Path Ch 7 Pg 260-316 (chapter continues a bit further)
Nomenclature
 Neoplasm – new growth; study thereof is oncology
 All tumors, benign and malignant, have 2 basic components: cloncal neoplastic cells that constitute parenchyma
and reactive stroma made of CT
o Growth and evolution critically dependent on stroma
 Adequate stromal blood supply required to live and divide
 Stromal CT provides essential structural framework
 Cross-talk between tumor cells and stromal cells influences growth of tumors
o Desmoplasia – abundant collagenous stroma; firm, stony (schirrhous)
 Benign tumors designated by attaching –oma to cell of origin
 Adenoma – benign epithelial neoplasm derived from glands (may or may not form glandular structures)
 Papilloma – benign epithelial neoplasm producing finger-like or warty projections from epithelial surface
 Cystadenomas – benign epithelial tumor that forms large cystic masses
 Polyp – neoplasm that produces macroscopically visible projection above mucosal surface and into lumen
 Sarcoma – malignant tumor arising in mesenchymal tissue (e.g., fibrosarcoma, chondrosarcoma, etc.)
 Carcinoma – malignant neoplasm of epithelial cell origin derived from any germ layer
o Squamous cell carcinoma – tumor cells resemble stratified squamous epithelium
o Adenocarcinoma – neoplastic epithelial cells grow in glandular patterns
 Mixed tumor – divergent differentiation of single neoplastic clone along 2 lineages
o Pleomorphic adenoma – salivary gland tumor w/epithelial components in a myxoid stroma that contains
islands of cartilage or bone
 Teratoma – contains recognizable mature or immature cells or tissues representative of more than one germ cell
layer; originate from totipotential cells (from ovary, testis, midline embryonic rests)
o Benign (mature) teratoma – all component parts well-differentiated
o Malignant teratoma – immature, less well-differentiated
o Cystic teratoma (ovarian) principally differentiates along ectodermal lines (skin, hair, sebaceous, tooth)
 Hamartoma – disorganized, benign-appearing masses of cells indigenous to that particular site
 Choristoma – heterotopic rest of cells
o Small nodule of well-developed and normally organized pancreatic substance in submucosa of stomach
Characteristics of Benign and Malignant Neoplasms
 Anaplasia: lack of differentiation in a tumor; more mitoses than well-differentiated tumors; associated w/
o Pleomorphism – variation in size and shape of cells
o Abnormal nuclear morphology – hyperchromatic nuclei disproportionately large for cell; nuclear shape
variable and irregular; chromatin clumped and distributed along nuclear membrane; large nucleoli
o Mitoses – expansion of parenchymal cells; atypical mitotic figures producing multipolar spindles
o Loss of polarity – sheets or large masses of tumor in disorganized fashion
o Formation of tumor giant cells (single huge polymorphic nucleus or large hyperchromatic nuclei)
o Vascular stroma scant; large central areas undergo ischemic necrosis
 Metaplasia – replacement of one type of cell w/another (esophagus in GERD)
 Dysplasia – disordered growth; often occurs in metaplastic epithelium; loss of uniformity of individual cells as
well as loss of architectural orientation; pleomorphism and large hyperchromatic nuclei
o More abundant mitotic figures (normal configuration, but abnormal location)
o Mild to moderate changes can revert back to normal if harmful stimulus removed
 Carcinoma in situ – marked dysplastic changes; involve entire thickness of epithelium; pre-invasive neoplasm
o Once tumor cells breach BM, tumor is invasive (may take years or never happen)
 Well-differentiated tumors follow the function of whatever cell they are (not necessarily regulated); anaplastic
tumors can have completely different functions start (carcinomas of non-endocrine organs can produce variety
of hormones)
 By the time a solid tumor clinically detected, it has already completed a major portion of its life span
o Dividing cells don’t complete cell cycle more rapidly (actually take longer for tumor cells)
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Growth fraction – proportion of cells in tumor in proliferative pool; often 20% by time tumor detected
Fast-growing tumors have high cell turnover (rates of proliferation and apoptosis high)
Growth fraction has effect on susceptibility to chemotherapy because this it only works on cells in cycle
 In low growth fraction tumors (breast, colon), shift cells from G0 into cell cycle by debulking
tumor in surgery or w/radiation
 More aggressive tumors can be cured by chemotherapy alone
 Hormonal stimulation, adequacy of blood supply, and unknown influences affect growth rate (unpredictable)
o Leiomyoma (benign smooth muscle tumor in uterus) grows faster when estrogen higher; after
menopause, may atrophy and be replaced by collagenous (sometimes calcified) tissue
 Some cancers express MDR1 that counteract effects of chemotherapeutic drugs
 Chronic myelogenous leukemia (CML) originates from malignant counterpart of normal hematopoietic stem cell
 Acute myeloid leukemias (AMLs) derived from more differentiated myeloid precursors that acquire abnormal
capacity for self-renewal
 Tumor initiating cells (T-ICs) – cells that allow human tumor to grow and maintain itself indefinitely
 BMI1 – component of polycomb chromatin-remodeling complex that promotes stem-ness in hematopoietic and
leukemic stem cells
 WNT pathway – key regulator of normal colonic crypt stem cells implicated in maintenance of colonic
adenocarcinoma stem cells
 Benign tumors develop rim of compressed CT (fibrous capsule) because they grow so slowly; derived largely
from ECM of native tissue due to atrophy of normal parenchymal cells under pressure of expanding tumor
 Growth of cancers accompanied by progressive infiltration, invasion, and destruction of surrounding tissue
o Invasiveness differentiates benign from malignant tumors
o In situ epithelial cancers display cytologic features of malignancy w/o invasion of BM
 All malignant tumors can metastasize except gliomas and basal cell carcinomas of skin (still locally invasive)
 Dissemination occurs through direct seeding of body cavities or surfaces, lymphatic spread, or hematogenously
o Pseudomyxoma peritonei – when mucus-secreting appendiceal carcinomas fill peritoneal cavity
w/gelatinous neoplastic mass
o Transport through lymphatics is most common way of initial dissemination of carcinomas (sarcomas use
both lymphatic and hematogenous)
o Skip metastasis – when local lymph nodes bypassed because of venous-lymphatic anastomoses,
inflammation, or radiation obliterating lymph channels
o Sentinel node biopsy – biopsy of first node in regional lymphatic basin that receives lymph flow from
primary tumor
o Enlargement of lymph nodes may be caused by spread and growth of cancer cells or reactive
hyperplasia (cancer cells have come here but are trapped and being killed by immune system)
o Arteries less easily penetrated than veins (thicker walls); blood-borne cells follow venous flow draining
site of neoplasm; tumor cells often come to rest in first capillary bed they encounter (liver & lungs)
o Cancers arising in close proximity to vertebral column often embolize through paravertebral plexus;
involved in frequent vertebral metastases of carcinomas of thyroid and prostate
Epidemiology
 High dietary fat and low fiber implicated in development of colon cancer
 Pre-neoplastic disorders – diseases associated w/increased risk of developing cancer
 Most common tumors in men: prostate, lung, and colorectum
 Most common tumors in women: breast, lung, and colorectum
 Obesity – significant risk factor for developing cancer
 Alcohol and tobacco synergistically increase danger of incurring cancers in upper aerodigestive tract
 Autosomal dominant inherited cancer syndromes – inherited mutation occurs on one allele, and silencing of
second allele occurs in somatic cells as result of deletion or recombination
o Childhood retinoblastoma is example caused by mutation in RB tumor suppressor gene; also have
greatly increased risk of developing second cancer (esp. osteosarcoma)
o Familial adenomatous polyposis – mutation of APC tumor suppressor gene
o Li-Fraumeni syndrome – germline mutations in p53 gene
o MEN-1 caused by mutation in gene that encodes menin transcription factor
o MEN-2 caused by mutation in RET tyrosine kinase gene
o Hereditary nonpolyposis colon cancer (HNPCC) caused by inactivation of DNA mismatch repair gene
