Chapter 9 Notes

• DNA repair


  Mechanism to maintain genome integrity and prevent cancer

  
• Cancer must compromise genome integrity to acquire mutator phenotype

  
• Tumour suppressor genes (TSGs)


  Gatekeepers and Caretakers
• Gatekeepers


  • Encode proteins that directly inhibit cell proliferation and survival
  • Loss of gatekeepers directly opens the gates to tumour formation
  • Examples: Rb, p53 and SMAD4
• Caretakers


  • Encode proteins that are involved in DNA repair and the maintenance of chromosome integrity
  • Loss of caretakers indirectly affects tumour formation by permitting an increased mutation rate for all genes
  • Examples: BRCA1, BRCA2 and ATM

  
• Genetic instability is caused by the loss of caretakers
• Cancer cells accumulate mutations at rates that can be hundreds or even thousands of times higher than normal = genetic instability

  
• Elevated mutation rates increase the probability that occasional mutations will arise and allow cells to escape from normal constraints on cell proliferation and survival
• Elevated mutation rates facilitate tumour progression and cells acquire additional traits – faster growth rate, increased invasiveness, ability to survive in the bloodstream, resistance to immune attack, ability to grow in other organs, resistance to drugs, evasion of apoptosis

  
• Telomere attrition


  Leading to aneuploidy
• DNA repair mechanisms


  • Nucleotide excision repair
  • Base excision repair
  • Mismatch repair
  • Homologous recombination

  
• Hydrolysis: Depurination


  Loss of the base adenine or guanine caused by hydrolysis of the bond linking it to the DNA chain

  
• Hydrolysis: deamination


  Removal of an amino group (-NH2) by hydrolysis

  
• In a human cell, rate of DNA damage by deamination is approximately 100 deaminations per day
• Oxidation


  Reactive oxygen species (ROS) attack bases within DNA (guanine) and causes single- and double-stranded DNA breaks

  
• Inflammation results in the formation of oxidants that can introduce mutations
• Pyrimidine dimers

  Covalent bond formed between two adjacent pyrimidine bases by UV radiation, distorting DNA structure and inhibiting polymerases

  
• Alkylation


  Transfer of an alkyl group from one molecule to another, destabilizing the covalent bond of the base to deoxyribose

  
• DNA adducts


  Chemical entities formed after reaction of a carcinogen with a DNA base

  
• DNA adducts


  • Benzo(a)pyrene (coal tar)
  • Aflatoxin (liver carcinogenesis)

  
• Single strand (SS) repair mechanisms

  • Nucleotide excision repair
  • Base excision repair
  • Mismatch repair

  
• Base excision repair (BER)


  Corrects single damaged bases in DNA, lesions derived from endogenous sources
• Nucleotide excision repair


  Used for removing bulkier DNA lesions, e.g. pyrimidine dimers

  
• Xeroderma pigmetosum


  Familial cancer syndrome with defects in NER genes, 2000-fold higher skin cancer rates

  
• Mismatch repair


  Corrects errors in DNA replication, defects lead to microsatellite instability

  
• Hereditary non-polyposis colon cancer (HNPCC)

  Familial cancer syndrome with defects in mismatch repair genes, 80% lifetime risk of colon cancer

  
• Double strand (DS) repair mechanisms

  • Non-homologous end joining
  • Microhomology–mediated end joining
  • Homologous recombination

  
• BRCA1 and BRCA2


  Tumour suppressor genes that are caretakers for double-strand DNA damage repair, mutations increase breast and ovarian cancer risk

  
• Homology-directed repair


  Late S phase and G2 phase, uses undamaged sister chromatid as template, least error-prone dsDNA repair mechanism

  
• Double strand (DS) repair mechanisms

  • Non-homologous end joining
  • Microhomology–mediated end joining
  • Homologous recombination

  
• dsDNA damage


  • Caused by X-rays and chemotherapeutic drugs
  • Unchecked dsDNA damage causes gross abnormalities in chromosome structure

  
• BRCA1 and BRCA2 TSGs

  Caretakers for dsDNA damage repair
• Women who inherit a mutation in one of the BRCA genes exhibit a lifetime cancer risk of 40-80% for breast cancer and 15-65% for ovarian cancer

  
• dbreaks useDNA damage


  Leads to gene fusion, loss of tumor suppressor

  
• Homology-directed repair


  1. Occurs in late S phase and G2 phase

  2. Uses undamaged sister chromatid as a template

  3. Least error prone dsDNA repair mechanism

  
• BRCA1 and BRCA2


  Scaffold proteins that assemble a cohort of other DNA repair proteins into large physical complexes necessary for DNA repair to occur

  
• Still not understood why inheritance of mutant alleles of BRCA1 and BRCA2 genes leads preferentially to cancers of the breast and ovary

  
• Multiple additional genetic and epigenetic changes are required for full malignant transformation of normal cells

  
• Types of genetic variation

  • Structural nucleotide variants (SNVs)
  • Copy number variations (CNVs)
  • Structural variants (chromosome)

  
• Structural nucleotide variants (SNVs)


  Point mutations, Insertion/deletions (indels)