EPI242 CANCER EPIDEMIOLOGY
STAGES IN NEOPLASTIC DEVELOPMENT
Zuo-Feng Zhang, MD, PhD
Fall, 2009
CONTENTS


Carcinogenesis and stages for cancer
development
Molecular genetic alterations in cancer
development

Carcinogens

Precursors
CARCINOGENESIS

The transformation of normal to neoplastic cells
is caused by both endogenous and exogenous
factors, including chemical and physical agents,
viruses, activation of cancer-promoting genes,
and inhibition of cancer-suppressing genes.
LATENCY
For infectious disease, the initial insult is the
entrance of the infectious organism into the host
and the latent period is the time during which
the infectious organism multiplies and alters the
host’s capacity for response, and manifested as
the clinical apparent disease.
 For cancer, it refers a period of time between the
initial etiologic insult and the clinical appearance
of cancer, e.g., lack of parity (hormone, breast
feeding) in Catholic nuns and high incidence of
breast cancer; chimney sweeps (soot) as young
boy and high incidence of cancer of the scrotum
among adult males; and A-bomb survival and
increased incidence of leukemia in Japan.

MULTISTAGE CARCINOGENESIS

Two stages: Initiation and Promotion. The
initial two stage theory base don the natural
history of epidermal carcinogenesis in the mouse
has lasted for many years. The limitation of the
theory is that it does not emphasize the latent
period of the development of other types of
neoplasms.
STAGES OF CARCINOGENESIS

Initiation is the first critical carcinogenic event
and it is usually a reaction between a carcinogen
and DNA. Two or more agents (chemicals,
viruses, radiation) may act together as
carcinogens. The process of initiation, the first
stage in the natural history of neoplastic
development, is permanent and irreversible.
STAGES OF CARCINOGENESIS

Promotion is induced by a stimulator of cell
proliferation and enhances the carcinogenic
process. A promoter, not carcinogenic in itself,
enhances other agents’ carcinogenicity. The
progression in promotion process is reversibility
and instability. The stage of promotion can be
continually modulated by a variety of
environmental alterations.
MULTISTAGE CARCINOGENESIS


Three stages: Initiation, promotion, and
progression. The two stage concept was modified
and the original stage of promotion was then
divided into two phases: promotion and
progression. Progression phase is irreversible.
Progression is a irreversible stage with
demonstrated changes in the structure of the
genome of the neoplastic cells. Such changes are
directly related to increased growth rate,
invasiveness, metastatic capability and
biochemical changes of the neoplastic cell.
TUMOR GROWTH
In the normal proliferating tissue, a balance
between cell renewal and cell death is strictly
maintained
 In tumor growth, more cells are produced than
die in a given time

HUMAN CANCER-BASED TWO-HIT
MODEL

Two-hit model. Tumor suppressor gene such as
RB gene. Knudson (1971) suggested that all
types of retinoblastoma (RB) involve two
separate mutations that are carried by all
retinoblastoma tumor cells. In the case of
sporadic retinoblastoma, he argued that both
mutations occur somatically in the same retinal
precursor cell. In heritable retonoblastoma, he
suggested that one of the two mutations is
already present at conception (germline
mutation), and the second mutation occur as a
somatic event (post conception).
HUMAN COLON CANCER MULTIPLE
STAGE MODEL

Colon cancer model. The development of human
cancer is a multistage process, involving a series
of genetic molecular alterations.
EARLY STAGE VERSUS LATE STAGE

“Early stage” versus “late stage” carcinogens
in epidemiologic stages. If an agent is “early
stage” carcinogen, both the increase in incidence
beginning with and during exposure and decrease
in incidence after cessation of exposure will be
delayed. If an agent is a “late stage” carcinogen,
responses both to starting and ceasing of
exposure will be much more rapid. The terms,
“early” and “late”, are used to correlate
multistage models with epidemiologic results.
Those may not necessary relate directly to the
stages of initiation, promotion, and progression.
EARLY STAGE VERSUS LATE STAGE
MOLECULAR GENETIC OF CANCER
 It
is now recognized that the unregulated
growth of cancer cells results from the
sequential acquisition of somatic
mutations in genes that control cell
growth, differentiation, and apoptosis or
that maintain the integrity of the genome
 Similar mutations may also be present in
the germline of persons with hereditary
predisposition to a variety of cancers
MOLECULAR GENETIC OF CANCER
Mutations can be produced by environmental
mutagens such as chemical carcinogens or
radiation
 Mutations can also arise during normal cellular
metabolism, particularly from the formation of
activated oxygen species