 In inherited cancer syndromes, tumors arise in specific sites and tissues; no increase in predisposition to cancers
in general (MEN-2 affects thyroid, parathyroid, and adrenals; MEN-1 affects pituitary, parathyroid, & pancreas)
o Often associated w/specific marker phenotype; sometimes abnormalities in tissue that isn’t target of
transformation (Lisch nodules and café-au-lait spots in neurofibromatosis)
 Defective DNA-repair syndromes – autosomal recessive pattern of inheritance; include xeroderma
pigmentosum, ataxia-telangiectasia, and Bloom syndrome; also HNPCC (but is autosomal dominant)
 Familial cancers – occurs in families w/o clearly defined pattern of transmission; early age at onset, tumors in 2+
close relatives of index case, sometimes multiple or bilateral tumors
o Susceptibility to cancer depends on multiple low-penetrance alleles, each contributing to small increase
in risk of tumor development
o BRCA1 and BRCA2 mutations increase susceptibility to breast cancer, but only present in 3% of pts
o Mutation in p16 tumor suppressor gene accounts for only 20% of familial melanomas
 Cancer develops at sites of chronic inflammation – compensatory proliferation of cells, aided by growth factors,
cytokines, chemokines, and other bioactive substances produced by activated immune cells that promote cell
survival, tissue remodeling, and angiogenesis
o May increase pool of tissue stem cells (subject to effect of mutagens)
o Activated immune cells produce ROS that are directly genotoxic
o Mediators promote cell survival, even w/genetic damage
o COX-2 (induced by inflammatory stimuli) increased in colon cancers
 Precancerous conditions – certain non-neoplastic disorders w/well-defined association w/cancer; include
chronic atrophic gastritis of pernicious anemia, solar keratosis of skin, chronic ulcerative colitis, and leukoplakia
of oral cavity, vulva, and penis
o In great majority of lesions, no malignant neoplasm emerges, but risk is just increased
Molecular Basis of Cancer
 Nonlethal genetic damage is basis of carcinogenesis; environmental risks: any acquired defect caused by
exogenous agents or endogenous products of cell metabolism
 Most commonly used method to determine tumor clonality involves analysis of methylation patterns adjacent
to highly polymorphic locus of human androgen receptor gene (AR)
 4 classes of normal regulatory genes are principal targets of genetic damage
o Growth-promoting proto-oncogenes (dominant because they transform cells despite presence of
normal gene on other allele)
o Growth-inhibiting tumor suppressor genes (recessive; requires loss of both alleles; loss of gene function
because of damage to single allele = haploinsufficiency)
o Genes that regulate apoptosis; may behave as proto-oncogenes or tumor suppressor genes
o Genes involved in DNA repair; don’t directly transform cells, but can let those w/mutations live
 Mutator phenotype – cells w/mutations in DNA repair genes
 Certain miRNAs can act as either oncogenes or tumor suppressors by affecting translation of other genes
 Carcinogenesis is multistep process resulting from accumulation of multiple mutations; by time malignant
tumors clinically evident, constituent cells extremely heterogeneous (affected by immune and nonimmune
selection pressures)
Essential Alterations for Malignant Transformation
 Self-sufficiency in growth signals – proliferate w/o external stimuli, usually as consequence of oncogene
activation
 Insensitivity to growth-inhibitory signals – may not respond to TGF-β or direct inhibitors of CDKIs
 Evasion of apoptosis – can be inactivation of p53 or activation of anti-apoptotic genes
 Limitless replicative potential – avoids cellular senescence and mitotic catastrophe
 Sustained angiogenesis
 Ability to invade and metastasize – depends on processes intrinsic to cell or initiated by signals from tissues
 Defects in DNA repair – leads to genomic instability and mutations in proto-oncogenes and tumor suppressors
Self-Sufficiency in Growth Signals: Oncogenes
 Oncogenes – genes that promote autonomous cell growth in cancer cells; unmutated yet = proto-oncogenes
o Characterized by ability to promote cell growth in absence of normal growth-promoting signals
o Products of oncogenes often devoid of important internal regulatory elements; production in
transformed cells doesn’t depend on growth factors or other external signals
 Under physiologic conditions, cell proliferation takes place by: binding of growth factor to specific receptor,
transient and limited activation of growth factor receptor (activates several signal-transducing proteins on inner
leaflet of PM), transmission of transduced signal across cytosol to nucleus via second messengers or by cascade
of signal transduction molecules, induction and activation of nuclear regulatory factors that initiate DNA
transcription, and entry and progression of cell into cell cycle, resulting in cell division
 Proteins encoded by proto-oncogenes function as growth factors or their receptors, signal transducers,
transcription factors, or cell cycle components
o Mutations convert proto-oncogenes into constitutively actively cellular oncogenes involved in tumor
development because oncoproteins endow cell w/self-sufficiency in growth
 Most soluble growth factors made by one cell type and act on neighboring cell to stimulate proliferation
(paracrine action); many cancer cells acquire ability to synthesize same growth factors to which they are
responsive (autocrine loop)
o Many glioblastomas secrete PDGF and express receptor
o Many sarcomas make TGF-α and its receptor
o In most instances, growth factor gene itself is not altered or mutated; products of other oncogenes that
lie along many signal transduction pathways (RAS) cause overexpression of growth factor genes, forcing
cells to secrete large amounts
o Growth factor proliferation increases risk of spontaneous or induced mutations in proliferating cells
 Growth factor receptors – transmembrane proteins w/external ligand-binding domain and cytoplasmic tyrosine
kinase domain; in normal forms, kinase transiently activated by binding of specific growth factors, followed
rapidly by receptor dimerization and tyrosine phosphorylation of substrates of signaling cascade
o Oncogenic versions of receptors associated w/constitutive dimerization and activation w/o binding
o RET proto-oncogene (receptor tyrosine kinase) has oncogenic conversion via mutations and gene
rearrangements
 RET protein – receptor for glial cell line-derived neutrotrophic factor and structurally related
proteins that promote cell survival during neural development; normally expressed in
neuroendocrine cells (parafollicular C cells of thyroid, adrenal medulla)
 Point mutations associated w/dominantly-inherited MEN types 2A and 2B and familial medullary
thyroid carcinoma
 MEN-2A – point mutations in RET extracellular domain cause constitutive dimerization and
activation, leading to medullary thyroid carcinomas and adrenal and parathyroid tumors
 MEN-2B – point mutations in cytoplasmic catalytic domain alter substrate specificity of tyrosine
kinase and lead to thyroid and adrenal tumors w/o involvement of parathyroid
o Point mutations in FLT3 lead to constitutive signaling detected in myeloid leukemias
o Certain chronic myelomonocytic leukemias – entire cytoplasmic domain of PDGF receptor fused
w/segment of ETS family transcription factor, resulting in permanent dimerization of PDGF receptor
o 90% of GI stromal tumors have constitutively activating mutation in receptor tyrosine kinase for c-KIT
(receptor for stem cell factor) or PDGFR (receptor for PDGF); amenable to specific inhibition by tyrosine
kinase inhibitor imatinib mesylate (targeted therapy)
 ERBB1 (normal EGF receptor gene) overexpressed in up to 80% of squamous cell carcinomas of lung, in 50% of
glioblastomas, and in 80-100% of head and neck tumors
o ERBB2 (HER-2/NEU) amplified in 25% of breast cancers and human adenocarcinomas arising in ovary,
lung, stomach, and salivary glands; molecular alteration specific for cancer cells
 Some oncoproteins mimic function of normal cytoplasmic signal-transducing proteins; most located on inner
leaflet of PM, where they receive signals from outside cell (activation of growth factor receptors) and transmit
them to cell nucleus
 RAS genes (HRAS, KRAS, and NRAS) – point mutation single most common abnormality of proto-oncogenes in
human tumors
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Carcinomas (particularly colon and pancreas) have mutations of KRAS
