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Cancer Etiology
1. Chemical Factors in Carcinogenesis 2. Physical Factors in Carcinogenesis3. Viral Oncogenesis4. Genetic Predisposition
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Hallmarks of cancer (Weinberg, Cell, 2000)
Figure 1. The Hallmarks of Cancer. This illustration encompasses the six hallmark capabilities originally proposed in our 2000 perspective. The past decade has witnessed remarkable progress toward understanding the mechanistic underpinnings of each hallmark. (Hanahan D, Weinberg RA. Hallmarks of Cancer: The Next Generation. Cell 2011, 144:646)
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Introduction
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Hallmarks of cancer (Weinberg, Cell, 2011)
. (Hanahan D, Weinberg RA. Hallmarks of Cancer: The Next Generation. Cell 2011, 144:646)
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The Hallmarks of Cancer Self-sufficiency in growth signals
Cancer cells do not need stimulation from external signals (in the form of growth factors) to multiply.
Insensitivity to anti-growth signals
Cancer cells are generally resistant to growth-preventing signals from their neighbours.
Tissue invasion and metastasis
Cancer cells can break away from their site or organ of origin to invade surrounding tissue and spread (metastasis) to distant body parts.
Limitless reproductive potential
Non-cancer cells die after a certain number of divisions. Cancer cells escape this limit and are apparently capable of indefinite growth and division (immortality).
Sustained angiogenesis
Cancer cells appear to be able to kickstart this process, ensuring that such cells receive a continual supply of oxygen and other nutrients.
Evading apoptosis
Apoptosis is a form of programmed cell death, the mechanism by which cells are programmed to die after a certain number of divisions or in the event they become damaged. Cancer cells characteristically are able to bypass this mechanism.
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Deregulated metabolism
Most cancer cells use abnormal metabolic pathways to generate energy, a fact appreciated since the early twentieth century with the postulation of the Warburg hypothesis, but only now gaining renewed research interest.
Evading the immune system
Cancer cells appear to be invisible to the body’s immune system.
Unstable DNA
Cancer cells generally have severe chromosomal abnormalities, which worsen as the disease progresses.
Inflammation
Recent discoveries have highlighted the role of local chronic inflammation in inducing many types of cancer.
(Hanahan, D.; Weinberg, R. A. (2011). "Hallmarks of Cancer: The Next Generation". Cell 144 (5): 646–674. doi:10.1016/j.cell.2011.02.013 )
Chemical Carcinogenesis
Multi-stage Theory of Chemical Carcinogenesis Classification of chemical carcinogens Mechanisms of Chemical Carcinogenesis DNA Damage Induced by Ultimate Carcinogens DNA Repair
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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
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Molecular Basis of multistep carcinogenesis
• Alterations in DNA cause changes in one or both of the following types of genes:– Proto-oncogenes– Tumor suppressor genes Best example is colonic cancerAPCRAS18qp53
Molecular Basis of Multistep Carcinogenesis
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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 4-aminobiphenyl ( 苯并芘 , 4- 氨基联苯 )
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Malignant conversion • The transformation of a preneoplastic cell into that
expresses the malignant phenotype
• Further genetic changes
• The further genetic changes may result from infidelity of DNA synthesis
• May be mediated through the activation of proto-oncogene and inactivation of tumor-suppressor 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 proto-oncogenes and the inactivation of tumor-suppressor 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
Tumor Progression & Heterogeneity
• Tumor progression: means increase aggressiveness & and is acquired occurring in an increasing fashion
• Development of new subset of cells that are different in aspects such as invasivness,ability to Mets, hormonal response-Heterogeneous group
• Results from multiple mutations occurring independently in different cellssubclone of cells that is different
