Carcinogenesis Cont'd - DSPR 139 PDF

Carcinogenesis (cont’d) DSPR 139: Neoplasia and Genetics Daria Vasilyeva, DDS In the context of carcinogenesis, a mutated gene that -- “gains a function” and enables the cell to proliferate is likely to be _______ -- “loses a function” and can no longer regulate cell proliferatio...

Carcinogenesis (cont’d) DSPR 139: Neoplasia and Genetics Daria Vasilyeva, DDS In the context of carcinogenesis, a mutated gene that -- “gains a function” and enables the cell to proliferate is likely to be _______ -- “loses a function” and can no longer regulate cell proliferation is likely to be _______ -- allows the neoplastic cell to escape cell death is likely to be _______ -- allows errors in the DNA sequence to go unchecked and to accumulate is likely to be _______  1. Epigenetics At a glance 2. Carcinogens 3. Immunosurveillance If all cells in the body have essentially the same DNA, - how do our organs and tissues serve such vastly different functions? - how are they able to retain their identity as they divide, if all cells of the body have essentially the same DNA?  Epigenetics: Beyond Genetics Most cancers have somatic genetic changes, e.g. ○ Inappropriate activation or silencing of gene ○ Copy number changes amplifying or removing a gene ○ Chromosomal rearrangements randomly connecting two non-consecutive DNA sequences, resulting in abnormal fusion transcript A small but important number of genetic changes dysregulates transcription across entire genome ○ Genome: complete set of genetic material in a cell ○ Transcription: copying of genetic information into RNA for protein synthesis Epigenetics: factors other than the DNA sequence itself that regulate gene expression Each human diploid cell (= 23 pairs of chromosomes) has about 1.8 meters of DNA; wound on the histones, the diploid cell has about 90 micrometers (0.09 mm) of chromatin DNA and histones can be modified in ways that regulate gene accessibility and, as a result, gene expression Nucleosome: basic unit of chromatin; DNA wrapped around histone Chromatin = DNA + histones  Epigenetics: Beyond Genetics 1. DNA hypermethylation and demethylation 2. Post-translational histone modifications 3. micro RNA 1. DNA hypermethylation and demethylation ○ May result in aberrant silencing of critical tumor suppressor genes in the absence of DNA sequence alteration ○ May result in aberrant over-expression of oncogenes in the absence of DNA sequence alteration Cytosine nucleotides can be covalently modified by adding a methyl group CpG = cytosine-phosphate-guanine  TET2 enzyme → demethylation ○ More potent in hyperglycemia Links higher cancer susceptibility in patients with diabetes to high blood glucose levels  There is a strong correlation (0.8) between Much of that correlation can be accounted average number of stem cell divisions per for by aberrant methylation changes in tissue and rate/risk of cancer in that tissue stem cells during cell division Methylation of cytosines on DNA is also a common physiologic mechanism of silencing gene expression If all cells in the body have essentially the same DNA, - how do our organs and tissues serve such vastly different functions? - how are they able to retain their identity as they divide, if all cells of the body have essentially the same DNA? 70-80% of CpGs are methylated in human adult cells  2. Post-translational histone modifications ○ Post-translational histone modifications can open chromatin for gene expression or close it for gene silencing ○ Lysine rich histone ‘tails’ are a frequent site of post-translational changes Histone acetylation → open, active chromatin state Histone methylation → more diverse, complex effects Cancers often have widespread alterations in DNA methylation patterns and in post-translational modification of histone proteins → aberrant gene expression - Beige dots: DNA methylation - Green dots: histones with post-transcriptional