Oncogenes and signalling pathway 1

• Oncogenes and signalling pathways 1
• Discovery of oncogenes stemmed from research into transforming properties of viruses
• First identified oncogene: Src
• Ellermann and Bang (1908)

  Reported that particles that could pass through filters (viruses) caused erythro-myeloblastic leukemia in chickens
• Peyton Rous (1911)


  Isolated filterable agent from the sarcoma in the breast muscle of a hen. Rous Sarcoma Virus
• Temin and Rubin (1950s)

  RSV can cause cancer like foci in normal cells
• Oncogenes


  Initially identified as genes carried by viruses that cause transformation of target cells. Known as v-onc or viral oncogenes
• Viral oncogene of RSV had been captured from host cells and modified
• Proto-oncogenes


  Cellular counterparts of viral oncogenes, involved in normal signalling in the cell
• Activated oncogene


  Switched off less frequently or not at all
• How oncogenes are activated


  Variety of ways genes can change
• Platelet derived growth factor (PDGF)

  Has A chain and B chain, mechanism of activation is quantitative (amplify rather than genetic changes in coding sequence), occurs in osteosarcoma (A chain dimers) and fibrosarcoma (B chain dimers)
• Epidermal growth factor (EGF)


  Quantitative, occurs in breast cancer
• Erb-B (EGFR)


  Qualitative: Truncated receptor, occurs in avian erythroblastosis v-erb-B. Quantitative: Increased expression-gene amplification, occurs in squamous carcinoma, skin, lung
• Signalling by ErbB homodimers vs ErbB2-containing heterodimers

  ErbB2 can dimerise with any of the other erb - hetero, resultant signalling is long lasting, receptor expressed for a prolonged period of time
• Normal vs mutated HER protein

  Mutation turns valine to glutamine, causes ligand independent homodimerisation and continuous phosphorylation and signalling. Mutation can also cause truncated protein to be expressed
• Signalling pathway of EGF binding to its receptor

  Receptor dimerisation → GRB2 binds pTyr and brings Sos to Ras → GEFs activate GTPases
• Ras


  A GTP binding transducer, oncogene
• GTP Binding transducers: G protein cycle


  In normal: RAS is bound to GDP, when activated GDP is replaced by GTP, leads to phosphorylation of Raf kinase leading to mapk/erk kinase phosphorylation
• Failure of GAP (GTPase activating protein) can cause uncontrolled Ras signalling and lead to diseases like Neurofibromatosis and Proteus Syndrome
• Signal transduction


  Flips molecular switches regulated by phosphorylation of tyrosine/serine residues or GDP/GTP switch
• Result of RAS getting switched on

  Activates RAF kinase, MEK, ERK, leading to transcription factor activation and cell proliferation
• MAP kinase cascade


  Mitogen-Activated Protein: activated by a mitosis-stimulating growth factor
• itch


  Resembles ras
• Change from GTP to GDP

  Facilitation by GAP and GEF
• RAS getting switched on results in...
• RAS


  • Small (molecular weight 21 kDa) proteins that bind guanine nucleotides and have GTPase activity
  • Mammalian cells contain 3 RAS genes: H-RAS, K-RAS, N-RAS
  • All have similar function and sequence
• Incidence of RAS mutation in human cancers

  • No to low: Ovarian, breast, gastric
  • Low to moderate: Lung (K-RAS), Urothelial (H-RAS)
  • High: Pancreas (K-RAS), Colon (K-RAS), Melanoma (H,N, K-RAS), Thyroid (H,N, K-RAS)
• Normal RAS


  Codes for amino acid 12 glycine which is located in the GTP binding domain of the protein, crucial for GTP hydrolysis
• Mutant RAS


  RAS becomes an oncogene by amino acid 12 being changed to code for any other amino acid besides proline, no longer able for GTP hydrolysis, therefore constantly bound to GTP
• RAS effectors and their biological responses

  • Raf
  • PI3K
  • RalGDS
  • Rac/Rho
• BCR-ABL


  Fusion chromosome leads to CML chronic myelogenous leukemia
• Src


  • SH2 domains bind to phosporylated tyrosines
  • SH3 Domains bind to prolinery regions
  • Kinase domain - also known as SH1 domain
• Mutated Src as an oncogene

  Loses the tyr527 by truncating it, making the kinase domain of src inaccessible and inactive
• Raf


  Immediately downstream of ras, activated by ras
• Mechanism of activation of turning raf to oncogene

  Rearrangement leads to replacement of N-terminal region and deregulated kinase, or point-mutation
• Myc


  Gene amplification leads to 200-2000 kbp amplicon, correlates with poor prognosis
• AP1


  Made of 2 components c-fos and c-jun, mutations in either can dysregulate AP1 and lead to overexpression of target genes
• Myb


  Transcriptional regulator that induces the expression of multiple pro-oncogenic drivers
• How signals get turned off

  • Phosphatases
  • GAPs
  • Degradation
  • Ubiquitination