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Cancer Stem Cells• Stem cells self-renew
– Cell divisions create new stem cells
• Stem cells are multipotent
– Ability to differentiate into multiple different cell types
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Classification of chemical carcinogens1. Based on mechanisms(1) Genotoxic carcinogen (DNA-reactive)• Direct-acting: intrinsically reactive
N-methyl-N’-nitro-N-nitrosoguanidine (MNNG, 甲基硝基亚硝基胍 )
methyl methanesulfonate (MMS, 甲基磺酸甲酯 ), N-ethyl-N-nitrosourea (ENU, 乙基 -N- 亚硝基脲 ),
nitrogen and sulfur mustards ( 氮和硫芥 )
• Indirect-acting: require metabolic activation by cellular enzyme to form the DNA-reactive
metabolite (members of the cytochrome P450 family) benzo[]pyrene, 2-acetylaminofluorene, benzidine, Aflatoxin B1, B2. ( 苯并芘， 2- 乙酰氨基芴，联苯胺，黄曲霉毒素 B1 ， B2)
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(2) Epigenetic carcinogens
• Those substances which 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 ( 有机氯农药 ), saccharin ( 糖精 ), estrogen, cyclosporine A ( 环孢素 A), azathioprine( 硫唑嘌呤 )
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2. Based on sturcture(1) Nitrosamines (NA, 亚硝胺类 )
MNNG, MMS (direct carcinogen)
(2) Polycyclic aromatic hydrocarbons (PAH, 多环芳烃 )
Benzo(a)pyrene (indirect carcinogen)
(3) Aromatic amines (AA, 芳香胺类 )
2-acetylaminofluorene, benzidine (indirect carcinogen)
（ 2- 乙酰氨基芴，联苯胺）
(4) Aflatoxin (AF, 黄曲霉素 )
(5) Inorganic elements and their compounds: arsenic, chromium,
and nickel are also considered genotoxic agents
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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
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(1) Alkylating agents ( 烷化剂 ) are electrophilic compounds with affinity
for nucleophilic centers in organic macromolecules.
(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;
Intrastrand cross-link: on the same polynucleotide chain of a DNA duplex.
such as Nitrogen and sulfur mustard, mitomycin, cis-platinum
( 氮和硫芥，丝裂霉素，顺铂 )
Direct Chemical Carcinogens
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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
---Monofunctional alkylating agents
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---Bifunctional alkylating agents
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Indirect Chemical Carcinogensand Their Phase I Metabolic derivatives
直接环氧化
N- 羟基化
二甲基亚硝胺
黄樟油
芳香类
脂类
两步环氧化
联苯胺
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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 ( 嵌入 , 嵌接 )
Bulky aromatic-type adducts, Alkylation (small adducts),
Oxidation, Dimerization, Deamination
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Repair systems• Direct DNA repair/ Direct reversal :
– DNA alkyltransferase （烷基转移酶） (O6-alkylguanine-DNA alkyltransferase)
– One enzyme per lesion
O6 甲基鸟嘌呤甲基转移酶，能直接将甲基从 DNA 链鸟嘌呤 O6 位上的甲基移到蛋白质的半胱氨酸残基上而修复损伤的 DNA 。这个酶的修复能力并不很强，但在低剂量烷化剂作用下能诱导出此酶的修复活性。
DNA Repair
• Base excision repair (BER ，碱基切除修复 )– small adducts,
– overlap with direct repair
– glycosylase to remove the adducted base
（糖基化酶）
• 所有细胞中都带有不同类型、能识别受损核酸位点的糖苷水解酶，它能够特异性切除受损核苷酸上的 N-β- 糖苷键，在 DNA 链上形成去嘌呤或去嘧啶位点，统称为 AP 位点（ apurinic/apyrimidinic ，缺嘌呤或缺嘧啶）。
• DNA 分子中一旦产生了 AP 位点， AP 核酸内切酶就会把受损核苷酸的糖苷 - 磷酸键切开，并移去包括 AP 位点核苷酸在内的小片段 DNA，由 DNA 聚合酶Ⅰ合成合成新的片断，最终由 DNA 连接酶把两者连成新的被修复的 DNA 链，这一过程即为碱基切除修复
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---TAGC------ATCG---
---TAGC------A CG---U
---TAGC---ung-ase
①
GCUA
U
---TAGC------A CG---
AP 内切酶(Apurinase)
②---TAGC------ATCG---
DNApol
Ligase
③
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• Nucleotide excision repair (NER, 核苷酸切除修复 ): – involves recognition, preincision, incision, gap-filling,
and ligation,
– large distortions ( 扭曲 )
– strand specific, the transcribed strand is preferentially repaired
– xeroderma pigmentosum (XP): NER deficiency
• 损伤识别 --- 蛋白复合体结合到损伤位点 ---- 在错配位点上下游几个碱基的位置上（上游 5’ 端和下游 3‘ 端）将 DNA 链切开 ---- 将两个切口间的寡核苷酸序列清除 ----DNA 聚合酶合成新的片段填补 gap---- 连接酶将新合成片段与原 DNA 链连接起来。
• Mismatch repair (MMR, 错配修复 )
– 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 （同源重组修复）– non-homologous end joining （非同源末端连接）
• 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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Translesion DNA synthesis （ 跨损伤修复）
• SOS 应答机制，可诱导一些DNA 聚合酶表达，包括大肠杆菌细胞中的 DNA 聚合酶IV （ DinB) 和 DNA 聚合酶V （ UmuC) ，以及真核细胞内的 Rev32 。
• 这些聚合酶大多数都不具有“校对” 功能，也不严格依赖模板，合成的 DNA 子链中的碱基可能不符合沃森 - 克里克碱基配对要求，表现出“合成错误风险”。
