GenPath Chapter 7

Question 1

Correlate growth factors and cancer

Answer 1

Some cancer cells synthesize the same growth factor to which they are responsive, creating and autocrine loop…

e. g. glioblastomas often express both PDGF and PDGFreceptor
e. g. sarcomas over express TGF-alpha and its receptor EGFR

Question 2

What are receptor tyrosine kinases?

Answer 2

Transmembrane proteins with an extracellular growth factor-binding domain and a cytoplasmic tyrosine kinase domain

Activated by binding of a specific growth factor

e. g. ERBB1 encodes EGFR
e. g. ERBB2 encodes HER2
e. g. ALK

Question 3

What does receptor tyrosine kinase activation stimulate?

Answer 3

RAS

…and 2 downstream “arms” the MAPK cascade and PI3K/AKT pathway

Question 4

What constitutes the most common type of abnormality involving proto-oncogenes in human tumors?

Answer 4

RAS family

Question 5

What prevents uncontrolled RAS activity?

Answer 5

GAPs (GTPase-activating proteins)
- normally RAS flips back and forth between an excited signal-transmitting state in which it is bound to GTP and a quiescent state in which it is bound to GDP

Question 6

Describe the MOA of mutated RAS (point mutations in RAS)

Answer 6

Reduced GTPase (GAP) activity

These mutated forms of RAS are trapped in the activated GTP-bound form, and as a result the cell receives pro-growth signals continuously

(NF1 is an example of a tumor suppressor gene that acts through negative regulation of RAS)

Question 7

Are the MAPK and P13K/AKT down or upstream of RAS

Answer 7

Downstream of RAS are present with active RAS

RAS -> RAF -> MAPK -> activation of transctiption
RAS -> PI3K -> Akt -> progrowth metabolism

Question 8

BRAF mutations MOA

Answer 8

BRAF (member of RAF family) found in leukemia, melanoma, some colon carcinomas

Activating mutations in BRAF stimulate downstream kinases and ultimatley activate transcription factors –> growth

Question 9

Describe MOA of mutations in kinases of the PI3K family

Answer 9

PI3K is downstream of RAS and activates Akt which leads to pro-growth metabolism

w/ mutations (30% of breast carcinomas) gain-of-function mutations of PI3K as the inhibitor of PI3K (PTEN) is mutated (e.g. endometrial carcinoma) and therefore PTEN cannot inhibit PI3K –> unregulated growth

Question 10

What orchestrates cells through the cell cycle?

Answer 10

Cyclin-dependent kinases (CDK) are activated by binding to cyclins

CKD-cyclin complexes phosphorylate cricual target proteins that drive cells forward through the cell cycle

Question 11

What are the two main cell checkpoints?

Answer 11

G1S - main cell checkpoint, is DNA damaged

• gain of function mutations in D cyclin genes and CDK4/6 occur in G1
• CDK2 in S phase

G2M - did chromosomes form properly?
- CDK1 occur at the end of G2 phase

Question 12

CDK4; D cyclins main function

Answer 12

Form a complex that phosphorylates RB, allowing the cell to progress through the G1 restriction point

Question 13

Function of cell cycle inhibitors and types?

Answer 13

CIP/KIP family: p21, p27 (CDKNIA-D): block cell cycle by binding to cyclin-CDK complex; p21 induced by p53 and p27 responds to TGF-beta

INK4/ARF family (CDKN2A-C) - p16 inhibits RB and p14 increases p53 levels

Question 14

What are the three tumor suppresors genes?

Answer 14

Retinoblastoma (Rb)

p53

P16/Ink4a/AF (CDKN2A)

Question 15

Function of the cell cycle checkpoint component Rb

Answer 15

Retinoblastoma

tumor suppressive protein that binds E2F transcription factors in its hypophosphorylated state, preventing G1/S transition

Interacts w/ transcription factors that regulate differentiation

Question 16

Function of the cell cycle checkpoint component p53

Answer 16

Tumor suppressor altered in the majority of cancers

Induced by DNA damage

Causes cell cycle arrest by upregulating the CDK inhibitor p21

induces apoptosis by upregulating BA and other pro-apoptotic genes

Question 17

What is the function of Rb (retinoblastoma)

Answer 17

Is a tumor suppressor gene

Inhibits G1/S transition - it exists in an active hypophosphorylated state in quiescent cells and an inactive hyperphosphorylated state in cells passing through the G1/S cell cytle

Function can be compromised in (1) loss-of function mutations involving both RB alleles (2) shift from teh active hypophosphorylated state to the inactive hyperphosphorylated state caused by gai-of-function mtations that upregulate CDK/cyclin D activity or by loss-of function mutations that abrogate the activity of CDK inhibitors

Question 18

What happens when the cell enters the S phase?

