section s: tumor viruses and oncogenesyang xu, college of life sciences section s tumor viruses and...

Section S: Tumor Viruses and Oncogenes Yang Xu, College of Life Sciences

Section S Tumor Viruses and Oncogenes

S1 Oncogenes Found in Tumor Viruses S2 Categories of Oncogenes S3 Tumor Suppressor Genes

Section S: Tumor Viruses and Oncogenes

S1 Oncogenes Foundin Tumor Viruses

• Cancer

• Oncogenic retroviruses

• Oncogenes

• Isolation of oncogenes

Yang Xu, College of Life Sciences


CancerDefinition: Cancer is a disease that

results from the breakdown of the regulations and controls of normal cell growth.

Evidence: It has long been recognized that cancer is a disease with a genetic element.

• The tendency to develop certain types of cancer may be inherited;

• In some types of cancer the tumor cells possess characteristically abnormal chromosomes;

• There is a close correlation between the ability of agents to cause cancer and their ability to cause mutations.


Section S: Tumor Viruses and Oncogenes Yang Xu, College of Life Sciences


Oncogenic retrovirusesOncogenic viruses: The basic

concepts of oncogenes were come from the studies on oncogenic retroviruses. The retroviruses were found to contain an extra gene, not present in closely related but non-oncogenic viruses. This extra gene was shown to be an oncogene by transfecting it into non-cancerous cells which then became tumorigenic.

Function: Oncogenic viruses are important cause of cancer in animals, although only a few rare forms of human cancer have been linked to viruses.

Section R: Bacteriophages and Viruses 华中师范大学生科院杨旭

5’ 3’gag pol env v-onc+RNA3’ 5’gag pol env v-onc- DNA

3’ 5’gag pol env v-onc- DNA5’ 3’gag pol env v-onc+DNA

3’ 5’gag pol env v-onc

5’ 3’gag pol env v-onc宿主DNA

宿主DNA

病毒RT

宿主DNA pol

病毒Integase

宿主RNA pol. II

结构基因表达蛋白

U3 R U5

调控基因

Oncogenic retroviruses

LTR



OncogenesDefinition: Oncogenes are genes

whose expression causes cells to become cancerous.

Tumorigenic mechanism: The normal version of the gene (termed a proto-oncogene) becomes mutated so that it is overactive. Because of their overactivity, oncogenes are genetically dominant over proto-oncogenes, that is only one copy of an oncogene is sufficient to cause a change in the cell's behavior.



OncogenesRelationship between virus oncogene and proto-oncogenes:• V-onc: virus oncogene, v-onc• P-onc: proto-oncogenes, p-onc Finding: The first oncogenes to be isolated were those present

in oncogenic retroviruses. When these had been cloned and were used as hybridization probes, a discovery was made: The genes with DNA sequences homologous to retroviral oncogenes were present in the DNA of normal cells.

Extrapolate: It was then realized that retroviral oncogenes must have originated as proto-oncogenes in normal cells and been incorporated into the viral genome when the pro-virus integrated itself nearby in the cellular genome. Subsequently, similar oncogenes were isolated from non-virally caused cancers.



OncogenesThe differences between onc and normal proto-onc: • Quantitative differences: The coding function of the gene

may be unaltered but, for example, it is under the control of a viral promoter/enhancer or it has been trans-located to a new site in the genome, it is transcribed at a higher rate. This results in overproduction of a normal gene product.

For example: int-2 is a such kind of oncogene.

MMTV

Enhancer Int-2 Low ExpressionH



OncogenesQualitative differences: The coding sequence may be altered,

for example by deletion or by point mutation, so that the protein product is functionally different, usually hyperactive.

For example: the erbB oncogene codes for a truncated growth factor receptor. Because the missing region is responsible for binding the growth factor, the oncogene version is constitutively active, permanently sending signals to the nucleus instructing the cell to 'divide'.

Normal erbB truncated erbB



Oncogenes



Isolation of oncogenesAdvantages of this assay are:

• It is a cell culture rather than a whole animal test and so particularly suitable for screening large numbers of samples;

• Results are obtained much more quickly than with in vivo tests;

• The NIH-3T3 cells are good at taking up and expressing foreign DNA;

• It is a technically simple procedure compared with in vivo tests.



Isolation of oncogenesHowever, extensive use has revealed some drawbacks, both

real and potential:

• Some oncogenes may be specific for particular cell types and so may not be detected with mouse flbroblasts;

• Large genes may be missed because they are less likely to be transfected intact;

• The NIH-3T3 cells are not 'normal' cells since they are a permanent cell line and genes involved in early stages of carcinogenesis may therefore be missed;

• The assay depends upon the transfected gene acting in a genetically dominant manner and so will not detect tumor suppressor genes.



S2 Categories of Oncogenes

• Categories of Oncogenes

• Oncogenes and growth factors

• Nuclear oncogenes

• Co-operation between oncogenes



Categories of oncogenes

Onc

Onc of coding protein like growth factors

Onc of coding protein like transcription factors

mRNA mRNA

Onc of coding protein like membrane receptors

Onc of coding protein like signal moleculesGF




Growth Factors

Transcription factors

Membrane receptors

Signal molecules

sis,/PDGF int-2/FGF-F hst (KS3) int-1/GF

bZIP: fos, jun bHLH: myc, N-myc, L-myc lyl-1, fal, scl ZF:myl/RARA, erb-A, vav, gli-1 HD: pbx, Hox-2, Other: myb, rel, est-1, est-2, spi-1

