Telomeres

What are they?

• Ends of linear chromosomes
  • Repetitive DNA sequence: TTAGGG in vertebrates
  • Specialized proteins
  • Form a "capped" end structure

Why are telomeres important?

• Telomeres allow cells to distinguish chromosome ends from broken DNA
  • If DNA is broken there are two options after the cell cycle is stopped: Repair or Death
    • Repair can occur in two ways:
      • Homologous Recombination (HR) -- Error-free but need homologue nearby
      • Non-homologous end-joining (NHEJ) -- Error-prone but saves chromosome from degradation
• Telomeres prevent chromosome fusions by NHEJ
  • Fusion-bridge-breakage cycles leads to genomic instability which in turn can result in cell death or neoplastic transformation
• Telomeres are specialized structures that are essential for protecting chromosome ends and ensuring chromosome stability
• Telomeres also provide a mechanism for "counting" cell divisions

Telomere shortening - the end replication problem

• Telomeres shorten with each cell division (S phase)
  • The "end replication" problem:
    • DNA replication is bidirectional
    • DNA polymerases are unidirectional
    • DNA polymerases must initiate replication from a primer
  • Therefore: each round of DNA replication leaves 50-200 bp DNA unreplicated at the 3' end
• Cells with telomeres that are 10-12 kb in length (average) divide 50-60 times
  • Telomeres are 4-6 kb [5-7 kb] in length (average)
• Cellular senescence is triggeredwhen telomeres are on average 4-6 kb

How do immortal cells avoid telomere loss (i.e. solve the end replication problem)?

• Enzyme (reverse transcriptase) with protein + RNA subunits
• Adds telomeric repeats directly to 3' overhang (uses its own RNA component as a template)
  • Vertebrate telomere repeat: TTAGGG
  • Vertebrate telomerase RNA template: AATCCC
• Overcomes telomere shortening/end replication problem
  • Added back to somatic cells
    1. Prevemts telomere shortening
    2. Prevents replicative senescence
  • However cells that express telomerase still undergo cellular senescence in response to DNA damage, oncogenes, etc.
• Expressed by germ cells and early embryonic cells
• NOT expressed by most somatic cells (human)
• Expressed by certain stem cells, but highly regulated
• Expressed by 80-90% of tumor cells!
  • remaining 10-20% still need to overcome end replication problem -- do so by alternative telomere lengthening mechanism (ALT), probably recombination

Telomere Hypothesis of Aging

• Hypothesis: Telomeres shorten with age (T cells, tissue with high cell turnover)
  • Therefore, telomere shortening is a cause of aging
• What is wrong with this hypothesis?
  • Telomere contributes to aging ONLY IF replicative senescence contributes to aging
    • Therefore, telomerase will prevent aging and/or restore youthfulness
  • Telomerase protects against replicative, but not other forms of cellular senescence

Telomerase: Useful for expanding cells for medical use

Telomerase Inhibitors: Useful for inhibiting or killing tumor cells

Summary

• Telomeres are essential for chromosome stability
• Telomere shortening causes replicative senescence. Other inducers of cellular senescence are telomere - independent
• Telomerase prevents telomere shortening and replicative senescence
• The telomere hypothesis of aging hinges on the cellular/replicative senescence hypothesis of aging

Dr. Campisi's diagrams from last year's lecture: Campisi diagrams from February 2000