Multistep Carcinogenesis - mutation double hit gene

1. MULTISTEP
CARCINOGENESIS
MODERATOR-DR.KANCHANA U T
BY- NAYANTARA M NIRGUDE

2. NEOPLASIA-
Is a disorder of cell growth that is triggered by a series of acquired
mutations affecting a single cell and its clonal progeny.
It literally means ‘new growth’.
Malignant tumors are collectively referred to as cancers.
Derived from the Latin word for “CRAB”.
They adhere to any part that they seize in an
Stubborn manner - similar to a crab’s behavior.

3. It can be -
Benign Malignant.
Benign-
A tumor is said to be benign when its
microscopic and gross characteristics are
considered to be relatively innocent,
implying that it will remain localized and is
amenable to local surgical removal.
Malignant-
Malignant neoplasm implies that the lesion can
invade and destroy adjacent structures and
spread to to distant sites (metastasize) to cause
death.

4. Carcinogenesis or oncogenesis or tumorigenesis means
mechanism of induction of tumours.
 Agents which can induce tumours are called carcinogens

5. A. Molecular pathogenesis of cancer (genes and cancer)
B. Chemical carcinogens and chemical carcinogenesis
C. Physical carcinogens and radiation carcinogenesis
D. Biologic carcinogens and viral oncogenesis

6.  Genes that are recurrently affected by genetic aberrations in cancers
 Contribute directly to the malignant behavior of cancer cells
 Causative mutations –give rise to cancer genes may be acquired –
I. Acquired -Environmental agents –
a) Chemicals
b) Radiation
c) Viruses
II. Spontaneously
III. Inherited in the germ line

7. ONCOGENES
I.-Mutated or overexpressed versions –PROTO-
ONCOGENES
II.-Induce a transformed phenotype when expressed in cells
by promoting increased cell growth
III.-Encode transcription factors
IV.DOMINANT GENE
TUMOR SUPPRESSOR GENES
-Normally prevent uncontrolled growth
-both normal alleles must be damaged for
transformation
2 general groups- Governor genes
a.Guardian genes
b.
Genes that regulate apoptosis
-Genes of this class that protect against apoptosis-
overexpressed
-Genes promote apoptosis tend to be underexpressed
or functionally inactivated by mutations
Genes that regulate interactions between tumor
cells and host cells
-Recurrently mutated or functionally
-Important genes are which enhance or inhibit
recognition of tumors cells by the host immune
system
Cancer
genes

8. The genetic changes found in cancers vary from point mutations involving single
nucleotides to abnormalities large enough to produce gross changes in chromosome
structure.
I. Driver and Passenger Mutations
II. Point Mutations
III. Gene Rearrangements
IV. Deletions
V. Gene Amplifications
VI. Aneuploidy
VII. MicroRNAs and Cancer
VIII.Epigenetic Modifications and Cancer

9. DRIVER MUTATIONS PASSENGER MUTATIONS
alter the function of cancer genes do not affect cellular behavior
directly contribute to the development or
progression of cancer
are neutral in terms of fitness
Usually acquired and occasionally inherited Acquired mutations
tightly clustered within cancer genes, sprinkled throughout the genome

10. POINT MUTATIONS-
 Either activate or inactivate protein products of the affected genes depending on
their precise position and consequence
proto-oncogenes oncogenes
 Produce a GAIN-OF-FUNCTION by altering amino acid residues in a domain that
normally holds the protein’s activity in check.
 By contrast- point mutations (insertions and deletions) in tumor suppressor genes-
reduce or disable the function of the encoded protein. TP53 is most commonly affected by
point mutations
convert

11. GENE REARRANGEMENTS-
 Produced by chromosomal translocations or inversions.
 Activate proto-oncogenes in 2 ways-
I. Overexpression of proto-oncogenes by removing them from their normal regulatory
elements and placing them under control of an inappropriate, highly active promoter
or enhancer.
Example- B cell lymphoma –
a) Burkitt lymphoma- translocation is usually between chromosomes 8 and 14-
overexpression of the MYC gene on chromosome 8 by juxtaposition with
immunoglobulin heavy chain gene regulatory elements on chromosome

12. b) Follicular lymphoma- a reciprocal translocation between chromosomes 14 and 18 leads to
overexpression of the anti-apoptotic gene.
BCL2, on chromosome 18, also driven by immunoglobulin gene regulatory elements.
II) Other oncogenic gene rearrangements create fusion genes encoding novel chimeric
proteins
Example- Philadelphia (Ph) chromosome in chronic myeloid leukemia, consisting of a
balanced reciprocal translocation between chromosomes 9 and 22
results in the fusion of portions of the BCR gene on chromosome 22 and the ABL gene on
chromosome 9.