TPK: erb-B/EGF-R Neu, Fms/CSF-1, Ros / Non-TPK: mas

src yes fps abl met mos raf ras, crk




1. Onc of coding protein like growth factors

4. Onc of coding protein like transcription factors

3. Onc of coding protein like signal molecules

2. Onc of coding protein like membrane receptors

DNA

mRNA Protein

nucleus



mRNA

p28sis

Oncogenes (growth factors)• The sis oncogene which codes for a subunit (p28sis) of

platelet-derived growth factor. Over-production of this growth factor auto-stimulates the growth of the cancer cell, if it has receptors for PDGF

v-sis

Over expression



mRNA

Fms

Oncogenes (membrane receptor)• The fms oncogene which codes for a mutated version of the receptor for

colony-stimulating factor-1 (CSF-1). The growth factor stimulates bone marrow cells during blood cell formation. The 40 ammo acids at the carboxyl terminus of the normal CSF-1 receptor are replaced by 11 unrelated amino acids in the Fms protein. As a result, Fms protein is constitutively active regardless of the presence or absence of CSF-1

v-fms

CSF-1 receptor



mRNA

Oncogenes (signal molecules)The various ras oncogenes which code for members of the G-protein family of

plasma membrane proteins that transmit stimulation from many cell surface receptors to enzymes that produce second messengers. Normal G-proteins bind GTP when activated and are inactivated by their own GTPase activity, ras oncogenes possess point mutations which inhibit their GTPase activity so that they remain activated for longer than normal.

v-ras

GDPGTP



Oncogenes (transcription factors)Another group of oncogenes codes for nuclear DNA-

binding proteins that act as transcription factors regulating the expression of other genes.

They can divided into 5 classes:• bZIP: fos, jun;• bHLH: myc, N-myc, L-myc, lyl-1, fal, scl;• ZF: myl/RARA, erbA, evi-1, gli-1;• HD: pbx, Hox-2,4;• Others: myb,rel, est-1, est-2, spi-1, ski.



Nuclear oncogenes (bZIP-fos, jun)

The fos and jun oncogenes code for subunits of a normal transcription factor, AP-1. In normal cells, expression of fos and jun occurs only transiently, immediately after mitogenic (分裂剂的 ) stimulation. The normal cellular concentrations of the fos and jun gene products are regulated not only by the rate of gene transcription but also by the stability of their mRNA. In cancer cells, both processes may be increased.

Rate of gene transcription: Slow and short Fast and quick

Stability of their mRNA: Very low Very high

In normal cells In cancer cells



Nuclear oncogenes (bHLH-myc)

The expression of the myc gene in normal cells is induced by a variety of mitogens (agents that stimulate cells to divide), including PDGF. The myc-encoded protein binds to specific DNA sequences and probably stimulates the transcription of genes required for cell division.

In cancercells

Influence of a viral enhancer

Over-expression of myc

Translocation of the coding sequencefrom its normal site on chromosome8 to a site on chromosome 14

Deletion of 5'-noncoding sequenceof the mRNA, which increases the life time of the mRNA



Nuclear oncogenes (ZF-erbA) • The erbA oncogene is a second oncogene (besides erbB)

found in the avian erythroblastosis virus. It codes for a truncated version of the nuclear receptor for thyroid hormone. Thyroid hormone receptors act as transcription factors regulating the expression of specific genes, when they are activated by binding the hormone.

• The ErbA protein lacks the carboxyl-terminal region of the normal receptor so that it cannot bind the hormone and cannot stimulate gene transcription. However, it can still bind to the same sites on the DNA and appears to act as an antagonist of the normal thyroid hormone receptor.



Co-operation between oncogenes• Transformation: The transformation of a normal cell into a

fully malignant cancer cell is (1) a multi step process (2) involving alterations in the expression of several genes.

• For example, when cultures is from normal rat fibroblasts. Neither the ras nor the myc oncogene on its own is able to induce full transformation in the normal cells, but simultaneous introduction of both oncogenes does achieve fully malignant.

• Pairs of oncogenes: A variety of other pairs of oncogenes are able to achieve together what neither can achieve singly, in normal rat fibroblasts.

• Co-operation: Interestingly, to be effective, a pair must include (1) one growth factor-related onc and (2) one nuclear oncogene. It seems that any one activated oncogene is only capable of producing a subset of the total range of changes necessary to convert a completely normal cell into a fully malignant



S3 Tumor Suppressor Genes

• Overview

• Evidence for tumor suppressor genes

• RB1 gene

• p53 gene



Tumor suppressor: Overview• Definition: A tumor suppressor gene acts, in a normal cell, to

restrain the rate of cell division. Tumor suppressor genes cause cells to become cancerous when they are mutated to become inactive.

• Function: Tumor suppressor genes act in a fundamentally different way from oncogenes:

Function: promote cell division inhibit cell division

Proto-oncogenes Suppressor genes

Cancerous : by mutated active by mutated inactive



p53 gene (As an oncogene) Features: The gene for p53 is located on the short arm

of chromosome 17, and deletions of this gene have been associated with nearly 50% of human cancers. Its mRNA codes for a 52 kDa nuclear protein. The protein is found at a low level in most cell types and has a very short half-life (6-20 min).

Functions: Confusingly, p53 has some of the properties of both oncogenes and of tumor suppressor genes:

• As an oncogene: many mutations (point mutations, deletions, insertions) have been shown to occur in the p53 gene, when co-transfected with the ras onc, they will transform normal rat fibrobiasts. In cancer cells, p53 has an extended half-life (4-8 h), resulting in elevated levels of the protein. All this seems to suggest that p53 is an oncogene.

p53 gene



p53 gene (As a tumor suppressor gene)• As a tumor suppressor gene: A consistent deletion of the short

arm of chromosome 17 has been seen in many tumors. In brain, breast, lung and colon tumors, where a p53 gene was deleted, the remaining allele was mutated. This suggests that p53 is a tumor suppressor gene:

• The explanation is the dominant-negative effect.



That’s all for Section S


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