case 4 - bowel cancer Flashcards
what is the majority of CRCs
sporadic
what percentage of bowel cancers are FAP
1%
what surface of the colon do the polyps line
the lumen surface of the colon
what is HNPCC
hereditary non-polyposis colon cancer
what is HNPCC
hereditary non-polyposis colon cancer
what percentage of CRCs are HNPCCs
2-3%
what is Lynch Syndrome
when you are predisposed to other cancers
what do these other cancers include
ovary, small intestine, urinary tract, skin and brain cancer
what is the difference between FAP and HNPCC
there are very few polyps in HNPCC but the progression is fast, 2-3 years instead of the 8-10 years in FAP
what are FAP and HNPCC
both familial forms of cancer
what are the hallmarks of cancer
evading apoptosis
self sufficiency in growth signals
insensitvity to anti growth signals
sustained angiogenesis
tissue invasion and metastasis
avoiding immune destruction
genomic instability
what is the implication gene type for oncogenesis and TSG
diploid
is is stage G1 in the cell cycle
cell is born and each chromosome is a single chromatid
what is the S phase of the cell cylce
synthesis - DNA replication (replicate the chromosomes)
what happens when the S phase is over
each chromosome is represented by two sister chromatids
what happens after S phase
G2
what happens after G2
mitosis
what happens during mitosis
division: chromatids get pulled apart in opposite directions, mean that each new cell inherits one copy of each chromosome in them. once they go back to G1, each cell yet again is represented by one chrmoatid
what are the steps from gene to tissue
- genome
- transcriptome
- proteome - via process of translation
- cell function - biogenesis and metabolism
- tissue architecture - proliferation
what happens if there is mutations in these genes
we have changes and this affects cell function and proliferation
what turns into malignancy
hyperplasia
what are most tumours made up fo
lots of different clones - tumour heterogeneity
what does multi step tumorgenesis involve
clonal expansion
what are oncogenes encoded by
viruses that can cause cancer e.g SV40 virus
what happens when viral genes are introduced into cells
they have a dominant transformative effect e.g Src, Ras, Myc
what is the case of oncogenes
it is sufficient to only have one copy of a mutated gene to cause cancer
what are examples of single activating mutations
B catenin and KRAS
what are tumour suppressor genes linked with
retinoblastoma and Knudson’s two hit hypothesis
what is retinoblastoma
a type of cancer that affects children and either comes in a familial or sporadic form
what did Knudson notice
that individuals with familial disease had eye tumours bilaterally and were more likely to have other tumours
what is retinoblastoma caused by
a mutation on gene 13 (Rb)
what happens with indiviudals with familial retinoblastoma
all inherent one mutant copy of this gene
This mutation is recessive but if one cell requires a second copy, Rb function is lost and this not only predisposes them to bilateral eye cancer, but to other cancers as well
what type of gene is Rb
a tumour supressor gene
what has to happen in sporadic Rb disease
you must first pick up a mutant Rb allelle and then you need a second mutant copy- much more rare event and wont get other cacers
what is the basis of Knudson’s theory
in familial disease, only one random hit is required but in sporadic disease, two hits are required
what do the signalling pathways include;
- ligands
- receptors
- signalling cascade
- transcription factors
- alters cellular function and gene expression
what are the different types of cell biological processes
growth factor receptors and hormone receptors
Receptor tyrosine kinase
G-protein coupled receptors
Phosphatase
GTpase switches e.g Ras
Cytoplasmic -nucelar shuttling
Serine/threonine phosphorylation cascades
All the elements of transcriptional control
Micro RNAs
Translational control
Sumolyation
(Don’t need to know just be aware)
what is RAS
a switch
what does ras bind to
GTP
what does RAS do to GTP
Ras has GTPase activity, it can hydrolyse GTP
what does it hydrolyse GTP to
GDP
what does this release
an organic phosphate
what happens when ras is bound to GTP
it adopts an active conformation and stimulates downstream pathways
what does ras’s own GTPase activity turn it into
GDP/RAS complex which is inactive
the GDP is then exchanged for a fresh molecule of GTP
what happens when these downstream RAS pathways are active
leads to stimulation of cell proliferation and protein synthesis
what happens if there is a single mutation in RASS
locks it in the active form which means these downstream signals continually on, even in the absence of upstream signals. so when RAS is locked on, proliferation is stimulation
how many copies of Ras need to be mutated
only one copy of the RAS gene
what is Wnt
a secreted factor that promotes proliferation by binding to its receptor
what is the Wnt complex made up of
active GSK-3 beta, APC, beta catennin
what happens when this Wnt complex is activated
GSK3 beta phosphorylates beta catenin and leads to degradation.