Bladder tumors have HRAS mutations
Hematopoietic tumors bear NRAS mutations
Plays role in signaling cascades downstream of growth factor receptors, resulting in mitogenesis;
abrogation of RAS function blocks proliferative response to EGF, PDGF, and CSF-1
 Normal RAS proteins tethered to cytoplasmic aspect of PM as well as ER and Golgi membranes;
activated by growth factor binding to receptors at PM
 Bind guanosine nucleotides
 Normally flip back and forth between excited signal-transmitting state and quiescent state; in
inactive state, RAS bind GDP; stimulation changes GDP for GTP (activating RAS)
 Activated RAS stimulates downstream regulators of proliferation (MAP kinase cascade)
 Orderly cycling of RAS protein depends on nucleotide exchange (GDP for GTP) and GTP
hydrolysis which converts GTP-bound active RAS to GDP-bound inactive form
o GTPase activity intrinsic to normal RAS proteins dramatically accelerated by GTPase-activating proteins
(GAPs); function as brakes that prevent uncontrolled RAS activity
o Mutations that cause cancer either in GTP-binding pocket or enzymatic region essential for GTP
hydrolysis; mutated RAS trapped in activated GTP-bound form, forcing cell to proliferate continuously
o Mutations in downstream members of RAS signaling cascade (RAS/RAF/MAP kinase) altered in some
cancer cells
 Mutations in BRAF (RAF family) detected in 60% of melanomas and 80% of benign nevi; BRAF
mutations along lead to oncogene-induced senescence giving rise to benign nevi rather than
malignant melanoma
 Oncogene-induced senescence barrier to carcinogenesis that must be overcome by mutation
and disabling of key protective mechanisms (provided by p53 gene)
C-ABL tyrosine kinase – in CML and some acute lymphoblastic leukemias, ABL gene translocated to fuse w/BCR
gene; resultant chimeric gene encodes constitutively active oncogenic BCR-ABL tyrosine kinase
o BCR moiety promotes self-association of BCR-ABL
o Tx w/imatinib mesylate (low toxicity and high therapeutic efficacy that inhibits BCR-ABL kinase)
o Signaling through BCR-ABL gene required for tumor to persist
Polycythemia vera and primary myelofibrosis associated w/activating point mutations in tyrosine kinase JAK2,
which activates STAT transcription factors, which promote growth factor-independent proliferation and survival
of tumor cells
All signal transduction pathways converge to nucleus, where responder genes that orchestrate cell’s cycle
activated; ultimate consequence of signaling through oncogenes is inappropriate continuous stimulation of
nuclear transcription factors that drive growth-promoting genes
o Transcription factors contain specific amino acid sequences that allow them to bind DNA or dimerize for
DNA binding; binding of proteins to specific sequences of genomic DNA activates transcription of genes
o Growth autonomy can occur as result of mutations affecting genes that regulate transcription (MYC,
MYB, JUN, FOS, and REL oncogenes)
MYC proto-oncogene expressed in virtually all eukaryotic cells; immediate early response genes (rapidly induced
when quiescent cells receive signal to divide)
o After transient increase of MYC mRNA, expression declines to basal level
o Some target genes (ornithine decarboxylase and cyclin D2) associated w/cell proliferation
o Activities modulated by MYC include histone acetylation, reduced cell adhesion, increased cell motility,
increased telomerase activity, increased proein synthesis, decreased proteinase activity, and changes in
cell metabolism that enable high rate of cell division
o Can act in concert to reprogram somatic cells into pluripotent stem cells
o Cells in culture undergo apoptosis if MYC activation occurs in absence of survival signals (growth factors)
 Proto-oncogene contains domains that encode growth-promoting and apoptotic activities
o Persistent expression or overexpression commonly found in tumors
Cyclin-dependent kinases (CDKs) orchestrate progression of cells through cycle; activated by binding to cyclins
o CDK-cyclin complexes phosphorylate crucial target proteins that drive cell through cell cycle, then cyclin
levels decline rapidly
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Mutations in cyclin D or CDK4 common in neoplastic transformation
Inhibitors (CDKIs) silence CDKs and exert negative control over cell cycle
 CIP/WAF family of CDKIs composed of p21 (CDKN1A), p27 (CDKN1B), and p57 (CDKN1C); inhibits
CDKs broadly
 INK4 family of CDKIs made of p15, p16, p18, and p19; selective effects on cyclin D/CDK4 and
cyclin D/CDK6; expression down-regulated by mitogenic signaling pathway, promoting
progression of cell cycle
 p27 inhibits cyclin E; expressed throughout G1; mitogenic signals dampen activity of p27,
relieving inhibition of cyclin E-CDK2, allowing cell cycle to proceed
 CDKIs frequently mutated or silenced in malignancies
 Germline mutations in p16 associated w/25% of melanoma-prone kindreds
 Inactivation of p16 in pancreatic carcinoma, glioblastoma, esophageal cancer, acute
lymphoblastic leukemia, and non-small-cell lung carcinomas, soft-tissue sarcomas, and
bladder cancers
 S phase is point of no return in cell cycle; cell checks for DNA damage at G1/S checkpoint, and if damage, DNArepair mechanisms arrest cell cycle; if damage not repairable, apoptosis pathways activated
 G2/M checkpoint monitors completion of DNA replication and checks whether cell can safely initiate mitosis and
separate sister chromatids; particularly important in cells exposed to ionizing radiation
o Cells damaged by ionizing radiation activate G2/M checkpoint and arrest in G2
o Defects in checkpoint give rise to chromosomal abnormalities
 Sensors at checkpoints include RAD family of proteins and ATM; transducers are CHK kinase families
o Cell cycle arrest mediated through p53, which induces cell cycle inhibitor p21
Insensitivity to Growth Inhibition and Escape from Senescence: Tumor Suppressor Genes
 Products of tumor suppressor genes form network of checkpoints that prevent uncontrolled growth
 Expression of oncogene in otherwise normal cell leads to quiescence (permanent cell cycle arrest)
 Growth-inhibitory, pro-differentiation signals originate outside cell and use receptors, signal transducers, and
nuclear transcription regulators to accomplish effects
 Protein products of tumor suppressor genes function as transcription factors, cell cycle inhibitors, signal
transduction molecules, cell surface receptors, and regulators of cellular responses to DNA damage
 2-hit hypothesis of oncogenesis: 2 mutations (hits) involving both alleles of RB required to produce
retinoblastoma; in familial cases, children inherit one defective copy of RB gene and retinoblastoma develops
when normal RB allele mutated in retinoblasts as result of spontaneous somatic mutation (autosomal
dominant); in sporadic cases, both normal RB alleles must undergo somatic mutation in same retinoblast
o RB exists in active hypophosphorylated state in quiescent cells and inactive hyperphosphorylated state
in G1/S cell cycle transition
o In G1, cells can exit cell cycle temporarily (quiescence) or permanently (senescence); RB is key node in
this decision
o Initiation of DNA replication requires activity of cyclin E-CDK2 complexes; expression of cyclin E
dependent on E2F family of transcription factors; early in G1, RB in hyperphosphorylated active form;
binds to and inhibits E2F family of transcription factors, preventing transcription of cyclin E
o Active RB blocks E2F-mediated transcription by sequestering E2F (preventing it from interacting w/other
transcriptional activators) and recruits chromatin-remodeling proteins (histone deacetylases and histone
methyltransferases) which bind promoters of E2F-responsive genes (cyclin E)
o Enzymes modify chromatin to make promoters insensitive to transcription factors
o Mitogenic signaling leads to cyclin D expression and activation of cyclin D-CDK4/6 complexes that
phosphorylate RB, inactivating it and releasing E2F to induce cyclin E
o Expression of cyclin E stimulates DNA replication and progression through cell cycle
o During ensuing M phase, phosphate groups removed from RB by cellular phosphatases, regenerating
active form of RB
o RB also controls stability of cell cycle inhibitor p27
o Mutations of RB genes found in tumors localized to RB pocket involved in binding to E2F
o RB pathway couples control of cell cycle progression at G1 w/differentiation (stimulates diff.)
