Transcript Slide 1

Lecture: 1
Molecular Basis of Cancer
What Is Cancer?
• Cancer is a group of diseases caused by the uncontrolled
multiplication of abnormal cells in the body, a process called
neoplasia.
• Abnormal new tissues called neoplasms are formed.
• Neoplasms usually form masses called tumors that may be
benign (non cancerous) or malignant (cancerous).
• Malignant or cancerous tumors grow rapidly,are invasive
(to surrounding tissue) and metastatic (traveling via
blood/lymph to invade distant tissues).
• Cancers destroy healthy tissues causing loss of function and
death.
• Cancer is the 2nd major killer in populations of developed
countries & the leading cause of death in children 3-15 (US).
• Cancers are genetic disorders caused by accumulation of
somatic mutations (gene & chromosome) in a person’s cells.
• Inherited mutations give a predisposition for certain cancers.
Characteristics of Cancer Cells
• Cancer cells are genetically altered via gene or
chromosome mutations so:
- lack normal controls over cell division or apoptosis.
- may express inappropriate genes (e.g. for telomerase,
enzyme that maintains length of DNA for continued division)
- are genetically unstable due to loss of DNA repair
mechanisms (so are more susceptible to radiation damage
than normal cells).
• Divide excessively (proliferate) & indefinitely producing
neoplasms.
• Live indefinitely (do not show apoptosis).
• Lose the normal attachment to other cells so become
metastatic (travelling via blood/lymph to invade distant
sites).
• Secrete signals for angiogenesis (growth of blood vessels
into tumor).
Cancer Cells are Undifferentiated & Malignant
• Cancer cells are
undifferentiated to
varying degrees
(even anaplastic, like
stem cells) so divide
& do not perform the
normal function of
mature cells.
• The less
differentiated the
cancer cell the more
malignant the
cancer (the more
rapidly growing is the
tumor).
What Causes Cancer?
Inherited mutations in genes that affect cell cycle,
DNA repair, or apoptosis: these mutations give a
genetic predisposition for cancer.
Somatic mutations to these same genes caused by:
• Exposure to risk factors
- environmental mutagens (carcinogenic chemicals,
radiation)
- hormones
- weakening of immune system (as in AIDS).
• Oncogenic (tumor) Virus infections
- Epstein Barr virus (causes Burkitt lymphoma)
- Human Papilloma Virus (causes cervical cancer).
Tumor viruses transform human cells into cancer cells by:
•Introducing viral cancer - causing oncogenes into host cell DNA
•Causing Translocation and overexpression of host protooncogenes.
Normal cell cycle is controlled by signal transduction:
•
Growth factors bind to surface receptors on the cell; transmembrane proteins relay
signals into the cell.
•
Two types of growth factors:
1.
Growth factors
2.
Growth-inhibiting factors
stimulate cell division.
inhibit cell division.
•
Healthy cells divide only when growth factor and growth-inhibiting factor balance
favors cell division.
•
Cancer cells divide without constraint
(e.g., mutations in growth and growth-inhibiting factor genes).
Regulation of cell division by signal transduction.
The Cell Cycle
Oncogenes
M (mitosis)
G2
(cell growth)
S (synthesis)
DNA repair
genes
G1
REPAIRS
AHEAD
Tumor suppressor genes
G0 (resting)
Cell cycle
CDK
p53 is known as the ‘ guardian of the genome ‘
CONTROL of CELL CYCLE
The Cell has 3 major
“checkpoints”
that are
sensitive to signals, the
G1, the G2 and the M
checkpoints.
• If a cell does not
pass G1 checkpoint
it enters a nondividing GO Phase.
• Most somatic cells
are in GO.
• Some cells in GO
(e.g. liver cells) can
reenter cell cycle if
needed.
G0
G1
checkpoint
G1
Differentiated
Cells enter
non dividing
G0 phase-
Control
system
M
S
G2
M checkpoint
In cancer cells genes that
so cells divide excessively
G2 checkpoint
control cell cycle have
producing
neoplasms.
mutated
Cancer and genes:
Three classes of genes are frequently mutated in cancer:
•
Proto-oncogenes ( oncogenes)
•
Tumor suppressor genes
•
Mutator genes
Proto-oncogenes  oncogenes:
Proto-oncogenes
•
Proto-oncgenes are genes that possess normal gene products and stimulate normal
cell development.
Oncogenes
•
Oncogenes arise from mutant proto-oncogenes.
•
Oncogenes are more active than normal or active at inappropriate times and stimulate
unregulated cell proliferation.
