5 October 2009

Unravelling the secret of ageing

By
COSMOS Magazine
More than 30 years after discovering an enzyme that prevents chromosomes from fraying, Elizabeth Blackburn is still unravelling the mystery of why our cells age.
Elizabeth Blackburn

Australian researcher Elizabeth Blackburn, whose co-discovery of telomeres has won the 2009 Nobel Prize for Physiology or Medicine. Credit: Elizabeth Finkel/COSMOS

Elizabeth Blackburn is not a household name. But the string of illustrious science awards she holds already suggest she is a hot favourite for a Nobel Prize. And that’s exactly what happened – finally in 2009, more than 27 years after her initial research.

My first impression of Blackburn is her naturalness. She’s kindly-looking with soft, even features and unlined pink skin framed by a natural fall of shoulder-length greying hair. She speaks with a velvety English-accented voice, which readily breaks into a chuckle.

But for all the ready chuckles, Blackburn has a commanding presence. One imagines that Leon Kass, chair of former U.S. President George W. Bush’s bioethics committee, got more than he bargained for after persuading her to join in 2001.

In 2004, when her concerns went unheeded, Blackburn publicly criticised two reports bearing her name as being scientifically misleading. She was promptly fired. Though relieved to be off the committee, she wondered at the wisdom of the Bush administration; her sacking served as a beacon of Bush’s war on science. “The phone rang hot for weeks”, she told me.

AS FAR AS FAMILY history, Blackburn was born in Tasmania into a family of physician parents and seven children. This makes her English accent somewhat of a mystery to her; perhaps it was imprinted during the year she spent in the U.K. when she was six, or later in her twenties, in the early 1970s, when she did her PhD at Cambridge in the laboratory of Fred Sanger.

Sanger was developing methods to read the sequence of letters that make up the DNA code, a feat for which he would eventually be acknowledged with a Nobel Prize. At the time Blackburn was just one of several students testing various approaches. Today a project like that would seem awfully boring. But not then. “You can’t imagine; it was reading the code of life.”

Though researchers had learnt to translate those parts of the DNA sequence which coded for proteins, the vast majority of DNA sequence remained indecipherable. Blackburn recalls her thrill at reading a sequence of DNA 48 letters long. Overall there was the reigning belief, that reading the sequence of the letters of DNA would reveal something – perhaps a clue to a new code or a three dimensional structure.

And if a structure could be inferred, then that might predict the function. That was what led to Watson and Crick’s breakthrough two decades prior. Once they figured out the double helix structure of DNA, the mechanism by which life replicated itself became obvious: one strand of the helix provided the template to synthesise the matching strand.

There wasn’t too much Blackburn could predict about the structure of her 48-letter sequence. But she was to get more of a chance in her next project. One of her colleagues in Sanger’s lab was an American, John Sedat. Romance blossomed and Blackburn decided to pursue her post-doctoral research in the U.S., both to be with John and because she had a sense that was where career opportunities lay for women scientists. “By then, John and I were an item; love and science luckily coincided.”

At Yale University, Blackburn turned her interest to the DNA sequence at the tips of chromosomes. DNA, like any length of thread, needs to be spooled to keep it from getting unruly. Cells spool it around a wad of proteins to form the structure known as the chromosome. In the 1930s geneticists had wondered how the tips of chromosomes avoided fusing with each other, because when chromosomes accidentally broke, their edges fused willy-nilly.

And there was another mystery. The enzyme that copies a DNA strand had been discovered in the 1960s. But this nano-machine had a glaring defect. It could not make a complete copy – the very tips of the chromosome missed out. Every time a cell divided, its newly spun DNA copy would grow shorter and shorter. So with all the division that gives rise to a trillion-cell creature like us, what happens to the DNA? It was known as the ‘end replication problem’.

Blackburn launched into sequencing the tips of the chromosomes of a single-celled pond swimmer called Tetrahymena. And there at the ends of the chromosomes she did find something intriguing. The DNA code had turned into a stutter. A six-letter motif was repeated over and over again, on average about 50 times. It was 1977 and Blackburn had found a doozy of a structure. But what was she to divine of its function?

UNBEKNOWNST TO BLACKBURN, Moscow biochemist Alexey Olovnikov, had an answer for her. He had formulated his answer not in response to her findings, but to those of Leonard Hayflick. In 1961 Leonard Hayflick, working at the Wistar Institute in Philadelphia, was culturing cells from aborted human foetuses to use them as pristine incubators for making polio vaccine.

