Cytokinesis-Block Micronucleus (CBMN) Assay for Quantitative Chromosomal Damage & Genotoxicity
🔬 Principle & OECD 487 Guidelines
Micronuclei (MN) are small, extranuclear round bodies composed of chromatin that lag behind during mitotic anaphase and fail to be incorporated into either daughter nucleus during telophase. Micronuclei originate through two distinct genotoxic mechanisms:
- Clastogenic Mechanism: Induced by double-strand DNA breaks that lead to chromosomal breakage. Acentric chromosome fragments lack a functional kinetochore, preventing attachment to mitotic spindle fibers. They fail to migrate to poles and form small micronuclei.
- Aneugenic Mechanism: Induced by compounds that disrupt spindle microtubules (colchicine, vinblastine) or kinetochore protein assemblies. Whole chromosomes fail to attach or lag at the metaphase plate and form large, kinetochore-positive micronuclei.
A primary confounding variable in conventional micronucleus assays is dividing cell kinetics: cells that have divided zero times or multiple times dilute the accurate scoring of mutational events. To resolve this, Michael Fenech developed the Cytokinesis-Block Micronucleus (CBMN) assay utilizing Cytochalasin-B ($3-6,mutext{g/mL}$).
Cytochalasin-B inhibits actin polymerization, completely halting cleavage furrow formation and cytokinesis without disrupting nuclear karyokinesis. Consequently, cells that complete exactly one round of mitosis are unambiguously trapped as binucleated (BN) cells. Scoring micronuclei strictly within binucleated cells guarantees that scored damage occurred during the preceding mitotic division!
📋 Standard Protocol & Scoring Criteria
- Culture Initiation: Initiate human lymphocyte cultures with PHA as in Exp 4. At 44 hours of culture, add test mutagen (e.g., Mitomycin C at $0.05,mutext{M}$ or EMS at $1,text{mM}$; include vehicle negative control).
- Cytochalasin-B Addition (44 Hours): Add Cytochalasin-B to each culture flask at a final concentration of $4.5,mutext{g/mL}$. Incubate for an additional 28 hours (total culture time = 72 hours).
- Harvesting: Centrifuge at $1000,text{rpm}$ for 8 minutes. Treat with mild hypotonic shock ($0.075,text{M KCl}$ diluted $1:1$ with saline) for 3 minutes (avoid excessive swelling which ruptures binucleate cell membranes).
- Fixation & Staining: Fix in Carnoy’s solution (3:1 methanol:acetic acid), prepare air-dried slides, and stain with $5%$ Giemsa in pH 6.8 buffer or DAPI fluorochrome.
- Microscopic Scoring (OECD Guideline 487): Under $100times$ oil immersion, identify at least $1000$ consecutive binucleated (BN) cells per test concentration. Record the number of BN cells containing 0, 1, 2, or $ge 3$ micronuclei.
📊 Micronucleus Frequency Calculation
Where $M1, M2, M3, M4$ represent the number of cells with 1, 2, 3, and 4 nuclei respectively. NDI quantifies cytotoxicity and cell proliferation kinetics.
