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Cancer Etiology
1. Chemical Factors in Carcinogenesis
2. Physical Factors in Carcinogenesis
3. Viral Oncogenesis
4. Genetic Predisposition
Jimin Shao
[email protected]
Chemical Carcinogenesis
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Multi-stage Theory of Chemical Carcinogenesis
Classification of chemical carcinogens
Mechanisms of Chemical Carcinogenesis
Types of DNA Damage
DNA Repair
Multi-stage Theory of
Chemical Carcinogenesis
Initiation -----------Genetic events
Chemical Carcinogens (Direct and Indirect Carcinogens)
Promotion -------Epigenetic events
Tumor promoters
e.g. Murine skin carcinogenesis model:
• A single dose of polycyclic aromatic hydrocarbon (PAH,
initiator)
• Repeated doses of croton oil (promoter)
Malignant conversion
Progression ------Genetic and epigenetic events
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Carcinogensis is multistep process, involving the multiple genetic and /or
epigenetic changes, leading to the activation of oncogenes and the inactivation
of tumor suppressors in cells.
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Initiation
• Irreversible genetic damage:
A necessary, but insufficient prerequisite for tumor
initiation
• Activation of proto-oncogene, inactivation of a
tumor suppressor gene, and etc
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Promotion
• Promotion: Selective expansion of initiated cells, which
are at risk of further genetic changes and malignant
conversion
• Promoters are usually nonmutagenic, not carcinogenic
alone, often do not need metabolic activation, can
induce tumor in conjuction with a dose of an initiator
that is too low to be carcinogenic alone
• Chemicals capable of both initiation and promotion are
called complete carcinogens: benzo[a]pyrene and 4aminobiphenyl
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Malignant conversion
• The transformation of a preneoplastic cell into
that expresses the malignant phenotype
• Further genetic changes
• Reversible
• The further genetic changes may result from
infidelity of DNA synthesis
• May be mediated through the activation of
proto-oncogene and inactivation of tumorsuppressor gene
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Progression
• The expression of malignant phenotype, the
tendency to acquire more aggressive
characteristics, Metastasis
• Propensity for genomic instability and
uncontrolled growth
• Further genetic changes: the activation of protooncogenes and the inactivation of tumorsuppressor genes
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• Activation of proto-oncogenes:
– Point mutations: ras gene family, hotspots
– Overexpression:
• Amplification
• Translocation
• Loss of function of tumor-suppressor genes:
usually a bimodal fashion
– Point mutation in one allele
– Loss of second allele by deletion, recombinational
event, or chromosomal nondisjunction
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Classification of chemical carcinogens
1. Based on sturcture
(1) Nitrosamines (NA)
MNNG, MMS (direct carcinogen)
(2) Polycyclic aromatic hydrocarbons (PAH)
Benzo(a)pyrene (indirect carcinogen)
(3) Aromatic amines (AA)
2-acetylaminofluorene, benzidine (indirect carcinogen)
(4) Aflatoxin (AF) (indirect carcinogen)
(5) Inorganic elements and their compounds: arsenic, chromium,
and nickel are also considered genotoxic agents
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2. Based on mechanisms
(1) Genotoxic carcinogen (DNA-reactive)
• Direct-acting:
intrinsically reactive
N-methyl-N’-nitro-N-nitrosoguanidine (MNNG),
methyl methanesulfonate (MMS),
N-ethyl-N-nitrosourea (ENU), nitrogen and sulfur mustards
•
Indirect-acting:
metabolic activation by cellular enzyme to form the DNA-reactive
metabolite (members of the cytochrome P450 family)
benzo[a]pyrene, 2-acetylaminofluorene, benzidine, Aflatoxin B1, B2.
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直接致癌物
间接致癌物
代谢激活
水解，氧化，还原
终致癌物
前致癌物
（procarcinogen） 混合功能氧化酶系统 （ultimate carcinogen）
（CYP450和P448 等）
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(2) Epigenetic carcinogens
• Promotes cancer in ways other than direct DNA damage/
do not change the primary sequence of DNA
• Alter the expression of certain genes and cellular events
related to proliferation and differentiation
• Promoters, hormone modifying agents, peroxisome
proliferators, cytotoxic agents, and immunosuppressors
• Organochlorine pesticides, estrogen, cyclosporine A,
azathioprine
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Mechanisms of Initiation in
Chemical Carcinogenesis
(1) DNA damages:
Pro-carcinogen metabolic activation (Phase I and II)
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Ultimate carcinogen (electrophiles)
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Interaction with macromolecules (nucleophiles)
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DNA damage, mutations, chromosomal aberrations, or cell death
(2) Epigenetic changes
(3)Activation of oncogenes; inactivation of tumor suppressor genes, etc
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Direct Chemical Carcinogens
(1) Alkylating agents are electrophilic compounds with affinity for
nucleophilic centers in organic macromolecules.