modifications leading to gene activation: - Red dots: histones with post-transcriptional modifications leading to gene silencing In cancer, epigenetic modifiers such as environmental exposures, aging, and mutations in epigenetic modifier genes lead to increased, random gene expression in cancer cells. This promotes tumor-cell heterogeneity and survival  3. microRNAs ○ Small RNA molecules (20-23 nucleotides in length) that act as post-transcriptional regulators of gene expression Base-pairing with complementary sequences of mRNA ○ Silenced mRNA molecule → impaired translation into protein (e.g., tumor suppressor) Which of these is not epigenetic? A: Silencing of mRNA by miRNA, B: 12q13-15 amplification in liposarcoma, C: Demethylation in hyperglycemia Carcinogens and their cellular interactions Chemical carcinogenesis Radiation carcinogenesis Microbial carcinogenesis  Targets of Chemical Carcinogens Mutations are generally random ○ Mutations in RAS, TP53, or other important genes give the cell: Potential selective growth advantages Higher risk for malignant transformation Cigarette smoke is a major cancer risk factor: huge mutational potency ○ Lung cancers in smokers -- 10x higher mutational burden than in non-smokers ○ Accelerated rate of mutations → higher risk of acquiring driver mutations  Targets of Chemical Carcinogens Some carcinogens, because of their chemical structure, interact with particular DNA sequences/bases ○ Mutations are clustered at ‘hotspots’ Aflatoxin B1 ○ Produced by some strains of Aspergillus (mold) Common food contaminant in parts of Africa and Far East ○ Results in increased risk of hepatocellular carcinoma in these regions Hepatocellular carcinoma worldwide: p53 mutations are generally uncommon Aflatoxin B1- associated hepatocellular carcinoma: Characteristic p53 R249S mutation (arginine in place of serine)  Chemical Carcinogens: Initiators and Promoters Chemical carcinogens may directly damage DNA or enhance proliferation of cells ○ Initiation: genetic alteration of a cell exposed to sufficient dose of chemical carcinogen Permanent and irreversible Necessary for tumor formation Not sufficient for tumor formation ○ Promotion: stimulation and alteration of an already initiated cell by a chemical carcinogen Cellular changes are reversible Not carcinogenic if cell is not initiated  Chemical Carcinogens: Initiators All initiating chemical carcinogens are highly reactive electrophiles ○ Electron-deficient chemicals that are attracted to electrons Electrophiles harvest electrons from (and mutate) nucleophiles in a cell ○ DNA, RNA, proteins (Nucleophile: chemical species that donates electron to electrophile) Direct and indirect initiators: ○ Direct-acting carcinogens don’t require metabolic conversion to become carcinogenic ○ Indirect-acting carcinogens require metabolic conversion of pro-carcinogen to become carcinogenic  Chemical Carcinogens: Initiators Direct-acting carcinogens don’t require metabolic conversion to become carcinogenic ○ Most are weak carcinogens ○ Risk of induced secondary cancer is low (but clinically significant) ○ Some are used in cancer chemotherapy (e.g. alkylating agents) Patients are cured of primary cancer but may develop second form of cancer Often acute myeloid leukemia  Chemical Carcinogens: Initiators Polycyclic hydrocarbons: Indirect-acting carcinogens require metabolic present in fossil fuels, formed during high-temperature tobacco combustion, conversion of pro-carcinogen to become carcinogenic produced from animal fats during broiling/grilling/smoking of meats Most are metabolized in the liver by cytochrome P-450-dependent enzymes Polymorphisms in genes encoding these enzymes may influence susceptibility to developing cancer e.g., 10% of white population has a highly inducible form of CYP1A1 (one of P-450 genes) Increased risk of lung cancer in smokers 7-fold risk of lung cancer among light smokers  Chemical