Unit 4 - General Concepts of Cancer and Protecting the Genome

Question 1

cancer - biology vs molecular level

Answer 1

out of control cellular proliferation (bio)

damage to genetic material (mutations and epigenetic) that affect cellular proliferation

Question 2

3 types of mutations in cancer cells

Answer 2

ONCOGENIC

TUMOUR SUPPRESSIVE

NEUTRAL

Question 3

oncogenic mutation

Answer 3

ras

myc

cyclin D

normal growth promoting genes (proto-oncogenes) become either hyperactive or inappropriately active

dominant mutations = mutation of only 1 allele required

Question 4

tumour suppressive mutation

Answer 4

normal growth restraining genes become lost or down-regulated

recessive mutation = mutation of both alleles required (to lose tumour suppressive function)

BRCA1 and 2

TP523

Question 5

neutral mutation

Answer 5

cancer cells have 1000s of mutations to genes that have little or no effect on aetiology of cancer

cancer evolves a mutagenic phenotype

Question 6

protooncogenes when mutated =

Answer 6

driving with foot on the accelerator

Question 7

normal (stem) cells

Answer 7

low instability

no cells have genetic alteration required to overcome the selection barrier

NO TUMOUR

Question 8

increased genetic instability

Answer 8

at least 1 cell contains the requisite genetic alteration to overcome the selection barrier (clonal selection)

barrier traversed and population of mutant cells eventually accumulates the new mutations required to cross the next selection barrier

significant step towards tumourigenesis

Question 9

too much genetic instability

Answer 9

too many mutations accumulate to allow viability

cells die - apoptosis, necrosis

NO TUMOUR

Question 10

6 selection barriers

Answer 10

reduced requirement for growth factors - autocrine stimulation

insensitivity to inhibitory signals

escape from senescence (cellular immortality)

evasion of apoptosis

stimulated angiogenesis

invasion/metastases

Question 11

how are these selection barriers overcome

Answer 11

by inactivating tumour suppressors and activating oncogenes

tumours - hostile cellular environments

e.g. periods of anoxia, malnutrition

fluctuating hormonal influences and immune attack

a fertile breeding ground for mutations

similarly, bacteria with higher levels of genomic instability, but not too high, adapt to and eventually dominate new environments

Question 12

challenge posed by the somatic mutation hypothesis

Answer 12

very low mutation rate - 1 x 10-10 nucleotides/cell/division for human somatic cells

diploid human genome = 6.4 x 109

1016 cell divisions in a human lifetime are insufficient to permit a single cell to obtain the estimated 5-7 advantageous mutations required to produce a cancer

⇒ cancer shouldn’t occur with such a low mutation rate

Question 13

the mutator (or genetic instability) hypothesis

Answer 13

cancer cells have significantly elevated (just-right) mutation rates

Question 14

Min tumours

where is there instability

type of karyotype

prevalence

Answer 14

microsatellite instability

instability at the nucleotide level e.g. mutation of MMR results in 10-100 fold increase in mutation

most easily seen at microsatellites

normal karyotype

relatively uncommon e.g. 15% colorectal cancers

Question 15

Cin tumours

where is there instability

karyotype

prevalence

Answer 15

chromosomal instability

instability at chromosomal level

not clear what mutational events initiate Cin tumours - loss of p53, mitotic checkpoints?

abnormal karyotype

most frequent - 85% of colorectal cancers

Question 16

similarity between min and cin

Answer 16

NOT FOUND TOGETHER

same oncogenes and tumour suppressors appear to be targeted in both min and cin tumours

Question 17

what is the lifetime risk of many cancers dependent on

Answer 17

the total number of divisions of adult stem cells in the particular tissue rather than on environmental or inherited mutations

the more cell divisions the more likely that random mutation to key cancer driver genes will occur during cell division - 65% of cancer

Question 18

what explains the extreme variation in cancer incidence across different tissues

Answer 18

> cell divisions ⇒ the more likely that random mutation to key cancer driver genes will occur during cell division

