Oncogenes and tumour suppressor genes Flashcards

Question

how do mutagens and viral proteins cause cancer?

Answer 1

they induce mutations - they do not directly cause cancer themselves

Answer 2

Ras is a small GTPase - Ras cycles between active GTP-bound and inactive GDP-bound

Answer 3

- GEF = guanine nucleotide exchange factors – catalyses exchange of GDP to GTP - GAP = GTPase-activating proteins – hydrolyses GTP to GDP

Answer 4

In normal cells, under growth factor influence, Ras promotes cell division and proliferation when active, and when there are no growth factors, Ras will be bound to GDP and inactive, so proliferative signalling will be off

Answer 5

Ras G12V or G12C (mutation at G12 position) = Ras unable to hydrolyse GTP to GDP, so is constitutively active and cannot be turned off

Answer 6

If Ras is in its oncogenic form, then it is always bound to GTP and therefore proliferative signalling is always on, even in absence of growth factors

Answer 7

1. H-ras 2. K-ras 3. N-ras

Answer 8

point mutation in one of three codons: 12 most frequent, than 13 and 61 ~20% of all tumours have undergone an activating mutation in one of the RAS genes

Answer 9

- Mutations in K-ras are the most frequent drivers of tumour development across the spectrum of human cancers - K-RasG12C mutation is most prevalent in lung cancers

Answer 10

AMG 510 inhibitor forms a covalent bond with GDP-bound KRASG12C  high specificity for the mutant protein - Prevents K-rasG12C from binding to GTP - AMG 510 is specific for G12C Ras mutation only

Answer 11

as normal cells do not have the Ras G12C mutation - AMG 510 is specific for G12C Ras mutation only

Answer 12

- AMG 510 inhibits downstream signalling and cell proliferation in lung cancer cell lines in vitro - AMG 510 inhibits tumour growth in mice - tumour regression in 8/10 mice - AMG 510 efficiency improves in combination with standard chemotherapy (carboplatin) or a drug that inhibits MEK, which acts downstream of Ras

Answer 13

AMG 510 is not effective on mice lacking an immune system – Ras-inhibitor requires an immune system

Answer 14

- AMG 510 in combination with an immunotherapy called anti-PD1 - harnesses the immune system to combat cancer - complete tumour regression in 9/10 immunocompetent mice - AMG 510 boosts the expression of pro-inflammatory cytokines in tumour-bearing animals - Increase infiltration of the tumours by T cells and dendritic cells - When KRASG12C cancer cells are reintroduced in animals cured by the combination therapy, there was no tumour formation - long term T-cell response to KRAS tumour cells induced by the treatment

Answer 15

adaptive resistance to the treatment occurred rapidly - Multiple mechanisms of resistance identified in 41% of patients during clinical trials - in the 41% of patients, there were more than one potential mechanism of resistance

Answer 16

- On-target mutations disrupting the binding site – AMG 510 can no longer bind to KRASG12C - Drug-resistant mutations on KRAS - KRAS downstream effector mutations - Amplification of KRASG12C

Answer 17

- Need second-generation inhibitors to against drug-binding mutants - Combination therapies are essential

Answer 18

- Myc is an proto-oncogene which is activated and overexpressed in >70% of human tumours - myc is a transcription factor which promotes growth

Answer 19

1. C-myc 2. N-myc 3. L-myc

Answer 20

they are localised in the nucleus

Answer 21

- Many times higher than the 2 copies present in the normal human genome. - The increased gene copy numbers results in corresponding increases in the level of the gene product, myc protein. - When expressed at excessive levels, myc drives uncontrolled cell proliferation

Answer 22

1. expression is driven by its normal promoter, but the gene is amplified, resulting in an elevated copy number 2. chromosomal translocation 3. pro-virus integration - insertional mutagenesis

Answer 23

N-myc amplification occurs in 30% childhood neuroblastoma - increased levels of the N-myc gene product due to more gene copies - therefore more N-myc protein to promote cell growth and proliferation

Answer 24

chromosomal translocation = a region of one chromosome is fused with a region from a second, unrelated chromosome

Answer 25

c-myc is under the control of a foreign transcriptional promoter: - Myc gene is translocated from one chromosome to another - Burkitt lymphoma: myc gene becomes under the control of the promoter of an immunoglobulin gene = very active - Immunoglobulins are expressed at high levels, so the myc gene will be expressed highly as well, leading to high proliferation - Structurally normal myc protein, but abnormally high amounts = relentless proliferation of lymphoid cells - T8:14 most common translocation = highly active Ig promoter = relentless proliferation of lymphoid cells.

