In this blog post I’m going to write about some of the types of living cells we grow in the lab, which are undoubtedly amongst the most important research tools that any leukaemia researcher has at their disposal.
Leukaemia patients frequently consent to give samples of their blood and bone marrow for research and these are absolutely vital for learning more about leukaemia. However these types of cells cannot be grown for very long in the artificial environment of the laboratory. Although scientists try our hardest to provide nutrients and growing conditions to mimic those found in the human body, cells from patients often do not adapt well to growing in the laboratory. They also normally stop growing, or grow much more slowly than they do in the human body, which is challenging as we often want to study why leukaemia cells grow differently to normal blood cells, or maybe test a new drug on them. There is little point testing a new drug on cells which have already stopped growing! Blood and bone marrow cells from patients are vital for our continuing research work, but they are normally best suited for experiments that can be done quickly, such as getting DNA from the cells, and analysing this for mutations to explain why the cell is leukaemic.
Cell lines are different from patient samples, because they can be grown indefinitely in specialist incubators with liquid food medium and all cells within a cell line are genetically identical, whereas in leukaemia cells directly from a patient, this may not be the case.
There are hundreds of leukaemia cell lines with a variety of genetic abnormalities that have caused the leukaemia. For example a cell line called NB-4, which I am currently using, contains a DNA mutation called a translocation, which is where two chromosomes get stuck together abnormally (in this case chromosomes 15 and 17) causing the PML-RARA fusion gene, which almost all people with acute promyelocytic leukaemia (APL) have. NB-4 has been vitally important for researching this type of leukaemia and for testing the effect of drugs such as all-trans-retinoic acid, which anyone who has undergone APL treatment recently will be familiar with.
There are limitations to using cell lines to investigate new drugs and abnormal DNA, as each cell line only accurately represents just the one patient that it originally came from, and we are increasingly learning that although patients with a similar type of leukaemia may have some common DNA mutations (such as PML-RARA in APL), each patient will also have additional DNA mutations which may affect the way their leukaemia behaves and responds to treatment.
So for that reason, if researchers are looking at a new drug, where the aim was to treat all patients with acute myeloid leukaemia (AML) for example, as Leukaemia and Lymphoma Research-funded scientists in my group at Newcastle University currently are – they would use a wide-range of AML cell lines with lots of different DNA mutations to check that any effect of the drug was likely to be useful in lots of patients, not just ones with very specific DNA abnormalities.
Increasingly however, researchers are looking to design drugs which are targeted to a specific DNA mutation that is present in leukaemia cells, but not normal healthy cells. These kind of drugs are called ‘targeted therapies’ and they generally have far fewer side-effects than conventional chemotherapies because they preferentially target cancer cells rather than healthy cells.
The earliest example of this is a drug called Imatinib which most people with Chronic Myeloid Leukaemia (CML) will likely have been treated with. Imatinib directly affects a protein called ABL, which in CML is joined abnormally with a protein called BCR. ABL regulates the growth of a new blood system in babies and young children, and again when a woman is pregnant, but in adult blood stem cells, ABL is fairly dormant and inactive.
When the ABL DNA becomes abnormally joined to BCR DNA, the BCR-ABL fusion is formed, making the normally sleepy, dormant ABL hyperactive and driving the development of CML. Imatinib binds to ABL and slows it down again, resulting in remission in most CML patients. Imatinib was revolutionary, changing the 5-year survival rate of people with CML from approximately 15% to around 90% in the present day. More about the discovery of BCR-ABL and Imatinib can be found in one of my previous blog posts here.
Cell lines are also very useful for investigating which abnormal proteins are especially important in the leukaemia. By using something called RNA interference (I will write a future blog post about this in more detail), we can directly target the bit of DNA (a gene, which is like a blueprint for a protein) with a small molecule which stops that protein then being formed. This is especially great for seeing what fusion proteins do to cells because we take a cell line which has a fusion gene in it, split the cells into two dishes, use RNA interference to reduce the amount of fusion protein in one dish of cells and then we can directly compare this to the un-treated dish of cells to look for changes that we can attribute to the fusion protein in the other. We can ask questions like does the fusion protein make the cells divide quicker, or does it make it more resistant or sensitive to certain chemotherapy drugs?
Some of the cell lines that I use have been alive longer than I have (!) but these combined with samples generously donated by leukaemia patients continue to be incredibly useful for researchers in improving the lives of people who are battling leukaemia.