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Hit cancer where it hurts
03 July 2004
From New Scientist Print Edition.
Garry Hamilton

It seemed like the perfect target. An enzyme that gives cancer cells their everlasting ability to divide, but which is missing from most normal cells. Could this be the one weak spot common to all cancers, the route to a universal cure with no serious side effects?

Discovered in the mid-1980s, this enzyme, called telomerase, generated a great deal of excitement among cancer researchers. Telomerase is largely missing from healthy cells, but 90 per cent of cancers churn it out by the bucketload. It gives them the ability to keep dividing long after they should have aged and died. Telomerase's discovery raised the hope that it could somehow be used to single out cancer cells from normal cells, and perhaps even strip them of their deadly immortality.

In the early 1990s, telomerase was the focus of great optimism. But several major setbacks have been enough to scare away drug companies and convince some researchers that telomerase as a cancer therapy was a dead end. One of the biggest blows was the discovery that it takes months for a cancer cell to stop dividing after telomerase activity has ceased. For most cancer patients, that is simply too slow.

But now comes a new twist. Researchers delving deeper into basic telomerase biology have uncovered evidence that this enzyme's role in cancer is a great deal more complex - and possibly even more critical - than originally thought. At the same time, there is growing excitement over recent experiments that have uncovered new and more efficient ways of killing cancer cells by exploiting telomerase. One approach, an anti-cancer vaccine that trains immune cells to zero in on cells making telomerase, already looks promising in clinical trials on cancer patients.

As a result, some researchers are now quietly optimistic that telomerase may yet emerge as a powerful tool in the fight against cancer. "As our understanding of telomerase has become more sophisticated, its attractiveness as a target has not gone away," says Elizabeth Blackburn, co-discoverer of telomerase. "It's gotten stronger."

Telomerase's basic appeal is the unique role it plays in maintaining telomeres, the molecular caps found at the ends of chromosomes. Apart from cancer cells, most cells don't make telomerase and don't repair their telomeres.

Telomeres consist of numerous proteins clustered around a single-stranded DNA backbone built from a six-nucleotide sequence repeated thousands of times. They perform at least two vital functions. One is hiding the ends of the chromosomes from the cell's damage detection machinery. Normally, cells are highly efficient at seeking out broken fragments of DNA and either repairing them or forcing the damaged cell to commit suicide, a process known as apoptosis. The other function is related to the fact that each time a cell copies its chromosomes, fragments of DNA at the ends are lost. Telomeres thus act as sacrificial caps at the ends of chromosomes that gradually erode away while keeping the chromosome's more valuable DNA out of harm's way.

You can think of telomeres as a fuse that limits the number of times a cell can divide. When they have been whittled down to a critical length, typically 20 to 30 divisions, the cell stops dividing and eventually self-destructs. The shortening of telomeres, therefore, probably performs an important role in preventing cancer. As cells age, they accumulate genetic errors, or mutations, that could turn them cancerous. Rather than risk cancer, it is better for cells to have a built-in expiry date.

Some normal cells, most notably stem cells, escape this fate, which implies there must be a way of circumventing the telomere clock. In 1985 Carol Greider, then a graduate student in Blackburn's lab at the University of California, discovered an enzyme whose job appeared to be preventing telomeres from shrinking. That enzyme was telomerase. Normally, only a small fraction of body cells, including stem cells, have telomerase and thus the ability to repair their telomeres. Most cells in the body do not, unless they turn cancerous.

Telomerase is unlike any other known enzyme. For one thing it is more complex, consisting of several different components. It contains a stretch of RNA that spells out the six-letter code of the repeat sequences of the telomere's DNA. A catalytic protein uses this RNA "template" to make the pieces of DNA used to rebuild telomeres. Intriguingly, this protein is similar to the reverse transcriptases that retroviruses such as HIV use to make copies of themselves inside cells. Together it is a highly efficient, one-molecule repair crew for tending telomeres whenever DNA replication takes its toll. In healthy cells, it strikes a delicate balance, making sure the telomeres are just the right length. "They don't get too long. They don't get too short," marvels Blackburn. "They really have a lot of checks and balances."

