Lec 15- cancer aeitology

Question 1

Targeted cancer therapy

Answer 1

Question 2

Mutations and cancer

Genes implicated in mutation

Answer 2

• Main suspects
• Mutations in
  
  • Oncogenes
  • Tumour suppressor genes
  • DNA repair genes
• Other contributing genes (mechanisms)
  
  • Cell death genes
  • Cell signalling genes
  • Cell cycle checkpoint genes
  • Cellular differentiation genes
  • MetastasisInvasion genes
  • Carcinogen- activating genes, deactivating

Question 3

Oncogenes

Answer 3

• Growth factor receptor (EGFR), RAS, Src
• A gene (proto-oncogene- gene before ) that promotes cell growth
• When mutated or altered expression enhance tumour formation or growth
• Mutation usually acquired through DNA damage
• Dominant Genetically - only need to change one allele for the phenotype to show up in the cell
• Discovered as a result of virus induced tumours (vRas, vSrc)

Question 4

Oncogenes often encode proteins involved in cell signalling

There are 4 classes of oncogene

Answer 4

• Class 1- Growth factors e.g. sis B chain of PDGF (platelet derived growth factor)
• Class 2- Growth factor receptors e.g. erbB membrane receptor for EGF (epidermal growth factor)
• Class 3- Intracellular transducers e.g. abl tyrosine protein kinase, ras-G protein
• Class 4- Nuclear transcription factors e.g. fos, myc, myb

Question 5

Oncogenes can be activated

Answer 5

1. Chromosomal translocation
2. Excessive production of proteins
3. Point mutation

Question 6

All of these mechanisms lead to altered sequence and aberrant protein behaviour

How oncogenes are activated

Answer 6

• Simplest one is point mutation resulting in the activation of that gene
  
  • Constitutively activated= constantly turned on
• Gene amplification- the multiplication in the number of copies of the gene- excess protein being produced= excess signal being produced leading to excessive growth
• Chromosomal translocation
• Insertional mutagenesis- insertion of viral DNA into the cell
  
  • Leads to insertion of proteins and DNA which promotes the transcription of genes downstream causing the production of those genes
• Local DNA re-arrangement
  
  • Insertion
  • Deletion
  • Inversion or transposition

Question 7

Examples of proto-oncogenes that are signal transducers

Answer 7

• Ras is a G-protein that signals on the ras-MAPK pathway
• Its activity is regulated by guanosine nucleotides (GDP and GTP)
• ras binds to GTP and becomes active, ras has intrinsically phosphatase activity turning GTP=> GDP, GDP is then released with ras becoming inactivated
• Without constant GTP to activate, ras system will inherently be turned off, this is what we want because we don’t want this proliferation signal
• A point mutation causes constitutive activity- mutation causes loss of phosphatase activity, this means when GTP is bound and ras is active, ras can’t inactive itself so proliferation goes out of control
• This is a dominant process as even if there is one allele being mutated 50% of the proteins being produced will be constitutively activated
• Src is a cellular tyrosine kinase involved in signalling from EGF and PDGF receptors
• The truncated form has constitutive activity and causes sarcoma in chicken (Rous sarcoma virus)

Question 8

Tumor suppressors

Answer 8

• Tumour suppressor genes- RB1 (retinoblastoma), p53
  
  • Genes that control cellular growth and death as well as DNA damage repair
  • Turn off pathways, suppress pathways, this is another way of stopping pathways being turned on accidentally
  • Normal gene controls growth and death (apoptosis) of cells
  • Suppress tumour formation
  • Recessive- only when both alleles of the tumour suppressor gene are changed is when we see the effect within the cell
    
    • These genes can be inherited- one parent each giving mutated allele
  • Mutation usually acquired through DNA damage
  • Can be inherited (usually one allele)- second is acquired

Question 9

Rb (retinoblastoma) was the 1^st tumour suppressor discovered

Answer 9

• Rb binds to a nuclear transcription factor (E2F), when bound it prevents the transcription of particular genes
• Normal function = tumour suppressor- prevents activation of nuclear transcription factor
  
  • When mutated Rb gene can’t bind to nuclear transcription factor and so can be turned on far easier
    
    • Rb becomes phosphoralted therefore can’t bind E2F, so genes are transcribed

Question 10

DNA repair genes

Answer 10

• Mistakes frequently occur during genome replication
• Radiation and chemicals (including O₂) can induce damage
  
  • UV light can cause dimerisation of the thymine base = wrong base = mutation
  • There are conditions where patients (often children) often get melanoma’s (skin cancer) due to an error in DNA repair where they can’t identify T-dimers and remove them leading to mutations- xeroderma pigmentosum
• Mismatch repair proteins correct errors in copying
• Other proteins repair radiation and chemical damage
• Other proteins sense DNA damage and induce apoptosis- double strand breaks in the DNA causes apoptosis- virtually impossible to put the 2 parts of the DNA back together correctly
• Recessive
• Can be inherited or acquired

Question 11

Cell cycle checkpoints and cancer

Answer 11

• One of the sensing mechanism of DNA damage
• ATM and ATR sense different types DNA damage
  
