Molecular basis diagnostic marker and therapeutic role in glial tumours and gliomas md pathology exam 10 marks

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IDH1 R132H immunohistochemistry staining glioma diffuse astrocytoma

This composite figure presents diagnostic neuroimaging and histopathology of a low-grade glioma (LGG) in the left frontal lobe. (A) Axial T2-weighted MRI shows a well-circumscribed hyperintense lesion. (B) Contrast-enhanced T1-weighted MRI reveals only patchy, faint enhancement, indicating an intact blood-brain barrier. (C) 7 Tesla Susceptibility Weighted Imaging (SWI) demonstrates a paucity of intratumoral hypointense structures, suggesting low microvascular density. (D) A color-coded SWI-Local Image Variance (SWI-LIV) map overlays the tumor ROI, showing predominantly cool colors (purple/blue) consistent with a low mean value (32.2), typical for low-grade pathology. (E) Histopathological examination (H&E stain, 200x) displays a diffuse astrocytoma (WHO Grade II) with moderate cellularity and no evidence of necrosis or vascular proliferation. (F) Immunohistochemistry (200x) confirms positive IDH1-R132H mutational status, represented by brown cytoplasmic staining. The figure illustrates how advanced imaging techniques like SWI-LIV correlate with molecular markers and histopathology to differentiate low-grade from high-grade gliomas.

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

(Pathology - 10 Marks)

1. Introduction

Gliomas are the most common primary CNS neoplasms, arising from glial cells (astrocytes, oligodendrocytes, ependymal cells). Since the WHO CNS Tumour Classification (5th edition, 2021), gliomas are no longer classified purely on histomorphology - molecular/genetic alterations now form the backbone of diagnosis, grading, prognostication, and treatment selection ("integrated diagnosis").

2. WHO Molecular Classification of Adult-Type Diffuse Gliomas

Tumour	Key Molecular Signature
Astrocytoma, IDH-mutant	IDH1/IDH2 mutation, ATRX loss, TP53 mutation, no 1p/19q loss
Oligodendroglioma, IDH-mutant and 1p/19q-codeleted	IDH1/IDH2 mutation + 1p/19q co-deletion, TERT promoter mutation
Glioblastoma, IDH-wildtype (Grade 4)	IDH-wildtype + TERT promoter mutation, EGFR amplification, or +7/-10 copy number changes

Pediatric-type diffuse gliomas are grouped separately (H3 K27-altered midline glioma, H3 G34-mutant hemispheric glioma, MAPK pathway-altered low-grade glioma) - Harrison's Principles of Internal Medicine 22E; GOLDMAN-CECIL Medicine.

3. Key Molecular Markers - Diagnostic Significance

a) IDH1/IDH2 mutations (Isocitrate Dehydrogenase)

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

IDH1 R132H immunohistochemistry in low-grade glioma

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.

b) 1p/19q co-deletion

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

c) ATRX loss / TP53 mutation

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

d) TERT promoter mutation

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

e) EGFR amplification

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

f) MGMT promoter methylation

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

g) H3 K27M mutation (histone H3.3/H3.1)

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

h) BRAF alterations

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

Marker	Therapeutic Implication
MGMT methylation	Predicts response to temozolomide (alkylating agent); unmethylated MGMT = temozolomide resistance, worse prognosis
IDH mutation	Better response to radiotherapy/chemotherapy; IDH inhibitors (e.g., vorasidenib) under trial for lower-grade IDH-mutant gliomas
1p/19q co-deletion	Predicts favorable response to PCV chemotherapy (procarbazine, CCNU, vincristine) and radiotherapy in oligodendroglioma
EGFR amplification/BRAF V600E	Potential target for EGFR inhibitors / BRAF-MEK inhibitors (e.g., dabrafenib-trametinib in BRAF-mutant gliomas)
VEGF pathway (angiogenesis)	Bevacizumab (anti-VEGF monoclonal antibody) used in recurrent glioblastoma - improves progression-free survival and reduces edema/steroid requirement, though no overall survival benefit
H3 K27M	Guides prognosis; investigational targeted/epigenetic therapies (ONC201/dordaviprone) being studied

Standard therapeutic paradigm for glioblastoma: maximal safe surgical resection -> concurrent radiotherapy + temozolomide -> adjuvant temozolomide for 6 months, with MGMT status guiding expected benefit (Washington Manual of Medical Therapeutics; Harrison's 22E).

5. Adverse Prognostic Factors in Glioblastoma

Older age, unmethylated MGMT promoter, poor Karnofsky performance status, unresectable tumor, IDH-wildtype status - Harrison's Principles of Internal Medicine 22E.

6. Summary Diagram (Exam Point Form)

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

Recent evidence note: A 2025 CNS-guideline systematic review (PMID: 39747718) updates neuropathology recommendations for WHO grade II diffuse gliomas, and a 2024 systematic review (PMID: 39063221) discusses glioma stem cell pathways and targeted therapies - both consistent with, and extending, the textbook framework above without contradicting it.

Sources: Harrison's Principles of Internal Medicine 22E; Robbins & Kumar Basic Pathology; Bradley and Daroff's Neurology in Clinical Practice; Grainger & Allison's Diagnostic Radiology; GOLDMAN-CECIL Medicine; Washington Manual of Medical Therapeutics; Henry's Clinical Diagnosis and Management by Laboratory Methods; Adams and Victor's Principles of Neurology 12th ed.

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