34 Cancer

Lecture Outline, 11/30/05 Review the Cell cycle Cancer is a genetic disease Oncogenes and proto-oncogenes Normally promote cell growth. Become oncogenic after point mutations, duplications, deletion of silencer, etc Tumor Suppressor genes Normally inhibit cell growth. Allow cell growth when damaged or deleted. Mutator genes The multi-step model of cancer

Cancer is caused by mutant genes Mutations in regulatory genes lead to uncontrolled cell growth Understanding gene regulation is one key to understanding cancer All cancers trace back to single cell Must accumulate multiple mutations, all in the same cell lineage

Changes in growth properties of cancer cells

The incidence of human cancers increases markedly with age It takes a long time for the causative mutations to accumulate in a cell

The Human Cell Cycle ~ 1 hour ~ 4 hours ~ 10 hours ~ 9 hours

Cell Cycle Regulators and Cancer

Free E2F is a transcription activator Rb represses E2F Cyclin/Cdk inactivates Rb by phosphorylation

P53 is a transcription factor that activates p21, an inhibitor of cyclin/cdk2

Signaling pathways that regulate cell division 1 Growth factor 2 Receptor p GTP Ras 3 G protein Hyperactive Ras protein (product of oncogene) issues signals on its own 4 Protein kinases (phosphorylation cascade) 5 Transcription factor (activator) NUCLEUS DNA Gene expression Protein that stimulates the cell cycle UV light DNA damage in genome 1 Active form of p53 Defective or missing transcription factor, such as p53, cannot activate MUTATION inhibits EFFECTS OF MUTATIONS Protein overexpressed Cell cycle overstimulated Increased cell division Cell cycle not inhibited Protein absent (a) Cell cycle–stimulating pathway. This pathway is triggered by a growth factor that binds to its receptor in the plasma membrane. The signal is relayed to a G protein called Ras. Like all G proteins, Ras is active when GTP is bound to it. Ras passes the signal to a series of protein kinases. The last kinase activates a transcription activator that turns on one or more genes for proteins that stimulate the cell cycle. If a mutation makes Ras or any other pathway component abnormally active, excessive cell division and cancer may result. (b) Cell cycle–inhibiting pathway. In this pathway, DNA damage is an intracellular signal that is passed via protein kinases and leads to activation of p53. Activated p53 promotes transcription of the gene for a protein that inhibits the cell cycle. The resulting suppression of cell division ensures that the damaged DNA is not replicated. Mutations causing deficiencies in any pathway component can contribute to the development of cancer. (c) Effects of mutations. Increased cell division, possibly leading to cancer, can result if the cell cycle is overstimulated, as in (a), or not inhibited when it normally would be, as in (b). MUTATION Figure 19.12

Stimulation versus inhibition of G1 progression Stimulation of G1 progression Inhibition of G1 progression mitogens DNA-damage anti-mitogen (TGF-b) myc ras PKB p53 p15 ? cyclin D / cdk4 p27 p21 cyclin D / cdk4 p16 P P pRB pRB pRB pRB P P P P Inhibition S-phase entry Inhibition S-phase entry S-phase entry allowed S-phase entry allowed oncogenes tumor suppressor genes

Oncogenes All are involved in positive control of cell growth and division. About 100 different oncogenes have been identified Can be various kinds of proteins: Growth factors, regulatory genes involved in the control of cell multiplication. Protein kinases, add phosphate groups to target proteins, important in signal transduction pathways. “Proto-oncogenes” Normal form of the gene that is involved in positive regulation of the cell cycle

Genetic changes that can turn proto-oncogenes into oncogenes DNA Translocation or transposition: gene moved to new locus, under new controls Gene amplification Point mutation within a control element within the gene Oncogene Normal growth-stimulating protein in excess Hyperactive or degradation- resistant protein New promoter Figure 19.11

Translocation puts abl under the control of a different promoter

Translocation puts bcl near a new enhancer

Ras Proto-oncogene 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.

Receptor tyrosine kinases can activate ras ras is a monomeric G-protein “molecular switch” You’ve seen RAS before . . .

Mitogen Activated Protein Kinases MAPKs Ras activation sets off a phosphorylation cascade Mitogen Activated Protein Kinases MAPKs 10 100 MAPKKK MAPKK 1,000 Controls: -Transcription Factors -Translation -Cell Division MAPK 100,000

- broken ras – won’t shut off PROBLEMS IN CANCER: - broken ras – won’t shut off Broken receptor – thinks ligand there even when it isn’t broken MAPK – on all the time, even when not phosphorylated RESULT: continuous signal for cell to divide

Oncogenes act cooperatively in tumor-induction

Tumor Suppressor Genes Normally inhibit cell growth Example: retinoblastoma RB protein normally blocks a transcription factor, E2F

p53 Gene Detects DNA damage The “Last Gatekeeper” Involved in 50% of cancers Often not malignant despite other cancer-causing mutations until p53 is inactivated by mutation. Two possible responses to DNA damage: 1) Acts as a Transcription Factor to activate expression of p21, which inhibits CDK/G1 cyclin to halt the cell cycle; then activates DNA repair. 2) Triggers Apoptosis (programmed cell death) if damage can’t be repaied.

Apoptosis = controlled cell death “executioner” proteins (caspases) break down the cell Reduced cell death can also lead to cancer

Apoptosis pathways http://www.cell-research.com/20014/20014cover.htm

Mutator genes Cancer is caused by mutations, so factors that increase mutation rate will increase cancer rate. What kinds of genes would increase mutation rate? Example: BRCA1 and BRCA2 Many environmental factors (carcinogens) also cause DNA damage or mutations, that can lead to cancer

A multistep model for the development of colorectal cancer (1) The clonal origin of tumors: each individual cancer is a clone that arises from a single cell. The progeny cells have growth advantage over the surrounding normal cells. Colon (2) Cancer development is a multi-step process. Multiple mutations accumulated over periods of many years ----“multi-hit” model. 1 Loss of tumor-suppressor gene APC (or other) 2 Activation of Ras oncogene 4 Loss of tumor-suppressor gene p53 Colon wall 3 Loss of tumor- suppressor gene DCC 5 Additional mutations Normal colon epithelial cells Small benign growth (polyp) Larger benign growth (adenoma) Malignant tumor (carcinoma) Figure 19.13