Chapter 23

• DNA repair mechanisms


  • Proofreading
  • Mismatch repair
  • Direct repair
  • Excision repair
  • Double-strand break repair

  
• Proofreading


  1. Wrong base is incorporated

  2. Polymerase activity stalls

  3. Exonuclease activity removes last few bases

  4. Polymerase continues synthesis
• Due to proofreading, the error rate is only about 1 in 10,000,000 nucleotides
• Mismatch repair


  • Hemimethylated DNA marks the template strand
  • Mismatch complex brings mismatched bases close to methylated GATC
  • Exonucleases remove nucleotides between methyl group and mismatch
  • DNA polymerase replaces nucleotides

  
• Direct repair


  • Restores correct structures of altered nucleotides without removing them
  • Methyltransferase removes methyl group and restores guanine

  
• Thymine dimer (UV light)

  • Covalent bonds between consecutive thymines
  • Causes lesions and stalls replication
  • Photolyase uses light energy to break covalent bonds
• Excision repair


  1. Recognize damaged DNA

  2. Chop out damaged DNA

  3. Fill in the gap

  
• Base excision repair


  • Repairs sites with abnormal or modified bases
  • Glycosylases recognize lesions and cleave off single base
  • DNA polymerase fills in gap
  • Ligase ligates gap

  
• Nucleotide-excision repair


  • Removes small patch of bases around bulky DNA lesion
  • Enzyme complex recognizes bulky lesion
  • Damaged strand is cleaved on both sides
  • Damaged portion is removed
  • Gap is filled by DNA polymerase and ligase

  
• Double-strand break (DSB) repair

  • Nonhomologous end joining: proteins recognize broken ends and join them
  • Homology directed repair: uses homologous chromosome or sister chromatid as template

  
• Summary of DNA repair mechanisms

  • Mismatch repair
  • Direct repair
  • Base excision repair
  • Nucleotide excision repair
  • Homology directed repair
  • Nonhomologous end joining

  
• Xeroderma pigmentosum


  • Autosomal recessive disorder
  • Defective nucleotide excision repair
  • Freckle-like spots, sensitive to sunlight, predisposition to skin cancer
• BRCA1 and BRCA2


  • Proteins involved in homology directed repair
  • Frequently mutated in breast cancer
• Genetic diseases associated with DNA repair defects

  • Xeroderma Pigmentosum
  • Cockayne syndrome
  • Trichothiodystrophy
  • Hereditary nonpolyposis colon cancer
  • Fanconi anemia
  • Li-Fraumeni syndrome
  • Werner syndrome
• Cancer is a genetic disease characterized by cell proliferation without regard to normal controls on cell division

  
• Cancer is the result of DNA mutations in somatic cells, generally requiring multiple mutations

  
• Cancer is not itself heritable, but increased risk is heritable
• Tumors


  • Benign: abnormal growth at primary site, don't spread
  • Malignant: capable of escaping primary site and spreading (metastasis)

  
• Effects of loss of growth control


  • Cancer cells grow without regard to normal cell cycle controls
  • Cancer cells take on different morphologies
  • Tumor suppressors guard genome integrity and prevent cell division with mutations
  • Loss of tumor suppressors increases mutation rate

  
• Anything that increases the risk of DNA mutations, increases the risk of cancer

  
• Chromosome rearrangements


  • Some are causative in cancer progression (BCR-ABL fusion)
  • Many are likely an effect of higher mutation rates in cancer

  
• Cancer cells often display genetic instability, a higher rate of genetic change than normal cells

  
• Tumor suppressors


  • Genes that help guard the integrity of the genome and prevent cell division when mutations are present

  
• Loss of function of tumor suppressor genes

  Increases the rate of mutation

  
• Increased rates of mutation

  Increases the risk of developing cancer

  
• Tumor suppressor genes


  • BRCA1/2 help repair DNA double strand breaks

  
• Mutations that increase the mutation rate promote cancer development

  
• Environmental factors can increase mutation rate
• In some cases, chromosome rearrangements are causative in cancer progression (BCR-ABL fusion protein)

  
• In many cases chromosome rearrangements are likely the effect of cancer (higher mutation rates)

  
• Spectral karyotype of a tumor cell

  • Chromosomes are color coded by WT chromosome identity
  • Chromosomes with multiple colors indicates rearrangements

  
• Transformation


  (in cancer biology) refers to cells becoming malignant, not the same as bacterial transformation

  
• Sometimes cancer appears in families due to inheritance of cancer risk
• Cancer often involves more than one mutation
• Those who inherit one mutation are at greater risk of developing cancer

  
• Multistep model


  Starting out with a mutation in a cancer pathway gives a "head start"

  
• Retinoblastoma


  • Rare and only affects one eye in most people
  • Common and affects both eyes in families carrying the first mutation in their germ line
• Proto-oncogene


  A gene whose gene product acts as a proliferative signal
• Oncogene


  A mutated proto-oncogene that acts dominantly to promote tumor formation

  
• Tumor suppressor


  A gene that normally acts to block cell proliferation