♋ Lecture 17: Cancer Continued

Metastatic Cell Migration

Invadopodia Formation
 Metastatic cells form invadopodia to penetrate basement membranes and migrate along the extracellular matrix (ECM) away from the primary tumor

Chemotactic Attraction
 Cells can be attracted by signals such as epidermal growth factor (EGF), sometimes secreted by macrophages

Matrix Degradation
 Metastatic cells release matrix metalloproteases (MMPs) and other proteases to degrade the ECM, creating paths for themselves and other tumor cells

Extravasation
 Tumor cells adhere to blood vessel linings in a new location and migrate through the vessel wall to colonize underlying tissue
  Uses similar mechanisms as invasion of the primary tumor

Survival Rate
 Fewer than 1 in 10,000 cells that escape survive to form a secondary tumor

Basement Membranes
 Specialized ECM structures that separate cells from underlying connective tissue

Cancer Cell Karyotypes

Abnormal Karyotypes
 Cancer cells often exhibit highly abnormal karyotypes due to loss of genome stability mechanisms

Characteristics
 Abnormal copy number of most chromosomes
 Some chromosomes contain portions of other chromosomes

Caveat
 The figure shown is from a long-established colorectal adenocarcinoma cell line
 Many chromosomal defects may have accumulated over time in culture
 The original cell line was derived from a female patient

Oncogenic Driver Mutations in Cancer

Data Overview
 Analysis based on 2,583 cancer genomes

Key Concept
 Identifies oncogenic driver mutations that contribute to cancer development and progression
 Helps distinguish driver mutations from passenger mutations that do not confer growth advantage

Cancer Incidence and Age – Multi-Hit Model

Concept
 Cancer rates increase with age due to the Multi-Hit Model, where tumors arise through a recurring clonal selection process

Mutation Progression
 First Mutation
  Gives a cell a slight growth advantage

 Second Mutation
  Allows cells to grow more uncontrollably and form a small benign tumor

 Third Mutation
  Enables cells to overcome constraints imposed by the tumor microenvironment

 Fourth Mutation
  Allows cells to enter the bloodstream and establish daughter colonies at other sites, a hallmark of metastatic cancer

Development and Metastasis of Human Colorectal Cancer

Concept
 Example of the Multi-Hit Hypothesis explaining tumor development and progression

Genetic Basis
 APC = adenomatous polyposis coli protein
  (Not to be confused with Anaphase Promoting Complex)

Key Idea
 Mutations accumulate in specific genes like APC, driving the formation of colorectal tumors and potentially metastasis

Colorectal Cancer Development – APC Loss

Normal Colon Cells
 Cells grow and function normally, maintaining tissue architecture

Polyp Formation
 A polyp (small growth) forms on the colon wall

APC Gene Loss
 Loss of APC (adenomatous polyposis coli protein, chromosome 5)
  (Not the Anaphase Promoting Complex)
 APC function: suppresses cell growth via cell-to-cell contact and signal transduction pathways

Signaling Reminder
 Signaling by plasma-membrane-attached proteins involves a signaling cell and an adjacent target cell

Colorectal Tumor Progression – K-ras Activation

Polyp Formation
 A polyp (small growth) forms on the colon wall
 Develops into a benign, precancerous tumor

K-ras Oncogene Activation
 K-ras (chromosome 12) becomes activated, promoting growth of a class II adenoma (benign)

Ras Signaling Pathway
 Ras transmits growth signals from growth factors via RTK (receptor tyrosine kinase) receptors
 GRB2 binds activated receptor
 Sos promotes dissociation of GDP from Ras
 GTP binds Ras, activating it and allowing it to dissociate from Sos

Key Concept
 Activated Ras drives cell proliferation, contributing to tumor growth

