Molecular Basis, Diagnostic Markers, and Therapeutic Role in Glial Tumours / Gliomas

(Pathology - 10 Marks)

1. Introduction

2. WHO Molecular Classification of Adult-Type Diffuse Gliomas

3. Key Molecular Markers - Diagnostic Significance

• Most common: IDH1 R132H (~90%); detected by immunohistochemistry (mutant-specific antibody) or sequencing if IHC-negative
• Early, defining event in diffuse gliomas - distinguishes IDH-mutant (younger patients, better prognosis) from IDH-wildtype tumours
• ~95% of glioblastomas are IDH-wildtype - Bradley and Daroff's Neurology in Clinical Practice; Grainger & Allison's Diagnostic Radiology

Diffuse astrocytoma (WHO Grade II): H&E showing moderate cellularity without necrosis/vascular proliferation, and IHC showing brown cytoplasmic positivity for IDH1-R132H, confirming molecular diagnosis.

• Defining marker of oligodendroglioma (always occurs with IDH mutation)
• Best prognosis among lower-grade gliomas (mean survival ~8 years)
• Predicts increased chemosensitivity and radiosensitivity - Henry's Clinical Diagnosis and Management by Laboratory Methods

• Loss of ATRX expression (by IHC) + TP53 mutation characterizes IDH-mutant astrocytomas (mutually exclusive with 1p/19q co-deletion)

• Found in IDH-wildtype glioblastoma and in IDH-mutant, 1p/19q-codeleted oligodendroglioma
• One of three genetic criteria (with EGFR amplification or +7/-10) sufficient to designate a histologically lower-grade astrocytoma as glioblastoma, Grade 4 - Robbins & Kumar Basic Pathology

• Present in ~40% of IDH-wildtype glioblastomas (chromosome 7)
• Along with PTEN mutation and CDKN2A deletion, drives growth factor signaling in primary (de novo) glioblastoma

• O6-methylguanine-DNA methyltransferase is a DNA repair enzyme that removes alkylating adducts
• Promoter hypermethylation -> reduced MGMT expression -> tumor cells cannot repair temozolomide-induced DNA damage -> better response to alkylating chemotherapy
• Strongest predictive and prognostic biomarker in glioblastoma management - Harrison's Principles of Internal Medicine; Washington Manual of Medical Therapeutics

• Defines diffuse midline glioma (e.g., DIPG - diffuse intrinsic pontine glioma), automatically assigned WHO Grade 4 regardless of histology
• Dismal prognosis; detected by mutant-specific IHC

• KIAA1549-BRAF fusion: characteristic of pilocytic astrocytoma (WHO Grade 1)
• BRAF V600E point mutation: seen in pleomorphic xanthoastrocytoma and a subset of low-grade gliomas - actionable with BRAF inhibitors

4. Therapeutic Role of Molecular Markers

5. Adverse Prognostic Factors in Glioblastoma

6. Summary Diagram (Exam Point Form)

1. Classification: IDH status -> 1p/19q status -> grade
2. Diagnostic IHC panel: IDH1-R132H, ATRX, p53, Ki-67, GFAP, H3K27M
3. Molecular tests: FISH/PCR for 1p/19q, MGMT methylation assay (PCR/pyrosequencing), NGS panel for IDH1/2, TERT, EGFR, BRAF
4. Prognostic use: IDH-mutant > IDH-wildtype; 1p/19q co-deleted = best prognosis
5. Predictive/therapeutic use: MGMT methylation -> temozolomide response; BRAF/EGFR -> targeted therapy; VEGF -> bevacizumab

Gliomas - Robbins & Kumar Basic Pathology (Textbook-Based Notes)

1. Astrocytoma, IDH-Mutant

• Grade 2-3: poorly defined, gray, infiltrative tumors that expand/distort brain without a discrete mass; well-differentiated cells with mild-moderate nuclear pleomorphism and a fibrillary background of GFAP-positive astrocytic processes; indistinct tumor-normal margin with infiltration many centimeters from the main lesion
• Grade 3: denser cellularity, more nuclear pleomorphism, mitoses present
• Grade 4: cellular crowding, cytologic atypia, increased proliferation, microvascular proliferation and/or necrosis - but (unlike IDH-wildtype glioblastoma) usually lacks large areas of central necrosis/hemorrhage

2. Glioblastoma, IDH-Wild-Type

• Evasion of senescence: TERT mutations or alternative telomere lengthening
• Escape from growth control: biallelic CDKN2A deletion (loss of p16)
• Growth factor signaling activation: EGFR or PDGFR amplification
• Resistance to apoptosis: TP53 mutation
• MGMT promoter methylation - downregulates a DNA repair enzyme and increases sensitivity to alkylating chemotherapy (temozolomide)

Fig 21.31 - Glioblastoma: (A) necrotic, hemorrhagic, infiltrating mass; (B) serpiginous pseudopalisading necrosis and microvascular proliferation (inset).