DNA ENDOGENOUS DAMAGE AND
REPAIR
 Approximately
lesions/cell/day
20,000 DNA damage
1
billion DNA damage lesions/human
body/second
Lindahl T, Quart Biol 2000,65,127-33
GENETIC MUTATIONS AND
TUMOR DEVELOPMENT


Most of these mutations are of no consequence,
because they either do not affect the function of
the cell or are repaired by DNA repair genes, or
are lost as a result of the death of the cell
However, if the mutation involves genes that
control growth or that protect the stability of the
genome, it may give rise to a clone of cells that
possess a growth advantage over their normal
neighbors. Successive mutations in similar genes
result in increasingly aberrant clones until a
malignant phenotype eventually emerges.
CELL TRANSFORMATION


Malignant transformation involves somatic
mutations that confer a set of common properties
It is estimated that a minimum of 4-7 mutated
genes are required for the transformation of a
normal cell into a malignant phenotype
TRANSFORMED CELLS SHARE
COMMON ATTRIBUTES
 Autonomous
generation of mitogenic
signals
 Insensitivity to exogenous antigrowth
signal
 Resistance to apoptosis
 Limitless replicative potential
(immortalization)
 Blocked differentiation
 Ability to sustain angiogenesis
 Capacity to invade surrounding tissues
 Potential to metastasize
ONCOGENES


Oncogenes are altered version of normal genes,
termed protooncogene, that regulate normal cell
growth, differentiation, and survival
Gain-of-function (dominant) mutations activate
protooncogenes to become oncogenes and are
positive effectors of the neoplastic phenotype
MECHANISMS OF ONCOGENE ACTION
Growth factors (IGF-1)
 Cell surface receptors
 Intracellular signal transduction pathways
 DNA-binding nuclear protein (transcription
factors)
 Cell cycle proteins (cyclins and cyclin-dependent
protein kinases)
 Inhibitors of apoptosis (bcl-2)

TUMOR SUPPRESSOR GENES

Tumor suppressor genes are normal genes whose
products inhibit cellular proliferation. Loss-of –
function (recessive) mutations inactivate the
inhibitory activities of tumor suppressor genes,
thereby permitting unregulated cell growth
TUMOR SUPPRESSOR GENES
A mutation that creates a deficiency of a normal
gene product that exerts a negative regulatory
control of cell growth and thereby suppresses
tumor formation.
 Such genes encode negative transcriptional
regulators of the cell cycle, signal-transduction
molecules, and cell surface receptors.

TUMOR SUPPRESSOR GENES
 Since
both alleles of such tumor
suppressor genes (“gatekeeper” genes)
must be inactivated to produce the deficit
that allows the development of a tuor, it is
inferred that normal suppressor gene is
dominant.
 The loss of heterozygosity in a tumor
suppressor gene by deletion or somatic
mutation of the remaining normal allele
predisposes to tumor development
EXAMPLE: RETINOBLASTOMA
GENE (RB) AND P53 GENE


The function of RB is the most critical checkpoint
in the cell cycle, the inactivating mutations in RB
permits unregulated cell proliferation.
The mutations of p53 seem to be the most
common genetic changes in human cancer.
EXAMPLE: RETINOBLASTOMA GENE
(RB) AND P53 GENE
 The
p53 molecule is a negative regulator
of cell division. In response to DNA
damage, oncogenetic activation of other
proteins, or other stresses, p53 levels rise
and prevent cells from entering S phase of
the cell cycle, thereby allowing time for
DNA repair to take place.
 Inactivating mutations of p53 allow cells
with damaged DNA to progress through
the cell cycle.
• DNA methylation—The
covalent addition of a
methyl group to the 5th
position of cytosine
within CpG dinucleotides,
which are frequently
located in the promoter
regions of genes.
Methylation can also
occur in other parts of
genes. DNA methylation
play a key role in a
number processes,
including DNA repair,
genome stability, and
regulation of chromatin
structure.
(Laird PW. Molecular Medicine Today. 1997:223-229)
DNA methylation and Cancer—
tumor suppressor gene
(Laird PW..Molecular Medicine Today. 1997:223-229)
CARCINOGENS
An agent that can cause cancer. The International
Agency for Research on Cancer (IARC) classifies
carcinogens as follows:

