LEC52: Oncogenes

Question 1

who first proposed oncogene notion?

Answer 1

1) percival potts, 18th century london noticed increased incidence of testicular cancer in chimney sweeps -> proposed that cancer’s caused by carcinogens
2) peyton rous in early 20th century identified a virus that caused sarcomas in chickens, RSV, proposed cancer can be caused by viruses

these two ideas together became oncogene hypothesis

Question 2

what are proto-oncogenes?

what happens to them to cause cancer?

Answer 2

specific cellular genes that can become mutated

by carcinogens

or co-opted - by viruses

to trigger tumorigenesis

Question 3

what is rous sarcoma virus?

Answer 3

first RNA tumor virus to be identified

its viral genome’s oncogene is src; when mutated, leads to cellular transfomration

Question 4

what is a proto oncogene?

Answer 4

a cellular gene within the genome that can be mutated, altered, or somehow manipulated by the virus or by the cancer cell, results in having oncogenic properties, tumorigenesis

Question 5

what are the oncogenic mechanisms of an RNA virus?

Answer 5

1) the viral genome can contain the oncogene
2) the virus can integrate in a genomic locus that results in aberrant expression of a cellular gene

Question 6

what is C-src vs V-src?

Answer 6

V-src: version of the oncogene in the virus

C-src: normal cellular gene, the counterpart of V-src, becomes mutated and thus the oncogene

Question 7

what is ALV?

Answer 7

class of RNA tumor viruses whose genome doesn’t have a corresponding src

works by insertional mutagenesis

Question 8

what is insertional mutagenesis?

Answer 8

mechanism of some RNA tumor viruses, like ALV

its provirus has a strong promoter enhancer that drives expression of its own genes

so when inserts randomly in a host genome, often has no effect, but if it inserts upstream of an oncogene such as c-myc, it causes aberrant overexpression, uncontrolled proliferation, oncogenesis

Question 9

mechanisms of oncogene activation in human cancer?

Answer 9

1) missense mutation
2) gene amplification
3) chromosomal rearrangement
4) uncommon, but also: regulatory mutation

Question 10

what does a missense mutation in a proto-oncogene cause?

Answer 10

constituitive activation

hyperactive protein, made in normal amounts

Question 11

what are common missense mutations in Ras? result?

Answer 11

G12V or G13V

these residues are adjacent to GTP binding site

results in loss of GTPase activity, to hydrolyze GTP to GDP

Ras-GTP is always active, so ERK pathway is always ON -> Cyclin D constituitively high -> E2F active -> cell undergoes uncontrolled proliferation

Question 12

will a cell with a Ras missense mutation be growth factor dependent or independent?

Answer 12

growth factor independent - do not need growth factor binding for RTK to be on

Question 13

what Raf missense mutation commonly occurs in cancer?

Answer 13

V600E mutation

leads to its constitutive Raf kinase activity

this Raf mutation is commonly found mutation in melanoma

Question 14

what is frontline melanoma therapy?

Answer 14

Raf kinase inhibitors; prevent ATP-kinase binding

Question 15

what is regulatory mutation’s result?

Answer 15

normal protein’s over produced

Question 16

broadly, how does gene amplification lead to cancer?

methods of occurence?

Answer 16

single copy of proto-oncogene undergoes amplification due to aberrant DNA replication, leading to increase in copy number

it’s an overexpression of a normal protein

1) gene in its normal locus is amplified and overexpressed
2) tyrosine kinase receptor constituitive activation without a ligand
3) amplification of the growth factor - autocrine loop

Question 17

what is a double minute?

Answer 17

small fragment of extrachromosomal DNA

are a manifestatin of gene amplification

this homogeneous staining region contains an amplified particular gene or oncogene

are the result of aberrant replication of a chromosome

Question 18

how are Myc, Cyclin D1, Cdk 4 potentially cancer causing?

Answer 18

amplification of a protein that is normally regulating like Mdm2 or myc or cyclin D can cause gene amplification, -> cancer

Question 19

how might EGFR amplification result in cancer?

Answer 19

EGFR is the furthest upstream signaler in the ERK pathway; EGF binding causes EGFRs’ dimerization, cross-phosphorylation on cytoplasmic side, signaling Ras, Raf, Mek, Erk activation

If EGFR amplified, localized concentration is high, they cluster, get spontaneous, constituitive, self-activating dimerization

results in Erk pathway activation

Question 20

when is EGFR gene amplification common?

Answer 20

lung cancer

Question 21

when is her2 often amplified?

Answer 21

breast cancer

Question 22

what is herceptin?

Answer 22

antibody that binds to her2 receptor, prevents its dimerizing

used as breast cancer therapy for patients w/ overexpression of her2 due to gene amplification

Question 23

what is autocrine loop?