Answer 18

obligated to complete mitosis

Question 19

One of 4 key regulators is typically dysregulated in the vast majority of cancers….list the 4

Answer 19

Retinoblastoma (Rb)

p16/INK4a

Cyclin D

CDK4

Question 20

What gene is considered the guardian of the genome?

Answer 20

TP53

Question 21

Function of TP53

Answer 21

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

is the most frequently mutated gene in human cancers

Question 22

What does p53 do?

Answer 22

Tumor suppressor gene

p53 is a focal point of a large network of signals that sense cellular stress, primarily DNA damage, but also shortened telomeres, hypoxia, and stress caused by excessive pro-growth signaling as may occur in cells bearing mutations in genes such as RAS and MYC

Question 23

What holds p53 at bay?

Answer 23

MDM2 an enzyme the ubiquitinates p53 – > leading to its degradation by teh proteasome

Question 24

When a cell is stressed, p53 is released from the inhibitory effects of MDM2 by 2 mechanims…

Answer 24

1. DNA damage and hypoxia - key initiator of p53 activation following DNA damage or hypoxic stress are two related protein kinases, ataxia-telangiectasia mutated (ATM) and ataxia-telgiectasia and Rad3 related (ATR)….p53 accumulates
2. Oncogenic stress - activation of oncoproteins such as RAS leads to sustained, supraphysiologic signaling through pro-growth pathways such as the MAPK and PI3K/AKT cascades

Question 25

What happens once p53 is activated

Answer 25

Once activated, p53 thwarts neoplastic transformation by inducing transient cell cycle arrest, senescnce (permanent cell cycle arrest), or programmed cell death

Question 26

What is the central monitor of stress in the cell?

Answer 26

p53

Question 27

What senses DNA damage and what is the mechanism?

Answer 27

sensed by complexes vontaining kinases of the ATM/ATR family
…these kinases phosphorylate p53 liberating it from inhibitors such as MDM2

Active p53 then upregulates the expression of proteins such as the cyclin-dependent kinase inhibitor p21
–> cell cycle arrest at the G1/S checkpoint –> this pause allows cells to repair DNA damage

Question 28

What is adenomatous polyposis coli (APC)

Answer 28

member of the class of tumor suppressors that function by downregulating growth-promoting signaling pathways

loss-of-function mutation involving the APC locus on chromosome 5q21 are associated with familial adenomatous polyposis

APC is a component of the WNT signaling pathway (major role in controlling cellular growth and differentiation during embryonic development

Question 29

What is the WNT pathway?

Answer 29

Plays a major role in controlling cellular growth and differentiation during embryonic development

WNT molecules signal by binding to cell surface receptors of the frizzled (FRZ) family –> this stimulate several pathways…the central one involving APC and B-catenin

APC major function is to hold the B-catenin activity in check

In absence of WNT signaling, APC particpates in the formation of a “destruction complex” that leads to the proteasomal degradation of beta-catenin

Question 30

What blocks the formation of the destruction complex

Answer 30

in absence of WNT signaling, APC partcipates in teh destruction complex that leads to proteasomal degradation of beta-catenin

WNT signaling blocks the formation of the destruction complex, stabilizing B-catenin and allowing it to translocate from the cytoplasm to the nucleus –> promotes growth of colonic epithelial cells by increasing epression of MYC, cyclin D1 and other genes

Question 31

Beta catenin function?

Answer 31

Basically stimulates cell growth and APC holds it in check

many colon cancers are b/c of beta-catenin unregulation

Question 32

Relationship of E-cadherin and beta catenin

Answer 32

w/ loss of cell-to-cell contact or epithelial junction disruption B-catenin is able to translocate to teh nculeus and stimulate genes that promote proliferatio –> wound repair

restablishment of these E-cadherin contacts as the wound heals leads to sequestration of beta-catenin at teh membrane and reduces proliferative signaling

Question 33

What is a characteristic of many carcinomas?

Answer 33

Loss of contact inhibition, by mutation of the E-cadherin/B-catenin axis or by other changes

Question 34

Germine loss of function mutations of the E-cadherin gene are known as_______

Answer 34

CDH1

Question 35

MEchanism of sensecent cells?

Answer 35

Upregulation of p53 and INK4a/p16

Maintain RB in a hypophosphorylated state. –> which favors cell cycle arrest