The discovery that telomerase plays a vital role in cancer came at a time when hopes of ever finding a universal target were fading. One of the main reasons cancer is so hard to cure is its reliance on multiple mutations to escape the many anti-cancer mechanisms built into a cell's normal circuitry. By altering several of these circuits, a cancer cell is more likely to survive if one of them becomes disabled. The nature of the mutations varies between different types of cancer and even between tumours within a single kind of cancer. This is why tumours often develop resistance to anti-cancer therapies, and why some treatments failed to live up to their initial promise (see "Not so simple"). Switching on telomerase emerged as the first true bottleneck that a large majority of cells must pass through to become malignant.

The unique structure of telomerase made it an appealing target for anti-cancer treatment. Earlier success targeting reverse transcriptases - notably in HIV - hinted that telomerase could be inhibited using "small molecule" drugs. They do not usually get digested in the gut, which means they can be taken as a pill. They are also relatively easy and quick to make. "There are a lot of interesting genes or proteins that contribute to many diseases," says Murray Robinson, a cancer researcher at Genpath Pharmaceuticals in Cambridge, Massachusetts. "But only a small subset of those are really approachable with small molecules."

Robinson was on the front line of telomerase research during the late 1990s at the drug company Amgen in Thousand Oaks, California. He set up the company's cancer research programme in 1997 with telomerase as its central component. "What we had was an essential gene that's tumour specific," he says. "It was a very exciting target."

Then came a string of setbacks. One was the failure, despite massive efforts, to find a small molecule that could inhibit telomerase efficiently. And even when Robinson succeeded in blocking telomerase with more elaborate molecules injected directly into cultured cells, cell division carried on for a long time until the telomeres had worn down. Telomerase also had to be continually inhibited to get this result. That would mean patients having to take large amounts of a drug at high levels for weeks if not months. Discouraged, Robinson recently decided to throw in the towel on telomerase. "I think it's going to be a very difficult target to approach," he now says.

Despite all this, other approaches to exploiting telomerase, for example targeting the enzyme with immunotherapy, are beginning to show unexpected promise. For years scientists have been trying to find a way of persuading a patient's own immune system to fight cancer. One goal has been to find a protein that is specific to cancer cells, and then train immune cells to go after it. In the past cancer vaccines have failed because cancer cells with their multiple circuits can easily evade the immune system by ditching whatever molecule happens to be under attack.

Precision strike

This shouldn't happen with telomerase. After all, if cancer cells rid themselves of telomerase, they will cease to be immortal and will die. But it is possible that a vaccine designed to rally immune cells against telomerase would trigger autoimmunity, where the immune system attacks healthy tissue. In lab tests, however, such immune cells target cancer cells but not stem cells, although it is not exactly clear why. Nor have there been any signs of autoimmunity after injecting anti-telomerase immune cells into lab mice with cancer.

The big test is how the human body will respond to such treatment. In February scientists at the University of Pennsylvania's Abramson Cancer Center in Philadelphia did a preliminary study with seven patients. First they extracted a patient's dendritic cells. These cells coordinate the deadly efforts of killer T-cells, which hunt down and kill infected or abnormal cells. Researchers then primed dendritic cells by injecting them with fragments of telomerase and replaced them in the patient. All seven patients showed a clear immune response to telomerase without serious side effects. One patient's tumours even started to shrink. "So far we have seen no toxicity," says Robert Vonderheide, a medical oncologist who is leading a larger safety trial of the vaccine.

Clinical results from a similar vaccine being developed by biotech company Geron of Menlo Park, California, and scientists at Duke University in Durham, North Carolina, were released in June when the American Society of Clinical Oncology met in New Orleans. According to Geron CEO Thomas Okarma, prostate cancer patients in the trial have demonstrated dramatic immune responses to the vaccine. Patient blood tests have also revealed a drop in both the number of circulating cancer cells and the level of prostate-specific antigen (PSA), a protein used to identify the presence of prostate cancer. "What we're seeing in the group that gets six injections a week is without precedent in the field of cancer immunotherapy," says Okarma, adding that no serious side effects have been seen so far. "We've studied 20 patients, some for as long as 24 months, and there isn't a single adverse reaction."

The next step is to maximise the vaccine's potency and develop a more commercially viable immunisation process. Ultimately, Geron hopes the vaccine will target metastasis, the deadliest and most difficult-to-treat stage of cancer when cells break off from a primary tumour and invade other tissues in the body. Such a vaccine may be able to train immune cells to root out single cancer cells wherever they may lurk. What is more, those involved are confident it will be useful against most cancers.

