Molecular neuropathology of gliomas

Abstract

Gliomas are the most common primary human brain tumors. They comprise a heterogeneous group of benign and malignant neoplasms that are histologically classified according to the World Health Organization (WHO) classification of tumors of the nervous system. Over the past 20 years the cytogenetic and molecular genetic alterations associated with glioma formation and progression have been intensely studied and genetic profiles as additional aids to the definition of brain tumors have been incorporated in the WHO classification. In fact, first steps have been undertaken in supplementing classical histopathological diagnosis by the use of molecular tests, such as MGMT promoter hypermethylation in glioblastomas or detection of losses of chromosome arms 1p and 19q in oligodendroglial tumors. The tremendous progress that has been made in the use of array-based profiling techniques will likely contribute to a further molecular refinement of glioma classification and lead to the identification of glioma core pathways that can be specifically targeted by more individualized glioma therapies.

Keywords: 19q; 1p; Glioblastoma; MGMT; biomarker; ependymoma; genetics; molecular diagnostics; oligodendroglioma; profiling.

Figure 1.

Schematic representation of the molecular pathogenesis and progression of diffusely infiltrating astrocytic gliomas.

Figure 2.

Schematic representation of the molecular pathogenesis of pilocytic astrocytomas.

Figure 3.

Schematic representation of the molecular pathogenesis and progression of oligodendrogliomas.

Figure 4.

Schematic representation of the molecular pathogenesis and progression of oligoastrocytomas.

Figure 5.

Schematic representation of the molecular pathogenesis and progression of ependymal gliomas.

Figure 6.

Core pathways involved in the pathogenesis of gliomas. Note the interrelationships between p53, RB, growth factor receptor, PTEN/PI3K/AKT and RAS signaling on the regulation of cell proliferation and apoptosis. While TP53 mutation, amplification of MDM2/MDM4 or p14^ARF deletion/methylation inhibits apoptosis, alterations in p16^INK4a, p15^INK4b, p18^INK4c and p21^waf1 disinhibit cell cycle progression at the G₁/S-phase checkpoint via cyclin-dependent kinases by phosphorylation of RB1 and release of E2F transcription factors. Amplification, overexpression or mutation of growth factor receptors stimulates cell proliferation and inhibits apoptosis through both the RAS as well as the PI3K/AKT signaling pathway. The RAS signaling pathway can be alternatively activated by mutations in the NF1, the PI3K/AKT signaling pathway by mutations in the PTEN gene and less commonly, the PIK3CA or the PIK3R1 gene.