This document discusses the molecular basis of cancer. It states that genetic damage underlies carcinogenesis and tumors are monoclonal, arising from a single damaged cell. Four classes of genes are commonly mutated in cancer - proto-oncogenes, tumor suppressor genes, genes regulating apoptosis, and DNA repair genes. Carcinogenesis is a multistep process involving accumulation of multiple genetic mutations over time that result in cancer hallmarks like self-sufficiency in growth signals and evasion of programmed cell death.

1. Neoplasia II
Dr Bahoran Singh

2. Molecular Basis of Cancer
Some fundamental principles
• Nonlethal genetic damage lies at the heart of
carcinogenesis
• Tumor is formed by the clonal expansion of a single
precursor cell that has incurred genetic damage (i.e.,
tumors are monoclonal)

3. • Four classes of normal regulatory genes are principal
targets of genetic damage
1. The growth-promoting proto-oncogenes,
2. The growth-inhibiting tumor suppressor genes,
3. Genes that regulate programmed cell death (apoptosis)
4. Genes involved in DNA repair
• Carcinogenesis is a multistep process at both the
phenotypic and the genetic levels, resulting from the
accumulation of multiple mutations

4. • Carcinogenesis results from the accumulation of
complementary mutations in a stepwise fashion over
time.
• Malignant neoplasms have several phenotypic
attributes referred to as cancer hallmarks.
• Mutations that contribute to the development of
the malignant phenotype are referred to as driver
mutations.
• Loss of function mutations in genes that maintain
genomic integrity appear to be a common early step
on the road to malignancy, particularly in solid
tumors.

5. Development of a cancer through stepwise acquisition of complementary
mutations.

6. • Once tumor evolved, its progression under the
pressure of Darwinian selection (survival of the
fittest).
• Selection of fittest explain the natural history of
cancer and changes in tumor behaviour following
therapy.
• In addition to genetic mutations, epigenetic
aberrations also contribute to the malignant
properties of cancer cells ( like DNA methylation and
histone modification.)

7. Cellular and Molecular Hallmarks of Cancer
The eight key changes in cell physiology that together
determine malignant phenotype
1. Self-sufficiency in growth signals
2. Insensitivity to growth-inhibitory signals
3. Altered cellular metabolism (Warburg effect)
4. Evasion of apoptosis
5. Limitless replicative potential ( immortality)
6. Sustained angiogenesis
7. Ability to invade and metastasize
8. Ability to evade the host immune response.

9. Hallmarks of cancer

11. Normal Cell Cycle and Its Regulation

12. SELF-SUFFICIENCY IN GROWTH SIGNALS:
ONCOGENES
• Genes that promote autonomous cell growth in cancer
cells are called oncogenes
• Oncogenes are created by mutations in proto-oncogenes
• Ability to promote cell growth in the absence of normal
growth-promoting signals
• Their products, called oncoproteins, resemble the normal
products of proto-oncogenes except
1. Oncoproteins are often devoid of important internal
regulatory elements
2. Their production in the transformed cells does not
depend on growth factors or other external signals

13. Under physiologic conditions cell proliferation
can be readily resolved into the following steps
• Binding of a growth factor to its specific receptor
• Transient and limited activation of the growth factor
receptor
• Activation of several signal-transducing proteins
• Transmission of the transduced signal across the
cytosol to nucleus via second messengers or by
cascade of signal transduction molecules
• Induction and activation of nuclear regulatory factors
that initiate DNA transcription
• Entry and progression of the cell into the cell cycle,
ultimately resulting in cell division

14. Proto-oncogenes, Oncogenes, and Oncoproteins
• Proto-oncogenes have multiple roles, participating in
cellular functions related to growth and proliferation
• Oncoproteins encoded by oncogenes generally serve
functions similar to their normal counterparts
• Mutations convert proto-oncogenes into
constitutively active cellular oncogenes involved in
tumorogenesis because oncoproteins endow cell with
self-sufficiency in growth

15. Growth Factors
• Normal cells require stimulation by growth factors to
undergo proliferation
• Many cancer cells acquire the ability to synthesize
same growth factors to which they are responsive,
generating an autocrine loop
• In most instances the growth factor gene itself is not
altered or mutated
• Products of other oncogenes that lie along many
signal transduction pathways, cause overexpression
of growth factor genes

16. • Increased growth factor production is not sufficient
for neoplastic transformation
• Growth factor driven proliferation contributes to
malignant phenotype by increasing the risk of
spontaneous or induced mutations in the
proliferating cell population

17. Growth Factors
CATEGORY Proto-
Oncogene
Mode of Activation in
Tumor
Associated Human
Tumor
Growth Factors
PDGF- β chain PDGFB Overexpression Astrocytoma
Fibroblast
growth factors
HST1
FGF3
Overexpression
Amplification
osteosarcoma
Stomach cancer
Bladder cancer
Breast cancer
Melanoma
TGF- α TGFA Overexpression Astrocytoma
HGF HGF Overexpression Hepatocellular
carcinomas
Thyroid cancer

18. Growth Factor Receptors

19. • Oncogenic versions of these receptors are associated
with constitutive dimerization and activation without
binding to the growth factor
• Hence, the mutant receptors deliver continuous
mitogenic signals to the cell, even in the absence of
growth factor in the environment
• Growth factor receptors can be constitutively activated in
tumors by multiple different mechanisms, including
mutations, gene rearrangements, and overexpression
• Far more common than mutations of these proto-
oncogenes is overexpression of normal forms of growth
factor receptors.
• Point mutations of RAS family genes constitute the most
common type of abnormality involving protooncogenes
in human tumors.

20. Growth Factor Receptors
CATEGORY Proto-Oncogene Mode of Activation in
Tumor
Associated Human Tumor
EGF-receptor
family
ERBB1 (EGFR)
ERBB2 (HER)
Mutation
Amplification
Adenocarcinoma of lung
Breast carcinoma
FMS-like tyrosine
kinase 3
FLT3 Point mutation Leukemia
Receptor for
neurotrophic
factors
RET Point mutation Multiple endocrine
neoplasia 2A and B, familial
medullary thyroid
carcinomas
PDGF receptor PDGFRB Overexpression,
translocation
Gliomas, leukemias
Receptor for KIT
ligand
KIT Point mutation Gastrointestinal stromal
tumors, seminomas,
leukemias
ALK receptor ALK Translocation, fusion Adenocarcinoma of lung,

21. Signal-Transducing Proteins
• Oncoproteins that mimic the function of normal
cytoplasmic signal-transducing proteins have been
found
• Most such proteins are strategically located on the
inner leaflet of the plasma membrane
• They receive signals from outside the cell ( by GFR
activation) and transmit them to the cell's nucleus
• Biochemically, the signal-transducing proteins are
heterogeneous

22. Proteins Involved in Signal Transduction
CATEGORY Proto-Oncogene Mode of Activation in
Tumor
Associated Human Tumor
GTP-binding (G)
proteins
KRAS
HRAS
NRAS
GNAQ
GNAS
Point mutation
Point mutation
Point mutation
Point mutation
Point mutation
Colon, lung, and pancreatic
tumors
Bladder and kidney tumors
Melanomas, hematologic
malignancies
Uveal melanoma
Pituitary adenoma, other
endocrine tumors
Nonreceptor
tyrosine
kinase
ABL Translocation
Point mutation
Chronic myelogenous leukemia
Acute lymphoblastic leukemia
RAS signal
transduction
BRAF Point mutation,
Translocation
Melanomas, leukemias, colon
carcinoma, others
Notch signal
transduction
NOTCH1 Point mutation,
Translocation
Gene rearrangement
Leukemias, lymphomas, breast
carcinoma
JAK/STAT signal
transduction
JAK2 Translocation Myeloproliferative disorders
ALL

