Hallmarks of Cancer I

Question 1

The Big 8 - Hallmarks of Cancer

Answer 1

1) Avoiding immune destruction
2) Evading growth suppressors
3) Enabling replicative immortality
4) Activating invasion and metastasis
5) Inducing angiogenesis
6) Resisting cell death
7) De-regulating cellular energetics
8) Sustaining proliferative signaling

Question 2

Who Rb?

Answer 2

A governor of proliferation and differentiation

A key negative regulator of the G1-S transition

Cancer inactivates it => two possible ways:

1) loss-of-function mutation involving both Rb alleles
2) A shift from the active, hypo-phosphorylated state to the inactive, hyper-phosphorylated state by gain-of-function mutations that up-regulate CDK/cyclin D activity or by loss-of-function mutations that inhibit CDK inhibitors

Question 3

Hypo-phosphorylated Rb in complex with E2F transcription factor means what for the cell cycle?

Answer 3

This complex inhibits transcription of genes whose products are required for the S phase of the cell cycle.

Question 4

Phosphorylation of Rb: process and consequence

Answer 4

Rb is phosphorylated by cyclin D/CDK4, cyclin D/CDK6, and cyclinE/CDK2 complexes. When these complexes phosphorylate Rb, they are inhibiting it from keeping hold of E2F.

Upon phosphorylation, Rb releases the E2F transcription factor.

E2F activates the transcription of S-phase genes.

Question 5

Who inhibits phosphorylation of Rb?

Answer 5

Cyclin-dependent kinase inhibitors phosphorylate Rb indirectly because CDKIs inhibit cyclin-CDK complexes, which are the complexes that phosphorylate RB.

Question 6

What does an active Rb look like, and where is it found?

Answer 6

A active Rb is hypo-phosphorylated, attached to E2F, and found in quiescent cells.

Question 7

What does an inactivate Rb look like, and where is it found?

Answer 7

An inactive Rb is hyper-phosphorylated, thanks to high levels of the cyclinD/CDK4, cyclinD/CDK6, and cyclinE/CD2 complexes (these complexes are up-regulated by growth factors). It has released the E2F transcription factor.

It is found in cells passing through the G1/S cell cycle transition.

Question 8

Growth factor signaling pathway vs. Growth inhibition signaling pathway

What does Rb have to do with these pathways?

Answer 8

Growth factors up-regulate the cyclins and CDKs required for transitioning through phases in the cell cycle, whereas growth inhibitors up-regulate CDKIs to stop progression through cell cycle phases.

Rb is the point of integration of these opposing signals, making it the key regulator of cell cycle progression.

Question 9

According to current cancer data, which molecules are typically mutated or affected in patients with cancer?

Answer 9

p16/INK4a
cyclin D
CDK4
Rb

Either these guys have mutations, or they are affected by an abnormal molecule upstream of them in the signaling pathway (meaning they’re normal, but cannot be activated/deactivated due to the struggler ahead in line).

Question 10

What can oncogenic viruses do to Rb?

Example?

Answer 10

The viral protein can bind to Rb, functionally inhibiting it so that it can no longer act as a cell cycle inhibitor (it can no longer hold on to E2F because the virus took that spot, so E2F goes off and transcribes things needed for the S phase of the cell cycle).

Example: HPV types express a protein called E7 that binds with higher affinity to Rb than E2F does, leading to a high risk for cervical carcinoma.

Question 11

blue 52! blue 52! Who p53?

Answer 11

Ye ol’ p53 is the guardian of the genome and the central monitor of stress in the cell.

Activated by anoxia, inappropriate signaling by mutated oncoproteins, or DNA damage

Many jobs:

1) Prevents propagation of genetically damaged cells
2) Binds to DNA
3) Arrests cell cycle for DNA repair
4) Initiates apoptosis if repair impossible

He lives only for half-life of 20 minutes and is killed by ubiquitin proteolysis.

Question 12

Is p53 affected in patients with tumors? If so, how?

Answer 12

Yes, it most certainly can be affected; 70% of the time there is bi-allelic loss of p53.

Example: HPV expresses E6 protein, which degrades p53 (recall there is also an E7 protein associated with HPV).

Question 13

TP53

Answer 13

A tumor suppressor gene that regulates cell cycle progression, DNA repair, cellular senescence, and apoptosis

A gene that encodes p53.

