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.


Peer review gone (terribly) wrong

Over the past few decades, the position of women in the sciences, especially fields such as biology, has steadily improved. Young female scientists today have several role models in their fields that provide encouraging evidence that women can be great scientists. And while females may still be the minority in certain settings, we are definitely receiving more encouragement and support, especially at younger ages. Every so often, however, something happens to remind us that not everyone shares the viewpoint that women can and should do science. Last week, such an event took place yet again.

On April 29th, Dr. Fiona Ingelby posted a shocking series of tweets about a review that accompanied the rejection of a research article she had co-written. Ingelby studies evolutionary genetics and statistics as a post-doctoral researcher at the University of Sussex. Dr. Meagan Head, co-author on the study, is a post-doctoral researcher in evolutionary and behavioral ecology at Australian National University. The study utilized survey data to examine gender differences in the Ph.D. to post-doctoral transition. The results of the study have yet to be published, but this transition is an important point in any academic career where gender differences could be a factor. Aside from making vague comments about the quality of the study, the review included comments that can only be interpreted as blatantly sexist, which Ingelby posted on Twitter:

“It would probably be beneficial to find one or two male biologists to work with (or at least obtain internal peer review from, but better yet as active co-authors), in order to serve as a possible check against interpretations that may sometimes be drifting too far away from empirical evidence into ideologically biased assumptions”

The review goes on to suggest that it may not be surprising that male Ph.D. students publish one more paper on average than their female colleagues, just as (the reviewer claims) the average male student can likely run a mile faster than a female student. The review also suggests that males tend to publish more often in better journals not because of any bias but because “men, perhaps, on average work more hours per week than women due to marginally better health and stamina.”

These statements incited a wave of shocked responses on Twitter and various news outlets. One issue that upset people was that the reviewer insists that the authors need to include researchers of both genders to add legitimacy to their interpretations, while not indicating whether both males and females were consulted in writing the review. The reviewer then proceeds to question the legitimacy of a study that is likely based on careful analysis of carefully collected survey data while making highly speculative statements about the physical fitness of graduate students based on no evidence whatsoever.

To my knowledge, there have not been any studies on the physical fitness of doctoral students, though I would be very interested if such a study exists. It has been well established in the general population that men are faster than women, largely due to differences in anatomy. However, doctoral students are not the general population. They are an eclectic mix of people with wide variations in nutrition, physical fitness, and mental health, and who often suffer health consequences from long hours and less-than-ideal eating habits. With this in mind, I would not be surprised if male and female students had similar mile times, or if women were even slightly faster. Additionally, the reviewer does not provide any evidence to support the claims that running times or stamina would have any influence on success in a laboratory. The ability to run a five-minute mile does not make a researcher any more qualified to run a gel, operate a microscope, or sit at a computer analyzing data.

The issue at stake here is not the quality of the study. As it is based on survey data, the possibility of non-response bias and other problems with similar studies may or may not be influencing the data or the interpretation of the results. It is possible that the study is very well done and that the results could have an important impact on gender equality in science. However, it is possible that the conclusions made in the article are not correct. These are the issues that a properly conducted peer review is meant to address. What is at stake is whether or not statements such as those made in this review will be tolerated and whether it will be acceptable to make important decisions such as accepting or rejecting an article based on biased views of female intelligence and capability.

At first, it appeared that the review would stand. Ingelby and Head appealed the rejection, but received no response for three weeks before Ingelby publicized the content of the review. A couple of days after she did so, the journal (PLOS ONE) announced that it had removed the reviewer from their reviewer database, removed the review from the record, sent the manuscript out for re-review, and asked the editor who handled the review to step down. The journal is also considering policy changes that would make the review process more transparent and institute more accountability for the reviewers. It is unfortunate that the review had to be posted on Twitter to motivate such a response. Yet it is encouraging that Ingelby’s voice was heard by the scientific community and that this could change the peer review process for the better.

