LEC53: Tumor Suppressors

Question 1

what experimentally suggests tumor suppressors exist?

Answer 1

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

Question 2

what about inherited syndromes tells us about tumor suppresor genes?

Answer 2

there are multipkle inherited syndromes that confer hereditary susceptibility to particular tumor types; are consistent w/ existence of tumor suppressor genes

Question 3

what are examples of inherited syndromes that confer hereditary susceptibility to paticular tumor types / what are their associated genes?

Answer 3

familial retinoblastoma - pRb

Wilms’ tumor of kidney - Wt1

hereditary breast cancer - BRCA1, BRCA2

hereditary melanoma - p16, p15

Question 4

what is inheritance pattern of susceptibility to tumor formation?

Answer 4

mendelian dominant

but not all who have inherited defective gene get cancer because of incomplete penetrance

Question 5

do inherited susceptibility to particular tumor types tend to be linked w/ single types of cancer, or more general?

Answer 5

single types of cancer

EXCEPTION THOUGH: LI-FRAUMENI SYNDROME, p53 mutation, results in increased susceptibility to many kinds of tumors

Question 6

what are DNA tumor viruses

Answer 6

contain novel oncogenes within their genomes that don’t have cellular counterparts

i.e. SV40, HPV

Question 7

what is SV40, how does it work?

Answer 7

DNA tumor virus, simin virus 40

expresses a laretumor antigen - T antigen - that binds cellular p53 and pRb and thereby inactivates them

Question 8

what is HPV, how does it occur?

Answer 8

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

Question 9

where does familial vs. sporadic retinoblastoma occur?

Answer 9

familial: both eyes, involves secondary non-ocular tumors
sporadic: one eye, not associated w/ secondary tumors

Question 10

what does mutation in Rb predispose someone to?

Answer 10

retinal cancer, and sometimes osteosarcoma if it’s familial - bone cancer

Question 11

what is knudsen’s 2 hit hypothesis? what does it explain re: retinoblastoma?

Answer 11

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

Question 12

what is different about what is required to activate an oncogene vs. tumor suppressor? and what is mutation result?

Answer 12

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

Question 13

what does Rb do?

Answer 13

it inhibits E2F, which is a gene regulatory protein that controls entry into S phase

thus Rb controls entry into S phase

Question 14

mechanisms for tumor suppressor genes loss in human cancer?

Answer 14

1) deletion
2) missense mutation
3) frameshift muation
4) haplounsufficiency

Question 15

what type of mutation does gene for p53 usually have?

Answer 15

missense muation

Question 16

what is p53 function? when is it upregulated?

Answer 16

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

Question 17

what does a mutational spectrum of p53 show?

Answer 17

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”

Question 18

where on p53 are mutations?

Answer 18

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

Question 19

are hot spots the same in all cancers?

Answer 19

no

different carcinogens are associated w/ different hot spots

Question 20

what are p53’s functions re: oncogenes?

Answer 20

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

Question 21

what does p53’s persistence in cancer state suggest?

Answer 21

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

Question 22

what causes a frameshift mutation?

what is result of a frameshift?

Answer 22

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

Question 23

what is most common type of mutation for most oncogenes?

Answer 23

for p53: missense

for others: frameshift

Question 24

what is APC, adenomatous polypopsis coli protein? its function in healthy vs cancer state?

Answer 24

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

Question 25

what happens to pRb to cause cancer?

Answer 25

usually deletion

Question 26

what is Loss of Heterozygosity?

Answer 26

when there’s a missense or frameshift muation of 1 allele of a tumor suppressor, the remaining WT allele is deleted

Question 27

what does haploinsufficiency differently require for tumor suppressor inactivation?

Answer 27

only 1 hit, not 2 hit- only 1 allele needs to be altered for haploinsufficiency to manifest

Question 28

what are the mechanisms of haploinsufficiency?

Answer 28

1) reduced expression
2) dominant negative effects
3) transcriptional silencing

Question 29

what is “reduced expression” method of haploinsufficiency?

Answer 29

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

Question 30

what is “dominant negative effect” method of haploinsufficiency?

what is required?

Answer 30

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

Question 31

what is “transcriptional silencing” method of haploinsufficiency?

how might it occur?

Answer 31

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

Question 32

why are cancer genes like BRCA1 called “susceptibility genes”?

Answer 32

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