13. GENE DELETION-
Deletion of specific regions of chromosomes may result in the
loss of particular tumor suppressor genes
Another prevalent abnormality in tumor cells.
Require inactivation of both alleles
Common mechanism –
I. Inactivating point mutation in one allele
II. Followed by deletion of the other, nonmutated allele

14. Gene amplification-
1. Proto-oncogenes may be converted to oncogenes by gene
amplification, with consequent overexpression and hyperactivity of
otherwise normal proteins
2 mutually exclusive patterns are seen microscopically:
a. Multiple small, extrachromosomal structures called double minutes
b. Homogeneously staining regions

15. ANEUPLOIDY
Aneuploidy is defined as a number of chromosomes that is not a
multiple of the haploid state
 Errors of the mitotic checkpoint, the major cell cycle control
mechanism that acts to prevent mistakes in chromosome
segregation

16.  Overexpression of mirna-leads to
carcinogenesis by-
 Reducing the expression of tumor
suppressors
 Deletion or loss of expression of
mirnas can lead to
overexpression of proto-
oncogenes.

17. All cancers display eight fundamental
changes in cell physiology:
1. Self-sufficiency in growth signals
2. Insensitivity to growth-inhibitory
signals
3. Altered cellular metabolism
4. Evasion of apoptosis
5. Limitless replicative potential
(immortality)
6. Sustained angiogenesis
7. Invasion and metastasis
8. Evasion of immune surveillance

18. Acquired from gain-of-function mutations that convert proto-oncogenes to
oncogenes
ONCOGENES encode ONCOPROTEINS promote cell growth
(even in the absence of normal growth-promoting signals)

19. The binding of a growth factor to its specific receptor
Transient and limited activation of the growth factor receptor
Activates several signal-transducing proteins on the inner leaflet of the plasma membrane
Transmission of the transduced signal across the cytosol to the nucleus via second
messengers or by a 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

20. Cancers may secrete their own growth factors or induce
stromal cells to produce growth factors in the tumor
microenvironment
 Normally- cells that produce the growth factor do not
express the cognate receptor, preventing the formation of
positive feedback loops within the same cell.
 RULE- may be broken by cancer cells in several different
ways
 Cancer cells acquire the ability to synthesize their own
GFs generating an autocrine loop.
Examples: - Glioblastomas secrete PDGF
Sarcomas secrete TGF-α

21. Growth factor binds to growth receptor
Activates growth factor receptor
Tyrosine kinase activity Downstream proteins activity
 Many growth factors – act as Oncoproteins – when Mutated or Overexpressed
Example-
Epidermal growth factor (EGF) receptor family- ERBB1-
overexpressed - 80% of squamous cell carcinomas of the lung,
50% or more of glioblastomas
80% to 100% of epithelial tumors of the head and neck

22.  Cancer cells often acquire growth autonomy as a result of mutations in genes that
encode components of signaling pathways downstream of growth factor receptors
 The signaling proteins that couple growth factor receptors to their nuclear targets are
activated by ligand binding to growth factor receptors.
 The signals are trasnmitted to the nucleus through various signal transduction
molecules.
 Two important oncoproteins in the category of signaling molecules are
i. RAS
ii. ABL.

23. RAS is the most commonly mutated proto-oncogene in human tumors.
Approximately 30% of all human tumors have mutated versions of the RAS gene.
The incidence is even higher in some specific cancers (e.g., colon and pancreatic
adenocarcinomas).
RAS is a member of a family of small G proteins that bind guanosine nucleotides
(guanosine triphosphate [GTP] and guanosine diphosphate [GDP]).