1) Sufficient evidence. A positive causal
relationship has been established between
exposure and occurrence of cancer.
CARCINOGENS

2) Limited evidence. A positive causal association
has been observed between exposure to the
agent, for which a causal interpretation is
credible, but chance, bias, confounding cannot be
rolled out.
CARCINOGENS

3) Inadequate evidence. Available studies are of
insufficient quality, consistency or statistical
power to permit a conclusion regarding the
presence or absence of a causal relationship.
CARCINOGENS

4) Evidence suggesting lack of carcinogenicity.
Several adequate studies covering the full range
of doses to which humans are known to be
exposed are mutually consistent in not showing a
positive association between exposure to the
agent and any studied cancer at any level of
exposure.
OVERALL EVALUATION OF CARCINOGEN
Taking all the evidence into account, the agent is assigned to one of the
following categories:

Group 1. The agent is carcinogenic to humans.

Group 2.
2A. The evidence for human carcinogenicity is almost
sufficient (probably carcinogenic).
2B. There are no human data but there is experimental
evidence of carcinogenicity (possibly carcinogenic).



Group 3. The agent is not classifiable as to its human
carcinogenicity.

Group 4. The agent is probably not carcinogenic to humans.
CLASSIFICATION OF CARCINOGENIC AGENTS IN
RELATION TO THEIR ACTION ON ONE OR MORE STAGES
OF CARCINOGENESIS
CARCINOGENS IN TOBACCO SMOKE
(According to International Agency for Research
on Cancer, unless otherwise noted)
GROUP 1: CARCINOGENIC TO HUMANS
 Tobacco Smoking
 Tobacco Products, Smokeless
 4-Aminobiphenyl (4-ABP)
 Benzene
 Cadmium
 Chromium
 2-Naphthylamine (2-NA)
 Nickel
 Polonium-210 (Radon)
 Vinyl Chloride
GROUP 2A: PROBABLY CARCINOGENIC TO
HUMANS
 Acrylonitrile
 Benzo[a]pyrene
 Benzo[a]anthracene
 1,3-Butadiene
 Dibenz(a,h)anthracene
 Formaldehyde
 N-Nitrosodiethylamine
 N-Nitrosodimethylamine
GROUP 2B: POSSIBLY CARCINOGENIC TO
HUMANS
Acetaldehyde
 Benzo[b]fluoranthene
 Benzo[j]fluoranthene
 Benzo[k] fluoranthene
 Dibenz[a,h]acridine
 Dibenz[a,j]acridine
 7H-Dibenz[c,g]carbazole

GROUP 2B: POSSIBLY CARCINOGENIC TO
HUMANS
Dibenzo(a,i)pyrene
 Dibenzo(a,l)pyrene
 1,1-Dimethylhydrazine
 Hydrazine
 Indeno[1,2,3-cd]pyrene
 Lead
 5-methylchrysene

GROUP 2B: POSSIBLY CARCINOGENIC TO
HUMANS
 4-(Methylnitrosamino)-1-(3-pyridyl)-1-
butanone (NNK)
 2-Nitropopane
 N-Nitrosodiethanolamine
 N-Nitrosomethylethylamine
 N-Nitrosomorpholine
 N-Nitrosonornicotine (NNN)
 N-Nitrosopyrrolidine
GROUP 2B: POSSIBLY CARCINOGENIC TO
HUMANS
Quinoline
 ortho-Toluidine
 Urethane (Ethyl Carbamate)

GROUP 3: UNCLASSIFIABLE AS TO CARCINOGENICITY
HUMANS (LIMITED EVIDENCE)
Chrysene
 Crotonaldehyde
 N-Nitrosoanabasine (NAB)
 N-Nitrosoanatabine (NAT)