Question 19

cancer risk for different tissues

Answer 19

6. 9% - lung
7. 08% - thyroid
8. 6% - brain
9. 003% - pelvic bone
10. 00072% - laryngeal cartilage

Question 20

proportion of cancers that have an inherited component

Answer 20

5-10% of cancers

Question 21

what effect does exposure to mutagens have

Answer 21

mutagens or viruses cannot account for a 24x variation of lifetime risk throughout alimentary canal

LI - 4.82%

stomach - 8.6%

oesophagus - 0.51%

SI - 0.2% - 3x LESS COMMON than brain tumours even though the brain is protected from environmental mutagens by the BBB

Question 22

lifetime risk vs total stem cell divisions

Answer 22

Question 23

tumour

Answer 23

an abnormal uncontrolled growth without physiological function, that can be either benign or malignant

Question 24

benign tumour

Answer 24

confined and not life threatening

Question 25

malignant tumour

Answer 25

invades surrounding tissue and may spread to other parts of the body

Question 26

neoplasia

Answer 26

process of forming tumours (benign or malignant)

Question 27

hyperplasia

Answer 27

small abnormal growth in a part of the body caused by an excessive multiplication of phenotypically normal cells

Question 28

metaplasia

Answer 28

appearance of invading, microscopally normal cells of a type not normally encountered at that site most frequent at epithelial transition zone e.g. oesophagus/stomach, cervix/uterus

Question 29

dysplasia

Answer 29

small abnormal growth in a part of the body caused by an excessive multiplication of cytologically abnormal (variable size and shape, bigger nuclei, increased mitotic) cells transitional state between benign and pre-malignant growths

Question 30

transitional state between benign and pre-malignant growths

Answer 30

dysplasia

Question 31

adenomas, polyps, papillomas

Answer 31

large benign tumours of epithelial origin dysplastic but not malignant usually grow to a certain size and them stop growing

Question 32

carcinoma

Answer 32

most common (80% of cancer deaths) malignant tumour derived from epithelial tissue → surface layer of an organ/body part e.g. GI tract skin breast pancreas lung liver ovary bladder

Question 33

adenocarcinoma and squamous cell carcinoma

Answer 33

2 main classes of carcinoma that derive from epithelia secrete substances into cavities/ducts e.g. breast or from simple protective layers e.g. skin

Question 34

leukemia (dispersed)/lymphoma (solid)

Answer 34

2nd most common (16%) malignant tumour type derived from haematopoietic and lymphatic tissues

Question 35

sarcoma

Answer 35

rare (1%) malignant tumour derived from CT/muscle e.g. fibroblast adipocyte osteocytes myocytes

Question 36

neuroectodermal tumours

Answer 36

rare (2.5%) malignant tumours derived from central and peripheral nervous system e.g. gliomas, neuroblastomas

Question 37

most common tumours

Answer 37

1. carcinoma 2. leukemia/lymphoma

Question 38

germ cell tumours (GCT)

Answer 38

tumours derived from germ cells germ cell tumours can be cancerous or non-cancerous normally occur inside gonads (ovary and testis) testicular cancer - curable

Question 39

teratoma

Answer 39

tumours with tissue or organ components resembling normal derivative of all 3 germ layers although resembling normal tissues, often dissimilar to surrounding tissues (teeth and hair) encapsulated and hence usually benign multiple fluid-filled cysts can form within the capsule teratoma within a large cyst can sometimes form a structure resembling a foetus immature teratomas occasionally malignant, rare but slightly more common in males

Question 40

mature vs immature teratomas

Answer 40

**_mature_** typically benign rare more common in females **_immature_** malignant rare more common in males

Question 41

colorectal cancer - cin occurence structures in colon

Answer 41

10⁷ crypts in colon each crypt has 1000s of differentiated cells (fast growing - lot of apoptosis - 10¹⁰ cells are lost) 1-10 stem cells (slow growing and self renewing) mutation can occur in any 1 cell cycle early studies of tiny adenomas ⇒ that \>90% had allelic imbalances of 1+ of 5 chr tested ⇒ supports the idea that CIN occurs early 85% of cases of sporadic colorectal cancer initiated by inactivation of APC (adenomatous polyposis coli) \< 10% by activation of β-catenin as both function in WNT signalling

