MOLECULAR BASIS of CANCER • NON-lethal genetic damage • A tumor is formed by the clonal expansion of a single precursor cell (monoclonal) • Four classes of normal regulatory genes – PROTO-oncogenes – Oncogenes Oncoproteins – DNA repair genes – Apoptosis genes
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MOLECULAR BASIS of CANCER NON-lethal genetic damage A tumor is formed by the clonal expansion of a single precursor cell (monoclonal) Four classes of normal.
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MOLECULAR BASISof CANCER
• NON-lethal genetic damage• A tumor is formed by the clonal expansion
of a single precursor cell (monoclonal)• Four classes of normal regulatory genes– PROTO-oncogenes– Oncogenes Oncoproteins– DNA repair genes– Apoptosis genes
• Carcinogenesis is a multistep process
TRANSFORMATION &PROGRESSION
• Self-sufficiency in growth signals• Insensitivity to growth-inhibiting signals• Evasion of apoptosis• Defects in DNA repair: “Spell checker”• Limitless replicative potential: Telomerase• Angiogenesis• Invasive ability• Metastatic ability
3Hanahan and Weinberg, Cell 100: 57, 2000
Apoptosis
Oncogenes
Tumor Suppressor
Inv. and MetsAngiogenesis
Cell cycle
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ONCOGENES
• Oncogenes are mutated forms of cellular proto-oncogenes.
• Proto-oncogenes code for cellular proteins which regulate normal cell growth and differentiation.
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Class I: Growth Factors
Class II: Receptors for Growth Factors and Hormones
Class III: Intracellular Signal Transducers
Class IV: Nuclear Transcription Factors
Class V: Cell-Cycle Control Proteins
Five types of proteins encoded by proto-oncogenes participate in control of cell growth:
Mutations that confer these properties fall into two categories
• Oncogene• : a cancer-causing gene that has been mutated to cause an
increase in• activity, or the activity becomes constitutive, or a new
activity is acquired.• -a mutation in a single allele is sufficient to transform cells
(dominant).• -originally identified as viral proteins that resembled
normal human proteins.• -the term "proto-oncogene" refers to the normal protein
that has not been mutated
• tumor Suppressor gene• : cancer-causing gene that has been mutated
to cause a loss of activity.• -mutations are required in both alleles to
transform cells (recessive)
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4 types of genetic mutations that contribute to cancer
• Categories of oncogenes• A. Growth factors• -generally not directly involved
transformation, but increased expression seen as part of
• an autocrine loop due to changes in other steps in the same pathway
growth factor receptors
• -They are transmembrane proteins with an external ligand binding domain and an
• internal tyrsosine kinase domain.• -oncogenic mutations can result in
dimerization and activation in the absence of • ligand• -more commonly, increased activity is a result
of overexpression of receptors
Growth factor receptors
• They are transmembrane proteins with an external ligand binding domain and an
• internal tyrsosine kinase domain.• -Oncogenic mutations can result in
dimerization and activation in the absence of • ligand• -More commonly, increased activity is a result
of overexpression of receptors.
signal transducers
• -Activated directly or indirectly by growth factor receptors
• -Activation of signal transducers triggers a phosporylation cascade that ultimately
• results in changes in gene expression at the transcriptional level.
• -mutations in RAS• , a GTPase, are the most common oncogenic
abnormality in tumors• -failure to hydrolyze GTP locks RAS in its active form.
Transcription factors
• -Transcription factors contain DNA binding domains.
• Sequences• Regulate expression of genes essential for
passage through the cell cycle, or• regulation of apoptosis.
• -
Normal CELL CYCLE Phases
INHIBITORS: Cip/Kip, INK4/ARF
Tumor (really growth) suppressor genes: p53
cyclins and cyclin-dependent kinases
• -cyclins are only expressed at specific stages of the cell cycle
• -cyclin-dependent kinases are expressed constitutively, but must bind cyclins for
• activation; phosphorylation of target proteins essential for progression through
• cell cycle
Regulation of G1/S cell cycle transition
Cell cycle arrest at G1/S (in response to DNA damage or other stressors) is medicated through which gene?
