18 Cancer

Chapter 18 Cancer: Student Learning Outcomes:

• Explain causes, development of cancer (clonal):• Chemicals, radiation, virus, spontaneous

• Decribe diversity of tumor viruses (model systems)

• Explain essential features of Oncogenes:• Derived from proto-oncogenes, examples

• Explain essential features of tumor suppressors:• Normal cell functions, examples

• Describe molecular approaches to cancer treatment

Introduction

18.1 Cancer cells have abnormalities in multiple cell regulatory systems.

• Breakdown of regulatory mechanisms that govern normal cell behavior:

• Grow, divide in uncontrolled manner, • Spread throughout body• Interfere with function of normal tissues, organs

• Understand cancer cells at molecular, cellular levels.• Studies of cancer cells illuminate mechanisms of

normal cell behavior

The Development and Causes of Cancer

Tumor: abnormal proliferation of cells:

Benign tumor • confined to original location, not invade normal tissue, not spread to distant sites

Malignant tumor • invades surrounding normal tissue, spreads

through body via circulation or lymphatics (metastasis)

• termed cancers.

Fig. 18.1Pancreatic cancer (purple stained nuclei) in normal

Development and Causes of Cancer

Most cancers three main groups:

Carcinomas: malignancies of epithelial cells (about 90% of human cancers).

Sarcomas (rare in humans): solid tumors of connective tissue, (muscle, bone, cartilage, fibrous tissue)

Leukemias and lymphomas: blood-forming cells and cells of immune system, respectively.

• Tumors are further classified according to tissue of origin and type of cell involved.

Note most common cancers; most lethal

Fig 18.2 Tumor clonality

Tumor clonality• Fundamental feature of cancer• Tumors develop from single cell that

proliferates abnormally (evidence from X-inactivation pattern)

Most cancers develop late in lifeCancer is multistep process: • Cells gradually become malignant through progressive alterations• Multiple abnormalities accumulate• Selection for growth advantage

Ex: colon cancer increases with age.

Fig. 18.2 clonalityFig. 18.3 age and colon cancer

Fig 18.4 Stages of tumor development

Tumor initiation: genetic alteration → abnormal proliferation of single cell, → population of clonal tumor cells.

Tumor progression: additional mutations occur within cells of population

• Growth advantage,• Survival• Invasion• Metastasis

Fig. 18.4

Fig 18.5 Development of colon carcinomas

EX. Colon cancer• Proliferation of epithelial cells → small benign neoplasm

(adenoma or polyp).• Clonal selection → • Growth of adenomas

of increasing size,

proliferative potential• Becomes carcinoma• metastasis

Fig. 18.5

The Development and Causes of Cancer

Carcinogens - substances that cause cancer.Radiation, chemicals, viruses can damage DNA,

induce mutations.• Solar UV radiation is major cause of skin cancer.• Aflatoxin produced by some molds that contaminate

peanuts and other grains• Viruses can cause cancer People can inherit cancer-susceptibility genes (oncogenes,

damaged tumor suppressor genes)]Tobacco smoke carcinogenic chemicals:benzo(a)pyrene, dimethylnitrosamine, nickel compounds (nearly 1/3 cancer deaths).

Fig. 18.6

The Development and Causes of Cancer

Tumor promoters stimulate cell proliferation:.

• Hormones, particularly estrogens, are tumor promoters in some human cancers.

exposure to excess estrogen increases likelihood woman will develop uterine cancer.

• Some viruses cause cancer:liver (HBV), cervical carcinoma (HPV)

• Bacterium Heliobacter pylori causes stomach cancer.

* Studies of tumor viruses identify molecular events in development of cancers.

Properties of cancer cells

Characteristic properties of Cancer cells:

1. lack density-dependent inhibition of proliferation

2. reduced requirements for growth factors

3. less regulated cell-cell, cell-matrix (less adhesive)

4. not sensitive to contact inhibition

5,6. secrete proteases for invasion, growth factors for angiogenesis

7. don’t differentiate normally, stay undifferentiated

8, 9. not undergo apoptosis; even after DNA damage

10. unlimited DNA replication; (over) express telomerase

Cancer cells have lost normal control – 10 properties

1. Cancer cells lack Density-dependent inhibition - Continue growing to high densities

• Normal cells proliferate to finite cell density, (availability of growth factors).