Hayflick noticed that from the time the cells were extracted, they divided 50 times and then stopped – like clockwork. Prior to Hayflick, it was assumed cells stopped growing in the culture dish because they sickened. But Hayflick reached a different conclusion.

He theorised that the cells weren’t sick; they were senescing. In other words, growing old right there in the dish. It seemed to Hayflick that just as the sands of the hourglass count the days of a human life, there was something in the cell counting its lifespan. Olovnikov had thought of an answer to what that clock might be.

He knew that chromosomes might have trouble replicating their ends. He theorised the existence of terminal buffer sequences that he dubbed ‘telogenes’. One of these telogenes would be sacrificed with each cell division. But once 50 cell divisions had taken place (enough to make over a trillion cells), the telogenes would run out and any further divisions would eat into the crucial genetic payload.

As he wrote in a 1973 issue of the Journal of Theoretical Biology: that would result in the “various disorders of age of the ageing of multicellular organisms.” Olovnikov dubbed his idea the theory of ‘marginotomy’. Like a slow burning fuse, the lifespan of a cell was measured by the eroding telogenes at the ends of the chromosomes. It seemed the ‘telogenes’ of Olovnikov’s theory had been found by Blackburn.

In 1977, when Blackburn first reported her telomere structures, she was unaware of Olovnikov’s theory. Even so, it is unlikely she would have jumped to the same conclusions as Olovnikov. For all her notion of inferring function from structure, Blackburn is really not all that inclined to grand theories. Her musings were more mechanistic. She wondered how the ‘stutters’ were generated.

Her hunch was that it was the work of a new type of enzyme, an enzyme that knew how to finish off the ends of the DNA thread. Soon she would have a chance to test that hunch, at her own laboratory at Berkeley.

Looking back from her current height, one might imagine it was all plain sailing, but Blackburn confesses the stresses of securing this first crucial position were huge. In fact, when at one point she thought she was pregnant, she was ready to escape the oppressively competitive and uncertain world of research. But she wasn’t pregnant, the Berkeley job came up, and the rest is history. (Some nine years later in 1986, Blackburn and Sedat’s son Benjamin was born).

Her preliminary experiments at Berkeley suggested that her hunch about an enzyme might be correct. Soon she was joined by a daring young student, Carol Greider, who tenaciously tracked this enzyme down several blind alleyways until she and Blackburn found a way to finally snare it. The new enzyme could indeed add more stutters to the tips of DNA. And it was a very novel kind of enzyme indeed. Most enzymes are nano-machines made from proteins. But one of the components of this nano-machine was made from a cousin of DNA – RNA.

The extraordinary enzyme was dubbed ‘telomerase’. And after Greider and Blackburn reported its existence in 1985, there was no stopping the grand theories. One of the most fervent proselytisers was Michael West, the man who started the world’s most high profile anti-ageing company, Geron.

The vision outlined by West and others was a modern-day revival of the fountain of youth. It went something like this: cells aged because their telomeres ran down like a burning fuse. But telomerase was an enzyme that restored those tips. And if telomerase restored a cell’s lifespan, then it might hold back the ravages of ageing.

It was a story used to great effect to raise money from investors in order to back Geron’s quest to clone the human gene for telomerase. Once the gene was cloned, telomerase might be produced in limitless quantities. And any company that could get its hands on intellectual property for the ‘fountain of youth’ would be set for life, as it were.

There was also an equally propitious flip side. Cancer cells had found a way to live for ever. It turns out 90 per cent of cancer cells produce telomerase. So if a way could be found to turn telomerase off, that could be used to put the brakes on cancer cells.

It wasn’t just Geron that was excited about the telomerase hypothesis – it gripped scientists and lay people alike. “The hype got mixed up with the science”, says Blackburn. And who could blame them? It was such a great story. Blackburn recalled a French film producer for whom the telomerase story was profoundly resonant. “He saw it as the metaphor of the life-candle burning down.”

THE MAJOR INCONVENIENT fact was this: though cells in a culture dish may run out of oomph, people don’t age and die because their cells stop proliferating. Skin cells from a centenarian will still proliferate. And for organs such as the brain or pancreas, most of these cells don’t proliferate whether the person is young or old.

Still, many of Blackburn’s colleagues went along for the telomere/ageing ride. Some became Geron employees; others joined the advisory board. Blackburn stayed clear, both because she didn’t want her academic freedom compromised, and because she felt it was far too soon to be joining the dots.

“I saw from my colleagues who were funded by companies, the pressure they were under to get certain results. I didn’t want to go there. The right stage to get involved [with a company] is when you have a well-defined product. It struck me that we were so far from that.”

