Albert Lasker
Basic Medical Research Award

The discovery of telomerase centers around four principal characters—a single-cell protozoan that swims in freshwater ponds, propelled by hairlike projections called cilia; a Tasmanian-born devourer of Amanda Cross mystery novels; a triathlete and competitive vaulter who performs handstands on the back of a speeding horse; and a 53-year-old virgin prizewinner.

So what is telomerase and why is it Lasker Prize-worthy? As you all know, life depends on the genetic instructions carried in the DNA of the genome.

In the human genome, our 25,000 genes are distributed over 23 different pairs of chromosomes. Each chromosome consists of a single, enormously long DNA molecule that encodes a linear array of many genes. The tips of each chromosome terminate in a specialized structure called the telomere. Telomeres act like protective shields in much the same way as plastic tips safeguard the ends of shoelaces; they stop chromosomes from fraying. The crucial role of telomeres in maintaining the integrity of chromosomes was first recognized in the early 1940s by two famous Nobel Prize-winning geneticists, Herman J. Muller and Barbara McClintock. Studying different organisms under the microscope, Muller with fruit flies and McClintock with corn observed that when chromosomes lack telomeres they stick to one another, producing catastrophic fusions and other abnormal structural changes that threaten cell replication and survival. Muller coined the term "telomere" from the Greek word for "end" (telos). His choice of telos may have been more apt than he intended; after his and McClintock's work in the early 1940s, all research on telomeres came to an abrupt end and did not resume for another 30 years.

Then, in 1972, James Watson of Double Helix fame and Alexey Olovnikov, a young Russian scientist with no fame, independently made an important theoretical advance that reignited interest in telomeres. Watson and Olovnikov noted that the polymerase enzymes that normally replicate DNA every time a cell divides are not able to copy the chromosomes all the way to the very tip, leaving a small region at the end of each telomere uncopied, much like a tape recorder cannot play the last few centimeters of tape in a cassette. If cells had no way of compensating for this quirk in the replication machinery, chromosomes would shorten with each round of cell division, and ultimately there would be an erosion of telomeric DNA such that the telomeres would no longer perform their protective function. Watson's and Olovnikov's theory, which became known as the "end-replication problem," predicted that cells possess a special strategy to maintain their telomeres during normal DNA replication.

And that special strategy turned out to be the enzyme telomerase. In 1975, Elizabeth Blackburn, a native of Tasmania and a newly minted Ph.D. from Fred Sanger's laboratory at the MRC in Cambridge, crossed the Atlantic to begin a postdoctoral fellowship with Joseph Gall, then at Yale University. Gall was—and still is—a renowned cell biologist whose career you'll hear about in a few minutes when he receives the Lasker Special Achievement Award. As an avid reader of Amanda Cross mystery novels, Blackburn was anxious to solve a mystery of her own.

She came to Gall's lab to work on an organism with highly unusual chromosomes—a ciliated protozoan called Tetrahymena. Unlike typical eukaryotic organisms, Tetrahymena has two nuclei—a micronucleus that contains normal chromosomes and a macronucleus whose chromosomes are fragmented into thousands of small pieces of DNA that all encode the same ribosomal RNA gene. Because of the high abundance of this one extrachromosomal gene, Blackburn and Gall were able to purify the molecule and determine the DNA sequence at its tips. The sequence turned out to be unique—a short simple sequence of six nucleotides, TTGGGG, which was repeated over and over 50 times at both ends of the molecule. This type of tandem repeat had never been seen before, and its discovery immediately raised the question as to whether it was a peculiar property of an unusual extrachromosomal gene in a ciliated organism far removed from the main line of eukaryotic evolution—or was it an authentic telomere sequence that would have universal relevance to the chromosomes of humans and other higher organisms? Blackburn no longer had time for Amanda Cross mysteries; she her own mystery to solve.

The first clue came in 1982 when Blackburn began collaborating with Jack Szostak, a yeast geneticist and molecular biologist who was just starting his own lab at Harvard Medical School. Blackburn, too, was just starting her own lab at the University of California in Berkeley. At that point in time—1982—cloning of recombinant DNA molecules was done in bacteria with circular plasmids that held only about one or two gene's worth of DNA. Szostak had started a bold project aimed to construct artificial chromosomes that would enable scientists to clone a large cluster of human genes—10 to 20—on a single linear molecule of DNA in yeast cells. The first versions of Szostak's artificial chromosomes were unstable and did not propagate in the yeast cells, presumably because they lacked telomeres.

When Szostak heard about Blackburn's Tetrahymena repeat sequences, he proposed testing whether they would function as telomeres in his yeast system. This was a stunningly novel experiment, and the results were spectacular. The linear yeast plasmids containing the Tetrahymena repeat ends replicated in stable fashion. Szostak and Blackburn had now developed the first functional assay for telomeres. This experiment also formed the basis for Szostak's later work on the creation of the first yeast artificial chromosomes—the famous YACs that played a key role in the Human Genome Project by enabling scientists to clone long pieces of human DNA that were used for sequencing.