Answer 23

mechanism of growth factor amplification when the same cell expresses both the growth factor and the receptor, whereas normally growth factors come from other cells & interact w/ the receptor on the cell surface

ex: PDGF, platelet-derived growth factor

Question 24

what are mechanisms of chromosomal rearrangements that -> cancer?

Answer 24

1) chromosomal translocation: piece of 1 chromosome reassociates w/ another chromosome
2) inversion: a piece of DNA switches around within the same chromosome

Question 25

how might chromosomal translocations occur?

Answer 25

reciprocal translocation between 2 different chromosomes OR inversion within the same chromosome

rearrangements can involve regulatory regions or coding regions

Question 26

what is result of a translocation involving a regulatory vs a coding region?

Answer 26

if involves regulatory region: alters expression of a normal protein

if involves coding region: results in expression of a novel, fusion protein

Question 27

what part of chromosome does gene rearrangment involving regulatory often involve?

what is the result?

Answer 27

immunoglobulin enhancer or T-cell receptor enhancer

results in a B cell or T cell lymphoma

Question 28

what do B cells do

Answer 28

make antibodies

undergo genomie rearrangements to create high expression of immunoglobulin, high level of transcription through enhancer

Question 29

what causes Burkitt’s lymphoma?

Answer 29

B-cell lymphoma. cause:

reciprocal translocation of myc gene on chr 8 and immunoglobulin enhancer on chr 14

Question 30

what does this show

Answer 30

reciprocal translocation causing burkitt’s lymphoma is:

c-myc (coding region) on chr 8 and Ig enhancer (regulatory region) on chr 14

Question 31

what causes follicular b-cell lymphoma?

Answer 31

reciprocal translocation of bcl-2 gene on chr18 and Ig enhancer on chr14

results in overexpression of Bcl-2 which inhibits apoptosis

thus suppresses cell death

Question 32

what causes parathyroid adenoma?

Answer 32

intrachromosomal inversion of chr11

puts promoter of parathyroid hormone gene upstream of cyclin D1 gene

puts cyclin D1 under parathyroid hormone promoter control, leads to overexpression of Cyclin D in parathyroid, constituitive E2F transcription

this is a cell-type specific cancer, as promoter is only activei n parathyroid cell

Question 33

what does gene rearrangement involving coding regions result in?

Answer 33

transcriptional over-expression

Question 34

what creates a fusion protein?

Answer 34

chromosomal rearrangement

overexpression of fusion protein can -> cancer

Question 35

what is the philadelphia chromosome

Answer 35

chromosomal abnormality of CML

results from reciprocal translocation of c-abl gene on chr9 and bcr gene on chr22

results in expression of novel, fusion protein, Bcr-abl

results in Abl kinase being constituitively active

Question 36

what is Abl

Answer 36

a tyrosine kinase on chr9 w/ an amino terminus that normally regulates kinase domain on the C terminus

Bcr-abl fusion protein makes kinase constituitively active because loses C terminus domain in fusion

thus get constitutive Abl kinase activity in CML

Question 37

what is Gleevec/Imatanib, how does it work?

Answer 37

inhibitor that’s highly selective for Bcr-abl fusion protein; blocks Bcr-Abl binding to substrate protein, thus cannot have ATP binding and activating phosphorylation of substrate protein

it prevents leukemia

Question 38

why is Bcr-Abl protein usually active? result?

Answer 38

Abl loses its ability to regulating its tyrosine kinase activity when fuses w/ Bcr

so it undergoes activating phosphorylation

results in signal for cell proliferation, survival, so leukemia

Question 39

are fusion proteins a big drug therapy target?

Answer 39

no, because chromosomal rearrangements involving coding regions resulting in fusion proteins like Bcr-abl in CML cases are very rare

Question 40

what occurs in APL, acute promyelocytic leukemia?

Answer 40

reciprocal translocation of PML gene on chr15 and retinoic acid receptor gene on chr17

results in expression of a novel, fusion protein, PML-RAR

Question 41

what is the idea of oncogene cooperation?

Answer 41

full tumorogenesis requires several genetic changes

a single oncogene activation isn’t sufficient for a person to get cancer

Question 42

how can cooperating oncogene principle be shown experimentally?

Answer 42

if incubate primary oncoblasts with myc - oncogene - or E1A - viral product that’s oncogeneic - either myc or E1A alone doesn’t yield transformation in an anchorage independent environment

Ras alone also doesn’t transform cells

however myc or E1A plus Ras gives cooperation

ras shows: need a 2nd oncogene to observe transformation

Question 43

how can cooperation btwn 2 oncogenes be shown experimentally in vivo?

Answer 43

mouse model:

mutant, activated Ras expressed in mouse mammary gland results in some tumorigenesis; overexpression of myc has minimal tumorigenic effect; but Myc overexpression at same time as mutant, activated Ras, tumor formation is accelerated