Lecture 15 (Matuschewski) Flashcards
Cancer Biology
Cancer Cells
Cancer Cells
- Each body cell normally behaves in a socially responsible manner: resting, growing, dividing, differentiating, or dying—for the good of the organism.
- Cancer cells bypass normal proliferation controls and colonize other tissues
- Most cancers originate from a single aberrant cell
Cancer Phenotypes
Cancer Phenotypes
- “Neoplasia”: Cells grow as compact mass and remain at their site of origin (Benign = Gutartig)
- Malignant phenotype (actual cancer): An uncontrolled proliferation of cells, resistant to apoptotic death, and ability to invade host tissues and metastasize to distant sites.
- Metastasis: Cells lose their origin and spread through the tissue to form secondary tumors
5 hallmarks of cancer cells
5 hallmarks of cancer cells
- Altered homeostasis that results in cells growing and dividing at a faster rate than they die
- Bypass of normal limits to cell proliferation
- Evasion of cell-death signals
- Altered cellular metabolism
- Manipulation of the tissue environment to support cell survival and to evade a deleterious immune response
- Escape of cells from their home tissues and proliferation in foreign sites (metastasis)
Chromosomal translocation in chronic myelogenous leukemia (Philadelphia chromosome)
Chromosomal translocation in chronic myelogenous leukemia (Philadelphia chromosome)
- In cancer patients there are portions of chr22 in chr 9 and chr 22 is much shorter
- created by a translocation between the long arms of chromosomes 9 and 22
- shows genetic instability
Causes of cancer
Causes of cancer
- Chemical carcinogenesis: mutagens: Benzene, Arsenic, Aflatoxin
- Physical carcinogenesis: UV radiation, asbestos
- Infectious pathogens: HPV, HBV, EBV
Stages of tumor progression
Stages of tumor progression (Cervix)
- At each stage of progression, some individual cell acquires an additional mutation or epigenetic change that gives it a selective advantage over its neighbors, making it better able to thrive in its environment
- an environment that with low levels of oxygen, scarce nutrients, and the natural barriers to growth presented by the surrounding normal tissues
- Normal Epithelium: In a stratified squamous epithelium, dividing cells are confined to the basal layer.
- Low-Grade Intraepithelial Neoplasia: dividing cells can be found throughout the lower third of the epithelium; the superficial cells are still flattened and show signs of differentiation
- High-Grade Intraepithelial Neoplasia: cells in all the epithelial layers are proliferating and exhibit defective differentiation
- Invasive Carcinoma: cells move through or destroy the basal lamina that underlies the epithelium and invade the underlying connective tissue (initial step in metastasis)
- Next stage would be Metastasis: cancer cells spread from the primary tumor to distant sites in the body via blood, lymphatic vessels, or direct invasion, forming secondary tumors
Clonal evolution during tumor progression
Clonal evolution during tumor progression
- A tumor develops through repeated rounds of mutation and proliferation
- At each step, a single cell undergoes a mutation that either enhances cell proliferation or decreases cell death, so that its progeny become the dominant clone in the tumor
- Most cancers have minimum 2 up to 8 mutations that develop over the course of 20 - 30 years
- Transformed cells no longer require all of the positive signals from their surroundings that normal cells require and fail to recognize some negative influences
- Loss of contact inhibition (proliferation depends on contact with the dish and is inhibited by contacts with other cells, Cancer cells disregard these restraints and continue to grow, so that they pile on top of one another)
The Warburg effect in tumor cells
The Warburg effect in tumor cells
- Non-proliferating cells oxidize most glucose to produce ATP via oxidative phosphorylation in mitochondria.
- When deprived of oxygen, these cells generate ATP through glycolysis, converting pyruvate to lactate to regenerate NAD⁺.
- Tumor cells produce abundant lactate even with oxygen present due to a high rate of glycolysis and increased glucose import.
- Tumor cells resemble embryonic cells, which require glucose for biosynthesis and building blocks.
- This metabolic rewiring supports the rapid proliferation of cancer cells
The Tumor microenvironment
The Tumor microenvironment
- Tumors consist of many cell types, including cancer cells, endothelial cells, pericytes (vascular smooth muscle cells), fibroblasts, and inflammatory white blood cells.