When Blackburn and Szostak sequenced their newly manufactured yeast plasmid containing the Tetrahymena repeat sequence, the results were totally unexpected: The replicated plasmid was longer than it should have been; the yeast cells had actually added many copies of a new type of repeat sequence onto the end of the Tetrahymena repeat sequence. This discovery led Szostak and Blackburn to predict that the yeast cells contained a telomere-synthesizing enzyme—soon to be called telomerase. The hunt for the telomerase had now begun. Blackburn took a biochemical approach, and Szostak a genetic approach.

Blackburn found the biochemical search to be exceptionally difficult. She began with crude extracts of nuclei that contained many other enzymatic activities unrelated to telomerase, including nucleases that degrade any newly made nucleic acid. She also had to worry that any telomerase activity she saw was an aberrant result caused by one of the DNA polymerases that replicate the middle of the chromosomes. It was like looking for a needle in a haystack. After six months of looking, Blackburn never found the needle, but instead she turned up the farmer's daughter—Carol Greider.

In 1984, Greider entered graduate school at Berkeley. Trained as an equestrian vaulter, Greider proved to be the white knight in shining biochemical armor who galloped into Blackburn's cold room and four years later emerged with the mythical telomerase in hand—purified and characterized. The key to Greider's success was the development of an innovative assay, not to mention a lot of hard work and ice-cold fingers. The telomerase that Greider and Blackburn had discovered was more interesting than anyone could have imagined.

To make a long story short, telomerase is a multi-subunit enzyme composed of both RNA and protein components. The most unique component—the one that Greider purified and cloned—is an RNA molecule that contains a built-in template that ensures that the catalytic protein component of the enzyme adds the correct repeat sequence to the ends of the chromosome. The catalytic component of telomerase, a reverse transcriptase-like enzyme, was subsequently purified and cloned by Tom Cech and colleagues. Telomerase also contains several regulator proteins essential for its function.

Meanwhile, Jack Szostak was not resting on his laurels at Harvard. Together with a talented postdoctoral fellow, Victoria Lundblad, Szostak had embarked on a clever genetic strategy designed to detect mutants in yeast that would be defective in any essential component of the putative telomerase enzyme and its regulators. Their first mutant, called EST1 for "ever shorter telomeres," had a dramatic phenotype. Telomeric DNA gradually disappeared, chromosomes became shorter with each succeeding cell division, and the cells underwent premature aging as their telomere reserve was depleted. These findings provided the first experimental evidence linking the length of telomeres to the aging of cells—a subject I'll say more about in a moment.

Later work by Lundblad showed that the protein lacking in the EST1 mutant was one of the accessory regulator proteins in the telomerase complex. Another EST mutant, EST2, actually lacked the catalytic component of telomerase—the same protein that Tom Cech had purified, providing a neat genetic validation of the complex biochemistry pioneered by Greider and Blackburn.

The basic discoveries of Blackburn, Szostak, and Greider stimulated hundreds of scientists to enter the field in 1990s, and this explosion of research has led to a new model of how the life span of normal cells is regulated and how this regulation goes astray in cancer. As cells in the body grow old, telomeres progressively shorten because normal cells contain only minute amounts of telomerase. Once the telomeres shorten beyond a critical point, aging cells undergo a crisis, and the vast majority of them die. The few cells that survive are the ones that have activated their telomerase. These surviving cells become immortalized, divide indefinitely, and are now poised to become malignant. More than 85 percent of all human cancers have elevated telomerase that propels their growth. Inhibition of telomerase provides an exciting new drug target for cancer therapy, which many pharmaceutical companies are actively pursuing.

Telomerase strikes at the very core of our longevity and our mortality. Its popularity has inspired thousands of newspaper and magazine articles as well as several books, including Merchants of Immortality by Stephen Hall, an engaging science writer who is here with us today.

I can't resist ending this talk without a telomeric ending that contains a few repeats. Remember at the beginning when I mentioned those four principal characters. I've already told you about the ciliated protozoan in Joe Gall's lab that got the ball rolling, I've told you about Liz Blackburn and her obsession to solve scientific mysteries, and I've told you about Carol Greider and her handstands in the cold room. But I didn't explain the 53 year-old virgin prizewinner. In the past 10 years, telomerase has been honored with many prizes—all of them going to Blackburn alone or Blackburn and Greider. This year's Basic Lasker Award is the first of these telomerase prizes that Jack Szostak shares with Liz Blackburn and Carol Greider.

To be completely honest, Jack is not really a virgin prizewinner. He has received many distinguished prizes for his other research, not involving telomerase. When Jack left the telomerase field 17 years ago, he went on to pioneer several other bold endeavors—the most recent involving an attempt to unravel the origins of life in a test tube by mixing together three elements that existed in the Earth's ancient chemical soup 4 billion years ago. The goal is to get these three primordial elements—RNA, fatty acids, and a special type of clay—to assemble into a so-called "proto-cell" that will replicate genetic material, form cell membranes, and divide into daughter cells—all in a test tube. If Jack is successful, the proto-cell will speak to the origins of life in much the same way that telomerase speaks to our longevity and our mortality.

This discovery of telomerase is a great story in the history of biomedical science. Congratulations to all three of you—Liz, Carol, and Jack.