  • ATM- DNA itself is damage
  • ATR- Replication fork arrest- when being copied there is a new strand being formed (known as replication fork)- if DNA is damaged the replication fork stops so no production of proteins
    
    • If either of these protein senses damage apoptosis occurs
• If ATR/M is faulty or doesn’t sense damage we introduce genomic instability- more likely to introduce errors into the DNA if there are already errors- accumulation of errors = more likely to become cancerous
  *

Question 12

Cell cycle checkpoints and cancer

Answer 12

Question 13

Cancer treatment

Answer 13

• Surgery- when solid
  
  • The mainstay of cancer treatment, and gives the highest chance of cure
  • Only useful if the tumour is localised
  • Defining boundaries can be difficult- need to remove as much as possible without causing excessive damage
• Radiotherapy
• Drug treatment (chemotherapy)

Question 14

Cancer treatment

When do we use drugs

Answer 14

• To prevent a tumour developing e.g. tamoxifen for healthy women at a high risk of breast cancer
• Before surgery to shrink the tumour (neoadjuvant)
• To kill any cancer cells remaining after surgery and radiotherapy (adjuvants)
• To treat inoperable cancer (e.g. brain), disseminated cancers (e.g. leukaemia) or metastasis

Question 15

More rapid growth and division than corresponding normal cells (through probably slower than GI and blood cells)

Targets

Answer 15

• DNA synthesis with anti-metabolites
• DNA replication and processing with alkylating agents
• Block metabolic pathways- need to produce more membranes, proteins etc all require energy if we can block this we stop cancer producing enough energy to multiply / function
• Alter stability of micro-tubles inhibit cell division (mitosis)

Question 16

Disrupted metabolism of cancer cells may lead to specific metabolic or nutritional requirements

Answer 16

• Asparagine is a necessary requirement for some leukaemic cells
• Shift towards biosynthesis- enhanced glucose uptake, lactate production and anabolism (Warburg effect)
  
  • Use asparaginase to degrade asparagine- cells stop growing
  • Inhibit pyruvate kinase M2 (Resveratrol)- inhibition of glycolysis in cancer cell so can’t make energy required to proliferate

Question 17

Nutritional requirements

Answer 17

• Asparaginase is injected for treatment of leukemias which are highly-dependent on dietary asparagine
• Asparagine supports rapid maligant cell growth
• Injected asparaginase metabolises blood asparagine withdrawing it from tumour cells, and causing asparagine-dependent tumour cells to die
• Normal cells provide enough asparagine through biosynthesis and are less affected this treatment

Question 18

Exploitable characteristics

chemicals

Answer 18

• Some chemicals become enriched in tumour tissue- often due to low pH
• Light-absorbing compounds are administered to tissue or whole body (some are selectively retained by tumours)
• Light is shone on the area and induces oxygen free radial oxygen formation: Photodynamic therapy (PDT)
  
  • Radicals damage tissue and induce immune response
    *

Question 19

Chromosomes ends are stabilized by repetitive sequence at end- TELOMERES

Answer 19

• Bits on the ends of chromosomes- that indicates how many times the cell has divided- with each division part of the telomere will be removed- before long you run out of telomere- once this happens apoptosis occurs
• Telomeres shorten with each cell division until a critical length reached
  
  • Cells then undergo cell cycle arrest (senescence) or apoptosis
• Somatic mutations block senescence induction, cells continue to divide and telomere erosion continues until cells stop dividing again- this 2^nd block is called “chromosomes crisis”
• The unprotected chromosome ends cause chromosome fusions, breakage, and re-arrangements and tumour development

Question 20

Telomeres

Answer 20

• Telomere length is maintained by a ribonucleoprotein called telomerase, which adds new DNA ends to chromosomes
• During development, telomerase activity is detected in almost all tissues
  
  • In most normal adult human somatic tissues it is repressed
• In adults, telomerase is significantly expressed in germline and stem cells of renewable tissues such as bone marrow, skin and GI tract
• Some cancer cells express active telomerase avoiding senescence (or apoptosis); becoming “immortal”

Question 21

Cancer cells often display different surface proteins/Ags

Answer 21

• Overproduction of HER2 in ~25% of metastatic breast cancer
• Abnormal Bcr-Abl tyrosine kinase encoded by the Philadelphia chromosome in chronic myeloid leukaemia
• Cancer cell specific cell surface Ag’s
• Specific cell surface proteins- are certain cancer cells producing certian Ag’s = target

Question 22

Hormone dependence

Answer 22

• Some cancers derived from hormone dependent tissue require the presence of the hormone to grow- particular breast, ovarian and testicular
• ²/₃^rds of all breast cancer are ER positive i.e. hormone receptor BC, characterised by dependence on estrogen for growth
  
  • Block the hormone receptors with anti-hormone (e.g. tamoxifen cyproterone acetate)
  • Swamp cells with excessive doses of hormones (downregulate receptors)
    *

Question 23

Hormones and hormone antagonist

Answer 23

• Prednisolone- A synthetic corticosteroid used in the treatment of blood cell cancers (leukaemias) and lymph gland cancers (Lymphomas)
• Stilboestrol (artificial oestrogen)- Effective against prostate cancer. Surprisingly effective against breast cancer in postmenopausal women
  
