Path Chapter 7: Cancer (pages 309-327)
The 2 major steps in carcinogenesis are initiation and promotion:
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Inititation – happens from exposure of cells to a high enough of a dose of a carcinogenic agent,
called an initiator
o The initiated cell is changed in a way that it can give rise to a tumor
o Initiation alone is not enough to form a tumor though
o Initiation causes permanent DNA damage, called mutations
o All initiating chemical carcniogens are highly reactive electrophiles (have atoms
deficient in electrons) that react with nucleopilic (electron rich) sites in the cell
 They target DNA, RNA, and proteins
o An initiator cause damage to DNA that can’t be repaired
 The mutated cell then passes on the DNA lesions to its daughter cells
o Chemicals that initiate carcinogenesis can be either direct acting or indirect
o Direct acting initiators – don’t need any metabolic conversion to become carcinogenic
 Most are weak carcinogens, but are important because some cancer chemo
drugs (like alkylating agents) have successfully cured, controlled or delayed
some cancers, only to then later evoke another kind of cancer
 The cancer they cause is usually acute myeloid leukemia
o Indirect-acting agents – chemicals that need metabolic conversion to a carcinogen
before they become active
 Polycyclic hydrocarbons are some of the most potent indirect chemical
carcinogens, and are in fossil fuels
 Can also get them from animal fats when you broil meats, and in
smoked meats and fish
 Hydrocarbons have epoxides, which form adducts on molecules in the
cell, mainly DNA, but also with RNA and proteins
 Benzopyrene is a polycyclic aromatic hydrocarbon that is an indirect-acting
carcinogen from cigarettes
 Most chemical carcinogens need metabolic activation for conversion into the
carcinogen – chart of how this happens page 310
 Other metabolic pathways though can inactivate (detox) the procarcinogen
 So the carcinogenic potency of a chemical is determined by both the inherent
reactivy of its electrophile derivative, and by the balance between metabolic
activation and inactivation rxns
 Most carcinogens are metabolized by cytochrome P-450 enzymes
 So susceptibility to cancers can be affected by the many differences
people have in the genes that code for P-450 enzymes
 Ex: 1/10 of people have a mutation to CYP1A1 that will cause it to
increase activation of benzopyrene, so smokers will have increased risk
for lung cancer
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Since malignant transformation happens from mutations, most initiating chemicals are
mutagenic and target DNA
 Alfatoxin B1 is an aspergillus mold that grows on improperly stored rice and nuts
in Africa and China, that causes hepatocellular carcinoma
 Aflatoxin mutates the p53 gene, by causing a “signature mutation”
called the 249 (ser) mutation, that won’t be seen by any other cause
o Unrepaired changes in DNA are an essential first step in initiation
 For the change to be inheritable, the DNA must be replicated
 So for initiation to happen, carcinogen-altered cells must undergo at least one
cycle of proliferation, so that the change in DNA becomes fixed
 Ex: in the liver, many chemicals are activated to reactive electrophiles, but most
won’t cause cancer unless the liver cells proliferate within a few days of the
making of DNA adducts
Carcinogenesis promoters can induce tumors in initiated cells, but promoters themselves are
not mutagenic
o Things that don’t cause mutation, but instead stimulate division, are called promoters
o Examples of promoters: phorbol esters, hormones, phenols, and drugs
o Promoters only cause problems when they show up after initiation, not before it
o So unlike initiators, cell changes from applying a promoter don’t affect DNA directly and
are reversible
o Promoters can make initiators more potent
o Application of promoters leads to proliferation and clonal expansion of initiated
(mutated) cells
 These cells don’t need as much growth factor to be stimulated, and can also be
less responsive to growth-inhibitor signals
o Since the cell is being driven to proliferate, the population gains more mutations,
developing eventually into a malignant tumor
Radiant energy is a carcinogen
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Radiant energy includes UV rays in sunlight and ionizing electromagnetic radiation
o UV light causes skin cancers, ionizing radiation exposure happens in medicine or at work
Radiation can also add to the effect of other carcinogens
UV rays from the sun increase the chance for squamous cell carcinoma, basal cell carcinoma,
and melanoma of the skin
The degree of risk depends on the type of UV ray, the intensity of exposure, and the amount of
melanin you have to absorb it and protect you
o Fair skinned people who are exposed to a lot of sun are most susceptible
o Melanoma is caused by an intense single exposure
o Nonmelanoma skin cancers are more from accumulation of UV exposure
UV-B radiation is the type of UV wave that causes skin cancer
