Lifespan. David Sinclair
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few months—work that had its roots in the workable genetic map that Mortimer had completed in the mid-1970s.
That was it. I had a passion for understanding aging, and I knew something about wrangling a yeast cell with a microscope and micromanipulator. Those were essential skills needed to figure out why yeast age. That night, Guarente and I agreed on one thing: if we couldn’t solve the problem of aging in yeast, we had no chance in humans.
I didn’t just want to work with him. I had to work with him.
Dawes wrote him to tell him that I was keen to join his lab and I was “skilled at the bench.”
“It would be a pleasure to work with David,” he replied a few weeks later, the same way he probably did to so many other enthusiastic applicants. “But he’s got to come with his own funding.” Later I learned he had been excited only because he’d thought I was the other student he’d met at dinner.
I had a foot in the door, but my chances were slim. At the time, foreigners weren’t considered for prestigious postdoctoral awards in the United States, but I insisted I be interviewed and paid for a flight to Boston myself. I was interviewed by a giant in the stem cell field, Douglas Melton, for a Helen Hay Whitney Foundation Fellowship, which has been providing research support to postdoctoral biomedical students since 1947. After waiting in line outside his office with the other four candidates, I had my chance. This was my moment. I don’t remember being nervous. I figured I probably wouldn’t get the award anyway. So I went for it.
I told Melton about my lifelong quest to understand aging and find “life-giving genes,” then sketched out on his whiteboard how the genes work and what I’d be doing for the next three years if I got the money. To show my gratitude, I gave him a bottle of red wine that I’d brought from Australia.
Afterward, two things became clear. One, don’t bring wine to an interview because it can be seen as a bribe. And two, Melton must have liked what I said and how I said it, because I flew home, got the fellowship, and then got onto a plane back to Boston. It was, without a doubt, the most life-changing meeting of my life.52
At the time of my arrival, in 1995, I had expected to build our understanding of aging by studying Werner syndrome, a terrible disease that occurs in less than 1 in 100,000 live births, with symptoms that include a loss of body strength, wrinkles, gray hair, hair loss, cataracts, osteoporosis, heart problems, and many other telltale signs of aging—not among folks in their 70s and 80s but rather among people in their 30s and 40s. Life expectancy for someone with Werner is 46 years.
Within two weeks of my arrival in the United States, though, a research team at the University of Washington, headed by the wise and supportive grandfather of aging research, George Martin, announced that they had found the gene that, when mutated, causes Werner syndrome.53 It was deflating at the time to have been “scooped,” but the discovery allowed me to take a bigger first step toward my ultimate objective. Indeed, it became the key to formulating the Information Theory of Aging.
Now that the Werner gene, known as WRN, had been identified in humans, the next step was to test if the similar gene in yeast had the same function. If so, we could use yeast to more rapidly determine the cause of Werner syndrome and perhaps help us better understand aging in general. I marched into Guarente’s office to tell him I was now studying Werner’s syndrome in yeast and that’s how we would solve aging.
In yeast, the equivalent of the WRN gene is Slow Growth Suppressor 1, or SGS1. The gene was already suspected to code for a type of enzyme called a DNA helicase that untangles tangled strands of DNA before they break. Helicases are especially important in repetitive DNA sequences that are inherently prone to tangling and breaking. Functionality of proteins, such as the ones coded for by the Werner gene, is therefore vital, since more than half of our genome is, in fact, repetitive.
Through a gene-swapping process in which cells are tricked into picking up extra pieces of DNA, we swapped out the functional SGS1 gene with a mutant version. In effect, we were testing to see if it was possible to give the yeast Werner syndrome.
After the swap, the yeast cells’ lifespan was cut in half. Ordinarily, this would not have been news. Many events unrelated to aging—such as being eaten by a mite, drying out on a grape, or being placed in an oven—can and do shorten the lifespan of yeast cells. And here we’d messed with their DNA, which could have short-circuited the cells in a thousand different ways to cause early death.
But those cells weren’t just dying. They were dying after a precipitous decline in health and function. As the SGS1 mutants became older, they slowed down in their cell cycle. They grew larger. Both male and female “mating-type” genes (descendants of gene A) were switched on at the same time, so they were sterile and couldn’t mate. These were all known hallmarks of aging in yeast. And it was happening more quickly in the mutants we’d made. It certainly looked like a yeast version of Werner’s.
Using specialized stains, we colored the DNA blue and used red for the nucleolus, which sits inside the nucleus of all eukaryotic cells. That made it easier to see under the microscope what was happening at a cellular level.
And what was happening was fascinating.
The nucleolus is a part of the nucleus in which ribosomal DNA, or rDNA, resides. rDNA is copied into ribosomal RNA, which is used by ribosome enzymes to stitch amino acids together to make every new protein.
In the aged SGS1 cells, the nucleolus looked as if it had exploded. Instead of a single red crescent swimming in a blue ocean, the nucleolus was scattered into half a dozen small islands. It was tragic and beautiful. The picture, which would later appear in the August 1997 issue of the prestigious journal Science, still hangs in my office.
What happened next was both enchanting and illuminating. In response to the damage, like rats to the call of the Pied Piper, the protein called Sir2—the first known sirtuin, which is encoded by the gene SIR254 and descended from gene B—had moved away from the mating genes that control fertility and into the nucleolus.
That was a beautiful sight to me, but it was a problem for the yeast. Sir2 has an important job: it is an epigenetic factor, an enzyme that sits on genes, bundles up the DNA, and keeps them silent. At the molecular level, Sir2 achieves this via its enzymatic activity, making sure that chemicals called acetyls don’t accumulate on the histones and loosen the DNA packaging.
When sirtuins left the mating genes—the ones descended from gene A that controlled fertility and reproduction—the mutant cells turned on both male and female genes, causing them to lose their sexual identity, just as in normal old cells, but much earlier.
I didn’t understand at first why the nucleolus was exploding, let alone why the sirtuins were moving toward it as the cells grew older. I agonized over the question for weeks.
And then one night, after a long day in the lab, I woke up with an idea.
It came in the space between sleep-deprived delirium and deep dreaming. The wisps of a concept. A few words jumbled together. A muddled picture of something. That was enough, though, to jolt me awake and pull me from my bed.
I grabbed my notebook and went to the kitchen. There, hunched over the table in the early morning hours of October 28, 1996, I began to write.
I wrote for about an hour, jotting down ideas, drawing pictures, sketching out graphs, formulating new equations.55 Scientific observations that had previously made no sense to me were falling perfectly into a larger picture. Broken DNA causes genome instability, I wrote, which distracts the Sir2 protein, which changes the epigenome, causing the cells to lose their identity and become sterile while they fixed the damage. Those were the analog scratches on the digital DVDs. Epigenetic changes cause aging.
There was, I imagined, a singular process that controlled them all. Not a countless
For four years after that, I sent Professor Melton a bottle of red wine for New Year’s, just to say thanks for changing my life. He never acknowledged any of them or ever smiled at me, either because he didn’t think that’s what an awardee should do or because he’s a very private person. At least he knew I was grateful. The selection of red wine turned out to be ironic, as that foodstuff helped propel my career a second time nine years later.
C. E. Yu, J. Oshima, Y. H. Fu, et al., “Positional Cloning of the Werner’s Syndrome Gene,”
In a paper published in late 1997, I showed how ERCs—rDNA circles—cause aging and shorten the life of yeast cells. D. A. Sinclair and L. Guarente, “Extrachromosomal rDNA Circles—A Cause of Aging in Yeast,”