Lecture 8

Oncogene and

anti-oncogene

Zhihong Li（李志红）

•Cells normally grow, reproduce and die in response to signals inside and outside the body in an orderly way.

•Malignant tissues can invade and damage other tissues and organs. Cancer cells can then break away from the tumour and enter the bloodstream or lymphatic system, spreading the cancer to other parts of the body. (metastasis)Benign Malignant

cancer

Normal cells

Cancer cells vs. normal cells: what's different?

Cancer Cell Do Not Grow Faster ThanCancer Cell Do Not Grow Faster Than

Normal CellsNormal Cells

Rather, Their Growth is Just Rather, Their Growth is Just

UncontrolledUncontrolled

Proliferation Differentiation Death

Cancer: disruption of cellular equilibrium

Proliferation Differentiation Death

Causes of Cancer

Heredity

Cancer - multi-step and multi-gene diseaseAll types of cancers are caused by changes in

genes.

Cancer - a genetic disease of somatic cells.

Cancer - the most common human genetic disease. Not every gene is involved in

carcinogenesis!Predominant mechanisms for cancers are: impairment of a DNA repair pathway transformation of normal genes into oncogenes malfunction of anti-oncogenes (tumor suppressor genes)

Relationship of oncogene, anti-oncogene and growth factor

Cellular control depends on a constant balance

Imbalance: proto-oncogenes over-expressed,

anti-oncogenes inactivated or lost.

proto-oncogenes - favour cell reproduction,

anti-oncogenes - prevent cell reproduction.

§1 Oncogene

oncogene: These genes code for proteins that are capabl

e of stimulating cell growth and division. In normal tissues and organisms, such growt

h-stimulating proteins are regulated, so that growth is appropriately limited.

However, changes/mutation in these genes may result in loss of growth regulation, leading to uncontrolled cell proliferation and tumor development.

Dominant mutation: one copy is sufficient to cause cancer. (different than tumor-suppressors, recessive mutation)

Oncogene

• found in viruses or as part of the normal genome. – v-onc: virus oncogene – c-onc: cellular-oncogene or proto-oncoge

ne

Virus- oncogene (v-onc)

• Genes are in viruses that can cause tumors in vivo and transform the cell in vitro.

• First link between viruses and cancer proposed by Francis Peyton Rous in 1910: – cell-free extracts from chicken

tumors injected into healthy chickens caused new tumors.

•In 1910, Peyton Rous found the first retrovirus (Rous Sarcoma Virus, RSV) in a chickens filtrated sarcoma.

•Rous got the Nobel Prize in Physiology and Medicine in 1966.

•Structure of RSV genome

9392 bp

LTR LTRgag pol env src

tyrosine kinase

526 residue

60 kD

wide type viral genes oncogene

to initiate

and regulate

transcription

Cellular Oncogene (c-onc)• Or proto-oncogene

– code for proteins that help to regulate cell growth and differentiation.

• Genes are in static or low-level expression state in normal cells under the normal situation.

• The proto-oncogene can become an oncogene due to mutations or increased expression.

Product and Function of Proto-oncogene

• Extracellular growth factors • Transmembrane growth factor recep

tors • Intracellular signal transduction prot

eins • Intranuclear transcription factors

Category and major product of c-oncogene

1. src family — tyrosine protein kinase2. ras familiy — P21 （ GTPase activity ）3. myc familiy — intranuclear DNA bindin

g protein4. sis familiy — P28 (similar platelet deriv

ed growth factor)5. myb familiy — intranuclear transcripti

on factor

Mechanisms of Oncogene Activation

1. Obtaining a strong promoter or enhancer

2. Group translocation or chromosome rearrangements

3. Proto-oncogene amplification

4. Gene mutation

avian leukemiavirus genome

host cellgenome

ssRNA

dsDNA

c-mycLTR

to increase c-myc gene expression, 30-100 times, compare with no infection.

1) Obtaining a strong promoter or enhancer

c-Myc

• Myc (c-Myc) codes for a protein that binds to the DNA of other genes and is therefore a transcription factor.

• Myc protein regulate expression of 15% of all genes through binding on Enhancer Box sequences (E-boxes).

2) Gene translocation• Chronic myelogenous leukemia (CML) is a can

cer of the blood system in which too many white blood cells (WBCs) are made in the bone marrow.

• What causes CML?

– In almost everyone with CML, the genetic material (chromosomes) in the leukemia cells has an abnormal feature called the Philadelphia(Ph) chromosome.