24.  Model for action of RAS:
I. When a normal cell is stimulated
through a growth factor
receptor, inactive (GDP-bound)
RAS is activated to a GTP
bound state.
II. Activated RAS transduces
proliferative signals to the
nucleus along two pathways:
i. RAF/ERK/MAP kinase pathway”
ii. PI3 kinase/AKT pathway

25. The ABL proto-oncogene has tyrosine
kinase activity that is reduced by internal
negative regulatory domains.
In chronic myeloid leukemia (CML) and
acute lymphoid leukemias.
When ABL gene is translocated from its
normal site on chromosome 9 to
chromosome 22, where it fuses with part
of the breakpoint cluster region (BCR)
gene= Philadelphia (Ph) chromosome

26. The BCR-ABL hybrid protein has potent, unregulated tyrosine kinase
activity, which activates several pathways, including the RAS-RAF
cascade.
Normal ABL protein localizes in the nucleus, where its role is to
promote apoptosis of cells that suffer DNA damage.
The BCR-ABL gene cannot perform this function, because it is retained
in the cytoplasm as a result of abnormal tyrosine kinase activity.

27. Growth autonomy may occur as a consequence of mutations affecting genes
that regulate transcription of DNA.
MYC, MYB, JUN, FOS, and REL oncogenes, function as transcription factors
that regulate the expression of growth-promoting genes, such as cyclins.

28. the MYC gene is involved most commonly in human tumors.
The MYC proto-oncogene is expressed in virtually all cells, the MYC protein is
induced rapidly when quiescent cells receive a signal to divide.
In normal cells, MYC levels decline to near basal level when the cell cycle begins.
In contrast, oncogenic versions of the MYC gene are associated with persistent
expression or overexpression, contributing to sustained proliferation.

29. Dysregulation of the C-MYC gene resulting from a t(8;14) translocation occurs
in Burkitt lymphoma, a B-cell tumor.
MYC is also amplified in breast, colon, lung, and many other cancers;
N-MYC and L-MYC genes are amplified in neuroblastomas and small-cell
cancers of lung.

30. Cancers may become autonomous if the genes that drive the cell cycle
become dysregulated by mutations or amplification.
Progression of cells through the various phases of the cell cycle is
controlled by CDKs.
CDKs are activated by binding to cyclins, so called because of the cyclic
nature of their production and degradation

31. The CDK-cyclin complexes phosphorylate crucial target proteins
that drive the cell through the cell cycle.
On completion of this task, cyclin levels decline rapidly.
More than 15 cyclins have been identified; cyclins D, E, A, and B
appear sequentially during the cell cycle and bind to one or more
CDK.

32. Mishaps affecting the expression of cyclin D or CDK4 seem to be a common event
in neoplastic transformation.
The cyclin D genes are overexpressed in many cancers, including those affecting
the breast, esophagus, liver, and a subset of lymphomas.
Amplification of the CDK4 gene occurs in melanomas, sarcomas, and
glioblastomas.
Mutations affecting cyclin B and cyclin E and other CDKs also occur, but they are
much less frequent than those affecting cyclin D/CDK4.

33. CDK INHIBITORS
Cyclins activate the CDKs .
CDK inhibitors (CDKIs) silence the CDKs and exert negative control
over the cell cycle.

34. The RB pathway is important to:
1- Control cell cycle progression at G1.
2- Induce cell differentiation.
3- Induce senescence.
Mutations in other genes that control RB phosphorylation can mimic
the effect of RB loss, such genes are mutated in many cancers that seem
to have normal RB genes.

35. Retinoblastoma (RB) gene, the first and prototypic cancer suppressor gene to be
discovered.
Retinoblastoma is an uncommon childhood tumor.
Approximately 60% of retinoblastomas are sporadic, and 40% are familial,
The predisposition to develop the tumor being transmitted as an autosomal
dominant trait.
To account for the sporadic and familial occurrence of an identical tumor,
Knudson, in 1974, proposed his two-hit hypothesis

36. Two mutations (hits): are required to produce retinoblastoma.
These involve the RB gene, located on chromosome 13q14.
Both of the normal alleles of the RB locus must be inactivated (two hits)
for the development of retinoblastoma.
In familial cases, children inherit one defective copy of the RB gene in
the germ line; the other copy is normal, retinoblastoma develops when
the normal RB gene is lost in retinoblasts as a result of somatic mutation.

37. The p53 tumor suppressor gene is one of the most commonly mutated genes in
human cancers.
P53 prevents (OK) neoplastic transformation by three interlocking mechanisms:
1-activation of temporary cell cycle arrest (termed quiescence),
2-induction of permanent cell cycle arrest (termed senescence),
3-triggering of programmed cell death (termed apoptosis).

