Week 10: Cancer

Question 1

What is cancer?

Answer 1

• The uncontrolled/sustained proliferation of abnormal cells that destroy the function of normal healthy tissues in the body
• Cancerous cells have replicative immortality and tend to acquire further mutations with each division
• The sequential acquisition of mutations gives cancers their “hallmarks” which are particular properties/sequences that allow them to proliferate

Question 2

What are the 2 types of malignancies?

Answer 2

1. Solid malignancies:
   - Mass of abnormal cells, generally without cysts or liquid
   - The cancer cells are normally derived from epithelial cells
   e. g. Breast, colon, lung and prostate cancers
2. Haematological malignancies:
   - Non-solid abnormal cells
   E.g. Cancers of the blood, bone marrow and lymphatic system

Question 3

What are the differences between benign and malignant cancers?

Answer 3

1. Benign cancers:
   - Slow growing (but still hyperproliferative)
   - Encapsulated by a membrane
   - Non-invasive
   - Still resembles cell of origin
   - Have the suffix oma
   E.g. polyp in colon
2. Malignant cancers:
   - Fast growing
   - Cells are variable in size and shape and poorly differentiated (do not resemble cells of origin)
   - Non-encapsulated
   - Invasive
   - Metastasize
   - Have the suffix carcinoma or sarcoma

Question 4

What are carcinomas in situ?

Answer 4

• A form of cancer that lies between benign and malignant cancers
• Tend to evolve into malignant tumours
• They do not undergo cell invasion (like benign tumours) but have the characteristics of malignant tumours meaning they have the potential to undergo cell invasion

Question 5

What are the most common types of cancer?

Answer 5

• Cancers that originate from surface epithelium (papilloma/carcinoma) or gland/ductal epithelium (adenoma/adenocarcinoma)

Question 6

What is stroma?

Answer 6

• Stroma are the supportive cells around tissues such as connective tissue and fibroblasts
• Plays an important role in cancer by releasing growth factors and cytokines that allow tissues to continue to proliferate

Question 7

What are the microscopic characteristics of cancer cells?

Answer 7

1. Large number of dividing cells
2. Large variable shaped nuclei (due to genetic instability creating more genes/chromosomes)
3. Small cytoplasmic volume compared to nuclei (the cells do not spend enough time in G1 or G2 as the cell cycle checkpoints are typically lost)
4. Variation in cell size and shape
5. Loss of specialised cell structures (are undifferentiated)
6. Disorganised arrangement of cells
7. Poorly defined tumour boundary

Question 8

What factors can cause the development of cancers?

Answer 8

1. Environmental factors:
   - Radiation (UV, ionising)
   - Tobacco smoke
   - Obesity
   - Alcohol
2. Immune deficiency
3. Genetic factors
   - Inherited mutations in tumour suppressor genes such as BRACA1 and BRACA2
4. Viruses
   E.g. HPV: associated with cervical cancer, leads to the production of protein E6 which ubiquitinates p53 and targets it for degradation by the proteosome pathway

Question 9

List the 6 hallmarks of cancer and give an example of what can cause them:

Answer 9

1. Sustained proliferative signalling
   e. g. overexpression of growth factors
2. Evading growth suppressors
   e. g. via TGFB
3. Activating invasion and metastasis
   e. g. via MMPs
4. Enabling replicative immortality
   e. g. via telomerase
5. Inducing angiogenesis
   e. g. via VEGF
6. Resisting cell death
   e. g. via p53 loss

Question 10

How do cancers have sustained proliferation?

Answer 10

• This is due to a loss of proliferative regulation:
  1. Cancer cells can do this by secreting growth factors such as EGF (epidermal growth factor- the master regulator of cell proliferation) which act in an autocrine manner to increase tumour growth
  2. Cancer cells can also have increased EGF receptor expression
  3. Cancer cells can increase IGF secretion to stimulate growth factor production (paracrine)
  4. Loss of negative regulators that inhibit proliferation such as TFGBeta e.g. by downregulating the receptor for TGFB

Question 11

How do cancers evade growth suppressors?

