Chapter 8 Notes

• Process of tumour formation generally requires decades to reach completion
• Risk of cancer increases exponentially with age


  Example: Risk of individuals dying from colon cancer grows with age. If RR of 10 year old = 1, RR of 70 year old is 1000

  
• Normal cells evolve into cells with increasing neoplastic (cancerous) phenotype through the process of tumour progression

  
• Tumor progression is driven by randomly occurring mutations and epigenetic alterations of DNA

  
• Cancer incidence increases with age

  
• Disease incidence


  The rate at which the disease is diagnosed
• The process of tumor formation requires years to develop

  
• Both graphs indicate that cumulative exposure to a carcinogenic stimulus determines the likelihood of developing a detectable tumor

  
• Tumour progression is driven by a sequence of randomly occurring genetic and epigenetic alterations in proto-oncogenes and TSGs

  
• Process by which normal cells evolve into cells with increasing neoplastic (cancerous) phenotype
• Normal intestinal wall


  • Epithelial cells are anchored to the basement membrane
  • The growths are composed of various cell types in malignant tissue
• Colon adenocarcinoma is developed through a series of intermediate growths = precursor lesions

  
• Polyp


  Early adenomatous crypts can diverge into one of 2 pathways: Small tubular adenoma & Villous adenoma

  
• Removal of benign polyps during colonoscopy: 80% reduction in colon cancer incidence

  
• Hyperplasia


  Increase in epithelial cell division, which results in the production of thicker epithelial cells
• Dysplasia


  The epithelial layers are arranged in an orderly manner, this results in the formation of polyps and subsequently, adenoma

  
• Polyps can also be referred to as adenomas, which are benign growths
• Cells accumulate genetic and epigenetic alterations as tumour progression proceeds

  
• Assessing the status of proto-oncogenes and TSGs

  1. Isolate DNA from normal colon, precursor lesions and carcinoma

  2. Mutations, loss, amplifications, translocations?

  
• LOH (Loss of Heterozygosity)

  Indicative of a tumor suppressor gene in this region

  
• Many of the steps in tumor progression are driven by heritable alterations that accumulate in the genome of tumor cells

  
• Genetic Alterations of Tumor Progression

  • Activation of proliferative signalling pathways (Wnt, MAPK)
  • Inactivation of growth suppressor pathways (TGF-β)
  • Inactivation of the p53 pathway

  
• APC


  80% of colon carcinomas suffer inactivation of the tumor suppressor gene on chromosome 5q

  
• DPC4/SMAD4


  Only about 35% of colon carcinomas acquire a K-ras mutation

  
• Example of alternative paths during pancreatic cancer progression: Ras-Raf-PI3K signaling

  
• In vitro experimental modeling of multi-step tumourigenesis

  1. Activating oncogenes and inactivating TSGs in normal cells (gene transfer)

  2. Assessing the resulting effects on transformation and tumorigenicity

  
• In vivo experimental modeling of multi-step tumourigenesis

  1. Generation of transgenic mice (i.e. with activated oncogenes or inactivated TSGs)

  2. Assessing the resulting effects on tumour formation in mice

  
• Expression Vector (Plasmid)


  A. Increase expression: 1. Introduce a gene into cells (overexpression of transgene), 2. Drug selection

  B. Decrease expression: 1. RNAi (siRNA, shRNA, miRNA), 2. CRISPR/Cas9
• siRNAs


  Double stranded RNAs, 21-22 nt in length, ~100% perfect match to its target mRNA, Cleaves mRNA
• CRISPR/Cas9 gene disruption
• The chimeric mice is a mixture of modified and unmodified cells
• Germ-line: the modified cells can be transferred to progeny, resulting in pure genetically modified mice
• Normal cells are resistant to transformation with a single oncogene
• Monozygotic twins who develop the same type of leukemia share common chromosomal marker, derived from the same clonal precursor, difference in time of disease onset
• Initiating somatic mutations not sufficient to trigger formation of clinically detectable tumour

  
• Cell transformation to tumour phenotype requires collaborative effects of more than one oncogene
• Soft agar assay (anchorage independence as transforming ability): Single oncogene = no change in anchorage independence, Multiple oncogenes = increased anchorage independence
• Synergistic effect of combining oncogenes results in potent effects on survival curve in transgenic, cancer-prone mice
• Human cells are highly resistant to immortalization and transformation
• Rodent fibroblasts spontaneously immortalize in vitro, easily transformed, while human cells do not spontaneously immortalize in vitro and cannot be transformed unless immortalized