ELGP 21_Molecular basis of cancer.pdf
- 1. MODULE 5. NEOPLASIA Molecular basis of cancer Department of General and Clinical Pathology Medical University - Varna Peter Ghenev MD, PhD
- 2. P Wheater, G Burkitt, A Stevens, J Lowe. Basic Histopathology, Second Edition, ELBS with Churchill Livinstone. 1991:2 (Acute cell injury) (Sublethal damage)
- 4. P Wheater, G Burkitt, A Stevens, J Lowe. Basic Histopathology, Second Edition, ELBS with Churchill Livinstone. 1991:2 (Acute cell injury) (Sublethal damage) Metaplasia
- 5. 5 Fundamental principles of the molecular basis of cancer •Nonlethal genetic damage determines carcinogenesis •A tumor is formed by the clonal expansion of a single genetically damaged precursor cell (monoclonality) •Four classes of normal regulatory genes are the principal targets of genetic damage the growth-promoting protooncogenes the growth-inhibiting tumor suppressor genes the genes regulating apoptosis the genes involved in DNA repair TRANSFORMATION
- 7. 7 ...molecular basis of cancer • Carcinogenesis is a multistep process at both the genetic and phenotypic levels •A malignant neoplasm has several phenotypic attributes: - excessive growth - local invasiveness - the ability to form metastases •These features are acquired in a stepwise fashion, a phenomenon called tumor progression
- 8. 8 •GF binding to specific receptor •signal-transducing proteins on the inner leaflet of the membrane are activated • transmission of the signal to the nucleus • DNA transcription is initiated • the cell enters into cell cycle, resulting in cell division Steps in cell proliferation
- 9. 9 How do cells acquire uncontrolled proliferation potential? TNF IL-6...
- 10. 10 How do cells acquire uncontrolled proliferation potential? +intra-S phase checkpoint
- 11. 11 The progression of cells through the cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs) •CDKs drive the cell cycle by phosphorylating critical target proteins • expressed constitutively in an inactive form •activated by phosphorylation after binding to proteins called CYCLINS How do cells acquire uncontrolled proliferation potential?
- 12. 12 •synthesized during the cell cycle •they activate the CDKs •cyclin levels decline rapidly on task completion •more than 15 cyclins have been identified - cyclins D, E, A, and B appear sequentially during the cell cycle and bind to one or more CDKs CYCLINS
- 13. 13 •Cyclin D appears first in mid G1 •cyclin D binds to CDK4, forming a cyclin D-CDK4 complex •This complex phosphorylates the retinoblastoma susceptibility protein (RB) •The phosphorylation of RB is a molecular ON-OFF switch for the cell cycle Cyclin D and RB Phosphorylation * •Cyclins D genes are overexpressed in breast, liver, and esophagus Ca, and mantle-zone lymphoma •amplification of CDK4 gene - sarcoma, glioblastoma
- 14. 14 •In its hypophosphorylated state, RB prevents cells from replicating by forming a tight, inactive complex with the transcription factor E2F (a family of transcription factors) •Phosphorylation of RB dissociates the complex and releases the inhibition on E2F transcriptional activity Thus phosphorylation of RB eliminates the main barrier to cell-cycle progression and promotes cell replication Cyclin D and RB Phosphorylation
- 16. 16 E2F-mediated transcription of cyclins E and A, and of genes required for DNA replication, permit the passage through the G1 restriction point
- 17. 17 Expression of cyclin-CDK complexes in cell cycle Further progression through the S phase and the initiation of DNA replication needs an active complex between cyclin E and CDK2
- 18. 18 G2/M transition point is passed by transcription of cyclin A, and the cyclin A-CDK2 complex
- 19. 19 The activity of cyclin-CDK complexes is regulated CDK inhibitors There are two main classes of CDK inhibitors: the Cip/Kip and the INK4/ARF families These inhibitors function as tumor suppressors and are frequently altered in tumors * INK4a/ARF locus = ("inhibitor of k inase 4/alternative reading frame")
- 20. 20 Role of cyclins, CDKs and CDK-inhibitors in regulating the G1/S cell-cycle transition SUMMARY
- 21. 21 Main Function • CDK4 - Forms a complex with cyclin D - phosphorylates RB, allowing pass through G1 restriction point • CDK2 - Forms a complex with cyclin E in late G1, for the G1/S transition - Forms a complex with cyclin A at the S phase for G2/M transition • CDK1 - Forms a complex with cyclin B, which acts on the G2/M transition SUMMARY