Gerty Cori: Foundational sugar metabolism researcher

Understanding how the human body processes sugars, particularly glucose, is fundamental to our knowledge of human health and disease. The supply of glucose can vary greatly based on diet or time of day. However, the levels of glucose in the bloodstream must remain fairly constant in order to maintain a steady energy supply and prevent damage to various organs. To do this, the human body has an intricate system of checks and balances to regulate how much glucose is allocated to storage versus allowed to enter the bloodstream. The scientist featured in today’s article was instrumental in discovering how this system functions. In 1947, Gerty Theresa Cori was awarded the Nobel Prize in Physiology or Medicine “for [her] discovery of the course of the catalytic conversion of glycogen.” She was the first woman to receive this award, and the third woman to receive a Nobel Prize. She shared the prize with Carl Cori, her husband and collaborator for decades, and Bernardo Houssay, a sugar metabolism researcher from Argentina.

Life and Career

Gerty met her husband while they were both attending medical school in Prague at the beginning of World War I. After being separated when Carl was drafted into the Austrian army, they married in 1920 and worked as both physicians and researchers in Vienna. Gerty remained in Vienna until she was able to obtain a position at the State Institute for the Study of Malignant Disease in Buffalo, where Carl had secured a position in 1922. Gerty and Carl collaborated extensively for the rest of their research careers, eventually moving to Washington University of St. Louis in 1931, where they worked together until Gerty’s death in 1957. As a female scientist working in the early 20th century, Gerty faced a fair degree of gender discrimination. For example, when they started at Washington University, Gerty was paid only one tenth of what Carl was paid, despite having similar education and research experience. The Coris also faced resistance to collaborating in their research, as many scientists thought it strange, counter-productive, and even un-American for a husband and wife to be working together. However, Gerty did not allow the opinions of her colleagues to hamper her research. Far from riding on her husband’s coattails, Gerty worked tirelessly to earn every honor she received, eventually being promoted to professor at Washington University. She was an equal partner in every collaboration with Carl and conducted large amounts of her own research. Though not always treated as such, Gerty eventually proved that she was a great biochemist, both to her colleagues, to those she mentored, and to the Nobel Prize committee.

Before the Cori’s research into the biochemistry of glucose metabolism began, it was known that defects in glucose metabolism cause diabetes and that administration of the hormone insulin can keep diabetes in check. It was also known that glucose is stored in the liver and muscle as glycogen, which is a polymer, or long chain, of glucose molecules analogous to starch in plants. What was not known was exactly how levels of glucose and glycogen are maintained or the mechanisms of conversion between glucose and glycogen. This is where the Coris stepped in.

Glucose shuttle: The Cori Cycle

While not the research that earned her the Nobel Prize, the proposal and the demonstration of the Cori cycle is still a fundamental part of Gerty’s contribution to this field. The cycle that the Coris proposed begins in skeletal muscle, where glucose is made from stored glycogen and utilized to produce the large amounts of energy that muscles need to function. If oxygen supplies in the muscle are low, as often happens during exercise, a byproduct of the reactions that harvest energy from glucose is lactic acid. This lactic acid is shuttled to the liver via the bloodstream, where it is converted back to glucose through a long, multistep process known as gluconeogenesis. This newly re-formed glucose is either used to make more liver glycogen or sent via the bloodstream back to the skeletal muscle, where it is used to replenish glycogen stores. In this way, the function of the liver as an energy storehouse and the function of the muscle as an energy producer are coordinated to help meet the demands of daily life as a human. Gerty, working closely with her husband, helped to theorize and later prove the existence of this cycle. They also discovered that insulin increases the synthesis of glycogen in both the liver and muscle, while another hormone known as adrenaline or epinephrine decreases muscle and liver glycogen stores. These discoveries increased our knowledge of how sugar metabolism works in general, which helped us to understand the negative effects of diabetes and many other metabolic diseases.

