1Cancer

• Unregulated cell growth• Family of >100 related diseases

– Cells multiplying out of control

• Tumor: mass of cells from repeated cell division– A tumor can be benign (not called “cancer”):

• Grows slowly• Tissue remains differentiated• Enclosed in a capsule

– A benign tumor can still be dangerous• E.g. a brain tumor, slowly crushes brain

• Cells from tumor spread elsewhere: metastasis– Tumor that metastasizes = malignant

2Cancer cells are different

• In culture, are rounded and float rather than stick to surfaces.

• Become de-differentiated• Immortal• Do not respond to cell signals

– Do not stop growing when crowded or touching• No contact inhibition

– No dependence on normal growth signals

• Loss of normal number and structure of chromosomes

3Types of cancer

http://www.willamette.edu/~stas/physiology/labs/lab1/epithelial2a.jpg

Cancer is named from the type of tissue in which it arises. There are various historical classifications.

Carcinomas: from epithelial tissue; 90% of all cancers.

Leukemias, lymphomas, and myelomas: from bone marrow and blood cells; 8% of all cancers.

Sarcomas: solid connective tissue such as muscle and bone; 2% of all cancers.

Cells that multiply often are more likely to form cancers.

4Cancer and aging

http://ovid.iss.it/html/lecture/pc0161/img003.GIF

As people age, more mutations accumulate, increasing the risk of cancer.

Also, as cells age, they too can become cancerous.

5Cancer is a genetic disease

• Predisposition to cancer can be inherited– Breast cancer, retinoblastoma, Li-Fraumeni

• Clonal expansion: all cells in a tumor descend from the abnormal cell, all inherit bad genes.

• Agents that damage, mutate DNA cause cancer– Chemicals, UV and ionizing radiation

• Certain genes brought into DNA by viruses cause cancer

• Cancer cells show chromosomal loss, damage

6Review: The Cell Cycle

•G1: a time of cell growth and general functioning.

•S: all the DNA in the cell is doubled to prepare for division.

•G2: cell prepares for division.

•M: mitosis, actual dividing up of the copied chromosomes and distribution to daughter cells.

http://www.med.unibs.it/~marchesi/cellcycle.gif

7Review: Cell cycle, continued

• G0: cells not dividing; may never divide again, or may re-enter cycle when needed.

Some cells divide nearly continuously; others enter G0 once mature.

8Cell cycle (continued)

• From cancer research, we have learned that the cell cycle is tightly regulated!– Checkpoints exist: at G1/S, at G2/M, and late in

Mitosis (the “M” checkpoint)• At each checkpoint, progress evaluated.

– G1/S checkpoint: is DNA in good condition? – Mutations that allow progression past this

checkpoint with damaged DNA lead to downward spiral of inherited mutations.

9Molecular regulation

• Two kinds of proteins work together: – cdc kinases and cyclins– Kinases: proteins that phosphorylate other proteins

• Adding a PO4 turns molecules “on” or “off”• Kinases always present in the cell

– Cyclins are proteins that change in amounts during the cell cycle• Specific cyclins accumulate at different times.

• Kinase combines with cyclin– Kinase is activated, given directions– Kinase phosphorylates proteins controlling cell

division

10Kinase/cyclin combination acts during checkpoint

Picture based on Hartl & Jones, 5th edition.

G1/S checkpoint important in cell cycle regulation. Important “tumor suppressor” genes involved in regulation at this step.

11The multi-hit hypothesis

• Cancer results from at least 2 “hits”, 2 instances of damage to the DNA.– Some cancers involve several.

• The gas pedal/ brake analogy– For the car to move, gas pedal doesn’t do much if

brake is on; taking brake off doesn’t get car to move if gas pedal isn’t pressed.

