Interview with Carolyn Bertozzi

‘Cancer cells trick immune cells via sugars on their surface’

Cells communicate with each other through molecules on their surface. Biochemist Carolyn Bertozzi studies exactly how they do this and how to influence this communication. In doing so, she is laying the groundwork for revolutionary cancer treatments and rapid tests for tuberculosis, among other things. She recently received the Dr H.P. Heineken Prize for Biochemistry and Biophysics for her research. In early October, it was announced that she will also receive the Nobel Prize in Chemistry.

At Stanford University’s beautiful campus, full of palm trees, conifers, and countless meticulously maintained lawns, Carolyn Bertozzi is working on something that she herself regularly compares to a varied landscape: the cell surface. This surface is covered with a wide variety of molecules. Some short like freshly cut grass, some long and slow-moving, like trees in the wind. Through these molecules, cells communicate with each other. ‘When cells become part of tissue, for example, they have to organise themselves,’ says Bertozzi. ‘Muscle cells end up in your muscles, neurons in your nervous system, blood cells in your blood. Thanks to the molecules on the cell surface, these cells are in the right place at the right time.’ Sugar molecules play an important role in this. Because there is a huge diversity in the structure of sugar molecules, the pattern of sugars on the cell surface can pass on complex information.

These sugars are also essential for your immune system. ‘Your immune system needs to be able to distinguish which cells belong in your body and which don’t,’ says Bertozzi. ‘The good guys are healthy cells, which your body needs to function. But there are also bad guys – disease-causing bacteria, cancer cells, or cells infected by a virus. By tasting the sugar molecules on the cell surface, so to speak, immune cells can recognise the bad guys, and then make short work of them. They leave the good guys alone.’

But as we see in films and comic books, bad guys always find ways to avoid getting caught. For example, Bertozzi discovered that cancer cells sometimes mask the suspicious sugars on their cell surface by making large amounts of unsuspicious sugar sialic acid appear on their surface. ‘It has been known since the 1960s that the sugar molecules on cancer cells are often very different from those on healthy cells,’ Bertozzi explains. ‘One of those differences is that there is more sialic acid on the cell surface. But no one understood why. A group of receptors – proteins in the cell membrane that can trigger a reaction when a specific molecule binds to them – known as the siglec family was discovered in the late 1980s. These receptors are on immune cells and were found to suppress the activity of those immune cells when they detect sialic acid on other cells. This way, your immune cells make sure they don’t accidentally attack your own cells, which would lead to autoimmune disease. In the early 2000s, we put these two together in my lab: if cancer cells have lots of sialic acid and sialic acid can suppress immune cells, maybe the cancer cell uses that sialic acid to protect itself from our own immune system.’ This turned out to be correct, and Bertozzi has since used this insight to develop a new form of immunotherapy, which is currently being tested in cancer patients. ‘In this treatment, an enzyme mows the sialic acid off the cancer cells like a lawnmower,’ says Bertozzi. ‘The cancer cell can then no longer bind to the siglec receptors on immune cells to suppress the immune system.’

The lawnmower is not the only gardening metaphor Bertozzi enjoys using when she talks about her technologies. ‘We are also currently developing another method of immunotherapy, which works more like a chainsaw: it cuts down the longer molecules. But we don’t just cut molecules from the cell, we can also plant new ones. For example, we created polymers with sialic acid attached. We plant this combination on cells, and thanks to the sialic acid, immune cells leave these cells alone. You can use this in a transplant, for example. In that case you want to make sure that the organ you implant is not attacked by the recipient’s immune system.’

Giving up control
In addition to her work on the cell surface and immunity, Bertozzi was a pioneer in so-called bioorthogonal chemistry, which involves performing chemical reactions in a biological environment, such as a cell or an animal, without affecting that environment. For her work in this field she receives the 2022 Nobel Prize in Chemistry. ‘When we started this in the late 1990s, people said: “What are these people doing?” Chemists were used to being able to control all components of a chemical reaction – the solvent, temperature, pH, etc. But if you want a reaction to take place in a living cell or in a human cancer patient, you have to give up all that control. You can’t heat the cells to 110 degrees Celsius or add a strong acid – you will kill the patient.’

Bertozzi set out to find reactions that could take place under the conditions in human cells without disrupting the regular processes in the cell. ‘We found some reactions that were close, but not perfect,’ says Bertozzi. ‘Then we perfected them until they were completely bioorthogonal and you could perform them in cells and humans. That is how that field of research was born.’ Since then, many more people have entered the field and built on the bioorthogonal reactions developed by Bertozzi and colleagues. Currently, several biotech companies are using these reactions in human patients, for drug delivery and cancer treatment, among other things.