And it may not be the only telomerase-based treatment available. For one thing there is mounting evidence that worries over the time lag associated with telomere shortening may have been at least partly exaggerated. In the past researchers focused on the average length of telomeres in a cell. Now evidence suggests it takes only one telomere to become critically short for the cell to press its self-destruct button. What is more, cancer cell telomeres are much shorter than those found in normal cells, and also vary more in length, with some being longer and some being perilously short. "This means that even though the tumour cell is making lots of telomerase," says Okarma, "it's actually dangerously close to apoptosis all the time."

Partly because of this researchers have not given up on finding a cancer drug that works by inhibiting telomerase. Indeed, more sophisticated telomerase inhibitors can kill cancer cells in less time. And although such drugs are more complex and so tougher to develop, Geron is close to clinical trials on its version, which Okarma says could have an even bigger impact than the vaccine. "It's not dependent on the immune responsiveness of the patient," he says. "It's a direct hit at the heart of the cancer cell."

Other research, meanwhile, suggests it may also be possible to kill cancer cells by attacking the structure of telomeres. Blackburn suspects telomerase may also help telomeres maintain their proper structure, especially as they become shorter and increasingly unstable. This shoring up may be vital since it seems that just one abnormal telomere is enough to launch the entire cell on the road to apoptosis.

This discovery has opened up a whole new territory for potential treatments. At the School of Pharmacy, University of London, for example, chemical biologist Stephen Neidle has demonstrated that cancer cells grown in the lab can be killed within days using small molecules that alter how the tips of the telomeres are folded. The molecules have no effect on normal cells, possibly because telomeres are folded differently in the absence of telomerase.

The molecules, which are being developed as a drug in partnership with London-based Antisoma, have also shown promise against human tumours grafted into mice.

In Blackburn's lab researchers are taking a different approach. They have created a mutant telomerase gene, one that carries an altered sequence in the RNA template used for generating fresh telomere tips. Insert the altered gene into a cell and the mutant telomerase enzyme adds the wrong DNA sequence to telomeres, which makes the cell stop growing and self-destruct. Blackburn suspects telomeres carrying the incorrect sequence cannot fold properly, and that this triggers apoptosis even though the chromosome tips have not worn away.

So far the technique has been tried successfully on human breast and prostate cancer cells. Even low levels of mutant telomerase were enough to work as well as chemotherapy drugs such as tamoxifen or mifepristone - evidence, says Blackburn, that a cell is highly sensitive to changes in just one or a few of its telomeres. When cells containing the mutant gene were grafted into mice, tumours grew at a dramatically reduced rate for a month before beginning to shrink.

No one yet knows if these treatments will work, or be safe, in people. Some, like those involving the insertion of genes, still face enormous technological hurdles before they can be considered viable. And it is possible that tampering with the enzyme will have unforeseen side effects.

Still, for those committed to telomerase like Blackburn, the realisation that the perfect target is not easy to hit does not diminish its appeal. "Initially there was this excitement, which was a bit of a fantasy," she says. "Now we're in the real world."

From issue 2454 of New Scientist magazine, 03 July 2004, page 40

Not so simple Several anti-cancer strategies have been hailed as "cure-alls" in the past, and initially failed to live up to the hype. But as researchers have discovered more about the subtleties of cancer biology, they have become valuable tools in the fight against the disease. MONOCLONAL ANTIBODIES These proteins, produced by white blood cells, are designed to seek out and stick to specific proteins on cancer cells. They can be used to draw fire from immune cells onto tumours, block key tumour proteins, or can be engineered to carry drugs to kill cancer cells. Hailed as the ideal targeted therapy in the 1980s, they soon ran into problems. The technology to produce the antibodies was originally developed in mice, but patients developed an immune response to antibodies made this way. Today, human antibody technology has been developed, and a handful of therapies licensed for use. Antibody therapy produces fewer side effects than conventional treatments and can be effective, but tumours often become resistant. INTERFERON Interferon is a protein produced by the immune system during infection. It caused a stir in the 1970s when researchers found it could shrink cancers in some patients. However, not all tumours respond to interferon and high doses of the drug cause unpleasant flu-like symptoms. Today, interferon has a valuable role to play in some therapies, notably against lymphomas and leukaemias. ANGIOGENESIS INHIBITORS These drugs "starve" tumours of a blood supply by stopping them from growing new blood vessels. There was great excitement 15 years ago when tumours in mice treated with these drugs disappeared. However, the drugs failed to shrink tumours in human patients. Today, angiogenesis inhibitors are firmly back on the map as they have shown promise when used in combination with other therapies.

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