Answer 11

1. Overcome the activity of tumour suppressors such as p53 and Rb via gene mutations:
   - p53 stalls the cell cycle and is usually increased in production when DNA is damaged)
2. Loss of negative regulators that inhibit proliferation
   e. g. Loss of TGFB
3. Overcoming contact-mediated inhibition of proliferation

Question 12

How do cancer cells resist cell death (apoptosis)

Answer 12

1. Loss of p53 tumour suppressor genes
2. Loss of pro-apoptotic regulators (Bax, Bad)
3. Gain of anti-apoptotic regulators (BCL2)
4. Overcoming signals from ligand-activated death receptors (Fas)

Question 13

How do cancer cells induce angiogenesis?

Answer 13

• Solid tumours require waste removal and nutrients/oxygen from growth greater than 1-2mm and this is achieved by inducing angiogenesis (the formation of new blood vessels)
  1. Tumours express pro-angiogenic factors such as VEGF and FGF
  2. Tumours suppress antiangiogeneif factors such as thrombospondin-1
  3. Angiogenic factors are upregulated by oncogene expression and also hypoxia

Question 14

How do cancer cells achieve replicative immortality?

Answer 14

• Telomeres are a region at each end of the chromosome that protects it
• As cells divide telomeres shorten, when telomeres shorten to a critical length the cell undergoes replicative senescene (and it stops dividing)
• Cancer cells often express the enzyme telomerase, which regenerates the telomeres

Question 15

How do cancer cells activate cell invasion and metastasis?

Answer 15

• Metastasis is a multistep process involving local invasion, intravasation into vessles, extravasation from vessels and then growth in distant tissue sites
• Cancer cells achieve this by activating EMT (epithelial to mesenchyme transition) which downregulates E-cadherin
• Cancer cells are increase expression/activation of proteins that promote cell invasion such as matrix metalloproteases (MMPs) and migration (Rac)

Question 16

What are the 4 new hallmarks of cancer that have been introduced?

Answer 16

1. Avoiding immune destruction
2. Tumour-promoting inflammation
3. Genoome instability and mutation
4. Deregulating cellular energetics: cancer cells favour glycolysis (even with O2)

Question 17

Describe the typical progression of solid cancers:

Answer 17

1. Primary tumour forms and establishes its own vascular network via secretion of VEGF
2. Cells undergo EMT and secrete MMPs which allow them to degrade endothelial blood vessel walls and enter the circulation
3. The cancerous cells are transported through circulation
4. The cancerous cells arrest in the microvessles of various organs e.g. lungs, brain and bone
5. The cancer undergoes extravasion and forms a micrometastasis in the new tissue
6. The cancer forms a macrometastasis in the new tissue that undergoes angiogenesis

Question 18

What is generally mutated first in a cancer, a tumour suppressor gene or a protooncogene?

Answer 18

• Tumour suppressor genes are generally mutated first e.g. p53
• Induction of oncogenes and epigenetic changes generally occur later

Question 19

What is an oncogene?

Answer 19

• A mutated version of a normal gene (proto-oncogene)
• Activation requires only 1 defective allele
• The mutations are gain of function and dominant
• Most protooncogenes are receptor tyrosine kinases

Examples:

1. Ras (signal transducer)
   - pancreatic, lung and colon
2. BRAF (signal transducer)
   - melanoma
3. Abl (signal transducer)
   - CML
4. HER2 (growth factor receptor)
   - breast cancer

Question 20

What are tumour suppressor genes?

Answer 20

• Normal cellular genes required for hometostasis that act as ‘breaks’ on cancer development/progression e,g. by restricting cell division, repairing DNA and inducing apoptosis
• Involves loss of function mutations to both alleles of the gene (recessive)
• Mutation of one allele is followed by the deletion of epigenetic silencing of the second (2 hit hypothesis)

Examples:

• BRACA1/2 (DNA repair genes)
• Rb
• p53
• APC (signal regulator)
• TP53 (transcription factors)

Question 21

How are oncogenes altered in cancer?

Answer 21

1. Gain of function mutations:
   - Increased enzyme activity, increased protein stability or loss of regulation
2. Translocation of the gene to a different area of the genome:
   - Under different control mechanisms
   - Leads to gene fusion and expression of aberrant chimeric protein
3. Gene amplification
4. Increased expression at the mRNA/protein level without gene amplification

Question 22

How are tumour suppressor genes altered in cancer?

