Just a few weeks ago, we provided a comprehensive update on the discoveries and advances being made by Defeat GBM (and and infographic depicting these, as well), our flagship research program. Defeat GBM utilizes a unique infrastructure consisting of four teams, or “Cores,” of research teams that work in concert on complementary research projects that combined, seek to discover and then test in clinical trials new potential treatments that can help glioblastoma patients. Now we want to break down the progress more completely by looking, in context, at the accomplishments of each Core.
We start today, with Core 1, or the “Discovery Core,” which is led by the laboratory of Dr. Frank Furnari of Ludwig Cancer Research, San Diego.
The most common mutations found in glioblastoma cells are alterations to a gene called epidermal growth factor receptor or “EGFR.” Despite these mutations being well-characterized, and despite a number of existing drugs that target EGFR mutations that have found some success treating other cancers that have these mutations, glioblastoma patients typically do not benefit significantly from these drugs. Contrary to the successes with other cancers, glioblastoma cells seem able to resist efforts to block the effect of EGFR mutations.
Defeat GBM’s Discovery Core is taking a laser focus on this vexing issue to determine ways that glioblastoma cells with EGFR mutations avoid or escape medicines designed to treat them, and finding new ways to target these rogue cells.
Dr. Frank Furnari, his lab at the Ludwig Institute for Cancer Research in San Diego, and with support from Defeat GBM’s other Cores and advisors devised three specific projects and angles to study this critical challenge. Combined, these aims will help the Discovery Core to determine how resistance occurs (both when a tumor doesn’t respond to the treatment from the get-go, and when the tumor develops resistance to the medicine over time), determine if resistance to one drug extends to others, and to test whether blocking target and resistance mechanisms can improve how patients respond to treatment regimens involving EGFR-targeting drugs.
AIM 1 – Hypothesis & Progress
Aim 1 is to understand how molecules that exist within cells – called “signaling molecules” – that help different parts of the cell communicate and control basic functions, contribute to how glioblastoma cells escape from EGFR targeting drugs.
The Discovery team studied resistance in new pre-clinical models of glioblastoma tumors that no longer respond to treatment with EGFR inhibitors. They found that in the drug resistant tumors, a molecule called urokinase (uPA) appeared frequently at abnormally high levels. The researchers were able to determine that high levels of uPA were associated with increased activity in a specific signaling pathway called the MAPK pathway in the tumor cells which in turn caused a decrease in levels of an important protein called Bim. (Bim has been shown to be key to a process that causes other types of tumor cells to die in response to targeted treatments).
The conclusion: when you treat GBM tumors with EGFR inhibitors, the tumor subsequently increases its level of uPA, which triggers a process that results in the decrease of Bim. Without enough Bim the tumor cells are able resume growing.
However, the Discovery team also believes that it might be possible to restore the function of Bim using additional medicines, and thus block the tumors growth yet again. “By using drugs that mimic the activity of Bim, or block the pathway initiated by uPA activity, we feel there is an opportunity to overcome EGFR inhibitor resistance,” said Dr. Furnari.
The Defeat GBM team is now trying to identify drugs that could mimic Bim.
AIM 2 – Hypothesis & Progress
The team also wanted to know what role the tumor’s “microenvironment” may have in facilitating or influencing glioblastoma cells’ ability to resist drugs targeting EGFR mutations. A tumor’s microenvironment is simply the cellular environment in which the tumor exists, including surrounding blood vessels and other types of cells.
Based on their research, what the team found was that glioblastoma cells that have EGFR mutations are actually able to “talk to” other cells around the tumor by secreting special molecules. Among these, the team showed that a molecule known as “interleukin 6 (IL-6)” is secreted by cells with EGFR mutations. IL-6, in turn, can stimulate the production of a protein called “survivin.” Survivin is aptly named because it counteracts the effects of drugs that target EGFR by blocking cell death, helping the cells survive.
The team then sought to unravel how IL-6 stimulated the production of survivin. What they found was that a transcription factor protein known as “NF-kappa B” was induced by IL-6 which turned on the gene that codes for survivin in cells without an EGFR mutation, and also drove the production of IL-6, itself, in cells with the EGFR mutation.
