Telomeres and telomerase

• Senescence

  Cells stop dividing - p53 derived response
• Immortal human cancer cells

  • HCT116 - colorectal cancer
• Population doubling (PD)


  How many times the cells divide
• Cancer cells continuously divide if there were enough reagents
• Normal human cells will only divide a limited number of times even in enough reagent present
• Replicative senescence


  Senescent cells are unable to divide any further, remain alive for years, and change phenotype
• Senescent fibroblasts spread out in the culture and become large and can stay in senescence for years
• Telomeres


  A special structure required for some essential "chromosome function" at the end of linear chromosomes
• Telomeres


  • Stop ends of a chromosome from fusing with other chromosomes
  • Stop ends activating genome damage checkpoints
  • Prevent loss of sequence by exonuclease attack
• Telomere


  Whatever structure is present at the natural end of a linear chromosome that enables it to behave differently from a simple double-stranded DNA break in the genome
• Telomere structure


  • Has TTAGGG sequence that is repeated again and again
  • Has a single-stranded 3' overhang - G rich
  • Creates a T loop structure - tucks the ends away from itself
• T loops


  The single-strand G-rich extension is tucked back into the double-stranded region of the telomere
• Semi-conservative DNA replication


  1. Leading strand

  2. Lagging strand - Okazaki fragments
• RNA primers are degraded and filled in again and ligated together on the lagging strand
• Telomeres shorten with cell division


  This triggers the replicative senescence phenotype of cells
• Telomerase


  A reverse transcriptase that adds TTAGGG onto chromosome ends
• Telomerase


  • Has hTERT: Catalytic protein subunit
  • Has hTERC: RNA molecule that provides the template for the synthesis of TTAGGG
• Mechanism of action of telomerase

  Telomerase's catalytic subunit binds to the end of the chromosome and catalyses the addition of the repeat units TTAGGG continuously
• Introducing telomerase into normal human fibroblasts makes the cells live forever as their telomeres are elongated
• Telomerase expression


  • Not expressed in normal human cells
  • Expressed in male and female germ line
  • Expressed in most immortal cell lines
  • Expressed in 85% of malignancies
  • Expressed in stem cells
• Cancer cells have a mutation that makes their telomeres longer due to upregulation of telomerase
• Telomeres shorten with age, and females have longer telomeres than males
• Erosion rates of telomeres vary between tissues, depending on cell turnover
• Telomere length is associated with many aspects of the human condition, but is not a good biomarker of biological age
• Cancer cells have a mutation that makes their telomeres longer
• Telomerase


  Upregulated in cancer cells
• Telomeres shorten with age
• Females have longer telomeres than males
• Telomere erosion rates vary between tissues
• Telomere erosion


  • Dependent on cell turnover
  • Tissues that proliferate a lot show telomere erosion
  • Little telomere erosion in neuronal tissues with age
• Telomere length


  Associated with many aspects of the human condition
• Telomere length is not a good biomarker of biological age in humans
• Shorter telomere length


  Higher risk of lung/liver disease
• Longer telomere length


  Higher risk of cancer
• Telomere shortening


  Occurs in atherosclerosis
• Obesity, cigarette smoking


  Impacts telomere length in women
• Telomere length in early life predicts lifespan
• Birds with shorter telomere length do not live as long as those with longer telomeres
• Telomerase can be used as a cure for aging in animals
• Short telomeres in mice

  • Intestinal atrophy
  • Hair falls out
  • Increased cancer incidence
  • Decreased lifespan