– cease proliferating, arrest in G0

Fig. 18.7

The Development and Causes of Cancer

2. Cancer cells reduced requirements for growth factors contributes to unregulated proliferation.

Can stimulate own proliferation (autocrine stimulation).

Fig. 18.8

The Development and Causes of Cancer

3. Cancer cells are less regulated by cell-cell, cell-matrix interactions:• Reduced expression of adhesion molecules contributes to

invasion, metastasis• Loss of E-cadherin (main adhesion molecule), aids

development of carcinomas (epithelial cancers).

4. Cancer cells lack Contact inhibition

• Normal fibroblasts migrate until contact neighbor cell, stop, adhere.• Tumor cells move after contact with neighbor cells, migrate over adjacent cells, grow in disordered, multilayered patterns.

Fig. 18.9

The Development and Causes of Cancer

• Permits invasion of normal tissues

• Digest collagen allows penetration of basal laminae, invasion of connective tissue.

• New blood vessels needed after tumor about 106 cells; penetrate capillaries

5,6. Cancer cells secrete: Proteases to digest extracellular matrixGrowth factors for angiogenesis

The Development and Causes of Cancer

7. Cancer cells don’t differentiate normally.Most fully differentiated cells cease cell division.Ex. Leukemias:

• Blood cells from hematopoietic

stem cells in bone marrow. • Leukemic cells don’t undergo

terminal differentiation;

arrest at early stages,

retain capacity for proliferation

Fig. 18.10

The Development and Causes of Cancer

8.9. Cancer cells fail to undergo programmed cell death or apoptosis:

• Longer life span than normal; not require growth factors • Lack of apoptosis after DNA damage increases resistance of

cancer cells to irradiation, chemotherapeutic drugs, (which damage DNA)

10. Cancer cells overexpress telomerase: • Capacity for unlimited DNA replication • Telomerase required to maintain ends of chromosomes after replication

Fig. 6.16 telomerase

The Development and Causes of Cancer

Ex. Assay for cell transformation in vitro:

• Study of tumor induction by radiation, chemicals, or viruses

• Detect conversion of normal cells to tumor cells in culture (altered growth properties)

• Focus assay (1958) recognizes group of transformed cells morphologically distinct “focus” versus normal cells on dish.

Fig. 18.11 Focus: RSV and chicken fibroblasts

Tumor Viruses

18.2 Tumor viruses directly cause cancer in humans or animals• Critical role in research - models for cellular, molecular study • Small genomes allowed identification of viral genes responsible

for cancer induction.

Tumor Viruses

Hepatitis B and C viruses • Principal causes of liver cancer.• Viruses infect liver cells, long-term chronic infections,

associated with high risk of liver cancer.

• HBV is DNA virus HCV is RNA virus

Tumor Viruses

Figs. 18.12,13

Simian virus 40 (SV40, monkey) (and polyomavirus, mice)

• not cause human cancer, important model• small genome sizes • Replicates in permissive host• Transforms non-permissive host

(inactivates Rb)

Early region encodes proteins (small and large T antigens)Proteins stimulate host cell gene expression, DNA synthesis.

Tumor Viruses

Papillomaviruses are small DNA viruses.• About 100 different types infect epithelial cells. • Some cause benign tumors (warts); others cause

malignant carcinomas, particularly cervical cancer• Transformation by expression of early genes, E6,E7:

• E7 binds Rb; E6 stimulates degradation of p53.

Fig. 18.14

Tumor Viruses

Adenoviruses large family of DNA viruses not associated with human cancer, important models.

• Adenoviruses lytic in cells of their natural hosts, can induce transformation in nonpermissive cells.