38. P53 can be viewed as a central monitor of stress, directing
the stressed cells toward an appropriate response.
A variety of stresses can trigger the p53 response pathways
including: anoxia, inappropriate oncogene expression (e.g.,
MYC or RAS), damage to the integrity of DNA.

40. TGF-β is a potent inhibitor of proliferation in most normal epithelial,
endothelial, and hematopoietic cells.
It regulates cellular processes by binding to a complex composed of TGF-β
receptors I and II.
Dimerization of the receptor upon ligand binding leads to a cascade of
events that result in: transcriptional activation of CDKIs and suppression of
growth-promoting genes such as MYC, CDK2, CDK4, and those encoding
cyclins A and E.

41. Contact inhibition is abolished in cancer cells allowing them to pile on top of one
another.
Cell-cell contacts in many tissues are mediated by homodimeric interactions
between transmembrane proteins called cadherins.
E-cadherin mediates cell-cell contact in epithelial layers by mechanism not fully
understood.
One mechanism that sustains contact inhibition is mediated by the tumor
suppressor gene NF2.

42. Adenomatous Polyposis Coli – b Catenin pathway:
APC is tumor supressor gene
APC gene loss is very common in colon cancers
It has anti-proliferative action through inhibition of b-Catenin
which activate cell proliferation
Individuals with mutant APC develop thousands of colonic
polyps

44. In Resting colonic
epithelial cells (not
exposed to WNT)
β-catenin forms a
macromolecular
complex containing the
APC protein.
This complex leads to
the destruction of β-
catenin, and
intracellular levels of β-
catenin are low
When normal colonic
epithelial cells are
stimulated by WNT
molecules, the destruction
complex is deactivated,
β-catenin degradation
does not occur, and
cytoplasmic levels
increase.
β-catenin translocates to
the nucleus, where it
binds to TCF, a
transcription factor that
activates genes involved
in cell cycle progression
 When APC is mutated or
absent, as frequently occurs
in colonic polyps and
cancers, the destruction of β-
catenin cannot occur.
 βcatenin translocates to the
nucleus and coactivates
genes that promote entry
into the cell cycle,
 cells behave as if they are
under constant stimulation
by the WNT pathway

46. Warburg effect and also known as aerobic glycolysis:
Even in the presence of ample oxygen, cancer cells
demonstrate a distinctive form of cellular metabolism
characterized by high levels of glucose uptake and increased
conversion of glucose to lactose (fermentation) via the
glycolytic pathway

47.  Warburg metabolism is a form of pro-growth metabolism favoring
glycolysis over oxidative phosphorylation.
 It is induced in normal cells by exposure to growth
factors and becomes fixed in cancer cells due to the
action of certain driver mutations.
 • Many oncoproteins (RAS, MYC, mutated growth factor
receptors) induce or contribute to Warburg metabolism,
and many tumor suppressors (PTEN, NF1, p53) oppose
it.

48.  Stress may induce cells to consume their components in a
process called Autophagy. Cancer cells may accumulate mutations
to avoid autophagy, or may corrupt the process to provide
nutrients for continued growth and survival.
 Some oncoproteins such as mutated IDH act by causing the
formation of high levels of “oncometabolites” that alter the
epigenome, thereby leading to changes in gene expression that are
oncogenic.

49.  Apoptosis in normal cell is by cell death receptor (CD95),
resulting in DNA damage.
 Other pro-apoptotic factors (bad, bax, bid and p53)
 Apoptosis-inhibitors (b-cell lymphoma 2, bcl-x).
In cancer cells, the function of apoptosis is interfered due to
mutations in bcl2 & cd95

50.  There are 2 distinct programs that activate apoptosis:
1- Extrinsic pathway (death receptor CD95/Fas).
2- Intrinsic pathway (DNA damage)
Evasion of cell death by cancers mainly involves acquired abnormalities
that interfere with the intrinsic (mitochondrial) pathway of apoptosis.
The most common abnormalities involve loss of p53 function, either
by way of tp53 mutations or overexpression of the p53 inhibitor mdm2.

