Download DNA Oncovirus

Cancer Etiology
1. Introduction
2. Chemical Factors in Carcinogenesis
3. Physical Factors in Carcinogenesis
4. Viral Oncogenesis
5. Genetic Predisposition
邵吉民，教授，病理学与病理生理学系
[email protected]
Introduction
Tumor
 Benign tumor
 Malignant tumor
2
Cancer Incidence and Mortality
 Siegel R, Naishadham D, Jemal A. Cancer statistics, 2012.
CA Cancer J Clin. 2012;62(1):10-29.
1,638,910 new cancer cases and 577,190 deaths from cancer are
projected to occur in USA in 2012.
One in 4 deaths in USA is due to cancer.
 2010年国际抗癌联盟（UICC）：
2008年全世界1270万新增癌症患者，死亡人数760万。
 全国肿瘤登记中心《2012中国肿瘤登记年报》
每年新发肿瘤病例约312万例，每天约8550人;
每年因癌死亡270万例,居民因癌死亡率13%，即每7-8人中有1人因癌死亡。
恶性肿瘤发病:第一位肺癌，其次胃癌、结直肠癌、肝癌和食管癌;
恶性肿瘤死亡：第一位肺癌，其次肝癌、胃癌、食管癌和结直肠癌；
3
中国近20年来癌症呈现年轻化及发病率和死亡率“三线”走高的趋势。癌
症种类呈现地域化特点。
History of Cancer Research
4
Kiberstis P, Marshall E. Cancer crusade at 40. Celebrating an anniversary. Introduction. Science. 2011;331(6024):1539.
Chemical Carcinogenesis





Multi-stage Theory of Chemical Carcinogenesis
Classification of chemical carcinogens
Mechanisms of Chemical Carcinogenesis
DNA Damage Induced by Ultimate Carcinogens
DNA Repair
Multi-stage Theory of
Chemical Carcinogenesis
Initiation -----------Genetic events
Chemical Carcinogens (Direct and Indirect Carcinogens)
Promotion -------Epigenetic events
Tumor promoters
– 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
6
7
Initiation
• Irreversible genetic damage:
A necessary, but insufficient prerequisite for tumor
initiation
• Activation of proto-oncogene, inactivation of a
tumor suppressor gene, and etc
8
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
9
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
10
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
11
• 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
12
Gene-environmental interactions
• The metabolism of xenobiotics by biologic systems
– Individual variation
– The competition between activation and detoxication
• The alteration of genes by xenobiotics
13
Classification of chemical carcinogens
1. 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:
require 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.
14
15
(2) Epigenetic carcinogens
• Promotes cancer in ways other than direct DNA damage/
do not change the primary sequence of DNA
• Alter the expression or repression of certain genes and
cellular events related to proliferation and differentiation
• Promoters, hormone modifying agents, peroxisome
proliferators, cytotoxic agents, and immunosuppressors
• Organochlorine pesticides, [saccharin], estrogen,
cyclosporine A, azathioprine
16
2. 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)
(5) Inorganic elements and their compounds: arsenic, chromium,
and nickel are also considered genotoxic agents
17
18
Mechanisms of Initiation in
Chemical Carcinogenesis
(1) DNA damages:
Pro-carcinogen metabolic activation (Phase I and II)

Ultimate carcinogen (electrophiles)

Interaction with macromolecules (nucleophiles)

DNA damage, mutations, chromosomal aberrations, or cell death
(2) Epigenetic changes
(3)Activation of oncogenes; inactivation of tumor suppressor genes
19
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 (although
varied).
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: the two sites are on opposite polynucleotide
strands;
20
Intrastrand cross-link: on the same polynucleotide chain of a DNA duplex.
---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:
MNNG
N-Methyl-N-nitroso-N'-nitroguanidine
21
---Bifunctional alkylating agents
22
Indirect Chemical Carcinogens
and Their Phase I Metabolic derivatives
23
BPDE binds DNA covalently,
resulting in bulky adduct
damage
BPDE intercalates into dsDNA
non-covalently, leading to
conformational abnormalities
24
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
25
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
26
• 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
27
• 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
28
Translesion DNA synthesis
29
Hormones and the etiology of cancer
• Major carcinogenic consequence of hormone
exposure: cell proliferation
• The emergence of a malignant phenotype
depends on a series of somatic mutation
• Germline mutations may also occur
• How to get exposure: contraceptives, hormone
replacement therapy, or during prevention of
miscarriage
• Epidemiological studies
30
Hormone-related cancer
•
•
•
•
•
Breast cancer and estrogen
Endometrial cancer: Estrogen replacement therapy
Ovarian cancer: follicle stimulating hormone
Prostate cancer and androgen
Vaginal adenocarcinoma: in utero
diethylstilbestrol (DES) exposure
31
Other hormone-related cancers
• Cervical cancer: OC use might increase the
risk, still a lot complicating factors
• 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
32
Physical factors
in carcinogenesis
33
Physical carcinogens
– Corpuscular radiations
– Electromagnetic radiations (EMF)
– Ultraviolet lights (UV)
– Low and high temperatures
– Mechanical traumas
– Solid and gel materials
34
35
Viral Oncogenesis
• RNA Oncovirus (Retrovirus)
• DNA Oncovirus
36
RNA Oncovirus
Retroviruses:
ssRNA viruses
Reverse transcriptase
Oncogenes
Rous sarcoma in chickens (RSV): in 1911
Human T-cell lymphotropic virus (HTLV-I,II)
Human immunodeficiency virus (HIV)
Classification of retrovirus
38
Structure of RNA Oncovirus
39
Genome of RNA Oncovirus and
Gene Products
Genome of Human T-cell Leukemia virus (HTLV)
40
Life cycle
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
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
41
Replication of RNA Oncovirus
42
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
43
• 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)
44
•Insertional activation
–
–
–
–
–
–
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;
45
•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
46
• 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.
47
DNA Oncovirus
Papilloma virus
Polyoma virus
Adenovirus
Herpes virus: EB virus
Hepatitis B virus
48
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
49
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
50
Genome and Products of HBV
Transforming gene: X gene
X protein activates gene transcription via XRE
51
Genetic Predisposition
• Hereditary Cancer
• Tumor Genetic Susceptibility
---Tumor susceptibility genes (Cytochrome P450 family,
DNA repair genes, Tumor suppressor genes, etc)
•
•
•
•
Immunity
Hormones and metabolism
Psychological factors
others
52