Carcinogenesis

Characteristics of Cancer

Disorder of altered cell differentiation and growthResults in neoplasia (“new growth”)

Growth is uncoordinated and relatively autonomousLacks normal regulatory controls over cell

growth and divisionTends to increase in size and grow after

stimulus ceases or needs of organism are met

Components of Tissue Renewal and Repair

Cell proliferationProcess of cell division Inherent adaptive mechanism for replacing body

cellsCell differentiationProcess of specializationNew cells acquire the structure and function of

cells they replaceApoptosisForm of programmed cell death to eliminate

unwanted cells

Types of Stem Cells

Unipotent: give rise to one type of differentiated cellMuscle satellite cellEpidermal stem cellSpermatogoniumBasal cell of the olfactory epithelium

Oligopotent: produce small number of cells

Pluripotent: give rise to numerous cell types

Determination and Differentiation

The Cell CycleDefinition: The interval between each cell division

Genetic information is duplicated Duplicated chromosomes are appropriately aligned

for distribution between two genetically identical daughter cells

Checkpoints in cycle provide opportunities for monitoring the accuracy of deoxyribonucleic acid (DNA) replication Edited and repaired defects ensure full complement of

genetic information to each daughter cell

Phase of the Cell Cycle

G1 (gap 1): the post mitotic phase DNA synthesis ceases while ribonucleic acid (RNA) and

protein synthesis and cell growth take place

S phase: DNA synthesis occurs, giving rise to two separate sets of chromosomes, one for each daughter cell

G2 (gap 2): the premitotic phase DNA synthesis ceases; RNA and protein synthesis

continues

M phase: the phase of cellular division or mitosis

Control of Cell Cycle

Control of Cell CycleThe cell cycle is controlled by many proteins from inside & outside the cell.

Intracellular cyclins and cyclin dependent kinases (CDKs) control the checkpoints.

Extracellular proteins from other cells called Growth Factors signal the target cell to divide.

Binding of growth factors to membrane receptor proteins of the target cell triggers a molecular signaling pathway - a series of proteins which allows the cell to pass the checkpoints of the cell cycle.

Cell Cycle is controlled by genes.

Signal Transduction

Cell Cycle: RegulationCyclines: regulatory proteins, active at specific stages in the cell cycle CDK: cycline dependent kinases CDI: CDK inhibitor

Tumor Suppressor Proteins Inhibit CellDivision & Prevent Cancer

Tumor suppressor proteins are proteins that bind to checkpoint proteins to stop the cell cycle & prevent cell division.

An important function of tumor suppressor proteins is to stop the division of mutated cells until mistakes in DNA are repaired by enzymes.

TS proteins keep most mutations from being passed on to daughter cells & developing into cancer.

If the genes for TS proteins mutate or are deleted cancers may result.

Two important TS proteins are the p53 protein & the RB protein.

Normal (interphase) cells are ‘parked’ in

G1 (G0) and will not proceed to S phase

unless induced.

eg by growth factors

• Therefore normal cells grown in tissue

culture need growth factors in order to

divide (proliferate)

Cancer cells have lost the G1

checkpoint regulation

• Therefore cancer cells will proliferate (in

culture) in the absence of growth factors

Cell Proliferation

DefinitionThe process by which cells divide and

reproduce

RegulationRegulated in normal tissue, so the number of

cells actively dividing equal the number of cells dying or being shed

Two Major Categories of Cells Existing in Humans

Gametes (ovum and sperm) Haploid (containing one set of chromosomes

from one parent) Designed for sexual fusion forming a diploid

cell (containing both sets of chromosomes)

Somatic cell The diploid cell that forms the rest of the

body

Categories of Cell Types of the Body

Well-differentiated neurons and cells of skeletal and cardiac muscle unable to divide and reproduce

Parent or progenitor cells that continue to divide and reproduceBlood cells, skin cells, liver cells

Undifferentiated stem cells that can be triggered to enter cell cycle and produce large numbers of progenitor cells when needed

Types of Tumors

Adenoma: benign tumor of glandular epithelial tissue

Adenocarcinoma: malignant tumor of glandular epithelial tissue

Carcinoma: malignant tumor of epithelial tissue

Osteoma: benign tumor of bone tissue

Sarcoma: malignant tumors of mesenchymal origin

Papillomas: benign microscopic or macroscopic fingerlike projections growing on a surface