51. Evasion of apoptosis by cancer cells
occurs
mainly by acquired mutations & changes in
gene expression
a. disable key components of the intrinsic
pathway
b. reset the balance of regulatory factors
favor cell survival in the face of intrinsic
stresses

52.  Cancer cells are capable of limitless replication.
 In normal cells, which lack expression of telomerase, the shortened
telomeres generated by cell division eventually activate cell cycle
checkpoints leading to senescence and placing a limit on the number
of divisions a cell may undergo.
 In cells that have disabled checkpoints, DNA repair pathways are
inappropriately activated by shortened telomeres, leading to massive
chromosomal instability and mitotic crisis.
 Tumor cells reactivate telomerase, thus staving off mitotic
catastrophe and achieving immortality.

53.  After each mitosis (cell doubling) there is progressive
shortening of telomeres which are the terminal tips of
chromosomes.
 Telomerase is the RNA enzyme helps in repair of such
damage to DNA and maintains normal telomere length in
successive cell divisions.
After repetitive mitosis for a maximum of 60 to 70 times,
telomeres are lost in normal cells and the cells cease to
undergo mitosis.
Cancer cells ,telomere length is maintained.
 cancer cells avoid aging , mitosis does not slow down or
cease

54. Tumors cannot enlarge beyond 1-2 mm in diameter unless they are
vascularized.
Cancer cells can stimulate neo-angiogenesis during which new vessels
sprout from previously existing capillaries or in some cases
vasculogenesis in which endothelial cells are recruited from the bone
marrow.
Angiogenesis is thus a necessary for both benign and malignant.
Angiogenesis is required not only for continued tumor growth but also
for access to the vasculature and hence for metastasis.

55.  Vascularization of tumors is essential for their growth and is
controlled by the balance between angiogenic and anti-
angiogenic factors that are produced by tumor and stromal cells.
 Hypoxia triggers angiogenesis through the actions of HIF-1α on the
transcription of the proangiogenic factor VEGF.
 Many other factors regulate angiogenesis; for example, p53 induces
synthesis of the angiogenesis inhibitor thombospondin-1, while RAS,
MYC, and MAPK signaling all upregulate VEGF expression and
stimulate angiogenesis

56. The metastatic cascade can be subdivided
into two phases:
I. Invasion of ECM and
II. vascular dissemination & Homing of
tumor cells.

57. Malignant cells first
breach the underlying
basement membrane
Traverse the interstitial
tissue
Penetrate the vascular
basement membrane
Gain access to the
circulation

58. Invasion of the ECM has four steps:
1. Detachment of
tumor cells
from each other
2. Attachments of
tumor cells to
matrix
components

59. 3.Degradation of
ECM by collagenase
enzyme
4. Migration of tumor
cells

60. Vascular dissemination and homing of tumor
cells:
Tumor cells frequently escape their sites of
origin and enter the circulation- because of
invasive properties.
May form emboli or travel as single cells
Adhesion to vascular endothelium
Extravasation

61.  The site at which metastases appear is related
to two factors:
I. The anatomic location and vascular
drainage of the primary tumor, and
II. The tropism of particular tumors for
specific tissues
Tropism depends on-
a. Adhesion molecules on endothelial cells on
target organ
b. Chemokines . Ex: cancers express the
chemokine receptor CXCR4

62. The clonal evolution model suggest
as mutations accumulate
in genetically unstable cancer cells and the tumor become heterogeneous,
A subset of tumor cell subclones develop the right combination of gene
products to complete all the steps involved in metastasis.

63. Metastasis is caused by the gene expression pattern of most cells of the
primary tumor, referred to as a metastatic signature;
This signature may involve not only properties intrinsic to the cancer
cells but also the characteristics of their microenvironment, such as the
components of the stroma, the presence of infiltrating immune cells, and
angiogenesis

64.  Tumor cells can be recognized by the immune system as nonself and
destroyed.
 Antitumor activity is mediated by predominantly cell-mediated
mechanisms. Tumor antigens are presented on the cell surface by
MHC class I molecules and are recognized by CD8+ CTLS.
 The different classes of tumor antigens include
a. Products of mutated genes,
b. Overexpressed or aberrantly expressed proteins, and
c. Tumor antigens produced by oncogenic viruses.

65.  Immunosuppressed patients have an increased risk for development of
cancer, particularly types caused by oncogenic DNA viruses.
Immunocompetent patients : tumors may avoid the immune system by
several mechanisms
Tumors may avoid the immune system by several mechanisms
a. selective outgrowth of antigen-negative variants,
b. loss or reduced expression of histocompatibility molecules
c. immunosuppression mediated by expression of certain factors (e.g., TGF-
β, PD-1 ligands) by the tumor cells.
Antibodies that overcome some of these mechanisms of immune evasion are
now approved for treatment of patients with advanced forms of cancer.

