Is it possible that abnormal activity of microRNA molecules in the cancer stem cells is what causes the wild behavior of these cells?
It is a small group of cancer stem cells that is responsible for the "maintenance" of cancerous tumors, just as small groups of normal stem cells are the ones that strengthen all the tissues and organs in the body. This theory is becoming more and more established, and in recent years it has even been proven for several types of cancer such as leukemia, breast cancer, certain brain tumors and more. Scientists who injected cancer cells into mice discovered that injecting a relatively small amount of cancer stem cells into healthy mice is enough to cause the development of a cancerous tumor, while injecting a much larger amount of "normal" cancer cells (which are not stem cells), does not necessarily cause a tumor to appear. The theory of cancer stem cells has medical significance, since the destruction of the small population of cancer stem cells will make it possible to uproot the cancerous tumor from the root and prevent its renewed development. This is in contrast to the treatments used today, which are directed against all tumor cells, and in many cases fail to damage all cancer stem cells.
The cancerous stem cells retain the ability to self-renew and divide, and thus they "strengthen" and "overcome" the cancerous growth. Most of the "normal" cancer cells, on the other hand, are cells that have undergone partial differentiation, and thus have lost the ability to self-renew, and with it their ability to divide and contribute to the development of the cancer tumor.
This "maturing" process that cancer cells go through is now also an accepted route for the development of medical treatments that encourage the differentiation of cancer stem cells, with the aim of stopping the development of the tumor. To find the best way to intervene in the differentiation processes of the cancer stem cells, the genetic differences between them and the cancer cells that are not stem cells must be found. Indeed, recently the studies that deal with the genetic mapping of these two types of cells, the expression of the genes in them, and the identification of the genetic mechanisms responsible for the self-renewal capacity of the cancer stem cells are multiplying.
The attempts to understand the factors that regulate the expression of genes in cancer stem cells, compared to normal cancer cells, lead the scientists to test the microRNA (miRNA) molecules. MicroRNAs are short sequences of RNA that attach to messenger RNA molecules, from which the protein molecules are translated, and interfere with the translation activity. In this way, they regulate gene expression, that is, they affect the amount of active protein produced by certain genes. The important role of these molecules, which are known to regulate many vital processes in the body, has become increasingly clear in recent years. So far, about 1,000 such molecules have been discovered, each of which affects dozens or hundreds of proteins found in cellular "activity nodes". Among other things, they are known to play an important role in regulating development. It was also found that abnormal activity or expression of microRNA is also associated with the development of cancerous tumors.
Is it possible that abnormal activity of microRNA molecules in the cancer stem cells is what causes the wild behavior of these cells? This is the question that Prof. David Gavol from the Department of Molecular Biology of the Cell, and the research student (at the time) Dr. Hila Gal (who is currently conducting post-doctoral research in the Institute's Department of Immunology) tried to answer. The scientists, who collaborated with Prof. Gideon Ravavi's group at the Sheba Sheba Medical Center in Tel Hashomer, used genetic chips that make it possible to compare the expression of microRNA molecules in cancer stem cells to those in normal cancer cells. The cells were taken from glioblastoma tumors - an invasive and violent brain cancer for which no treatment has been found to date.
In the first step, the researchers used a method developed by Dr. Gal to separate the cancer stem cells from the rest of the tumor cells. The separation was made through the identification of a certain protein that appears on the cell membrane of the stem cells but not on the other cells. The researchers discovered that, after separation, the stem cells can be easily distinguished from the cancer cells: when the cancer stem cells are grown in plates, they form round clusters (neurospheres), while the cancer cells that are not stem cells do not cluster in such clusters. Later, the scientists collaborated with scientists from "Rosetta Genomics" to compare the amounts of microRNA molecules in both types of cells. The scientists identified six molecules whose concentration in the cancer stem cells was considerably lower than their concentration in the normal cancer cells of the same tumor. What is the role of these microRNA molecules? The researchers assumed that their increased expression in the cancer stem cells is what "pushes" them to differentiate.
In other words, it is possible that these microRNA molecules cause the cancer stem cells to lose their self-renewal properties - and to differentiate. This is how they "disconnect" the tumor from the supply of stem cells that regenerate and maintain it.
To test the hypothesis, the researchers injected some of the microRNA molecules they identified into the cancer stem cells. As a result, the round aggregates dispersed into individual cells – leading the researchers to assume that they had lost their stem cell properties – and subsequently died. When the researchers tried to use a combined treatment of the microRNA molecule together with the anti-cancer drug "Gleevac" (see box), the effect of the double treatment was much faster and stronger than the effect of each individual treatment.
Prof. Gavol hopes that the research findings will open a door to the development of new healing methods, based on the administration of microRNA, possibly in combination with other drugs. "MicroRNA has several notable advantages: the small amount of molecules needed to achieve an effect, the ease of introducing the material into the cancer cells, and the rapid effect - cell death occurred after just one day. All of this gives us hope that further research will lead to the development of new drugs to treat cancer."
Gleebak - the story of a medicine
Research by Prof. Eli Kanani, from the department of molecular biology of the cell, and the members of his group, in the 08s, laid the foundation for the development of the drug "Gleebak", which received a lot of attention because it was the first to be based on understanding the molecular mechanism that causes the type a certain type of cancer. Prof. Kanani was able to identify and isolate two genes that undergo a process of fusion - as a result of an abnormal exchange of genetic material between two different chromosomes. The replacement of the DNA segments leads to the formation of a fusion protein: one part of which includes most of the ABL enzyme, which activates various target proteins through phosphorylation. However, the regulatory region of the enzyme, responsible for its selective activation, is replaced by a part of the BCR protein, which causes its uncontrolled activation. The overactivity of the phosphorylating enzyme in a certain type of blood cells causes an outbreak of chronic myelogenous leukemia (CML) cancer.
Through the use of new cloning techniques for that time, the members of Prof. Kanani's group were able to show the fusions of the two genes, and predict the amino acid sequence of the destructive enzyme. Based on these findings, the pharmaceutical company "Novartis" developed a molecule that is able to block the activity of the enzyme in a selective and efficient manner, and ultimately causes the death of the cancer cells that produce it. In addition, the drug manages to prevent the transition from the chronic phase of leukemia to the acute, fatal phase of the disease.
Following Prof. Kanani's findings - which showed for the first time that cancer can be caused by protein fusion - a similar mechanism was identified in dozens of other types of cancer, mainly in various leukemias, soft tissue tumors and prostate cancer. In 1992, the drug was approved by the US Food and Drug Administration (FDA) for the treatment of CML patients. "Gleevec", which is given as a pill, is now the standard medicine for CML patients, and is also given to patients with a certain type of cancer in the gastrointestinal tract. "Gleevec" is an example of the enormous potential of drugs directed against the products of destructive molecular events, which lead directly to specific types of cancer. The development of similar drugs is a central direction in modern pharmacology.
