Cancer and the Cell Cycle

1. Cancer and the Cell Cycle

2. Outline of the lecture • What is cancer? • Review of the cell cycle and regulation of cell growth • Which types of genes when mutated can  cancer? • Roles for screening for mutations in specific genes • Tumor suppressor p53 • Have you figured it out?

3. Overview of Cancer Cancer: Abnormal proliferation of cells forming cell masses known as tumors. Tumors Malignant Benign Slow growth rate Fast growth rate

4. Malignant Tumors Malignant tumor cells have the ability to travel through the corporal fluids settling on other tissues. This process is known as metastasis. From a single cell new tumors can arise

5. According to the American Cancer Society 25 % of deaths in U.S. are caused by cancer. ~ 564,630 Americans are expected to die of cancers in 2006. American Cancer Society (ACS): www:cancer.org

6. What is cancer? • Cancer = uncontrolled proliferation of cells within the body  tumor. • Tumor = clone of cells resulting from series of sequential genetic mutations  loss of growth control. • Cancer is also known as malignancy. • Development of cancer = oncogenesis • Study or treatment of cancer = oncology

7. Cancer is a multi-step process.

8. Non-dividing cells 1 2 1 2 1 1 1 2 1 2 1 123 123 123 123 123 123 123 123 123 This process continues, with each successive mutation leading to a faster rate of cell division, slower rate of cell death, and eventually loss of cell adhesion.

9. Review of the Eukaryotic Cell Cycle

10. The cell cycle • The cell cycle has four phases: • M, during which the cell divides (undergoes mitosis); • G1, during which the cell grows larger; • S , during which DNA synthesis occurs; • G2, during which the cell continues to grow and prepare for mitosis.

11. The cell cycle- regulation at several points • G1 Restriction Point • time when the decision is made whether to continue the cycle or to to exit the cycle in a nondividing state called G0. • Once the cell passes the restriction point in G1, the cycle will continue until it is arrested at one of several later checkpoints in response to some problem that needs to be corrected.

12. The cell cycle- regulation at several points (cont’d.) • Late G1 and late G2 checkpoints • If DNA damage has occurred, these checkpoints allow time for the damaged DNA to be repaired before the cycle resumes. • Late G2 checkpoint also responds to the presence of unreplicated DNA and prevents mitosis from occurring until all of the DNA has been copied. • In the event that a cell enters an S phase with damaged DNA that can’t be repaired, apoptosis may be triggered to prevent the mutant cell from reproducing itself.

13. The cell cycle- regulation at several points (cont’d.) • Late M phase checkpoint • Halts the cell cycle until all of the chromosomes are properly aligned.

14. What is G0 ? • Cells in G0 may differentiate and assume specialized functions. • A cell can remain in G0 indefinitely, or it may re-enter the cell cycle in response to signals from a variety of growth factors.

15. Seven levels of regulation of cell growth An unrepaired mutation in a gene for a DNA-repair protein, a cell-cycle control protein, or an anti-apoptosis protein can increase the likelihood of a cancer developing.

16. An Example of Cell Cycle Regulation by a Serum Growth Factor • Cyclin D is made following the binding of the serum growth factor to its receptor and the ensuing cascade of phosphorylations.

17. An Example of Cell Cycle Regulation by a Serum Growth Factor Where and what kinds of mutations would lead to permanent expression of Cyclin D?

18. Phosphorylation of Rb is Regulated in turn by Cyclin D Genes whose products move the cell through S phase. Note multiple examples of cell cycle regulation by tumor suppressors (white circles)

19. The Cell Cycle • Cyclins, CDKs, and CDKIs • Cyclins and Cyclin Dependent Kinases (CDKs) interact to move the cell cycle forward. • Cyclins, and Cdks act together as a dimer, functioning as the regulatory and catalytic subunits, respectively. • Cyclins are degraded at the end of their functional period, thus inactivating their Cdk partner in the dimer. • The assembly of the dimers is regulated by other proteins.

20. The Cell Cycle • Cyclins, CDKs, and CDKIs • Cyclins, CDKs, and cyclin dependent kinase inhibitors, (CDKIs) interact to block phases of the cycle.

