Zooming in on micro-RNA
As well as studying the genes involved in driving cancer and using this information to drive the development of new cancer drugs that target faulty proteins, researchers are also taking a more direct approach, investigating how to turn down the activity of genes themselves. One of them is Professor Peter Leedman from the Harry Perkins Institute of Medical Research at the University of Western Australia in Perth, who explained his ideas to Chris Smith...
Peter - We’re fascinated with the concept of using part of the genome. That is the DNA blueprint as a new therapy in cancer. And, in particular, some of the solid tumours like liver cancer, pancreatic cancer, head and neck cancer, and brain cancer, for which there are very few treatments, and the prognosis and the outlook may be very poor.
Chris - We know cancer is a genetic disease. It’s when genes go wrong and, therefore, cells begin to obey the wrong sets of instructions or disobey the normal sets of instructions. So when you’re say you’re trying to use the genome to come up with a new way to treat cancer, what do you mean?
Peter - Some of these solid tumours, they are driven by factors such as receptors that sit on the outside of the cell, and they drive the cells to proliferate or grow in an unrestrained way. We’re looking at that as the set of targets. The second part of the story is saying what can we discover inside the genome or the DNA blueprint that might give us an answer for slowing down that receptor which drives the growth. And the dogma for 50 years was that a lot of the DNA was said to contain junk, so called “junk DNA.” What’s been discovered over the last 15 years is that about 2% of the DNA is really important and made into protein, but that 98% of the non protein-making DNA is really important.
Chris - Just to clarify that a bit then. So you're saying that 2% of the DNA code actually are genes, and those genes turn into proteins in cells; they’re like recipes that make proteins. The 98% is really important but how? What is the 98% making then, or doing that makes it so important?
Peter - The 98% tends to make what we call non-coding RNA. So DNA is made into RNA - this copy - and it’s that copy that’s not turned into protein in the 98% of the genome, and it’s that that contains this rich tapestry of little molecules called RNA. And that is an area that’s of great interest globally because we believe that if we can untap some of the secrets in that 98% of the DNA, so called junk, that some of them might be useful for clinical treatments.
Chris - Right, so the dogma of for what’s useful about DNA is that it contains recipes that make proteins. That’s going out the window and what we now realise is that it might not be making proteins but this DNA sure as hell is making something important. It’s making these RNA molecules and what, do they stay in the cell and do something?
Peter - What we know: some of them are really small, only 22 or 23 bases long. What we found is that one of them called microRNA-7 is particularly powerful at slowing down and inhibiting the growth receptor that drives a number of these cancers, and it does it in a very sophisticated way. It basically takes out the key receptor itself and multiple friends of the receptor that help drive the growth and promote the tumour, and what we find when we add microRNA-7 to a number of these cancers, we can stop the growth of the cancer in the test tube. We can also stop it in mice and that’s particularly exciting.
Chris - What do they do physically inside the cell to achieve that effect? Do we understand chemically what they are doing?
Peter - What we know is that the microRNAs work inside the cell and they target the message and basically blow it apart. That protein being a key growth receptor that drives the cancer, that now is taken out of the cell and so the cancer undergoes what we call cell death.
Chris - So it’s a way of specifically, and in a highly targeted way, taking down a discrete genetic message so it robs the cell of the ability to have that genetic message that it was making too much of to make it at all? The only problem is that that would happen in every cell in your body, if you just deluged a person in this microRNA-7, they would be robbed of the ability to make that gene product everywhere. And that would be bad, wouldn’t it?
Peter - That’s a great question. What we know about these tumours, these nasty tumours such as liver cancer and head and neck cancer, for example, is they’re characterised by having lots, and I mean sometimes millions, of these growth promoting receptors on the surface of the cell. In contrast, normal cells which might have a thousand or much, much lower amounts and the principle here is to replace back something that the cancer cell used to make, and that’s a really important point.
The cancer cells we’re looking at used to make microRNA-7 and they’ve lost it and it provides this incredible break on growth promotion, and when you lose it you provide the opportunity for the cell to grow in an unrestrained way. Now, if we add back microRNA-7 it targets the growth receptor promoting pathway and all it’s friends in that, but it typically will target those cells in the body that have lots, and lots of the receptor. Much, much more than the normal cells, and so we believe that’s why it would be better tolerated and, of course, fight the cancer very effectively.
Chris - And you can get this message into the cell sufficiently, can you?
Peter - We can. There is, when we give the microRNA intravenously in model systems, it’s very well taken up into the cancer. So we believe this will be a wonderful way of treating human cancer.
Chris - One of the things that cancer is notorious for is it’s genetic diversity. And one of the reasons why cancers respond poorly in the end to chemotherapy is because, eventually, you get cancer cells that have evolved and been selected out, and they have an ability to rid the cell of whatever the agent is you're putting into it to try and kill it. So, is there a possibility that all you’re doing is kicking the can down the road and we’re going to select using your agent cancer cells that can just make even more of the gene for the receptor and you'll be back where you started?
Peter - Great question. One of the key things in cancers is they often develop resistance to the first therapy or therapies that they might be given. One of the things we’ve found with microRNA-7 is that when we give it to cells that have become resistant to a chemotherapy agent such as cisplatin, which is often used for head and neck cancer, we find that we can re-sensitize the cells to the chemotherapy agent by giving microRNA-7.
That’s really exciting because that suggests that we can rescue patients from the terrible traumas of resistance. And in real terms, when cancer kills people, it kills people because it comes back and it comes back in dreadful areas and spreads all over the body. You don’t die from the first time you get a cancer, you’d actually die from the complications of the cancer occurring - so called metastasis or metastatic disease. And we’re very excited about this molecule, microRNA-7, because of it’s ability to work with other agents and make the tumours resensitized to those agents and therefore, hopefully, improve the outcome.