LEC54: Cancer Pathways and Tumor Biology Flashcards
what are the components of the Rb pathway
how are Rb pathway targets altered in cancer?
in a mutually exclusive manner: it’s not merely Rb that’s the target, the whole pathway is the target
i.e. deletion of p16 is mutually exclusive w/ D1 and Rb, and the same tumor won’t sustain mutations in multiple components of the same pathway unless the pathway’s inactivated
what mutations happen to p16? result?
deletion, missense mutation, promoter methylation
it is deleted in sporadic melanoma, muated in many melanomas
what mutations happen to cyclin D1 gene?
amplification, chromosomal translocation
what muations occur to genes encoding Cdk4 or Cdk6? result?
amplification, missense mutation
prevents binding of p16
occurs in glioblastoma and other brain cancers
mutations that occur to Rb?
deletion, mutation
what does human papilloma virus (HPV) lead to?
expression of E7 protein
this inhibits pRb
what is pRb? when is it deleted -> result?
tumor suppressor
deleted in retinoblastoma, osteosarcoma
what happens re: cyclins in breast cancer?
cyclin D1 is amplified in some breast cancers
overexpression of D1 inactivates Rb
what does p53 pathway normally cause?
arrest or apoptosis
components of the p53 pathway?
what are mechanisms to arrest the p53 pathway?
1) ARF gene deletion, missense mutation, or promoter methylation
2) Mdm2 gene amplification and Mdm2 protein overexpression
3) p53 missense mutation
4) HPV E6 protein targets p53 for degredation
* ONE OF THESE THINGS, NOT ALL, OCCUR, SHOWING MUTUAL EXCLUSIVITY AND THAT THE PATHWAY IS THE TARGET IN CANCER*
what does cervical cancer inactivate in p53 pathway?
p53
cells w/ a mutant Mdm2 have what kind of p53?
what does this demonstrate?
WT p53
demonstrates the PATHWAY is the target, not the actual GENE or PROTEIN
what is the APC pathway?
what is APC, what is its normal function re: beta-catenin?
APC: adenomatous polyposis coli protein
normally, is a scaffold that interacts w/ Beta-catenin, facilitates its degredation, by bringing GSK-3-beta and beta-catenin teogether
what does loss of APC cause?
upregulation of Myc and Cyciln D, via transcriptional effects of beta-catenin
what do mutations in APC cause?
expression of a truncated protein that can’t bind beta-catenin
what do mutations in beta-catenin cause?
changes in its phosphorylation sites, so it cannot be degraded
if it’s not phosphorylated°raded, doesn’t matter if APC or Axin is present
can cause regulatory issues w/ Myc, Cyclin D
mutations in which parts of the Apc pathway are mutually exclusive in many cancers?
mutations in Apc, Axin, B-catenin
what activates the Erk pathway?
growth factors binding to receptor on cell surface
cause pathway to go
what are the components of the Ras/PI3K pathways?
what does Ras pathway result in cellularly?
cell growth
what is the result of the PI3K pathway?
cell survival
what muations occur to the Erk pathway in human cancers?
1) GENE AMPLIFICATION b/c of GROWTH FACTOR expression
2) CONSTITUITIVELY ACTIVE RTK if GENES encoding CELL SURFACE RECEPTOR are amplified, missense mutated
3) Ras MISSENSE MUTATION -> CONSTITUITIVE ACTIVITY b/c cannot hydrolyze GTP
4) Raf can have missense mutation causing constituitive activity
what is Mek’s relevance to human cancers?
Mek is not usually altered, however can be a good drug target b/c Mek inhibitor will deal with a hyperactive Raf or Ras growth factor receptor that’s constituitively signaling or overexpressed GF
what is the PI3K pathway?
what mutations can occur in the PI3K pathway to cause human cancer?
1) PI3K and Akt can be muated to be constituitively active
2) PTEN, a phosphotase, prevents PI3K signaling and PTEN is often deleted in tumors
what are the mutually exclusive changes to the PI3K pathway that can cause cancer?