• Adenoviruses potential gene therapy vector

Tumor Viruses

Herpesviruses among the most complex viruses, enveloped DNA genomes 100 to 200 kb:

• HSV-1, HSV-2 cause cold sores, genital sores

• Varicella zoster virus (VZV) causes chicken pox, shingles

• Kaposi’s sarcoma-associated herpesvirus causes Kaposi’s sarcoma (common with AIDS)

• Epstein-Barr virus cause mononucleosis (transient) and Burkitt’s lymphoma

Tumor Viruses

Retroviruses (RNA genome, DNA intermediate) cause cancer in animals, including humans.

• HTLV-I causes adult T-cell leukemia• Human immunodeficiency virus (HIV) causes AIDS.• HIV not cause cancer directly, but infects & destroys T cells;

AIDS patients malignancies (lymphomas, Kaposi’s sarcoma)Most retroviruses only 3 genes (gag, pol, and env): for virus replication, not transformation; rarely induce tumors• Other retroviruses have specific extra genes induce cell transformation, oncogenes (carcinogens

Fig. 18.15

Fig 18.16 Cell transformation by RSV and ALV

18.3 Oncogenes - genes that transform cells:• Rous sarcoma virus (RSV) • Prototype highly oncogenic retrovirus• 1st oncogene identified by comparison of RSV to ALV

(avian leukosis virus does

not induce tumors)

** Cellular oncogenes are involved in development of non-virus-induced cancers.

Fig. 18.16

Fig 18.17 The RSV genomeRSV mutants revealed gene responsible for tumors• RSV causes sarcomas: oncogene is src. (not in ALV)

• Src was 1st protein-tyrosine kinase identified.

* other oncogenic retroviruses have key proteins of cell signaling path: ras, raf, myc, erbB, fos, jun,

Fig. 18.17

Fig 18.18 Isolation of Abelson leukemia virus

Viral oncogenes are derived from genes of host cell:

• Key expt: isolation of oncogenic retrovirus Abelson leukemia virus from mice injected with a nontransforming MuLV virus.

• One mouse developed lymphoma from which a new, highly oncogenic virus was isolated:

• Virus contained an oncogene (abl)

Abl is related to normal cell geneNormal gene called proto-oncogene

Fig. 18.18

Oncogenes

*18.3 Proto-oncogenes: • Normal-cell genes from which oncogenes originated• Often proteins of signal transduction pathways that control

cell proliferation (e.g., src, ras, and raf )Retroviral oncogenes differ from proto-oncogenes:• transcribed under control of viral promoter, enhancer• expressed at much higher levels, or in inappropriate cells.• point mutations lead to unregulated activity (ex. Ras)• can differ in structure and function from normal proteins:

Fig. 18.19 Raf oncogene: loss of regulatory domain makes oncogene protein unregulated

Detection of human tumor oncogene by gene transfer

Evidence cellular oncogenes in human tumors (gene transfer experiments 1981)• DNA from human bladder carcinoma induced transformation of mouse cells in culture, indicating tumor had oncogene

• Many other examples later:• ras, raf, c-myc• erbB, cdk4, PDGFR

Fig. 18.20

Fig 18.21 Point mutations in ras oncogenes

First human oncogene: homolog of rasH oncogene of Harvey sarcoma virus.

• 3 members of ras gene family (rasH, rasK, rasN) are oncogenes most frequent in human tumors.

• ras oncogenes have point mutations critical sites: • Mutations maintain Ras proteins constitutively in

active GTP-bound conformation

Fig. 18.21 mutation in RasH of bladder cancers

Oncogenes

*Cancer cells have abnormal chromosomes: translocations, duplications, deletions.• Creates oncogenes from proto-oncogenes: altered promoters, gene fusions, amplification of normal gene

Ex Burkitt’s lymphomas: translocations involve genes encoding immunoglobulins.• c-myc transcription factor• Expressed abnormally from immunoglobulin promoter• Tumors of B cells

Fig. 18.22

Oncogenes

Translocations rearrange coding sequences → abnormal gene products.

Ex: Translocation of c-abl proto-oncogene causes chronic myeloid leukemia (CML);

• fusion protein has constitutive Abl tyr-kinase activityFig. 18.23 Bcr-Abl

Oncogenes*Proto-oncogene proteins have normal roles in growth factor-stimulated signal transduction pathways.

