Molecular Oncogenesis Flashcards
What is the molecular genetic basis of cancer?
A combination of acquired (somatic), inherited (germline), and other infections/epigenetics all contribute to the formation of cancer
**Nonlethal genetic damage lies at the heart of carcinogenesis
What are the characteristics of oncogenes?
- Drug targets
- One mutated gene (dominant; only need to lose one copy to lose function) “DRIVER MUTATION”
- Constitutive activation (fuel)
- Growth factors/receptors
- Signaling molecules
- Transcription factors
Contrast a proto-oncogene and oncogene
Most oncogenes began as proto-oncogenes, normal genes involved in cell growth and proliferation or inhibition of apoptosis. If normal genes promoting cellular growth, through mutation, are up-regulated, (gain of function mutation) they will predispose the cell to cancer and are thus termed oncogenes
What are the characteristics of tumor suppressor genes?
- Two mutated genes (recessive; need to lose BOTH copies for loss of function) “Loss of heterozygosity”
- Loss of function (brakes)
- Tumor suppressors (TP53)
- Cell cycle controllers (RB)
What are two examples of small genetic lesions?
**They are simple to detect
- Single nucleotide variant (e.g. BRAF, KRAS, NRAS, EGFR)
Can cause CRC, thyroid, melanoma, lung - Insertion/deletion “Indel” (e.g. EGFR -> lung cancer)
What are two examples of large genetic lesions?
**Complex to detect
- Copy number variants (e.g. HER2 amplification, 1p/19q deletion) Causes breast cancer/glioma
- Structural variants (translocations e.g. EWSR1 -> ewing sarcoma)
What are the types of single nucleotide variants?
- Silent (AA not changed; clinically benign and not reported)
- Missense (AA changed; benign, uncertain, or pathogenic; MD determines if reported)
- Nonsense (Stop codon instead of AA; pathogenic and always reported)
What is the usual tumor progression? How long does this take?
Transformation of normal to tumor cell -> progression -> proliferation of genetically unstable cells -> tumor cell variants; heterogeneity -> clonal expansion of surviving cell variants
**decades while cancer develops
**only ~2 yrs from invasive presentation to death
**clinically detectable cancer ~6 months prior to death for invasive pancreatic cancer
What are driver mutations?
Mutations that alter the function of cancer genes (oncogenes), primarily responsible for the cancer phenotype
**represent therapeutic targets
What is a passenger mutation?
Acquired mutation that does not contribute to cancer phenotype but may synergize with driver mutations
Contrast the different ways receptors can affect cancer
- Normal receptors have transient activation (“lights on and off”)
- Mutant receptors are constitutively activated (“lights always on”)
- Overexpression of receptors results in increased sensitivity to growth factors (“lights turn on easier/need less stimulation”)
**tumors can synthesize and secrete their own GF (paracrine loop)
Describe the major characteristics of the ErbB1 gene
- EGFR mutation; predicts responsiveness to anti-EGFR TKI
- mutation causes lung cancer
- overexpressed in many cancers
- targeted drug therapy= erlotinib
Describe the major characteristics of ErbB2
- Also called Her2/Neu
- Amplification in breast cancer; poor prognostic sign
- Predicts lack of response to estrogen therapy
- Targets drug therapy= trastuzumab
What is KIT?
KIT mutation is seen in GIST (GI stromal tumors) and can be specifically targeted by the drug imatinib (a CKIT tyrosine kinase inhibitor)
What are signal transducing proteins?
- couple receptors to nuclear targets
- mutations “short circuit” the receptor
- constitutive signalins is observed in cancer
- E.g. RAS, BRAF
What is the rule of thumb for ATP and GTP hydrolysis?
ATP hydrolyzed for work
GTP hydrolyzed for signaling
Describe the RAS oncogene family
RAS are GTP binding proteins; mutations affect GTP binding/hydrolysis (lock RAS in permanent activation)
Describe the effects of mutations in BRAF and 2 drugs used for targeted BRAF therapy
BRAF is a ser/threonine protein kinase that is activated by Ras
There are mutations in BRAF in 100% of hairy cell leukemias and 60% of melanomas
**Targeted BRAF therapy by vemurafenib and dabrafenib
Describe “oncogene addiction” and one example
When a cancer is dependent on the fusion/crossing of genes;
E.g. Normally ABL oncogene (a non-receptor tyrosine kinase) is at the end of a normal chromosome and not expressed… cross-over switches ABL from 9 to 22 and allows its constitutive expression thanks to the BCR locus ->
Chronic myelogenous leukemia (CML) or
Acute lymphoblastic leukemia (ALL)
**Targeted therapy= imatinib
What is the role of transcription factors in cancer?
TFs are the endpoint of signal transduction and oncogenes lead to the continuous stimulation of TFs (drive expression of growth promoting genes)
**MYC, MYB, JUN, ROS, REL
What is the result of the t(8;14) MYC mutation?
Increased MYC protein -> increased expression of pro-growth genes (upregulation of cyclin D; stimulates cell cycle progression)
**observed in burkitt lymphoma
Describe the cell cycle regulators
- Cyclin dependent kinases (CDKs)
- constitutively expressed
- phosphorylate target proteins
- Cyclins
- transient expression, unstable
- activate CDKs
- **CDK + cyclin= cell cycle advances
- Cyclin dependent kinase inhibitors
- inhibit CDK
- **CDK + CDKI= cell cycle delay
**Tumors upregulate CDK/cyclins and downregulate CDKIs
Describe the 2 major checkpoints in the cell cycle
- G1 -> S (CDK4/6 and cyclin D allow passage through G1 restriction point; inhibited by p16)
- G2 -> M (CDK1/cyclin B)
How does the G1-S checkpoint work? (what protein?)