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At least 1 of 4 key regulators of cell cycle (p16/INK4a, cyclin D, CDK4, or RB) dysregulated in vast
majority of cancers
LOH – loss of heterozygosity; when one gene was inherited defective and other gene mutates
Von Hippel-Lindau (VHL) gene – tumor suppressor, causes familial and sporadic clear cell renal carcinomas
Transforming proteins of several oncogenic DNA viruses act by neutralizing growth-inhibitory activities of RB; RB
protein functionally inactivated by binding of viral protein and no longer acts as cell cycle inhibitor
o Polyomavirus large T antigens, adenoviruses EIA protein, and HPV E7 protein all bind active RB
o HPV binding particularly strong for type 16 HPV; high risk for development of cervical carcinoma
 RB protein unable to bind E2F and is functionally inactivated
A little over 50% of human tumors contain p53 mutation; homozygous loss seen in virtually every type of cancer
o Li-Fraumeni syndrome – inherit only 1 working p53 copy; 25x greater chance of developing malignant
tumor by age 50 than general population
 Most common tumors are sarcomas, breast cancer, leukemia, brain tumors, and carcinomas of
adrenal cortex
 Tumors occur at younger age and may develop multiple primary tumors
o p53 acts as molecular policeman that prevents propagation of genetically damaged cells
 Transcription factor at center of large network of signals that sense cellular stress (DNA damage,
shortened telomeres, and hypoxia)
o 80% of mutations in cancer located in DNA-binding domain
o Transforming proteins of several DNA viruses (i.e., E6 protein of HPV) bind to and promote degradation
of p53
o MDM2 and MDMX stimulate degradation of p53; proteins frequently overexpressed in malignancies
o p53 thwarts neoplastic transformation by activation of quiescence, induction of senescence, or
triggering apoptosis
o In non-stressed cells, p53 has short half-life because of association w/MDM2 (targets it for destruction)
 When stressed, p53 undergoes post-transcriptional modifications that release it from MDM2
and increase half-life; also becomes activated as transcription factor by detaching
 Genes triggered by p53 cause cell cycle arrest or initiate apoptosis
o p53 activates transcription of mir34 family of miRNAs (mir34a-mir34c) that bind cognate sequences in 3’
untranslated region of mRNAs, preventing translation
 Blocking mir34 severely hampers p53 response in cells; ectopic expression of mir34 w/o p53
activation sufficient to induce growth arrest and apoptosis
 Targets of mir34s include pro-proliferative genes (cyclins) and anti-apoptotic genes (BCL2)
o Key initiators of DNA-damage pathway: ATM and ATR
 ATM is germline mutation in ataxia-telangiectasia (inability to repair certain kinds of DNA
damage, increased incidence of cancer)
 Once triggered, both ATM and ATR phosphorylate targets (p53 and DNA-repair proteins)
o Cell cycle arrest occurs late in G1 phase; caused mainly by p53-dependent transcription of CDKN1A
(p21), which inhibits cyclin-CDK complexes and phosphorylation of RB, preventing cells from G1
 p53 induces proteins (GADD45) that help in DNA repair
 If DNA damage repaired, p53 upregulates MDM2, leading to destruction of p53; if not repaired,
cell enters p53-induced senescence or undergo p53-directed apoptosis
o Senescence due to p53 – requires activation of p53 and/or RB and expression of mediators (CDKIs);
generally irreversible (may require continued expression of p53)
 Involves epigenetic changes that result in formation of heterochromatin at different loci
throughout genome; senescence-associated heterochromatin foci include pro-proliferative
genes regulated by E2F and permanently alters expression of E2F targets
 Stimulated in response to unopposed oncogene signaling, hypoxia, and short telomeres
o Apoptosis due to p53 – p53 directs transcription of pro-apoptotic genes (BAS and BBC3 (PUMA))
 Affinity of p53 for promoters and enhancers of DNA-repair genes stronger than affinity for proapoptotic genes, so DNA-repair pathway stimulated first
 If DNA not repaired, enough p53 accumulates to stimulate transcription of apoptotic genes
o Family members of p53: p63 and p73 show more tissue specificity
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p63 essential for differentiation of stratified squamous epithelia
p73 has strong pro-apoptotic effects after DNA damage induced by chemotherapeutic agents
Both p63 and p73 expressed as different isoforms (some act as transcriptional activators and
others function as dominant negatives)
 Perturbation of p53-p63-p73 network contributes to chemoresistance and poor prognosis
APC – germline mutations associated w/FAP; polyps undergo malignant transformation  colon cancer
o Both copies must be lost for tumor to arise
o 70-80% of non-familial colorectal carcinomas and sporadic adenomas show homozygous loss of APC
o APC is component of WNT signaling pathway; has role in controlling cell fate, adhesion, and cell polarity
during embryonic development
o WNT signaling required for self-renewal of hematopoietic stem cells
o WNT signals through FRZ family of cell surface receptors, stimulates several pathways (including one
w/β-catenin and APC)
o In absence of WNT, APC causes degradation of β-catenin, preventing its accumulation in cytoplasm
 Forms macromolecular complex w/ β-catenin, axin, and GSK3β, leading to phosphorylation and
ubiquitination of β-catenin and destruction by proteasome
 Signaling by WNT blocks APC-AXIN-GSK3β destruction complex, allowing β-catenin to
translocate from cytoplasm to nucleus, where it forms complex w/TCF (upregulates cellular
proliferation by increasing c-MYC, cyclin D1, etc.)
o Colon tumors w/normal APC genes have β-catenin mutations that prevent destruction by APC
o Mutations in β-catenin gene present in hepatoblastomas and hepatocellular carcinomas
o β-catenin binds to cytoplasmic tail of E-cadherin (surface protein that maintains intercellular
adhesiveness); loss of cell-cell contact disrupts interaction between E-cadherin and β-catenin, allowing
β-catenin to travel to nucleus and stimulate proliferation
 Re-establishment of E-cadherin contacts as wound heals; leads to β-catenin being sequestered
at membrane and reduction in proliferative signal (contact inhibition)
 Loss of contact inhibition by E-cadherin/ β-catenin mutation key in carcinomas
o Loss of cadherins favors malignancy by allowing easy disaggregation of cells (invade or metastasize)
o Germline mutations in E-cadherin predispose to familial gastric carcinoma
INK4a/ARF locus encodes p16/INK4a CDKI (blocks cyclin D/CDK2-mediated phosphorylation of RB, keeping RB
checkpoint in place) and P14/ARF (activates p53 pathway, inhibiting MDM2 and preventing destruction of p53)
o Both products function as tumor suppressors; mutation or silencing here affects both RB and p53
o Mutations at p16 lack induction of senescence (tumors of head, neck, bladder; cholangiocarcinoma)
o In cervical cancer, p16/INK4a silenced by hypermethylation of gene w/o presence of mutation
TGF-β – potent inhibitor of proliferation; binds to serine-threonine kinase complex composed of TGF-β receptors
I and II  dimerization of receptor  activation of kinase and phosphorylation of R-SMADs  R-SMADs enter
nucleus and bind to SMAD-4  activate transcription of genes (CDKIs p21 and p15/INK4b)
o TGF-β signaling leads to repression of c-MYC, CDK2, CDK4, and cyclins A & E (decreased phosphorylation
of RB and cell cycle arrest)
o Mutations affect type II TGF-β receptor or interfere w/SMAD molecules that transduce anti-proliferative
o In 100% of pancreatic cancers and 83% of colon cancers, at least one component of TGF-β pathway
mutated
o In many cancers, loss of TGF-β mediated growth inhibition occurs downstream of core signaling pathway
(loss of p21 and/or persistent expression of c-Myc)
 Tumor cells can use other elements of TGF-β-induced program, including immune system
suppression/evasion or promotion of angiogenesis, to facilitate tumor progression
o TGF-β functiosn as either prevention or promotion of tumor growth depending on state of other genes
PTEN – membrane-associated phosphatase encoded by gene mutated in Cowden syndrome (autosomal
dominant disorder marked by frequent benign growths (tumors of skin appendages) and increased incidence of
epithelial cancers (esp. breast, endometrium, and thyroid)
o PTEN acts as tumor suppressor by serving as brake on pro-survival/pro-growth PI3K/AKT pathway
(normally stimulated w/RAS and JAK/STAT pathways) when ligands bind receptor tyrosine kinases 
cascade of phosphorylation events

PI3K phosphorylates inositide-3-phosphate to give rise to inositide-3,4,5-triphosphate, which
binds and activates PDK1
 PDK1 and other factors phosphorylate and activate AKT; by phosphorylating substrates
(including BAD and MDM2), AKT enhances cell survival and inactivates TSC1/TSC2 complex
 TSC1 and TSC2 – products of tumor suppressor genes mutated in tuberous sclerosis (autosomal
dominant disorder w/developmental malformations and benign neoplasms such as cardiac
rhabdomyomas, renal angiomyolipomas, and giant cell astrocytomas
o Inactivation of TSC1/TSC2 unleashes activity of mTOR (potent immunosuppressant) that stimulates
uptake of nutrients such as glucose and amino acids needed for growth and augments activity of factors
required for protein synthesis
 NF1 – mutation here develops numerous benign neurofibromas and optic nerve gliomas as result of inactivation
of second copy of gene (neurofibromatosis type 1)
o Some neurofibromas later develop into malignant peripheral nerve sheath tumors
