3.8.2.3: Gene expression & cancer

• Proto-oncogenes:

  • Code for proteins that stimulate the cell cycle, promoting cell division
• Mutations in a proto-oncogene:

  • Proto-oncogene -> oncogene
  • Consequently, gene is permanently active

  Receptor protein effect:

  • receptor proteins (that some proto-oncogenes normally code for) to growth factors of cell cycle are permanently activated, even in the absence of growth factors
  • -> uncontrolled cell division

  Growth factor effect:

  • Excessive growth factor production -> uncontrolled cell division
• Oestrogen & breast cancer

  • Oestrogen production falls in the ovaries but fat cells in breast tissue produce more even after menopause
  • Oestrogen PROMOTES transcription -> hyperpromote proto-oncogene expression -> become oncogenes
• The Two Hit Hypothesis:

  • A mutation of one allele of proto-oncogene is sufficient to induce cancer
  • A mutation of both alleles is required to deactivate tumour suppressor genes
  • Gain-of-function mutations that convert proto-oncogenes act dominantly (i.e mutation in only one of the two alleles is sufficient for inducing cancer)
  • Mutations in tumour suppressor genes act recessively so two recessive alleles required (to induce cancer)