what happens when Wnt is bound to this complex
it disrupts this complex, now GSK-3 beta is iz inactive and beta catenin doesnt get degraded and beta catenin builds up in the cell
moves into the nucleus and drive the processes for cell proliferation
what is beta catenin
an oncogene
what is APC
a tumour supressor gene
what are the target genes
cyclin D and myc
when is entry into the cell cycle regulated
G1
what happens during this period
cells are responsive to mitogenic GFs and to TGF beta
what happens once it passes the R point in G1
it is now committed to compete the entire process of the cell cycle
what happens if you withdraw stimulatory processes beyond R
the cell cycle will still complete
what do CDK4 and 6 bind to
cyclin D
what does this complex activate
the cell to drive it through G1
what is the CDK2 complex bound to
cyclin E
what does CDK2/cyclin E complex trigger
entry into the S phase
what does CDK2 also bind to
cyclin A
what does this complex do
drives the cell through the S phase
what does CDK1 bind to
cyclin B
what does this CDK1 cyclin B complex drive
cell into mitosis
what happens to the levels of cyclin and the levels of CDK
the levels of cyclin moves up and down but the CDK levels stay relatively the same
what is the rise and fall of cyclin crucial to
the regulation of the cell cycle
what inhibits the cyclin-CDK complex
CKIs
what blocks the activity of multiple CDKs
p21
what do growth factors signal
RAS, which along with Wnt signals, signals beta catenin
what does RAS signal
fos/jun
what does the beta-catenin signal
tcf/lef
what do both ras and wnt signal
signal cyclin d1
what happens once cyclin D is synthesised
it binds to CDK4, which forms an active protein kinase complex
what can this active protein kinase complex phosphorylate
a protein called Rb
what happens when rb gets phosphorylated
it lets go of its binding partner E2F
what is E2F and what happens once it is liberated
it is a transcription factor and once liberated, it can go onto stimulate the expression of genes like the cyclin E gene
what does this expression of cyclin E lead to
the cyclin E- CDK2 complex
what can this complex go on to do
phosphorylate Rb which leads to positive feedback cycle
what can this cyclin E -CDK2 complex also do
phosphorylates origins of replication to trigger S phase
what is Rb in the cell cycle
a break, and if you remove this break then you have uncontrolled replication in the cell cycle (tumour supressor)
what happens once the second copy of Rb is hit
the break is lost
what is the most important cell cycle checkpoint
p53
what is p53
a transcription factor
it is normally degraded
what is p53 activated by
‘stabilisation’
what are the ligases
kinases and ubiquitin ligases
what happens once p53 is activated and stabilised
it can lead to
- cell cycle arrest
- DNA repair
- block of angiogenesis
- apoptosis
OR
- senescence
- return to proliferation
what is a very important target of p53
the p21 gene which is a cyclin dependent kinase inhibitor which can block the activity of multiple CDKs
what does p21 do
puts a block on the cell cycle, giving it enough time to repair and then the cell cycle continues
what happens if the damage is too bad to be repaired
p53 leads to apoptosis, and stimulates the expression of Bax etc, which induces cell death
what is the guardian of the genome
p53
what is the genome being continuously damaged by
xoxidation
Replication errors
UV
X-rays
Chemicals
Mitosis
what continuously repairs the genome
BER
NER
Proof reading
NHEJ
HR
Checkpoints
what is the very important repair mechanism on the context of colon repair
the mismatch repair and HNPCC
how many cases of bowel cancer are there a year
40,000
what percentage of people will be alive after their initial diagnosis ten years on
57%
what is the abnormal cell growths:
Hyper proliferation
Adenomatous polyps (small)
Adenomatous polyps (large)
Severe dysplasia (precancerous polyp)
Adenocarcinoma - malignant
Invasive cancer - malignant
what are the colonic crypts
invaginations where the epithelial sheet has folded back upon itself multiple times
there is an intricate substructure to this structure
what kind of cells are there between the Paneth cells in these crypts
rapidly dividing stem cells
what do these stem cells produce
produce two daughter cells one stays behind to maintain the stem cell compartment and the other migrates up and out and eventually is shed off at the top of the lumen
what is the most important feature of the tissue
it is highly proliferative, and maintained by balancing loss at the top with growth at the bottom
what protein do all these stem cells express at the base of the crypts
Lgr5+
what is the gene that is responsible for FAP
5q21
what does APC stand for
adenomatous polyposis coli and this is the name of 5q21
in what percentage of sporadic CRCs is APC mutated in
80-90%
what type of gene is APC
tumour suppressor gene
what happens to individuals with the disease
they inherit one mutated copy and then the second allele undergoes LOH,
what happens to individuals with the disease
they inherit one mutated copy and then the second allele undergoes LOH, APC protein function is lost and tumour suppressor function is lost and then will lead to cancer
what happens when Wnt is not bound to its receptor
the APC complex is attacked and beta catenin is targeted for degradation
what happens when Wnt signals bind to their receptors
beta-catenin no longer gets phosphorylated and no longer gets degraded, it moves into the nucleus and up regulates cyclin D - proliferation
what cells secrete the Wnt signals
the surrounding stromal cells
what do the Wnt signals engage with
the stem cells
when is proliferation stimulated
when beta catenin is switched on
what happens when beta catenin is switched on
the cells then migrate up and out of the crypt and because they are still in close proximity to the Wnt signals they keep on proliferating
what happens when they are too far up and far away from the Wnt signals
beta catenin is switched off and gets degraded. the cells stop dividing and disappear out of the top
what happens once the cell loses APC function
it is no longer reliant on Wnt signals for proliferation.