[Fu D, Calvo JA, Samson LD. Balancing repair and tolerance of DNA damage caused by alkylating agents. Nat Rev Cancer. 2012 Jan
12;12(2):104-20. doi: 10.1038/nrc3185.]
(2) These agents can be either monofunctional or bifunctional.
---Monofunctional alkylating agents have a single reactive group and
thus interact covalently with single nucleophilic centers in DNA .
such as MNNG
---Bifunctional alkylating agents have two reactive groups, and each
molecule is potentially able to react with two sites in DNA.
Interstrand DNA cross-link;
Intrastrand cross-link.
such as Nitrogen and sulfur mustard, mitomycin, cis-platinum
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---Monofunctional alkylating agents
Numerous potential reaction sites for alkylation have been
identified in all four bases of DNA (not all of them have equal
reactivity):
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---Bifunctional alkylating agents
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Indirect Chemical Carcinogens and
Their Phase I Metabolic derivatives
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BPDE binds DNA covalently,
resulting in bulky adduct
damage
BPDE intercalates into dsDNA
non-covalently, leading to
conformational abnormalities
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Types of DNA Damage Induced by
Ultimate Carcinogens
• DNA Adduct Formation
• DNA Break
Single Strand Break
Double Strand Break
• DNA Linkage
DNA-DNA linkage
DNA-protein Linkage
• Intercalation
Bulky aromatic-type adducts, Alkylation (small adducts),
Oxidation, Dimerization, Deamination
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DNA Repair
Repair systems
• Direct DNA repair/ Direct reversal :
– DNA alkyltransferase (O6-alkylguanine-DNA alkyl transferase)
– One enzyme per lesion
• Base excision repair (BER)
– small adducts,
– overlap with direct repair
– glycosylase to remove the adducted base
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• Nucleotide excision repair (NER):
– involves recognition, preincision, incision, gap-filling,
and ligation,
– large distortions
– strand specific, the transcribed strand is preferentially
repaired
– xeroderma pigmentosum (XP): NER deficiency
• Mismatch repair (MMR)
– transition mispairs are more efficiently repaired (G-T or
A-C) than transversion mispairs
– microenvironment influences efficiency
– similar to NER
– involves the excision of large pieces of the DNA
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• Double-strand breaks (DSBs)
– homologous recombination
– non-homologous end joining (NHEJ): DNA-PK
• Postreplication repair
– a damage tolerance mechanism
– occurs in response to replication of DNA on a damaged template
– the gap
• either filled through homologous recombination with parental
strand
• or insert an A residue at the single nucleotide gap
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Extended Reading
Translesion DNA synthesis
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1.DNA damage blocks the progression of the replication fork.
2.PCNA plays a central role in recruiting the TLS polymerases
(translesion DNA synthesis) and effecting the polymerase switch from
replicative to TLS polymerase (low stringency DNA polymerases).
3. TLS polymerases carry out TLS, either singly or in combination, past
different types of DNA damage.
4.Such regulation must ensure that (1) the specialized polymerases act
only when needed, and (2) that polymerases act only at the right
location in DNA.
5.TLS evolved in mammals as a system that balances gain in survival
with a tolerable mutational cost, and that disturbing this balance causes
a potentially harmful increase in mutations, which might play a role in
carcinogenesis.
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Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4):Y-family of DNA polymerases.
Cellular responses evoked by DNA
damaging agents are very complex events
• Responses may triggered by the signals originated from:
genomic and mitochondrial DNA damages,
malfunction of signaling molecules,
endoplasmic reticulum stress, and others.
• Networks between different signaling pathways;
• Cellular responses are the comprehensive and integrated
consequences.
