Cancer cells must overcome cellular defense mechanisms that usually lead to self-destruction (apoptosis) or an irreversible arrest in their growth (senescence) when problems occur. Abnormal cells have to undergo more than one transforming event to escape these mechanisms and successfully survive and replicate. Klaus Rajewsky's group at the MDC and Harvard Medical School has now gained new insights into the way certain immune system cells evade defenses in a type of cancer called Burkitt lymphoma. This is crucial information because it may shift the focus of treatments for this deadly form of cancer. The work appeared in the August 14 edition of Cancer Cell.
Mouse Burkitt lymphomas are highly aggressive tumors that destroy normal tissue architecture. The tubular structures (blue) represent invaginations of the lining of the bowel and are surrounded by a massive infiltrate of lymphoma cells (brown). As in human Burkitt lymphomas, most of the mouse tumors originated from the small intestine.
Burkitt lymphoma originates from B cells, which are responsible for the production of antibodies during an infection. Since the early 1980s, researchers and clinicians have known that Burkitt lymphoma cells produce unusually high amounts of a protein called MYC. This molecule is a transcription factor that binds to DNA and activates genetic programs that drive cell replication and other biological processes. In many types of cancer, MYC is abundantly expressed and causes tumor cells to rapidly divide.
"This is interesting because in non-malignant cells, the overexpression of MYC would cause the cells to self-destruct," Klaus says. "Since that doesn't happen in Burkitt lymphoma cells, we knew that B cells with abnormal MYC levels evade cell death by undergoing additional transformations."
In other types of cancer, the detrimental effect of elevated MYC expression is balanced by the activation of a "pro-survival" factor called NF-kB. Although NF-kB activation is crucial for healthy B cells, other laboratories had shown that Burkitt lymphoma cells weren't using this strategy. So Sandrine Sander and additional members of the Rajewsky lab went in search of another factor that might be contributing to the cells' survival.
In 2009, while still at Harvard, the lab had discovered that a cellular signaling pathway controlled by the protein PI3K plays an essential role in the survival of B cells once they have been released from the bone marrow. Now Sandrine and her colleagues wanted to have a closer look at whether PI3K activation rescues the B cells that produce high levels of MYC and finally give rise to Burkitt lymphomas.
B cells undergo important stages of maturation in specific sites of the lymphatic system. In so-called germinal centers they modify their genome to create millions of antibodies with different shapes. This permits the immune system to recognize and combat a vast array of foreign targets. However, DNA remodeling can cause errors and may lead to an abnormal expression of proteins in the B cells, as happens with MYC in Burkitt lymphoma. To mimic the development of the disease in a model organism, the scientists generated a strain of mouse whose B cells start to produce MYC and an active version of PI3K specifically while they are in the germinal center.
To accomplish this the researchers took advantage of a technology called conditional mutagenesis, in whose development Klaus played a pioneering role. The method uses genetic engineering to attach specific genes to "control regions" that govern when and where in the body cells produce specific molecules. This allowed the scientists to control the production of MYC and PI3K at a specific stage of B cell life in the new study.
"We found that the altered B cells thrive in the germinal centers and went on to spawn large tumors in the small intestine, but also in the spleen and lymph nodes of the animals," Sandrine says.
Not only did this pattern of growth imitate that of Burkitt lymphoma, the tumor cells had characteristics that were nearly identical to changes found in human Burkitt lymphoma cells. They had the same shape, showed the same global changes in the expression of genes, and experienced a deregulation of several important proteins. Finally, the scientists demonstrated that in cases of Burkitt lymphoma, human tumors also show increased activity of the cellular signaling pathway activated by PI3K.
The study suggests that changes in the activity of both MYC and PI3K are necessary to trigger Burkitt lymphoma. Once they occur, a much wider range of genes begin to behave erratically and the tumor progresses.
"The PI3K signal needs to be present for MYC-expressing B cells to progress to cancer, and it may also be necessary for the tumor cells to survive," Klaus says. "Our work suggests that blocking this signal with an inhibitor could be an useful new clinical strategy for combating this deadly disease."