Carcinogens: Promoters Promoter is a chemical agent that: ○ Stimulates cell proliferation ○ Is not mutagenic Promoter action in initiated/mutated cell results in: ○ Clonal proliferation of initiated/mutated cell ○ Increased risk of acquiring additional carcinogenic mutations e.g., exposure to estrogen ○ Breast cancer was noted to be 6x more prevalent among nuns than general population (18th century, Bernardino Ramazzini) ○ Modern increase in breast cancer incidence is associated with estrogen-driven proliferation: Earlier menarche Fewer children Shorter periods of breastfeeding Postmenopausal hormone replacement therapy Chronic Inflammation as Promoter Inflammatory bowel disease: chronic inflammation associated with chronic wound repair/regeneration ○ 1.5-2-fold risk of developing colorectal cancer Chronic Inflammation as Promoter Sjögren syndrome: chronic autoimmune disorder 5-10% of patients develop MALT lymphoma 16-44x higher risk than normal population Normal stem cell divisions as a promoter Carcinogens and their cellular interactions Chemical carcinogenesis Radiation carcinogenesis Microbial carcinogenesis  Radiation Carcinogenesis Two forms of radiation energy are carcinogenic: ○ UV (ultraviolet) light ○ Ionizing (electromagnetic and particulate) radiation  Ultraviolet Radiation UV portion of solar spectrum divided into three ranges: ○ UVA (320-400 nm) ○ UVB (280-320 nm) Responsible for induction of cutaneous tumors ○ UVC (200-280 nm) Potent carcinogen, but filtered out by ozone layer  Ultraviolet Radiation Increased risk of skin cancer ○ Basal cell carcinoma ○ Squamous cell carcinoma ○ Melanoma Risk is greatest in fair-skinned individuals ○ Less light-absorbing melanin in skin Non-melanoma skin cancers ○ Cumulative UV exposure Melanoma ○ Intense intermittent exposure (sunbathing) UVB is carcinogenic due to formation of pyrimidine dimers → distort DNA helix and prevent proper pairing Thymine-thymine Cytosine-cytosine This overwhelms nucleotide excision repair pathway → mutations accumulate  Which of these has a defective nucleotide excision repair system? A: Gorlin syndrome, B: Xeroderma pigmentosum, C: Familial adenomatous polyposis, D: Retinoblastoma  Ionizing Radiation 1950-1953: peak incidence of leukemia diagnosis in Japan following Hiroshima and Nagasaki atomic bombs Chernobyl disaster: worst nuclear power plant accident in history ○ Chernobyl, Ukraine: April 26, 1986 ○ Approximately 40,000 cancer cases predicted by 2065 Ionizing Radiation Electromagnetic (x-rays, γ rays) and particulate (α-particles, β-particles, protons, neutrons) radiation is carcinogenic Oxygen free radicals damage DNA Radiated cancer cells cannot divide → die ○ Basis of radiation therapy Normal cells included in radiation field cannot divide → die ○ Basis of side effects Some normal cells may incidentally acquire cancer-promoting DNA damage ○ e.g., post-radiation sarcoma following radiation for breast cancer  Radiation Carcinogenesis Certain human tissues are more sensitive to radiation than others ○ Bone marrow is most sensitive ○ Thyroid is sensitive in young people ○ Breast, lungs, salivary glands have intermediate sensitivity Any cell exposed to enough radiant energy can be transformed into cancer cell Marie Curie and her daughter both died of leukemia ○ Many individuals pioneering use of x-rays developed cancer CT scans ○ 3x higher risk of leukemia in children after 2-3 scans ○ 3x higher risk of brain tumors in children after 5-10 scans What about dental and maxillofacial X-rays? Dental x-ray Single PA/bitewing = 6 hrs of background radiation Full mouth series = 4.3 days of background radiation Panoramic Xray = 1.7 days of background radiation CBCT = 8 days of background radiation Normal yearly background radiation Approximately 330 million radiographs are taken annually in US ○ 95 million FMX ○ 19 million BWX Approximately 1000 cancer cases caused by dental radiography annually Proper