Question 42

sporadic colorectal cancer causes

Answer 42

85% of cases of sporadic colorectal cancer initiated by inactivation of APC (adenomatous polyposis coli) \< 10% by activation of β-catenin as both function in WNT signalling

Question 43

Wnt signalling regulated by target what is included in its complex

Answer 43

proliferation of epithelial cells is regulated by mitogen Wnt key target = β-catenin - activates transcription of genes involved in proliferation no Wnt ⇒ no β-catenin - constitutively associated with an inhibitory cytoplasmic complex including APC, Axin and GSK3β (glycogen synthase kinase 3β) which regulates phosphorylation of β-catenin and thus targets it for destruction via the SCF E3 ubiquitin ligase

Question 44

Wnt present ⇒

Answer 44

dissociation of APC complex and GSK3β inactivated, therefore releasing β-catenin to perform its function

Question 45

loss of APC or activating mutations of β-catenin itself in colon cancer

Answer 45

constitutive activation of β-catenin signalling

Question 46

where else does β-catenin function

Answer 46

in cell adhesion

Question 47

where also does GSK3β function

Answer 47

in glycogenesis

Question 48

familial adenomatous polyposis (FAP) associated gene

Answer 48

early tumour initiation APC gene regulates colorectal cell proliferation via wnt signalling referred to as gatekeeper

Question 49

hereditary non-polyposis colorectal cancer (HNPCC) gene also known as

Answer 49

rapid tumour progression MMR mutations cause the MIN class of genome instability 15% of sporadic colorectal carcinomas display MIN phenotype Lynch syndrome

Question 50

function of gatekeeper genes

Answer 50

required for net cellular proliferation maintenance of a constant cell number in renewing populations

Question 51

mutation of a gatekeeper leads to

Answer 51

a permanent imbalance of cell division over cell death

Question 52

role of gatekeepers in different tissues

Answer 52

can be expressed ubiquitously may function as gatekeepers in only 1 or a few tissues redundant, expendable or perhaps play different roles in other cell types

Question 53

examples of gatekeeper genes

Answer 53

NF1 in schwann cells Rb in retinal epithelium VHL gene in kidney cells \*\* not yet reported for majority of human malignancies but will be important for our understanding and future treatment of cancer

Question 54

Rb and gatekeeper concept how can a retinoblastoma arise

Answer 54

human retinal progenitor (stem) cells give rise to 7 different cell types but only in 1 of these, **the cone-precursor,** does loss of Rb result in transformation (in others - either no effect or apoptosis) molecular circuitry of cone-precursor (possibilities identified by authors include - high expression of N-myc, SKP2 and MDM2) allows them to proliferate and become transformed when Rb is lost specific molecular circuitry likely to be a paradigm for all so-called gatekeeper genes

Question 55

what is DNA damage response (DDR)

Answer 55

biochemical signalling pathways that respond to structural perturbations in the genetic material e.g. DNA damage transient "normal" structures generated during cell proliferation also sensed e.g. ongoing DNA replication prevents exit from S phase until replication is complete DDR regulates co-ordinated cellular responses to DNA damage and replication structures = checkpoint responses once repair is complete cells re-enter the cell cycle and resume proliferation in a regulated process - recovery

Question 56

inefficient DDR =

Answer 56

genome instability easily repairable lesions are converted into mutations

Question 57

biological response to DNA damage

Answer 57

Question 58

checkpoint in G1

Answer 58

prevents cells going into replication - damage is repaired

Question 59

checkpoint in S

Answer 59

detects any damage

Question 60

checkpoint in G2/M

Answer 60

prevents cells going into mitosis e.g. DNA double strand break part of the chromosome is not attached to centromere - during mitosis chromosomes would segregate but broken fragment of chromosome would stay in the middle of the cell BRCA1 and BRCA2

