Researchers at Children's Hospital of Philadelphia (CHOP) have identified tiny messenger RNA fragments that are missing in pediatric high-grade glioma tumors but present in normal brain tissue, offering a potential breakthrough for immunotherapy targeting these aggressive brain cancers. The findings, published in Cell Reports, demonstrate how these missing RNA pieces could make difficult-to-treat tumors more vulnerable to immune-based treatments.
Addressing Critical Challenges in Brain Tumor Immunotherapy
One of the most significant obstacles in treating aggressive brain tumors lies in developing safe and effective therapies. Current adoptive immunotherapies using CAR-T cells show promise but often target healthy cells that share surface proteins with cancerous cells. While this collateral damage might be acceptable for certain blood cancers, destroying healthy neurons in the brain is unacceptable, making the identification of tumor-specific targets crucial.
The research team focused on alternative splicing, a cellular process where a single gene produces multiple proteins by rearranging exons—the building blocks of messenger RNA—in different combinations. The researchers suspected that splicing patterns in glioma cells differ from those in normal brain cells, potentially revealing new therapeutic opportunities.
Discovery of Tumor-Specific Microexons
Previous RNA sequencing analyses of high-grade gliomas failed to account for very short exons called "microexons." The CHOP team's deeper analysis revealed that in glioma, many of these microexons are not incorporated into messenger RNAs encoding important surface proteins, including the neuronal cell adhesion molecule NRCAM.
In normal brain cells, full-length NRCAM is essential for forming synapses—the close contacts between neurons. However, in pediatric high-grade gliomas, two NRCAM microexons are consistently skipped, resulting in a distinct protein structure with unknown function.
Functional Significance and Therapeutic Potential
The researchers discovered that the shortened version of NRCAM generated through microexon skipping is essential for cancer cell migration and invasion in laboratory studies and for tumor growth in preclinical mouse models implanted with glioma cells. This makes the glioma-specific version of NRCAM an especially attractive immunotherapy target because tumors cannot easily shut it down.
"While microexons may be small, the effects they have on the overall protein structure are quite profound," said senior study author Andrei Thomas-Tikhonenko, PhD, chief of the Division of Cancer Pathobiology at CHOP and Professor in the Department of Pathology and Laboratory Medicine at the Perelman School of Medicine of the University of Pennsylvania.
Development of Targeted Antibody Therapy
Because the skipping of NRCAM microexons profoundly changes protein conformation, the research team developed a mouse monoclonal antibody against the glioma-specific version of NRCAM. When mixed with glioma cells, the antibody functions like a highlighter, "painting" glioma cells and marking them for killing by T cells armed with an immune receptor for mouse antibodies.
"In addition to developing these immune receptors clinically, we are actively using our proof-of-principle experiments to design traditional CAR T cell-based immunotherapeutics that selectively target glioma cells," said first study author Priyanka Sehgal, PhD, a research scientist in the Thomas-Tikhonenko laboratory at CHOP. "This could also change the way we find new targets in other solid tumors."
Broader Applications and Future Directions
The researchers noted that similar molecular mechanisms have been observed in other tumors including glioblastoma multiforme and cancers of neuroendocrine origin, which could also be targeted with NRCAM-directed immunotherapeutics.
The next steps involve expanding preclinical research and identifying a specific form of immunotherapy that could potentially be explored in clinical trials. The research team is actively applying these findings to create CAR-T cell-based immunotherapeutics that would selectively target glioma cells while sparing healthy brain tissue.
This study was supported primarily by the CureSearch for Children's Cancer Foundation Acceleration Initiative and multiple National Institutes of Health grants, along with additional funding from the National Science Foundation Graduate Research Fellowship Program and other organizations focused on pediatric cancer research.