Answer 22

1. Loss of function mutation:
   - Loss of enzyme activity, reduced protein stability and disrupted subcellular localisation
   - Most TSGs are transcription factors that lose their ability to transcribe their genes
2. Deletion of the gene
3. Epigenetic regulation:
   - CpG methylation (blocks transcription)
   - Histone modification (blocks transcription)
   - MicroRNAs that cause post transcriptional silencing or mRNA degradation

Question 23

How does epidermal growth factor receptor function in cancers?

Answer 23

• EGFR is mutated in 50% of lung adenocarcinomas
• Protooncogenic EGFR:
• In the absence of a ligand, the receptor is not phosphorylated and downstream signalling proteins are not activated
• If the ligand is present, the intracellular domains dimerise and phosphorylate which activates downstream molecules that signal the cell to grow and proliferate
• Oncogenic EGFR:
• Often has a mutation in the kinase domain of the receptor tyrosine kinases
• Even without the ligand binding, phosphate groups are present on the intracellular domain leading to the recruitment of effector proteins
• Mutant EGFR is active in the absence of EGF ligand and is further activated by ligand binding

Question 24

Describe how p53 tumour suppressor gene is inactivated and its effect:

Answer 24

• p53 loss of function mutations are the most common type of mutated tumour suppressor genes in cancers
• Its normal function is activated when the DNA repair pathway is activated and the translocation of p53 to the nucleus will cause the transcription of growth arrest genes which gives the cell time to repair DNA lesions
• p53 is regulated by the heterodimer MDM2-MDM4 which ubiquinate p53 normally but when the cell is under stress they do not and p53 expression is allowed
• When p53 has loss of function mutations it allows the cells to evade cell death and DNA repair

Question 25

What is the process of epigenetic regulation of gene expression (occurs in tumour suppressor genes)

Answer 25

1. Methylation of CpG dinucleotides:
   - Done by DNMT
   - Represses transcription of a gene as it causes compaction of DNA in the nucleosomes
2. Histone Modification:
   - Histone tails can lose their histone aceylation which results in a compaction of nucleosomes and inhibition of transcription
3. MicroRNAs:
   - MicroRNAs can bind to the 3’UTR or target mRNA causing either mRNA cleavage, degradation of translational repression

Question 26

What genes tend to undergo epigenetic repression in cancers?

Answer 26

• Tumour suppressor genes undergo epigenetic repression

Question 27

How many different TSG or oncogenic mutations are typically required for the progression of a cancer?

Answer 27

2-7

Question 28

What are the main methods for screening cancers?

Answer 28

1. Cytogenetics
2. Fluroescence in situ hybridisation
3. Array comparative genomic hybridisation (aCGH)
4. Immunohistochemistry and proteomics

Question 29

How does cytogenetics screen for cancer?

Answer 29

• Method for detecting chromosomal abrnomalities such as translocations, explansions and duplications
• Done by Karyotyping and G-banding
• Useful for CML where 95% of patients have a Philadelphia chromosomes
• Not good at detecting mutations in specific genes

Question 30

What is the Philadelphia chromosome?

Answer 30

• A chromosome occurring in 95% of CML patients
• Chromosome 9 (which expresses abl tryrosine kinase) and chromosome 22 (which encodes Bcr gene) both experience breaks
• This results in translocations resulting in an altered chromosome 22 (the Philadelphia chromosome) and an altered chromosome 9
• Within the altered chromosome 22 there is now the chimeric gene bcr-abl

Question 31

How does fluorescence in situ hybridisation (FISH) screen for cancer?

Answer 31

• Can be used to detect specific genes on chromosomes
• Useful for detecting copy number and gene translocations
• Is achieved through conjugating the gene of interest to a flurophore or through spectral karyotyping
• Does not allow analysis of single gene changes

Question 32

What is Array Comparative Gene Hybridisation?