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Gene-environmental interactions
• The metabolism of xenobiotics by biologic systems
– Individual variation
– The competition between activation and detoxication
• The alteration of genes and epigenetics by xenobiotics
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Physical factors
in carcinogenesis
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Physical carcinogens
– Corpuscular radiations
– Electromagnetic radiations
– Ultraviolet lights (UV)
– Low and high temperatures
– Mechanical traumas
– Solid and gel materials
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Ionizing radiation (IR)
• Penetrate cells, unaffected by the usual cellular barriers
to chemical agents
• A relatively weak carcinogen and mutagen
• The initial critical biologic change is damages to DNA
• It takes place in a matter of the order of a microsecond
or less
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Electromagnetic fields (EMF)
Remains controversial:
• Minimal increase in relative risk of brain tumor and leukemia in
electric utility workers
• Also relatively increased risk for acute lymphoblastic leukemia
by EMF exposure during pregnancy or postnatally
• However, some studies lend no support for this proposition
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Ultraviolet (UV)
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Sunlight and skin cancer
Well established for basal and squamous cell cancers
Some controversy remains for melanoma
Nonmelanoma skin cancers are the most common cancer in
the US (45%)
• Usually occurs at the age of 50 – 60
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Sunlight spectrum and wavelength
• UVA (320-400)
– photocarcinogenic
– weakly absorbed in DNA and protein
– active oxygen and free radicals
• UVB (290-320)
– overlaps the upper end of DNA and protein absorption spectra
– mainly responsible through direct photochemical damage
• UVC (240-290)
– not present in ambient sunlight
– low pressure mercury sterilizing lamps
– experimental system
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Shielding us from the sun
• Ozone: shorter than 300 nm cannot reach the earth’s surface
• UVA and UVB: only a minute portion of the emitted solar
wavelengths ( 0.0000001%)
• Skin:
– melanin pigment
– keratin layers
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Xeroderma pigmentosum (XP)
(着色性干皮病)
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Autosomal recessive disease, 1/250,000
Obligate heterozygotes (parents): asymptomatic
Homozygotes: skin and eyes, even neurologic degeneration
Onset at 1-2 year of age
2,000 times higher frequency for cancer
30-year reduction in lifespan
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• 7 complementation groups, with various reduced
rates for excision repair
• An 8th, the XP variant, has a defect in replication
of damaged DNA (polymerase h)
• Groups A and D are very sensitive to UV killing
• Group C is the largest group, or called the
common/classic form, only shows skin
disorders, preferentially repairs transcriptionally
active genes
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Cancer-associated Pathogens
Persistent infection with some pathogens is an important cause of about 20
percent of cancers worldwide.
This knowledge has enabled the development of new cancer prevention strategies
that use medicines and vaccines to eliminate or prevent infection with these agents.
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Viral Oncogenesis
• RNA Oncovirus (Retrovirus)
• DNA Oncovirus
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RNA Oncovirus
Rous sarcoma in chickens (RSV): in 1911
Human T-cell lymphotropic virus (HTLV-I,II);
Human immunodeficiency virus (HIV)
Classification of retrovirus
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Structure of RNA Oncovirus
Retroviruses:
• ssRNA viruses
• Reverse transcriptase
• Oncogenes
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Genome of RNA Oncovirus and Gene Products
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Life cycle
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Receptor binding and membrane fusion
Internalization and uncoating
Reverse transcription of the RNA genome to form double-stranded
linear DNA
Nuclear entry of the DNA
Integration of the linear DNA into host chromosomal DNA to form the
provirus
Transcription of the provirus to form viral RNAs
Splicing and nuclear export of the RNAs
Translation of the RNAs to form precursor proteins
Assembly of the virion and packaging of the viral RNA genome
Budding and release of the virions
Proteolytic processing of the precursors and maturation of the
virions
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Replication of RNA Oncovirus
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Mechanisms of Oncogenesis
Induced by RNA Oncovirus
• Transducing Retrovirus
v-onc
• cis-Activating Retrovirus
c-onc
• trans-Activating Retrovirus
tax trans-acting x p40tax
rex repressive expression x p27rex, p21rex
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• Oncogene transduction
– Acutely transforming in vivo and in vitro
– Transform cells by the delivery (transduction) of an
oncogene from the host cell (v-onc) to a target cell
– Cause the formation of polyclonal tumors
– Most of this group of viruses are replication defective
(the requirement of a helper virus)
– Examples:
RSV (v-src);
Abelson murine leukemia virus (v-Abl)
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•Insertional activation
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Long latent periods, Less efficient
Do not induce transformation of cells in vitro
Usually are replication competent
No oncogenes
Tumors are usually monoclonal
Provirus (LTR) is found within the vincity of a protooncogene (c-myc)
– Examples: lymphoid leukosis virus;
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•Grow stimulation and two-step oncogenesis
– The defective spleen focus-forming virus (SFFV) and its helper,
the Friend murine leukemia virus (Fr-MuLV)
– Induce a polyclonal erythrocytosis in mice
– Require the continued viral replication
– A mutant env protein gp55 of SFFV binds and stimulated the
erythropoietin receptor, thus inducing erythroid hyperplasia
– Fr-MuLV or SFFV integration inactivates p53
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• Transactivation
– HTLV-1 and 2
– Like cis-activation group: replication competent, carries no oncogene,
induces monoclonal leukemia, and latent
– Like transducing group: can immortalize cells in vitro, has no specific
integration site
– Unique 3’ genomic structure: the X region; Encodes at least three proteins:
Tax (p40), Rex (p27, p21)
– Tax is the focus
– Transactivate the viral LTR, results in a 100- to 200-fold increase in the
rate of proviral transcription
– Transactivate cellular enhancers and promoters, including genes for IL-2,
granulocyte-macrophage colony-stimulating factor (GM-CSF), c-fos, and
others.