Glycogen breakdown: The Cori Ester

The other fundamental discovery made by Gerty and her husband was the first product of glycogen breakdown. This product is similar in structure and size to glucose, but with a phosphate group (a large, negatively charged structure) attached. This phosphate group and its position on the molecule allow the proteins involved in glycogen metabolism to recognize it and shuttle it to the next step in making a form of glucose the body can use for energy. The Coris were able to discover this molecule, which they named glucose-1-phosphate, from preparations of frog skeletal muscle, which they used extensively in their biochemical research. The Coris also established the structure of this molecule and the enzyme that produces it, which they named phosphorylase. The discoveries of glucose-1-phospate (also termed the Cori esther) and phosphorylase was one of the first steps toward understanding the intricate regulation of glucose metabolism, an understanding which is taken for granted by biochemistry students today. The discovery was honored when the US Postal Service included Gerty on a stamp in April of 2008, despite a small error in the structure. Her greatest honor, despite having two craters named after her and her husband, was to have her name attached to two such fundamental aspects of glucose metabolism.

Stamp honoring Gerty Cori. From the Washington University Magazine.

Sources and further reading

  1. http://en.wikipedia.org/wiki/Gerty_Cori
  2. http://en.wikipedia.org/wiki/Cori_cycle
  3. http://en.wikipedia.org/wiki/Glucose_1-phosphate
  4. American Chemical Society National Historic Chemical Landmarks. Carl and Gerty Cori and Carbohydrate Metabolism. http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/carbohydratemetabolism.html (accessed April 4, 2015)

Hello world!

The impetus for starting this new blog came from the announcement that Maryam Mirzakhani had become the first woman, and first Iranian, to win the Fields Medal. I joined in the excitement of many of my colleagues that one of the most male-dominated STEM fields had at last chosen to honor one of its high-achieving females. Because the Fields Medal had been described to me as the Nobel Prize for mathematics, I wondered how many women had been awarded the Nobel Prize in Physiology or Medicine, the equivalent honor in my field. To my chagrin, I could not think of a single female awardee. I could recall several male awardees whose work had been highlighted in my coursework: Yamanaka, Schekman, Warren, and of course Watson and Crick. Naturally, I thought of Rosalind Franklin, who is mentioned by nearly every biology professor as the woman who should have won the prize (rightly so, given the groundbreaking nature of her work). However, I could not remember the name of any woman that had actually been awarded this honor. I was not alone, as no one in my lab could either (disclaimer: I did not ask my PI [Principal Investigator] who would likely have listed half of them without blinking an eye).

This lack of knowledge about the most highly honored women in my profession irked me for several days. I wanted to be able to speak of female Nobel Laureates in the same way that female math graduate students can now speak of Dr. Mirzakhani. And so I looked them up. Since the creation of the award in 1901, 207 individuals have been awarded the Nobel Prize in Physiology or Medicine. Of these, 11 are women, starting with Gerty Cori in 1947 and ending most recently with May-Britt Moser in 2014. At face value, this number seems shockingly low at just over five percent. The temptation here, for me, is to become angry at the injustice and discrimination behind that low number. However, I think it is also important to remember that the number of female laureates, while small, is still not zero.

Academic departments in the life sciences in the U.S. are in an interesting state in which women are well represented, sometimes even being the majority, at the undergraduate, graduate, postdoctoral, and even junior faculty levels. But the proportion of women steadily declines as one climbs further up the ladder, which can be attested anecdotally by many women in the life sciences and by research into the issue. As a female biologist in the lower ranks of academia, where women are no longer a minority, it is easy to think of the disadvantages people like me will face as we march steadily towards the gender gap at the top of our fields. How mentors of our gender are harder to find. How advancing in our careers may involve breaking into an “old boys’ club.” How we may have to decide, willingly or otherwise, to choose between career and family. I think that these issues and other related ones are important and should be addressed. However, I think it should not be forgotten in the midst of these efforts that many women have been and will continue to be successful in the life sciences. I believe that if we can remember these women and celebrate their accomplishments, the problems listed above, while still important, will seem less intimidating.

That is why I created this blog. I wanted to create a space for myself and other biologists, both men and women, can recognize and celebrate the women who have been successful in our field and to learn about their research. I hope this space can highlight how far women in biology have come and continue to go, despite how much farther they have left to travel. I will also share my perspective on current events and issues that affect women in the life sciences. Every once in a while, I will also include articles on different topics in biology that I find fascinating, just for fun. It is my wish that anyone and everyone can walk away from this site having learned something valuable. Enjoy!