1. Tumor Suppressors Versus Oncogenes

2. The Cancer Phenotype is Usually Recessive R. Weinberg, Cancer Biology

3. The story of Retinoblastoma Tumor Suppressor Genes

4. Retinoblastoma is a cancerous disease 1/20,000 children; 300 per year Average age is 18 months Treatment: enucleation = eye removal Prognosis is good after enucleation over 90% survival with early detection and treatment Leukocoria or “white pupil”

5. - Sporadic cancer in 55-65% of all cases - Sporadic cancers are unilateral Rb is either sporadic or familial Hereditary childhood cancer: - bilateral tumors in ~75% of cases - unilateral tumors in ~25% of cases

6. Children with bilateral (familial) Rb have a high risk of developing non-retinal tumors Germ-line mutations in the Rb gene lead to predisposition to cancer Sporadic Familial

7. In cancer patients with a family history of Retinoblastoma: the inheritance seems to be ?

8. Rb tumors are associated with a deleted region in chromosome 13 Deletion = loss-of-function probably a recessive mutation in the Rb gene

9. Knudson’s “Two Hit” Hypothesis for the Generation of RB Alfred Knudson, PNAS 68:820 (1971)

10. Knudson’s “two hit” hypothesis for the generation of RB

11. Retinoblastoma is inherited as a dominant trait, but it is recessive at the cellular level People with familial Retinoblastoma carry one mutated copy in ALL their cells. Cells that would get a second hit will develop Rb or other cancers later in life.

12. Retinoblastoma is Associated with Loss of Heterozygosity (LOH) at the RB Locus ~40% of the time the wild type allele is mutated 4% of these are deletions R. Weinberg, Cancer Biology (LOH)

13. (nondisjunction) ~30% of the time Mitotic Nondisjunction Causes (LOH) at the RB Locus

14. Mitotic Recombination Causes LOH at the RB Locus ~30% of the time

15. Rb is just one example Inheritance of brca1(lf) mutation results in predisposition for breast cancer Rb = A Tumor Suppressor Gene Predisposition is inherited dominantly, but cancer is not inherited The offspring CANNOT inherit two mutated genes

16. How can we clone a tumor-recessive gene? Oncogenes transform cells into cancerous cells But TSGs are recessive How do we test candidate genes?

17. Rb tumors are associated with a deleted region in chromosome 13

18. Use a fragment of the candidate gene as a probe for hybridization analysis Search for absence of the gene in tumors (hoping both mutated copies are deletions) Testing a candidate gene More on this in Angier book, starting p. 334

19. Rb gene expression is absent or altered in retinoblastoma tumors Friend et al. Nature (86) Lee et al. Science (87) Northern blots (mRNA expression) Rb tumorsWTOther tumors

20. Huang et al Science, 242:1563-6 (1988) Saos-2 cells Saos-2 cells infected with RB retrovirus Expression of the RB Gene in RB Mutant Cells Corrects the Cancer No virus RB virus Control virus Parental = WERI-Rb27 cells Lux = control virus infected Rb = RB virus infected Tumor formation assayed in “nude” mice

21. Dr. David Abramson, RB expert at New York Hospital (ca. 1986, According to Natalie Angier) “I believe that in fifteen years, at the outside, we’ll be able to stop retinoblastoma before it begins. I’m so sure that I’ve already given the drug a name. I call it retino-revert, or retino-prevent. The drug will be an analogue of the natural protein that is missing in retinoblastoma cells … We’ll be able to diagnose a child prenatally and start giving this retino-revert to the mother to prevent retinoblastomas from growing as the fetus is developing. I know I’m going out on a limb with this one, but … Come back to me in 2001 and tell me if I wasn’t right.” Bold Predictions, Further Work

22. pRb: What does it do? pRb is a nuclear protein that undergoes phosphorylation and dephospharylation in concert with the cell cycle

