WHO 2016 Classification of Infiltrating Gliomas

Diagnosis	Common Mutations	Additional Characteristics
ASTROCYTOMAS
Diffuse astrocytoma, IDH-mutant (WHO grade II)	IDH1/IDH2 ATRX TP53 TERT promoter EGFR amplification +7/−10 PTEN mutation/loss H3.3 p.K27M	Usually younger patients (40–50 years old); present as lower gliomas; most glioblastomas in this group are secondary
Anaplastic astrocytoma, IDH-mutant (WHO grade III)
Glioblastoma, IDH-mutant (WHO grade IV)
Diffuse astrocytoma, IDH-wildtype (WHO grade II)		Usually older patients (>55 years old); present as high-grade gliomas; most glioblastomas in this group are primary
Anaplastic astrocytoma, IDH-wildtype (WHO grade III)
Glioblastoma, IDH-wildtype (WHO grade IV)
Diffuse midline glioma, H3 -K27M-mutant	Midline location, infiltrating on histology, H3.3 p.K27M mutation
OLIGODENDROGLIOMAS
Oligodendroglioma, IDH-mutant, 1p/19q codeleted (WHO grade II)	IDH1/IDH2 1p/19q codeletion	ATRX is wild-type (ATRX mutations are mutually exclusive with 1p/19q codeletion), strong correlation with perinuclear clearing histology
Anaplastic oligodendroglioma, IDH-mutant, 1p/19q codeleted (WHO grade III)

Low Grade Glioma

1° goal in initial Tx of LGG is max. safe resection → assoc. w/ prolonged survival, sz pPx, ↓ risk of transformation;
Qx for suspected LGGs challenges:
- GTR is difficult d/t the diffusely infiltrative growth pattern of LGG → difficult to id the exact tumor borders; ∴ image guidance w/ FLAIR is crucial.
- Histopathological undergrading is common w/ Bx, as LGG often exhibits focal areas of malignant transformation (anaplastic foci). To avoid this and reduce subsequent treatment failure, the surgical goal is to perform precise tissue sampling from a potential anaplastic focus.
- "Eloquent" localization → awake Qx ± functional mapping

The O(6)-Methylguanine-DNA Methyltransferase (MGMT) is a DNA repair enzyme that counteracts the chemotherapeutic effects of alkylating agents.
Cancer-related methylation of the MGMT promoter region results in less DNA repair activity, including that induced by temozolomide (TMZ).
MGMT promoter methylation in GBM is a prognostic and predictive biomarker indicating response to chemoradiation.
Patients with MGMT promoter methylated tumors benefit most from treatment with TMZ.
Absence of MGMT promoter methylation → ↓ benefit from chemoradiation.
MGMT promoter methylation is a useful predictive biomarker for stratifying elderly GBM patients for RT versus alkylating agent chemotherapy.

Telomeres, at the ends of chromosomes, shorten w/ each cell division - limit to cell replication.
Telomerase, an enzyme that helps to maintain the length of telomeres;
Gliomas can bypass this limit via overexpression of TERT due to mutations in the promoter.
overactive TERT → maintaining telomere length → facilitating tumor expression

IDH 1 & 2 glu metabolism enzymes
Mutations in IDH1/2 → abnml form of IDH enzyme → ↑ 2-hydroxyglutarate (2-HG)
- 2-HG is a toxic metabolite → promote tumor growth.
IDH mutations assoc. w/ better prognosis (LGG)
IDH wild type (no mutations of the IDH) - worse prognosis because assoc. w/ HGG