Factors differentiating Benign and Malignant Neoplasms

Cell characteristics

Manner of growth

Rate of growth

Potential for metastasizing or spreading

Ability to produce generalized effects

Tendency to cause tissue destruction

Capacity to cause death

Characteristics of Benign Neoplasms

A slow, progressive rate of growth that may come to a standstill or regress

An expansive manner of growth

Inability to metastasize to distant sites

Composed of well-differentiated cells that resemble the cells of the tissue of origin

Characteristics of Malignant Neoplasms

Tend to grow rapidly and spread widely

Have the potential to kill regardless of their original location

Tend to compress blood vessels and outgrow their blood supply, causing ischemia and tissue necrosis

Rob normal tissues of essential nutrients

Liberate enzymes and toxins that destroy tumor tissue and normal tissue

Methods by which Cancer Spreads

Direct invasion and extension

Seeding of cancer cells in body cavities

Metastatic spread through the blood or lymph pathways

Factors Affecting Tumor Growth

The number of cells that are actively dividing or moving through the cell cycle

The duration of the cell cycle

The number of cells that are being lost compared with the number of new cells being produced

Carcinogenesis Hypotheses of the Origin of Neoplasia

1. Oncogenes and Tumor Suppresor Genes

2. Viral Oncogene Hypothesis

3. Epigenetic Hypothesis

4. Failure of Immune Surveillance

1. Oncogenes and Tumor Suppresor Genes

Genes that Control Cell Growth and Replication

Genes control cell division by cytokines.

Three classes of regulatory genes.1. Promotors – Proto-oncogenes

2. Inhibitors – Cancer-suppressor genes – p53

3. DNA stability genes.

Non-lethal Genetic damage lies at the center of carcinogenesis.

Loss/damage to suppressor genes,

Duplication of promotor genes

Loss/damage of DNA stability genes.

Gene Mutations That Cause Cancer

Mutations in 4 types of genes cause CancerProto - oncogenes: genes that code for normal proteins used in cell division Growth factors Growth factor membrane receptors Signaling proteins ras proto- oncogene in 30% of cancers.

Tumor Suppressor genes: gene that code for proteins that help prevent uncontrolled cell division by blocking key steps (e.g. DNA replication).

Retinoblastoma susceptibilty (RB) gene p53 gene in >50% of cancers

DNA stability genes

Oncogenes

Activation

Tumor Suppressor Genes

Inactivation

Differentiation Apoptosis/Proliferation

CANCER

Alterations of Specific Cellular Alterations of Specific Cellular Functions in CancerFunctions in Cancer

Proto-oncogenes

Oncogenes: Viral proteins which interact with the cellular controll

mechanisms to overcome the strict regulation of proliferation (v-ras, v-myc, v-abl, ...)

Proto-Oncogenes: Cellular proteins which correspond to the viral

Oncogenes but which are strictly regulated. Mutations in this genes could transform a cell into a tumor cell (c-ras, c-myc, c-abl, ...).

Tumor genes: Knowledge from Viruses

Proto-oncogenes

TYPES OF ONCOGENES1. Growth factors

2. Growth factors receptors

3. Intracellular signaling transduction factors

Proteins with GTPase activity

Cytoplasmic serine threonine kinases

4. DNA-binding nuclear proteins

5. Cell cycle factors

Growth factors eg IGFGrowth factor receptors Eg erb-2, ret

Signal transducing factorsEg cytoplasmic kinases

DNA binding proteins concerned with transcription

cell cycle proteins eg cyclin D

Relationship between gene products of proto oncogene

Proto-oncogenes

FUNCTION OF ONCOGENES• Cancers have characteristics that indicate, at cellular

level, loss of the normal function of oncogene products consistent with a role in the control of cellular proliferation and differentiation in the process known as signal transduction. It is a complex multistep pathway from the cell membrane, through the cytoplasm to the nucleus.

• Proto oncogenes have been highly conserved during evolution, and the protein products they encode are likely to have essential biological functions.