23. Un- and Hypo- phosphorylated pRb inhibits the cell from entering a new cell cycle RB regulates progression through G1 phase Upon further phosphorylation at the R point, hyper-phosphorylated pRb becomes inert and the cell cycle can proceed

24. Figure 8.23c The Biology of Cancer (© Garland Science 2007) Un- and Hypo-phosphorylated Rb inhibits activity of the E2F family of transcription factors

25. Hyper-phosphorylated Rb cannot bind and inhibit E2F

26. RB/E2F complexes act as transcriptional repressors

27. Releasing Rb from the E2Fs leads to activation of transcription E2Fs have 100s of target genes, mostly involved in DNA replication

28. Rb, the retinoblastoma protein regulates the cell cycle Cell cycle = OFF Rb binds to E2F: no transcription, no entry into S phase Cell cycle = ON Rb does not bind to E2F: transcription and entry into S phase w/o 2 copies of Rb: no cell cycle arrest

29. Sites of RB Missense Mutations in Tumors A B The A and B domain = pocket = where E2F binds

30. Figure 8.23b The Biology of Cancer (© Garland Science 2007)

31. How is Rb activity regulated during the cell cycle?

32. How is Rb activity regulated during the cell cycle? By Cyclin/CDKs Hypo-phosphorylation is catalyzed by cycD-CDK4/6 Hyper-phosphorylation is catalyzed by cycE-CDK2

33. pRb is hyper-phosphorylated and inhibited (and released from its role as a guardian), only upon cycE expression

34. However, E-CDK2 can phosphorylate Rb, only AFTER Rb is phosphorylated by D-CDK4/6 How is Rb activity regulated during the cell cycle? By Cyclin/CDKs

35. One of the E2F targets: the cycE gene! Transcription of cycE starts a positive feedback loop

36. Rb control of E2F IS the restriction point

37. As E2Fs are necessary for expression of cycE, think how critical negative regulation by Rb is for cell cycle control E2Fs

38. Proliferation Signals The Canonical p16/pRB/E2F “Pathway”

39. Sherr and McCormick, Cancer Cell ·2:103 (2002) The RB Pathway in Human Cancer (p16)

40. Forward versus reverse genetics Forward Genetics: isolate mutants with a specific phenotype and then clone the gene Reverse Genetics: clone the gene and then make a mutant to determine phenotype How can we study retinoblastoma in the lab? with GEMs: Genetically Engineered Mice

41. “Knock Out” your gene in Cultured Pluripotent ES Cells

42. Formation of ES Cells Carrying a Knockout Mutation

43. Positive and Negative Selection of Recombinant ES Cells

44. Making the Mutant Mouse

45. The Phenotype of RB Mutant Mice Homozygous embryos die at day 13.5 massive apoptosis in the CNS anemia caused by lack of blood cells made by the fetal liver therefore RB is not essential for cell division Heterozygous animals are viable they develop pituitary tumors with near 100% penetrance these tumors lose the wild type RB allele also develop thyroid adenomas therefore the Rb-/+ mouse is not a model for retinoblastoma but it is a good model for tumor suppressors And can study it’s mechanism of action genetically

46. Tumorigenesis in RB Mutant Mice Requires E2F GenotypePituitary TumorsThyroid Tumors RB (+/-)19/20 (95%)10/19 (53%) RB (+/-); E2F1 (+/-)36/36 (100%)2/34 (6%) RB (+/-); E2F1 (-/-)16/26 (62%)0/22 (0%) Yamasaki et al, Nat. Gen. 18:360 (1998)

47. The Phenotype of RB Mutant Mice Homozygous embryos die at day 13.5 massive apoptosis in the CNS anemia caused by lack of blood cells made by the fetal liver therefore RB is not essential for cell division Heterozygous animals are viable they develop pituitary tumors with near 100% penetrance these tumors lose the wild type RB allele also develop thyroid adenomas therefore the Rb-/+ mouse is not a model for retinoblastoma but it is a good model for tumor suppressors Why don’t Rb-/+ mice get retinoblastoma?????!!!!!!