Glioblastoma (GBM), one of the most aggressive types of brain cancer, is one of the greatest challenges for medicine, both because it is difficult to treat and because of its high mortality rate. In Brazil, although no exact figures are available, it is estimated that between 10,000 and 12,000 new cases are diagnosed every year.
The disease, which accounts for nearly half (49%) of all brain tumors, has an extremely low survival rate, with most patients living only about 12 months after diagnosis. For this reason, scientists have been searching for new therapeutic targets for years to develop more effective treatments that can improve survival and quality of life for these patients.
Traditional treatment includes surgery to remove the tumor, chemotherapy, and radiotherapy. The main drug used is temozolomide (TMZ), a chemotherapy approved in the late 1990s and still used to control the disease. The problem is that although the patient may be free of the tumor for a few months, glioblastoma rarely responds completely to treatment, and it recurs months later – often in a more aggressive and invasive form.
With this in mind, the group led by Professor Marilene Hohmuth Lopes , of the Laboratory of Neurobiology and Stem Cells in the Department of Cell and Developmental Biology at the Biomedical Sciences Institute of the University of São Paulo (ICB-USP) in Brazil, decided to take a closer look at the mechanism of action of the tumor cells that remain in the brain tissue even after complete treatment.
In a study supported by FAPESP and published in the journal BMC Cancer, the team discovered that the prion protein plays a key role in the biology of glioblastoma.
"The treatment of glioblastoma has stagnated for more than 20 years. It's essential to discover new strategies to improve the chances of recovery and survival of patients," Lopes told Agência FAPESP.
Tumor stem cells
To understand the importance of the prion protein in cancer biology, it is first necessary to understand the mechanism of action of glioblastoma. As Lopes explains, surgery and treatment with temozolomide kill the cells that proliferate rapidly and form the "mass" of the tumor. However, the so-called tumor stem cells (or glioblastoma stem cells) remain dormant in the brain tissue. When they become active again, they are able to orchestrate tumor growth once more.
"It's important to remember that stem cells are very powerful and have the ability to self-renew. They stay quiet for a while, but when they 'wake up' they generate new cells that multiply rapidly and rebuild the entire cellular hierarchy of the tumor. That's what caught our attention," says the professor.
Prion protein
All humans produce a protein called prion, which has relevant and extremely important biological functions for the maintenance of the central nervous system: it affects the functionality and plasticity of the brain, is involved in cognitive processes (e.g. memory formation and consolidation) and contributes to communication between neurons.
"I was already studying this protein before I started to look at its role in cancer development. When we observed in patient samples that it was elevated in very aggressive tumors, we decided to better understand its relationship with glioblastoma and its influence on the tumor stem cells responsible for cancer recurrence," she says.
This is important because the prion protein is present on the surface of cells, which makes it "druggable" (a target that can be modulated by drugs). "This means that when thinking about a possible therapy, it's much easier to cross the blood-brain barrier and target a protein that's on the surface of the cell than one inside the cell, for example," explains the professor.
In the in vitro experiments, the group observed that when they cultured the glioblastoma stem cells, there was a significant increase in the levels of the prion protein, suggesting that it plays a fundamental role in regulating these cells.
Genetic editing
Based on this finding, the group used CRISPR-Cas9 technology to edit the genome of glioblastoma stem cells and block the production of the prion protein in these cells. This allowed the researchers to alter the functioning of these tumor stem cells, reducing their ability to invade and proliferate.
"This showed us that prion is a potential therapeutic target. But it's unlikely that a single protein alone is responsible for the development of the disease. We believe that it acts in different signaling pathways, which is why we're continuing to investigate other mechanisms and possible partners for the protein," explains Lopes.
The group then studied the interaction of prion with the CD44 protein, a well-known cancer stem cell marker involved in the collective invasion of breast and colon cancers.
"We recently discovered that one molecule modulates the other, and now we're trying to better understand this interaction. So far, we know that the prion protein can act as a scaffold, creating multiprotein signaling platforms in the membrane of cells so that they can survive and proliferate. When we altered the production of this protein [using CRISPR-Cas9], we found that its absence compromises the self-renewal, migration and invasion of tumor cells," she points out.
Despite the promising results, it is not yet possible to predict when these new discoveries will be applied in clinical practice. "We work with basic research. It takes many years to translate these discoveries into treatments. But we're in the process of understanding mechanisms, of understanding how this protein regulates other important genes in cell and tumor biology, and how it could become a potential therapeutic target in the future. The study continues," the researcher concludes.
About FAPESP
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.