what does this mean
it means that as it migrates up, it can still proliferate, even in the absence of the Wnt signals. so now the APC mutants move up beyond the zone of Wnt signalling, they just dont care
what stays on even if there are no Wnt signals and explain how the hyperproliferative zone comes to be
beta catenin - the cells dont differentiate, they keep on proliferating and they dont finish migrating, forming a hyperproliferative zone of cells when normally no division would take place
where do normal epithelial cells proliferate
only proliferate at the bottom when they are close to the secreted Wnt signals.
what is the difference to this in APC mutants
B-catenin is stabilised even in the absence of Wnt signals, cyclin D is synthesised, driving proliferation even when the cells have migrated beyond the normal proliferative zone
how many CRCs have beta catenin mutations
10-20%
what kind of mutation is a beta catenin mutation
single point mutation that stops it being phsophorylated - stops it being degraded.
what is an obligate step in colon cancer
deregulation of the Wnt pathway
where is the HNPCC gene found
found on chromosome 2p
what is the mismatch repair
error in newly made strand
binding of mismatch proofreading proteins
DNA scanning detects nick in new DNA strand
strand is removed
repair DNA synthesis
what is silenced by methylation in around 15% of sporadic CRCs
MLH1 promotor is silenced
what is the mutator phenotype of mismatch repair
mutation of MHL1 does not cause cancer.
but rather, by losing MSH2, or MLH1 function, the mutation rate goes up and that accelerates the accumulation in traditional tumour suppressor and oncogenes
what is mutated in about 90% of mismatch repair CRCs
the TGF beta receptor is mutated
what can we see when we sequence genes
whole genome - CNV
Exome - mutations
Promoter methylation
RNA - global gene expression profiling
Proteomics (protein level) - net effect of how mutations and changes in gene expression manifest
what are the hyper mutated colon cancers
the mismatch repair colon cancers
80% dont have mismatch repair defectd
what do the hyper mutated tend to be
diploid i.e the CNV is low
what do the non hypermutated tend to exhibit
CIN and therefore are aneuploid i.e the CNV is high
what are the specific mutations
APC, p53, and K-ras
what is the novel b catenin mutation
29 cases
deletions
spanning exon 3
what is the novel APC mutations
35 cases
upstream of exon 9
slicing defect
protein truncation
where are mutant proteins expressed
on the cell surface
what are these mutant proteins recognised by
T cells
what do tumour cells also express
PD-L1 which suppresses T cells
what can antibodies do
block the PD-1 interaction
what does this reuslt in
some cancers will benefit greatly from antibody therapies and given immune checkpoint inhibitors to resolve this inteaction
what is the antibody which blocks PD1
Pembrolizumbab
what are the 3 mutation subtypes
POLE ultra mutated <1%
MSI - hyper mutated due to micro satellite instability - 9% (mis match repair defect)
MSS - micro satellite stable - 90% (but high CNV due to CIN)
what kind of target is the Wnt pathway
an obligate and early target, either via APC or beta catenin
what tumours are prioritised for immune checkpoint therapy
MSI tumours
what are liquid biopsies
tumours shed cells and DNA into the blood
Minimally invasive and relative inexpensive biopsy
Advances in detection technology e.g NGS, mass spec
what are the benefits of liquid biopsies
early detection
Predictive biomarkers
Monitor response to therapy