The most frequently mutated gene in cancer; found in 50% of cancers

Mutation is not usually inherited, so it’s found in somatic cells, with both alleles of the gene mutated.

Question 14

Li-Fraumeni Syndrome

Answer 14

The syndrome you have when you DO end up inheriting a mutated TP53 allele.

You have a 25-fold greater chance of getting cancer because you’re now just one mutation away from inactivating TP53

Question 15

MDM2

Answer 15

An inhibitor of p53. Phosphorylation of p53 releases it from the clutches of MDM2

It is over-expressed in 33% of malignancies. It stimulates the degradation of p53.

Question 16

E6 and E7

Answer 16

Proteins expressed by HPV that bind to RB with higher affinity than E2F, causing inactivation of RB and therefore progression of cell cycle to S phase, as E2F is now free to transcribe the proteins required.

Question 17

p53 is a transcription factor. The key target genes that execute the functions of p53 are not really defined, but tend to fall in three categories:

p53 also affects genes that encode two kinds of regulator RNA. What are they?

Answer 17

KEY TARGET GENES
1) those that cause cell cycle arrest

2) those that cause apoptosis
3) those that enhance catabolic metabolism or inhibit anabolic metabolism

REGULATORY RNA

1) micro-RNA (mIRs)
2) long intervening noncoding RNAs (LINC RNAs)

These help to coordinate the p53-dependent cellular response to stress.

Question 18

How is DNA damage sensed?

Answer 18

Damage is sensed by complexes containing the kinases of the ATM/ATR family.

The kinases phosphorylate p53, freeing it from inhibitors such as MDM2.

Active p53 up-regulates p21, a CDKI that inhibits cell cycle progression to the S-phase, standing guard at the G1/S checkpoint.

DNA repair can now occur during the pause in cell cycle progression. Hopefully, for the cell’s sake. If no, then bye-bye cell, ‘ello apoptosis!

Question 19

Von Hippel Lindau

Answer 19

A gene product that causes ubiquination and degradation of hypoxia inducible transcription factor-1.

Result: Increased PDGF and VEGF => tumor angiogenesis.

Question 20

Adenomatous polyposis coli (APC) and consequence of its germline loss-of-function mutation

Answer 20

A tumor suppressor that down-regulates growth-promoting signaling pathways.

Molecular Consequence: loss of APC causes cells to behave as if they’re continuously stimulated by WNT.

Clinical Mutation Consequence: Familial adenomatous polyposis, an autosomal dominant disorder in which people born with one mutant allele develop thousands of polyps in the colon, one or more of which will invariably undergo mutant transformation, giving rise to colon cancer IF the other allele also ends up suffering a mutation.

Question 21

Does APC really have anything to do with colorectal carcinomas?

Answer 21

Fo sho’. 70% of non-familial colorectal carcinomas and sporadic adenomas show acquired defects in both APC genes; thus, APC loss-of-function mutations are important in the formation of colonic tumors.

Question 22

General relationship between Wnt/Frizzled, APC, and B-catenin

Answer 22

Wnt/Frizzled sends a signal to APC

APC responds by failing to form a proteosomal degradation complex to degrade B-catenin.

B-catenin heads to nucleus, forms a complex with DNA binding factor TCF, and they together promote colonic epithelial cell growth

Question 23

Consequence of loss of APC or lack of Wnt signaling to APC

Answer 23

If Wnt does not send signals to APC, APC forms a complex that degrades B-catenin. B-catenin can no longer promote colonic epithelial cell growth.

If lack APC, as many colorectal patients do, APC is not available to degrade B-catenin, and unfettered proliferation of colonic epithelial cell growth results.

Dysregulation of APC plays a role in cancers other than colorectal.

Question 24

B-catenin

Answer 24

A proto-oncoprotein that plays a role in colonic epithelial cell growth.

A gain-of-function mutation could lead to hepatoblastoma or hepatocellular carcinomas.

If APC is lost (it is upstream of B-catenin in signaling process), then it can also enjoy unfettered cell proliferation.

Question 25

The relationship between B-catenin and E-cadherin.

How can their relationship lead to cancer?

