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70 Cards in this Set
- Front
- Back
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Somatic mutation Hypothesis of Cancer:
1) What is under genetic control 2) What causes malignant transformation 3) What is the tumor mass made of |
Cell growth, differentiation and survival are under genetic control
Malignant transformation is the result of mutations accumulating in a specific class of genes The tumor mass is from clonal expansion of one progenitor cell that incurred the damage |
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6 Phenotypic Hallmarks of Cancer:
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1) Dysregulation of cell proliferation (contitutive activation of growth stimulatory pathways/GFs) OR independence of proliferation signals
2)Insensitivity to growth inhibitory signals or loss of growth inhibition pathways 3) Evasion of apoptosis 4) Limitless potential to replicate 5) Angiogenesis 6) Invasion and metastasis |
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Genes in carcinogenesis:
Oncogenes and proto-oncogenes= Tumor suppressor genes = |
required for and promote cell proliferation
inhibit cell proliferation or block a step in cell cycle |
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Genes involved in apoptosis or cell senescence =
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pro or anti apoptotic proteins and immortalization genes (telomerase)
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Caretaker Genes =
Landscaper Genes = |
DNA repair enzymes (looking inward to cell)
(looking outward to environment) proteins regulating angiogenesis, cell-cell and cell-matrix adhesion, proteolytic enzymes for invasion |
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Definitions:
Proto-oncogene= |
encodes a protein that mediates or stimulates cell proliferation
inappropriately activated proto-oncogene via mutation or aberrant expression (over-expression or ectopic expression) |
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How many Oncogenes?
Types of proteins encoded by oncogenes? |
~100 identified
growth factors, GF receptors, signal transduction molecules, steroid hormone receptors, transcription factors, cell cycle proteins |
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Growth Factors: example v-cis
Growth Factor Receptor: example transmembrane tyrosine kinase |
oncogene v-sis is from proto-oncogene c-cis -> encodes PDGF (autocrine stimulated loop)
oncogenic versions of transmemb kinase receptors are overactive, stimulate mostly EGF-R family (EGF-R (c-erbB1) overexpressed in 80% squamous cell carcinoma of the lung) (HER2-neu (c-erbB2) amplified in breast, ovary, lung, stomach cancers) |
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Cell cycle proteins =
Signal Transduction molecules: (3) |
cyclins and CDKs
1) Non-receptor protein-tyrosine kinases (src) 2) Cytoplasmic serine/threonine kinases (raf) 3) GTP-binding proteins (ras) |
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ras:
role mutations (examples) |
critical in signal transduction from RTKs
Mutations = single most common abnormality of dominant oncogenes in human tumors Ki-ras: involved in lung, ovarian, colon and pancreatic cancers N-ras: involved in leukemias |
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How are proto-oncogenes activated?
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gain of function mutations
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What are gain of function mutations?
2 types and example for each |
Dominant mutation
1) Qualitative: Loss of regulatory elements via changes in gene structure -> abnormal gene product (oncoprotein) with uncontrolled, aberrant function (e.g. ras) 2) Quantitative: upregulation of a structurally normal growth promoting protein (e.g. breast cancer cells often produce excess cyclin D and cyclin E) |
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Ras mutation that increases ras signal -
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decreased GTPase activity (because ras is a g protein activated by a growth factor receptor)
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2 mechanisms of oncogene activation and examples:
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gene amplification e.g. MDM2 in soft tissue sarcomas
Chromosome rearrangement e.g. IgH and c-myc in Burkitt's lymphoma, or EWS gene rearrangement in some pediatric sarcomas |
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Bcr-Abl:
What is it, what does it cause? What is the translocation? What is the resulting protein? |
An oncogene in CML
First chromosomal abnormality ever linked to a specific cancer Translocation: t(9;22)fusion between the proto-oncogene c-abl and bcr Overactive tyrosine kinase activity and abnormal cellular localization |
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What is STI157?
