Molecular biology of
Cancer
Raya Saab, MD
Cancer is not a uniform disease:
Tissue specific
Carcinomas versus Sarcomas
Age groups
Hallmarks of cancer:
Growth
Invasion
Metastasis
Overview of Cancer
Multistage process: inherited and somatic mutations of
cellular genes lead to repeated waves of clonal
selection in which cells with the most aggressive growth
and survival capabilities prevail
Basic mechanisms of cancer are similar:
Unchecked cellular
proliferation
Accumulation of
genetic changes Independence from
that endow a normal restrictions for
survival advantage growth
Independence from
Growth and normal restrictions for
Blood vessel invasion invasion
Abnormal cellular
differentiation
Hanahan & Weinberg, Cell 2000
Results in disorganized proliferation
Damage to surrounding tissues and organs
Dissemination to distant organs
Bloodstream
Lymphatics
Manifestations of cancer:
Primarilydue to invasion and tissue/organ
compromise
Site-specific
Age-specific
Organ failure
Death of the organism
What makes a cancer cell
cancerous?
Four major aberrations in cancer cell
regulation:
Proliferation
Apoptosis
Differentiation
Chromosomal and genetic disorganization
Four major aberrations in cancer cell
regulation:
Proliferation
Apoptosis
Differentiation
Chromosomal and genetic disorganization
Proliferation is at the heart of cancer
biology
Most cancers are clonal
Most therapies, in fact, are aimed at reducing
tumor cell number and preventing proliferation
Excessive proliferation early in cancer evolution
allows for further mutations by natural selection
As a cell accumulates genetic changes/
mutations in the process of transformation, two
classes of genes are primarily targeted:
Oncogenes
Tumor suppressor genes
These two classes of genes essentially control
the ratio of cell birth to cell death
Proliferation
Theinteraction of a growth factor with its
receptor activates a series of intracellular
events, leading to expression of specific genes
and resulting in a response
Proliferation
Abnormalities in cancer cells can be at the
level of
growth factors (PDGF, VEGF, EGF, FGF, SCF,
HGF, NGF, Ang, etc…)
receptors (PDGFR, VEFGR/FLT, EGFR, FGFR, c-
Kit, c-Met, Trk, Tie, etc…)
intracellular mediators of signal transduction (Akt,
Ras)
Cell cycle checkpoint controls (RB, TP53, CDKs,
CDKIs)
Apoptosis
Programmed cell death
Essential in every tissue, organ, and stage of
development
Activated by p53 in many cases
Activated in response to myriad stimuli,
including GF deprivation, oxidants, calcium
ion overload, DNA damage, microtubule
changes
Hanahan & Weinberg, Cell 2000
Checkpoints and checkpoint defects
Normal cell cycle:
Checkpoints and checkpoint defects
Normal cell cycle:
G1-S checkpoints:
Need for growth signals
Restriction point (R): Once the R point is
passed, the cell goes through S, G2, and M
without need for additional growth factors
The retinoblastoma (RB) gene (and to some
extent the TP53 gene), regulates this
restriction point
G1-S Checkpoint defects in tumor cells:
Cancer cells continue to proliferate in the
absence of mitogens
Relaxation of this checkpoint control is
essential for tumorigenesis
Disruption of the RB and/or p53 pathway
likely occurs in every tumor cell
The paradigm of tumor suppressor genes:
Retinoblastoma
The clinical phenotype:
Two-hit hypothesis
Trilateral Retinoblastoma (TRB)
Retinoblastoma
Recessive nature of tumorigenesis,
transmitted in an “autosomal dominant”
phenotype
Within each cell, both alleles need to be lost
Within each family, every child with one allele
mutation is affected
Retinoblastoma
Gene identified at locus 13q14, termed RB1
RB1 found to be expressed in all cells, but in
germline mutations, only limited spectrum of
tumors develop…
Retinoblastoma
RB1 subsequently shown to
be a principal controller of
the cell cycle in all human
tissues
pRB Cyclin/Cdk
pRB
(active)
pppRB
(inactive)
Cyclins + Cyclin-dependent kinases (CDK) together
inactivate RB to allow cell cycle progression
CDK-inhibitors activate RB to stop proliferation
Retinoblastoma
RB1 activity is regulated by phosphorylation,
in turn regulating entry into S phase of the cell
cycle
This suggests that RB1 would have to be
