Lecture 23

Question 1

what can explain many familial cancers

Answer 1

inheritance of mutant tsgs

Question 2

what do early stage cancer cells do and explain

Answer 2

find ways to eliminate wild type copies of tsgs
eg = mitotic recomb can lead to loh - can occur
can occur during g2 phase of cell cycel
Subsequent segregation of chromatics may yield a pair of daughter cells that have undergone loh
can be random but not always - can be due to improper recombination events - mitotic recomb = allows allele to be transfered = allele can end up in same cell - 2 mutated alleles = passed along to next chrom

Question 3

name and describe mechanisms of loh and wt copy inactivation of tsgs

Answer 3

terminal deletion - wild type lost
point mutation
indels
frameshift - premature stops

Question 4

describe chromatin structure

Answer 4

epigenetic mechanisms = no changes in dna sequence but changes in ability of dna to be expressed
tightly would arounf histones

Question 5

describe heterochromatin

Answer 5

compact
wound up
OFF
cytosine usually methylated
transcriptionally silent
ptms can lead to dna being compacted

Question 6

describe euchromatin

Answer 6

can be transcribed
ONN
transcriptionally accessible
where chromatin acessible

Question 7

how to lose your tsgs

Answer 7

promoter methylation can lead to tsg inactivation without mutation
ex of epigenetic silencing - how actively gene transcribed
normal cell = fine
tumoural cell = hypermethylated and inactivated tsgs = silenced by methylation
promoter methylation can lead to tsg inactivation without mutation = epigenetic regulation of gene expression present in all genes and usually controlled for development and differentiation * dna methyl transferases = highly upregulated in cancer cells DNMT1, and DNMT3B - high upreg in highly aggressive adenocarcinoma
sometimes = give meds that prevent methylation of cytosines but not always

Question 8

define epigenetic

Answer 8

control gene activity without changing dna sequence

Question 9

what leads to p53 stabilization

Answer 9

dna damage and dysregulated growth signals

Question 10

what is p53

Answer 10

tsg
form homotetramer to function - levels destabilize quickly usually
functions as a transcription factor that halts cell cycle - has dna binding domain

Question 11

what are p53 targets

Answer 11

growth arrest genes
dna repair genes
regulators of apoptosis

Question 12

Do all Tumor Suppressor Genes follow the Knudson 2-hit model??? Explains

Answer 12

HELL NAH
ex = p53, pten
deviates from knudson rule = loss of single tsg allele yueld abnormal cells
haploinsufficent
so one gene making p53 = not enough to do job
dicer syndrome - 2 hits needed

Question 13

what does dominant negative do

Answer 13

mutation whose gene product adversely affects the normal wild type gene product in the same cell
usually occurs if product can still interact with same elements as wildtype product but block some aspect of its function

Question 14

describe p53 - dom neg effect

Answer 14

normally works as tetramer
mutant p53 found in many human tumours usually carries aa substitutions in its dna binding domain

Question 15

describe tsgs

Answer 15

regulate cell proliferation through many mechanisms
unites them is the fact that the loss of any one of them increases a cell’s selective growth advantage
act as gatekeepers - control apoptosis, cell cycle, senescence, quiescence

Question 16

what happens when tsgs lost

Answer 16

usually affects cell phenotype only when both copies of such gene are lost - although there are exceptions that we discussed

Question 17

what is synthetic lethality

Answer 17

combined effect of 2 alleles - each of which is non lethal but when acting in combo = results in lethality

Question 18

describe how synthetic lethality found

Answer 18

first described by american geneticist calvin bridges in early 20th ce
Developed from genetic studies in model organisms = yeast, fruit flies

Question 19

describe synthetic lethality in context of cancer

Answer 19

inhibiting the activity of a protein that acts downstream of the missing tumour supressor gene product along a signalling pathway
if gene a knocked out by drug and gene b is cancer mutation
= leads to cellular death since cell relies on gene a since b is cancer

Question 20

Benefits to using synthetic lethality-based strategy

Answer 20

selective for cancer cell specific genetic mutations
strategy can be applied to any type of cancer mutation including tsgs and mutations deemed undruggable - like ras

Question 21

describe translating synthetic lethality into the clinic PARP inhibitors and DNA damage repair

Answer 21

first synthetic lethal therapy using parp inhibitors for patients with brca1/2 mutant ovarian and breast cancers
parp does ber and brca does homologous recomb cell repair
if brca not working - parp will take over, but since brca is cancer if parp inhibited = no repair and cell death
if brca out = use parp1, if inhibit parp, fine since has other means
but if breast cancer = more dependent on parp, so use inhibitor = target brca mutated cells - synthetic lethal

Question 22

describe screening for synthetic lethal interactors

Answer 22

oncogenic ras pushing cell
one compound that wont affect normal ras cells - does not have to affect ras pathway, can affect anything

Question 23

what are hela cells

Answer 23

henrietta lacks
cervical cancer
cells still used to this day
immortalized = cells can divide forever

Question 24

what is replicative senescence

Answer 24

irreversible halt in cell proliferation with retention of cell viability over extended periods of time - cells metabolically active but exited cell cycle forever
when quiescence = can go back into cell cycle

Question 25

what is crisis

Answer 25

genetic catastrophe that leads to death by apoptosis
cells must bypass this to become immortalized

Question 26

what is senescence

Answer 26

process of growing old
cells that remain metabolically active but have lost ability to reenter cell cycle

Question 27

what is hayflick limit

Answer 27

first demonstrated by leonard hayflick in 1960s = how many cell divisions can it do on plastic - ~50 before stop and enters senescence
number of time a normal human cell population will divide untill cell division stops

Question 28

describe the many roads to senescence

Answer 28

shortening of telomeres = also trigger senescence
cells have mechanisms to leave cell cycle and not contaminate pool and propagate
cytotoxic drugs, oncogene activation, telomere dysfunction, cell culture, oxidative stress, dna damage
oncogene induced senescence = must be bypassed for cells to be immortalized

Question 29

what is senescence triggered by

Answer 29

telomere shortening which can be bypassed by disruption of tsg pathways
usually p53 upregulated when telomeres shortened

Question 30

what happens after senescence bypass

Answer 30

cells undergo crisis
chromosomes fuse leading to apoptosis - p53 independent
ASYNCHRONOUS EVENT - happens in cells at diff times

Question 31

describe mechanisms of breakage fusion bridge cycles

Answer 31

chromatids ends start to fuse and during anaphase = snaps and breaks = new fusion products = mitotic arrest and die

Question 32

how can cells escape crisis

Answer 32

cancer cells escape by expressing telomerase enzyme to keep telomeres long - 85-90% of human tumours are telomerase positive
low survival is expresses telomerase

Question 33

when cells have escaped crisis what thennn?

Answer 33

can proliferate indefinitely and are said to be immortalized