- Communication among these and other cell types plays an important part in tumor development
- Tumor establishes an immunosuppressive microenvironment by blocking the activation of white blood cells that could lead to its destruction ⟶ Therapeutic target
Oncogenes (Cancer-critical genes)
Oncogenes (Cancer-critical genes)
- All genes whose alteration can contribute to the causation or evolution of cancer
- Proto-oncogenes: act in a dominant manner: a gain-of-function mutation in a single copy of the cancer-critical gene can drive a cell toward cancer
- Tumor suppressor genes: act in a recessive manner: the function of both alleles of the cancer critical gene must be lost to drive a cell toward cancer (no affect of mutation in one gene copy)
- Both the mutations lead toward cancer either directly (by causing cells to proliferate when they should not) or indirectly (by causing genetic or epigenetic instability) and so hastening the occurrence of other inherited changes that directly stimulate tumor growth.
Ways Genes Mutated in Cancer Can Be Made Overactive
Ways Genes Mutated in Cancer Can Be Made Overactive
- A pointmutation or deletion may produce a hyperactive protein when it occurs within a protein-coding sequence or lead to protein overproduction when it occurs within a regulatory region for that gene
- Gene amplification events, such as those that can be caused by errors in DNA replication, may produce extra gene copies; this
can lead to overproduction of the protein
- A chromosomal rearrangement involving the breakage and rejoining of the DNA helix—may either change the protein-coding region, resulting in a hyperactive fusion protein, or alter the con
trol regions for a gene so that a normal protein is overproduced.
IGF receptor
IGF receptor
- Receptor for the extracellular signal protein: epidermal
growth factor (EGF) is normally activated by EGF binding to its extracellular domain, triggering phosphorylation of its intracellular domain to regulate cell signaling.
- Can also be activated by a deletion that removes part of its extra cellular domain, causing it to be active even in the absence of EGF leading to hyperphosphorylation and inappropriate activation of signaling, which can drive uncontrolled cell growth and cancer.
Identification of tumor suppressor genes: Retinoblastoma
Retinoblastoma
- The first tumor suppressor gene, Rb, was discovered through studies of Retinoblastoma, a rare eye cancer.
Forms:
- Hereditary: Multiple tumors in both eyes due to one defective Rb gene copy in all somatic cells. Cancer develops when the second copy is lost.
- Nonhereditary: A single tumor in one eye. Rare, as it requires two independent inactivation events in the same retinal cell.
- Some individuals have a deletion on chromosome 13, linked to both hereditary and nonhereditary forms, suggesting loss of a critical gene causes cancer.
- Mapping this deletion led to the discovery of the Rb gene.
Rb Gene:
- Encodes a protein regulating the cell cycle by inhibiting E2F, which controls S-phase gene expression.
- When Rb is phosphorylated, it releases E2F, allowing the cell cycle to progress. Loss of Rb causes uncontrolled cell division.
- Inactivation occurs via epigenetic silencing, point mutations, deletions, gene conversion, or aneuploidy.
Oncogenes vs tumor suppressor genes detection
Oncogenes vs tumor suppressor genes detection
- DNA sequencing enables a survey of the genome for regions that have undergone deletion or duplication to reveal copy number variations and the loss or gain of chromosomes (aneuploidy)
- Focus on sequencing the genes in the human genome that code for protein (the so-called exome), looking for mutations in the DNA that alter the amino acid sequence of the product (missense) or prevent its synthesis (truncating = shorten)
- Oncogene mutations can be detected by the fact that the same nucleotide change is repeatedly found among the missense mutations in a gene
- Predominated by tumor suppressor genes missense mutations that abort protein synthesis by creating stop codons
Types of mutations in cancer
Types of mutations
- Passenger mutations: mutations that happen to have occurred in the same cell as the driver mutations, due to genetic instability, but are irrelevant to the development of the cancer.
- Driver mutations: Causal factors in the development of the disease. They fall into 3 core cellular processes (pathways) ⟶ Cell Survival (p53), Cell Cycle (Rb) & Cell Proliferation (Ras)
- Driver mutations will be seen repeatedly, in many individuals with a particular type of cancer.
- Passenger mutations is likely to be encountered only rarely.
- Third group of medium-impact putative passengers and undetected weak drivers
- Single Base Substitution is the majority of alterations per tumor (others are Indels, Amplifications, Deletions, Translocations)
- Up to 70 mutations occur in one tumor
The tumor suppressor p53
The tumor suppressor p53
- The p53 protein is a cellular stress sensor which rises from a low level in healthy cells to a high level in cells undergoing Stress in form of DNA damage, Hypoxia, telomere shortening etc.
