Chapter 6 Notes

• Tumour suppressor genes require two hits for inactivation
• Loss-of function mutation


  Mutation that results in the loss of normal function of a gene
• TSG


  Abbreviation for Tumour Suppressor Gene
• Gain-of function mutation


  Mutation that results in the gain of a new or abnormal function
• ONCOGENES ARE DOMINANT GENES
• TSGs ARE RECESSIVE GENES
• INACTIVATION of TSGs in cancer

  1. First hit: Loss of function mutations or chromosomal deletions, Epigenetic silencing (promoter methylation)

  2. Second hit (Loss of Heterozygosity-LOH): Mitotic recombination, Gene conversion, Chromosomal nondisjunction (LOH) Loss of wild type or normal copy of the gene
• p53 is the most studied single gene in history
• p53 is a tumour suppressor gene located on chromosome 17p13 that encodes p53 protein
• Activation of p53 leads to cell cycle arrest, apoptosis and/or senescence of the affected cells
• 50% of all tumours exhibit the loss of p53 function
• SV40- monkey virus causes cancer
• p53 is a transcription factor, active only as a homotetramer
• Specific p53 mutants can transform cells
• Mutations in p53 act as gain of function mutations
• p53 mutations are dominant negative
• The presence of even a single mutant protein subunit may compromise the functioning of the entire tetramer
• Tetramerization


  • Key is in the tetramerization
• p53 mutations are dominant negative (unlike other TSGs)
• p53 mutations


  • Mostly missense mutations in the DNA binding domain
• The majority of p53 mutations affect the DNA-binding domain
• p53


  • Responds to stress stimuli
• p53 protein levels quickly rise in response to stress
• How are p53 levels regulated in cells?

  1. p53 protein synthesis

  2. p53 protein degradation
• p53 protein is short-lived (half-life is 20 minutes)
• Ubiquitination


  Post-translational addition of ubiquitin to a substrate protein, signals for protein degradation via cytoplasmic proteasomes
• p53 levels are controlled by MDM2


  p53/Mdm2 negative-feedback loop
• Mdm2 catalyzes ubiquitination of p53 protein
• Cancer cells can deactivate p53 by several mechanisms
• p53 pathway: Genotoxic Stress (e.g. DNA damage) and Hyperproliferative Stress (e.g. Oncogenes) lead to Cell Cycle Arrest or Apoptosis
• Hyperproliferative stress response is mediated through ARF protein


  Excessively high levels of E2Fs raise a red flag in the cell, Cell turns on its suicide pathway: E2Fs turn on p14ARF gene, ARF protein associates with and inactivates Mdm2, p53 accumulates and turns on suicide genes, Cell self-destructs (apoptosis)
• Cell turns on its suicide pathway

  1. E2Fs turn on p14ARF gene

  2. ARF protein associates with and inactivates Mdm2

  3. p53 accumulates and turns on suicide genes

  4. Cell self destructs (apoptosis)
• Response to massive, irreparable genomic damage, anoxia, or severe signaling imbalances

  ps3 will trigger apoptosis
• ARF-p53 pathway simplified degradation

  Apoptosis
• ARF-p53 pathway accomplishes the goal of eliminating cells that lack proper pRb function
• p16INK4A and p14ARF


  • p14ARF shares the locus with the p16INK4A gene
  • p14ARF uses an alternative transcriptional promoter
  • p14ARF mRNA is translated using an Alternative Reading Frame
  • Many human tumours have p16INK4A/p14ARF locus loss
• p53 is a transcription factor that acts as a homotetramer
• p53 is synthesized constitutively, but degraded quickly
• p53 can be deactivated in cancer via multiple mechanisms
• p53 is induced by DNA damage and hyperproliferative stress