Question 61

key players in the DDR

Answer 61

recruitment and activation of protein kinases typical of signal transduction in general 2 TYPE OF PKs 1. phosphatidylinositol 3-kinase-like kinases or PIK kinases e.g. ATM and ATR - do not phosphorylate the lipid - phosphatidylinositol, at the 3 position -OH of inositol ring, but instead serine/threonine directed protein kinases 2. checkpoint kinases or CHK kinases e.g. CHK1 and CHK2

Question 62

2 types of PKs - DDR

Answer 62

1. phosphatidylinositol 3-kinase-like kinases or PIK kinases e.g. ATM and ATR - do not phosphorylate the lipid - phosphatidylinositol, at the 3 position -OH of inositol ring, but instead serine/threonine directed protein kinases 2. checkpoint kinases or CHK kinases e.g. CHK1 and CHK2

Question 63

ATM mutation names pattern of inheritance symptoms when does it develop

Answer 63

ataxia telangiectasia, BOder-Sedgwick or Louis-Bar syndrome rare, autosomal recessive disease (non-essential) progressive loss of purkinje cells in the cerebellum, affecting motor skills (ataxia = poor co-ordination) telangiectasia = prominent BVs in whites of eyes symptoms develop in toddler years and post patients die by age 20 from bronchopulmonary infection and/or malignancy increased incidence of tumours - lymphomas/leukemias half of patients have immune problems poor appetite hypersensitivity to ionising radiation

Question 64

ATR mutation names pattern of inheritance

Answer 64

seckel syndrome microcephalin primordial dwarfism harper's syndrome bird-headed dwarfism rare autosomal recessive disease (ATR is an essential gene - caused by hypomorphic rather than null mutations - wouldn't get past early development) 1 of several proportionate dwarfisms of prenatal onset severe microcephaly with a bird-like head appearance - protrusion of nose, large eyes, low ears, small chin, mental retardation

Question 65

activation of PIK kinases (ATM/ATR)

Answer 65

ATM - DNA double strand break ATR - single stranded DNA

Question 66

activation of CHK kinases (CHK1/CHK2) role of adaptors/mediators

Answer 66

CHK1 and CHK2 are released from lesion sites

Question 67

G1 checkpoint

Answer 67

drives expression of p21

Question 68

hypothesis - proto-oncogene and DNA

Answer 68

early in development of cancer oncogene activation of a proto-oncogene causes increased DNA replication stress resulting in activation of the DDR and cell cycle arrest or apoptosis

Question 69

evidence to support that DDR is activated at early stages of lung cancer

Answer 69

1. NSCLC, as well as for malignant melanoma that the DDR is activated at early (pre-neoplastic) stages of tumorigenesis 2. bladder carcinoma, as well as for carcinomas of the breast, colon and lung that the DDR is activated at early stages of tumorigenesis 3. DDR is not appreciably activated in any normal proliferating tissues e.g. normal colonic crypts, which have a higher proliferation index than most cancers, or even in tissues experiencing inflammation (hyperplasias) IMMUNOHISTOCHEMISTRY, EXPERIMENTALLY INDUCED HYPERPLASIA, OVEREXPRESSION OF ONCOGENES ⇒ constitutive activation of the DDR pathway commonly occurs at pre-invasive stages of major types of human tumours

Question 70

is the DDR activation in the earliest cancer lesions

Answer 70

yes - in hyperplasias as well as earliest cancer lesions

Question 71

S phase promoting oncogenes and DDR

Answer 71

DDR induced in cultured cells upon expression of S phase promoting oncogenes

Question 72

what does oncogenic activation result in

Answer 72

replication stress (SSBs/DSBs) which both activate DDR/checkpoint activation of DDR checkpoint dependent apoptosis (p53 dependent) and senescence suppress expansion of precancerous lesion i.e. tumour suppressive subsequent mutations of DDR results in loss of these crucial tumour suppressive mechanisms and further evolution of cancerous phenotype telomere attrition and hypoxia can also contribute to formation of DSBs

Question 73

conclusions

Answer 73