23. Receptor-mediated signaling

24. • Most well-studied example of a signal-transducing
oncoprotein is the RAS genes
• Normal RAS proteins are tethered to the cytoplasmic
aspect of the plasma membrane, as well as the
endoplasmic reticulum and Golgi membranes
• Normally RAS proteins flip back and forth between
an excited signal-transmitting and a quiescent state
• Activated RAS stimulates downstream regulators of
proliferation, as mitogen-activated protein (MAP)

25. • The removal of GDP and its replacement by GTP during
RAS activation are catalyzed by a family of guanine
nucleotide–releasing proteins
• Conversely, the GTPase activity intrinsic to normal RAS
proteins is dramatically accelerated by GTPase-activating
proteins (GAPs)
• These widely distributed proteins bind to the active RAS
and augment its GTPase activity leading to termination of
signal transduction
• GAPs function as “brakes” that prevent uncontrolled RAS
activity

26. Growth factor signaling pathways in
cancer

27. Alterations in Nonreceptor Tyrosine Kinases
• Mutations occur in several non-receptor-associated
tyrosine kinases, which normally function in signal
transduction pathways that regulate cell growth.
• As with receptor tyrosine kinases, in some instances
the mutations take form of chromosomal
translocations or rearrangements .
• They create fusion genes encoding constitutively
active tyrosine kinases.

28. The chromosomal translocation and associated oncogenes in
Burkitt lymphoma and chronic myelogenous leukemia

29. Transcription Factors
• All signal transduction pathways converge to the
nucleus.
• In nucleus large bank of responder genes that
orchestrate cell's orderly advance through the
mitotic cycle, get activated.
• Ultimate consequence of signaling through
oncogenes like RAS or ABL is inappropriate and
continuous stimulation of nuclear transcription
factors that drive growth-promoting genes

30. • Transcription factors contain specific amino acid
sequences or motifs that allow them to bind DNA or
to dimerize for DNA binding.
• Binding of these proteins to specific sequences in the
genomic DNA activates transcription of genes.
• Growth autonomy may thus occur as a consequence
of mutations affecting genes that regulate
transcription.

31. Nuclear Regulatory Proteins
CATEGORY Proto-Oncogene Mode of Activation
in Tumor
Associated Human
Tumor
Transcriptional
activators
MYC
NMYC
Translocation
Amplifiation
Burkitt lymphoma
Neuroblastoma

32. The MYC Oncogene
• MYC proto-oncogene expressed in all eukaryotic cells.
• The MYC proto-oncogene belongs to the immediate early
response genes, which are rapidly induced by RAS/MAPK
siganaling when quiescent cells receive a signal to divide.
• After a transient increase of MYC messenger RNA, the
expression declines to a basal level.
• Range of activities modulated by MYC includes histone
acetylation, reduced cell adhesion, increased cell
motility, increased telomerase activity, increased protein
synthesis, decreased proteinase activity, & changes in
cellular metabolism that enable a high rate of cell
division.

33. • MYC interacts with components of the DNA-
replication machinery, and plays a role in the
selection of origins of replication
• Thus, overexpression drive activation of more origins
than needed for normal cell division, or bypass
checkpoints involved in replication
• Leading to genomic damage and accumulation of
mutations

34. • MYC activates the expression of many genes that are
involved in cell growth.
• Some MYC target genes, like D cyclins, are directly
Involved in cell cycle progression.
• It up regulates the expression of rRNA genes
and rRNA processing, so enhancing assembly of
ribosomes needed for protein synthesis.
• It upregulates a program of gene expression that
leads to metabolic reprogramming and the Warburg
effect.
• MYC upregulates expression of telomerase.
• It can reprogram somatic cells into pluripotent stem
cells.

35. • MYC may also enhance self-renewal, block
differentiation, or both
• Persistent expression, overexpression, dysregulation of
MYC expression resulting from translocation of the gene
(Burkitt lymphoma) & MYC amplification
(neuroblastomas, breast, colon,& lung ) commonly found
in tumors.
• Constitutive RAS/MAPK signaling (many cancers), Notch
signaling (several hematologic cancers), Wnt signaling
(colon carcinoma), and Hedgehog signaling
(medulloblastoma) all transform cells in part through
upregulation of MYC.

36. Amplification of the NMYC gene in human neuroblastomas
Normally present on chromosome 2p, becomes amplified and is seen
either as extra chromosomal double minutes or as a chromosomally
integrated, homogeneous staining region (HSR). The integration involves
other autosomes, such as 4, 9, or 13.

37. Cyclins and Cyclin-Dependent Kinases
• The CDK-cyclin complexes phosphorylate crucial
target proteins that drive the cell through the cell
cycle
• Mishaps affecting the expression of cyclin D or CDK4
seem to be a common event in neoplastic
transformation
• Cyclin D genes overexpressed in Ca of breast,
esophagus, liver, and a subset of lymphomas
• Amplification of the CDK4 gene occurs in
melanomas, sarcomas, and glioblastomas

38. Role of cyclins, cyclin-dependent kinases (CDKs), and
CDK inhibitors in regulating the cell cycle.

39. • While cyclins arouse the CDKs, inhibitors (CDKIs),
silence the CDKs & exert negative control .
• CIP/WAF family of CDKIs, composed of three
proteins, called p21 , p27 , and p57 inhibits the CDKs
broadly.
• INK4 family of CDIs, made up of p15, p16, p18 , and
p19, selectively effects on cyclin D/CDK4 and cyclin
D/CDK6.
• Expression of these inhibitors is down-regulated by
mitogenic signaling pathways, thus promoting the
progression of the cell division.

40. • Internal controls of the cell cycle called checkpoints
• Two main cell cycle checkpoints, one at the G1/S
transition and the other at G2/M.
• S phase is the point of no return in the cell cycle.
G1/S checkpoint
• Checks for DNA damage before point of no return in
cell cycle.
• If damage is present  DNA-repair machinery and
mechanisms that arrest the cell cycle activated.

41. • The delay in cell cycle progression provides the time
needed for DNA repair.
• If the damage is not repairable, apoptotic pathways
activated to kill the cell.
• Thus, the G1/S checkpoint prevents the replication of
cells that have defects in DNA, which would be
perpetuated as mutations or chromosomal breaks in
the progeny of the cell.
• Gain of function mutations in D cyclin genes and
CDK4, oncogenes that promote G1/S progression.

42. G2/M checkpoint
• Monitors the completion of DNA replication and checks
whether cell can safely initiate mitosis and separate sister
chromatids.
• This checkpoint is particularly important in cells exposed
to ionizing radiation.
• Cells damaged by ionizing radiation activate the G2/M
checkpoint and arrest in G2.
• Defects in this checkpoint give rise to chromosomal
abnormalities
• To function properly, cell cycle checkpoints require
sensors of DNA damage, signal transducers, and effector
molecules

43. • Sensors and transducers of DNA damage seem to
be similar for G1/S and G2/M checkpoints.
• They include, as sensors, proteins of the RAD
family and ataxia telangiectasia mutated (ATM)
and as transducers, the CHK kinase families.
• In G1/S checkpoint, cell cycle arrest mediated
through p53, which induces cell cycle inhibitor p21.
• Arrest of cell cycle by G2/M checkpoint involves
both p53-dependent and p53-independent
mechanisms.
• Defects in cell cycle checkpoint components are a
major cause of genetic instability in cancer cells.