o UV-B rays form pyrimidine dimers in DNA
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This type of DNA damage is fixed by the nucleotide excision repair pathway
With excessive sun exposure, the pathway gets overwhelmed, and damaged DNA gets
replicated
Electromagnetic rays (x rays, γ-rays) and particulate radiation (α and β particles, protons, and
neutrons) are ionizing radiations that can cause cancer
o Most often, the result is acute and chronic myeloid leukemia
Many viruses can cause cancer:
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The only RNA virus (retrovirus) that causes cancer is human T-cell leukemia virus type 1 (HTLV1)
o HTLV-1 causes a T-cell leukemia/lymphoma that is endemic in Japan and the Caribbean,
but sporadic elsewhere
o HTLV-1 targets CD4+ T cells, and can be spread through blood, sex, or breastfeeding
o Leukemia develops in only up to 5% of people infected, & it takes 40-60 years to happen
o HTLV-1 causes cancer through its tax gene, which makes Tax protein that can activate
transcription of host genes for proliferation and differentiation of the T cell, and
promote movement through the cell cycle
 HTLV-1 activates genes for FOS, Il-2, GM-CSF, NF-Kb, and inhibits p16/INK4a
o HTLV-1 causes cancer by making Tax protein, which causes mutations and genome
instability, and then increases T cell proliferation, making it more likely these mutations
get passed on
DNA viruses that cause cancer – Epstein Barr (EBV), HPV, HBV, HSV-8, and merkel cell
polyomavirus
o HPV:
 Some types of HPV just cause benign warts, while high risk HPVs like 16 and 18
cause squamous cell carcinoma of the cervix and anogenital region
 1/5 of oral cancers are from HPV
 Genital warts have low malignant potential, and are from low risk HPVs, mainly
HPV 6 and 11
 In benign warts, the HPV genome is maintained in an episome that is not
integrated into the host DNA
 In HPV cancers, the HPV genome is integrated into the host genome
 HPV DNA integration causes loss of the E2 protein, which was repressing the
virus, and overexpression of E6 and E7 oncoproteins
 E7 binds to RB, releasing E2F transcription factors to promote the cell cycle
 E7 from higher risk HPVs has a higher affinity for RB than E7 from lower
risk HPVs
 E7 also inactivates CDKI’s p21 and p27
 E6 protein binds p53 and pro-apoptotic BAX, and marks them for breakdown by
proteasomes
 E6 from higher risk HPVs has more affinity for p53 than lower risk HPV’s
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People with p53’s that code for an arginine at amino acid position 72,
are at more risk for HPV caused cervical cancer
 Infection by HPV alone isn’t enough to cause carcinogenesis, you also need the
cell to get a RAS gene mutation
 Cigarrete smoking can help HPV cause a cancer
 Most people infected with HPV have their immune system clear the infection
Epstein-Barr virus (EBV) – can cause Burkitt lymphoma, B-cell lymphomas, Hodkin
lymphoma, nasopharyngeal carcinoma, gastric carcinoma
 All of these except for nasopharyngeal carcinoma are B cell tumors
 EBV infects B cells by using their CD21 complement receptor
 The infection is latent, meaning there is no viral replication and the cells aren’t
killed
 The infected B cells though are immortalized and get the ability to divide
indefinitely
 EBV have an oncogene called latent membrane protein-1 (LMP-1) that causes B
cell lymphoma
 LMP-1 can act as a constantly turned on CD40 receptor, therefore
activating the B cell to proliferate
 LMP-1 activates NF-Kb and JAK/STAT pathways for B cell proliferation
 LMP-1 prevents apoptosis by activating BCL-2
 EBV also has EBNA-2 gene, leading to proliferation
 EBV can also turn host Il-10 into viral Il-10 (vIL-10), which prevents macrophage
from activating T cells, and is needed to turn a B cell malignant
 In people with normal immune systems, the B cell proliferation by EBV is
controlled, & the person is either asymptomatic, or gets has mononucleosis
 Burkitt lymphoma – B cell tumor that is the most common childhood tumor in
Africa
 Relationship between Burkitt lymphoma and EBV:
o Over 90% of African tumors carry the EBV genome
o Every patient with Burkitt lymphoma has increased antibodies
against viral capsid antigens
o These antibody titers are correlated with the risk of developing
a tumor
 Burkitt lymphoma involves more than just EBV though, since it’s so
common Africa, but rare outside it, and of those cases outside Africa,
only 1/5 show EBV genome
o Also, Burkitt lymphoma behaves different from EBV
transformed cells, and lacks LMP-1 and EBNA2
 Burkitt lymphoma happens by another infection, like malaria, impairing
the immune system, allowing B cell proliferation from EBV
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Eventually, T cells kick in and get rid of most of the EBV infected
cells, by targeting their LMP-1 and EBNA2
o A few of the EBV infected cells though down-regulate
expression of LMP-1 and EBNA2 and persist indefinitely, even
once normal immune system is restored
o From these cells, lymphoma cells can emerge once they get