– Cell must get “ON” signals AND ignore “OFF” signals• On: growth factor stimulation• Off: tumor suppressors

12New insights into off/on regulation

• Recent research: effect of estrogen on entire genome of ovarian tissue– Estrogen is a growth factor, promotes cell division– Genes for promoting cell division AND stopping cell

division both turned on: a balance• Like “riding the brake” to keep car under control

13Genetics of on and off

• “On signal” mutations are dominant for cancer– If other mutations present, one mutated copy of the

gene is enough for cancer.

• “Off signal” mutations are recessive for cancer.– But are dominant for a susceptibility to cancer.– Both copies of tumor suppressor genes must be

bad for cancer to occur.– Familial predispositions: usually one copy is already

mutated, much more likely to get a mutation in one copy of the gene than both.

14Genetics visualized

15What are the “on” and “off” signals?

• “Off” signals: tumor suppressor genes– Genes (and their proteins) which act to prevent the

cell from proceeding through the cell cycle.• pRB, p53, BRCA1, BRCA2

• “On” signals: proto-oncogenes– Genes that stimulate cell division.– Mutated forms called oncogenes

• Through various malfunctions, stimulate cell division when inappropriate.

16Tumor suppressor genes

• Retinoblastoma– Cancer of the retina of the eye– Occurs in about 1 in 15,000, usually ages 1 – 3.– 40% inherited susceptibility; 60% sporadic

• RB1 gene codes for pRB, a tumor suppressor– Functions at the G1/S checkpoint

• Protein is active during G0 and G1– Binds to E2F, a transcription factor

• Prevents E2F from working, prevents transcription of genes that aid cell division.

17RB continued

At G1/S, cyclin complex phosphorylates pRB, inactivating it and releasing E2F.

E2F is a transcription factor controlling 30 genes needed for progression through cell cycle.

Mutation in both copies of pRB allows unregulated passage through G1/S checkpoint.

18p53: the Guardian of the Genome

• An important tumor suppressor; various signals including DNA damage activate p53 which then – Activates DNA repair– Blocks cell division

• Prevents replication of damaged DNA– Triggers apoptosis in damaged cells.

• Cells beyond repair are killed to avoid mutations

• Mutations in p53 account for >50% of cancers.– Single mutation is dominant negative because of

tetrameric structure.

19Action of p53

• Continually made but continually degraded– Also Inactive because NOT

phosphorylated

http://www.accelrys.com/webzine/01/q1/appnotes/geneatlas_japan/p53.jpg

DNA damage activates protein kinasesResults in phosphorylation (thus activation)

Degradation of p53 stops.Active p53 accumulates in cell, carries out its duties

20BRCA genes

• 10% of breast cancers involve a familial susceptibility; mutations in BRCA1 or BRCA2.– Woman with a faulty BRCA1 allele has an 85%

chance of developing mutation in 2nd allele, thus getting breast cancer (with risk of ovarian cancer)

• Proteins coded for by BRCA1 and 2 in nucleus– Abundant during S phase– Work with other proteins in DNA repair, etc.– Failure of these proteins to repair may result in the

mutations that lead to cancer.

21Cancer continued

• Proto-oncogenes to oncogenes– Proto-oncogenes are normal cell genes that turn on

cell division.– A mutated or over-expressed proto-oncogene

becomes an oncogene: gene leading to cancer

• Given other appropriate mutations:– These are dominant mutations for a cancer

phenotype; that is, only one bad copy is enough.• Two gas pedals: one turned on makes it go.

22Control of cell division

Proto-oncogene proteins found in membrane, cytoplasm, and nucleus.

23Cancer results from too much “ON”

Effect of mutations:

•Cancer cells produce their own growth factors.

•Increase in growth factor receptors.•ON signal sent to nucleus no matter what.

•Example: ras, described in text.

•Mutation in transcriptional activator; wrong genes turned on.

24Oncogenes: the rude surprise

• Peyton Rous, 1966 Nobel Laureate– Discovered that viruses could cause cancer in

chickens (e.g. Rous sarcoma virus)

• Molecular biologists identified genes in the viruses that were responsible: oncogenes.