44. Cell Cycle Components and Inhibitors

45. TUMOR SUPPRESSOR GENES
INSENSITIVITY TO GROWTH INHIBITION AND ESCAPE
FROM SENESCENCE
• Failure of growth inhibition is one of the fundamental
alterations in the process of carcinogenesis.
• Whereas oncogenes drive the proliferation of cells, the
products of tumor suppressor genes apply brakes to
cell proliferation.
• Tumor suppressor proteins form a network of
checkpoints that prevent uncontrolled growth.

46. • Many tumor suppressors, such as RB and p53, are
part of a regulatory network that recognizes
genotoxic stress from any source, and responds by
shutting down proliferation.
• Expression of an oncogene in an otherwise
completely normal cell leads to quiescence, or to
permanent cell cycle arrest (oncogene-induced
senescence), rather than uncontrolled proliferation.
• Ultimately, the growth-inhibitory pathways may lead
the cells into apoptosis.

47. • Another set of tumor suppressors seem to be involved
in cell differentiation, causing cells to enter a post-
mitotic, differentiated pool without replicative
potential.
• Similar to mitogenic signals, growth-inhibitory, pro-
differentiation signals originate outside the cell and
use receptors, signal transducers, and nuclear
transcription regulators .

48. Tumor Suppressor Genes and Associated Familial
Syndromes and Cancers

49. Gene Protein function Familial syndrome Sporadic Cancer

50. Retinoblastoma gene: Governor of Proliferation.
• RB locus on chromosome 13q14 .
• RB protein,product of RB gene, is ubiquitously expressed
nuclear phosphoprotein that plays a key role in regulating
the cell cycle.
• Active hypophosphorylated state in quiescent cells and
inactive hyperphosphorylated state in the G1/S cell cycle
transition .
• It is a key negative regulator of G1 /S cell cycle transition.
• In G1, however, cells can exit the cell cycle, either
temporarily  quiescence or permanently senescence.

51. • In G1, therefore, diverse signals integrated to
determine whether the cell should enter the cell
cycle, exit the cell cycle and differentiate, or die.
• RB is a key node in this decision process as once cells
cross the G1 checkpoint can be paused cell cycle for
a time, but they are obligated to complete mitosis.
• Mechanism of Retinoblastoma in initiating DNA
replication by activating cyclin E gene transcription.

52. The role of RB in regulating the G1-S checkpoint of the cell
cycle.

53. • If RB is absent (gene mutations) or its ability to
regulate E2F transcription factors is derailed, the
molecular brakes on the cell cycle are released, and
the cells move through the cell cycle.
• The mutations of RB genes found in tumors are
localized to a region of the RB protein, called the “RB
pocket,” that is involved in binding to E2F.
• However, the versatile RB protein has also been
shown to bind to a variety of other transcription
factors that regulate cell differentiation .

54. Pathogenesis of retinoblastoma. Two mutations of the RB locus on chromosome 13q14 lead to
neoplastic proliferation of the retinal cells. In the sporadic form, both RB mutations in the tumor-
founding retinal cell are acquired. In the familial form, all somatic cells inherit one mutated copy
of RB gene from a carrier parent, and as a result only one additional RB mutation in a retinal cell
is required for complete loss of RB function.
TWO HIT HYPOTHESIS

55. • RB stimulates myocyte-, adipocyte-, melanocyte-,
and macrophage-specific transcription factors.
• Thus, the RB pathway couples control of cell cycle
progression at G1 with differentiation, which may
explain how differentiation is associated with exit
from the cell cycle.
• In addition to these dual activities, RB can also
induce senescence.

56. p53: Guardian of the Genome
• The p53 gene is located on chromosome 17p13.1, and
most common target for genetic alteration in human
tumors.
• A tumor suppressor gene that regulates cell cycle
progression, DNA repair, cellular senescence, and
apoptosis.
• Homozygous loss of p53 occurs in virtually every type of
cancer, including carcinomas of the lung, colon, and
breast—the three leading causes of cancer death.
• In most cases, the inactivating mutations affect both p53
alleles and are acquired in somatic cells.

57. • Less commonly, some individuals inherit one mutant
p53 allele.
• Inheritance of one mutant allele predisposes
individuals to develop malignant tumors because
only one additional “hit” is needed to inactivate the
second, normal allele.
• Such individuals, said to have the Li-Fraumeni
syndrome, have a 25-fold greater chance of
developing a malignant tumor by age 50 than the
general population.

58. • p53 mutations are common in a variety of human
tumors suggests that the p53 protein functions as a
critical gatekeeper against the formation of cancer.
• Indeed, p53 acts as a “molecular policeman” that
prevents the propagation of genetically damaged
cells.
• P53 protein is a transcription factor that is at the
center of a large network of signals that sense
cellular stress, such as DNA damage, shortened
telomeres, and hypoxia.

59. • Many activities of the p53 protein are related to its
function as a transcription factor.
• Several hundred genes have been shown to be
regulated by p53 in numerous different contexts.
• 80% of the p53 point mutations present in human
cancers are located in the DNA-binding domain of
the p53 protein.

60. • In non-stressed, healthy cells, p53 has a short half-
life (20 minutes), because of its association with
MDM2, a protein that targets it for destruction.
• When the cell is stressed, by an assault on its DNA,
p53 undergoes post-transcriptional modifications
that release it from MDM2 and increase its half-life.
• Unshackled from MDM2, p53 also becomes activated
as a transcription factor.

61. • Hundreds of genes whose transcription is triggered
by p53 grouped into two broad categories-
• 1. Causes cell cycle arrest
• 2. Causes apoptosis
• If DNA damage can be repaired during cell cycle
arrest, the cell reverts to a normal state.
• If the repair fails, p53 induces apoptosis or
senescence.

62. The role of p53 in maintaining the integrity of the genome

63. • p53-mediated cell cycle arrest may be considered the
primordial response to DNA damage.
• It occurs late in the G1 phase and is caused mainly by
p53-dependent transcription of the CDK inhibitor
CDKN1A (p21).
• p21 inhibits cyclin-CDK complexes and
phosphorylation of RB, thereby preventing cells from
entering G1 phase.
• p53 also inducing certain proteins, such as GADD45
(growth arrest and DNA damage), that help in DNA
repair.

64. • p53 can stimulate DNA-repair pathways by
transcription-independent mechanisms as well.
• If DNA damage is repaired successfully, p53 up-
regulates transcription of MDM2, leading to its own
destruction and thus releasing the cell cycle block.
• If the damage cannot be repaired, the cell may enter
p53-induced senescence or undergo p53-directed
apoptosis.

65. • p53-induced senescence
• It is a permanent cell cycle arrest.
• Characterized by specific changes in morphology and
gene expression that differentiate it from quiescence
or reversible cell cycle arrest.
• Senescence requires activation of p53 and/or RB and
expression of their mediators, such as the CDK
inhibitors, and is generally irreversible.
• Mechanisms of senescence involve epigenetic changes
that result in the formation of heterochromatin at
different loci throughout the genome.

66. • These senescence-associated heterochromatin foci
include pro-proliferative genes regulated by E2F.
• This drastically and permanently alters expression of
these E2F targets.
• Like all p53 responses, senescence may be
stimulated in response to a variety of stresses, such
as unopposed oncogene signaling, hypoxia, and
shortened telomeres

67. • p53-induced apoptosis of cells with irreversible DNA
damage is the ultimate protective mechanism
against neoplastic transformation.
• p53 directs the transcription of several pro-apoptotic
genes such as BAX and PUMA.
• It appears that affinity of p53 for the promoters and
enhancers of DNA-repair genes is stronger than its
affinity for pro-apoptotic genes.
• Thus, the DNA-repair pathway is stimulated first,
while p53 continues to accumulate.