- p53 acts as a transcription regulator by inducing the transcription of p21, which encodes a protein that inhibits the cyclin-dependent kinase (Cdk) complexes required for progression through the cell cycle.
- This may arrest cell cycling in a way that allows the cell to adjust and survive, trigger cell suicide by apoptosis, or cause cell senescence
Cancer Research in genetically engineered mice
Cancer Research in genetically engineered mice
- To explore the function of a possible oncogene or tumor suppressor gene, one can make a transgenic mouse that overexpresses it or a knockout mouse that lacks it.
- then engineer mice in which the misexpression or deletion of the gene is restricted to a specific set of cells, to see whether and how tumors develop.
- Tumor growth can be tracked using fluorescent or luminescent markers in live mice.
Genetic heterogeneity in tumors
Genetic heterogeneity in tumors
Intratumoral Heterogeneity
- Describes genetic diversity within a single primary tumor.
- The tumor originates from “founder cells” and evolves into different subpopulations or “clones,” each with distinct characteristics.
Intermetastatic Heterogeneity
- Refers to differences between metastatic tumors at different sites in the same patient.
- For example, Metastasis 1 and Metastasis 2 in the liver may arise from the same primary tumor but evolve independently, resulting in different genetic profiles.
Intrametastatic Heterogeneity
- Refers to genetic diversity within a single metastatic lesion.
- A single metastasis can contain multiple clones, each with unique genetic changes.
Interpatient Heterogeneity
- Refers to genetic differences between tumors in different patients. - For example, Patient 1 and Patient 2 have tumors with distinct sets of clones, reflecting individual variations in tumor evolution.
Prime Targets for Cancer Therapy
DNA repair pathways
Prime Targets for Cancer Therapy I PARP activity
- Normal cells have two DNA repair pathways: Pathway 1 and Pathway 2.
- Pathway 1 repairs DNA single-strand (ss) breaks using PARP (an enzyme).
- Pathway 2 repairs double-strand (ds) breaks through homologous recombination involving Brca2 and Rad51 proteins.
- Brca1 and Brca2 genes were discovered as mutations that predispose humans to cancer.
- In tumor cells, repair Pathway 2 is often inactivated due to Brca gene mutations, making them dependent on Pathway 1 for DNA repair.
- Drugs that block PARP activity (Pathway 1) kill Brca-deficient tumor cells by preventing DNA repair, while normal cells are unharmed because they still have a functional Pathway 2.
Prime Targets for Cancer Therapy II
Imatinib (Gleevec)
Prime Targets for Cancer Therapy II
- Imatinib (Gleevec) is a drug that inhibits the activity of the Bcr-Abl protein, a key driver of Chronic myelogenous leukemia (CML).
- CML is caused by a chromosomal translocation that creates the Philadelphia chromosome, fusing two genes, Bcr and Abl, into a hybrid gene, Bcr-Abl.
- Abl is a tyrosine kinase involved in cell signaling. The substitution of the Bcr fragment for Abl’s normal sequence makes the resulting Bcr-Abl protein hyperactive.
- This hyperactive protein stimulates excessive proliferation of hemopoietic precursor cells and prevents apoptosis, causing an accumulation of white blood cells.
- Bcr-Abl is an obvious target for therapeutic attack and led to the invention of Imatinib (Gleevec), which specifically blocks the activity of Bcr-Abl by binding in the ATP-binding pocket of Bcr-Abl and thereby preventing it from transferring a phosphate group from ATP onto a tyrosine residue in a substrate protein
Prime Targets for Cancer Therapy III
Drug targets in the Ras–MAP kinase signaling pathway
Prime Targets for Cancer Therapy III
- Drugs ending in “mab” (monoclonal antibodies) target molecules outside the cell or on the cell surface, such as receptor tyrosine kinases (RTKs).
- Drugs ending in “nib” (small molecule kinase inhibitors) target proteins inside the cell, such as kinases like Raf, Mek, or Bcr-Abl.
Ras–MAP kinase signaling pathway
- Growth factor binds to RTK → receptor phosphorylation → signal cascade starts.
- Signal activates Ras (GTP-bound state).
- Ras activates Raf (a kinase).
- Raf activates Mek (downstream kinase).
- Mek activates Erk, which drives gene expression, cell proliferation, and survival.