o Neurofibromin (protein product of NF1) contains GTPase-activating domain that converts RAS from
active to inactive state
o Mutation of NF1 makes RAS trapped in active, signal-emitting state
 NF2 – mutations predispose to development of neurofibromatosis type 2; develop benign B/L schwannomas of
acoustic nerve; somatic mutations found in sporadic meningiomas and ependymomas
o Product of NF2 gene (neurofibromin 2 or merlin) shows homology w/RBC cytoskeletal protein 4.1 and is
related to ERM family of membrane cytoskeleton-associated proteins
o Cells lacking merlin not able to establishing stable cell-to-cell junctions and have no contact inhibition
o Merlin member of Salvador-Warts-Hippo (SWH) tumor suppressor pathway; signaling pathway controls
organ size by modulating cell growth, proliferation, and apoptosis
 VHL – mutations associated w/hereditary renal cell cancers, pheochromocytomas, hemangioblastomas of CNS,
retinal angiomas, and renal cysts
o VHL protein part of ubiquitin ligase complex
o HIF1α hydroxylated in presence of O2; binds to VHL protein, leading to ubiquination and proteasomal
degradation; can’t occur in hypoxic environments
o In hypoxia, HIF1α escapes recognition by VHL and subsequent degradation; can then translocate to
nucleus and turn on many genes (growth/angiogenic factors VEGF and PDGF)
 WT1 gene associated w/development of Wilms’ tumor (pediatric kidney cancer)
o WT1 protein – transcriptional activator of genes involved in renal and gonadal differentiation; regulates
mesenchymal-to-epithelial transition that occurs in kidney development
o Variety of adult cancers, including leukemias and breast carcinomas, overexpress WT1
 WT2 gene associated w/Beckwith-Wiedemann syndrome
 PTCH1 and PTCH2 – tumor suppressor genes that encode PM protein (PATCHED) that functions as receptor for
Hedgehog family of proteins
o Hedgehog/PATCHED pathway regulates TGF-β, PDGFRA, and PDGFRB
o Mutations in PTCH related to Gorlin syndrome (inherited condition also called nevoid basal cell
carcinoma syndrome)
o Mutations present in 20-50% of sporadic cases of basal cell carcinoma; half are type caused by UV
Evasion of Apoptosis
 Anoikis – loss of adhesion to BM; can trigger apoptosis
 Extrinsic pathway of apoptosis – death receptor CD95/Fas pathway; initiated when CD95/Fas binds ligand
(CD95L/FasL), leading to trimerization of receptor and cytoplasmic death domains, which attract intracellular
adaptor protein FADD (recruits procaspase 8 to form death-inducing signaling complex)
o Caspase 8 activates caspase 3 (executioner caspase) that cleaves DNA and other substrates to cause cell
death; can also cleave and activate BH3-only protein BID, activating intrinsic pathway as well
 Intrinsic pathway of apoptosis – triggered by DNA damage; triggered by withdrawal of survival factors, stress,
and injury; activation leads to permeabilization of mitochondrial outer membrane w/resultant release of
cytochrome c that initiates apoptosis
 Integrity of mitochondrial outer membrane regulated by pro-apoptotic and anti-apoptotic members of BCL2
family of proteins
o
Pro-apoptotic proteins BAX and BAK required for apoptosis; directly promote mitochondrial
permeabilization; action inhibited by anti-apoptotic members (BCL2 and BCL-XL)
o BH3-only proteins (BAD, BID, and PUMA) regulate balance between pro and anti-apoptotics
 Sense death-inducing stimuli and promote apoptosis by neutralizing actions of anti-apoptotic
BCL2 and BCL-XL
 When sum of all BH3 proteins expressed overwhelms anti-apoptotic BCL2/BCL-XL protein
barrier, BAX and BAK activated and form pores in mitochondrial membrane
 Cytochrome c leaks into cytosol where it binds APAF1, activating caspase 9
 Caspase 9 can cleave and activate executioner caspases
 Caspases inhibited by IAPs; some tumors upregulate to avoid apoptosis
o Reduced levels of CD95/Fas render tumor cells less susceptible to apoptosis by CD95L/FasL
 Some tumors have high levels of FLIP (protein that can bind death-inducing signaling complex
and prevent activation of caspase 8)
 Location of IgH genes also involved in pathogenesis of Burkitt lymphoma; juxtaposition of transcriptionally active
locus w/BCL2 causes overexpression of BCL2 protein, increasing BCL2/BCL-XL buffer, protecting lymphocytes
from apoptosis and allowing them to survive for long periods
o Results in lymphadenopathy and marrow infiltration
o Arise in large part from reduced cell death rather than explosive proliferation, so indolent (slowgrowing) compared w/other lymphomas
 Actions of p53 mediated by transcriptional activation of BAX; other connections as well
Limitless Replicative Potential: Telomerase
 Short elomeres recognized by DNA-repair machinery as ds DNA breaks, leading to cell cycle arrest mediated by
p53 and RB
 In p53 or RB1 mutations, nonhomologous end-joining pathway activated as last-ditch effort to save cell, joining
shortened ends of 2 chromosomes
o Inappropriately activated repair system results in dicentric chromosomes that are pulled apart at
anaphase, resulting in new ds DNA breaks
o Resulting genomic instability from repeated bridge-fusion-breakage cycles eventually produces mitotic
catastrophe, characterized by massive cell death
 If during crisis, cell manages to reactivate telomerase, bridge-fusion-breakage cycles cease and cell avoids death
o During period of genomic instability that precedes telomerase activation, numerous mutations could
accumulate, helping cell toward malignancy
o Telomerase active in normal stem cells; normally absent or very low in most somatic cells
o 85-95% of cancers have upregulation of telomerase
 Alternative lengthening of telomeres – depends on DNA recombination; seen in few tumors
 In progression from colonic adenoma to colonic adenocarcinoma, early lesions have high degree of genomic
instability w/low telomerase expression; malignant lesions had complex karyotypes w/high levels of telomerase
activity
Angiogenesis
 Cancer cells can stimulate neoangiogenesis (new vessels sprout from existing capillaries) or vasculogenesis
(endothelial cells recruited from bone marrow)
 Tumor vessels leaky and dilated w/haphazard pattern of connection
o Neovascularization gives needed nutrients and oxygen and newly formed endothelial cells stimulate
growth of adjacent tumor cells by secreting growth factors (IGFs, PDGF, and GM-CSF)
 Tumor angiogenesis controlled by balance between angiogenesis promoters and inhibitors
o Tumors remain small until angiogenic switch terminates vascular quiescence; switch involves increased
production of angiogenic factors and/or loss of angiogenic inhibitors, which may be produced directly by
tumor cells or inflammatory cells (macrophages) or other stromal cells associated w/tumors
o Proteases from tumor cells or stromal cells involved in regulating balance between angiogenic and antiangiogenic factors; many release pro-angiogenic bFGF stored in ECM
o Angiogenesis inhibitors (angiostatin, endostatin, and vasculostatin) produced by proteolytic cleavage of
plasminogen, collagen, and transthyretin
 Relative lack of oxygen stimulates HIF1α, which activates transcription of pro-angiogenic cytokines (VEGF, bFGF)
o
o
Stimulates proliferation of endothelial cells and guides growth of new vessels toward tumor
VEGF also increases expression of ligands that activate Notch signaling pathway (regulates branching
and density of new vessels)
 In normal cells, p53 can stimulate expression of anti-angiogenic molecules (thrombospondin-1) and repress
expression of pro-angiogenic molecules (VEGF)
o Loss of p53 in tumor cells provides more permissive environment for angiogenesis
 Transcription of VEGF influenced by signals from RAS-MAP kinase pathway; mutations in RAS or MYC upregulate
production of VEGF
 Elevated VEGF and bFGF can be detected in serum and urine of significant fraction of cancer patients
o Bevacizumab – anti-VEGF monoclonal antibody used to treat cancers
Invasion and Metastasis
 BM and interstitial CT – components of ECM; made of collagens, glycoproteins, and proteoglycans
 Dissociation of cells often result of alterations in intercellular adhesion molecules; cell-cell interactions mediated
by cadherin family of transmembrane glycoproteins
o E-cadherins mediate homotypic adhesions in epithelial tissue, serving to keep epithelial cells together
and relay signals between cells
o Intracellularly, E-cadherins connected to β-catenin and actin cytoskeleton
o In epithelial tumors, there is downregulation of E-cadherin, reducing ability of cells to adhere to each
other and facilitating detachment from primary tumor and advance into surrounding tissues
o E-cadherins linked to cytoskeleton by catenins (proteins under PM); normal function depends on this
o In some tumors, E-cadherin normal but expression reduced because of mutations in gene for α catenin
 Degradation of BM and interstitial CT – tumor cells secrete proteolytic enzymes or induce stromal cells to do so
o MMPs, cathepsin D, and urokinase plasminogen activator implicated in tumor cell invasion
o MMPs regulate tumor invasion by remodeling insoluble components of BM and interstitial matrix; also
release ECM-sequestered growth factors
o Cleavage products of collagen and proteoglycans chemotactic, angiogenic, and growth-promoting
 MMP9 – gelatinase that cleaves type IV collagen of epithelial and vascular BM; stimulates
release of VEGF from ECM-sequestered pools
 Ameboid migration – cell squeezes through spaces in matrix instead of cutting through; able to use collagen
fibers as railways; tumor cells switch between this and degradation of BM and CT
 Normal epithelial cells have receptors (integrins) for BM laminin and collagens that are polarized at basal surface