68. • If the DNA damage is not repaired, enough p53
accumulates to stimulate transcription of the pro-
apoptotic genes and the cell dies.
• p53 activates transcription of the mir34 family of
miRNAs.
• miRNAs, bind to cognate sequences in the 3′
untranslated region of mRNAs, preventing translation.
• Blocking targets of mir34s include pro-proliferative
genes such as cyclins, and anti-apoptotic genes such
as BCL2 to induce growth arrest and apoptosis.

69. • With loss of p53 function, DNA damage goes
unrepaired, driver mutations accumulate in
oncogenes and other cancer genes, and the cell leads
to malignant transformation.
• It also has therapeutic implication like wild type TP53
are more likely killed by radiation and chemotherapy.
• Eg. Testicular teratocarcinomas and childhood acute
lymphoblastic leukemias have wild typeTP53 alleles.
• Lung cancers and colorectal cancers, which
frequently carry TP53 mutations, are relatively
resistant to chemotherapy and irradiation.

70. APC: Gatekeeper of Colonic Neoplasia.
• A tumor suppressors.
• Main function is to down-regulate growth-promoting
signals.
• Germ-line mutations at the APC (5q21) loci are
associated with familial adenomatous polyposis.
• All individuals born with one mutant allele develop
thousands of adenomatous polyps in the colon
during their teens or 20s (familial adenomatous
polyposis).

71. • One or more of these polyps undergoes malignant
transformation, giving rise to colon cancer.
• As with other tumor suppressor genes, both copies
of the APC gene must be lost for a tumor to arise.
• 70% to 80% of nonfamilial colorectal carcinomas and
sporadic adenomas also show homozygous loss of
the APC gene.
• Implicating APC loss in the pathogenesis of colonic
tumors.

72. • APC is a component of the WNT signaling pathway.
• Major role in controlling cell fate, adhesion, and cell
polarity during embryonic development.
• WNT signaling also required for self-renewal of
hematopoietic stem cells.
• WNT signals through a family of cell surface
receptors called frizzled (FRZ), and stimulates several
pathways.
• The central one involving β-catenin and APC.

73. The role of APC in regulating the stability and function of β-
catenin

74. • Loss of cell-cell contact, such as in a wound or injury to
the epithelium, disrupts the interaction between E-
cadherin and β-catenin.
• This allows β-catenin to travel to the nucleus and
stimulate proliferation.
• Re-establishment of these E-cadherin leads to β-
catenin again being sequestered at the membrane and
reduction in the proliferative signal.

75. • This mechanism known as contact-inhibition
mechanism.
• Loss of contact inhibition, by mutation of the E-
cadherin/β-catenin axis, or by other methods, is
characteristic of carcinomas.
• Loss of cadherins can favor the malignant phenotype
by allowing easy disaggregation of cells, which can
then invade locally or metastasize.
• Reduced cell surface expression of E-cadherin has
been seen in Ca esophagus, colon, breast, ovary, and
prostate.

76. CDKN2A INK4a/ARF
• Also called the CDKN2A gene locus.
• INK4a/ARF locus encodes two protein products.
1. p16/INK4a CDKI, which blocks cyclin D/CDK2-mediated
phosphorylation of RB, keeping the RB checkpoint in
place.
2. p14/ARF, activates the p53 pathway by inhibiting MDM2
and preventing destruction of p53.
• Both protein products function as tumor suppressors.
• Mutation or silencing by hypermethylation of the gene
of this locus impacts both the RB and p53 pathways.

77. The TGF-β Pathway
• Occur in most normal epithelial, endothelial, and
hematopoietic cells.
• TGF-β is a potent inhibitor of proliferation.
• It regulates cellular processes by binding to a serine-
threonine kinase complex composed of TGF-β
receptors I and II.
• Dimerization of the receptor upon ligand binding
leads to activation of the kinase and phosphorylation
of receptor SMADs (R-SMADs).

78. • Upon phosphorylation, R-SMADs can enter the
nucleus, bind to SMAD-4, and activate transcription
of genes, including the CDKIs p21 and p15/INK4b.
• In addition, TGF-β signaling leads to repression of c-
MYC, CDK2, CDK4, and cyclins A and E.
• These changes result in decreased phosphorylation
of RB and cell cycle arrest.
• In many forms of cancer the growth-inhibiting effects
of TGF-β pathways are impaired by mutations in the
TGF-β signaling pathway.

79. • These mutations may affect the type II TGF-β
receptor or interfere with SMAD molecules.
• Mutations affecting the type II receptor are seen in
cancers of the colon, stomach, and endometrium.
• Mutational inactivation of SMAD4 is common in
pancreatic cancers. In 100% of pancreatic cancers
and 83% of colon cancers, at least one component of
the TGF-β pathway is abnormal.

80. NF1 gene
• Individuals who inherit one mutant allele of the NF1
gene develop numerous benign neurofibromas.
• Inactivation of the second copy of the gene results in
optic nerve gliomas.
• This condition is called neurofibromatosis type 1.
• Some of the neurofibromas later develop into
malignant peripheral nerve sheath tumors.

81. • Neurofibromin, a GTPase-activating domain,
regulating signal transduction through RAS proteins.
• Neurofibromin facilitates conversion of RAS from an
active to an inactive state.
• With loss of neurofibromin function, RAS is trapped
in an active, signal-emitting state.

83. NF2 gene
• Germline mutations in the NF2 gene predispose to
the development of neurofibromatosis type 2.
• Individuals develop benign bilateral schwannomas of
the acoustic nerve.
• Somatic mutations affecting both alleles of NF2 have
also been found in sporadic meningiomas and
ependymomas.

84. • Neurofibromin 2 or merlin, homologous with the red
cell membrane cytoskeletal protein 4.1.
• Also related to the ERM (ezrin, radixin, and moesin)
family of membrane cytoskeleton-associated
proteins.
• Cells lacking merlin incapable of establishing stable
cell-to-cell junctions and insensitive to normal
growth arrest signals by contact inhibtion.

85. VHL gene
• Von Hippel-Lindau (VHL) gene located on
chromosome 3p.
• Germline mutations associated with hereditary renal
cell cancers, pheochromocytomas,
hemangioblastomas of CNS, retinal angiomas, and
renal cysts.
• VHL protein is part of a ubiquitin ligase complex. A
critical substrate for this activity is HIF1α (hypoxia-
inducible transcription factor 1α).
• In presence of 02, HIF1α is hydroxylated and binds to
VHL protein, l/t ubiquitination and proteasomal
degradation.

86. • In hypoxic environments the reaction cannot occur,
and HIF1α escapes recognition by VHL and
subsequent degradation.
• HIF1α then translocate to the nucleus and turn on
many genes, such as the growth/angiogenic factors
vascular endothelial growth factor (VEGF) and PDGF.
• Lack of VHL activity prevents ubiquitination and
degradation of HIF1α and is associated with
increased levels of angiogenic growth factors.

87. WT1 gene
• WT1 gene, located on chromosome 11p13.
• Associated with development of Wilms' tumor, a
pediatric kidney cancer.
• Both inherited and sporadic forms of Wilms' tumor
occur & mutational inactivation of WT1 locus seen in
both forms.
• The WT1 protein is a transcriptional activator of
genes involved in renal and gonadal differentiation.