66. Individuals with inherited mutations of genes involved in DNA repair
systems are at greatly increased risk for the development of cancer
Hereditary Nonpolyposis Colon Cancer Syndrome
Patients have defects in mismatch repair system
leading to
development of carcinomas of the colon.
These patients genomes show microsatellite instability (MSI)
characterized by
changes in length of short tandem repeating sequences throughout the
genome.

67. XERODERMA PIGMENTOSUM
Patients have a defect in the nucleotide excision repair
pathway.
Increased risk for the development of skin cancers in sites
exposed to sunlight
Because of an inability to repair pyrimidine dimers induced
by UV

68. Infiltrating cancers provoke a chronic inflammatory reaction
cancer-enabling effects of inflammatory cells and resident stromal cells
include the following
1. Release of factors that promote proliferation
2. Removal of growth suppressors
3. Enhanced resistance to cell death
4. Angiogenesis.
5. Invasion and metastasis
6. Evasion of immune destruction

69. Carcinogenic agents inflict genetic damage.
 Three classes of carcinogenic agents have been identified:
(1) chemicals
(2) radiant energy
(3) microbial product

70.  Chemical carcinogens have highly reactive electrophile groups that directly
damage DNA, leading to mutations and eventually cancer.
 Direct-acting agents do not require metabolic conversion to become carcinogenic.
 Indirect-acting agents are not active until converted to an ultimate carcinogen by
endogenous metabolic pathways.
Polymorphisms of endogenous enzymes such as cytochrome P-450 may influence
carcinogenesis by altering the conversion of indirect-acting agents to active
carcinogens.

71. INITIATORS & PROMOTERS
Initiator
carcinogenic agent
not sufficient for tumor formation by
themselves
 permanent DNA damage (Mutations)
 rapid and irreversible
Promoters
 Changes are reversible
non- tumorigenic by themselves
 not affect DNA directly
 induce tumors in initiated cells &
reversible

74. Radiant energy, whether in the form of the UV rays of sunlight or as
ionizing electromagnetic and particulate radiation, is a well-established
carcinogen.
Ionizing radiation
causes-
I. chromosome breakage
II. chromosome rearrangements
III. less frequently, point mutations
any of which may affect cancer
genes and thereby drive
carcinogenesis.

75.  UV rays in sunlight
induce
Formation of pyrimidine dimers within DNA,
Leading
mutations that can give rise
squamous cell carcinoma and melanomas of the skin

76. Carcinogenesis
multistep process
accumulation of multiple genetic alterations
collectively give rise to
transformed phenotype & its associated hallmarks

77. Tumor progression
• Over a period of time, many tumors become more aggressive and
acquire greater malignant potential which is not simply represented
by an increase in tumor size.
• Tumor progression and associated heterogeneity results from
multiple mutations that accumulate independently in different
tumor cells, generating subclones with different characteristics

78. Even though most malignant tumors are monoclonal in origin, by the time
they become clinically evident, their constituent cells are extremely
heterogeneous.
During progression, tumor cells are subjected to immune and nonimmune
selection pressures.
E.g: cells that are highly antigenic are destroyed by host defenses, whereas
those with reduced growth factor requirements are positively selected.
A growing tumor tends to be enriched for subclones that are capable of
survival, growth, invasion, and metastasis.

79.  Combination of molecular events that lead to colonic adenocarcinoma is
heterogeneous and includes genetic and epigenetic abnormalities
 two distinct genetic pathways-
1. APC/β-catenin pathway
2. microsatellite instability pathway
 mutations involving the APC/β-catenin pathway lead to increased WNT
signaling
 microsatellite instability pathway are associated with defects in DNA mismatch
repair

80.  Both pathways involve the stepwise accumulation of multiple mutations
 but the genes involved and the mechanisms by which the mutations
accumulate differ
 Epigenetic events, the most common of which is methylation-induced gene
silencing, may enhance progression along both pathways

81.  The APC/β-catenin pathway.
 classic adenomacarcinoma sequence accounts for 80% of sporadic colon tumors
 typically involves mutation of the APC tumor suppressor early in the neoplastic
process
 For adenomas to develop, both copies of the APC gene must be functionally
inactivated, either by mutation or epigenetic events

82.  APC is a key negative regulator of β-catenin, a component of the WNT signaling
pathway
 The APC protein normally binds to and promotes degradation of β-catenin
 With loss of APC function, β-catenin accumulates and translocates to the nucleus,
where it activates the transcription of genes, such as those encoding MYC and
cyclin D1, that promote proliferation.