adherence to selection criteria and use of collimation could reduce this number by 75% Carcinogens and their cellular interactions Chemical carcinogenesis Radiation carcinogenesis Microbial carcinogenesis Microbial Carcinogenesis Only a few potentially oncogenic DNA viruses, RNA viruses, bacteria DOI:https://doi.org/10.1016/S2214-109X(19)30488-7  Human papillomavirus (HPV) Ubiquitous DNA virus infecting exclusively humans ○ 120 different HPV types described Affects epithelial surfaces of skin and mucosa ○ Verruca vulgaris ○ Condyloma acuminatum ○ Oropharyngeal cancer ○ Cervical cancer Human papillomavirus (HPV) HPV consists of a double-stranded, circular DNA genome containing 8 genes ○ E1, E2, E4, E5, E6, E7 ○ L1, L2 Certain ‘high-risk’ HPV types integrate into host DNA ○ Instead of remaining in cytoplasm as an episome To integrate: ○ Circular DNA breaks → HPV integrates as a linear strand ○ Breaks at part of DNA that encodes E2 → leads to viral overexpression of E6/E7, since E2 regulates E6/E7 expression  High-risk HPV E6: degrades p53 and stimulates TERT High-risk HPV E7: binds and inactivates Rb, allowing progression through cell cycle Human papillomavirus (HPV) HPV types (strains) are classified based on relative carcinogenic potential ○ High-risk HPV-16 HPV-18 A few others ○ Low-risk All others High risk HPV causes essentially all cervical and most oropharyngeal cancers HPV vaccine: If the virus can’t enter a cell, it can’t cause cancer 3 vaccines approved by FDA to prevent HPV infection: Cervarix (HPV-16, -18) Gardasil (HPV-16, -18, -6, -11) Gardasil 9 (HPV-16, -18, -6, -11, -31, -33, -45, -52, -58) CDC recommends that 11-12 year olds receive two doses of HPV vaccine 6-12 months apart Epstein-Barr virus (EBV) EBV associated with several cancers ○ Burkitt lymphoma (African form) ○ B-cell lymphomas in immunosuppressed patients ○ Some Hodgkin lymphomas ○ NK/T cell-lymphoma ○ Nasopharyngeal carcinoma EBV appears to act as potent mitogen Burkitt lymphoma: t(8:14) translocation Which of these is not caused by a translocation? A: Burkitt lymphoma, B: Mucoepidermoid carcinoma, C: Melanoma, D: Chronic myelogenous leukemia  Hepatitis B and C viruses (HBV, HCV) 70-85% of worldwide hepatocellular carcinoma is caused by HBV, HCV ○ HBV is endemic in Far East and Africa ○ Far East and Africa have highest rate of hepatocellular carcinoma Dominant oncogenic effect seems to be immunologically mediated chronic inflammation and liver regeneration  Helicobacter pylori Chronic H. pylori infection presents predominantly as antral gastritis Patients are at increased risk for: ○ Duodenal or gastric ulcers ○ Gastric adenocarcinoma ○ MALT lymphoma Chronic inflammatory state → pro-carcinogenic effect Which of these infectious agents has not been implicated in carcinogenesis? A: EBV, B: HPV, C: RSV, D: H. pylori  Cancer and Immunosuppression Immunodeficiency states are associated with increased cancer frequency ○ AIDS ○ Immunosuppressed transplant recipients ○ Patients on long-term pharmacologic immunosuppression ○ Congenital immunodeficiency Immunodeficient patients have decreased immune surveillance  Cancer and Immunosuppression Immune surveillance: constant scanning of body for abnormal cells and their destruction ○ Natural function of intact immune system ○ Immunodeficiency → immune surveillance is not intact In immunosuppressed host, antigens (such as those found in tumor cells) don’t elicit appropriate immune response ○ Favorable environment for infection ○ Favorable environment for tumorigenesis Cancer and Immunosuppression Class I MHC antigen processing pathway is critical in immune surveillance All nucleated cells express MHC I antigens on cell membrane ○ Present endogenous/self antigens Normal peptides Viral peptides if cell is infected by virus Abnormal/mutant proteins if DNA is mutated CD8+ (cytotoxic) T-cells bind MHC I ○ Trigger apoptosis if endogenous/ self antigens abnormal