PTEN alteration or AKT activation
what are PI3K and Akt considered?
oncogenes
what is PTEN considered?
a tumor suppressor b/c it can reverse teh AKT for PI3K
how is there redundancy in the PI3K pathway?
a receptor mutation means there is also a PTEN pathway mutation; not just a receptor mutation alone impacts teh PTEN pathway, and PTEN alone isn’t mutated
why is colorectal cancer so well documented and studied?
colon is easily accessible
can detect early, middle, late lesions, and analyze them for protein expression
what is the schematic from normal epithelium -> metastisis?
what occurs to the genetic/epigenetic characteristis of a pt as colorectal cancer progresses?
increasing genetic and epigenetic instability
what is the sequence of changes from earliest to latest that occurs in colrectal cancer progression?
1) loss of Apc
2) activation of K-Ras
3) loss of Smad4 and other tumor suppressors
4) loss of p53 (LATE EVENT!)
5) other unknown alterations
what is p16?
CDK inhibitor of the Ink class; tumor suppressor
suppressor of Rb pathway
what is ARF protein?
Mdm2 inhibitor, so can act as a tumor suppressor because it facilitates p53 activation
suppressor of p53 pathway
what is expressed from the CDKN2A locus?
what’s the structure of the locus?
both p16 and ARF
they share exon 2 and 3 but each have their own promoter and exon 1
how are different proteins produced from the CDKN2A locus given its structure?
both p16 and ARF share the CDKN2A locus
but have different 1st exons; 1-beta is ARF and 1-alpha is p16-INK
splicing of the locus puts the 1a-exon2 or 1b-exon2 in different reading frames, creates different RNA, different proteins
what’s relationship of transcriptional regulation of p16 and ARF?
completely independent because each has its own promoter
if a mutation occurs in exon 2 or 3 of CDKN2A locus, what occurs?
inactivation of both Rb via p16 and p53 via ARF pathways because both are encoded at this locus
how does upregulation of ARF occur?
loss of Rb -> activation of E2F -> ARF activation -> Mdm2 suppression -> p53 activation
what upregulates Arf? result?
Myc and Ras upregulate Arf
This inhibits Mdm2, thus activates p53 -> cell-cycle arrest or apoptosis
what is Arf a transcriptional target of?
what does loss of Arf result in?
Arf is target of E2F
E2F expression and inactivation of pRb upregulates p53
how is Arf an oncogene checkpoint?
must both upregulated Myc and mutate p53 ?
what mediates epithelial to mesenchymal transition during normal development?
Slug, Snail, Twist transcription factors
can be altered to have this change in cancer situation
what occurs to cancer cells of epithelial origin to make them behave like mesenchyme cells?
1) loss of cell-cell junctions due to loss of E-cadherin expression
2) loss of cell polarity
3) increased migratory behavior, more motility
what is metastitis dependent on in order to cause cancer-associated death?
1) invasiveness of the tumor
2) immune response and tumor vasculature of the host
what are the 6 key steps of metastisis?
1) angiogenesis
2) invasion
3) intravasation
4) metastasis
5) extravasation
6) secondary growth
when does angiogenesis occur?
what happens?
what is expressed, what is reduced?
whe ntumor mass reaches 1-2mm angiogenesis factors are produced that induce blood vessel formation
acidic FGF, basic FGF, VEGF are expressed factors
inhibitors such as thrombospondin are reduced
what occurs during invasion?
tumor cells become attached to sub-endothelial extracelular matrices via cell surface receptors
protease-mediated degredation of the matrix follows
migration via chemotaxis using the degredation products or tumor-associated autocrine motility factors occurs
what occurs during intravasation?
tumor cells invade through cascular endothelial cells and sub-endothelial basement membranes and enter vasculature
what happens during extravasation?
tumor cells extravasate out of vasculature, into pervascular stroma
this is reverse of invasion!
what occurs during secondary growth?
at distant sites, get formation and growth of secondary tumor metastasis
what is the rational basis of targeted therapies in cancer?
the tumor cell behaves different from the normal cell, it is dependent on the activated oncogene for proliferation and survival, and the normal cell is not
targeted therapies actually make cells w/ a mutation DIE, whereas normal cell’s pathway may be inhibited, but it’ll grow again