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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
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Hormone-related cancer
• Estrogen and breast cancer• Endometrial cancer: Estrogen replacement therapy• Ovarian cancer: follicle stimulating hormone• Prostate cancer and androgen• Vaginal adenocarcinoma: in utero diethylstilbestrol
(DES ，己烯雌酚 ) exposure
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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
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Physical factors in carcinogenesis
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Physical carcinogens
– Ionizing 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
• IR: 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)
• 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 )
• 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 )
• 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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Viral Oncogenesis
• RNA Oncovirus (Retrovirus)
• DNA Oncovirus
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)
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Classification of retrovirus
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Structure of RNA Oncovirus
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Genome of RNA Oncovirus and Gene Products
Genome of Human T-cell Leukemia virus (HTLV)
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Life cycle1. Receptor binding and membrane fusion 2. Internalization and uncoating 3. Reverse transcription of the RNA genome to form double-
stranded linear DNA 4. Nuclear entry of the DNA 5. Integration of the linear DNA into host chromosomal DNA to
form the provirus 6. Transcription of the provirus to form viral RNAs
7. Splicing and nuclear export of the RNAs
8. Translation of the RNAs to form precursor proteins
9. Assembly of the virion and packaging of the viral RNA genome
10. Budding and release of the virions
11. 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
• tax 基因能够编码一种反式激活因子，一方面活化 LTR ，促进病毒基因的转录，另一方面可活化宿主细胞 IL-2 及其受体的基因，发挥细胞促生长作用。• rex 基因可表达两种对病毒结构基因有调节作用的蛋白。
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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
– 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 proto-
oncogene (c-myc)– Examples: lymphoid leukemia virus
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• Transactivation – HTLV-1 and 2– Like cis-activation group: replication competent, carries no oncogene,
induces monoclonal leukemia– 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.
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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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DNA Oncovirus
Papilloma virus （乳头状瘤病毒）Polyoma virus （多瘤病毒）Adenovirus
Herpes virus: EB virus （疱疹病毒）Hepatitis B 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 （猴空泡病毒） Large T interact with RB Py virus ( 多瘤病毒 ) 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 Function
E1 Virus proliferationE2 Regulation of transcriptionE5 、 E6 、 E7 Cell transformationL1 、 L2 Encoding capsid proteinE4 Encoding late cytosolic proteinE3 、 E8 Unkown
E5: activates growth factor receptorE6: ubiquitin-mediated degradation of p53E7: binds and inactivates unphosphorylated pRb
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Genome of Adenovirus
Transformaing genes:E1A: Encoding intranuclear 26 and 30 kD phosphorylated proteinsE1B: Encoding a 19 kD protein located in nuclear and plasma membranes
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Gene Map of Polyoma and SV40 Virus
Transforming Genes SV40 virus: Large T Polyoma virus: Large and Middle T
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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 unknownLP: Leader Protein RNA ProcessingLMP: Latent Membrane Protein Activation of NF-κBTP: Terminal Protein Function unknown
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Genome and Products of HBV
Transforming gene: X gene X protein activates gene transcription via XRE (HBX ropnsive element, HBX反应元件 )
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Genetic Predisposition
• Hereditary Cancer• Tumor Genetic Susceptibility
---Tumor susceptibility genes:Cytochrome P450 family, DNA repair genes, Tumor suppressor genes
• Immunity• Hormones and metabolism• Psychological factors• others