Some tumor viruses that infect cells possess oncogenes:
•
RNA tumor viruses = possess viral oncogenes (derived form cellular protooncogenes) capable of transforming cells to a cancerous state.
•
DNA tumor viruses = another class of tumor viruses; do not carry oncogenes, but
induce cancer by activity of viral gene products on the cell (no transformation per
se).
Types & effects of different types of mutations:
1.
Point mutations: occur in protein coding or controlling sequences.
2.
Deletion: frameshifts may lead to defective proteins.
3.
Gene amplification: random over-replication of small segments of DNA results in
extra copies (up-regulates cell growth).
Mutator genes:
•
Mutator gene increases spontaneous mutation rate of other genes.
•
Mutator gene products are involved in DNA replication and repair; mutations make
the cell error prone.
Proteins
Control
Cell Division
The cell cycle is controlled
by proteins
from inside & outside the cell.
• Intracellular Cyclins and Cyclin Dependent Kinases
(CDKs) control the checkpoints.
• Hormones or extracellular proteins from other cells
(called Growth Factors) signal target cell to divide.
- Hormones (e.g. Growth Hormone) or Growth Factors
bind to receptor proteins of target cell membrane.
- This triggers a molecular signaling pathway.
- A series of linked proteins activate Cyclin- CDKs which
Allows Cells to Pass Cell Cycle Checkpoints & divide.
How Growth
Factors
Growth Factor
or Hormone
Receptor
protein
Signal
Transduction
Pathway
Trigger
Cell Division
Plasma
membrane
Relay
•G1 checkpoint prevents
Proteins open
damaged DNA from replicating
checkpoints
•Checkpoint controlled by
Cyclin - CDK
Control
system
G1
M
G2
S
Tumor
Suppressor Proteins Inhibit
Division & Prevent Cancer
Tumor suppressor proteins
bind to checkpoint
proteins
cycle & prevent cell division
Cell
are proteins that
to stop the cell
if DNA is damaged.
• Tumor suppressor proteins stop division of
mutated cells until mistakes in DNA are
repaired by enzymes.
• TS proteins keep most mutations from being
passed on to daughter cells & developing into
cancer.
• If the genes for TS proteins mutate the brake
on cell division is removed cancers may result.
• Two important TS proteins are the p53 protein
& the RB protein.
The p53 Tumor
Suppressor
Protein
The p53 tumor suppressor
protein is activated
when DNA is damaged. The p53 gene is called the
“guardian angel of the genome”
P53 protein
activates
genes for
proteins that
•Prevent cell
entering S
phase
•Repair DNA
•Cause
apoptosis (if
DNA is
irreparable)
Internal
signalling
pathway
DNA repair
Apoptosis
Cell cannot
enter
S phase
Gene Mutations That Cause Cancer
Mutations in 4 types of genes cause Cancer
• Proto - oncogenes: genes that code for normal proteins
used in cell division
–Growth factors
–Membrane Receptors for Growth Factors
–Signaling Proteins (e.g. ras proto- oncogene
mutates in 30% of cancers).
• Tumor Suppressor genes: gene that code for proteins that
help prevent uncontrolled cell division by blocking key steps
(e.g. DNA replication).
- Retinoblastoma susceptibilty (RB) gene
- p53 gene mutates in >50% of cancers.
• DNA Repair genes
• Genes for Apoptosis
How
Carcinogens
Cause
Cancer
Inactivation
of DNA
Repair Genes
Activation of
Oncogenes
Inactivation
of Genes for
Apoptosis
Inactivation of
Tumor
Suppressor
Genes
Oncogenes
Are Mutated
Proto-oncogenes
A cell can acquire a cancer - causing
from
•A virus
•A mutation in a proto-oncogene.
oncogene
Oncogenes still code for the proteins needed for
cell division but they cause cancer by producing
– Too much of the protein
– An abnormally active protein, e.g. protein that
activates division by itself
– Protein that is made when it is not needed
– Protein that should be made by a different (i.e.
dividing) cell.
Cancer causing
Mutations
•Proto-oncogenes form active oncogenes by
- being misplaced (e.g. by translocation) to a site where
the gene is continually expressed resulting in
overproduction of a protein that stimulates cell division
(e.g. in Chronic Myeloid Leukemia)
- By mutating to a form that is over expressed.
•Cancer causing Mutations in Tumor Suppressor genes
inactivate the genes so normal protein product is not formed.
