4.Aneuploidy in Cancer

1. Definition

• Aneuploidy = abnormal chromosome number that is not a multiple of the haploid set (23).
• A normal somatic cell has 46 chromosomes (23 pairs).
• Aneuploid cell may have 45, 47, or any non-diploid complement.
• Represents a deviation from normal chromosome complement.

2. Frequency in Cancers

• Aneuploidy is very common in malignant tumors → especially carcinomas.
• First proposed over a century ago as a causal mechanism in cancer.
• Modern evidence supports active involvement in tumor progression.

3. Mechanism of Development

Main mechanism

• Mitotic checkpoint failure (spindle assembly checkpoint).

Normal checkpoint role

• Ensures chromosomes are properly attached to mitotic spindle before anaphase.
• Prevents premature separation of sister chromatids.
• Acts as a chromosome gatekeeper controlling equal segregation.

When checkpoint fails

• Chromosomes mis-segregate during cell division.
• Daughter cells receive unequal chromosome numbers.
• Severe checkpoint failure → catastrophic mis-segregation → usually lethal to cells.

4. Biological Debate – Cause vs. Consequence

• Historically unclear whether aneuploidy was:
• a downstream consequence of cancer or
• a driver of carcinogenesis.
• Increasing evidence shows aneuploidy can actively promote cancer.

How it promotes cancer

• Chromosome copy number variation → altered gene dosage:
• ↑ oncogene copies (drive proliferation)
• ↓ tumor suppressor copies (remove brakes)

5. Key Gene Dosage Examples

• Chromosome 8 gain → MYC amplification
• MYC = potent pro-proliferative transcription factor.
• Frequently gained, rarely lost in aneuploid cancers.
• Chromosome 17 loss → TP53 deletion
• TP53 = major tumor suppressor involved in DNA damage control + apoptosis.
• Frequently lost, rarely gained.

6. Mechanistic Sequence Summary

Mitotic checkpoint failure

→ chromosome mis-segregation

→ abnormal chromosome number (aneuploidy)

→ altered oncogene / tumor suppressor balance

→ ↑ proliferation + ↓ growth control

→ tumor evolution and progression

7. Conceptual Memory Anchors

• Checkpoint = Chromosome gatekeeper.
• Oncogene MYC “multiplies” on chromosome 8.
• TP53 “tumbles” when chromosome 17 copy lost.
• Aneuploidy = “library missing or adding books” — disrupts function.

8. High-Yield Exam Points

• Aneuploidy is hallmark feature of many cancers.
• Checkpoint failure = strongest mechanistic link.
• Complete loss of checkpoint → cell death → tumors retain partial checkpoint dysfunction.
• Gene dosage imbalance is the mechanism linking aneuploidy to cancer.

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