5.3

• Cancers


  Evolving populations
• Learning Objectives


  • To provide evidence that cancers are a genetic disease
  • To know evidence that cancers are typically derived from a single cell (clonal population)
  • To understand that cancer cells undergo both Darwinian evolution (undergoing natural selection) and random drift during subsequent cell divisions
• Mutagens


  • UV/Alkylators
  • X/gRays
  • ROS
  • Virus
• Cancers are a genetic disease
• Cancers are typically derived from a single cell (clonal population)
• Cancer cells undergo both Darwinian evolution (undergoing natural selection) and random drift during subsequent cell divisions
• DNA mutations cause cancer
• Causes of DNA mutations that lead to cancer

  • Environmental Mutagens (Genotoxic)
  • Inheritance of cancer predisposition gene variants
  • Loss of DNA repair
  • Errors in mitosis (leading to aneuploidy)
  • Epigenetics (gene regulation)
• Environmental Mutagens


  Genotoxic - X/yRays (double stranded chromosome breaks), UV radiation and alkylators (base changes/point mutations), ROS (DNA oxidation, variety of mutations), Oncogenic viruses (add oncogenic genes/proteins and cause insertional mutagenesis)
• Inheritance of cancer predisposition gene variants

  1. 10% of all cancers, dependent on the cancer type, Examples: RB1 (Retinoblastoma syndrome), TP53 (Li Fraumeni syndrome)
• DNA repair


  Protective, normal process to repair damage to DNA, Strand breaks and point mutations can be repaired efficiently if detected in damaged cells before mitosis
• 25% heritable mutations in DNA repair machinery genes e.g. BRCA1, TP53, Mismatch Repair (MMR) defects lead to development of colon, stomach, uterine cancers (syndromes), These DNA repair machinery genes are also dysfunctional in sporadic cancers
• Errors in mitosis


  Highly mitotic cells are more likely to develop cancers, Can lead to aneuploidy, Copy number changes (loss/deletion) are seen in genes that normally pause the process of cell cycle and mitosis e.g. RB1 and TP53
• Epigenetics


  Changes to gene expression through regulation by promoter or histone modifications - methylation and acetylation of DNA bases or histones, Can be inherited from a cell to its daughters - permanent effect
• Some cancers have very low rates of mutations but they're still cancers, Can be driven through epigenetic changes
• Cancer cells


  Clones - A population of daughter cells descended from a single progenitor cell, Genetically related - but new mutations can be gained, or lost, from a cell leading to tumour lesions being genetically and phenotypically heterogenous, Most cancers are descended from a single precursor cell hence they're monoclonal
• Evidence of Clonality


  • X-inactivation (females have random X-inactivation, but tumours have the same X chromosome inactivated)
  • Antigen Receptor Gene (IGH, TCR) Recombination (lymphomas have monoclonal VDJ gene recombination)
  • Mitochondrial Inheritance (all cells can carry the same mitochondrial genome mutation)
• Tumour Evolution


  1. Spatial intratumour heterogeneity (not every tumour cell in a lesion is exactly the same, genetically and phenotypically)

  2. Cancer cells evolve through collection of gene variants and changes to their gene expression in order to adapt to and survive their environment (stress/treatment)

  3. Evolution also allows cells to successfully metastasise and seed new sites in the body
• Darwinian Evolution applies to cancer cells
• Tumour evolution


  • If you take multiple small biopsies of the SAME tumour mass and analyse the genetic and epigenetic information you will detect spatial intratumour heterogeneity - not every tumour cell in a lesion is exactly the same, genetically and phenotypically
  • Cancer cells evolve through collection of gene variants and changes to their gene expression in order to adapt to and survive their environment (stress/treatment)
  • Evolution also allows cells to successfully metastasise and seed new sites in the body
• Metastasis


  The spread of cancer cells to new locations in the body
• W. WHEWELL: 'Bat with regard to the material weed, we can at least go so far as this-we can perceive that events are levoght about not by Insolated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws.'
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• Natural selection


  The process whereby organisms better adapted to their environment tend to survive and produce more offspring
• Survival of the fittest

  The continued existence of organisms which are best adapted to their environment, with the extinction of others, as a concept in the Darwinian theory of evolution
• Normal epithelial cells


  • Have tight junctions and correct regulation of cell cycle and apoptosis checkpoints, requirement for growth factors to promote proliferation, adherence to basement membrane
• Tumour evolution


  1. Mutation in one cell confers freedom from normal growth control mechanisms

  2. It proliferates without restraint, and does not die

  3. During proliferation, new mutations can occur, some of which confer a growth, survival and invasion advantage - degrading the basement membrane

  4. Further mutations occur. Although cells are related to a single progenitor (i.e. they are clonal), the tumour cells are mixed for phenotype and genotype. Each new population has a survival advantage for the environment it encounters
• Clonal evolution


  Acquired genetic lability permits stepwise selection of variant sublines and underlies tumor progression
• Clonal succession theory


  Overgrowth of tumours with clones that have a survival advantage
• Tumour evolution theory


  Replaces clonal succession theory
• Phylogenetic tree


  • Trunk - shared, originating mutations present in every cancer cell - often called Founder, Clonal or driver mutations in oncogenes or tumour suppressors (key 'cancer genes')
  • Branches - early subclonal but common mutations
  • Twigs/leaves - recent, 'private' mutations present in a small subpopulation of cells
• Forms of evolution


  • Selective - In response to external environmental pressures or therapy induced selection. Allows survival of cells that have a proliferative/survival or resistance advantage.
  • Neutral - Gene Variants, often called passenger mutations, that are non-beneficial to the cell but maintained in genome. This is called Random Drift.
• Clonal tumour cells in a lesion/patient change in proportions in response to selective pressures
• A tumour mass at one time is NOT the same as at another time (Temporal Heterogeneity) -> tumours change and evolve to survive
• Methods to follow tumour evolution

  • Tissue biopsies throughout clinical care - patient morbidity
  • Autopsy analysis - end of life analysis - fixed timepoint
  • Liquid biopsy - minimally invasive analysis of tumour-shed DNA in biofluids
• Liquid biopsy


  • Analysis of cells in a biofluid = liquid aspirate/cytology
  • Cell-Free Tumour DNA (ctDNA): Gene variants are specific to tumour cells (cancer is a genetic disease!)
  • Captures information from all lesions in a patient (including small tumours that might not be visible by imaging or accessible for core biopsy)
  • Can track a single 'driver' mutation present in every tumour cell in the body to monitor overall tumour mass
  • Can identify mutations that can be used to select molecularly targeted systemic treatment
  • Can track emergence of resistance mutations in response to treatments
• EGFR therapy #1


  Leads to reduction in tumour size but emergence of resistant clone, through a new mutation, that overgrows the tumour (ctDNA allows early detection)
• EGFR therapy #2


  Again tumour shrinks but another new resistance mutation occurs and becomes dominant-present in all resistant cells which have a survival advantage