88. • It regulates the mesenchymal-to-epithelial transition
that occurs in kidney development.
• Tumorigenic effect of WT1 deficiency intimately
connected with role of gene in the differentiation
of genitourinary tissues.
• WT2, located on 11p15, is associated with the
Beckwith-Wiedemann syndrome .

89. PTEN gene: a tumor suppressor gene
• PTEN (phosphatase and tensin homologue) is a
membrane-associated phosphatase.
• Encoded by gene on chromosome 10q23.
• It is mutated in Cowden syndrome : an autosomal
dominant marked by frequent benign growth like
skin appendages.
• Has increased incidence of epithelial cancer
particularly of breast, endometrium, and thyroid.
• It act by serving a break on the PI3K/AKT arm of
receptor tyrosine kinase pathway.

90. Growth-Promoting Metabolic Alterations:
The Warburg Effect
• Even in presence of ample oxygen Cancer cells
have a distinct form of cellular metabolism
characterized by high level of glucose uptake
and increased conversion of glucose to lactose
(fermentation) via the glycolytic pathway.
• This is called Warburg effect or aerobic
glycolysis.
• Glucose hunger of the cancer cell is visualized
by PET scan.

91. • “Warburg metabolism” is not cancer specific,
but instead is a general property of growing
cells that becomes “fixed” in cancer cells.
• This metabolism occur In cancer cell because
aerobic glycolysis provides rapidly dividing
tumor cells with metabolic intermediates that
are needed for the synthesis of cellular
components, whereas mitochondrial oxidative
phosphorylation does not.
• Metabolic reprogramming is produced by
signaling cascades downstream of growth
factor receptors.

92. Relation between pro- growth
signaling factors and metabolism
• PI3K/AKT signaling- it up regulate the activity
of glucose transporters and multiple glycolytic
enzymes leading to-
– Increase glycolysis.
– Shunting of mitochondrial intermediates to
pathways leading to lipid biosynthesis.
– stimulates factors that are required for protein
synthesis.

93. • Receptor tyrosine kinase activity –
• It transmit growth signal to nucleus.
• This causing the alteration of M2 isoform of
pyruvate kinase ( catalyze the last step in
glycolytic pathway, PEP pyruvate.) causing
phosphorylation of M2 isoform, attenuating its
activity.
• This create a damming effect leading to build-up
of upstream glycolytic intermediates.
• Post-mitotic tissues with high demand for ATP
such as the brain express M1 isoform which is
insensitive to growth factor signaling pathways.

94. • MYC – it causes changes in gene expression that
support anabolic metabolism and cell growth.
• It upregulates the multiple glycolytic enzymes
and glutaminase , required for mitochondrial
utilization of glutamine.
• Tumor suppressors often inhibit metabolic
pathways that support growth like breaking effect
of PTEN on PI3K/AKT pathway.
• p53 target gene that inhibit glucose uptake,
glycolysis, lipogenesis, and generation of NADPH

95. Metabolism and cell growth Quiescent cells rely mainly on the Krebs cycle for ATP
production; if starved, autophagy. When stimulated by growth factors, normal cells
markedly upregulate glucose and glutamine uptake, which provide carbon sources
for synthesis of nucleotides, proteins, and lipids. In cancers, oncogenic mutations
involving growth factor signaling pathways and other key factors such as
MYC deregulate these metabolic pathways, an alteration known as the Warburg effect.

96. EVASION OF APOPTOSIS
• Apoptosis represents a barrier that must be
surmounted for cancer to occur.
• Signals, ranging from DNA damage to loss of
adhesion to the basement membrane (anoikis), can
trigger apoptosis.
• Two distinct programs that activate apoptosis, the
extrinsic and intrinsic pathways.
• CD95 receptor–induced and DNA damage–triggered
pathways of apoptosis .

97. Intrinsic and extrinsic pathways of
apoptosis
• (1) Loss of p53, leading to
reduced function of pro-
apoptotic factors such as BAX.
• (2) Reduced egress of
cytochrome c from
mitochondria as a result of
upregulation of anti-apoptotic
factors such as BCL2, BCL-XL,
and MCL-1.
• (3) Loss of apoptotic peptidase
activating factor 1 (APAF1).
• (4) Upregulation of inhibitors
of apoptosis (IAP).
• (5) Reduced CD95 level.
• (6) Inactivation of death-
induced signaling complex.
FADD, Fas-associated via death
domain.

98. Limitless Replicative Potential: The Stem
Cell–Like Properties of Cancer Cells
• All cancers contain cells that are immortal and
have limitless replicative potential.
• Three interrelated factors appear critical to
the immortality of cancer cells:
• (1) evasion of senescence;
• (2) evasion of mitotic crisis;
• (3) the capacity for self-renewal.

99. Evasion of senescence
• Normal human cells have the capacity to
divide 60 to 70 times.
• The senescent state is associated with
upregulation of tumor suppressors such as
p53 and INK4a/p16 (perhaps in response to
the accumulation of DNA damage over time).
• Maintaining RB in a hypophosphorylated
state.

100. Evasion of mitotic crisis
• Mitotic crisis- This phenomenon has been
ascribed to progressive shortening of telomeres
at the ends of chromosomes.
• When the telomeric DNA is eroded, the exposed
chromosome ends are “sensed” as double-
stranded DNA breaks. By p53 and cell arrest
occur.
• if p53 is dysfunctional, the nonhomologous
endjoining pathway is activated and may join the
“naked” ends of two chromosomes.

101. Escape of cells from senescence and mitotic catastrophe caused by
telomere shortening In the absence of checkpoints, DNA repair
pathways, such as the nonhomologous end-joining (NHEJ) pathway
are inappropriately activated, leading to the formation of
dicentric chromosomes.

102. • Inappropriately activated repair system results in
dicentric chromosomes that are pulled apart at
anaphase.
• Resulting in new double-stranded DNA breaks.
• The resulting genomic instability from the repeated
bridge-fusion-breakage cycles eventually produces
mitotic catastrophe, characterized by massive cell
death.

103. Self-renewal
• Self-renewal means that each time a stem cell
divides at least one of the two daughter cells
remains a stem cell.
• Symmetric division - both daughter cells remain
stem cells. Occur during embryogenesis.
• Asymmetric division - only one daughter cell
remains a stem cell. the non– stem cell daughter
proceeds along some differentiation pathway,
losing “stemness” but gaining one or more
functions in the process.

104. • Cancers cells are immortal and have limitless
proliferative capacity, they too must contain cells
that self-renew, so-called cancer stem cells.
• Cancer stem cells arises from transformation of
tissue stem cells like in CML and conversion of
conventional somatic cells to transformed cells
with acquired property of stemness. eg Acute
myeloid leukemia cancer stem cells in this disease
arise from more differentiated hematopoietic
progenitors that acquire an abnormal capacity for
self-renewal.

105. Origins of cells with self-renewing capacity in
cancer.

106. ANGIOGENESIS
• New blood vessel formation in adults k/a
angiogenesis or neovascularization
• Angiogenesis is controlled by a balance between
angiogenesis promoters and inhibitors; in angiogenic
tumors this balance is skewed in favor of promoters.
Two machenisms
• New vessels sprout from previously existing
capillaries( capillary growth)
• Vasculogenesis, in which endothelial cells are
recruited from the bone marrow (EPC)

107. Angiogenesis from pre existing vessels

108. • The local balance of angiogenic and
antiangiogenic factors is influenced by several
factors-
• Relative lack of oxygen due to hypoxia
stabilizes HIF1α, an oxygen-sensitive
transcription factor which then activates the
transcription of the proangiogenic cytokines
VEGF and bFGF.
• Transcription of VEGF is also influenced by
signals from the RAS-MAP kinase pathway,
and gain-of-function mutations in RAS or MYC
upregulate the production of VEGF.

