Oncogenes And Tumour Suppressor Genes Flashcards
State the major functional changes that occur in cancer?
- Increased growth (angiogenesis)
- Failure to undergo programmed cell death (apoptosis) or senescence
- Loss of differentiation (including alterations in cell migration + adhesion)
- Failure to repair DNA damage (including chromosomal instability)
What are the factors that regulate cell numbers?
- Growth
- Apoptosis
- Differentiation
What are the 2 major types of mutated genes that contribute to carcinogenesis?
- Oncogenes
- Tumour suppressor genes
What is an oncogene?
- Mutant form of proto-oncogene involved in the control of cell growth
- Component of growth factor signalling pathways
- Product formed has dominant and increased activity
- Uncontrollable cell divison - cancer
What is a tumour suppressor gene and what happens if mutated?
- A gene that controls cell division
- Both genes for TS must be mutated
- Causes Loss of function
- Increased likelihood that a cell can become cancerous
How sarcoma was induced into a chicken
- Chicken with sarcoma in breast muscle
- Remove sarcoma and break up into small chunks of tissue
- Grind up sarcoma with sand
- Collect filtrate that has passed through fine pore filter, bacteria can’t pass through filtrate, viruses can pass
- Inject filtrate into young chicken
- Observe sarcoma in injected chicken
What is c-src + v-src?
- C-src = Cellular oncogene
- V-src = Proto oncogene altered form transduced by retrovirus
Describe the capture of c-src by retroviruses and what this leads to?
- Virus acquires fragments of genes -> including C-src from host -> integrated with viral sequence -> creation of V-src -> Product is IC tyrosine kinase -> phosphorylates cellular proteins + uncontrolled growth -> cancer
What is Explanation of the oncogene hypothesis?
Final line is basically the hypothesis
- Following infection, however, the v-sc oncogene was expressed at high levels in the host cell, leading to uncontrolled host cell growth, unrestricted host cell division, and cancer
- Various agents, including radiation, chemical carcinogens, and, perhaps, exogenously added viruses, may transform cells by “switching on” the endogenous oncogenic information
Describe viral oncogenesis
- DNA viruses causes lytic infection (causes the death of host) OR DNA replication with host (increases neoplastic transformation)
- Viral oncogenes transmitted by DNA or RNA viruses
Describe how this differs between DNA viruses + RNA
viruses?
- DNA: encodes proteins with environmental factors which initiates and maintains tumours
- RNA: Integrate DNA copies of their genomes into the genome of the host cell (contain transforming oncogenes) causes cancer in host
Describe how oncogenes can become activated? (Steps)
- Oncogenes code for proteins in growth factor signal transduction pathway
- Genes captured (by virus) + altered
- Via mutation, amplification/duplication, translocation + insertion
- Alteration of at least one allele
- Loss of response to growth regulatory factors
State the 4 types of proteins involved in the transduction of growth signals?
- Growth factors
- Growth factor receptors
- Intracellular signal transducers + Nuclear transcription factors
Describe what the oncogenes code for within the Growth factor signal transduction pathways?
- OG proteins function as growth factors (e.g.EGF), growth factor receptors (e.g. ErbB) + intracellular signalling molecules (Ras and Raf)
- Regulates cell proliferation + survival in response to GF stimulation
Describe what ras and raf do
- Ras + Raf -> activate ERK MAP kinase pathway -> induction of additional genes (fos) -> encodes potentially oncogenic transcriptional reg. proteins -> More proliferation -> Cancer
What are RAS proteins?
small GTPases bound to GDP in a neutral state
Describe the ras protein mechanism of action
- Binding of EC GF signal
- Increases recruitment of RAS proteins to receptor complex
- Recruitment increases Ras to exchange GDP (inactive Ras) with GTP (active Ras)
- Activated Ras initiates remainder of the signalling
cascade (mitogen activated protein kinases) - Targets phos. - TF = increased expression of genes for cell growth + survival
- Ras hydrolyzes TP to GDP fairly quickly, turning itself “off
Describe the oncogenic activaton of Ras protein with the specific mutations that occurs?
- Point mutations in codons 12, 13 and 61 -> loss of GTPase activity normally required to return active RAS to the inactive RAS GDP -> Constitutive activation - hyperactivation of Ras + cell cycle
What cancer can arise from mutations in codon 12 of ras protein?
- Val -> gly = bladder carcinoma
- Cys -> gly = lung cancer
Outline the MYC oncogene family
- Oncogene family
- C-MYC, MYCN + MYCL
- Encodes c-Myc, N-Myc + L-Myc (TF)
- Regulate transcription for 15% of genome
- Identified in avian myelocytomatosis virus
Outline The effects of the MYC oncogene family
Downstream effects
- Ribosome biogenesis, protein translation, cell-cycle progression + metabolism
- Cell proliferation, differentiation, survival, + immune surveillance
Describe how MYC oncogene is activated
Encodes a helix-loop-helix leucine zipper TF that dimerizes with partner protein, Max, to transactivate (increase) gene expression
Describe how MYC oncogene is overexpressed?
- MYC activated via foreign transcriptional promotors -> chromosomal translocation -> deregulation of oncogene -> relentless proliferation
Describe how MYC is activated in Burkitt’s lymphoma?
