each of the X or O tiles in this game (displayed here in an animated cartoon version), researchers started with a single, long strand of DNA and many much shorter strands, called staples. When the sequence of DNA letters in each of those components is arranged just right, the longer strand will fold up into the desired 2D or 3D shape. This technique is called DNA origami because of its similarity to the ancient art of Japanese paper folding.

In the early days of DNA origami, researchers showed the technique could be used to produce miniature 2D images, such as a smiley face [2]. Last year, the Caltech group got more sophisticated—using DNA origami to produce the world’s smallest reproduction of the Mona Lisa [3].

In the latest work, published in Nature Communications, Qian, Philip Petersen and Grigory Tikhomirov first mixed up a solution of nine blank DNA origami tiles in a test tube. Those DNA tiles assembled themselves into a tic-tac-toe grid. Next, two players took turns adding one of nine X or O DNA tiles into the solution. Each of the game pieces was programmed precisely to swap out only one of the tile positions on the original, blank grid, based on the DNA sequences positioned along its edges.

When the first match was over, player X had won! More importantly for future biomedical applications, the original, blank grid had been fully reconfigured into a new structure, built of all-new, DNA-constructed components. That achievement shows not only can researchers use DNA to build miniature objects, they can also use DNA to repair or reconfigure such objects.

Of course, the ultimate aim of this research isn’t to build games or reproduce famous works of art. Qian wants to see her DNA techniques used to produce tiny automated machines, capable of performing basic tasks on a molecular scale. In fact, her team already has used a similar approach to build nano-sized DNA robots, programmed to sort molecules in much the same way that a person might sort laundry [4]. Such robots may prove useful in miniaturized approaches to drug discovery, development, manufacture, and/or delivery.

Another goal of the Caltech team is to demonstrate to the scientific community what’s possible with this leading-edge technology, in hopes that other researchers will pick up their innovative tools for their own applications. That would be a win-win for us all.

References:

[1] Information-based autonomous reconfiguration in systems of DNA nanostructures. Petersen P, Tikhomirov G, Qian L. Nat Commun. 2018 Dec 18;9(1):5362

[2] Folding DNA to create nanoscale shapes and patterns. Rothemund PW. Nature. 2006 Mar 16;440(7082):297-302.

[3] Fractal assembly of micrometre-scale DNA origami arrays with arbitrary patterns. Tikhomirov G, Petersen P, Qian L. Nature. 2017 Dec 6;552(7683):67-71.

[4] A cargo-sorting DNA robot. Thubagere AJ, Li W, Johnson RF, Chen Z, Doroudi S, Lee YL, Izatt G, Wittman S, Srinivas N, Woods D, Winfree E, Qian L. Science. 2017 Sep 15;357(6356).

Links:

Paul Rothemund—DNA Origami: Folded DNA as a Building Material for Molecular Devices (Cal Tech, Pasadena)

The World’s Smallest Mona Lisa (Caltech)

Qian Lab (Caltech, Pasadena, CA)

NIH Support: National Institute of General Medical Sciences


Mood-Altering Messenger Goes Nuclear

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Serotonin

Serotonin is best known for its role as a chemical messenger in the brain, helping to regulate mood, appetite, sleep, and many other functions. It exerts these influences by binding to its receptor on the surface of neural cells. But startling new work suggests the impact of serotonin does not end there: the molecule also can enter a cell’s nucleus and directly switch on genes.

While much more study is needed, this is a potentially groundbreaking discovery. Not only could it have implications for managing depression and other mood disorders, it may also open new avenues for treating substance abuse and neurodegenerative diseases.

To understand how serotonin contributes to switching genes on and off, a lesson on epigenetics is helpful. Keep in mind that the DNA instruction book of all cells is essentially the same, yet the chapters of the book are read in very different ways by cells in different parts of the body. Epigenetics refers to chemical marks on DNA itself or on the protein “spools” called histones that package DNA. These marks influence the activity of genes in a particular cell without changing the underlying DNA sequence, switching them on and off or acting as “volume knobs” to turn the activity of particular genes up or down.

The marks include various chemical groups—including acetyl, phosphate, or methyl—which are added at precise locations to those spool-like proteins called histones. The addition of such groups alters the accessibility of the DNA for copying into messenger RNA and producing needed proteins.

In the study reported in Nature, researchers led by Ian Maze and postdoctoral researcher Lorna Farrelly, Icahn School of Medicine at Mount Sinai, New York, followed a hunch that serotonin molecules might also get added to histones [1]. There had been hints that it might be possible. For instance, earlier evidence suggested that inside cells, serotonin could enter the nucleus. There also was evidence that serotonin could attach to proteins outside the nucleus in a process called serotonylation.

These data begged the question: Is serotonylation important in the brain and/or other living tissues that produce serotonin in vivo? After a lot of hard work, the answer now appears to be yes.

These NIH-supported researchers found that serotonylation does indeed occur in the cell nucleus. They also identified a particular enzyme that directly attaches serotonin molecules to histone proteins. With serotonin attached, DNA loosens on its spool, allowing for increased gene expression.

The team found that histone serotonylation takes place in serotonin-producing human neurons derived from induced pluripotent stem cells (iPSCs). They also observed this process occurring in the brains of developing mice.

In fact, the researchers found evidence of those serotonin marks in many parts of the body. They are especially prevalent in the brain and gut, where serotonin also is produced in significant amounts. Those marks consistently correlate with areas of active gene expression.

The serotonin mark often occurs on histones in combination with a second methyl mark. The researchers suggest that this double marking of histones might help to further reinforce an active state of gene expression.

This work demonstrates that serotonin can directly influence gene expression in a manner that’s wholly separate from its previously known role in transmitting chemical messages from one neuron to the next. And, there are likely other surprises in store.

The newly discovered role of serotonin in modifying gene expression may contribute significantly to our understanding of mood disorders and other psychiatric conditions with known links to serotonin signals, suggesting potentially new targets for therapeutic intervention. But for now, this fundamental discovery raises many more intriguing questions than it answers.

Science is full of surprises, and this paper is definitely one of them. Will this kind of histone marking occur with other chemical messengers, such as dopamine and acetylcholine? This unexpected discovery now allows us to track serotonin and perhaps some of the brain’s other chemical messengers to see what they might be doing in the cell nucleus and whether this information might one day help in treating the millions of Americans with mood and behavioral disorders.

Reference:

[1] Histone serotonylation is a permissive modification that enhances TFIID binding to H3K4me3. Farrelly LA, Thompson RE, Zhao S, Lepack AE, Lyu Y, Bhanu NV, Zhang B, Loh YE, Ramakrishnan A, Vadodaria KC, Heard KJ, Erikson G, Nakadai T, Bastle RM, Lukasak BJ, Zebroski H 3rd, Alenina N, Bader M, Berton O, Roeder RG, Molina H, Gage FH, Shen L, Garcia BA, Li H, Muir TW, Maze I. Nature. 2019 Mar 13. [Epub ahead of print]

Links:

Any Mood Disorder (National Institute of Mental Health/NIH)

Drugs, Brains, and Behavior: The Science of Addiction (National Institute on Drug Abuse/NIH)

Epigenomics (National Human Genome Research Institute/NIH)

Maze Lab (Icahn School of Medicine at Mount Sinai, New York, NY)

NIH Support: National Institute on Drug Abuse; National Institute of Mental Health; National Institute of General Medical Sciences; National Cancer Institute


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