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a pharmaceutical aka Gleevec which is the first drug to directly turn off the signal from the protein causing the cancer
It effectively lowered the white cell count during treatment |
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Insensitivity to growth inhibitory signals:
Tumor suppressor genes = |
inhibit cell proliferation
Inactivation or loss -> tumor development by eliminating negative regulatory signals |
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Evidence for tumor suppressor genes
What is most frequently observed genetic abnormality in solid tumors: Henry Harris's cell fusion experiment: did what suggests what Two-hot hypothesis of Alfred Knudson: |
deletion of genetic material
Fused one tumor and one normal cell -> a hybrid non-tumor cell Suggests that cancer is recessive trait explained that cancer is due to accumulated genetic mutations - children with retinoblastoma were born with one mutated gene, only needed one more mutation to develop disease (earlier onset, more affected) - adults with retinoblastoma needed two mutations to occur, more unlikely (older onset, less affected) |
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Loss of heterozygosity =
used to detect DNA sequences in various regions With one gene deletion PCR shows Where does LOH happen? |
- loss of Tumor Suppressor Genes (TSGs)
- Are usually slightly different (heterozygous) - One band - prevalence differs at different positions in genome, more prevalent at certain hot spots (TSG locations) |
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Possible ways to eliminate a normal gene:
Recessive mutations = |
nondisjunction -> chromosome loss
nondisjunction -> duplication mitotic recombination gene conversion deletion point mutation When Both copies need to be eliminated (as in tumors) |
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Cell cycle checkpoints
Involve what? Ensure what? Failure -> ? |
involve tumor suppressor genes
ensure no progression until preceding phase in completed with high fidelity apoptosis or genomic instability |
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RB/pRb =
process: Regulated by: Mutation of Rb -> LOH plays a role in: |
primary regulatory protein of G1/S phase transition
Rb represses transcription of many genes involved in cell cycle progression and DNA synthesis Regulated by cyclinD/CDK compelxes -> these phosphorylate and inactivate pRb -> release of critical transcription factors inactivation, removing a key regulator of cell cycle progression -> retinoblastoma Retinoblastomas, osteosarcomas, SCLC (small cell lung cancer), adenocarcinomas |
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Cyclin-dependent kinase inhibitors: (e.g.)
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induce cell cycle arrest
e.g. CDKI p16INK4a induces G1 arrest by inhibiting CDK4 and CDK6 when cyclins are bound to CDKs they phosphorylate serine and threonine and drive the cell cycle forward, when cyclins decrease they are no longer bound, CDKs are no longer active and the cell cycle halts |
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p53 =
responsible for process |
"Guardian of the genome"
a transcription factor that is responsible for - arrest of cell cycle (so DNA damage can be repaired before replication/division) - apoptosis upon DNA damage p53 activates transcription of CDK inhibitor p21 -> p21 blocks cell cycle in G1 and G2 -> allows time for damaged DNA to be repaired |
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Loss of p53 ->
involved in how many cancers? Most common genetic alteration for? Li-fraumeni syndrome = |
increased mutation frequencies, general genome instability
up to 50% of cancers sporadic human malignancies genetic mutation in p53 gene -> increase susceptibility to cancer |
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How does p53 trigger apoptosis?
Is p53 always recessive? |
by activation of transcription of pro-apoptotic genes (e.g. Bax)
No, can be dominant unlike other TSGs via a Dominant Negative Mutation in one allele that prevents the function of the other allele (usually by physical association such as dimerization) |
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why are p53 levels upregulated in cancer?