The RB protein is either…
-
hypophsophorylated
- stimulated by growth inhibitors such as TGFbeta/p53
- keeps transcription factor E2F near it, preventing S phase gene expression -> cell stays in G1
-
hyperphosphorylated
- stimulated by growth factors such as EGF/PDGF
- allows E2F to dissociate, travel to DNA and allows S phase gene transcription -> cell enters S phase)
How is the RB protein affected in cancer?
The antiproliferative effect of RB is gone in cancers by one or more of the following:
- loss of function mutation affecting RB
- overexpression of CDK4/6 and cyclin D genes
- loss of cyclin dependent kinase inhibitors (p16)
- E7 protein of HPV binds and inhibits RB
How is p53 the “guardian of the genome”?
- When cellular damage occurs, ATM/ATR family senses it and activates p53 (via release of MDM2)
- p53 arrests the cell cycle (p21) until the damage is repaired (GADD45)
- If damage can’t be repaired then apoptosis occurs (BAX)
- p53 is inactivated by viral oncoproteins (E6 protein of HPV)
**Most cancers have biallelic loss of function of TP53 (e.g. Li-Fraumeni syndrome inherits one defective copy of TP53 and has a very high incidence of a wide variety of cancers)
What are 3 p53 mediators and their functions?
- p21 (CDK inhibitor -> G1 arrest)
- GADD45 (DNA repair)
- BAX (apoptosis)
What is the function of MDM2?
MDM2 regulates the p53 dependent response to DNA damage (inhibits TP53… MDM2 inactivation/phosphorylation is required for TP53 response)
In what ways do HPV proteins increase one’s risk of cancer?
- Activation of TERT -> increased telomerase expression
- Inhibition of p53
- Inhibition of p21 -> increased CDK4/cyclin D -> inhibition of RB-E2F
- Direct inhibition of RB-E2F
What is the main homeostatic balance that is broken down in cancer?
Cell cycle control (RB) versus DNA damage sensor (p53)
What is the APC gene?
It negatively regulates the WNT pathway by promoting the degradation of beta catenin leading to no celllular proliferation
What is the result of APC mutation?
**APC= tumor suppressor gene (requires “two hits” for loss of function)…
- beta catenin accumulation
- WNT activation -> cellular proliferation
- upregulation of cyclin D1, MYC, TWIST, and SLUG
- reduced E-cadherin -> loss of contact inhibition (lobular breast cancer)
What proteins are commonly mutated in gliomas?
Isocitrate dehydrogenase 1/2 (IDH1 and IDH2); normally catalyze isocitrate -> alpha KG + NADPH
*TET inhibits DNA methylation, 2-HG inhibits TET, IDH mutation upregulates 2-HG -> increased DNA methylation
**NADPH helps glutathione formation and normally provides protection from oxidative stress… glioma mutations are all missense
What is the prognosis for a patient with an IDH1 mutation?
Although they develop gliomas, they have a better long term survival rate than cancer patients with wt IDH1
What is the result of upregulating the BCL-2 gene?
BCL-2 overexpression leads to tumor progression due to inhibition of normal apoptosis (BCL-2 inhibits pro-apoptotic BAX/BAK) -> follicular lymphoma
What two general mutations would allow a tumor “limitless replicative potential”?
A tumor wants to get rid of p53 and restore telomerase action to allow continued proliferation
What determines the angiogenesis of a tumor?
Beyond 1 mm tumors require vascularization:
- Normoxia= VHL protein targets HIF1a for degradation
- Anoxia= HIF1a is stabilized and drives expression of angiogenic and proliferative genes
What effect can inflammation have on cancer?
Chronic inflammation can promote tumorgenesis
E.g. in Hepatitis B and C (chronic hepatocellular injury/inflammation stimulates proliferation -> hepatocellular carcinoma)
What is lynch syndrome?
A hereditary nonopolyposis colon cancer syndrome due to mismatched repair genes (MMR) leading to a mutator phenotype
**“Mutators” accumulate mutations in the genome and experience microsatellite instability
What are the two major mechanisms by which microbes can be oncogenic?
- Viral genomic integration (overexpression of viral proteins that affect host cell growth/ disruption of proto-oncogene)
- Stimulation of host inflammatory response with subsequent regeneration (Hep B/C, H pylori, schistosoma)
**Epstein Barr virus -> burkitts lymphoma
What is the mechanism by which EBV can cause cancer?
Normally, latent Epstein Barr Viral infection leads to polyclonal B cell expansion which is self limiting via CTLs
**B cells can undergo mutation (e.g. MYC translocation) and continue to replicate uncontrollably -> burkitt lymphoma
How can helicobacter pylori cause cancer?
Host inflammatory response leads to carcinogenesis -> gastric adenocarcinomas or MALTomas
**Treat with eradiation; kill bacteria and make tumor regress
What is the difference between direct and indirect carcinogens?
- Direct acting
- Highly reactive (electrophillic) and can react with any electron rich site; DNA, RNA, protein
- Do NOT require enzymatic processing
- e.g. alkylating agents
- Indirect acting/ Procarcinogens
- Metabolism by cytochrome P450 systems to activate
**BOTH have the same target; nucleic acid
What is an initiator carcinogen?
A chemical that causes permanent DNA mutations (directly or indirectly… cannot be repaired)
**DNA -> initiator -> mutated DNA -> promoter ->
replicated DNA
What is a promoter carcinogen?
A nontumorigenic chemical that enhances the proliferation of mutated cells; effect is reversible
**DNA -> initiator -> mutated DNA -> promoter ->
replicated DNA