Colorectal Cancer Progression – Malignancy and Metastasis

Malignant Transformation
 A malignant carcinoma develops from the benign adenoma

Metastasis
 The cancer spreads to other tissues, establishing secondary tumors

Genetic Changes
 Loss of p53 (tumor-suppressor gene, chromosome 17)
 Other genetic and epigenetic changes accumulate

p53 Function
 p53 pauses the cell cycle in response to DNA damage
 Can induce apoptosis to prevent propagation of damaged cells

Key Concept
 Loss of p53 removes a critical growth checkpoint, allowing malignant progression and metastasis

Cancer-Inducing Mutations

Normal Genes
 Proto-oncogenes and tumor suppressor genes are normal genes that regulate cell growth and survival

Proto-Oncogenes
 Examples: HER2, Ras, Myc, Fos
  Promote cell survival
  Suppress apoptosis
 Mutation converts them into oncogenes
  Can be gain-of-function or hyperactivating mutations

Tumor Suppressor Genes
 Examples: p53, Rb, APC
  Pause or slow down the cell cycle
  Induce apoptosis
 Mutation reduces or eliminates their function
  Can be gene deletions or loss-of-function mutations

Key Concept
 Cancer arises when mutations disrupt the balance of cell proliferation and death, enabling uncontrolled growth

Mutations Affecting the Same Pathway

Signal Pathway Overview
 Mutations in different components of a signaling pathway can lead to similar oncogenic outcomes

Gain-of-Function Mutations
 Occur in proto-oncogenes such as:
  Signal receptors
  Signal transduction proteins
 These mutations are oncogenic, causing hyperactive signaling

Loss-of-Function Mutations
 Occur in negative regulators of signal transduction (tumor suppressor genes)
 These loss-of-function mutations remove inhibitory control, also resulting in oncogenic signaling

Outcome
 Both types of mutations disrupt normal cell growth control
 Leads to abnormal proliferation and potential tumor formation

Receptor Tyrosine Kinases (RTKs) – Structure and Activation

Inactive State
 RTKs exist without a bound ligand
 ATP is present but the kinase is inactive

Activation Step 1: Dimerization
 Ligand binding induces dimerization of RTKs
 Phosphorylation of activation loop tyrosines occurs
 ATP → ADP, transferring phosphate groups (P)

Activation Step 2: Tyrosine Phosphorylation
 Additional tyrosine residues in the receptor are phosphorylated
 Creates docking sites for downstream signal transduction proteins

Key Concept
 Activated RTKs initiate intracellular signaling pathways that regulate cell growth, survival, and differentiation

Oncogenic Mutations in RTKs (Proto-Oncogenes)

HER2 Mutation
 HER2 (Human Epidermal Growth Factor Receptor 2) can undergo a point mutation replacing valine with glutamine in the transmembrane domain
 Mutated form = NEU oncoprotein
 NEU can dimerize and become active without ligand

EGFR Mutation
 EGFR (Epidermal Growth Factor Receptor 1 / HER1) can mutate to produce an incomplete receptor lacking the ligand-binding domain
 Mutated form = ErbB oncoprotein
 Can dimerize with other copies and become active

Key Outcome
 Both mutations lead to hyperactivation of Ras, driving cell proliferation and potential tumorigenesis

Valine-to-Glutamine Substitution

Genetic Basis
 A single nucleotide mutation can convert valine to glutamine

Codons for Valine (Val)
 GTT, GTC, GTA, GTG

Codons for Glutamine (Gln)
 GAG, GAA

Key Concept
 This point mutation in the HER2 gene can create the NEU oncoprotein, leading to ligand-independent activation and hyperactive Ras signaling

RTK/Ras/MAPK Pathway in Cancer

Pathway Components
 Includes RTKs, Ras, MAPK, and regulators like NF1 (tumor suppressor, neurofibromin)
 NF1 functions as a Ras GTPase, inactivating Ras

Mutations
 Mutations in proto-oncogenes of this pathway often make the component hyperactive or constitutively active (always on)

Key Concept
 Constitutive activation of the RTK/Ras/MAPK pathway drives uncontrolled cell proliferation, a hallmark of cancer