MDM2? |
because many mutations increase p53 half life on top of reducing p53 function
altered MDM2 binding, ubiquitinization? |
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Viral Oncogenesis:
how? e.g. - |
some DNA viruses have transforming sequences not derived from cellular genes
code for proteins that complex with and inactivate p53 - HPV E6 binds to and promotes p53 degradation - HPV E7 binds pRb and displaces E2F transcription factors high risk genotypes found in 82-85% invasive squamous cell carcinomas of the cervix |
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Caretaker genes:
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DNA repair genes - maintain genetic stability and prevention of mutation
Don't directly promote tumorigenesis Inactivation -> increased gene mutation rate (all genes!) -> accelerated tumorigenesis |
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Caretaker genes examples:
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XP - nucleotide excision repair for UV light damage
BRCA1 and BRCA2 - recombinational repair for xray damage HNPCC - mismatch repair for replication errors |
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Evasion of Apoptosis
Apoptosis/stress pathway: steps and players in each step |
Triggers = chemo, XRT (radiation), hypoxia, genetic damage, GF or cytokine withdrawal, loss of cohesion/adhesion
Modulators = many factors/pathways including Bcl2/Bax family (Bax is part of the Bcl2 family, promotes apoptosis by competing with bcl2 proper Effectors: caspase cascade (essential for apoptosis, acts as a protease) Substrates: DNA, cytoskeleton (are destroyed by caspases) |
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Dysregulation of anti-apoptotic signals vs loss of proapoptotic signals: (examples)
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dysgreg of anti:
in 80% follicular lymphomas, there is a t(14;18) -> overexpression of Bcl2 (antiapoptotic) -> lymphocytes fail to undergo normal apoptotic death in lymphoid follicles Loss of pro-apoptotic: inactivated proteins can include PTEN, p53, Bas -> independence from survival factors |
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What are implications for therapy based on the lack of pro or increase of anti-apoptotic signals?
What are the most curable cancers? |
these mutations in the apoptotic pathway -> chemotherapy and radiotherapy resistance
cancers in which p53 is not mutated = hematopoietic and germ cell tumors |
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Limitless replicative potential
Senescence = How can the cell circumvent senescence? Crisis = Immortalization = |
cells have a finite replicative potential, and stop dividing after a certain number of divisions -> senescence
by disabling pRb (retinoblastoma tumor suppressor protein) and p53 cells that have circumvented senescence continue dividing until they enter crisis = massive cell death, karyotopic disarray, end to end fusion of chromosomes 1 in 10^7 cells will become an immortal variant that can multiple without limit |
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The role of telomeres in crisis:
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telomeres are a counting device, with each cell division they shorten by 50-100 BPs
This is because DNA polymerases can't completely replicate the 3' end during S phase Telomere erosion -> lose the ability to protect ends of chromosomal DNA End to end chromosomal fusions occur -> karyotopic disarray associated with crisis and death * therefore, normally cell death will occur after too many divisions due to loss of telomeres and crisis |
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Telomerase =
in connection with neoplasia - |
an enzyme that elongates telomeres, usually only present in germ cells and stem cells
telomerase levels generally higher in malignant than benign neoplasms Some malignant neoplasms don't have detectable telomerase -> suggests an alternate mechanism of telomere stabilization or lengthening |
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Angiogenesis
purpose: depends on: |
tumors MUST have blood supply to grow
local ratio of angiogenic inducers to anti-angiogenic agents |
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angiogenic inducer e.g.
normal rate of angiogenesis = "Angiogenic switch" a) in normal physiologic conditions caused by b)in pathologic conditions caused by |
VEGF
low to no vascular prolif Normal physiologic: Wound healing Normal development Physiologic hyperplasia Pathologic: tumorigenesis subclone develops ability to stimulate angiogenesis due to clonal progression |
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Tumors as angiogenesis inhibitors
what has been found to happen? why does this happen? |
the removal of certain tumors (breast, colon, and osteogenic sarcomas) has been shown to result in increased/rapid growth of distant metastases from different types of tumors
This suggests that e.g. a breast cancer can inhibit melanoma metastases Why? proposed mechanism is that the primary tumor secretes factors which directly inhibit vessel proliferation at secondary tumor sites called micrometastases via endocrine effects |
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Specific angiogenesis inhibitors:
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Angiostatin - from lewis lung carcinoma, 38kD internal fragment of plasminogen
(plasminogen is precursor of plasmin which is an enzyme that degrades fibrin clots) Endostatin - from a murine vascular tumor, a 20kD internal fragment of collagen XVIII |
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Antiangiongenic therapy:
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treat cancers with antiangiogenesis drugs, e.g. endostatin
this doesn't induce acquired drug resistance - where resistant cells will not die with treatment and create a new, resistant tumor Instead, all tumor cells are in need of nutrients from the blood in order to grow, and all could potentially be killed by this method |
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Invasion and Metastasis
How many tumor cells are in circulation and how often do they metastasize? |
millions are shed daily into circulation, these can even be detected in patients who don't develop overt metastases
Less than 1 in 10,000 circulating tumor cells survive to initiate a metastasis |
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Steps to metastasis (6):
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Dyscohesion: loss of cell-cell adhesion in tumor
Matrix Degradation: destruction of BM and ECM by tumor cells Cell motility: into capillaries Survival and adherence: to vascular wall Extravasation: out of capillaries Angiogenesis and Proliferation: forming metastasis |
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What causes loss of cell-cell dyscohesion in tumor?