Answer 32

• Is able to examine the expression pattern of thousands of genes at once
• Can identify genes expressed differently between normal and cancerous cells
• Can be used to identify specific subtypes of cancer
• aCGH involves:
  1. Isolation of mRNA expressed in the control and experimental samples
  2. The isolated mRNA undergoes reverse transcription to isolate cDNA
  3. The control sample is always conjugated to a green flurophore whilst the experimental sample is always conjugated to a red flurophore
  4. Equal amounts of the fluroscently labelled cDNA is exposed to an array disc coated with various gene targets
  5. If the expression of the gene in the control sample is higher- the gene target for that gene will show more green
  6. If the expression of the gene in the experimental sample is higher- the gene target for that gene will show more red
  7. If expression of the genes is comparable between the control and experimental samples, the gene target shows as yellow
• NGS and Sanger sequencing can also be used

Question 33

How are the colours on an aCGH microarray read?

Answer 33

• The spectrum of colours gives an indication of the copy number variations on the microarray
• If the gene target is yellow- it is equally expressed in the control and experimental sample
• If the gene target it green- the gene is highly underexpressed in the experimental sample
• If the gene target is red, it is highly overexpressed in the experimental sample

Question 34

How does immunohistochemistry and proteomics screen cancer?

Answer 34

1. IHC:
   - IHC uses specific antibodies to detect and quantify specific proteins in tissue samples from cancer biopsies
   - Important for cancer subclassification and personalised treatment
2. The tissue is first stained with H&E to show tissue architecture
3. An antibody against the specific protein of interest is detected where it is bound with a secondary antibody that is conjugated to either a flurophore or a enzyme
4. Proteomics:
   - High-throughput mass spectroscopy can be used to compare protein expression profiles between cancerous and normal tissue samples

Question 35

What are the pros and cons of cancer screening?

Answer 35

• Pros:
  1. Cure rates are up
  2. Death rates are down (though not as much as cure rates would predict)
  3. New screening techniques are very sensitive and allow for subtyping
• Cons:
  1. Some cancers are detected in screening that would never have been life threatening as they may become benign/are indolent and may be treated when they do not need to be
  2. Invasive screening procedures e.g. colonoscopy may potentially be more harm
  3. May give false positives

Question 36

What are the 3 types of skin cancer in order from most to least prevalent?

Answer 36

1. Basal cell carcinoma (BCC)
   - Cell of origin: basal cells
   - 80% of skin cancers
   - Lowest invasive potential
2. Squamous cell carcinoma (SCC):
   - Cell of origin: squamous cells
   - Accounts for 15% of skin cancers
   - Higher invasive potential than BCC but lower than MM
3. Malignant melanoma (MM):
   - Cell of origin: melanocytes
   - Accounts for 5% of skin cancers
   - Has highest invasive potential

Question 37

What are the shared risk factors for all 3 skin cancers?

Answer 37

1. UV radiation exposure
   - History of sunburn
   - Prolonged sun exposure (BCC is associated with intermittent intense UV exposure and SCC is associated with chronic UV exposure)
   - Indoor tanning habit
   - Certain types of moles (SCC develops from skin lesions called actinic keratosis)
2. Genetics:
   - Family history
   - Skin that freckles and burns easily
   - Light colouring

Question 38

How does UV exposure increase the risk of skin cancers?

Answer 38

• There are 3 subtypes of UV light: UVC, UVB and UVA
• The shorter the wavelength of light, the more it is absorbed by DNA
• UVC is the shortest wavelength but is filtered by the ozone
• UVB is readily absorbed by DNA and is associated with burning and the initiation of most skin cancers
• UVA penetrates deeper and can reach dermis of skin, is associated with wrinkling
• UV is linked to 86% of MM, 70% of BCC and 60% of SCC:
• MM= intermittent sun exposure/burning and dysplastic nevi
• BCC= intermittent UV exposure and childhood sunburn
• SCC= actinic keratosis and is related to chronic UV exposure and older age
• UV radiation induces cyclobutane pyrimidine dimers (DNA lesions)
• The UVB is absorbed by double bonds in C and T bases and opens the bond so the C or T can react to an adjacent base and form a covalent bond
• These lesions must be corrected with DNA/nucleotide excision repair (NER)
• If the lesions are not repaired CC-TT mutations occur

Question 39

How do UVB induced DNA mutations cause SCC?