certain mutations, especially when c-MYC oncogene is activated
o In cases where EBV didn’t cause Burkitt lymphoma, they still
have the c-MYC oncogene mutation
 So in Burkitt lymphoma, EBV isn’t directly oncogenic, but acts as a
mitogen that creates conditions that can lead to getting other
mutations that cause Burkitt lymphoma
 B cell lymphomas from being immunosuppressed can be caused by EBV
 Can happen in AIDS or those who are chronically immunosuppressed
because of a transplant
 These B cell lymphomas from EBV express LMP-1 and EBNA2, and aren’t
picked up by the T cells due to immunosuppression
 Every case of nasopharyngeal carcinoma is from EBV
 This is endemic in China, Africa, and the arctic
 You’ll see big increases in antibodies to EBV
 The EBV expresses LMP-1
Hepatitis B (HBV) and C (HCV)
 Most cases of hepatocellular carcinoma are caused by HBV
 Hepatitis causes liver cancer through chronic inflammation and damage
requiring constant proliferation to replace it, creating conditions for a mutation
to be replicated
 Inflammation causes repair, driven by growth factors, and uses immune
cells, which can make reactive oxygen species, which are toxic to genes
and mutagenic
 A key step in causing hepatocellular carcinoma is activation of the NF-kB
pathway in hepatocytes, which blocks apoptosis
 HCV isn’t a DNA virus, but also causes hepatocellular carcinoma
Helicobacter pylori is the only bacteria that is a carcinogen, and causes peptic ulcers, gastric
adenocarcinoma, and gastric lymphomas
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H. pylori causes chronic inflammation that increases epithelial cell proliferation to fix it
It starts as chronic gastritis, then gastric atrophy, then intestinal metaplasia of the lining cells,
then dysplasia, and then cancer
o This takes decades to happen, and only happens in 3% of infections
H. pylori also has cytotoxin-associated A gene (CagA)
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H. pylori is not invasive, but CagA penetrates into gastric epithelial cells, where it can
act as growth factor
H. pylori gastric lymphomas are from B cells
o Caused by promotion of Il-1β and TNF, and activation of T cells that then keep causing B
cell proliferation
o The B cell proliferation causes a “MALToma” that depends on continued stimulation by
T cells, especially activation of NF-Kb
o If by this point you cure the H.pylori with antibiotics, you cure the MALToma because it
no longer gets any stimuli
o If the MALToma gains a mutation from all this that then turns on NF-Kb continuously, it
no longer needs H. pylori, and becomes malignant and spreads
Host defense against tumors:
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Immune surveillance – the immune system normally surveys the body for developing malignant
cells, and destroys them
Many tumors have antigens that elicit an immune response to them
o Mutated protoncogenes and tumor suppressors create products that the immune
system has never seen before, so it goes and kills them
 These products are made in the cytoplasm, so just like any other protein in the
cytoplasm, they’re displayed by MHC1’s to be recognized by CD8+ T cells
 Phagocytes can also ingest these antigens and display them on MHC2’s to CD4+
T cells
o Tumor antigens can also be normal cell proteins that are overexpressed or
inappropriately expressed, triggering an immune response
 Ex: tyrosinase of melanin cells is overexpressed in melanomas, making products
that T cells go get
 Tyrosinase is so little made and in so few cells, that there’s probably no
immune tolerance to it, so it activates an immune response
 Ex: “cancer-testis” antigens are genes only found in the testis
 When in the testis, it’s not antigenic, because sperm don’t have MHC1’s
 So when these genes are expressed, there’s an immune response
 The main cancer-testis antigen is melanoma antigen gene (MAGE), seen
in many types of tumors
o Viruses that cause cancer can make proteins that the immune system reacts against
 The most potent viruses are latent DNA viruses, like HPV and EBV
 CTLs recognize their antigens and survey for tumors because they look for virusinfected cells
 Vaccines to these viruses can prevent the cancers caused by them
o Oncofetal antigens – proteins expressed highly by fetal and cancer cells, but not adult
tissues
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Genes for these antigens get silenced during development, and inhibited by
cancers
 The 2 most important oncofetal antigens are α-fetoprotein (AFP) and
carcinoembryonic antigen (CEA)
o Most tumors express more or abnormal forms of surface glycoproteins and glycolipids
 Can be used for diagnosis
 Antibodies can be made against these antigens
 Includes gangliosides GM2, GD2, and GD3
 Mucins are large glycoproteins with many O-linked carb side chains attached to
the core protein
 Tumors have enzymes that mess up the side chains, causing tumorspecific anitgens
 Tumor mucins include CA-125 and CA-19-9 in ovarian carcinomas, and
MUC-1 in breast carcinomas
