Oncogene etc

8/7/2019 Oncogene etc

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TUMOR SUPPRESSOR GENES


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Tumor suppressor genes encode proteins that have at least

one of the following functions:

1- Proteins that slow or inhibit progression through a

specific stage of cell cycle e.g. p2land p16.

2- Checkpoint control proteins that arrest cell cycle if DNA

is damaged or chromosomes are abnormal e.g. p53.

3- Receptors that function to inhibit cell proliferation.

4- Proteins that promote apoptosis e.g. protein Bax.

5- DNA repair enzymes (BRCA 1

).

They are termed anti-oncogenes, their mutations remove

certain control on growth and proliferation which play a

major role in the development of cancer.


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Tumor Suppressor Genes That

Directly Regulate the Cell Cycle

The two best understood cell cycle

regulators that are also tumor suppressorsare the retinoblastoma (Rb) andp53 genes.


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The proteins induced by E2F includecyclin E, cyclin A, cdc25A (an activating

protein phosphatase), and proteins requiredto bind at origins of replication to initiateDNA synthesis.

The synthesis of cyclin E allows it tocomplex with Cdk2, forming another activecyclin complex that retains activity into S

phase . Cyclin A activates Cdk2. Thus, eachphase of the cell cycle activates the nextthrough cyclin synthesis. The cyclins areremoved by regulated proteolysis.


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I- Rb (Retinoblastoma) Tumor

Suppressor Gene

The protein product (pRb) is a nuclear phosphoproteinand its phosphorylation oscillates during cell cycle.During the G0 or G1, the protein is present in

dephosphorylated form, its phosphorylation increasesin late G and S phases.

a- The dephosphorylated form of pRb:

1- It inhibits cell proliferation through binding with

certain transcription factors.2- It binds certain viral proteins and produces

inactivation of these proteins.


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I- Rb (Retinoblastoma) Tumor

Suppressor Gene

b- The phosphorylated form of pRb:Phosphorylation of pRb is initiated by CDK-cyclin

complexes in mid G1. Phosphorylation of pRbproduces release and activation of transcription

factors, which stimulate transcription of the geneswhose products are used in replication (enzymesneeded for DNA synthesis).

Phosphorylation of pRb is maintained throughout the

S, G2 and M phases by different CDK-cyclincomplexes. After cell mitosis, the fall of CDK cyclin complexes leads to dephosphorylation ofpRb.


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The main tumors in which inactivation of

pRb is an important cause include tumors of

the retina (retinoblastoma), lung cancers,

adenocarcinoma of prostate and tumors of

bone and connective tissues.


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Disease Example:

RetinoblastomaAn example of a cancer caused by a tumor suppressormutation is retinoblastoma. About one-half ofretinoblastoma cases are familial, with the phenotype

following an autosomal dominant inheritance pattern.Penetrance for heterozygous gene carriers isapproximately 90%. The predisposition toretinoblastoma is caused by the inheritance of amutation of RB1, a tumor suppressor gene onchromosome 13; however, inheritance of this mutationalone is not sufficient to produce a tumor. For this tohappen, the developing fetus must experience a secondmutation in the other copy of RB1 in a specific

retinoblast (i.e., a somatic mutation).


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II-p53 Tumor Suppressor Gene

This is an important example for tumor suppressor genes,

which encodes a protein termed p53 (its molecular weight

equals 53 kDa). It is a nuclear phosphoprotein and acts as a

transcriptional regulator (regulating certain genes thatparticipate in cell cycle).

The level of p53 increases after exposure to agents that

damage DNA. This causes G1 specific cell cycle arrest and

allows time for DNA repair to occur. If the damage cannot be

repaired, p53 can trigger apoptosis, so the damaged cells die.


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It acts as a guardian of the genome or a molecular

policeman by the following mechanisms:

1-Moderate DNA damage activates expression ofp53 that stimulates expression of a cyclin kinaseinhibitor protein (CIP) termed p21. The latter

binds to CDK-cyclin complexes, causing arrestin G1 and G2.


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2-In response to extensive DNA damage,

p53 activates genes that induce

apoptosis, i.e. bax gene.

3-It binds many viral proteins forming

inactive polymeric complex.


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Absence or mutations of p53 is associated with

genome instability and increases the occurrence oftumors. Mutations of p53 gene occur frequently in

may types of cancer. These mutations may be due

to exposure to environmental factors, for example:

- Aflatoxins (a fungal toxin) acts as a potent

hepatocarcinogen.

- Lung cancer in many cases is associated with

smoking or exposure to chemicals e.g.benzo(a)pyrene.


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ONCOGENESOncognes are genes that encode

proteins capable of causing cancer.