Answer 25

E-cadherin maintains cellular adhesiveness. B-catenin is bound to E-cadherin. Their association is disrupted upon injury or trauma. In response, B-catenin heads to the nucleus, where it stimulates genes that promote proliferation that help with wound repair. Once repair is finished, B-catenin re-associates with E-cadherin and therefore remains sequestered in the cell membrane; this means these cells are “contact-inhibited”. In the words of Dr. Nichols, “This is good.”

Cancer: Loss-of-contact inhibition or mutation of the E-cadherin/B-catenin complex can lead to cancer. Loss of E-cadherin (and therefore cell adhesion) also allows easy disaggregation of cells, which means that they can then head out to another site and invade (or invade locally).

Question 26

What is the consequence of reduced cell surface expression of E-cadherin?

Answer 26

Carcinoma, especially in esophagus, colon, breast, ovary, and prostate.

Question 27

CDH1

Answer 27

The E-cadherin gene

A germline loss-of-function mutation in this gene causes familial gastric carcinoma.

Question 28

TGF-B: Normal and Cancer

Answer 28

Normally an inhibitor of proliferation

1) Bind TGF-B receptors I and II, causing them to dimerize
2) Dimerization initials intracellular signals (involving the SMAD family of proteins) that turn ON anti-proliferative genes and turn OFF proliferative genes.
3) T These changes result in hypo-phosphorylation of Rb, causing it to hang on to E2F.

…But also a double-edged sword that can promote or prevent tumor growth

1) Loss-of-function mutation in the TGF-B signaling pathway
2) Mutations in TGF-B II receptor that lead to cancers of the colon, stomach, and endometrium
3) Mutational inactivation of SMAD4 leads to pancreatic cancer
4) Loss of TGF-B mediated growth inhibition via mutations causing loss of p21 or sustained expression of MYC (MYC is a growth promoter)
5) Sometimes preserved elements of TGF-B can allow for immune evasion and angiogenesis.

Question 29

PTEN

Answer 29

A membrane-associated phosphatase that acts as a tumor suppressor by serving as a brake on the P13K/AKT arm of the receptor tyrosine kinase pathway.

Gene function can be lost through deletion, deleterious point mutations, or epigenetic silencing; seen notoriously in endometrial carcinoma.

Can be mutated, causing Cowden syndrome

Question 30

Cowden syndrome

Answer 30

An autosomal dominant disorder marked by frequent benign growths, such as skin appendage tumors, as well as an increased incidence of epithelial cancers, particularly of the breast, endometrium, and thyroid.

Question 31

CDKN2A

Answer 31

This locus encodes:

1) p16/INK4a, a CDKI that augments Rb function (allows it to hold tight to T2F)
2) ARF, which stabilizes p53

Question 32

NF1

Answer 32

This gene encodes neurofibromin 1, a GTPase that acts as a negative regulator of Ras (recall that activation of Ras leads to signaling that allows for gene transcription and that Ras is active only when GTP is bound)

Germline loss-of-function mutations cause Neurofibromatosis Type I

Question 33

NF2

Answer 33

This gene encodes neurofibromin 2, a cytoskeletal protein involved in contact inhibition (recall the relationship of E-cadherin and B-catenin as an example of a contact inhibition relationship).

Germline loss-of-function mutation leads to Neurofibromatosis Type II.

Question 34

WT1

Answer 34

This gene encodes a transcription factor that is required for normal development of genitourinary tissues

Germline loss-of-function mutations are associated with a pediatric kidney cancer called Wilms tumor.

Question 35

PTCH1

Answer 35

This gene encodes a membrane receptor that is a negative regulator of the Hedgehog signaling pathway.

Germline loss-of-function mutations cause Gorlin syndrome

Acquired bi-allelic loss-of-function mutations of PTCH1 are seen frequently in basal cell carcinomas and medulloblastomas.

Question 36

Gorlin Syndrome

Answer 36

An autosomal dominant disorder associated with high risk of basal cell carcinoma and medulloblastomas.

Caused by a germline loss-of-function mutation in PTCH1.

Question 37

Wilms Tumor

Answer 37

A pediatric kidney cancer caused by a germline loss-of-function mutation in WT1.

Question 38

Neurofibromatosis I and II

Answer 38

Caused by germline loss-of-function mutations in NF1 and NF2