o Tumors also express antigens that are on the normal cell it came from
 These are used as differentiation antigens to figure out what lineage the tumor
came from
 These antigens are normal & don’t generate an immune response to the tumor
 Ex: Lymphomas can be determined to come from B cells if they have B cell
antigens, like CD20
Cell mediated immunity is the main antitumor response
o Includes TC, NK cells, and macrophage
o NK cells can kill tumors without ever having seen them before, so they’re the first line
of defense
 NK cells are activated by Il-2 and Il-15
 NK cells can use NKG2D proteins as activating receptors for tumors
o T cells and NK cells work together
 If a tumor doesn’t have MHC1, T cells can’t come get it, but this lack of MHC1 is
a trigger for NK cells
o T cells and NK cells also release IFN-γ, which activates macrophage
Immunosuppressed people have less immune surveillance, so an increased risk for cancers to
develop
o Most tumors from immunosuppression are B cell lymphomas
Most cancers develop in people who aren’t immunodeficient though, so most tumors have
developed ways to evade immune defense
o During tumor progression, the clones that strongly trigger the immune system may be
wiped out, while less immunogenic ones persist
o Tumor cells often lack costimulators, so when a T cell finds it, it can react to its MHC,
but doesn’t get the second signal
o Many things that trigger cancer development also impair the immune system
o Tumors can also make things to suppress the immune system
 Ex: when tumors secrete a lot of TGF-β, it suppresses the immune system
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Tumors can also mask their antigens with glycocalyx stuff like sialic acid
Some tumors express FasL, causing any T cell that comes to get it to undergo apoptosis
Tumors are bad because they can impinge on structures, make hormones, cause bleeding and
infections, block blood to cause infarcts, and cause cachexia
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Cachexia – loss and wasting away of body fat and lean body mass, along with weakness,
anorexia, and anemia
o Unlike starvation, the weight loss involves equal amounts of fat and protein loss
o In people with cancer, the basal metabolic rate is increased, despite decreased food
intake
 Unlike the lower metabolic rate that happens in starvation
o The main cause of the cachexia is release of cytokines from the tumor and host
 Especially TNF, which mobilizes fats from the tissues and suppresses appetite
 They also can release stuff that mess up the body’s natural balance between
body building and body breakdown
o Cachexia hampers chemotherapy by limiting the doses you can use
o 1/3 of cancer deaths are from cachexia, rather than direct effects of the tumor
Paraneoplastic syndromes – symptoms expressed in a person with cancer that can’t be
explained by spread of the tumor or hormone making by the original tissue it came from
o Page 321 – table of paraneoplastic syndromes
o Endocrinopathies are common paraneoplastic syndromes
 Since the hormone is made by the tumor and not the usual tissue that makes it,
it’s called ectopic hormone making
 Cushing syndrome is the most common endocrinopathy
 About half of people with Cushing syndrome from an ectopic release of
ACTH have a lung carcinoma, usually small cell
 Lung cancer ACTH release also shows high pro-opiomelanocortin, which
doesn’t happen with excess ACTH from the pituitary
o The most common paraneoplastic syndrome is hypercalcemia
 Suddenly symptomatic hyeprcalcemia is most often from cancer instead of
hyperparathyroidism
 Cancer can cause hypercalcemia by either causing bone resorption, or releasing
calcium from the tumor
 Hypecalcemia from spread to bone though is not a paraneoplastic syndrome
 The most common hormone tumors use to cause hypercalcemia is parathyroid
hormone-related protein (PTHRP)
 It’s similar to parathyroid hormone (PTH), but not the same
 PTHRP and PTH use the same receptor
 PTHRP is made in small amounts by normal tissues, including
keratinocytes, muscles, bone, and ovary
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PTHRP regulates calcium transport in the lactating breast, across the
placenta, and works in lung remodeling
 So this is why tumors that cause paraneoplastic hypercalcemia are
usually carcinomas of the breast, lung, kidney, and ovary
 The most common lung cancer that causes hypercalcemia is squamous
cell bronchogenic carcinoma
Tumors can cause neuromuscular problems that are considered paraneoplastic
syndromes
Acanthosis nigricans – gray/black patches of warty hyperkeratosis on the skin
 Acanthosis nigricans can rarely happen from genetics, but half of case are from
a cancer, especially in people over 40
Cancer can cause hypertrophic osteoarthropathy, which is characterized by new bone
making at the distal ends of long bones and fingers and toes, arthritis of the joints
nearby, and clubbing of the digits
Grading a cancer is based on the degree of differentiation of the tumor cells, and sometimes the # of
mitoses