  • Oestrogen down-regulates cells in the prostate reducing growth
• Tamoxifen- Partial anti-oestrogen
• Flutamide- Anti-androgen: palliative hormonal treatment of advanced prostate cancer and sometimes in the adjuvant and neoadjuvant hormonal treatment of earlier stages of prostate cancer

Question 24

Hormones and hormone antagonist

Goserelin

Answer 24

• Gonadatropin-releasing hormone analogues and used to block hormone production in the ovaries or testicales
• Used to treat hormone-sensitive cancers of the prostate and breast

Question 25

²/₃^rds of all breast cancers are HR positive i.e.g hormone receptor BC

Answer 25

• 25 years, selective estrogen receptor modulator tamoxifen has been the mainstay in the hormonal treatment therapy of breast cancer
• This a non-steroidal compound with both anti-oestrogen and oestrogen agonist activity
• Recently, third-generation aromatase inhibitors/inactivators suppress estrogen production and are being integrated in adjuvant treatment regimens

Question 26

Tamoxifen

Answer 26

• Tumours of hormone-dependent tissue (e.g. breast, prostate may still be hormone dependen)
• Anti-hormone: antagonist of steroid hormone action
• They interact with the ligand-binding domains of steroid hormone receptors and competitively inhibit the action of receptors by mechanisms (not understood)
• Therefore, target the hormone receptors with anti-hormones or swamp the cells with excessive doses of hormones

Question 27

Tamoxifen

Answer 27

Question 28

Breast cancer therapy

Answer 28

• Anastrozole (Arimidex)
• Blocks conversion of aromatizable steroids to oestrogen by inhibiting the enzyme aromatase (reduced production of oestrogen)
• Compared to anti-oestrogens like tamoxifen- stop cancer cells proliferating but doesn’t kill them
  
  • No oestrogen agonist effects
  • Protecting bone density
  • Decreasing serum ChE
  • Increasing endometrial cancer risk
• Clinical use- hormone-dependent breast cancer in post-menopausal women
• Other aromatase inhibitors- Letrozole, Exemestane

Question 29

Microtuble formation

Answer 29

• The miotic spindle is assembled from tubulins and then disassembled when cell division is complete
• Vinca alkaloids prevent the assembly of tubulin dimers
• Taxanes prevent disassembly
• For deatails about microtubules, consult kimball’s biology pages under M

Question 30

Model of a microtuble

Answer 30

Answer 31

Question 32

P53 and cancer therapy

Answer 32

• P53 is a tumourSuppressor gene (regulates cell division)
• Mutations of this gene are associated with many types of tumour
• ~50% of human tumours lack a functional p53 gene
• The expressed p53 protein is a transcriptional regulator that is activated in response to DNA damage
• Wild-type p53 prevents progression of cell cycle until DNA damage is repaired
• Cell cycle progression increases mutations, chromosomal anomalies and aneuploidy
• This serves to increase the chance that apoptosis will be blocked

Question 33

p53 and cancer therapy

Answer 33

• When p53 mutates the cell cannot control cell division
• Cancer cells lacking p53 initially are more resistant to DNA damage; they stay alive and reproduce anyway
  
  • With continuing treatment, the DNA damage accumulates to the point of non-viability
• Hepatitis B virus X protein implicated in liver cancer development inhibits p53 tumour suppressor protein by cytoplasmic retention of p53 protein
• p53 deficiency confers resistance to doxo (doxorubicin), a clinically active and widely used anti-tumour anthracycline anti-biotic
• Anti-cancer drug, adriamycin restores p53 activity

Question 34

Potentially exploitable characteristics

Answer 34

• An angiogenic inhibitor is a drug or dietary component that inhibits angiogenesis i.e. the growth of new blood vessels
• Inhibitors include bevacizumab which binds vascular endothelial growth factor (VEGF)
• Inhibiting it’s binding to the receptors that promote angiogenesis
• Tumour outgrows it’s blood supply
• THEREFORE
  
  • Inhibit vascularisation
  • Design prodrugs activated by the reducing conditions in the hypoxic interior of the tumour

Question 35

= chromosome

The Philadelphia chromosome

Answer 35

• Chronic Myelogenous Leukemia (CML) is ony cancer to occur from a single mutation- most have multiple pathways
• C-abl is a gene on #9 and bcr is on #22
• During cellular division- chromosomes get shared (bits get chopped off and placed on another of the same chromosome- between male and female)
  
  • this can go wrong and a combination of c-abl from 9 and bcr from 22 makes an extra long chromosome with a combination of c-abl and bcr proteins
  • c-abl-bcr fusion protein is a kinase which is switched on when it shouldn’t be

Question 36

Glivec

Answer 36

• Imatinib mesylate
• Inhibits Bcr-Abl tyrosine kinase
• Inhibits proliferation & induces apoptosis in Philadelphia chromosome-positive CML cells
• Not entirely selective, but side effects are mild
• Binds near where the ATP would normally bind in the normal kinase
  
  • Normally ATP phosphorylates a substrate causing signalling when the drug is given this is prevented