Oncogenes Are Mutated Proto-oncogenes

A cell can acquire a cancer causing oncogene from A virus A mutation in a proto-oncogene

Oncogenes still code for the proteins needed for cell division but they cause cancer by producing Increased In growth factor Increased In growth factor receptors Increased in signal transduction Increase in activation of transcription

IncreasedIn growth factor

IncreasedIn growth factorreceptors

Increased in signal transduction

Increase in activation of transcription

Cancer causing MutationsProto-oncogenes form oncogenes by

• being misplaced (e.g. by translocation) to a site where the gene is continually expressed resulting in overproduction of a protein that stimulates cell division (e.g. in CML*)

• By mutating to a form that is over expressed.Mutations in Tumor Suppressor genes cause cancer by inactivating the genes.

Tumor-suppressor genes

BIOLOGICAL FUNCTIONS OF TUMOR SUPPRESSOR GENES

1. Growth Inhibitors (e.g., TGF-β; glucocortocoids)

2. Growth Inhibitor Receptors

3. Signal Transduction Protein Inhibitors

4. Transcription Factors of Growth Inhibitors

Tumor-suppressor genesGeneproducts which are normaly responsible for negative controll of transcription and proliferation

Examples: pRb inhibits transcription factors of the E2F-family,

which are needed to get into the S-Phase of the cell cycle (Restriction Point)

p53 induces transcription of the CDK-inhibitor (CDI) p21 which causes a cell cycle arrest (one function)

p53 is found upregulated in cells with a high level of NAdamage

Rb protein

Tumor-suppressor genes

RETINOBLASTOMA• Retinoblastoma (Rb) is a relatively rare, highly

malignant childhood cancer of the developing retinal cells of the eye that usually occurs before the age of 5 years.

• Rb can occur either sporadically (non-hereditary form, ussually involve only one eye), or be familial (hereditary form, more commonly bilateral), which is inherited in an AD manner, and also tend to present at an earlier age.

Retinoblastoma

Retinoblastoma

Two hit hypothesis All cells in the hereditary form have one mutated copy

of the gene RB1,i.e. the mutation is in the germline.

Retinoblastoma

Two hit hypothesisIn the non-hereditary form a mutation in RB1 gene arises as a post-zygotic (somatic) event sometime early in development.

p53 Genep53 senses DNA damage, and induces G1 arrest and induces DNA repair process.

Cell with un-repairable DNA is directed to apoptosis by p53 gene.

“P53 is a guardian of the genome.

Its homozygous loss leads to accumulation of damaged DNA may result in malignancy”

Homozygous loss of p53 is seen in virtually every type of cancer.

Over half of human malignant cells show loss of p53 gene by special tests.

The p53 Tumor Suppressor Protein

The p53 tumor suppressor protein is activated when DNA is damaged. The p53 gene is called the “guardian angel of the genome”

P53 activates genes for proteins that Prevent cell

entering S phase Repair DNA Cause apoptosis

(if DNA is irreparable)

DNA Stability Genes

Monitor and maintain the integrity of the DNA.

Loss of function promotes mutationsDetection of DNA lesions decreasedRepair of damage decreased or improperDecreased apoptosis

Routes to Genetic Instability based on Defective DNA Repair

Carcinogenesis Hypotheses of the Origin of Neoplasia2 – Viral Oncogene HypothesisRNA Retrovirus – produces DNA provirus

DNA provirus containing viral oncogene (v-onc) is introduced, or

DNA provirus without v-onc is inserted adjacent to c-onc in host cell DNA

RNA viruses is thought to have acquired v-onc sequence by recombinant mechanism from animal cells

DNA virusDo not contain viral oncogenesAct by blocking suppressor gene productsExamples – HPV, EBV,HBV

Carcinogenesis Hypotheses of the Origin of Neoplasia3 – Epigenetic HypothesisChanges in the regulation of gene

expression rather than in the genetic apparatus

Pattern of gene expressions responsible for tissue differentiation (ie. epigenetic mechanism) are thought to be heritable

Carcinogenesis Hypotheses of the Origin of Neoplasia4 – Failure of Immune SurveillanceConcepts

Neoplastic changes frequently occur in cells

Altered DNA result in production of neoantigens & tumor-associated antigens

Immune response (cytotoxic) to neoantigens as foreign antigens

Neoplastic cells escaping recognition and destruction become clinical cancers

Causes of Neoplasia

Environmental causes: (Carcinogens)Chemicals Viruses Radiation

Hereditary causes- Genetic defects.