- 22. 22 •Cell-cycle arrest in response to DNA damage and other cellular stresses is mediated through p53 Defect in cell-cycle checkpoint components is a major cause of genetic instability in cancer cells
- 23. 23 Seven fundamental changes determine malignant phenotype •Self-sufficiency in growth signals •Insensitivity to growth-inhibitory signals •Evasion of apoptosis •Defects in DNA repair •Limitless replicative potential •Sustained angiogenesis •Ability to invade and metastasize 7
- 24. 24 Oncogenes = promote autonomous cell growth in cancer cells Protooncogenes = physiologic regulators of cell proliferation and differentiation Oncogenes are characterized by: - the ability to promote cell growth in the absence of normal mitogenic signals - oncoproteins, resemble the normal oncoproteins but are devoid of important regulatory elements and are produced in transformed cells constitutively Self-sufficiency in growth signals 1
- 25. 25 GAP = GTPase activating protein
- 26. 26 • Growth factors (PDGF, TGF-, EGF, FGF) - synthesis of GF for autocrine activation + targeted therapy - HER2 (Ab), c-KIT (TRK) • GF receptors - constitutive dimerization and activation without binding of ligand - mutations - gene rearrangement - overexpression Self-sufficiency in growth signals 1 Point mutation of RAS family genes is the single most common abnormality of dominant oncogenes in human tumors
- 27. 27 Insensitivity to growth-inhibitory signals The proteins that apply brakes to cell proliferation are the products of tumor suppressor genes The mutations required to produce retinoblastoma involve the RB gene, located on chromosome 13q14 2 •Cancer develops when the cell becomes homozygous for the mutant allele or when the cell loses heterozygosity for the normal RB gene (loss of heterozygosity - LOH) •Also referred to as a recessive cancer gene
- 28. 28 Insensitivity to growth-inhibitory signals The "two-hit" hypothesis of oncogenesis (Knudson) •in hereditary cases- one genetic change ("first hit") is inherited from an affected parent and present in all somatic cells •the second mutation ("second hit") occurs in one of the retinal cells (already has the first mutation) In sporadic cases, however, both mutations (hits) occur somatically within a single retinal cell, whose progeny then form the tumor RB 2
- 30. 30 in majority of human cancers dysregulated is at least one of the four key regulators of the cell cycle: - p16INK4a - cyclin D - CDK4 - RB In cells that harbor mutations in any one of these other genes, the function of RB is disrupted even if the RB gene itself is not mutated Tu suppressor genes in human neoplasms Insensitivity to growth-inhibitory signals 2
- 31. 31 Other pathways of cell growth regulation, also converging on RB •TGF-β inhibits cell proliferation by up-regulation of the CDK inhibitor p27 •human DNA viruses seem to act by neutralizing the growth inhibitory activities of RB Insensitivity to growth-inhibitory signals 2
- 32. 32 APC/β-Catenin Pathway •Down-regulation of growth-promoting signals is another potential area in which products of tumor suppressor genes may be operative •APC and NF-1 gene products fall into this category •Germ line mutations at the APC (5q21) and NF-1 (17q11.2) loci are associated with benign tumors that are precursors of Ca that develop later 2
- 34. 34 p53: Guardian of the Genome •p53 gene is located on chromosome 17p13.1 •most common target for genetic alteration in tumor •p53 acts as a "molecular policeman" that prevents the propagation of genetically damaged cells •p53 protein is a DNA-binding protein in nucleus •controls transcription of several other genes the growth-inhibiting effects of p53 are produced by up-regulating the synthesis of the CDK inhibitor p21 Evasion of apoptosis 3
- 35. 35 p53 3
- 36. 36 Subcellular localization and functions of major classes of cancer-associated genes protooncogenes - red cancer suppressor genes - blue DNA repair genes - green genes that regulate apoptosis - purple 4
- 37. 37 LIMITLESS REPLICATIVE POTENTIAL: TELOMERASE •After a fixed number of divisions, normal cells become arrested in a terminally nondividing state known as replicative senescence •With each division telomeres, at the ends of chromosomes shorten •In germ cells, telomere shortening is prevented by the enzyme telomerase •This enzyme is absent from most somatic cells, and hence they suffer progressive loss of telomeres 5
- 38. 38 5 Telomerase activity and maintenance of telomere length are essential for the replicative potential in Ca