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Loss of cadherins
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What do cadherins mediate/regulate?
What do they complex with and how? What results from a loss of cadherins? |
mediate homo-typic cell-cell interactions at adherence junctions, regulate cell proliferation, apoptosis, differntiation and cell motility via signaling pathways
Complex with cytoskeletons via alpha, beta, and gamma catenin (family of cytoplasmic proteins) cadherin loss is correlated with increased invasiveness and metastatic potential |
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What regulates cell-ECM attachments?
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integrins
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Integrins =
Focal adhesion kinase pathways = Integrin switching = |
transmembrane receptors for BM components and ECM molecules
intracellular pathways that regulate apoptosis, proliferation and cell motility altered integrin expression patterns by tumor cells which decreases adhesion to BM and increases adhesion to ECM -> increased migration over ECM |
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Matrix Degradation =
Process: secrete what? downregulate what? |
destruction of local BM abd ECM to invade underlying stroma
tumor cells secrete proteolytic enzymes (matrix metalloproteinases/MMPs and collagenases) Also, downregulate expression of tissue inhibitors of metalloproteinases (TIMPs) - natural inhibitors that control the activity of matrix metalloproteinases (MMPs) |
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Cell motility
Migration directed by: |
factors that regulate tumor cell adhesion and cytoskeleton activity
Autocrine and paracrine factors and Extracellular matrix factors |
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Autocrine and Paracrine regulators of Migration:
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Growth Factors (IGFs, FGFs, TGF-B): Stimulate tumor cell motility
Hepatocyte growth factor/scatter factor: ligand for c-met proto-oncogene receptor Cytokines: IL-8, Histamine |
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Extracellular Matrix factors regulating migration:
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Intact extracell molecules: vitronectin, fibronectin, laminin, type I collagen
Fragments of ECM molecules: released from matrix by MMPs |
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Adherence and vascular extravasation
Tumor cells interact with: What do these do for the tumor cell? What is responsible for adhesion to vascular membrane? |
fibrin, platelets and clotting factors
these protect tumor cells in circulation from immune and non-immune destruction (monocytes/macrophages, NK cells, activated T cells) They also facilitate attachment to endothelial cells probably integrins |
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Why is metastasis angiogenesis dependent?
Experimental model of this: |
immature and developing neovasculature in leaky and discontinuous allowing tumor cells to enter microcirculation
grow malignant human keratinocytes in immune deficient mice -> when VEGF-R is inactivated via blocking antibodies, angiogenesis is disrupted and invasion of malignant cells is prevented without decreasing cell proliferation -> functional reversion from malignant to benign phenotype |
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What determines distribution of metastases?