Ex. ERK pathway oncogenes:• polypeptide growth factors, • growth factor receptors (ErbB), • intracellular signaling proteins

(Ras, Raf, MEK, ERK),• transcription factors (fos), • transcription targets (cyclin D1).

Fig. 18.24 ERK pathSee also Fig. 15.34

Mechanism of Tel/PDGFR oncogene activation

Many oncogenes encode growth factor receptors, mostly protein-tyrosine kinases.

Ex. Receptor for PDGF converted to oncogene by chromosome translocation, replacement of amino terminus by transcription factor Tel.

• Fusion protein• Tel sequences dimerize in absence of PDGF, → activate oncogene tyr protein kinase.

Fig. 18.25

OncogenesOncogenes can encode transcription factors

normally induced by growth factors

EX. Transcription of fos proto-oncogene is induced by phosphorylation of Elk-1 by ERK (Fig. 18.24).

• Fos and Jun dimerize to form AP-1 transcription factor, activates transcription of cyclin D1

• Mutant fos or jun always activate

• Cyclin D1 proto-oncogene, can become oncogene (CCND1) by chromosome translocation or gene amplification

Fig. 18.26

Oncogenes

Oncogenic activity of transcription factor can result from inhibition of differentiation.

Ex. Mutated form of retinoic acid receptor (PML/RARa) oncoprotein in acute promyelocytic leukemia (APL)

• Mutated receptors interfere withaction of normal homologs, block cell differentiation, maintain leukemic state.• Treatment with retinoic acid induces differentiation, blocks cell proliferation.

Fig. 18.28

Tumor Suppressor Genes

*18.4 Tumor suppressor genes normally act to inhibit cell proliferation and tumor development.

• In many tumors, both genes are lost or inactivated, contributes to abnormal proliferation of tumor cells

Tumor suppression first noticed during somatic cell hybridization experiments in 1969; hybrids did not cause tumor, suggesting genes in normal cell suppressed tumors.

Fig. 18.30

Fig 18.31 Inheritance of retinoblastoma

• First tumor suppressor gene found was retinoblastoma, inherited childhood eye tumor.

• About 50% of children of affected parent develop retinoblastoma: inherited as dominant autosomal susceptibility to tumor development:

• Need somatic mutation of other copy

photo

Fig. 18.31; retinoblastoma;Purple, affected people

Tumor Suppressor Genes

Retinoblastoma requires loss of both functional copies of tumor susceptibility gene (Rb gene);

• People inherit one mutated gene, later other mutates

Fig. 18.32

Mutations of Rb during retinoblastoma development

Noninherited retinoblastoma is very rare:• needs two independent somatic mutations in cell.

Fig. 18.32

Tumor Suppressor Genes

Rb is tumor suppressor:Deletions of chromosome 13q14 in some

retinoblastomas suggested loss (rather than activation) of Rb gene led to tumor development

Mutations of Rb contribute to many human cancers

Oncogene proteins of some DNA tumor viruses, including SV40, adenoviruses, and human papillomaviruses, bind to Rb and inhibit it.

.Fig. 18.33,34

Mutations ruining tumor suppressor genes:common molecular alterations in human tumors

Tumor Suppressor Genes

Mutations ruining tumor suppressor genes are common molecular alterations in tumors

p53 - second tumor repressor gene identified. • inactivated in many cancers, leukemias, lymphomas,

sarcomas, brain tumors, carcinomas. • Mutations of p53 in about 50% of all cancers.

Proteins encoded by most tumor suppressor genes inhibit cell proliferation or survival.

Tumor suppressor proteins inhibit regulatory pathways stimulated by products of oncogenes.

Tumor Suppressor Genes

Tumor suppressor proteins inhibit pathways stimulated by products of oncogenes.

Ex. Products of Rb and INK4 (p16) tumor suppressor genes regulate cell cycle at restriction point in G1Point affected by cyclin D1/ Cdk4 (which can be oncogenes).