- 39. 39 “And of course... telomerase is required to allow cancer cells to remain immortal And so the successful clones in a cancer mutate so as to express telomerase It's a target for anti-cancer therapy today: •Nat Med 5: 1164, 1999 •Anticancer Res 20: 4419, 2000 Only cancer cells stain positive for telomerase •Cancer 90: 117, 2000 Is this the dream of a pathologist's "stain for malignancy"? *
- 40. 40 ANGIOGENESIS •Tumors stimulate the growth of host blood vessels •Tumors exist in situ without developing a blood supply for months to years •but some cells change to an angiogenic phenotype •this change is known as the angiogenic switch 6
- 41. 41 Seven fundamental changes determine malignant phenotype cont’d I. Self-sufficiency in growth signals: proliferation without external stimuli due to oncogene activation II. Insensitivity to growth-inhibitory signals: no response to proliferation inhibitors (TGF-β, direct inhibitors of cyclin-dependent kinases III. Evasion of apoptosis: due to inactivation of p53 IV. Defects in DNA repair: failure to repair DNA after carcinogens or unregulated cellular proliferation (genomic instability)
- 42. 42 Seven fundamental changes determine malignant phenotype cont’d V. Limitless replicative potential: maintenance of telomere length and function VI. Sustained angiogenesis: Tu are able to provide vascular supply, induced by various factors (VEGF) VII.Ability to invade and metastasize: depend on processes intrinsic to the cell and initiated from environment
- 43. 43
- 44. 44 A paraneoplastic syndrome - when a neoplasm elaborates a substance that results in an effect that is not directly related to growth, invasion, or metastasis •most paraneoplastic syndromes result from production of hormone-like substances, but a variety of effects are possible •paraneoplastic syndrome may precede diagnosis
- 45. 45 Paraneoplastic syndromes Syndrome Mechanism Example Cushing's Syndrome ACTH-like substance Lung (oat cell) carcinoma Hypercalcemia Parathormone-like substance Lung (squamous cell) carcinoma Hyponatremia Inappropriate ADH secretion Lung (oat cell) carcinoma Polycythemia Erythropoietin-like substance Renal cell carcinoma Trousseau's Syndrome Hypercoagulable state Various carcinomas Hypoglycemia Insulin-like substance Various carcinomas and sarcomas Carcinoid Syndrome 5-hydroxy-indoleacetic acid (5-HIAA) Metastatic malignant carcinoid tumors
- 46. 46 Oncogenesis Mechanism Action Example Overexpression of growth factor receptors (such as epidermal growth factor, or EGF) making cells more sensitive to growth stimuli c-erb-B2 Increased growth factor signal transduction by an oncogene that lacks the GTPase activity that limits GTP induction of cytoplasmic kinases that drive cell growth ras Overexpression of a gene product by stimulation from an oncogene (such as ras) c-sis Lack of normal gene regulation through translocation of a gene where it is controlled by surrounding genes to a place where it is no longer inhibited c-abl Growth Promotion Binding of oncogene product to the nucleus with DNA transcriptional activation to promote entry into the cell cycle c-myc Loss of normal growth inhibition BRCA-1 Lack of regulation of cell adhesion with loss of growth control through cell interaction APC Loss of down-regulation of growth promoting signal transduction NF-1 Loss of regulation of cell cycle activation through sequestation of transcriptional factors Rb Loss of Tumor Suppressor Gene Function Loss of regulation of cell cycle activation through lack of inhibition of cell proliferation that allows DNA repair p53 Limitation of Apoptosis Overexpression of gene, activated by translocation, prevents apoptosis bcl-2
- 47. 47 What Is Microsatellite Instability and Why Should We Care? from ACS Surgery: Principles & Practice Posted 11/19/2004 Dieter Hahnloser, MD, University Hospital Zurich, Switzerland; Stephen N. Thibodeau, PhD, Mayo Clinic College of Medicine ACS Surgery 2004. © 2004 WebMD Inc. All rights reserved. Microsatellites are short segments of repetitive DNA bases that are scattered throughout the genome; they are found predominantly in noncoding DNA. Microsatellite instability (MSI) can be defined as a change of any length resulting from either insertion or deletion of repeating units in a microsatellite within a tumor, as compared with normal tissue. MSI results from failure of the cell to repair errors made during DNA replication. A group of intranuclear proteins, collectively known as the mismatch repair (MMR) system, is responsible for removing these errors. Failure allows errors to accumulate and MSI to develop; this facilitates malignant transformation.