Answer 26

ALV = avian leukosis virus - the viral transcriptional promoter disrupted the mechanisms normally controlling the expression of myc, causing increased expression, extremely high levels of myc protein - In >80% ALV-induced leukaemia, integration of ALV provirus in the chromosomal DNA is seen immediately adjacent of the myc proto-oncogene - This means that myc is now under the control of the ALV promoter, which is very active – myc becomes oncogenic - this leads to uncontrolled cell proliferation, resulting in the aggressive cancer that constitutes a leukaemia

Answer 27

The formation of many tumours is linked to deregulation of the epidermal growth factor receptor (EGFR): - Normally, EGFR recognises the presence of its ligand (EGF) in the extracellular space - downstream signalling upon binding - In 1/3 glioblastomas, cancer has a truncated form of EGFR that is constitutively active, lacking most of the extracellular domain - Extracellular domain deletion is well documented in lung cancer and glioblastoma - constitutive signalling in the absence of the ligand - this enables growth-stimulatory signals to be promoted in the absence of EGF

Answer 28

1. activation of oncogenes 2. inactivation of tumour suppressor genes (TSGs)

Answer 29

for oncogenes, the cancer phenotype is dominant for TSGs, the cancer phenotype is recessive

Answer 30

- Regardless of the genes already present in the normal cells, the oncogene dominates and turns the cell cancerous - Viral oncogenes can dictate cellular behaviour in spite of the continued presence and expression of opposite cellular genes that usually mediate normal cell proliferation cell transformation = conversion of a normal cell into a cancer cell, despite the continued presence and expression of opposite cellular genes that usually mediate normal cell proliferation

Answer 31

a hybrid cell is formed that cannot form tumours - Therefore the normal cell phenotype dominates, as it has genes which counteract oncogenes - Normal cells carry genes that constrain their proliferation, which are inactivated during the development of a tumour = TUMOUR SUPPRESSOR GENES

Answer 32

genes that constrain proliferation of normal cells, and are inactivated during the development of a tumour

Answer 33

Favour: easier to lose a TSG by mutation than activating an oncogene (specific mutations required for oncogene activation) Against: 2 copies of TSG must be lost - 2 genetic alterations/mutations sound complex and improbable to occur in short period of time. - 2 mutations are needed to lose TSG, one mutation needed for oncogene

Answer 34

Retinoblasts = population of cells in the developing eye; normally stop growing during embryogenesis & differentiate - If cells keep growing, retinoblastoma can arise

Answer 35

Retinoblastoma: tumour of the retina, arising in the precursor of photoreceptor cells (retinoblasts), which fail to differentiate & divide - ~ 1:20,000 children - Diagnosed from birth up to the age of 6 to 8 years - Can be cured with radiation or surgery

Answer 36

1. sporadic form - unilateral retinoblastoma 2. familial form - bilateral retinoblastoma

Answer 37

Sporadic form: children from family with no history of retinoblastoma, developing a single tumour in one eye - unilateral retinoblastoma

Answer 38

Familial form: children having a parent who suffered from retinoblastoma, developing multiple foci of tumours in both eyes - bilateral retinoblastoma + elevated susceptibility of developing other tumours

Answer 39

Knudson used mathematics to determine whether the data followed a trend associated with a one-hit or two-hit model of gene mutation - Information from 2 groups: 23 patients with bilateral familial retinoblastoma, & 25 patients with unilateral sporadic retinoblastoma. - Time is required for mutations to accumulate, Knudson examined the age at which the children in these two groups were diagnosed with retinoblastoma. - Rate of appearance of familial tumours (bilateral) = a single random event needed to develop the disease – 1 hit - rate of tumour formed is faster - Sporadic tumours (unilateral) = 2 random events are required – 2 hits needed - takes longer for tumour to develop

Answer 40

a mutation in Rb gene, resulting in an inactive and recessive Rb allele

Answer 41

Children with 2 WT Rb from the parents require 2 successive alteration/mutations in the retinal cell lineage to inactivate the 2 copies of Rb.

Answer 42

If Rb mutations are present in the family, then the child already has one mutant Rb - the mutation of the other copy in enough to drive retinoblastoma

Answer 43

Probability of both random mutations occurring one after the other is 10^-12 per cell generation – very rare therefore, mitotic recombination is the cause

Answer 44

genetic material is exchanged between 2 homologous chromosomes through the process of genetic crossing over occurring during G2 phase or, less often, in M phase of the cell cycle

Answer 45

- the chromosomal arm carrying WT Rb allele might be replaced with the one carrying the mutant allele. - Occurs at a frequency of 10^-5 to 10^-4 per cell generation - far easier way than mutational inactivation -Loss of heterozygosity or LOH (observed in the DNA in the vicinity of TSGs) - Heterozygous cell with Rb is changed for a cell which is homozygous for mutant Rb – enough to cause retinoblastoma as 2 mutants are present in the DNA

Answer 46

1. Direct suppression of cell proliferation in response to growth-inhibitory and differentiation-inducing factors 2. Components of the cellular machinery that inhibits proliferation in response to metabolic imbalance and DNA damage

Answer 47

Rb and p53 - Both have an important role in the control of the cell cycle - Both are disrupted in most human tumours

Answer 48

familial cancer syndrome, - benign tumours in peripheral nervous system = neurofibromas, - some progress to malignant neurofibrosarcomas