Genome of HTLV
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•Immunodeficiency
• AIDS patients have an extraordinary increased rate of developing
high-grade lymphomas and Kaposi’s sarcoma (KS)
• Probably secondary
• However, Tat protein of HIV (the transactivating protein) may
induce KS-like lesions in mice.
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Endogenous retroviruses
• Exo or endo: somatic vs germline
• 0.5-1% mammalian genome is composed of retroviral
proviruses
• Some properties:
– Most are defective
– Great variations between species or within
– Variable level of expression
– Generally not pathogenic
– The potential to induce disease is notable
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DNA Oncovirus
Papilloma virus: HPV
Polyoma virus
Herpes virus: EBV
Hepatitis B virus
Hepatitis C virus
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Mechanism of Oncogenesis
Induced by DNA Oncovirus
Transforming proteins
1. HPV
E6 interact with P53
E7 interact with RB
2. Adenovirus
E1a interact with RB
E1b
3. Polyoma virus
SV40
Large T interact with RB
Py virus Large and Middle T
Transcription activators
1. EB virus
EBNA-2 and LMP
2. HBV
p28 X protein
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Gene Map and Function of HPV
ORF
E1
E2
E5、E6、E7
L1、L2
E4
E3、E8
Function
Virus proliferation
Regulation of transcription
Cell transformation
Encoding capsid protein
Encoding late cytosolic protein
Unkown
E5: activates growth factor receptor
E6: ubiquitin-mediated degradation of p53
E7: binds and inactivates unphosphorylated pRb
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Genome of EB Virus
EBNA (EB virus Nuclear Antigen)
EBNA-1 Immortalization of cell
EBNA-2 trans-acting transcription activator
EBNA-3 Function unknown
LP: Leader Protein RNA Processing
LMP: Latent Membrane Protein Activation of NF-κB
TP: Terminal Protein Function unknown
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Genome and Products of HBV
Transforming gene: X gene
X protein activates gene transcription via XRE
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Genetic Predisposition
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Hereditary Cancer
Tumor Genetic Susceptibility
Hormones
Metabolism
Immunity
Psychological factors
others
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Tumor Genetic Susceptibility
Some individuals are at increased risk of certain cancers because
they inherited a cancer-predisposing genetic mutation.
Tumor susceptibility genes (DNA repair genes, Tumor suppressor
genes, Cytochrome P450 family, etc).
Not all potentially inheritable causes of cancer have been
identified, but if an individual suspects that a relative has a cancer
caused by one of the 17 known cancer-predisposing genetic
mutations, he or she should consult a physician and consider genetic
testing for verification.
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Hormones and cancer
• Major carcinogenic consequence of hormone exposure:
cell proliferation
• How to get exposure: contraceptives, hormone replacement
therapy, or during prevention of miscarriage
• The emergence of a malignant phenotype depends on a series of
somatic mutation; Germline mutations may also occur;
• Epidemiological studies
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Hormone-related cancer
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Breast cancer and estrogen
Endometrial cancer: Estrogen replacement therapy
Ovarian cancer: follicle stimulating hormone
Vaginal adenocarcinoma: in utero diethylstilbestrol (DES)
exposure
• Prostate cancer and androgen
• Cervical cancer
• Thyroid cancer: the pituitary hormone thyroid stimulating
hormone (TSH)
• Osteosarcoma: incidence associates with the pattern of
childhood skeleton growth; and hormonal activity is a primary
stimulus for skeleton growth
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Some higher risks for certain types of cancer
Inflammation and Cancer：for example, ulcerative colitis and Crohn disease increase an
individual’s risk for colorectal cancer six fold.
Obesity and Cancer: Obesity increases risk for a growing number of cancers, most
prominently the adenocarcinoma subtype of esophageal cancer, and colorectal, endometrial,
kidney, pancreatic, and postmenopausal breast cancers. It also negatively impacts tumor
recurrence, metastasis, and patient survival for several types of cancers.
Type 2 Diabetes Mellitus and Cancer:
Those with type 2 diabetes are most at risk for developing liver, pancreatic, and
endometrial cancers, but also have an increased risk for developing biliary tract, bladder,
breast, colorectal, esophageal, and kidney cancers, as well as certain forms of lymphoma.
it is not well established how type 2 diabetes increases cancer risk.
Similar to obesity, type 2 diabetes increases levels of insulin and causes persistent
inflammation.
Energy balance is a complex dynamic that is not only influenced by calorie consumption
and physical activity, but also by other factors such as genetics, diet composition, body
weight or body composition, and sleep. How changes in energy balance promote cancer is an
area of intense research investigation.
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思考题：
1. 简述环境化学致癌因子分类及其致肿瘤机制。
2. 简述肿瘤病毒分类及其致肿瘤机制。
3. 肿瘤遗传易感性的概念。
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