83.  followed by additional mutations, including activating mutations in KRAS, which
also promote growth and prevent apoptosis
 Neoplastic progression also is associated with mutations in other tumor
suppressor genes such as SMAD2 and SMAD4, which encode effectors of TGF-β
signaling. Because TGF-β signaling normally inhibits the cell cycle, loss of these
genes may allow unrestrained cell growth.

84.  TP53 mutations also occur at late stages of tumor progression
 Loss of function of TP53 and other tumor suppressor genes is often caused by
chromosomal deletions, highlighting chromosomal instability as a hallmark of
the APC/β-catenin pathway
 Alternatively, tumor suppressor genes may be silenced by methylation of CpG
islands, a 5′ region of some genes that frequently includes the promoter and
transcriptional start site. Expression of telomerase also increases as lesions
become more advanced

87.  Endometrioid cancers arise in association with estrogen excess in the setting of
endometrial hyperplasia in perimenopausal women, whereas serous cancers arise in
the setting of endometrial atrophy in older postmenopausal women.
 The endometrioid type accounts for 80% of cases of endometrial carcinomas
 . Mutations in mismatch repair genes and the tumor suppressor gene PTEN are early
events in the stepwise development of endometrioid carcinoma.

88.  Women with germline mutations in PTEN (Cowden Syndrome) and germline
alterations in DNA mismatch repair genes (Lynch Syndrome) are at high risk for
this cancer. TP53 mutations occur but are relatively uncommon and are late events
in the genesis of this tumor Type
 The serous type of endometrial carcinoma is less common but also far more
aggressive.
 It accounts, for roughly 15% of tumors and is not associated with unopposed
estrogen or endometrial hyperplasia.

89.  Nearly all cases of serous carcinoma have mutations in the TP53 tumor
suppressor gene, whereas mutations in DNA mismatch repair genes and in PTEN
are rare.
 Serous tumors are preceded by a lesion called serous endometrial intraepithelial
carcinoma (SEIC) in which TP53 mutations are often detected, suggesting an
early role for such mutations in the development of this form of endometrial
carcinoma

90.  Progressive accumulation of genetic changes in pancreatic epithelium as
it proceeds from nonneoplastic, to noninvasive precursor lesions, to
invasive carcinoma
 Both intraductal papillary mucinous neoplasms and mucinous cystic
neoplasms can progress to invasive adenocarcinoma
 The most common antecedent lesions of pancreatic cancer arise in small
ducts and ductules, and are called pancreatic intraepithelial neoplasias
(panins).

91.  Pancreatic cancer genome has confirmed that four genes
are most commonly affected by somatic mutations in this
neoplasm: KRAS, CDKN2A/ p16, SMAD4, and TP53:
 KRAS -the most frequently altered oncogene in pancreatic
cancer
Activated by a point mutation in greater than 90% of cases
 These mutations impair the intrinsic GTPase activity of
the KRAS protein so that it is constitutively active
 In turn, KRAS activates a number of intracellular
signaling pathways that promote carcinogenesis

92.  P16 (CDKN2A)
 most frequently inactivated tumor suppressor gene in pancreatic
cancer
 P16 protein has a critical role in cell-cycle control; inactivation
removes an important checkpoint

93. SMAD4 tumor suppressor gene –
 inactivated in 55% of pancreatic cancers and only rarely in
other tumors;
codes for a protein that plays an important role in signal
transduction downstream of the transforming growth
factor-β receptor.
 Inactivation of the TP53 tumor suppressor gene occurs in
50% to 70% of pancreatic cancers
Its genes product, p53, acts both to enforce cell-cycle
checkpoints and as an inducer of apoptosis or senescence
 BRCA2 is also mutated late in a subset of pancreatic
cancers.

95. Robbins basic pathology 10th edition
 Hanahan D, Weiberg RA: The hallmarks of cancer: the
next generation. .