Oncogenes were first recognized as

unique Genes of tumor-causing viruses

that are responsible for the process oftransformation (viral oncogene)


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Proto-Oncogenes:

Normal cells containe DNA sequences similar-if not

identical-to those of the viral oncogenes.

The genes present in normal cells thus have been

designated proto-oncogenes, and their products arebelieved to play important roles in normal

differentiation and other cellular process.

The product of the myc oncogene, originally found

in chicken myelocytoma viruses, is a DNA-binding

protein, which may affect the control of mitosis.


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Mechanisms for Conversion of Proto-oncogenes

to Oncogenes Five mechanisms are mainly responsible for this conversion as follows:1- Promoter insertion: Some viruses act through this mechanism e.g. avian

leukemia virus.

2- Enhancer insertion: Many viruses act through this mechanism.

3- Chromosomal translocation: The basis of translocation is that a piece of onechromosome is split off, then joined to another chromosome that brings agrowth- regulatory gene under control of different promoter and causesinappropriate expression of the gene. For example, Philadelphia chromosomethat produces increased activity of the tyrosine kinase and transforms normalcells to leukemic cells.

4- Gene amplification: it is present in many tumor cells. The increased amount ofgene products produced by gene amplification may play a role in theprogression of tumor cells.

5- Point mutation: This produces changes in the protein product of the gene.


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Conversion of a Proto-oncogene to Oncogene


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Conversion of the Abl proto-oncogene into

oncogene in patients with chronic myelogenous

leukemia (CML) by chromosome translocation

(Philadelphia chromosome )

p

q


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Philadelphia chromosome The results from a reciprocal translocation

between the long arms of chromosome 9 and 22.As a consequence, a fusion protein is produced

containing the N terminal region of the Bcr proteinfrom chromosome 22 and the C-terminal region of

the Abl protein from chromosome 9. Ablis aproto-oncogene, and the resulting fusion

protein (Bcr-Abl) has lost its regulatory region andis constitutively active. When active, Ablstimulates the Ras pathway of signal transduction,leading to cell proliferation.


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Burkitts lymphoma, a general name for anumber of types of B cell malignancies, results from a

translocation between chromosomes 8 and 14. Thetranslocation of genetic material moves the proto-

oncogene transcription factorc-myc(normally found

on chromosome 8) to chromosome 14. The

translocated gene is now under the control of thepromoter region for the immunoglobulin heavy chain

gene, which leads to inappropriate and overexpression

ofc-myc.The result may be uncontrolled cell

proliferation and tumor development. All subtypes ofBurkitts lymphoma contain this translocation.

Epstein- Barr virus infection of B cells is also

associated with certain types of Burkitts lymphoma.


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Chromosomal translocation (Burkitts

lymphoma).

Burkitts lymphoma is a fast-growing cancer ofhuman B lymphocytes. In certain cases, a reciprocal

translocation between chromosomes 8 and 14 are

involved.

C-MYC

8

p

q

14

Break

H chain gene

Break

8 8

p

q

14 14


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Schematic representation showing how the translocation involved

in Burkitts lymphoma may activate the C-MYCproto-oncogene.

A: A small segment of chromosome 14 prior to the translocation.

The segment shown contains the genes encoding regions of heavychains of immunoglobulins.

B: Following the translocation, the previously inactive

C-MYCgene is placed under the influence of enhancer sequences

in the genes encoding the heavy chains and is thus activated,

resulting in transcription.


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A combination of activation of proto-

oncogenes combined with inactivation of

tumor suppressor genes is involved Incolorectal and probably most cancers


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Tumor Suppressor GenesOncogenes

Normal version of gene is typicallyinvolved in controlling cell

proliferation (e.g., cell cycle control,

cell adhesion control)

Mutation is typically a loss of

function

Mutations of both copies (two hits)

of the gene are typically necessary

to promote tumor growth

Normal version (proto-oncogene) isusually involved in promoting cell

growth/proliferation

Mutation is typically a gain of

function

A mutation of one copy of the gene

in a cell is sufficient to promote

tumor growth


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DEVELOPMENT OFCANCER


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Cell Proliferation Cell Death

Homeostasis

Regulation of Cell Cycle:

Cell Cycle Check points

Control of Apoptosis


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Cell Proliferation

Cell Death

Neoplasm


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Types of Tumor Cells Epithelial cells: Carcinoma

Connective tissues (fibroblast, muscle):

Sarcoma

Hematopoietic cells: Leukemia


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DEVELOPMENT OF CANCERCancer cells have the following characteristics:

1- Cancer cells can multiply in the absence of growth promotingfactors required for proliferation of normal cells and are resistant

to signals that normally program cell death or apoptosis.2- Cancer cells invade surrounding tissues, often by breaking the

basal laminas and spread through the body to establish secondaryareas of growth (metastases), this is due to secretion of proteasesthat degrade the surrounding extracellular matrix.