Fig. 18.36

Tumor Suppressor Genes

Ex. p53 gene product regulates both cell cycle progression and apoptosis (programmed cell death)

• DNA damage induces p53, • activates cell cycle inhibitory gene p21, • transcription of proapoptotic genes

• Cells lacking p53 do not cycle arrest, not do apoptosis after DNA damaging agents• p53 mutated in many cancers

(Fig. 16.20 p21 inhibits Cdk2/cycE)

Fig. 18.37

Tumor Suppressor Genes

Ex. BRCA1 and BRCA2 genes(responsible for some inherited breast and ovarian

cancers) function as stability genes that maintain integrity of genome:

• Checkpoints in cell cycle progression, • Repair of DNA double-strand breaks

Mutations inactivating these genes leads to a high frequency of mutations in oncogenes or tumor suppressor genes.

Tumor Suppressor Genes

MicroRNAs (miRNAs) also regulate gene expression

• post-transcriptionally, inhibit translation and/or induce mRNA degradation,

• contribute to regulation of about 1/3 of protein-coding genes.

Expression of miRNAs is lower in tumors, suggesting they are tumor suppressors.

Example: let-7 targets oncogene c-myc

[Other miRNAs may be oncogenes]

Fig. 18.38

Tumor Suppressor Genes

Development of cancer is multistep process: Accumulated damage in multiple genes → increased

proliferation, survival, invasiveness, metastatic potential.Large-scale genome sequencing detects frequency mutations• 100 colorectal cancers: each tumor ~ 15 mutations in genes

thought to be involved in cancer development: Oncogenes rasK and PI3K ; Tumor suppressor genes APC and p53.• Breast cancers ~ 14 mutations per tumor, p53 tumor suppressor and

PI3K oncogene

Fig. 18.39

Molecular Approaches to Cancer Treatment

18.5. Molecular approaches to cancer treatment:Molecular defects being deciphered-> new

approaches to prevention and treatment

• 1. prevent development of cancer

• 2. detect early premalignant, before metastasis

• 3. identify susceptible individuals (gene tests)

• 4. molecular diagnosis of oncogenes, tumor suppressor genes in patient

• 5. drugs specifically targeted to mutant proteins

Molecular Approaches to Cancer Treatment

Early detection of cancer:

• Cured by localized treatment, before metastasis.

Ex. Early stages of colon cancer (adenomas) usually curable by minor surgical procedures.

Fig. 18.40

Molecular Approaches to Cancer Treatment

Identify individuals with inherited susceptibilities to cancer development, diagnose tumors.

• Mutations in tumor suppressor genes (p53, Rb), oncogenes (myc and cdk4), stability genes (BRCA1 and BRCA2) detected with genetic testing.

• Molecular analysis of oncogenes, tumor suppressor genes used in diagnosis of tumor (Bcr-Abl, ErbB-2)

• Molecular markers monitor course of disease during treatment (loss of PML/RAR with treatment).

Molecular Approaches to Cancer Treatment

Treat with drugs that inhibit angiogenesis:

• Blocks proliferation of endothelial cells, less toxic to normal cells.

• Most cancer drugs damage DNA or inhibit DNA replication, are toxic to normal cells

• Especially cells continually replaced by division of stem cells (hematopoietic cells, epithelial cells of gastrointestinal tract, hair follicle cells)

New drugs targeted specifically against oncogenes recall proto-oncogenes important roles in normal cells.

Molecular Approaches to Cancer Treatment

Small molecule inhibitors of oncogene proteins, protein kinases:

• Imatinib or Gleevec, specific inhibitor of Bcr/Abl protein kinase, blocks proliferation of chronic myeloid leukemia cells (CML).

• Imatinib inhibits PDGF receptor and Kit protein-tyrosine kinases:

• Kit oncogene in most gastrointestinal stromal tumors.

• Imatinib active against tumors with PDGF receptor activated as oncogene

Catalytic domain abl plus imatinib

Molecular Approaches to Cancer Treatment

gefitinib and erlotinib, (small molecule inhibitors of EGF receptor), active against some lung cancers.

• Responsive lung cancers had mutations for constitutive activation EGF receptor tyrosine kinase.

Fig. 18.41