Answer 49

LOF mutation in NF1

Answer 50

NF1 (neurofibromin) is a Ras-GAP = GTPase activating protein - it is a negative regulator of Ras signalling

Answer 51

In a normal cell, when cell is stimulated with growth factors, NF1 is degraded, enabling Ras signalling to proceed. - After 60-90min, NF1 levels return to normal, shutting down Ras signaling. - NF1 is activated when needed

Answer 52

Mutations in NF1 result in a protein with 1000-fold decrease in GTPase stimulating activity and cannot inactivate Ras - When NF1 is lost, Ras remains activated for longer and loss of NF1 function in a cell mimic the hyperactivation of Ras observed in the presence of mutant ras oncogenes

Answer 53

Ras activates: - Raf-Erk pathway which leads to proliferation, - P13K-Akt-mTOR pathway which inhibits apoptosis In WT, NF1 is present, so there is less Akt and less mTOR, so apoptosis in induced

Answer 54

NF1 null cells show overactivation of AKT and the mTOR target S6K - NF1 null cells have an enhanced ability to form colonies in anchorage independent conditions - When NF1 is lost, Akt and mTOR is upregulated, leading to lack of apoptosis and uncontrolled growth

Answer 55

overactivation of Akt pathway is blocked by the mTOR inhibitor called Rapamycin: - Rapamycin inhibits mTOR, thus inhibiting the growth of the cancer - mTOR inhibition could be a potential strategy to target NF1 null tumours

Answer 56

- inheritable colon cancer - susceptibility to develop polyps in the colon, prone to develop into carcinomas. - The gene responsible is APC (Adenomatous Polyposis Coli)

Answer 57

*Loss of APC gene = first step in colon cancer progression by formation of benign polyps, followed by oncogenic mutations of Ras and mutations or loss of p53 forming larger polyp and then a malignant tumour

Answer 58

- stem cells are found at the bottom of crypts, and Wnt signalling triggers the stem cells to proliferate - When the cells proliferate, they migrate upwards, so lose Wnt signalling, stop self-renewing and differentiate - They localise at the top of the crypt, undergo apoptosis and die before any mutations can give rise to cancer - out-migration and death takes only 3 to 4 days - effective defence mechanisms: even if cells sustain mutations, they die within days without causing damage to the intestine.

Answer 59

Mutations in APC lead to the formation of cancer: - block of the out-migration of cells from the crypt - mutated cells accumulate in the crypt rather than being apoptosed

Answer 60

- Functional APC = part of the destruction complex, responsible for mediating the degradation of b-catenin, mediated by the proteasome - APC negatively controls levels of beta-catenin in cytosol - APC is not expressed in the cells at the bottom of the crypt - b-catenin can accumulate and move in the nucleus and promote proliferation - As the cells move upward, APC expression is increased, which blocks b-catenin TF and inhibits proliferation, promoting differentiation

Answer 61

- Inactivation of APC - b-catenin cytosolic accumulation and nuclear translocation -> transcription of growth-promoting genes, including myc - Accumulation of b-catenin = most important consequence of APC inactivation (90% of sporadic carcinomas) – beta-catenin is a transcription factor, so can target its growth-promoting downstream targets

Answer 62

Wild-type mice have small defined crypts in the small intestine with little nuclear β-catenin at the bottom of the crypt. The small intestine of APC-/- show the crypt-progenitor cell (CPC) phenotype with increased crypt size and nuclear β-catenin

Answer 63

- Hereditary predisposition to the development of a variety of tumours, including clear cell kidney carcinomas, pheochromocytomas (adrenal gland cell tumours) & hemangioblastomas (blood vessel tumours) in the CNS & retina - Caused by germline mutations in the tumour suppressor VHL, coding for pVHL protein - VHL gene also inactivated in ~ 70% of sporadic kidney carcinomas

Answer 64

pVHL main function = promoting the destruction of the transcription factor HIF-1a (hypoxia-inducible factor 1a) - If oxygen is present, HIF-1a is hydroxylated, and pVHL binds to HIF to mediate ubiquitination and degradation of HIF - If oxygen is absent in hypoxic conditions, hydroxylase is inactive, so HIF isn’t degraded by pVHL, accumulates and mediates transcription of downstream growth-promoting target genes

Answer 65

Expression of target genes: increase blood flow for uptake of more oxygen - Angiogenesis - Erythropoiesis - Glycolysis & glucose uptake To enable cells to survive under hypoxia (short term) + to acquire access to oxygen supply

Answer 66

there is a LOF point mutation in the amino acid residues of the hydrophobic pocket of VHL, which recognises the hydroxyproline residues of HIF-1A

Answer 67

Even in presence of oxygen, HIF is not degraded by VHL, so HIF signalling remains active and cell growth is promoted - Constitutive activation of HIF-1 transcription factors -> growth promoting genes to stimulate the proliferation of a variety of cell types

Answer 68

1. NF1 - negative regulator of Ras signalling 2. APC - negative regulator of b-catenin signalling 3. pVHL negtaive regulator of HIF-1A transcription factors