Breast Cancer and Stem Cells
Kat - And now, it's time to join Meera Senthilingam who's gone along to the Cancer Research UK Cambridge Research Institute this week, to find out how John Stingl and his team are investigating the role of stem cells in breast cancer development. Now, stem cells are known for their ability to regenerate and differentiate to form lots of the cells in our bodies. But as well as this crucial role in our growth and development, it seems that rogue stem cells might be at the heart of cancer formation in many cases including breast cancer. So, Meera spoke to John about how he's studying this kind of change in the human breast.
John - The breast has traditionally been considered to be composed of two types of cells. We have luminal cells which are cells that make milk and an underlying layer of myoepithelial cells, just the name implies, they're muscle-like cells, and they're literally a cell that contracts and squeeze the milk out of the mammary gland. So our hypothesis is that the mammary gland is probably more complex than just milk-producing and muscle-like cells and it's our hypothesis that there's also stem cells present - mother cells present in the mammary gland that can generate both these luminal cells and myoepithelial cells. It's been basically our research for the last number of years now, where we basically try to identify mammary gland stem cells, trying to figure out what all the different types of differentiated cells in the breast and as well, other cells in between stem cells and the differentiated daughter cells. Cells we would call actually, progenitor cells.
Meera - How are you trying to understand how this pathway between stem cell to differentiated cell varies when somebody gets breast cancer?
John - The normal mammary gland is under quite strict control of homeostasis. So for example, a stem cell, would normally divide and produce a new stem cell, and it also produces more differentiated daughter cells. So cells would have some proliferative capacity but not as important as a stem cell. We call these cells a progenitor cell. Then these progenitor cells would produce their daughter cells. And this balance between a stem cell producing one new stem cell and a more mature daughter cell, we think is perturbed in cancer. Such that in cancer, there may be more a shift to, say, producing more stem cells. Basically, we believe that cancer is a disease of cells that have proliferative capacity, other stem cells or cells with stem cell-like properties. And the reason for this is that you have to keep in mind that cancer is multistep process. We don't just get a mutation, then get cancer. If that's the case, we would probably all be dead. In fact, you probably need about say, five or six mutations to go from a normal cell to malignant cell. But the body has developed pretty good methods for protecting its DNA and the probability of getting a mutation, a meaningful mutation of cells, is actually quite low. Analogous to like winning a lottery. So how is a cell supposed to get say, five mutations in a row, it's like trying to win the lottery five times in a row. But if you have a cell that has that possibility to generate lots and lots of daughter cell, such as a stem cell. So say, just by unlucky chance, a stem cell got a mutation. That stem cell can generate a million daughter cells. All of them will have that mutation. Now, the probability that a certain daughter cell will get a second mutation is quite low, but the probability that one of those daughter cells out of that, you know whole population will get a second mutation is quite high. And maybe the second mutation's a mutation that reduces DNA repairability. And therefore, the problem in getting a third mutation is not one in a million, but say, maybe one in a hundred. Basically, if you target cells that have the ability to grow, you can increase your chances of getting multiple mutations.
Meera - Because whilst the probabilities are quite low, the fact that so many cells are being generated obviously makes that risk higher.
John - Exactly.
Meera - So how are you going about actually researching this?
John - We get normal breast tissue from the hospital. This is human breast tissue say, reduction mammoplasty specimens. So women who had their breast surgically reduced. We bring them back to the lab and we basically mince them up and we incubate ithem in an enzyme mixture which basically degrades all the extracellular matrix but doesn't hurt the cells. And we basically make a single cell suspension of human breast tissue. We use a machine called the fluorescence activated cell sorter and by tagging the cells with fluorescent molecules, we can purify different subfractions of breast cells.
Meera - But now, with your team having separated out to find all of these different cells that are present within a breast, how are you then going to go about actually researching the cancer side of it?
John - So for example again, there are many different types of breast cancers. Are they all rising in a common cell type or they're arising from different types of breast cancer cells? The way how we're going to test that is, say for example, you have an oncogene, gene X, and you know that gene X is somehow an associate of breast cancer but you don't know how. Now what we're going to do is we're going to purify subfractions of normal human breast cells and then using specialized viruses, we can actually infect those cells, such that they overexpress oncogene X. And we can examine, okay, does oncogene X for example, cause stem cells to expand the stem cell population or does it cause a more mature progenitor cell to turn into a stem cell. And then we can also, instead of just introducing oncogene X, we can also get the oncogene X, Y, and Z and seeing what type of cell generates what type of tumour.
Meera - So having identified the cause of the tumour like a particular gene that's causing a tumour, you're inserting it into different types of cells that are made within a breast to see which of those cells that gene actually affects to become cancerous, essentially.
John - Yeah, exactly. The problems we've had is that there's been a lack of understanding of the normal cellular context in which oncogenes and tumour suppressor genes exert their actions. For example, does oncogene X, you know, does it cause this one population to expand or does it cause another population to expand or does it cause one population to generate another population?
Meera - So you don't know whether it's causing one cell to just proliferate or if it's causing one cell to produce something that causes another cell to proliferate and so on?
John - Yes. So we don't understand the underlying mechanisms of what is actually happening. We only see the end stage result and it's very important to understand the molecular pathways that regulate stem cell behaviour because you want to target these pathways in order to stop tumour growth. Now it has been demonstrated and pretty conclusively that there is such thing as a breast cancer stem cell. The challenge now is to figure out how abundant are these breast cancer stem cells, are they identical between different types of breast tumours, and they're probably not, and basically, how do we isolate them and how do we target them. And that's basically the stage we are right now.
Chris - Well, that was John Stingl who's from the Cancer Research UK Cambridge Research Institute and he was talking to Meera Senthilingam to explain how he and his team are investigating the role of stem cells and breast cancer formation and how those cells change their regulation and how that could be targeted as a potential therapy for cancer treatment.