o Receptors help maintain cells in resting, differentiated state
o Loss of adhesion in normal cells leads to induction of apoptosis; tumor cells resistant to this
o Matrix modified in ways that promote invasion and metastasis – cleavage of collagen IV and laminin by
MMP2 or MMP9 generates novel sites that bind receptors on tumor cells and stimulate migration
 Migration involves many families of receptors and signaling proteins that eventually impinge on actin
cytoskeleton; cells must attach to matrix at leading edge, detach from matrix at trailing edge, and contract actin
cytoskeleton to ratchet forward
o Movement directed by tumor cell-derived cytokines (autocrine motility factors)
o Cleavage products of matrix components and some growth factors (IGF-I and IGF-II) have chemotactic
activity for tumor cells
o Proteolytic cleavage liberates growth factors bound to matrix molecules
o Stromal cells produce paracrine effectors of motility (HGF-scatter factor) that bind to tumor receptors
 Concentrations of scatter factor elevated at advancing edges of highly invasive glioblastoma
multiforme
 Tumor-associated fibroblasts exhibit altered expression of genes that encode ECM molecules, proteases,
protease inhibitors, and various growth factors
 In circulation, tumor cells tend to aggregate in clumps (favored by homotypic adhesions among tumor cells and
heterotypic adhesion between tumor cells and platelets
o Tumor cells may bind and activate coagulation factors, resulting in formation of emboli
o Arrest and extravasation of tumor emboli involves adhesion to endothelium then through BM
o CD44 adhesion molecule – expressed on normal T lymphocytes; used to migrate to selective sites in
lymphoid tissue by binding of CD44 to hyaluronate on high endothelial venules
o Overexpression of CD44 may favor metastatic spread
 Because first step in extravasation is adhesion to endothelium, tumor cells have adhesion molecules whose
ligands expressed preferentially on endothelial cells of target organ
 Chemokines have role in determining target tissues for metastasis
o Some breast cancer cells express chemokine receptors CXCR4 and CCR7; chemokines that bind them
highly expressed in tissues to which breast cancers commonly metastasize
o Some target organs liberate chemoattractants that recruit tumor cells to site (IGF-I and IGF-II)
 Tumor cells secrete cytokines, growth factors, and ECM molecules that act on resident stromal cells to make
metastatic site habitable for them
o Breast cancer metastases to bone osteolytic: cancer cells secrete PTHRP, which stimulates osteoblasts to
make RANKL, which activates osteoclasts, which degrade bone matrix and release growth factors
embedded in it (IGF and TGF-β)
 mir335 and mir126 suppress metastasis of breast cancer; mir10b promotes metastasis
 SNAIL and TWIST encode transcription factors whose primary function is to promote epithelial-to-mesenchymal
transition (EMT); carcinoma cells downregulate certain epithelial markers (E-cadherin) and upregulate certain
mesenchymal markers (vimentin and smooth muscle actin), favoring development of promigratory phenotype
o SNAIL and TWIST downregulate E-cadherin, which is key event in EMT and leads to metastasis
Genomic Instability – Enabler of Malignancy
 DNA-repair genes themselves not oncogenic, but abnormalities allow mutations in other genes during the
process of normal cell division
 Typically, genomic instability occurs when both copies of DNA repair gene lost; subset of genes may promote
cancer in haploinsufficient manner
 HNPCC characterized by familial carcinomas of colon affecting predominantly cecum and proximal colon; results
from defects in genes involved in DNA mismatch repair
o Hallmark of patients w/mismatch-repair defects is microsatellite instability; microsatellites are tandem
repeats of 1-6 nucleotides found throughout genome; in normal people, they are constant length, but
w/HNPCC, they are unstable and increase or decrease in length in tumor cells, creating alleles not found
in normal cells of same patient
o Germline mutations in MSH2 and MLH1 account for 30% of cases
o DNA-repair genes behave like tumor suppressor genes in mode of inheritance, but they affect cell
growth only indirectly (allowing mutations in other genes during cell replication)
 Xeroderma pigmentosum – increased risk for development of cancers of skin, particularly following exposure to
UV light, which causes linking of pyrimidine residues, preventing normal DNA replication; usually repaired by
nucleotide excision repair system
o Inherited loss of any protein involved in nucleotide excision repair
 Group of autosomal recessive disorders (Bloom syndrome, ataxia-telangiectasia, and Fanconi anemia)
characterized by hypersensitivity to other DNA-damaging agents, such as ionizing radiation (Bloom and ataxiatelangiectasia) or DNA cross-linking agents such as chemotherapeutic agents (Fanconi anemia)
o Features include neural symptoms (ataxia-telangiectasia), bone marrow aplasia (Fanconi), and
developmental defects (Bloom)
o ATM mutated in ataxia-telangiectasia; important in recognizing and responding to DNA damage caused
by ionizing radiation
o Pts w/Bloom syndrome have predisposition to broad spectrum of tumors; defective gene encodes
helicase that participates in DNA repair by homologous recombination
o Mutation in any one of 13 genes (including BRCA2) can cause Fanconi anemia
o BRCA1 mutations confer higher risks of breast and ovarian cancer in women and prostate in men
o BRCA2 mutation increases risk of breast cancer in both genders (and ovary, prostate, pancreas, bile
duct, stomach, and melanocyte cancer)
o Cells that lack BRCA1 and BRCA2 develop chromosomal breaks and severe aneuploidy
o Both BRCA1 and BRCA2 associate w/proteins involved in homologous recombination repair pathway
o Fanconi anemia proteins and BRCA proteins form DNA-damage response network whose purpose is to
resolve and repair intrastrand and interstrand DNA cross-links induced by chemical cross-linking agents;
failure to resolve cross-links before separation of strands leads to chromosome breakage and exposed
chromosome ends
 Generation of exposed ends leads to activation of salvage nonhomologous end joining pathway,
formation of dicentric chromosomes, bridge-fusion-breakage cycles, and massive aneuploidy
o Both copies of BRCA1 and BRCA2 must be inactivated for cancer to develop (rarely inactivated in
sporadic cases)
Stromal Microenvironment and Carcinogenesis
 Macrophages infiltrating tumor can be induced by tumor cells to secrete factors that promote metastasis
 Macrophages surrounding blood vessels secrete EGF, resulting in chemotactic migration of tumor cells toward
vasculature
 Fibroblasts secrete matrix that results in desmoplastic response to tumors
 Desmoplastic response to cancer may be stimulated by cancer cells to promote growth
Metabolic Alterations: the Warburg Effect
 Even in presence of ample oxygen, cancer cells shift glucose metabolism away from mitochondria to glycolysis
(Warburg effect or aerobic glycolysis)
o PET scan – patients injected w/18F-fluorodeoxyglucose (non-metabolizable derivative of glucose
preferentially taken up into tumor cells and actively dividing cells (bone marrow))
 Altered metabolism confers growth advantage in hypoxic tumor microenvironment; angiogenesis generates
increased vasculature, but vessels poorly formed (tumors still relatively hypoxic)
o Activation of HIF1α by hypoxia stimulates angiogenesis and increases expression of numerous metabolic
enzymes in glycolytic pathway as well as downregulates genes involved in oxidative phosphorylation
o Decreased demand by individual tumor cells increases oxygen supply, increasing number of tumor cells
that can be supported by vasculature and increasing size of tumor
 Continuous rounds of hypoxia/normoxia select for tumor cells that constitutively upregulate glycolysis
 Halting breakdown of glucose at pyruvate allows carbons to be shunted to anabolic pathways (lipid and
nucleotide production); tumor cells able to shunt glutamine into both glycolytic and anabolic pathways
o Increases ability to synthesize building blocks for cell division
o Normally, growth factors stimulate glucose and amino acid uptake through PI3K/AKT/mTOR pathway
(downstream of receptor tyrosine kinases and other growth factor receptors); in tumors, signals are cell
autonomous, leading to constitutive activation of pathways that favor survival and proliferation as well
as make glycolysis and anabolic biosynthesis permanent fixture of tumor cell
 LKB1 – tumorsuppressor gene encoding threonine kinase mutated in Peutz-Jegher syndrome (benign and
malignant epithelial proliferations of GI tract)
o Aspects of LKB1’s tumor suppressive activity mediated through ability to activate AMPK (conserved
sensor of cellular energy status that is important negative regulator of mTOR); LKB1 suppresses tumor
formation by halting anabolic metabolism
 TSC1 and TSC2 mutated in tuberous sclerosis; negatively regulate mTOR
 Autophagy – cells arrest growth and cannibalize their organelles, proteins, and membranes as carbon sources
o If this fails, cells die
o Tumor cells grow under marginal environmental conditions w/o triggering autophagy
o Several genes that promote autophagy are tumor suppressors (PTEN: negative reg of PI3K/AKT path)
o Under conditions of severe nutrient deprivation, tumor cells may use autophagy to become dormant;
resistant to therapies that kill actively dividing cells
Dysregulation of Cancer-Associated Genes
 Karyotypic abnormalities identified in most leukemias and lymphomas, many sarcomas, and carcinomas