Taken together, these findings show that NF-kappaB plays a key role in facilitating the “cross-talk” between cells within the tumor environment that have an EGFR mutation, and in the cells that don’t. This is important because it suggests that finding, or developing, a medicine that could block this cross-talk, could improve the effectiveness of EGFR targeted drugs – which the team will endeavor to do.
AIM 3 – Hypothesis & Progress
While studying glioblastoma cells’ resistance to EGFR drugs, Dr. Furnari and his team noticed that glioblastoma cells expressing a protein called PTEN which had undergone a chemical modification, called phosphorylation, at a specific site in its sequence i.e., “pY240-PTEN” are particularly resistant to not only treatment strategies based on EGFR mutations, but also the standard of care treatment for glioblastoma patients (radiation and chemotherapy). The team then showed that a protein called “FGFR” was facilitating the generation of the pY240-PTEN alteration, and that this modified PTEN played an important role in DNA repair, causing PTEN to protect the tumor cells from DNA damage when cells become stressed. Thus, the pY240 alteration to PTEN was enabling the protein to repair DNA in cancer cells which had been damaged by radiation, making the glioblastoma cancer cells more resistant to radiation treatment. The team now believes that if they can block FGFR from facilitating the pY240-PTEN alteration, that radiation can be more effective for glioblastoma patients.
The team is now working closely with the Defeat GBM’s “Drug Development” Core (Core 2) to assess drugs that could effectively form this blockade and make radiation work better for glioblastoma patients.
Other Findings & Work
Because PTEN alterations in tumors (see above in Aim 3) have also been shown by previous research to make cancer cells vulnerable to treatment with a class of drugs called “PARP inhibitors,” Dr. Furnari and his colleagues in Drug Development Core are also testing if PARP inhibitors can be combined with drugs that target FGFR to even more powerfully help radiation treatments work for glioblastoma patients.
Importantly, during his research Dr. Furnari independently noticed the role that “epigenetics” – which are changes, including gene expression and activity, that occur in cells that are not caused by alterations to the actual DNA sequence – may play in treatment resistance in glioblastoma, particularly the role that a family of proteins (known as “bromodomain” proteins) called the “BET” family play in resistance to treatment. This was a discovery originally made by Dr. Paul Mischel working in Core 3, but the independent validation by Dr. Furnari highlights to potential importance of this finding. (More on this in the forthcoming Core 3, Predictive Markers/Biomarkers blog).
Most recently, Dr. Furnari and his team made the surprising discovery that the loss of two normally-important tumor-suppressor genes, might actually be a positive for glioblastoma cells. This apparent paradox can be explained by the discovery of a previously unknown interaction between PTEN and another protein called DAXX by Dr. Furnari and a postdoctoral researcher in his lab, Dr. Jorge Benitez. Dr. Furnari and his team validated this through experiments using lab mice, where they demonstrated that if either PTEN or DAXX was eliminated in GBM cells, then tumor growth occurred. But, if both were eliminated, then tumor growth slowed. In these experiments the researchers used genetic engineering techniques to “knockdown” and inhibit DAXX. But for this approach to ultimately prove successful as a treatment strategy that could help GBM patients, they need to develop a new drug that achieves the same effect. > Read more
Summary & Moving Forward
All-in-all the Discovery Core, has identified five (5) potentially critical mechanisms whereby glioblastoma cells can avoid or escape treatments designed to target them, while also independently validating a sixth (6th) trick these tumors use. The team is already working on developing associated treatment strategies to attack and overcome these resistance mechanisms, and in conjunction with the Drug Development Core, are evaluating the potential of dozens of drugs to block these mechanisms.
Specifically, moving forward, the Discovery team will prioritize testing for the best FGFR targeting drugs – both alone and in combinations with other medicines – to make glioblastoma cells more sensitive to radiation treatment; testing BET targeting drugs in combination with EGFR-targeting drugs to see if they, together, can help overcome treatment resistance in glioblastoma cells; and screening for potential molecules that could be used to target DAXX.