Two classes of genes: tumor-suppressor genes and proto-oncogenesplay a key role in cancer induction. These genes encode manyproteins that help the control of cell growth and proliferation.Mutations affecting these genes may result in production ofcancer (converted into oncogenes).


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Several types of gene mutations may participate in cancer

development, which include the following:

1- Misexpressed growth factors: Cancer rarely arises from mutation

of genes encoding growth factors that result in autostimulation ofcell proliferation.

2- Mutation causing autoactivation of growth factor receptors: Some

types of mutations may result in production of receptors that

transmit growth signals in absence of a normal ligand that result in

production of some types of cancer.

3- Overexpression of growth factor receptors: Also, mutations

leading to overexpression of a normal receptor protein can be

oncogenic e.g. breast cancer.


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4- Active signal transduction protein: Many

oncogenes encode proteins that act as

intracellular transducers. These proteins aid intransmitting signals from a receptor to a cellular

target and stimulate growth e.g. Ras protein.


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Ras is a class of G-proteins. Mutated Ras

remain in the active form (Ras-GTP

complex) and acts as a growth-promoting

signal even in the absence of growth

factors. MAP-kinase is a protein kinase

that is activated in response to stimulationby many different growth factors and

mediates cellular response by

phosphorylating specific target proteinse.g. transcription factors.


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5- Inappropriate transcription factors: Most

oncogenes encode proteins that bind to

DNA and modify the rate of geneexpression.

6- Mutations causing loss of cell cycle

control: This may be due to one of the

following examples:

- Overexpression of cyclins.- Loss of genes encoding pRb and cyclin-kinase

inhibitors (p16 & p21), both defects can cause

unregulated cell cycle control.


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7- Mutations affecting genome stability:

Cancer cells are defective in one or more

of the check control points which normallyprevent cells with damaged DNA or

abnormal chromosomes from dividing or

that cause such cells to undergo

programmed cell death e.g. mutation

affecting p53 occurs in more than 50% of

human cancers.

8- Defects in DNA repair systems: as in

cases of xeroderma pigmentosum.


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Viral Infections and

CarcinogenesisSome viruses are carcinogenic (oncogenic

viruses), for example:

1- Some types of adenovirus are known to

cause cell transformation.

2- Epstein-Barr virus is associated with

lymphoma and nasopharyngeal carcinoma.

3- Hepatitis B virus is a major cause of many

primary liver cancer.


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TUMOR MARKERSMany tumors are characterized by the presence of abnormal genes,

enzymes, proteins and hormones, which can be used as markers forthese tumors.

For the detection of abnormal genes, we may use different techniquese.g. in situ hybridization or PCR based techniques (see recombinantDNA technology). Also, the detection of the specific DNA forcertain specific tumors in serum, urine and peritoneal fluid is nowavailable. These techniques help in the early detection of cancer.

The measurement of certain specific markers in serum is useful andnow it is essential for diagnosis and for follow up of cases, forexample:

a- Carcinoembryonic antigen (CEA) in cancer colon, lung, breast and

pancreas.b- Alpha-feto protein (AFP) in liver cancer.

c- Prostatic acid phosphatase in prostate cancer.

d- Prostatic specific antigen (PSA) in prostate cancer.


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Clinical Correlate Translocations

and Cancer

Although most of our discussion deals with inheritedchromosome alterations, rearrangements in somatic cellscan lead to the formation of cancers by altering the geneticcontrol of cellular proliferation.

A classic example is a reciprocal translocation of the longarms of chromosomes 9 and 22, termed the Philadelphiachromosome. This translocation alters the activity of theabl proto-oncogene . When this alteration occurs inhematopoietic cells, it can result in chronic myelogenous

leukemia. More than 100 different chromosome rearrangements

involving nearly every chromosome have been observed inmore than 40 types of cancer.


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One important translocation is the Philadelphia

chromosome, involving chromosomes 9 and 22

and occurring in chronic myelogenous leukemia.

This produces abnormal juxtaposition of the BCR

gene (breakpoint cluster gene) on chromosome 22

with part of the C-ABL gene on chromosome 9.Normally, C-ABL codes a protein-tyrosine kinase.

The juxtaposition resuIts in a chimeric BCR-ABL

mRNA, which encodes t bcr-abl fusion protein

displaying increased tyrosine protein kinaseactivity. This increased activity transforms the

normal cell to a leukemic cell.

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