Lecture 28

• Cancer is a heterogeneous group of diseases characterized by cells that do not respond to normal controls on cell division

  
• Cancer develops via clonal evolution, which is a stepwise attainment of mutations

  
• These mutations lead to the gain of traits which most cancer cells share, sometimes referred to as the hallmarks of cancer

  
• Different cancers can attain these traits through mutations in different genes

  
• Initiation of cancer is usually through loss of growth disinhibition

  
• Increased cell proliferation occurs through loss of a tumor suppressor function or gain of function mutation in a proto-oncogene

  
• Mutations that occur late in cancer evolution often include genomic instability and mutations in p53

  
• Mutations in signal-transduction pathways, i.e. Ras, and chromosomal rearrangements are both cause and effect of cancer

  
• Cell division cycle and checkpoints

  1. Various checkpoints halt the cell cycle or trigger apoptosis if there are problems

  2. Cyclin dependent kinases (CDKs) form complexes with cyclins to regulate progression through these checkpoints

  3. The abundance of cyclins is tightly regulated throughout the cell cycle

  
• RB


  A tumor suppressor that binds to E2F and keeps it inactive

  
• Loss-of-function mutations in RB lead to uncontrolled progression into S phase

  
• People who are heterozygous for an Rb mutation are at elevated risk for developing a retinoblastoma, earlier, and in both eyes

  
• p53 controls the cell cycle

  1. DNA damage is sensed by DNA binding proteins

  2. p53 is phosphorylated

  3. Phosphorylated p53 turns up expression of p21

  4. p21 encodes a CDK inhibitor

  5. CDK inhibition halts the cell cycle at the G1/S & G2/M checkpoints until DNA damage is corrected

  
• Mutations in p53 prevent it from upregulating p21, so there is no halt on the cell cycle in the presence of DNA damage

  
• p53 activates pro-apoptotic pathways in response to DNA damage

  
• 50% of cancers have a mutation in p53
• Li-Fraumeni Syndrome


  A condition where people have a mutation in p53 and develop tumors much more often and much earlier than normal

  
• Due to the end replication problem, linear chromosomes shorten as they replicate, leading to permanent cell cycle arrest

  
• Telomerase is expressed in 90% of all cancers
• As a tumor's mass grows, its access to blood declines, so successful tumors will activate angiogenesis pathways that promote blood vessel growth to supply the tumor

  
• For a growth to progress from a benign tumor to a malignant tumor, it must gain the ability to move to new tissues, which requires mutations that alter the cytoskeleton or extracellular matrix enzymes

  
• The development of cancer in the colon begins with the formation of polyps, patches of abnormal growth, that demonstrate the multistep model of cancer

  
• Early screening matters! Removing polyps/lumps can head off malignancies
• Key innovations in molecular genetics include the discovery of the structure of DNA, recombinant DNA technology, PCR, DNA sequencing, and CRISPR-Cas systems
• Recombinant DNA


  An arrangement of DNA that is not found in nature, created by joining together two or more pieces of DNA
• Outline of gene cloning technique

  1. Isolate DNA of interest and cloning vector

  2. Cut with restriction enzymes

  3. Ligate cut fragments into cloning vector

  4. Transfer vector into host using transformation

  5. As host replicates, it makes copies of the recombinant vector

  6. DNA of interest can be recovered, analyzed, or expressed in the host

  
• Restriction enzymes


  Bacterial enzymes that bind to DNA at specific sequences and cleave the DNA
• Essential features of a cloning vector

  • Origin of replication
  • Selectable marker gene
  • One or more unique restriction sites

  
• Human insulin was the first recombinant biopharmaceutical approved in the US in 1982
• Forward genetics


  Identifying a causative gene
• Reverse genetics


  Finding genes involved in producing a phenotype
• Genetic engineering


  Applications of molecular genetic techniques