Most frequent location of metastases? What are exceptions to location? |
histologic type, anatomic location of primary tumor
first capillary bed encountered by the tumor cells (usually lung and liver - extensive vascular beds and slow flow) Some primary tumors preferentially metastasize to specific sites (e.g breast and prostate cancers to bone) |
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Soil and seed hypothesis:
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A receptor or binding site for a protein on the surface of the tumor cell may be present at a distant location -> acts as a homing mechanism
Then chemokines activate cell signaling pathways that support the metastatic focus |
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Therapies targeting metastasis:
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Anti-adhesive agents
Matrix metalloproteinase inhibitors Anti-motility agents |
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Anti-adhesive Agents:
what are they, what do they target? |
peptidomimetics and monoclonal Abs targeted at integrins
Vitaxin (antagonist of avB3) induces vascular cell apoptosis and inhbits angiogenesis by blocking endothelial cell-matrix interactions |
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Matrix metalloproteinase Inhibitors:
Blocks what? What are its properties in vitro and in vivo? |
blocks degradation of matrix and activation of proteases and growth factors
in vitro - anti-invasive in vivo - anti-angiogenic |
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Anti-motility agents:
examples and what do they block |
Taxanes block microtubule cycling
Carboxyamido-triazole inhibits calcium influx |
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what mutations are required for the development of cancer?
How do you map mutations in each morphological stage? |
mutations arising early are also expected to be present later with each successive stage marked by additional mutations
by isolating precursor lesions - Adenoma-carcinoma sequence |
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What mutations are present in benign tissue surrounding malignant?
List certain mutations occurring with high probabiliy at specific definable stages of colon cancer progression: |
many of the same mutations as malignant but lacks at least 1
Early in the Process of transformation - converting colonic epithelial cells to hyperproliferative state: loss of tumor suppressor gene APC Early ademona stage: hypomethylation of DNA Carcinoma in situ: activation of oncogene Ki-ras Later in disease with eventual development of metastastic colon cancer: Lose tumor suppressor gene DCC on chromosome 18q and Lose p53 as well |
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Epigenetics of cancer
genetic change = Epigenetic change = characteristics of epigenetic changes |
alteration in DNA sequence
heritable modification of the genome, doesn't involve change in DNA sequence -unlike classical genetics, these are reversible, act over large distances, usually result in gene silencing |
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epigenetics =
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the study of all heritable and potentially reversible changes in genome function that do not alter the nucleotide sequence within the DNA[1]. When a cell undergoes an epigenetic change, it is the phenotype of the cell that is affected. Epigenetic events during embryo development lead to the differentiation of fetal cells. The combined processes of fetal development and cell differentiation are called epigenesis. The term is also sometimes used as a synonym for the closely related topic of chromatin remodeling.
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DNA Methylation:
what is hyper and what is hypomethylated? |
Tumor suppressor genes are hypermethylated (gene expression is shut down)
vast areas including proto-oncogenes are hypermethylated (expression is activated) |
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Reversal of epigenetic changes
how can you revert malignant to non malignant phenotype without correcting genetic abnormality? |
animal models have achieved this by reprogramming the epigenetic changes associated with differentiation (differentiation therapy)
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Differentiation Therapy:
What is it? Benefits? |
Many tumor cells are arrested at a premature development stage, therefore unable to control their own growth
Differentiation therapy is an attempt to enhance cell differentiation to mature cells via cytokines (e.g with myeloid leukemia cells) Could be target cell specific and less toxic than chemo *the development of differentiation inducing agents to treat cancer has been limited to this date |
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Acute Promyelocytic Leukemia (APML) and All-Transretinoic Acid (ATRA)
What is the translocation that causes APML? What do these genes normally do? |
t(15;17)
-PML gene (15q22): involved in protein-protein interactions -RAR-a gene (17q21): transcription factor, mediates differentiation and growth-suppression effects in response to retinoic acid |
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What occurs as a result of the fusion?
What is the treatment? |
PML-RAR-a fusion may inactivate RAR-a-ligand complexes (dominant negative interaction)
ATRA is treatment, restores some RAR-a-mediated transactivation, induces DIFFERENTIATION! |
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Example of treatment for liposarcomas:
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agonist ligand for the nuclear receptor (ligand activated transcription factor) "peroxisome proliferator-activated receptor-gamma" induces terminal differentiation
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Why is it important to study the molecular aspects of cancer?
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to develop therapies that target deranged molecular pathways and signals, instead of relying on traditional chemotherapy and/or radiation therapy
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