- Epstein barr virus associated with BL -> high grade lymphoma (2-16 yrs)
- BL: carries one of 3 chromosomal translocations involving Cr 2,14 OR 22 ->Region formed with section of Cr 8 -> Places MYC gene under regulation of Ig heavy chain -> deregulates c-myc expression
Describe the logic of Tumour suppressor gene
- A balance between growth promoting factors (Proliferation, Cell survival) and intrinsic tumour suppressor pathways (Cell cycle arrest, Apoptosis)
- Like other well designed control systems, biological systems follow a similar logic component. Promoting a process must be counterbalanced by others that oppose the process tumour suppressor genes
Describe features of Tumour suppressor genes
- TSG products act as stop signs to uncontrolled growth, promote differentiation or trigger apoptosis
- Regulators of cell cycle checkpoints (e.g. RB1), differentiation (e.g. APC) or DNA repair (e.g. BRCA1)
Describe the loss of TSG function
- Loss of TSG function requires inactivation of both alleles of the gene
- Inactivation can be a result of mutation or deletion
- TSG defined as recessive genes and ‘anti-oncogenes’
What is retinoblastoma?
- Rare childhood cancer
- When immature retinoblasts continue to grow very fast and don’t turn into mature retinal cells
- Eye contains a tumour which reflects light back in white colour (cat’s eye appearance - leukocoria)
State the mutation of retinoblastoma
- Two forms of the disease, familial (40%) and sporadic (60%)
- The hereditary mutation is on chromosome 13 (13q14), the retinoblastoma 1 (Rb1) gene
Describe the two-hit hypothesis of retinoblastoma
- Rb gene = TSG - mutations of both functional copies required -> Loss of function mutation (point or small) -> inactivates TSG -> disrupts function
- OR
- Loss of heterozygosity -> inactivates the second copy of TSG -> A heterozygous cell receives a second hit in remaining functional copy -> thereby becoming homozygous for mutated gene.
Describe the structure of the retinoblastoma protein RB structure and state its main binding partner?
- The Rb gene family includes three members: Rb/(p105/110), p107 and b2/p130 - collectively known as pocket proteins
- A transcriptional co factor that can bind to TF
- N-terminus, large pocket (small pocket (spacer) + C-terminus)
- Main binding partner is the E2F TF, interacting with the large pocket
Describe the main function of Rb within the cell cycle?
- Regulates cell cycle
- Inhibit G1 to S phase transition
What can lead to the phosphorylation of PRb?
- Cyclin D is the first cyclin to be synthesized and drive progression through G1 together with cdks4/6
- The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage
- Cyclin D (key substrate), E families + cdks phosphorylate RB
Describe the function of the retinoblastoma protein
- PRb regulates activity of E2F TF (expresses genes for S phase)
- Rb activity regulated via phosphorylation
Describe the activity of the retinoblastoma protein
- Dephosphorylated -> active -> Remains bound to E2F -> blocks progression to S phase and inhibits cell proliferation
- Hyperphosphorylated (via EC physiological signals) -> Inactive -> E2F released -> migrates to nucleus to induce transcription for Genes of S phase -> Cell cycle progression from G1 to S
Describe ways in how Rb can become inactivated and within retinoblastoma?
- Phosphorylation, mutation or viral oncoprotein binding
- Retinoblastoma -> pRb -> mutations OR partial deletions
- Viral inactivation found in small DNA tumour viruses -> disrupting E2F binding or destabilisation of Rb
- Cancer cells -> RB phosphorylation deregulated -> E2F TF induces deregulation of cell cycle -> cell move through G1 into S + not subjected to usual check
Describe the role of P53 and features of when P53 mutation arises from tumour cell genome?
- TSG -> Senses DNA damage + regulates cell death/apoptosis
- Frequent mutation of P53 - tumour cells try to eliminate P53 function before thriving
Describe the structure of a P53 gene?
Contains: Amino transactivation domain, a central DNA binding Domain, a tetramerization Domain + carboxyl regulatory Domain -> TF
What regulates P53 levels?
- Normally P53 levels decrease
- Kept via MDM2 protein - ubiquitin ligase (oncogene)
How does this occur?
- MDM2 binds p53
- Complex formed in nucleus
- MDM modifies the carboxyl terminus of p53 + targets it degradation via proteasome
- Wildtype p53 has a short 20 min half life
How is P53 tumour suppressor gene activated?
- Stress signals sensed by kinases
- Phosphorylation of p53
- Activated p53 disrupts the interaction between it + MDM2
- Regulates genes involved in DNA damage repair, apoptosis and cell cycle arrest
State an example of a stress signal that induces activation of Ras?
ionizing radiation signals through two kinases ATM/ATR activate oncogenes (ras) induce activity of p14arf responsible for sequestering MDM2.
State factors that can be targeted for P53 mutation/therapeutic strategies?
- Mutational inactivation (>1/2 of human cancer carry loss of function of p53), correcting P53 mutation, restoring wild-type P53 function by targeting its regulators
- 95% of mutations were detectable within the DNA-binding domain
Describe therapeutic strategies for TP53?
- Gene therapy
- Retroviruses integrate in stable form into genome of infected cell - retrovirus-mediated gene transfer of wildtype TP53 into human lung tumour cell lines + xenograft model - inhibition of tumour cell growth
- Use of inhibitors
State and describe inhibitors used for P53?
- PRIMA-1 - Restores mutant P53 by modifying thiol groups in core domain of protein
- Nutlin - Potent MDM2 antagonist
- RITA - binds P53 + restores mutp53 activity
- CRM1 inhibitor - nuclear accumulation of P53
