MN1 Fusion

MN1 stands for *Meningioma 1*, a gene located on chromosome 22q12.1.

In CNS tumors, MN1 rearrangement typically involves fusion with a partner gene (commonly BEND2) and defines a distinct molecular entity.

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MN1 fusions are pathognomonic for the tumor type:

Astroblastoma, MN1-altered

This is currently the only CNS tumor recognized by WHO (2021) that requires MN1 fusion for diagnosis.

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• MN1::BEND2 (most frequent)
• Other rare partners: CXXC5, ETV1, etc.

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To detect MN1 rearrangement:

• FISH (break-apart probe)
• RNA sequencing (preferred)
• RT-PCR (if fusion partner known)

Methylation profiling often clusters MN1-altered tumors into the “CNS high-grade neuroepithelial tumor with MN1 alteration” class.

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• Astroblastic pseudorosettes (perivascular)
• Epithelioid to spindle cell morphology
• Sharp tumor borders
• Young female predominance

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If MN1 rearrangement is absent, the tumor:

• Should NOT be labeled “astroblastoma” under WHO 2021
• May be considered:
  • High-grade glioma, NOS
  • HGNET, NOS
  • Another defined entity pending methylation/NGS results

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• MN1 fusion is REQUIRED for the diagnosis: 'Astroblastoma, MN1-altered'
• Use RNA-seq or FISH for confirmation
• MN1-altered tumors form a distinct molecular and clinical group