Elizabeth Blackburn and the Ends of Chromosomes

“Elizabeth Blackburn CHF Heritage Day 2012 Rush 001” by Chemical Heritage Foundation. Licensed under CC BY-SA 3.0 via Commons

How we age, and how the process may be slowed or prevented, has been an important focus of biomedical research for decades. A large part of what we know about aging at the level of individual cells is due to the research of Elizabeth Blackburn. Her work has helped us to understand how genetic information is protected from damage. Her work began in a single-celled organism and later became pivotal to understanding of human health and disease.

Human cells (and the cells of many other organisms) store genetic information as chromosomes, which are very long, double-stranded DNA molecules packed together with proteins.  The ends of chromosomes, referred to as telomeres, are important for protecting the genetic material stored on chromosomes. The molecular machinery that copies the DNA at each cell division is not able to copy the DNA all the way to the end. This is because the machinery requires a free piece of single-stranded DNA of a certain length to synthesize the other DNA strand. At the ends of chromosomes, not enough single-stranded DNA is available. Because of this, some DNA is lost from the ends of the chromosomes each time the cell divides. After enough divisions, important genetic material begins to be lost. This can lead to cellular malfunction and cell death.

“Tetrahymena thermophila” by see source – Ciliate Genome Sequence Reveals Unique Features of a Model Eukaryote. Robinson R, PLoS Biology Vol. 4/9/2006, e304. doi:10.1371/journal.pbio.0040304. Licensed under CC BY 2.5 via Commons

Blackburn began researching telomeres in the late 1970s, focusing her studies on the unicellular organism Tetrahymena thermophila. This organism has hundreds of short chromosomes, which means it has hundreds of chromosome ends. Combined with the fact Tertahymena grows easily in a laboratory setting, this made it relatively easy to obtain and study large numbers of telomeres, and any proteins that may be necessary for their function.

Blackburn, working with Joseph Gall, determined the order of nucleotides at these telomeres. Surprisingly, they found that the telomeres consisted of large numbers (20-70) of repeats of the same six nucleotides. Also, the number of repeats did not decrease significantly with repeated cell divisions, and the Tetrahymena could divide indefinitely in the lab without any telomere degradation. Blackburn and Gall believed that an undiscovered protein was adding more nucleotide repeats to the end of each chromosome after each cell division, thus maintaining telomere length and protecting important genetic material from damage.

Working with Carol Greider, Blackburn then did a series of experiments to identify this protein that replicates the chromosome ends and determine how it works. They employed a basic strategy still used today by molecular and cell biologists: they took extract from the Tetrahymena cells and divided it into parts, testing each part for a protein with the activity of interest.

In this case, they showed that something inside of Tetrahymena cells can add nucleotide repeats onto the ends of pieces of synthetic DNA made in the lab. Further testing showed that this entity has RNA and protein components, both of which need to be active in order for it to work. They found that the RNA component had the complementary sequence to the telomere repeats and could therefore act as the template for the addition of new repeats. They dubbed this protein “telomerase”, a combination of telomere and –ase, a suffix commonly used for proteins that perform chemical reaction such as synthesizing DNA.

Blackburn and her lab then changed various bases on the telomerase RNA and found this always led to a change in the sequence of the telomere repeats added to the synthetic DNA. This showed that the telomerase RNA is indeed a template for telomere repeat replication. Next, they selectively destroyed the function of the telomerase gene in Tetrahymena, leaving the other genes intact. The cells now had a limited number of cell divisions before their chromosomes shortened too much and the cells began to sicken.

Blackburn and Greider’s work was done concurrently with the work of Jack Szostak’s laboratory, which identified the genes in brewer’s yeast that are necessary for the function and replication of telomeres. The field of chromosome biology held the work of Blackburn and Szostak was held to a high standard, as proteins like telomerase that use RNA as a template for DNA replication had previously only been identified in viruses and were not thought to exist naturally in other organisms. Blackburn’s work in Tetrahymena contributed greatly to the body of evidence that supports the existence of telomerase and its mechanism of action. Blackburn, Greider, and Szostak shared the Nobel Prize in Physiology or Medicine in 2009 for their discovery.

The implications of this discovery for human health is still and active area of research. A telomerase gene has been identified in the genomes of all cells with linear chromosomes. However, cells do not always make telomerase. For example, most healthy cells in human adults do not contain active telomerase protein. The cells can only divide a certain number of times. This is thought to contribute to the normal process of aging in humans, as important groups of cells slowly reach the maximum number of divisions. In many cancer cells, however, telomerase is made when it should not be. This allows the cancer cells to divide many more times than they would otherwise and form tumors. Telomerase activity may also be important in diabetes, heart disease, and response to stressful situations.

Despite the important implications of her research, Blackburn advocates pursuing research not simply because of its applications, but instead because of a desire to discover how things work:

“What I take home as a message from [my work] is that one really wants to understand how biology works by working at it in the most curiosity-driven, question-driven ways, and not necessarily trying to ask the question of application, but simply trying to understand how things work. Because I think we won’t predict necessarily what the ramifications of that would be. That’s certainly been the case in our adventure in working with telomeres and telomerase.”

Blackburn, along with Greider and Szostak, also shared the Albert Lasker Basic Medical Research Award in 2006. She was elected president of the American Society for Cell Biology in 1998. She was also elected to the American Academy of Arts and Sciences, the Royal Society of London, the American Academy of Microbiology, and the American Association for the Advancement of Science. She was named one of Time Magazine’s 100 Most Influential People in 2007 and she was the North American Laureate for L’Oreal-UNESCO for Women in Science in 2008. She currently runs a laboratory in the Department of Biochemistry and Biophysics at University of California, San Francisco.

Sources and Further Reading

Blackburn and Gall. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. Journal of Molecular Biology. 1978.

Greider and Blackburn. Identification of a specific telomere terminal transferase activity in Tetrahymena extracts. Cell. 1985.

Greider and Blackburn. The telomere terminal transferase of Tetrahymena is a ribonucleoprotein enzyme with two kinds of primer specificity. Cell. 1987.

Greider and Blackburn. Recognition and elongation of telomeres by telomerase. Genome. 1989.

Greider and Blackburn. A telomeric sequence in the RNA of Tetrahymena telomerase required for telomere repeat synthesis. Nature. 1989.

Vicki Lundblad. Telomeres in the ’80s: a few recollections. Nature. 2006.

Elizabeth Blackburn: Discovery of Telomeric DNA and Telomerase.

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