109. • NO---> vasodilation
• VEGF---> ↑permeability
• MMP---> proteolytic degredation of basement memb
• Plasminogen Activator---> disruption of cell to cell
contact between endothelial cells
• EC migration towards angiogenic stimuli.
• Proliferation of endothelial cells just behind leading
front of migrating cells.

110. • Maturation of endothelial cells by growth inhibition
& remodeling into capillary tubes.
• Recruitment of periendothelial cells ( pericytes &
smooth muscles from mature vessels)
(B) Angiogenesis from Endothelial Precursor
cells
1. ECP---> angioblasts
2. Hematopoetic/ ECP---> hemangioblasts

112. • EPC----> moblise from BM --> site of tumor/injury -->
EPC diffrentiates into endothelial cells---> forms a
mature network by linking to existing vessels
• EPC expresses markers of
1. Hematopetic stem cells
2. VEGFR 2
3. V E cadherin
• Leads to re- endothelization of vascular implant and
neovascularization.

113. • Like normal tissues, tumors require delivery of
oxygen and nutrients and removal of waste products.
• Cancer cells can stimulate neo-angiogenesis.
• Tumor vasculature is abnormal, however with leaky
& dilated vessels & haphazard connection pattern.
• Neovascularization has dual effect on tumor growth
1. Perfusion supplies nutrients and oxygen.
2. Newly formed endothelial cells stimulate the
growth of adjacent tumor cells by secreting growth
factors, such as IGFs, PDGF, and GM-CSF.

114. • Angiogenic switch involves increased production of
angiogenic factors and/or loss of angiogenic
inhibitors.
• These factors may be produced directly by the
tumor cells themselves or by inflammatory cells
macrophages or other stromal cells associated with
the tumors.
• Proteases, elaborated from tumor cells directly /
from stromal cells involved in regulating the balance
between angiogenic and anti-angiogenic factors.

115. • Many proteases can release the proangiogenic basic
fibroblast growth factors (bFGF) stored in the ECM.
• Three potent angiogenesis inhibitors—angiostatin,
endostatin, and vasculostatin—are produced by
proteolytic cleavage of plasminogen, collagen, and
transthyretin, respectively.
• The angiogenic switch is controlled by several
physiologic stimuli, such as hypoxia..
• VEGF also increases expression of ligands activating
Notch signaling pathway, which plays crucial role in
regulating branching and density of the new vessels

116. INVASION AND METASTASIS
• Invasion and metastasis are biologic hallmarks of
malignant tumors.
• Major cause of cancer-related morbidity and
mortality..
• Millions of cells released into circulation each day
from 10 tumor, only few produces metastases .
• Each step in the process is subject to a multitude of
controls.

117. • Metastatic cascade divided into two phases
(1) Invasion of the extracellular matrix (ECM)
(2) Vascular dissemination, homing of tumor cells, and
colonization
Invasion of Extracellular Matrix
• Initiates the metastatic cascade and is an active
process that can be resolved into several steps
• “Loosening up” of tumor cell–tumor cell interactions
• Degradation of ECM
• Attachment to novel ECM components
• Migration and invasion of tumor cells

118. • Dissociation of cancer cells from one another
is often the result of alterations in intercellular
adhesion molecules and is the first step in the
process of invasion.
• E-cadherins mediate the homotypic adhesion
of epithelial cells, serving to both hold the
cells together and to relay signals between the
cells.

119. • Degradation of the basement membrane and
interstitial connective tissue is the second step
in invasion.
• By proteolytic enzymes themselves or by
inducing stromal cells (fibroblasts and
inflammatory cells) to elaborate proteases.
• Matrix metalloproteinases (MMPs), cathepsin
D, and urokinase plasminogen activator.

120. • The third step in invasion involves changes in
attachment of tumor cells to ECM proteins.
• Normal epithelial cells have receptors help to
maintain the cells in a resting, differentiated
state.
• Loss of adhesion in normal cells leads to
induction of apoptosis.
• Locomotion is the final step of invasion,
propelling tumor cells through the degraded
basement membranes and zones of matrix
proteolysis.

121. The metastatic cascade. Sequential steps involved in the
hematogenous spread of a tumor

122. Sequence of events in the invasion of epithelial basement
membranes by tumor cells.

123. Vascular Dissemination and Homing of Tumor Cells
• Within the circulation, tumor cells tend to aggregate in
clumps.
• Homotypic adhesions among tumor cells & heterotypic
adhesion between tumor cells and platelets l/t
formation of platelet-tumor aggregates enhancing
tumor cell survival and implantability.
• Tumor cells may also bind and activate coagulation
factors, resulting in the formation of emboli.
• Arrest and extravasation of tumor emboli at distant
sites involves adhesion to the endothelium, followed
by egress through the basement membrane.

124. • Involved adhesion molecules (integrins, laminin
receptors) and proteolytic enzymes.
• Particularly CD44 adhesion molecule, which is
expressed on normal T lymphocytes and is used by
these cells to migrate to selective sites in the lymphoid
tissue.
• Such migration is accomplished by the binding of CD44
to hyaluronate on high endothelial venules, and
overexpression of CD44 may favor metastatic spread.
• At the new site, tumor cells must proliferate, develop a
vascular supply, and evade the host defenses.

125. Molecular Genetics of Metastasis
Development
• Four models are presented
1. Metastasis is caused by rare variant clones that
develop in the primary tumor.
2. Metastasis is caused by the gene expression pattern
of most cells of the primary tumor, referred to as a
metastatic signature.
3. Combination of A and B, in which metastatic
variants appear in a tumor with a metastatic gene
signature.

126. 4. Metastasis development is greatly influenced by
the tumor stroma, which may regulate angiogenesis,
local invasiveness, and resistance to immune
elimination, allowing cells of the primary tumor, as in
C, to become metastatic.

127. Mechanisms of metastasis development within a
primary tumor
• A. Metastasis is caused by
rare variant clones that
develop in the primary
tumor.
• B. Metastasis is caused by
the gene expression pattern
of most cells of the primary
tumor, referred to as a
metastatic signature.
• C. A combination of A & B.
• D. Metastasis influenced by
stroma which may regulate
angiogenesis, local
invasiveness, and resistance
to immune elimination.

128. Evasion of Host Defense
• Tumor cells can be recognized as “foreign” and
eliminated by the immune system.
• Immune surveillance- it is the normal function of
the immune system is to constantly “scan” the
body for emerging malignant cells and destroy
them.
• Cancer immunoediting– it is the ability of the
immune system to shape and mold the
immunogenic properties of tumor cells in a
fashion that ultimately leads to the darwinian
selection of subclones that are best able to avoid
immune elimination.