inactivated, by some means, in all human
cancers, for cell proliferation and tumor
progression
Loss of RB tumor suppressor function:
RB mutation: genetic, sporadic
Inactivation of the RB protein:
Cyclin D1 overexpression in variety of cancers-
amplification (11q13), translocation
Inactivation of CDK inhibitor proteins:
INK4 family
Cip/Kip family
Checkpoints and checkpoint defects
Normal cell cycle:
Checkpoint defects:
DNA damage checkpoint:
Cell cycle arrest
DNA repair
Apoptosis
TP53, ATM
TP53 is a genomic stability gene
Genomic Stability genes protect against disruptions of
genomic integrity
Disruption of these genes allowed accumulation of
genetic changes, contributing to acceleration of
mutations in proto-oncogenes and tumor
suppressors, leading to cancer progression
TP53:
the master regulator of the DNA damage
checkpoint, leading to:
cell cycle arrest (to allow the cell to repair itself)
Apoptosis
senescence
TP53 is the most frequently mutated gene in
human cancer
Loss of TP53 leads to loss of genomic
stability accumulation of genetic mutations
required for malignant transformation
TP53: Master tumor suppressor
Found that the region containing TP53 on
chromosome 17p was lost (LOH) in the
majority of human cancers, with the remaining
allele being mutated
Conformed to Knudson’s hypothesis
Now thought to be the most commonly
mutated gene in human cancer
TP53
Germline mutations: Li-Fraumeni syndrome
TP53 can be inactivated by ways other than
gene mutations: mutation in other genes that
affect p53 activation, stability, or turnover
Function: transcriptional regulatory protein,
with direct effects on pathways of apoptosis
and cell cycle regulation
Functions at the G1/S and G2/M checkpoints,
as well as influencing the decision of the cell
to undergo apoptosis/programmed cell death
Loss of TP53 tumor suppressor:
deletion or mutation
other members of TP53 pathway:
MDM2 binds TP53 and targets its destruction
MDM2 amplification in cancer
ARF binds MDM2, preventing its action on TP53
p19ARF mutation in cancer
TP53
Lung cancers: GT mutations likely arise due
to direct interactions of TP53 gene sequences
with carcinogens in tobacco smoke
Squamous cell skin cancer arising in UV-
exposed skin: pyrimidine dimer premutagenic
lesions in TP53
Hepatocellular carcinoma due to aflatoxin
exposure: specific mutations also identified
during in-vitro studies
Environmental carcinogenesis
Chemicals: tobacco, asbestos, metals,…
Ionizing radiation: leukemia, thyroid cancer
UV radiation: skin cancer
Inflammation/Trauma
Tumor viruses
Infections:
Herpes Simplex Virus
Ebstein-Barr Virus
Human Papilloma Virus
Hepatitis viruses
Parasites
Hanahan & Weinberg, Cell 2000
Molecular Cancer Research:
To understand molecular events
underlying tumor progression, interaction of
oncogenic and tumor suppressor pathways,
and how they are overcome in cancer cells
Model systems are useful for
understanding disease biology
developing and testing novel therapies-
especially in tumors where clinical trials more
difficult due to limited patient population
A mouse model of pineal Rb inactivation
Cyclin D1 pppRB
pRB tumor
Progenitor tumor
cell tumor
tumor tumor
Normal Malignant
A mouse model of pineal Rb
inactivation
Saab et al, Cancer Research, 2009
Pineoblastoma in
Irbp-Cyclin
D1/p53-/- mouse
Pineoblastoma in
Irbp-Cyclin
D1/Ink4c -/- mouse
Saab et al, Cancer Research, 2009
Senescence is a tumor-
suppressive mechanism in Mature
Cyclin D1-driven
Cyclin D1-driven Cell?
tumors? Senescence
pineoblastoma
Prevention?
RB-pathway dependent Mature Mature
Therapy?
tumors? Cell? Cell?
Cyclin D1
?
p53, Ink4c tumor
pppRb
(inactive)
Progenitor
cell tumor
Progenitor
cell tumor
Progenitor
Progenitor
cell cell
tumor tumor
normal hyperplasia malignant
Conclusion
Cancer development depends on a balance between
proliferation and cell death
Abnormalities in tumor suppressors- the “watchdogs” of
the cellular genome- result in proliferation, accumulation
of genetic insults, and transformation
Two tumor suppressor pathways, RB and P53, have to
be overcome for tumor progression in almost all cancers
Targeted therapies to reactivate these pathways in
cancer cells may be useful in tumor prevention or
treatment