Instead, she continued amassing data – about the structure of telomeres, how they protect the chromosome and the mechanics of how the telomerase enzyme operated. As far as the relationship between telomeres and human ageing, Blackburn steered clear.

But Blackburn was not going to be able to keep her distance. In 2001 some dramatic studies saw her “dragged kicking and screaming into ageing research”.

What happened in 2001 was that some inadvertent human guinea pigs appeared on the scene: three families with a genetic defect that meant they produced half the normal level of telomerase.

These patients provided an opportunity to test the telomerase hypothesis. If it was correct – that telomere shortening is the cause of ageing – then these patients ought to have aged faster.

In fact these patients did die young: between 16 and 50 years of age. They suffered from the disease known as dyskeratosis congenita, which caused defects in their skin, gut, lung, liver, testis and bone marrow. They usually died of bone marrow failure – the inability to produce enough blood cells to fight infections.

The revelation came as a surprise to many. Prior to this, the view had been that most cells did not require telomerase: immortality was only needed for germ cells (eggs and sperm), stem cells and cancer cells. “It showed us how little we knew; 50 per cent telomerase is bad for you.”

The other surprise was that these patients were also cancer-prone. That flew in the face of the expectation that reducing telomerase would reduce the incidence of cancer. However, Blackburn was not surprised. The findings actually supported her line of thinking. Cancer is often caused when chromosomes are unstable; they start shuffling their contents around in a genetic lottery that produces an unhinged cell. Long telomeres keep the chromosomes stable, so telomerase actually helped normal cells resist cancer, rather than causing it.

Soon more papers appeared, showing a remarkable link between telomere lengths and human health and longevity. In 2004, Richard Cawthon at the University of Utah used blood from a blood bank to compare the length of telomeres with the life histories of the blood donors. When he sorted the data into long telomeres versus short telomeres, the results were striking. People with the shorter telomeres ended up with shorter lifespans and a higher risk of death from cardiovascular disease and infection.

Then Blackburn got involved in a really way-out study – looking at the effects of life stress on telomeres. It’s common wisdom that life stress ages a person – think of stories of people going grey overnight after trauma.

Less anecdotally, stress is a key risk factor for heart disease – an age-related disease. Researchers had noticed that cells exposed to stress in the culture dish shortened their telomeres. Elissa Epel, a psychiatrist at the University of California in San Francisco, wondered whether the same thing happened in stressed people.

To find out she teamed up with Blackburn and Cawthon. The stressed people in this case were mothers of sick children. The researchers took some of their blood and measured both the length of the telomeres and the telomerase activity. “We were stunned by the results,” says Blackburn. They showed that the more years of chronic stress, the shorter the telomeres. Comparing the most to the least stressed mothers, their telomere shortening corresponded to about ten years of accelerated ageing!

“It was chronic stress, not the sort that you need to win marathons; that’s the good stress”, Blackburn adds with a clearly relieved note to her voice. That first study appeared in 2004. A couple of years later the collaborators published another study on these women. They found that low levels of telomerase corresponded to an elevated heart rate and high levels of blood lipids and fasting glucose. This cluster of factors is known as metabolic syndrome and is often a harbinger of brewing heart disease. “What I loved about this experiment is that I totally thought nothing would come out of it. Even when we controlled for every variable like age, smoking and body mass index, we couldn’t make it [the link between low telomerase levels and CV disease risk] go away.”

The latest reports in medical journals corroborate these findings: blood cells and arterial cells of people with cardiovascular disease have short telomeres. The theory is that at the cellular level, stress unleashes potent chemicals known as free radicals, which damage telomeres. Once damaged, cells that repair the lining of blood vessels for instance, can’t do their job properly. The result is narrowed, hardened arteries.

SO ARE TELOMERES really barometers of ageing and stress? Blackburn is excited about the connection, but she’s still averse to drawing the big picture.

“Is what we see in the culture dish related to what happens in ageing? I would say we don’t know. Before there was a black and white answer. Now it’s shades of grey.”

But Blackburn, for one, is taking heed of her recent findings. One of the ongoing studies of the stressed mothers is to see if meditation will raise their telomeres levels and restore their telomeres. Blackburn has learnt the technique. Ever the scientist she says, “I needed to familiarise myself with the methodology.”

Elizabeth Finkel is a Melbourne-based science writer, a contributing editor of COSMOS and the author of Stem Cells: Controversy on the Frontiers of Science.
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