129. • Tumor Antigens`- Antigens found in tumors that
elicit an immune response.
• Classes of tumor antigens-
• 1. Products of mutated genes
• 2. Overexpressed or aberrantly expressed cellular
proteins
• 3. Tumor antigens produced by oncogenic viruses
• 4. Oncofetal antigens
• 5. Altered cell surface glycolipids and
glycoproteins
• 6. Cell type–specific differentiation antigens

130. • Products of mutated genes- due to genetic
alteration in proto- oncogene and tumor
suppressor gene, the new mutated protein is
produced that is non self.
• These may enter the class I MHC antigen-
processing pathway and be recognized by
CD8+ T cells. Or may enter class II antigen-
processing pathway in antigen presenting cells
that have phagocytosed dead tumor cells, and
thus be recognized by CD4+ T cells

131. Tumor antigens recognized by CD8+ T
cells

132. • 2. Overexpressed or aberrantly expressed
cellular proteins- Tumor antigens may also be
normal cellular proteins that are abnormally
expressed in tumor cell.
• Eg. Tyrosinase- when produced in excess
amount
• Cancer testis antigen- genes that are silent in
all tissues except germ cells in the adult testis
• Melanoma antigen gene (MAGE) family
expressed by variety of tumor like melanoma,
lung , liver, stomach..

133. • Oncofetal antigens- proteins that are
expressed at high levels on cancer cells and in
normal developing (fetal) tissues.
• These are specific and can serve as markers
that aid in tumor diagnosis and clinical
management.
• Eg. Carcinoembryonic antigen (CEA) and α-
fetoprotein (AFP).

134. • Altered cell surface glycolipids and
glycoproteins- these altered molecules
include gangliosides , blood group antigens
and mucin.
• Tumors often have dysregulated expression of
the enzymes that synthesize these
carbohydrate side chains, which leads to the
appearance of tumor-specific epitopes on the
carbohydrate side chains
• Eg. CA-125 and CA-19-9- in ovarian carcinoma

135. • Cell type–specific differentiation antigens-
antigen that are normally present on the cell of
origin.
• They are called differentiation antigens because
they are specific for particular lineages or
differentiation stages of various cell types.
• Their importance is as potential targets for
immunotherapy and for identifying the tissue of
origin of tumors
• Eg . Monoclonal antibody like anti CD20 have
broad cytocidal activity against mature B-cell
lymphomas and leukemias
• Anti CD30 in CD30positive lymphoma
• Toxin-conjugated antibodies specific for HER2

136. Antitumor Effector Mechanisms
• Cell-mediated immunity is the dominant
antitumor mechanism in vivo.
• Cytotoxic T lymphocytes- CD8+ CTLs have
protective role against virus-associated
neoplasms (e.g., EBV- and HPV-induced tumors).
• Natural killer cells- lymphocytes that are capable
of destroying tumor cells without prior
sensitization.
• Provide first line of defense against tumor cells.

137. • Macrophages- Activated macrophages exhibit
cytotoxicity against tumor cells in vitro.
• Interferon-γ, a cytokine secreted by T cells and
NK cells, is a potent activator of macrophages.

138. Immune Surveillance and Escape
• Persons with congenital immunodeficiencies
develop cancers at about 200 times the rate in
immunocompetent individuals.
• Mechanism of tumor cell to escape or evade the
immune system-
• 1. Selective outgrowth of antigen-negative
variants
• 2. Loss or reduced expression of MHC molecules-
by this escape by attack of cytotoxic T cells

139. • 3. Activation of immunoregulatory pathways
• Tumor cells actively inhibit tumor immunity by
engaging normal pathways of immune regulation
that serve as “checkpoints” in immune responses.
• Mechanism-1. Downregulate the expression of
costimulatory factors on antigen-presenting cells
eg in dendritic cells.
• 2. Upregulate the expression of PD-L1 and PD-L2,
cell surface proteins that activate the
programmed death-1 (PD-1) receptor on effector
T cells. PD-1, like CTLA-4, may inhibit T cell
activation.

140. • Secretion of immunosuppressive factors by
cancer cells-
• TGF-β- a potent immunosuppressant
• Galectins- sugar-rich lectin-like factors that
skew T-cell responses so as to favor
immunosuppression.
• Interleukin-10, prostaglandin E2, VEGF inhibit
the diapedesis of T cells from the vasculature
into the tumor bed.
• Induction of regulatory T cells (Tregs)
• factors that favor the development of
immunosuppressive regulatory T cells,

141. Mechanisms by which tumors evade
the immune system

142. Genomic Instability
• These are genetic alteration that increase the
mutation rate and expedite the acquisition of
driver mutation.
• Mutations in DNA-repair genes themselves are
not oncogenic, but their abnormalities greatly
enhance the occurrence of mutations in other
genes during the process of normal cell division.
• Genomic instability occurs when both copies of
the DNA repair gene are lost;

143. • Types of DNA repair system
– Mismatch repair
– Nucleotide excision repair
– Recombination repair
• Defect in any these of mechanism contributes
to different types of cancer

144. Hereditary Nonpolyposis Colon
Cancer Syndrome.
• It is an autosomal dominant disorder
characterized by familial carcinomas of the
colon affecting predominantly the cecum and
proximal colon.
• It occur due to defect in DNA mismatch repair.
• These proteins acts as a spell checker.
• Eg. if there is an erroneous pairing of G with T
rather than the normal A with T, the
mismatch-repair factors correct the defect.

145. • HNPCC syndrome inherit one abnormal copy
of a mismatch repair gene.
• when cells acquire loss of- function mutations,
in their normal alleles with loss of
“proofreading” function .
• These errors gradually accumulate and may
activate proto- oncogene or deactivate tumor
suppressor gene.
• So DNA-repair genes behave like tumor
suppressor genes in their mode of inheritance.

146. • Mismatch-repair defects has microsatellite
instability.
• Microsatellites are tandem repeats of one to six
nucleotides found throughout the genome.
• These are constant in normal people but may be
increase or decrease in length in HNPCC.
• DNA mismatch-repair genes involved in HNPCC
are MSH2 and MLH1, genes encoding TGF-β
receptor II, TCF component of the β-catenin
pathway, & BAX.

147. Xeroderma Pigmentosum
• It is a disorder of DNA repair.
• Increased risk for the development of cancers
of the skin particularly following exposure to
the UV light.
• UV radiation causes cross-linking of pyrimidine
residues, preventing normal DNA replication.

148. Diseases with Defects in DNA Repair by
Homologous Recombination
• Hypersensitivity to DNA-damaging agents, such
as ionizing radiation( Bloom syndrome and
ataxia-telangiectasia) or DNA cross-linking
agents, such as many chemotherapeutic drugs
(Fanconi anemia).
• Gene mutated in ataxia telangiectasia ATM
recognize and respond to DNA damage by
ionizing radiation.
• Mutation of BRCA1 and BRCA2 in breast cancer.

149. Cancers Resulting from Mutations Induced by
Regulated Genomic Instability: Lymphoid Neoplasms
• B cells and T cells express a pair of gene
product ,RAG1 and RAG2, it carry out V(D)J
segment recombination, permitting the
assembly of functional antigen receptor
genes.
• Eg. lymphoid cells mutation.

150. Cancer-Enabling Inflammation
• Infiltrating cancers provoke a chronic
inflammatory reaction leading to “wounds
that do not heal.”
• The cancer-enabling effects of inflammatory
cells are following:
• Release of factors that promote proliferation-
secreted by infiltrating lymphocytes
• Eg. EGF and proteases.

151. • Removal of growth suppressors- growth is
suppressed by cell- cell and cell-ECM interaction.
• Proteases degrade adhesion molecule, and
remove this barrier.
• Enhanced resistance to cell death –
• Detachment of epithelial cells from basement
membranes and from cell-cell interactions can
lead to death called anoikis.
• Tumor-associated macrophages may prevent
anoikis by expressing adhesion molecules like
integrins.

