LEC53: Tumor Suppressors Flashcards
what experimentally suggests tumor suppressors exist?
1) when fused a normal and tumor cell in lab, phenotype was of **normal cell; **this means tumor suppressors must exist
2) cancer susceptibility can be inherited, which wouldn’t make sense re: oncogenes, since there are events cause susceptibility to oncogene expression to occur
what about inherited syndromes tells us about tumor suppresor genes?
there are multipkle inherited syndromes that confer hereditary susceptibility to particular tumor types; are consistent w/ existence of tumor suppressor genes
what are examples of inherited syndromes that confer hereditary susceptibility to paticular tumor types / what are their associated genes?
familial retinoblastoma - pRb
Wilms’ tumor of kidney - Wt1
hereditary breast cancer - BRCA1, BRCA2
hereditary melanoma - p16, p15
what is inheritance pattern of susceptibility to tumor formation?
mendelian dominant
but not all who have inherited defective gene get cancer because of incomplete penetrance
do inherited susceptibility to particular tumor types tend to be linked w/ single types of cancer, or more general?
single types of cancer
EXCEPTION THOUGH: LI-FRAUMENI SYNDROME, p53 mutation, results in increased susceptibility to many kinds of tumors
what are DNA tumor viruses
contain novel oncogenes within their genomes that don’t have cellular counterparts
i.e. SV40, HPV
what is SV40, how does it work?
DNA tumor virus, simin virus 40
expresses a laretumor antigen - T antigen - that binds cellular p53 and pRb and thereby inactivates them
what is HPV, how does it occur?
DNA tumor virus, human papilloma virus
makes 2 proteins: e6 and e7
e6 targest p53 for degredation; e7 inactivates pRb
associated w/ almost all human cervical cancer, and some head and neck cancers
where does familial vs. sporadic retinoblastoma occur?
familial: both eyes, involves secondary non-ocular tumors
sporadic: one eye, not associated w/ secondary tumors
what does mutation in Rb predispose someone to?
retinal cancer, and sometimes osteosarcoma if it’s familial - bone cancer
what is knudsen’s 2 hit hypothesis? what does it explain re: retinoblastoma?
explains difference in manifestation of familial vs. sporadic inheritance of Rb cancer, where familial -> bilateral disease and sporadic -> unilateral disease
familially inherited retinoblastoma ppl begin w/ 1 mutant Rb allele in all retinal cells, so only takes 1 somatic mutation to get 2 mutant Rb gene copies in all cells
vs. sporadic retinoblastoma, needs to get 1 mutant Rb allele developed, then another somatic mutation to get 2; this is a rarer event, so unifocal disease results
what is different about what is required to activate an oncogene vs. tumor suppressor? and what is mutation result?
oncogene: single allele mutation event creates gain of function mutation -> cell transformation
tumor suppressor: mutation required on both alleles, causes loss of function -> cell transformation
what does Rb do?
it inhibits E2F, which is a gene regulatory protein that controls entry into S phase
thus Rb controls entry into S phase
mechanisms for tumor suppressor genes loss in human cancer?
1) deletion
2) missense mutation
3) frameshift muation
4) haplounsufficiency
what type of mutation does gene for p53 usually have?
missense muation
what is p53 function? when is it upregulated?
p53: tumor suppressor, involved in cellular response to DNA damage. it is a txn factor that binds to DNA, recruits coactivators to alter gene expression
p53’s upregulated through a disruption of its interaction w/ mdm2, the Ub-ligase
what does a mutational spectrum of p53 show?
shows each residue of the p53 protein (393 aa in p53), plots number of mutations at each residue
see frequency of alterations are clustered in evolutionarily conserved domains, “hot spots”
where on p53 are mutations?
6 hot spots that correspond to the central domain of the protein, where it binds to DNA - the DNA-binding domain
R175, G25, R248, R249, R273, R282 are the hot spots
R248 adn R273 directly contact DNA; remaining hot spots play role in structure pf 53 to allow these contacts w/ DNA
are hot spots the same in all cancers?
no
different carcinogens are associated w/ different hot spots
what are p53’s functions re: oncogenes?
1) cause cell cycle arrest: oncogene activation by proteins like Myc, Ras, E1a leads to upregulation of ARF; ARF inhbits Mdm2, thereby upregulating p53
2) cause apoptosis by upregulation as a txn factor, of target genes like p21 - a cdk inhibitor- or PUMA and Noxa - apoptosis promoters
what does p53’s persistence in cancer state suggest?
rather than being deleted altogether, p53 is mutated by missense mutation in cancer state
suggests that p53 muation doesn’t merely cause LoF; p53 in its mutated form may also confer additional activities that’re important for cancer progression
what causes a frameshift mutation?
what is result of a frameshift?
1 or 2 base insertions of deletions
changes the reading frame of a gene
results in expression of a truncated protein, and a STOP codon within ~30 bases
what is most common type of mutation for most oncogenes?
for p53: missense
for others: frameshift
what is APC, adenomatous polypopsis coli protein? its function in healthy vs cancer state?
tumor suppressor gene
HEALTHY: In Wnt pathway, APC binds to beta-catenin, targets beta-catenin for degredation by phosphorylating it, SCF ubiquitin ligase destroys it, keeps beta-catenin levels low & prevents tumor
CANCER: APC frameshift mutation results in truncated protein, it cannot bind to B-catenin, B-catenin get tumor formation
what happens to pRb to cause cancer?
usually deletion
what is Loss of Heterozygosity?
when there’s a missense or frameshift muation of 1 allele of a tumor suppressor, the remaining WT allele is deleted
what does haploinsufficiency differently require for tumor suppressor inactivation?
only 1 hit, not 2 hit- only 1 allele needs to be altered for haploinsufficiency to manifest
what are the mechanisms of haploinsufficiency?
1) reduced expression
2) dominant negative effects
3) transcriptional silencing
what is “reduced expression” method of haploinsufficiency?
mutation of 1 allele can result in reduced expression of that allele as the tumor suppressor protein
if need a certain amount of tumor suppressor protein for good functioning, it may not be sufficienct if it’s mutated
causes loss of growth control
what is “dominant negative effect” method of haploinsufficiency?
what is required?
occurs when 1 allele is mutated, the other is WT, and mutant allele in complex w/ WT results in mutant inhibiting the action of the WT protein or otherwise poisoning it
only relevant for an active mutated allele, not a deleted allele - altered allele must be making something for this effect
what is “transcriptional silencing” method of haploinsufficiency?
how might it occur?
when 1 allele is inactivated by mutation and remaining WT allele isn’t expressed b/c silenced through some epigenetic means
i.e. promoter methylation of WT allele -> silencing
net effect: no expression, even though looks like a normal and a mutated allele when look at the DNA
why are cancer genes like BRCA1 called “susceptibility genes”?
b/c they have incomplete penetrance - presentation of inherited bad copy of allele doesn’t mean disease state will result
penetrance in this case is highly variable
thus cannot tell woman with certitude what to do if she is a carrier of BRCA1 mutation- like angelina jolie