Answer 39

• UVB induced DNA lesions often lead to mutations in the small GTPase Ras as well as p53
  1. Ras oncogenic mutations:
• Ras is an activator of the MAP kinase pathway that becomes oncogenic in SCC and causes it to be consituently active and causing the proliferation of the cells
• Increased activation of MAP kinase pathways is observed as phosphorylation of ERK
• The activation of c-Fos and then AP-1 TFs causes increased COX-2 expression

2. UVB activation of phospholipase C
   - Produces AA which is converted to PGH2 and prostaglandins by COX1/2 enzymes
   - Prostaglandins mediate skin cell hyper-proliferation

Question 40

What occurs when nucleotide excision repair enzymes are dysfunctional?

Answer 40

• NER enzymes being dysfunctional means that DNA lesions can no longer be repaired
• Dysfunctional NER enzymes are the cause of Xeroderma Pigmentosa (XP) which is an autosomal recessive disease that results in severe sunburn upon minimal UV exposure and excessive freckling
• Greatly increased risk of MM, SC and BC
• Death usually occurs by 32 years

Question 41

What is Basal Cell Nevus Syndrome?

Answer 41

• A hereditary disease (also called Gorlin syndrome) associated with the development of BCC
• The major pathway de-regulated in BCC is the hedgehod signalling pathway
• In Gorlin syndrome there is a loss of function mutation in the PTCH1 gene which is a tumour suppressor gene and repressor of hedgehog signalling
• Loss of heterozygosity of PTCH1 results in expansive BCC and also the development of medulloblastoma in 4% of patients
• The patched receptor inhibits smoothened normally
• When patched function is lost Smo is constiuitively active
• A drug called LDE225 acts to inhibit the hedgehog signalling pathway that is abberantly activated in Gorlin syndrome by inhibiting Smo
• Shows a partial or complete response in the majority of patients

Question 42

What is melanoma?

Answer 42

• The most aggressive type of skin cancer
• The cell origin is the melanocyte (pigment producing cells) which are found in the basal layer of the skin and also the eye
• There is a strong environmental risk factor in UV light and family history, fair skin and number of moles (>20) are also important
• Melanocytes contain melanosomes which contain melanin that play a protective role against UV induced DNA damage by absorbing UV
• Melanosomes often concentrate over the top of the nucleus in melanocytes and protect the DNA, so having fair skin (less melanocytes) therefore can predispose someone to melanoma

Question 43

What is B-Raf and how is it related to melanoma?

Answer 43

• The RAF proteins are a family of serine, threonine-specific kinases that form a part of a signalling module that regulates cell proliferation, differentiation and survival
• Ras (a GTPase) activates RAF which forms dimers via kinase domains and is therefore activated
• B-RAF is a tyrosine kinase mutated in a significant proportion of MM
• B-Raf phosphorylates and activates MEK which then activates ERK which promotes cell proliferation and survival (MAP kinase pathway)
• Activating mutations in N-RAS also occur in human MM (a V600E in the kinase domain is the predominant mutation which constuitively activates B-raf indepdently of B-raf homodimerisation or binding to Ras

Question 44

How can can B-Raf Inhibitors treat melanoma?

Answer 44

• Standard therapies for melanoma patients have a poor overall response (5-10%)
• B-Raf is mutated in many melanomas so it a target by rational drug design
• Selective B-Raf inhibitors (e.g. Plexxikon) aim to target melanoma by competing with ATP for the binding domain in the kinase domain of B-Raf
• These are very effective (usually have a response in 81% of patients) but relapse occurs after 9 months

Question 45

What mechanisms underlie the rapid development of resistance in melanomas to selective B-Raf inhibitors?

Answer 45

• V600E mutated B-Raf in MM has a mutation in its kinase domain so that it functions as a monomer and can be activated independently of Ras binding
• Resistance of Plexxikon occurs in p61B-Raf(V600E) which is an alteratively spliced isoform with the deletion of the Ras-binding domain/mutation in the kianse domain but it can still homodimerise independently of Ras
• It is a result of deletions in exons 4-8 causing the deletion of the N-terminal Ras binding domain which is normally mutated