• DNA hybridization studies revealed: we all have comparable genes: proto-oncogenes.– In human cancers, viruses can bring oncogenes

into our DNA, or changes can occur in our DNA without virus involvement.

25How oncogenes are made

• Point mutations– Ras is a intracellular signal peptide

• Mutation makes it always stimulate cell division• Translocations

– Burkitts lymphoma: t(8;14) moves myc gene to a more active promoter, drives proliferation.• myc is a gene for a transcription factor.

– Philadelphia chromosome: t(9;22) creates a fusion protein (protein kinase) that activates cell division.

26How oncogenes are made-2

• Over-expression & Gene amplification: – too many copies of receptors, internal signalling

molecules, transcription factors.– Example: myc. Transcription factor that causes

transcription of genes involved in cell division.– Virus insertion may put active promoter next to

gene.– Virus may bring in extra copies of gene.

Cmyc-FISH

www.pathology.unibe.ch/.../ speztech/spez_fish.htm

27Another overview

• Historically, several models of cancer have been developed since War on cancer declared by Nixon, 1970.

• ON/Off switch idea

• Multi-hit hypothesis, textbook: colon cancer

• Gatekeeper and Caretaker genes– Gatekeepers: tumor suppressor genes. Prevent

cells from dividing when they’re not supposed to.– Caretakers: protect the DNA, repair damage.

28Review of Telomerase

• Because of discontinuous synthesis of linear DNA, each round of DNA would shorten the chromosome, eventually causing chromosome instability and cell death. Telomerase adds protective DNA to end, making cell “immortal”.

29Cancer and Telomerase

• Stem cells, frequently, rapidly dividing cells, maintain telomerase.

• Descendents have a life span.– Telomerase gene is shut off, enzyme not made.– Lack of telomerase is a cell clock.– Chromosome becomes unstable, cell undergoes

apoptosis and dies; replaced by new cells.

• Some cells cheat death.– Cancer cells turn telomerase back on, become

immortal. Area of active study.

30Cancer and the Environment

• Familial predispositions exist, but sporadic cancers are more common.– Environmental factors combine with genetic

background to produce cancer.– Microbes, chemicals, radiation all implicated.

• Direct experimentation not possible w/ humans.– Evidence is correlation, not causation.

• Statistical analysis of epidemiological data• Not “proved” that smoking causes cancer

– Need statistical evidence plus plausible mechanism

31Cancer and microbes

• Retroviruses and other inserting viruses– RNA to DNA, inserts into chromosome, directs

synthesis of new RNA. HIV, Rous sarcoma virus.– DNA viruses that insert: Hepatitis B

• Hepatitis B infected: 100x more likely to get liver cancer

• Epstein Barr Virus, Kaposi’s sarcoma virus– EBV associated with Burkitt Lymphoma (and mono)– Herpes virus KSV causes sarcoma in HIV infection

32Cancer and microbes-2

• Sex causes cancer: Papilloma virus and cancer of the cervix.– Papilloma virus causes warts, including genital

warts; sexually transmitted disease.– Subtypes, instead of causing warts, inserts into

DNA, causes increased expression of viral protein E7. See next slide.

• Helicobacter pylori– Bacterium now known to cause ulcers.– Highly correlated with stomach cancer; under study.

33Cancer and microbes-3

pRB is a tumor suppressor protein that prevents the action of transcription factor E2F by binding to it. Viral protein E7 bids tightly to pRB, freeing E2F to transcribe genes promoting cell division.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/E7_Rb.gif

34Cancer and the environment

• Relatively few cancers are “inherited”, most result from interactions with the environment or “lifestyles”

• Tobacco use– Benzo-a-pyrene, tar, radioisotopes, formaldehyde

• Diet: natural and produced carcinogens– Aflatoxins, nitrosoamines– Fiber, antioxidant compounds helpful.

• Radiation: UV , ionizing