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) BenignMalignant cancer Normal cells Cancer cells vs. normal cells: what's different?

Cancer Cell Do Not Grow Faster Than Normal Cells Rather, Their Growth is Just Uncontrolled

ProliferationDifferentiation Death Cancer: disruption of cellular equilibrium Proliferation Differentiation Death

Causes of Cancer Heredity

Cancer - multi-step and multi-gene disease All 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: TThese genes code for proteins that are capable of stimulating cell growth and division. IIn normal tissues and organisms, such growth- stimulating proteins are regulated, so that growth is appropriately limited. HHowever, changes/mutation in these genes may result in loss of growth regulation, leading to uncontrolled cell proliferation and tumor development. D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- oncogene

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 gag pol env src tyrosine kinase 526 residue 60 kD wide type viral genesoncogene 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.

Integration of RNA virus genome and host cell genome

Product and Function of Proto- oncogene Extracellular growth factors Transmembrane growth factor receptors Intracellular signal transduction proteins Intranuclear transcription factors

Category and major product of c-oncogene 1. src family — tyrosine protein kinase 2. ras familiy — P21 （ GTPase activity ） 3. myc familiy — intranuclear DNA binding protein 4. sis familiy — P28 (similar platelet derived growth factor) 5. myb familiy — intranuclear transcription 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 leukemia virus genome host cell genome ssRNA dsDNA c-myc LTR to increase c-myc gene expression, 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 cancer 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 GTCGTC Gly Val DNAProtein 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 several 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 retinoblastoma have one mutated copy of the Rb gene (Rb-) and one normal copy (Rb + ).People prone to retinoblastoma have one mutated copy of the Rb gene (Rb-) and one normal copy (Rb + ). Conversion of the Rb + copy to Rb - by mutation leads to uncontrolled growth of retinal cells. Conversion of the Rb + copy to Rb - by mutation leads to uncontrolled growth of retinal cells.

Knudson’s “Two-Hit” Model for Retinoblastoma Normal 2 intact copies Predisposed 1 intact copy 1 mutation Affected Loss of both copies Modified from Time, Oct. 27, 1986

Mechanism of action of Rb gene E2F = transcription factor Rb = 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 function 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 rearrangements –Proto-oncogene amplification –Gene mutation Tumor suppressor gene (anti-oncogene) –Rb gene and P53 gene