o Whole chromosomes may be gained or lost
o Certain karyotypic abnormalities specific enough to be of diagnostic value; in some cases predict course
 Activation of proto-oncogenes can happen by translocations and inversions; translocations more common
o Translocations activate proto-oncogenes because
 In lymphoid tumors, specific translocations result in overexpression of proto-oncogenes by
swapping regulatory elements w/those of another gene
 Many hematopoietic tumors, sarcomas, and certain carcinomas – translocation allows normally
unrelated sequences from 2 different chromosomes to recombine and form hybrid fusion genes







that encode chimeric proteins that promote growth and survival or enhance self-renewal and
block differentiation
o Burkitt lymphoma caused by one of 3 translocations in MYC gene and one of 3 immunoglobulin genecarrying chromosomes
 Most common form of translocation results in movement of MYC-containing segment of
chromosome 8 to chromosome 14q32 (close to IGH gene) (t(8:14)(q24;q32))
 Translocation causes mutation or loss of regulatory sequences of MYC gene, replacing them
w/control regions of IGH locus, which is highly expressed in B-cell precursors
o Mantle cell lymphoma – cyclin D1 gene (CCND1) overexpressed by juxtaposition to IGH locus
o Follicular lymphomas – translocation causes activation of BCL2 gene
o All tumors in which Ig gene involved are of B-cell origin
Philadelphia chromosome – characteristic of CML and subset of acute lymphoblastic leukemias; provides
prototypic example of oncogene formed by fusion of 2 separate genes
o Reciprocal translocation relocates truncated portion of proto-oncogene c-ABL to BCR
o Chimeric protein BCR-ABL has constitutive tyrosine kinase activity
Transcription factors often partners in gene fusions occurring in cancer cells
o MLL gene (component of chromatin-remodeling complex) involved in 50 different translocations
w/several different partner genes, some of which encode transcription factors
Ewing sarcoma/primitive neuroectodermal tumor (PNET) defined by translocation of Ewing sarcoma (EWSR1)
gene (involved in numerous translocations); all partner genes also encode transcription factor
o Ewing sarcoma/PNET – EWSR1 gene fuses w/FLI1 gene; resultant EWS-FLI1 has transforming ability
Fusion proteins resulting from translocations most often inhibit (occasionally increase) transcriptional function
o Translocation involving androgen-related gene (TMPRSS2) and one of 3 ETS family transcription factors
(ERG, ETV1, or ETV4) present in 50% prostate adenocarcinomas
 Removes ETS family gene from normal control region and fuses it to androgen-regulated
TMPRSS2, so ETS inappropriately expressed in prostate cells (transforming ability)
o Many fusion genes initiators in carcinogenesis; cancers addicted to their properties (oncogene addiction
in CML w/BCR-ABL fusion)
Deletions more common in non-hematopoietic solid tumors; deletion of specific regions of chromosomes
associated w/loss of particular tumor suppressor genes
o Deletions noted in retinoblastoma (RB), colorectal cancers (tumor suppressor genes), and small-cell lung
carcinomas
Amplification of proto-oncogenes may produce chromosomal changes
o Mutually exclusive patterns: double minutes (multiple small centric structures) and homogeneous
staining regions
o Homogeneous staining regions derive from insertion of amplified genes into new chromosomal
locations; because regions containing amplified genes lack normal banding pattern, they appear
homogeneous in G-banded karyotype
o N-MYC amplified in 25-30% of neuroblastomas; amplification associated w/poor prognosis; gene
present both in double minutes and homogeneous staining regions
o ERBB2 amplification occurs in 20% of breast cancers
Epigenetics – reversible, heritable changes in gene expression that occur w/o mutation
o Post-translational modifications of histones and DNA methylation (affect gene expression)
o In differentiated cell, majority of genome not expressed; some silenced by DNA methylation and histone
modifications that lead to compaction of DNA into heterochromatin
o Cancer cells characterized by global DNA hypomethylation and selective promoter-localized
hypermethylation; sometimes tumor suppressor genes become silenced by promoter hypermethylation
 CDKN2A: encodes 2 tumor suppressors (p14/ARF and p16/INK4a)
 p14/ARF epigenetically silenced in colon and gastric cancers; p16/INK4a silenced in variety
 Silencing CDKN2A removes p53 and Rb checkpoint w/single mutation
 BRCA1, VHL, and MLH1 can be silenced by hypermethylation
o Genomic imprinting – maternal or paternal allele of gene or chromosome inactivated by methylation

Demethylation of imprinted gene leads to biallelic expression (loss of imprinting); can occur in
tumor cells
o Histone code where acetylation or methylation of histone tail leads to activation or repression of gene
 EZH2: chromatin-modifying enzyme overexpressed in breast and prostate carcinomas;
enzymatic component of multiprotein polycomb repressive complex 2 (puts repressive
chromatin marks at promoter of genes)
 Overexpression leads to repression of tumor suppressors (p21)
 Polycomb repressive complexes required for maintenance of stem cells and to silence
lineage-specific transcription factors until proper cues signal differentiation
 Inappropriate repression or expression can give cancer cells stem-cell-like quality
 Placement of repressive chromatin marks by enzymes results in recruitment of DNA methylases,
methylation of promoters, and durable repression of gene expression
 miRNAs – small noncoding ssRNAs incorporated into RNA-induced silencing complex; mediate sequence-specific
recognition of mRNAs and (through action of RNA-induced silencing complex) mediate post-transcriptional gene
silencing
o miRNAs have frequent amplications and deletions in many cancers (either increase expression of
oncogenes or reduce expression of tumor suppressor genes)
o Can downregulate BCL2 (leukemias and lymphomas)
o Upregulation of RAS detected in lung tumors and MYC in certain B-cell leukemias
o Patterns of miRNA expression = miRNA profiling
Molecular Basis of Multistep Carcinogenesis
 No single oncogene can fully transform non-immortalized cells, but cells can generally be transformed by combo
of oncogenes
o RAS oncogene induces cells to secrete growth factors and permits them to grow without anchorage to
normal substrate
o MYC oncogene renders cells more sensitive to growth factors and immortalizes cells
o RAS and MYC in combo can cause neoplastic transformation
 In cells w/competent checkpoints, oncogenic signaling through RAS leads to senescence or apoptosis
 Colon cancer evolves – colon epithelial hyperplasia  adenomas that enlarge  malignant transformation
o Inactivation of APC tumor  activation of RAS 00> loss of
tumor suppressor gene on 18q and p53
 Most cells of most adenomas senescent; mutation of proto-oncogene
drives cell into senescence; loss of p53 prevents oncogene-induced
senescence, allowing adenomatous cells to continue to proliferate
(carcinoma)
Carcinogenic Agents and Their Cellular Interactions
 Initiation results from exposure of cells to sufficient dose of
carcinogenic agents (initiator); initiated cell is altered, making it
potentially capable of giving rise to tumor
 Initiation causes permanent DNA damage; tumors produced even if
application of promoting agent delayed for several months after single
application
 Promoters induce tumors in initiate cells, but non-tumorigenic themselves;
tumors don’t result when promoting agent applied before rather than
after initiating agent; cellular changes resulting from application of
promoters don’t affect DNA directly and are reversible
o Promoters enhance proliferation of initiated cells; contributes to
development of additional mutations
o Tumors fail to develop in initiated cells if time between promoters
sufficiently spaced out
 All initiating chemical carcinogens are highly reactive electrophiles that can
react w/nucleophilic sites in cell
o Initiation inflicts nonlethal damage to DNA but can’t be repaired
o Mutated cell passes DNA to daughter cells
 Direct-acting agents – require no metabolic conversion to become carcinogenic; most weak carcinogens; some
cancer chemotherapy drugs (alkylating agents) that control or cure certain types of cancer to evoke a second
form of cancer later (usually acute myeloid leukemia); risk of induced cancer low, but existence dictates
judicious use
 Indirect-acting agents – chemicals that require metabolic conversion to ultimate carcinogen before active
o Polycyclic hydrocarbons present in fossil fuels; produced from animal fats during broiling process
 Principal active products are epoxides (form covalent adducts w/DNA (some RNA & protein))
o Benzo[a]pyrene and carcinogens in tobacco smoke
 Metabolism is by CYP1A1 – 10% of white population has highly inducible form (associated
w/increased risk of lung cancer)
o Aromatic amines and azo dyes in dye and rubber industries
o Carcinogenic potency determined by balance between metabolic activation and inactivation reactions
o Most carcinogens metabolized by cytochrome P450-dependent monooxygenases
 Susceptibility to carcinogenesis regulated in part by polymorphisms in P450 genes
 Aflatoxin B1 (naturally occurring agent produced by Aspergillus mold that grows on improperly stored grains and
nuts) – causes hepatocellular carcinoma; produces mutations in p53 gene (specifically 249(ser) mutation)
 Nitrites (preservatives) cause nitrosylation of amines contained in food; nitrosoamines carcinogenic