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These grades attempt to judge how much the tumor cells resemble the normal tissue it came
from
Ex: well-differentiated mucin-secreting adenocarcinoma vs poorly differentiated
adenocarcinoma
Staging of cancers is based on the size of the primary lesion, how much it has spread to reginal lymph
nodes, and the presence or absence metastases
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The major staging system used is the TNM system (T-primary tumor, N-node involvement, Mmetastasis)
o It goes T1-T4
o T0 is carcinoma in situ
o N0 – no nodes involved, N3 – many nodes involved
o M0 – no metastasis, M1 – spread
Histologic and cytologic methods of diagnosing cancer:
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Can do excision, biopsy, needle aspiration, and cytologic smears
You have to preserve the sample right by immersing it in a fixative solution (usually formalin) or
refrigerate it
“quick-frozen section” diagnosis allows histo evaluation within minutes and is highly accurate
o Used when it’s better to act quick, rather than waiting for a longer but probably more
thorough look at the sample
Fine needle aspiration is most often used for easily palpable lesions, like in the breast or lymph
nodes
o They’re less invasive and quicker than needle biopsies
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Cytologic (Pap) smears are used to screen for carcinoma of the cervix, but can also be used for
other cancers
o Shed cells on the smear are examined for anaplasia, which indicates they came from a
tumor
o Page 323 – difference between a normal and a mailignant pap smear of the cervix
 A normal smear shows large, flattened squamous cells with possibly some
metaplatic cells (you’re in the transformation zone)
 Malignant cells show pleomorphic, hyperchromatic nuclei
Immunochemistry – using antibodies to identify cell markers
o Antibodies can help you tell the difference between types of undifferentiated tumors,
because solid tumors often have intermediate filaments characteristic of the cells they
came from
 Ex: immunochemistry can find cytokeratins, telling you it came from an
epithelial cell (carcinoma), and desmin, telling you it came from a muscle cell
o Antibodies can tell you where a metastatic tumor came from (ex: PSA and thyroglobulin)
o Antibodies can detect things in tumors that help you give a prognosis of the tumor
Molecular techniques – used for malignant cancers and give their prognosis, and look for
hereditary predispositions to cancer
o Molecular techniques are used to differentiate benign lymphocyte proliferation
(polyclonal) from malignant (monoclonal), by looking at their antigen receptor
(TCR/BCR)
o One important molecular technique is PCR
 PCR can be used to look for minimal residual disease, which is when after
treatment there’s still a tiny bit of cancer left that can lead to recurrence
o “Special karyotyping” can detect all types of chromosomal rearrangements in tumor
cells, and is very sensitive
o Comparative genomic hybridization lets you look at chromosome gains and losses in
tumor cells
o FISH is another important molecular technique
o Molecular techniques can be used to see if you have a hereditary predisposition to
getting cancer
Being able to look at all the genes in a tumor at once, and create a molecular profile, is a great way to
examine tumors now
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This is done by DNA microarray – page 326 is a diagram of this
You put either PCR products from cloned genes, or oligonucleotide homologs of genes you
wanna look at, onto a slide
A gene chip is then made with complementary probes to the genes and to control samples
o The probes are usually complementary DNA copies of RNAs extracted from tumor and
uninvolved tissues, that have been labeled with a fluorochrome
After hybridization, you read the chip with a laser scanner
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This can allow to form a hierarchical clustering, which shows a short list of genes that are
differentially expressed between types of tumors cells, or tumor and nontumor cells
This “signature” can then be used to predict the behavior of the tumor
Tumor markers like enzymes and hormones can’t be used to definitively diagnose a cancer, but helps
you diagnose one and can help see how effective therapy is or look for recurrence – page 327
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Prostate specific antigen (PSA) is used to screen for prostate adenocarcinoma
o Prostate cancer can be suspected when you find increased PSA in the blood
o PSA can also be increased though in benign prostatic hyperplasia
o Also, there is no PSA level that ensures you don’t have cancer
o So PSA tests have both low sensitivity and specificity
Other widely used markers include:
o Carcinoembryonic antigen (CEA) – colon, pancreas, stomach, and breast cancer
o α-fetoprotein (AFP) – hepatocellular carcinoma, yolk sac tumors, and the gonads
o hCG – testicular cancer
o CA-125 – ovarian tumors
o Antibodies – multiple myeloma