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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


The Cancer Genome Atlas Symposium 2018

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NIH Researchers Recognized for Service to America

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Steve Rosenberg

Caption: Steve Rosenberg receiving his Sammie as 2015 Federal Employee of the Year.
Credit: Aaron Clamage/clamagephoto.com

It was a pleasure for me last night to attend the Samuel J. Heyman Service to America Medals, also known as “the Sammies.” This Washington, D.C. event, now in its 12th year as the “Oscars of American government service,” was a big night for NIH. Steven Rosenberg, a highly regarded physician-scientist at NIH’s National Cancer Institute (NCI), took home the evening’s highest honor as the 2015 Federal Employee of the Year.

Also hearing their names called were NCI’s Jean Claude Zenklusen and Carolyn Hutter of NIH’s National Human Genome Research Institute (NHGRI). They received the inaugural People’s Choice Award. It marks the highest vote-getter from the general public, which was invited to choose from among this year’s 30 finalists in eight award categories.

Jean Claude Zenklusen and Carolyn Hutter

Caption: Francis Collins presenting 2015 People’s Choice Award medals to Jean Claude Zenklusen and Carolyn Hutter.
Credit: Aaron Clamage/clamagephoto.com


Precision Oncology: Creating a Genomic Guide for Melanoma Therapy

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Melanoma cell

Caption: Human malignant melanoma cell viewed through a fluorescent, laser-scanning confocal microscope. Invasive structures involved in metastasis appear as greenish-yellow dots, while actin (green) and vinculin (red) are components of the cell’s cytoskeleton.
Credit: Vira V. Artym, National Institute of Dental and Craniofacial Research, NIH

It’s still the case in most medical care systems that cancers are classified mainly by the type of tissue or part of the body in which they arose—lung, brain, breast, colon, pancreas, and so on. But a radical change is underway. Thanks to advances in scientific knowledge and DNA sequencing technology, researchers are identifying the molecular fingerprints of various cancers and using them to divide cancer’s once-broad categories into far more precise types and subtypes. They are also discovering that cancers that arise in totally different parts of the body can sometimes have a lot in common. Not only can molecular analysis refine diagnosis and provide new insights into what’s driving the growth of a specific tumor, it may also point to the treatment strategy with the greatest chance of helping a particular patient.

The latest cancer to undergo such rigorous, comprehensive molecular analysis is malignant melanoma. While melanoma can rarely arise in the eye and a few other parts of the body, this report focused on the more familiar “cutaneous melanoma,” a deadly and increasingly common form of skin cancer [1].  Reporting in the journal Cell [2], The Cancer Genome Atlas (TCGA) Network says it has identified four distinct molecular subtypes of melanoma. In addition, the NIH-funded network identified an immune signature that spans all four subtypes. Together, these achievements establish a much-needed framework that may guide decisions about which targeted drug, immunotherapy, or combination of therapies to try in an individual with melanoma.


Different Cancers Can Share Genetic Signatures

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Cancer types floating over a cell with unraveling DNA

NIH-funded researchers analyzed the DNA of these cancers.

Cancer is a disease of the genome. It arises when genes involved in promoting or suppressing cell growth sustain mutations that disturb the normal stop and go signals.  There are more than 100 different types of cancer, most of which derive their names and current treatment based on their tissue of origin—breast, colon, or brain, for example. But because of advances in DNA sequencing and analysis, that soon may be about to change.

Using data generated through The Cancer Genome Atlas, NIH-funded researchers recently compared the genomic fingerprints of tumor samples from nearly 3,300 patients with 12 types of cancer: acute myeloid leukemia, bladder, brain (glioblastoma multiforme), breast, colon, endometrial, head and neck, kidney, lung (adenocarcinoma and squamous cell carcinoma), ovarian, and rectal. Confirming but greatly extending what smaller studies have shown, the researchers discovered that even when cancers originate from vastly different tissues, they can show similar features at the DNA level


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