•produces a new, abnormal gene called bcr-abl. This abnormal gene produces Bcr-Abl tyrosine kinase, an abnormal protein that causes the excess WBCs typical of CML.

ras or c-myc

expression of ras or c-myc is increased obviously

amplification

3) Proto-oncogene amplification

4) Point mutation

• Mutated in 30% of all cancers.

• A “molecular switch” in the signal transduction pathway leading from growth factors to gene expression controlling cell proliferation:

– GF receptor Ras TF target genes growth.

• A single amino acid change in Ras protein can cause constant stimulation of the pathway, even in the absence of growth factors.

Ras Proto-oncogene

Ras Proto-Oncogene

In response to growth factor binding at receptor, the Ras gene product combines with GTP

to promote cell division

In cancer cells, the RAS gene product is locked into its GTP-binding shape and does not require a signal at

the receptor in order to stimulate cell division

H-ras

GGC

GTC

Gly

Val

DNA Protein

carcinoma

normal

Genetic Disease Associated with Ras

• The altered H-Ras protein is permanently activated within the cell.

• This overactive protein directs the cell to grow and divide in the absence of outside signals, leading to uncontrolled cell division and the formation of a tumor---Bladder cancer.

• Ras family: H-Ras, K-Ras, R-Ras and N-Ras • The presence of ras mutations is detected in sev

eral human tumors- 90% of pancreatic, 50% of colon and 30% of lung.

§2 Anti-oncogene

Tumor suppressor gene (anti-oncogene)

• A gene whose protein products inhibit cell division, thereby preventing uncontrolled cell growth.

Tumour suppressor genes

Act as a brake for cell division

“Guardian of the genome”

PROBLEM:

Mutation in tumour suppressor genes = brakes don’t work, or there is an accumulations of mutations (DNA repair enzymes)

• Coding the restrained protein relating to cell cycle control.

• When tumor suppressive gene is deleted and mutated, there is an induced occurrence of tumors.

• Rb gene and P53 gene.

Functions of tumour suppressor genes

Retinoblastoma (Rb gene)

• Diagnosis: “Cat’s eye” reflection in affected eye.

• Most common cancer of infants and children (1/20,000 U.S. live births).

• Survival > 90% with early diagnosis and treatment.

• Individuals at greater risk of developing other cancers.

Recessive mutation

•People prone to retiPeople prone to retinoblastoma have noblastoma have onone mutated copy of the mutated copy of the Rb gene (Rb-)e Rb gene (Rb-) and and one normal copy (Rbone normal copy (Rb++). ).

• Conversion of the Conversion of the RbRb++ copy to Rb copy to Rb- - by mby mutation leads to uncoutation leads to uncontrolled growth of retntrolled growth of ret

inal cells.inal cells.

Knudson’s “Two-Hit” Model for Retinoblastoma

Normal Normal 2 intact copies2 intact copies

Predisposed Predisposed 1 intact copy1 intact copy1 mutation1 mutation

Affected Affected Loss of both Loss of both copiescopies

Modified from Modified from Time, Time, Oct. 27, 1986Oct. 27, 1986

Mechanism of action of Rb gene

E2F = transcription factorRb = product of Retinoblastoma

gene, inhibits action of E2F until chemically modified

P53 gene

• P53 gene encodes the protein which molecular weight is 53kD.

• The “Last Gatekeeper” gene since malignant state not attained despite the presence of other cancer-causing mutations until p53 is inactivated by mutation.

Biologic function of P53 protein

• Suppressing cell cycle• Suppressing transformation fun

ction of some oncogenes • Monitoring cell DNA damage • Inducing the cell apoptosis

Function of Tumor Suppressor Gene p53

• p53 initiates repair of damaged DNA• if DNA cannot be repaired, it initiates apoptosis

•Cancer is a multistep process

Oncogene activation

Oncogenic viruses

Environmental factors (physical and chemical)

Mutations

Inactive antioncogenes Carcinogenesis

Diminished regulation by apoptosis genes

A simplified hypothesis for the development of cancer

Points• Oncogene

– v-onc– c-onc (proto-oncogene)– Product and Function of Proto-oncogene

• Mechanisms of Oncogene Activation– Obtaining a stronge promoter or enhancer– Group translocation or chromosome rearra

ngements – Proto-oncogene amplification– Gene mutation

• Tumor suppressor gene (anti-oncogene)– Rb gene and P53 gene