Glioblastoma Physiology

Cheatsheet Content

### Introduction to Glioblastoma (GBM) - **Definition:** Glioblastoma (GBM) is the most common and aggressive primary malignant brain tumor in adults. It is a Grade IV astrocytoma, characterized by rapid growth and poor prognosis. - **Origin:** Believed to originate from glial cells (astrocytes or neural stem cells) within the brain. - **Key Features:** Highly infiltrative, characterized by cellular pleomorphism, high mitotic activity, microvascular proliferation, and necrosis. ### Molecular Pathogenesis - **Genetic Alterations:** GBM is highly heterogeneous with complex genetic landscapes. - **IDH1/2 Wildtype:** Accounts for ~90% of GBMs, generally primary GBMs, more aggressive. - **IDH1/2 Mutant:** Accounts for ~10% of GBMs, often secondary GBMs evolving from lower-grade gliomas, associated with better prognosis. - **Commonly Mutated Genes:** - **EGFR:** Amplification and mutations (e.g., EGFRvIII truncation) are frequent, leading to constitutive activation of signaling pathways. - **TP53:** Tumor suppressor gene mutations are common. - **PTEN:** Tumor suppressor gene mutations, leading to activation of PI3K/Akt pathway. - **RB1:** Retinoblastoma gene mutations. - **TERT Promoter:** Mutations are very common, associated with telomerase activation. - **Signaling Pathways:** Dysregulation of several key pathways drives GBM growth: - **EGFR/RAS/MAPK pathway:** Promotes cell proliferation and survival. - **PI3K/Akt/mTOR pathway:** Involved in cell growth, survival, and metabolism. - **p53 pathway:** Crucial for cell cycle arrest and apoptosis. - **Retinoblastoma (Rb) pathway:** Regulates cell cycle progression. ### Cellular Characteristics - **Heterogeneity:** GBM cells exhibit significant intra-tumoral heterogeneity, with distinct subpopulations of cells (e.g., glioblastoma stem cells, differentiated tumor cells). - **Glioblastoma Stem Cells (GSCs):** - Possess self-renewal capacity and multipotency. - Highly resistant to conventional therapies (chemotherapy, radiotherapy). - Contribute to tumor initiation, progression, and recurrence. - Express markers like CD133, SOX2, Nestin. - **Angiogenesis:** GBM is highly vascularized due to significant angiogenesis. - **VEGF (Vascular Endothelial Growth Factor):** Overexpression is a hallmark, promoting new blood vessel formation. - **Hypoxia:** Tumor hypoxia drives VEGF expression and promotes an aggressive phenotype. - **Invasion:** GBM cells are highly infiltrative, migrating into surrounding healthy brain tissue. - **Mechanisms:** Upregulation of matrix metalloproteinases (MMPs), altered cell adhesion molecules, and interaction with the extracellular matrix. - **Clinical Impact:** Makes complete surgical resection impossible and contributes to recurrence. ### Tumor Microenvironment - **Immune Suppression:** GBM creates an immunosuppressive microenvironment. - **Tumor-Associated Macrophages/Microglia (TAMs):** Abundant in GBM, polarized to an M2-like phenotype, promoting tumor growth, angiogenesis, and immune evasion. - **Regulatory T cells (Tregs):** Increased infiltration, suppressing anti-tumor immune responses. - **Myeloid-Derived Suppressor Cells (MDSCs):** Accumulate in tumor and periphery, inhibiting T-cell function. - **Extracellular Matrix (ECM):** GBM cells interact extensively with the ECM, influencing migration, proliferation, and survival. - **Astrocytes and Neurons:** GBM cells interact with surrounding non-malignant brain cells, co-opting them to support tumor growth. ### Therapeutic Challenges - **Blood-Brain Barrier (BBB):** Limits drug delivery to the tumor. - **Infiltrative Nature:** Makes complete surgical removal impossible, leading to recurrence from residual tumor cells. - **Therapy Resistance:** - **MGMT Methylation Status:** Methylation of the O6-methylguanine-DNA methyltransferase (MGMT) promoter predicts response to temozolomide (TMZ) chemotherapy. Unmethylated MGMT is associated with resistance. - **GSCs:** Resistance to radiation and chemotherapy. - **Molecular Heterogeneity:** Different tumor cells respond differently to treatments. - **Recurrence:** Almost universal, often more aggressive than the primary tumor. ### Current Standard Treatment - **Maximal Safe Surgical Resection:** To reduce tumor bulk and relieve symptoms. - **Radiation Therapy:** Followed by concomitant and adjuvant Temozolomide (TMZ) chemotherapy. - **Tumor Treating Fields (TTFields):** An anti-mitotic treatment delivered via cap. - **Bevacizumab:** Anti-VEGF antibody, used for recurrent GBM to reduce edema, but does not improve overall survival.