Combination – common.

Obscure defects

Carcinogenesis:

Chemical Carcinogenesis:Chemical Carcinogenesis:

Initiation DNA damage eg.Benzpyrene

Promotion Histologic change – eg.

Turpentine (co-carcinogens)

Malignant transformation: Visible tumor formation –

further DNA damage.

Chemical Carcinogenesis:

Direct Acting Carcinogens:Alkylating Agents: Cyclophosphamide

Procarcinogenes (needs activation)Polycyclinc hydrocarbons – BenzpyreneAromatic amines, dyes - BenzidineNatural products: AflotoxinOthers: Vinyl chloride, turpentine etc.

Viral Oncogenesis:

Insertion of viral nucleic acids mutation

Alterations in Oncogenes, cancer suppressor genes and genes regulating DNA repair resulting in up-regulation of cell division Carcinogenesis.

Nobel Laureates – Varmus and Bishop v-fes, v-sis proto-oncogenes.v-sis sis PDGF Brain tumours.

Viral Oncogenesis:

Human Papilloma Virus Cervical neoplasia – warts, papilloma, ca cx

Epstein-Barr virus – Burkitts Lymphoma, Nasopharyngeal ca.

Hepatitis B & C virusHepatocellular carcinoma.

Radiation Carcinogenesis:

Ionizing radiation dysjunction random fusion mutation.

X Ray workers – LeukemiaRadio-isotopes – Thyroid carcinomaAtomic explosion – Skin cancer, Leukemia

Mutations

Neoplasia

Hereditary Causes:

Due to inhereted abnormal genes.

FAP – gene C5, polyposis Adenocarcinoma colon

Retinoblastoma – Rb gene – (C13)

Neuroblastoma – (C17)

Trisomy 21 – Down’s syndrome – Leukemias in infants.

Clinical Manifestations of Cancer

Tissue IntegrityCompressed and eroded blood vessels, ulceration

and necrosis, frank bleeding, and hemorrhage

Cancer CachexiaWeight loss and wasting of body fat and muscle

tissue; profound weakness, anorexia, and anemia

Paraneoplastic SyndromesManifestations in sites not directly affected by the

disease

Molecular Basis of Neoplasia:

Proto-oncogeneProto-oncogene

OncogeneOncogene

V-Onc

V-Onc

Other

Other

Heredity

Heredity

Radiation

Radiation

Chemical

Chemical

Multiple Genetic Changes Cause Cancer

Cancers result from a series of genetic changes in a cell lineage

Some cancers begin with an inherited germ line mutation.

Some inherited cancers follow a dominant pattern, e.g. inherited retinoblastoma caused by a mutation in the Rb tumor supressor gene increases cancer risk 10,000 times.

More than one somatic mutation is often necessary.

Accumulation of mutations over time leads to uncontrolled cell division.

Example: Colon cancer develops in a stepwise fashion.

Multiple Genetic Changes Cause Cancer

Multiple Hits and Multiple Factors Knudson proposed that carcinogenesis requires 2 hits

1st event – initiation Carcinogen = initiator

2nd event – promotion Agent = promoter

Multiple hits occur – 5 or more Each hit produces a change in the genome which is

transmitted to its progeny (ie. clone) Lag period

Time between exposure (first hit) and development of clinically apparent cancer

Altered cell shows no abnormality during lag period

Multiple Genetic Changes Cause Cancer

Multiple Genetic Changes Cause Cancer

1. DNA of a normal cell This piece of DNA is an exact copy of the DNA from which

it came. When the parent cell divided to create two cells, the cell's DNA also divided, creating two identical copies of the original DNA.

2. Mutation of DNAThis DNA has suffered a mutation, either through mis-copying (when its parent cell divided), or through the damaging effects of exposure to radiation or a chemical carcinogen.

3. Genetically altered cell• Body cells replicate through mitosis, they respond to their surrounding

cells and replicate only to replace other cells. Sometimes a genetic mutation will cause a cell and its descendants to reproduce even though replacement cells are not needed.The DNA of the cell highlighted above has a mutation that causes the cell to replicate even though this tissue doesn't need replacement cells at this time or at this place.

