molecular biologyofca csbrp
TRANSCRIPT
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OncogenesOncogenesDr.CSBR.Prasad, M.D.,
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Four classes of normal regulatory genes
• Growth-promoting Proto-oncogenes, • Growth-inhibiting Tumor suppressor genes, • Genes that regulate Apoptosis, and • Genes involved in DNA repair
They are the principal targets of genetic damage
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Oncogenes encode proteins that control Cell proliferation,
Apoptosis, Or both
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Products of oncogenes
The products of oncogenes can be classified into six broad groups:
1.Growth factors2.Growth factor receptors 3.Transcription factors4.Signal transducers5.Apoptosis regulators and 6.Chromatin remodelers
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Activation of oncogenes
They can be activated by structural alterationsresulting from:• Mutation or gene fusion • by Juxtaposition to enhancer elements or • by AmplificationTranslocations and mutations can occur as initiating
events or during tumor progression, whereas amplification usually occurs during progression
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Transformation & Progression of cancerTransformation & Progression of cancer
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Transformation & Transformation & Progression of Progression of
cancercancer
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Two main properties of cancers
They are:• Autonomous &• Eternal
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Oncogenes
Genes that promote autonomous cell growth in cancer cells are
called oncogenes
In short, cancer causing genes
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Terms
• Proto-oncogenes
• Oncogenes
• Cellular oncogenes
• Oncoproteins
Genes that promote autonomous cell growth in cancer cells are called
oncogenes
Unmutated cellular counterparts of
oncogenes are called proto-oncogenes
Produced by oncogenes, resemble the normal products of proto-oncogenes except that oncoproteins are often
devoid of important internal regulatory elements, and their
production in the transformed cells does not depend on growth factors or
other external signals
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The sequential steps that characterize normal cell proliferation
Under physiologic conditions cell proliferation can be readily resolved into the following steps:
• The binding of a growth factor to its specific receptor • Transient and limited activation of the growth factor receptor, which, in
turn, activates several signal-transducing proteins on the inner leaflet of the plasma membrane
• Transmission of the transduced signal across the cytosol to the nucleus via second messengers or by a cascade of signal transduction molecules
• Induction and activation of nuclear regulatory factors that initiate DNA transcription
• Entry and progression of the cell into the cell cycle, ultimately resulting in cell division
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Proto-oncogenes, Oncogenes, and OncoproteinsProto-oncogenes, Oncogenes, and Oncoproteins
Proteins encoded by proto-oncogenes may function as growth factors or their receptors, signal transducers, transcription factors, or cell cycle components
Oncoproteins encoded by oncogenes generally serve functions similar to their normal counterparts
However, the oncoproteins endow the cell with self-sufficiency in
growth
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Growth Factors• Most soluble growth factors are made by one cell
type and act on a neighboring cell to stimulate proliferation - paracrine action
• Cancer cells, however, acquire the ability to synthesize the same growth factors to which they are responsive, generating an autocrine loop
• Note: – In most instances the growth factor gene itself is not altered or
mutated– They are forced to secrete large amounts of growth factor
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Growth Factors
Increased growth factor production is not sufficient for neoplastic transformationBut, growth factor driven proliferation may contributes to spontaneous or induced
mutations in the proliferating cell population
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Growth Factor Receptors• GF receptors are transmembrane proteins with
an external ligand-binding domain and a cytoplasmic tyrosine kinase domain
• GF binding results in dimerization of and tyrosine phosphorylation of several substances down the signalling cascade
• The oncogenic versions of these receptors are associated with constitutive dimerization and activation without binding to the growth factor
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Growth Factor ReceptorsGrowth factor receptors can be constitutively
activated in tumors by multiple different mechanisms:–Mutations–Gene rearrangements–Overexpression
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Growth Factor ReceptorsExample: RET oncogeneIn MEN-2A: mutations in the RET extracellular domain
cause constitutive dimerization and activation, leading to medullary thyroid carcinomas and adrenal and parathyroid tumors.
In MEN-2B: mutations in the RET cytoplasmic domain alter the substrate specificity of the tyrosine kinase and lead to thyroid and adrenal tumors without involvement of the parathyroid
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Growth Factor Receptors
Example: c-Kit, PDGFR oncogene• Greater than 90% of GIST have a
constitutively activating mutation in the receptor tyrosine kinase c-KIT or PDGFR, which are the receptors for stem cell factor and PDGF, respectively
• Targeted therapy: Imatinib
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Signal-Transducing Proteins
• Most such proteins are strategically located on the inner leaflet of the plasma membrane, where they receive signals from outside the cell (e.g., by activation of growth factor receptors) and transmit them to the cell's nucleus
Eg: RAS family of guanine triphosphate (GTP)-binding proteins (G proteins)
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Signal-Transducing Proteins - The RAS Oncogene
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Alterations in Nonreceptor Tyrosine Kinases
• Non-receptor-associated tyrosine kinases normally function in signal transduction pathways that regulate cell growth
• Mutations take the form of chromosomal translocations or rearrangements that create fusion genes encoding constitutively active tyrosine kinases
Eg: c-ABL tyrosine kinase: BCR-ABL fusion gene - constitutively active, oncogenic BCR-ABL tyrosine kinase - Imatinib
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ABL-BCR fusion in CML
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Transcription Factors
• A host of oncoproteins, including products of the MYC, MYB, JUN, FOS, and REL oncogenes, are transcription factors that regulate the expression of growth-promoting genes, such as cyclins
• Of these, MYC is most commonly involved in human tumors
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Transcription FactorsThe MYC Oncogene (Chr#8)
Range of activities modulated by MYC:
• > cell motility, • > telomerase activity, • histone acetylation, • < cell adhesion,• > protein synthesis, • < proteinase activity,changes that enable a high
rate of cell division
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Cyclins and Cyclin-Dependent Kinases
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END
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