3. Oligodendroglioma, IDH-Mutant and 1p/19q-Codeleted

4. Pilocytic Astrocytoma (Circumscribed Glioma, WHO Grade 1)

Quick Comparison Table

Gliomas - Combined Robbins Coverage

(Robbins & Kumar Basic Pathology + Robbins, Cotran & Kumar Pathologic Basis of Disease)

1. Astrocytoma, IDH-Mutant

• Basic Pathology: peak in 4th-6th decades, cerebral hemispheres
• Pathologic Basis of Disease adds precision: less common than glioblastoma; median age 38 years; cerebral hemispheres of young-to-middle-aged adults

• IDH1 (or IDH2) gain-of-function mutation - the earliest tumorigenic event. Mutant IDH produces 2-hydroxyglutarate, an oncometabolite (Cotran/Pathologic Basis, Ch. 7 cross-reference)
• Concurrent TP53 mutation and ATRX inactivation (mechanism of senescence evasion) - Cotran describes these three alterations as occurring "simultaneously"
• Homozygous CDKN2A/CDKN2B deletion upgrades the tumor to CNS WHO grade 4 by definition, even without high-grade histology, because prognosis is markedly worse

• Gross: poorly defined, gray, infiltrative masses that expand and distort brain without a discrete margin; cut surface firm or soft/gelatinous, sometimes with cystic degeneration; size ranges from a few cm to near-total hemispheric replacement
• Microscopic: hypercellular compared to normal white matter; enlarged, elongated/irregular hyperchromatic nuclei in a GFAP-positive fibrillary background; tumor cells infiltrate well beyond the visible margin (individual cells seen many cm away, including "perineuronal satellitosis" around cortical neurons)
• IDH1 p.R132H immunostain positive in up to 90% of cases; IDH sequencing reserved for immunonegative cases to detect less common mutations
• Nearly all cases show ATRX loss and p53 overexpression by IHC
• Grade 3: denser cellularity, more pleomorphism, mitoses appear
• Grade 4: cellular crowding, cytologic atypia, microvascular proliferation and/or necrosis (though typically less necrosis/hemorrhage than IDH-wildtype glioblastoma)

Fig. 28.46 (Cotran) - Coronal section showing expanded, infiltrated left frontal white matter; histology shows hyperchromatic tumor nuclei among native fibrillary matrix; inset shows positive IDH1 p.R132H immunostain.

2. Glioblastoma, IDH-Wild-Type

• Basic Pathology: ~50% of all primary malignant brain tumors in adults
• Cotran/Pathologic Basis: ~14% of all primary CNS tumors and over half of all CNS malignancies; occurs mostly in cerebral hemispheres of adults >55 years; arises de novo and does not evolve from a lower-grade astrocytoma (unlike the old "secondary glioblastoma" concept)

• Gross: strikingly heterogeneous - firm/white regions next to soft/yellow necrotic areas, cystic degeneration, and hemorrhage
• Microscopic: high cellularity, poorly differentiated pleomorphic cells, brisk mitoses, serpiginous "pseudopalisading" necrosis (tumor nuclei lined around anucleate necrotic zones), and microvascular proliferation

Fig. 21.31 (Basic Pathology) - Necrotic, hemorrhagic infiltrating mass with pseudopalisading necrosis and microvascular proliferation (inset).

3. Oligodendroglioma, IDH-Mutant and 1p/19q-Codeleted

• Grade 2: gelatinous, gray, infiltrative mass; cysts, hemorrhage, calcification (up to 90% of cases)
• Classic "fried egg" cells - round nuclei with finely granular chromatin and a clear cytoplasmic halo - set within a "chicken-wire" capillary network
• Grade 3: higher cellularity, nuclear anaplasia, more mitoses, microvascular proliferation and/or necrosis

Combined Comparison Table (Table 28.5, Cotran - Pathologic Basis of Disease)

4. Pilocytic Astrocytoma (WHO Grade 1) - circumscribed, non-diffuse glioma

Key synthesis point (why both editions matter for exam purposes)

• Basic Pathology gives the more streamlined, exam-friendly version (definitions, grading rules, survival numbers).
• Pathologic Basis of Disease (Cotran) adds mechanistic depth: the oncometabolite 2-hydroxyglutarate, precise epidemiologic proportions (14% of CNS tumors for GBM), the explicit de novo origin of glioblastoma (refuting the old low-grade-to-high-grade progression model for GBM), and the IHC correlate panel (IDH1 R132H, ATRX loss, p53 overexpression) used for diagnosis in practice.