TO
2-1. Background: Theoretical model of gene-gene/environmental interaction pathway
Tobacco consumption
Occupational
Exposures
Environmental Carcinogens /
Procarcinogens Exposures
Ile105Val 
Ala114Val
Environmental Exposure
Null 
GSTP1
GSTM1
CYP1A1
MspI
Ile462Val 
Tyr113His
His139Arg
PAHs,
Xenobiotics,
Arene,
Alkine, etc
Detoxified
carcinogens
Active carcinogens
Pro187Ser
mEH
mEH
NQO1
DNA damage
repaired
DNA Damage
Tyr113His
His139Arg
Normal cell
Defected DNA
repair gene
If DNA damage not
repaired
XRCC1
Arg194Trp,
Arg399Gln,
Arg280His
M
G
G2
P53
P16
Arg72Pro
Ala146Thr
S
G870A
Cyclin D1
If loose cell cycle
control
Carcinogenesis
Programmed cell
death
CHEMICAL/ENVIRONMENTAL
CARCINOGENS
 Smoking
and lung cancer
 Sun exposure and squamous cell
carcinoma of skin
 Asbestos exposure and lung cancer
 Smoke food risk with nitrosamines and
adenocarcinoma of the stomach
 Alcohol drinking and squamous cell
carcinoma of esophagus
 Aflatoxin B1 and liver cancer
 Low fiber diet and adenocarcinoma of
colon
RADIATION
Exposure to ultraviolet radiation (in the form of
sunlight) and squamous cell carcinoma of skin
 Ionizing radiation is related to skin cancer and
leukemia in radiologist

VIRAL FACTORS
 HPV
(human papilloma virus) and Cervix
cancer
 EBV(Epstein-Barr virus) and
Nasopharyngeal cancer, Burkitt’s
lymphoma
 HBV (hepatitis B virus) and
hepatocellular carcinoma
 HIV (human immunodeficiency virus) and
Kaposi’s sarcoma
PRECURSORS
Is a condition which be associated with the
development of cancer (Stout, 1932)
 Visible steps in a dynamic process of neoplasia
that may or may not undergo progression to a
more advanced stage of neoplasia (Foulds, 1958)
 All morphologic lesions on the pathway from
normal tissue to cancer, up to but not including
invasive cancer

PRECURSORS
Two groups:


less advanced lesions, which do not include
abnormal clones
more advanced lesions or dysplasia, which
include abnormal clones and are considered
dangerous if untreated
SIGNIFICANCE
Elucidation of the etiology of precursors provides
insight into etiology of the corresponding cancer
 Identification of etiology of precursors may
provide opportunity for primary prevention for
both precursors and invasive cancer
 If precursors are defined, they can provide
targets for screening and early detection and
chemoprevention of these at an increased risk of
cancer
 They can provide functional inside into the
nature of carcinogenesis

ORAL LESIONS
Incidence rates of leukoplakia (/1000) are 3.3-5.5
for men and 1.9-3.6 for women. 11-43% of
individuals with leukoplakia may have
histologically defiend dysplasia.
 Incidence rates for erythroplakis have not clearly
defined yet. More than two thirds of cases with
histologically defined dysplasia or carcinoma
 Incidence rates for oral submucous fibrosis (OSF)
(/1,000) are 8-21 in men and 29-46 in women

Oral Lesions
Oral Leukoplakia
2. Invasive Head & Neck
Cancer
Oral submucous
fibrosis
Erythroplakia
3. Second Primary Cancers following a first
primary oral cancer
Esophageal Cancer
Oral Cancer
Lung Cancer
PRECANCEROUS LESIONS OF CERVIX
RESEARCH OPPORTUNITIES FOR
PRECURSOR
 Precursor
cellular changes hold the clue to an
understanding of the mechanisms of multistep
carcinogenesis
 Abnormal proteins formed by precursor lesions
can help in the exploration of the neoplastic
process as well as in the identification of high
risk individuals
 The precursor lesions can be employed to study
genetic susceptibility and environmental
exposure.
RESEARCH OPPORTUNITIES FOR
PRECURSOR
 Study
of progression of precursor lesions can
help to determine if the etiological factors under
study influence the initiation or the promotion of
the neoplastic process
 Precursor lesions have a shorter latency period to
develop as cancer. Study of precursor lesions
will give researcher shorter follow-up time.
 The identification of precursor lesions changes
offers hope of inducing the regression of lesions.
This is explored in chemoprevention trials.