152. • Inducing angiogenesis- due to release of
angiogenic factors like VEGF.
• Activating invasion and metastasis- Proteases
foster tissue invasion by remodeling the ECM,
while TNF and EGF may directly stimulate tumor
cell motility.
• TGF-β, may promote epithelial-mesenchymal
transitions, process of invasion and metastasis.
• Evading immune destruction-
immunosuppressive factors like TGF- β, suppress
the function of CD8+ cytotoxic T cells.

153. Dysregulation of Cancer-Associated
Genes
• Chromosomal Changes-
• 1. Chromosomal Translocations – defined as
a chromosome abnormality caused by
rearrangement of parts between nonhomologous
chromosomes.
• Translocations can activate proto-oncogenes in
two ways:
• 1. By promoter or enhancer substitution-
• Translocation results in overexpression of a
protooncogene by swapping its regulatory
elements with those of another gene.

154. • 2. By formation of a fusion gene- in which coding
sequences of two genes are fused in part or in
whole, leading to the expression of a novel
chimeric protein with oncogenic properties.
• Overexpression of a proto-oncogene caused by
translocation is exemplified by Burkitt lymphoma
• Overexpression of MYC located on chromosome
8q24.

155. Oncogenes Created by Translocations
Malignancy Translocation Affected Genes
Chronic myelogenous
leukemia (CML)
(9;22)(q34;q11) ABL 9q34
BCR 22q11
Acute myeloid leukemia
(AML)
(8;21)(q22;q22)
(15;17)(q22;q21)
AML 8q22
ETO 21q22
PML 15q22
RARA 17q21
Burkitt lymphoma (8;14)(q24;q32) MYC 8q24
IGH 14q32
Mantle cell lymphoma (11;14)(q13;q32) CCND1 11q13
IGH 14q32
Follicular lymphoma (14;18)(q32;q21) IGH 14q32
BCL2 18q21
Ewing sarcoma (11;22)(q24;q12) FLI1 11q24
EWSR1 22q12
Prostatic adenocarcinoma (7:21)(p22;q22)
(17:21)(p21;q22)
TMPRSS2 (21q22.3)
ETV1 (7p21.2)
ETV4 (17q21)

156. • The Philadelphia chromosome- characteristic of CML
and a subset of B-cell acute lymphoblastic leukemias is
an example of chromosomal rearrangement that
creates a fusion gene encoding a chimeric oncoprotein.
• The ABL gene on chromosome 9 and BCR on
chromosome 22.
• There is Non-homologous end-joining then leads to a
reciprocal translocation that creates an oncogenic BCR-
ABL fusion gene.
• The BCR-ABL fusion gene encodes a chimeric BCR-ABL
protein with constitutive tyrosine kinase activity.

157. • Fusion gene encode nuclear factor that regulate
transcription or chromatin structure.
• Eg. Acute promyelocytic leukemia (APML)
associated with reciprocal translocation between
chromosomes 15 and 17 that produces a PML-
RARA fusion gene.
• Mechanism of action of fusion gene-
• The fusion gene encodes a chimeric protein
consisting of part of a protein called PML and
part of the retinoic acid receptor-α (RARα).

158. Molecular pathogenesis of acute promyelocytic leukemia and
basis for response to all-trans retinoic acid. ATRA

159. • Deletions -Deletion of specific regions of
chromosomes is associated with the loss of
particular tumor suppressor genes.
• eg. RB gene deletion on 13q14- retinoblastoma
• VHL gene deletion on 3p in RCC.
• Few deletion causing activation of proto-
oncogene like in T- cell acute lymphoblastic
leukemias have small deletions of chromosome 1,
leading to overexpression of the TAL1
transcription factor.

160. • Gene Amplification- A selective increase in
the number of copies of a gene coding for a
specific protein without a proportional
increase in other genes.
• Overexpression of oncogenes may also result
from reduplication and amplification of their
DNA sequences.
• Two mutually exclusive patterns are seen:
• (1) double minutes.
• 2. homogeneous staining regions

161. • The affected chromosomal regions lack a normal
pattern of light and dark-staining band appearing
homogeneous in karyotypes.
• Eg. NMYC in neuroblastoma and ERBB2 in breast
cancers.
• Chromothrypsis- phenomenon by which up to
thousands of clustered chromosomal
rearrangements occur in a single event in
localised and confined genomic regions in one or
a few chromosomes.

162. Epigenetic Changes
• epigenetics” refers to factors other than the
sequence of DNA that regulate gene
expression.
• It include histones modifications catalyzed by
enzymes associated with chromatin regulatory
complexes;
• DNA methylation, a modification created by
DNA methyltransferases;

163. • epigenetic alterations in cancers can be divided
into the following categories:
• 1. Silencing of tumor suppressor genes by local
hypermethylation of DNA - hypermethylation of
the promoters of tumor suppressor genes that
results in their transcriptional silencing.
• Eg. Hypermethylation of CDKN2A,which encodes
p14/ARF and p16/INK4a( tumor suppressor gene)

164. • Global changes in DNA methylation- this is
abnormal patterns of DNA methylation
throughout the genomes. in the form of
hypermethylation or hypomethylation.
• Eg. Acute myeloid leukemia
• The consequence is altered expression of
multiple genes may be overexpressed or
underexpressed compared to normal.
• Hypomethylated genomes also exhibit
chromosomal instability

165. • Changes in histones these changes in histones
near genes that influence cellular behavior.
• mutations that affect the activities of protein
complexes that “write”, “read” and “erase”
histone marks, or that position nucleosomes
on DNA.

166. Epigenomic Regulatory Genes that
Mutated in Cancer
Gene(s) Function Tumor (Approximate Frequency of
Mutation)
DNMT3A DNA methylation Acute myeloid leukemia (20%)
MLL1 Histone methylation Acute leukemia in infants (90%)
MLL2 Histone methylation Follicular lymphoma (90%)
CREBBP/ EP300 Histone acetylation Diffuse large B cell lymphoma (40%)
ARID1A Nucleosome
positioning/chromatin
remodeling
Ovarian clear cell carcinoma (60%),
endometrial carcinoma (30%-40%)
SNF5 Nucleosome
positioning/chromatin
remodeling
Malignant rhabdoid tumor (100%)
PBRM1 Nucleosome
positioning/chromatin
remodeling
Renal carcinoma (30%)

167. Noncoding RNAs and Cancer
• OncomiRs- miR-200 promote epithelial-
mesenchymal transitions which is important in
invasiveness and metastasis.
• miR-155- overexpressed in many human B cell
lymphomas and indirectly upregulate genes that
promote proliferation, including MYC.
• Tumor suppressive miRs- miR-15 and miR-16 in
chronic lymphocytic leukemia,
• their loss leads to upregulation of the anti-
apoptotic protein BCL-2.

168. • Tumor suppressive properties of mIR
processing factors families that are prone to
the development of an unusual collection of
neoplasms have heterozygous germline
defects in DICER.
• DICER encodes endonuclease, required for the
processing and production of functional mIRs.

169. Molecular Basis of Multistep
Carcinogenesis
• cancers result from the stepwise accumulation of
multiple mutations that act in complementary
ways to produce a fully malignant tumor.
• Eg Colon cancer- colon epithelial hyperplasia
formation of adenomas  progressively
enlarge malignant transformation.
• inactivation of the APC gene  activation of RAS
 loss of a tumor suppressor gene on 18q and
loss of TP53.

170. Molecular model for the evolution of colorectal cancers through
the adenoma-carcinoma sequence

171. Thank you