4. Spread and second mutationThe genetically altered cells have, over time, reproduced unchecked, crowding out the surrounding normal cells. The growth may contain one million cells and be the size of a pinhead. At this point the cells continue to look the same as the surrounding healthy cells.

After about a million divisions, there's a good chance that one of the new cells will have mutated further. This cell, now carrying two mutant genes, could have an altered appearance and be even more prone to reproduce unchecked.

5. Third mutationNot all mutations that lead to cancerous cells result in the cells reproducing at

a faster, more uncontrolled rate. For example, a mutation may simply cause a cell to keep from self-destructing. All normal cells have surveillance mechanisms that look for damage or for problems with their own control systems. If such problems are found, the cell destroys itself. Over time and after many cell divisions, a third mutation may arise. If the mutation gives the cell some further advantage, that cell will grow more vigorously than its predecessors and thus speed up the growth of the tumour.

6. Fourth mutationThe new type of cells grow rapidly, allowing for more opportunities for

mutations. The next mutation paves the way for the development of an even more aggressive cancer.

At this point the tumour is still contained.

7. Breaking through the membraneThe newer, wilder cells created by another mutation are able to push their way through

the epithelial tissue's basement membrane, which is a meshwork of protein that normally creates a barrier. The invasive cells in this tumour are no longer contained.

At this point the cancer is still too small to be detected.

8. AngiogenesisOften during the development of earlier stages of the tumour, or perhaps by the time the tumour has broken through the basement membrane (as pictured above), angiogenesis takes place. Angiogenesis is the recruitment of blood vessels from the network of neighbouring vessels. Without blood and the nutrients it carries, a tumour would be unable to continue growing. With the new blood supply, however, the growth of the tumour accelerates; it soon contains thousand million cells and, now the size of a small grape, is large enough to be detected as a lump

9.Invasion and dispersalThe tumour has now invaded the tissue beyond the basement membrane.

Individual cells from the tumour enter into the network of newly formed blood vessels, using these vessels as highways by which they can move to other parts of the body. A tumour as small as a gram can send out a million tumour cells into blood vessels a day.

10. Tumour cells travel – metastasis

What makes most tumours so lethal is their ability to metastasize -- that is, establish new tumour sites at other locations throughout the body.Secondary tumours.

Metastasis is now underway, as tumour cells from the original cancer growth travel throughout the body. Most of these cells will die soon after entering the blood or lymph circulation.

11. MetastasisTo form a secondary tumour, a tumour cell needs to leave the vessel system and invade tissue. The cell must attach itself to a vessel's wall. Once this is done, it can work its way through the vessel and enter the tissue.

Although perhaps less than one in 10,000 tumour cells will survive long enough to establish a new tumour site, a few survivors can escape and initiate new colonies of the cancer.

Multi-step Theory

Initiation

Promotion

Progression

InitiationAn initiated cell is one in which a chemical carcinogen has interacted with DNA to produce a mutation, often a single base alteration, in the genome.

An initiated cell is not a tumor cell because it has not yet acquired autonomy of growth.

The DNA alteration may remain undetected throughout the life of the organism unless further events stimulate development of a tumor.

Tumor PromotionIn general, tumor promotion can be viewed as the clonal expansion of an initiated cell via altered gene expression that gives the cell a selective growth advantage.

Tumor promoters cause cells to proliferate but not to terminally differentiate, resulting in proliferation of preneoplastic cells (benign lesions).

Unlike initiators, most promoters do not bind covalently to DNA and usually do not cause mutations.

Tumor ProgressionTumor progression describes the process whereby tumors acquire the ability to grow, invade local tissue and establish distant metastases.

Increased genetic instability and karyotypic alterations are hallmarks of progression.

Inherited or acquired mutations in genes such as p53 or DNA mismatch repair can increase the rate of mutation in other genes (mutator phenotype) and, therefore, promote the tumor progression.

Phenotypic characteristics of cancer cells

Immortalization

Transformation

Loss of contact growth inhibition

Autonomy of proliferation

Avoidance of apoptosis

Aberrant differentiation

Induction of angiogenesis

Overview of Carcinogenesis