 For change to be heritable, damaged DNA template must be replicated; for initiation to occur, carcinogenaltered cells must undergo at least one cycle of proliferation so that change in DNA becomes fixed
o In liver, many chemicals activated to reactive electrophiles, but most don’t produce cancers unless liver
cells proliferate within a few days of formation of DNA adducts
o In tissues normally quiescent, mitogenic stimulus may be provided by carcinogen itself because many
cells die as result of toxic effects of chemical, stimulating regeneration in surviving cells
o Cell proliferation may be induced by concurrent exposure to biologic agents (viruses, parasites, dietary
factors, or hormonal influences)
o Promoters – agents that don’t cause mutation but stimulate division of mutated cells
 Carcinogenicity of some initiators augmented by subsequent administration of promoters (phorbol esters,
hormones, phenols, and drugs) that are nontumorigenic by themselves
o Cells stimulated by promoters have reduced growth factor requirements and may be less responsive to
growth-inhibitory signals
o Initiated clone of cells suffers additional mutations, developing eventually into malignant tumor
Radiation Carcinogenesis
 UV rays cause squamous cell carcinoma, basal cell carcinoma, and possibly melanoma of skin
o Degree of risk depends on type of UV rays, intensity of exposure, and quantity of light-absorbing
protective mantle of melanin in skin
o Nonmelanoma skin cancers associated w/total cumulative exposure to UV radiation
o Melanomas associated w/intense intermittent exposure (sunbathing)
o UV classified by wavelength into UVA, UVB, and UVC; UVB responsible for induction of cancers
o UVC filtered out by ozone shield around earth
o UVB forms pyrimidine dimers in DNA; usually repaired by nucleotide excision repair pathway
 w/excessive sun exposure, capacity of nucleotide excision repair pathway overwhelmed, and
error-prone nontemplated DNA-repair mechanisms operative that provide for survival of cell at
cost of genomic mutations that lead to cancer
 Electromagnetic (x-rays, γ-rays) & particulate (α particles, β particles, protons, neutrons) radiation carcinogenic
o Most frequently cause acute and chronic myeloid leukemia
o Cancer of thyroid is close second, but only in young people
o Can cause cancers of breast, lungs, and salivary glands
o Skin, bone, and GI tract relatively resistant to radiation-induced neoplasia; GI epithelial cells vulnerable
to acute cell-killing effects of radiation
o Practically any cell can be transformed into cancer by sufficient exposure to radiant energy
Microbial Carcinogenesis
 Human T-cell leukemia virus type 1 (HTLV-1) causes T-cell leukemia/lymphoma; tropism for CD4+ T cells
o
o
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RNA retrovirus
Human infection requires transmission of infected T cells via sexual intercourse, blood products, or
breastfeeding
o Leukemia develops in 3-5% of infected individuals after 40-60 year latent period
o Viral integration shows clonal pattern in leukemic cells; although site of viral integration in host
chromosomes random, site of integration identical w/in all cells of given cancer
o HTLV-1 genome contains gag, pol, env, and long-terminal-repeat regions typical of retroviruses
 Also contains tax region; product stimulates transcription of viral mRNA by acting on 5’ long
terminal repeat; Tax protein
 Activates transcription of host cell genes involved in proliferation and differentiation of
T cells, including immediate early gene FOS, genes encoding IL-2 and its receptor, and
gene for myeloid growth factor GM-CSF
 Inactivates cell cycle inhibitor p16/INK4a and enhances cyclin D activation, dysregulating
cell cycle
 Activates NF-κB (regulates pro-survival/anti-apoptotic genes)
 Interferes w/DNA-repair functions and inhibits ATM-mediated cell cycle checkpoints
activated by DNA damage
o Infection by HTLV-1 causes expansion of nonmalignant polyclonal cell population through stimulatory
effects of Tax on cell proliferation
 Proliferating T cells increase risk of mutations and genomic instability induced by Tax
 Instability allows accumulation of mutations and chromosomal abnormalities, and eventually
monoclonal neoplastic T-cell population emerges
 Malignant cells replicate independently of IL-2 and contain molecule and chromosomal
abnormalities
HPV – causes benign squamous papillomas (warts); types 16 & 18 implicated in cervix and anogenital cancer
o At least 20% of oropharyngeal cancers associated w/HPV
o HPV-6 and HPV-11 cause warts that are low-risk (low malignant potential); HPV genome maintained in
nonintegrated episomal form
 Cancers – HPV genome integrated into host genome
o DNA virus; site of viral integration in host chromosomes random; pattern of integration clonal
 Cells w/viral integration show more genomic instability
 Integration interrupts viral DNA in E1/E2 open reading frame, leading to loss of E2 viral
repressor and overexpression of oncoproteins E6 and E7
o E7 protein binds RB protein and displaces E2F transcription factors normally sequestered by RB,
promoting progression through cell cycle; E7 from high-risk HPV has higher affinity for RB than E7 from
low-risk HPV and bind and activate cyclins E and A
 E7 inactivates CDKIs p21 and p27
o E6 binds to and mediates degradation of p53 and BAX (pro-apoptotic member of BCL2 family) and
activates telomerase
 E6 from high-risk HPV has higher affinity for p53 than E6 from low-risk HPV types
 p53 Arg72 variant more susceptible to degradation by E6 than p53 Pro72
EBV – DNA virus implicated in pathogenesis of African Burkitt lymphoma, B-cell lymphomas in
immunosuppressed individuals, subset of Hodgkin lymphoma, nasopharyngeal and some gastric carcinomas,
and rare forms of T cell lymphomas and NK cell lymphomas
o EBV infects B lymphocytes using complement receptor CD21 to attach; infection latent; infected cells
immortalized and acquire ability to propagate indefinitely
o LMP-1 – EBV gene that acts as oncogene; expression induces B-cell lymphomas
 Activates NF-κB and JAK/STAT signaling pathways autonomously
 Prevents apoptosis by activating BCL2
o EBNA-2 – EBV gene that transactivates cyclin D and src family of proto-oncogenes
o vIL-10 – part of EBV genome hijacked from host genome; prevents macrophages and monocytes from
activating T cells; required for EBV-dependent transformation of B cells
o
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In immunologically normal people, EBV-driven polyclonal B-cell proliferation controlled and person
asymptomatic or develops self-limited episode of mono
o Burkitt lymphoma – neoplasm of B lymphocytes; most common childhood tumor in central Africa and
New Guinea; more than 90% of African tumors carry EBV genome; concomitant infections or HIV cause
o T-cell immunity directed against EBV antigens (EBNA2 and LMP-1) eliminates most EBV-infected B cells;
small number of cells downregulate expression of immunogenic antigens
o All tumors possess translocations that dysregulate c-MYC
o Immunosuppressed patients have multifocal B-cell tumors in lymphoid tissue or in CNS
 Tumors polyclonal at outset but can develop into monoclonal neoplasms
 Tumors uniformly express LMP-1 and EBNA2 recognized by CTLs
o Nasopharyngeal carcinoma associated w/EBV infection; 100% of these from endemic parts of world
contain EBV DNA; viral integration in host cells clonal
 Patients develop IgA before appearance of tumor
 Only happens in S China, some parts of Africa, and Inuits in Arctic (environmental cofactors)
 LMP-1 expressed in epithelial cells as well (activates NF-κB pathway and induces expression of
VEGF, FGF-2, MMP9, and COX2 (angiogenic))
70-85% of hepatocellular carcinomas worldwide due to infection w/HBV or HCV
o Genomes don’t contain viral oncoproteins; HBV DNA integrated into human genome, but no consistent
pattern of integration in liver cells
o Dominant effect is immunologically mediated chronic inflammation w/hepatocyte death leadgin to
regeneration and genomic damage
o In unresolved chronic inflammation, immune response may become maladaptive, promoting tumors
o Key molecular step = activation of NF-κB pathway in hepatocytes in response to mediators derived from
activated immune cells
 Blocks apoptosis; allows dividing cells to incur genotoxic stress and accumulate mutations
o HBV contains HBx gene that can directly or indirectly activate transcription factors and signal
transduction pathways
 Viral integration can cause secondary rearrangements of chromosomes
o Components of HCV genome (HCV core protein) have effect on tumorigenesis by activating growthpromoting signal transduction pathways
H. pylori – infection implicated in genesis of both gastric adenocarcinomas and gastric lymphomas
o Increased epithelial cell proliferation w/chronic inflammation
o Inflammatory milieu contains numerous genotoxic agents (ROS); initial development of chronic gastritis,
followed by gastric atrophy, intestinal metaplasia of lining cells, dysplasia, and cancer
o Strains associated w/gastric adenocarcinoma contain pathogenicity island that contains CagA gene,
which penetrates gastric epithelial cells, initiating signaling cascade that mimics unregulated growth
factor stimulation
o Gastric lymphomas of B-cell origin; because tumors have features of normal Peyer’s patches, sometimes
called MALTomas; dependent on T-cell stimulation of B-cell pathways that activate NF-κB
 At early stages, eradication of H. pylori by antibiotics cures lymphoma by removing antigenic
stimulus for T cells; later, additional mutations may be acquired to cause NF-κB to be activated
constitutively