For example, the Shasqi company, for which Bertozzi is a strategic advisor, has developed a cancer treatment that can deliver chemotherapy drugs very precisely into the tumour. This treatment consists of two steps. First, they inject a polymer in the tumour environment. To this polymer, they have added a component that forms one half of a bioorthogonal reaction pair. They then administer a chemotherapy drug. Again, they have added an extra component: the other half of the reaction pair. This component ensures that the drug is not toxic (yet). It spreads throughout the body but does no harm. Once near the tumour, the two halves of the reaction pair click together, so to speak. They engage in a reaction that unblocks the drug. This releases the drug locally in a very high concentration. The drug kills the tumour without exposing the rest of the body to the aggressive drug. This allows you to administer higher concentrations, while the patient experiences far fewer side effects.

Trojan molecule
The original inspiration to start bioorthogonal chemistry came from Bertozzi’s research on sugars on the cell surface. ‘We wanted to image these sugars, in living cells or even in animals,’ says Bertozzi. ‘The sugars on the cell surface are long sugar chains made up of simple building blocks. And these building blocks come from the food you eat.’ So, Bertozzi took one such sugar building block, and tied an extra component to it, which again was half of two molecular clasps that could snap together. She then fed that sugar building block to cells or animals, which incorporated it into the sugar chains on the cell surface. The extra component hitched a ride without the cell noticing. Now all she had to do was add the other half of the molecular clasp, with a fluorescent molecule attached, and the sugar molecules lit up like a lighthouse in the night. ‘This was the first technology to image living cells and the sugars on their surface,’ says Bertozzi.

Recently, this led to an entirely new application: a rapid test for tuberculosis. Tuberculosis (TB) is caused by a bacterium. And like human cells, bacteria have sugars all over their surface. Bertozzi’s team cleverly exploited the fact that the sugar trehalose is common on the surface of tuberculosis bacteria, but absent on human cells. ‘We discovered that we could feed tuberculosis bacteria chemically modified trehalose and they put this all over their surface.’ As in the earlier experiment, they added a clasp to the sugar to which they could later click fluorescent molecules. ‘These fluorescent molecules have a special property,’ says Bertozzi. ‘They are not fluorescent in water, but when they reach the cell membrane, the fluorescence switches on. So, you can see the tuberculosis bacteria light up under the microscope.’ This enables rapid TB diagnoses, by examining patients’ blood or coughed-up mucus. It also allows you to monitor whether TB medication is taking effect. ‘You want to know this as soon as possible because there are drug-resistant forms of tuberculosis,’ says Bertozzi. ‘In addition, another research group has shown that you can also use this technique to detect tuberculosis cells in air filtration systems. TB is spread through the air. We may soon be able to detect TB outbreaks at an early stage, through the air filtration system in schools, hospitals or railway stations, and isolate people as soon as possible. Wouldn’t that be great?’

CV
Carolyn Bertozzi (Boston, 1966) studied chemistry at Harvard University in the United States. She received her Ph.D., also in chemistry, from Berkeley University in California in 1993. In 1996, she was appointed professor of chemistry and molecular cell biology there. In 2015, she moved from Berkeley University to Stanford University and became professor of chemistry and of chemical systems biology. Her scientific work forms the basis of more than fifty patents, and she is co-founder of several biotech companies, where her research results find their way into medical applications. Bertozzi received the Wolf Prize, the Solvay Prize, and the Nobel Prize in Chemistry, among others.

Research
Carolyn Bertozzi is developing chemical methods to study and influence the cell surface. Our cells communicate with each other through molecules on the cell surface. Immune cells, for example, can tell from these molecules whether a cell belongs in the body or not. Bertozzi discovered that cancer cells fool immune cells by presenting large amounts of a specific sugar molecule on the cell surface. She developed technologies to ‘mow down’ these sugar cells, which allow immune cells to recognise the cancer cells as dangerous and attack them. In addition, she developed many other technologies to influence proteins inside the cell and on the cell surface. In doing so, she was at the forefront of so-called bioorthogonal chemistry.

Heineken Prizes
Every two years, Heineken Prizes are awarded to five renowned international scientists and one artist. In 1964, Alfred Heineken established the Dr H.P. Heineken Prize for Biochemistry and Biophysics as a tribute to his father. Later, Heineken Prizes for the Arts, Medicine, Environmental Sciences, Historical Sciences, and Cognitive Sciences followed.

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