p53 (levels of p53 under negative regulation by MDM2 and p14 ARF)
• a second level of control is achieved by CDK inhibitors
• -p21 family (broad specificity) and the INK4 (p16) family (CDK4/6
• specific)• -overexpression of cyclin D and CDK4 common.• -phosphorylate and inactivate • Rb
CategoryPROTO- Oncogene
Mode of Activation
Associated Human Tumor
GFsPDGF-β chain SIS Overexpression Astrocytoma
OsteosarcomaFibroblast growth factors
HST-1 Overexpression Stomach cancer
INT-2 Amplification Bladder cancer
Breast cancerMelanoma
TGFα TGFα Overexpression Astrocytomas
Hepatocellular carcinomas
HGF HGF Overexpression Thyroid cancer
CategoryPROTO- Oncogene
Mode of Activation
Associated Human Tumor
GF ReceptorsEGF-receptor family
ERB-B1 (ECFR)
Overexpression Squamous cell carcinomas of lung, gliomas
ERB-B2 Amplification Breast and ovarian cancers
CSF-1 receptor FMS Point mutation Leukemia
Receptor for neurotrophic factors
RET Point mutation Multiple endocrine neoplasia 2A and B, familial medullary thyroid carcinomas
PDGF receptor PDGF-R Overexpression Gliomas
Receptor for stem cell (steel) factor
KIT Point mutation Gastrointestinal stromal tumors and other soft tissue tumors
CategoryPROTO- Oncogene
Mode of Activation
Associated Human Tumor
Signal TransductionProteins
GTP-binding K-RAS Point mutation Colon, lung, and pancreatic tumors
H-RAS Point mutation Bladder and kidney tumors
N-RAS Point mutation Melanomas, hematologic malignancies
Nonreceptor tyrosine kinase
ABL Translocation Chronic myeloid leukemia
Acute lymphoblastic leukemia
RAS signal transduction
BRAF Point mutation Melanomas
WNT signal transduction
β-catenin Point mutation Hepatoblastomas, hepatocellular carcinoma
CategoryPROTO- Oncogene
Mode of Activation Associated Human
TumorNuclear Regulatory Proteins
Transcrip.activators
C-MYC Translocation Burkitt lymphoma
N-MYC Amplification Neuroblastoma, small cell carcinoma of lung
L-MYC Amplification Small cell carcinoma of lung
2) Activation Growth-Promoting OncogenesWhich signal transduction pathway is continuously activated by mutant RAS?
MAP kinase pathway
Point mutations of ras are seen in what % of all human malignancies?
15-20%
Tumor supressor gene
• . Tumor suppressor were originally identified as inherited mutations that confer a
• predisposition to cancer (familial form).• -inheritance is dominant, meaning a single
defective allele is sufficient to confer• the predisposition
• Inactivation of tumor suppressors can occur • Sporadically• -sequential inactivation of both alleles in somatic cells• You may hear the term • haploinsufficiency• , which refers to inactivation of a single• allele contributing to malignancy.• -usually not the initiating event, but exacerbating.• Viral inactivation• -HPV expresses proteins that inhibit Rb and p53 function.
P53 and RASp53
• Activates DNA repair proteins
• Sentinel of G1/S transition
• Initiates apoptosis• Mutated in more than
50% of all human cancers
RAS• H, N, K, etc., varieties• Single most common
abnormality of dominant oncogenes in human tumors
• Present in about 1/3 of all human cancers
RB gene
• a.Loss of RB function confers a predisposition to retinoblastoma.
• occurs in both the familial form (early onset) and sporadic fromthe basis for tissue specificity of some tumor suppressors is unknown, but
• presumably is due to the transcriptional profile of the tissue, determined by tissue
• function
P53
• p53 is the most commonly mutated gene in tumors
• -over 50% of all tumors lack functional p53• -• Li-Fraumeni syndrome• : inheritance of a single defective copy of p53
results in a • predisposition to a wide spectrum of cancers.• -p53 is a transcription factor.
1: Failure of DNA Repair (acquired)
Normal function of p53 is to upregulate activity of which 2 genes to allow repair of DNA?
p21
GADD45
• Unlike Rb, p53 inhibits G1 progression only in response to DNA damage
• -normally p53 is very unstable, due to proteolytic degradation triggered by
• mdm2• .• -p53 is phosphorylated in response to DNA damage; mdm2 no
longer binds p53• -p53 upregulates expression of p21, which in turn inhibits G1/S
CDKs.• c. In response to excessive DNA damage, p53 can trigger
apoptosis
• Some other tumor suppressors found to be inactivated in tumors inhibit proliferation by
• various mechanisms:• -APC: degradation of • b• -catenin, a transcriptional activator anchored to E-cadherins• -NF-1: activates GTPase activity of ras• -TGF-• b• receptor: a tyrosine kinase that upregulates expression of CDK
inhibitors• -
• -PTEN: dephosphorylates inositol phospholipids, which act as docking sites for