21. Examples of Cell Cycle Regulation by a Serum Growth Factor and by Tumor Suppressors • Cyclin D associates with either Cdk4 or Cdk6. • P16 may block the assembly. • After assembly the Cdk becomes phosphorylated. • This may be blocked by either p21 or p27 • The target of the active dimer is Rb1 which is bound to a transcription factor called E2F. • The Rb1/E2F dimer blocks transcription of genes needed to enter the S phase. • Phosphorylation of Rb results in its dissociation from E2F. • This results in activation of S phase genes. • In addition to its ability to block the association of cyclin D with a Cdk, P16 can also directly block the phosphorylation of Rb.

22. Examples of Cell Cycle Regulation by Tumor Suppressors • P16, p21, and p27 are regulated by p53 (more on this later) which blocks the cell cycle in the G1 phase if there is DNA damage. • P53, Rb1, p21, p16, and p27 are called tumor supressors because their normal function is to prevent the growth of cells with damaged DNA.

23. An Example of Cell Cycle Regulation by Tumor Suppressors • P53 also responds to unrepaired DNA damage by triggering apoptosis (programmed cell death) of the injured cell. • P53 interacts with Bax, which, in turn activates special enzymes called caspases. • Bax is member of the Bcl-2 family of proteins, but unlike Bcl-2, which prevents apoptosis, Bax activates apoptosis. • Caspases initiate a protease cascade that results in digestion of the DNA. • This ultimately leads to cell death.

24. Bax Apoptosis – in response to irreparable DNA damage Note the role of tumor suppressor p53.

25. Which types of genes when mutated can  cancer? • Oncogenes = genes whose products turn DNA synthesis ON • Tumor suppressors/anti-oncogenes = genes whose products turn DNA synthesis OFF • Genes whose products contribute to genomic stability, (e.g.) • repair DNA, • limit synthesis to doubling the DNA content • make sure DNA is completely doubled • Genes whose products contribute to cell longevity • In each case, ask yourself: would the mutation contributing to the development of cancer be an activating mutation or an inactivating mutation?

26. Which types of genes when mutated can  cancer? • Oncogenes (turn DNA synthesis ON) • In progression towards cancer, a gene for a protein that normally stimulates DNA synthesis (proto-oncogene) is either • consitutively expressed at high levels or • Change in the regulation of the gene • mutated such that protein product is constitutively active, i.e., can not be inactivated • Change in the protein for which the gene codes • Note: be sure you can distinguish between these two types of changes – in the regulation, or in the protein product itself. • Mutations in classes I-IV from Slide # 15 generally give rise to dominantly active oncogenes. • Examples: see next slide.

27. Oncogenes

28. Which types of genes when mutated can  cancer? • Tumor suppressors/anti-oncogenes (turn DNA synthesis OFF) • In the progression towards cancer, a gene for a protein that normally inhibits DNA synthesis is either • permanently inactivated or • mutated such that the protein product is inactive • Mutations in Class VI, cell-cycle control proteins, from Slide #15. • Examples: • APC inhibits Wnt gene product from activating myc • Rb1 inhibits activation of transcription of DNA synthesis genes

29. Which types of genes when mutated can  cancer? • Contributors to genomic stability • Some tumor suppressors turn DNA synthesis off when DNA is damaged. • The progression toward cancer occurs when a gene for a protein which contributes to DNA repair is • permanently inactivated or • mutated such that protein product is inactive

30. Which types of genes when mutated can  cancer? • Contributors to genomic stability • Mutations in repair genes increase likelihood of mutations in proto-oncogenes and tumor suppressors. • Examples: • p53 gene product induces genes for DNA repair • MDM2 gene product destabilizes p53 • MutS and MutL gene products repair UV or chemically damaged DNA

31. Which types of genes when mutated can  cancer? • Contributors to cell longevity (anti-apoptosis genes) • Progression toward cancer can occur when an anti-apoptosis gene is • constitutively expressed or • mutated such that protein product is constitutively active • Allows survival of cells with oncogenic mutations • Example: Bcl2

32. Roles for screening for mutations in specific genes • To determine • Type of cancer • Familial predispositions • Progression of the cancer