Mutated
Tumor
Suppressor
gene
oncogene
neoplasm
Over
Stimulation
of Cell
Division by
Oncogene
Growth
factor
Membrane
Receptor
Normal product
of ras gene
Relay
proteins
Transcription factor
(activated)
DNA
Transcription
Protein that
stimulates
cell division
Translation
Hyperactive
relay protein
(product of
ras oncogene)
issues signals
on its own.
Tumor-suppressor gene
Mutated tumor-suppressor gene
Cancer
from
Mutation
of Tumor
Supressor
Gene
Normal
Protein
prevents
cell
division
if DNA is
damaged
Cell division
allowed if DNA
repaired
Defective,
nonfunctioning
protein
Protein
absent
(cell division
not blocked)
Mutations accumulate
in cancer cells
Multiple
Genetic
Changes
Cause Cancer
Cancers result from a series of genetic changes in a cell lineage
• Inherited (germline) cancers begin with an inherited cancer
susceptibility mutation in every cell that is passed on to
offspring.
• Inherited cancers may follow a dominant pattern, e.g. Inherited
Retinoblastoma caused by a mutation in the Rb tumor
supressor gene increases cancer risk 10,000 x.
• However, Inherited cancers need at least one more somatic
mutation for cancer to develop (“2 hit hypothesis for cancer
causation”) .
• Sporadic cancers are caused solely by somatic mutations
occurring in certain body cells so are not passed on to
offspring.
• Accumulation of somatic mutations in a cell over time
eventually leads to uncontrolled cell division and cancer.
• Therefore sporadic cancers tend to appear much later in life than
inherited cancers.
Accumulation
of Mutations
Cause Cancer
• inheritance
of a germ cell mutation
acts as a risk
factor for cancers by reducing the number of
somatic mutations
required to cause cancer.
• Early mutations
show up in all subsequent stages
of a cancer.
1
Normal
Chromosomes mutation
Normal cell
2
mutations
3
mutations
4
mutations
Malignant cell
Oncogenes
Normal genes
(regulate cell
growth)
1st mutation
(leads to
accelerated cell
division)
1 mutation sufficient for role in cancer development
Tumor Suppressor Genes
Normal genes
(prevent cancer)
1st mutation
(susceptible carrier)
2nd mutation or loss
(leads to cancer)
The Two-Hit Hypothesis
First hit
First hit in
germline of
child
Second hit
(tumor)
Pathogenesis
of Colon
Cancer
Colon
Colon Cancer is usually
Sporadic & develops in a series
of steps caused by a series of
somatic cell mutations
Loss of
tumorsuppressor
Colon wall gene (e.g. APC)
Normal colon
epithelial cells
APC: adenomatous polypodsis coli
Removal of polyps
prevents cancer
Small benign
growth (polyp)
= adenoma
Pathogenesis
of Colon
Activation of
ras oncogene
Cancer -2
Loss of
tumorsuppressor
gene p53
Campbell, & Reece
Biology fig. 19.13
Loss of
Additional
tumormutations
Malignant tumor
suppressor
Larger benign
(adenocarcinoma)
Small benign gene
growth
(adenoma)
growth (polyp)
Environmental
Risk factors
for Colon
Cancer
• Low fiber diet
• Smoked meats (contain heterocyclic aromatic amines that are
converted to mutagens in the liver)
• Low intake of fruits & vegetables (antioxidants)
• Low intake of cruciferous vegetables (a chemical in brocolli,
brussels sprouts, cabbage activates enzymes that block
formation of mutagens)
Genetic Abnormalities Associated
With Hematologic Malignancies
A- Point Mutaion
Mutaions within the RAS oncogenes or P53 tumorsuppressor gene are common in many haempoietic
malignancies. The point mutation may involve several
base pairs. In 35% of cases of AML the nucleophosmin
gene shows an insertion of 4 base pairs.
B- Translocation
Includes two main mechanisms:
1- Fusion of parts of two genes to generate a chimeric
fusion gene that codes a novel fusion protein. Ex:
BCR- ABL in t(9; 22) in chronic myeloid leukaemia.
2- Overexpression of a normal cellular gene. Ex:
overexpression of BCL-2 in the t(14; 18) translocation
of follicular lymphoma or MYC gene in Burkitt,s
lymphoma.
C- Deletions
May involve a small part of a chromosome, the short or long
arm or the entire chromosome. Losses most commonly affect
chromosomes 5, 6, 7, 11, 20 and Y. The critical event is probably
loss of a tumor suppressor gene.
D- Duplication or amplification
Gains are common in chromosomes 8, 12, 19, 21 and Y. It is not
a common feature in haematologic malignancy but has been
described involving the MLL gene.
E- Epigenetic alterations
Means alterations in the mechanism by which genes are
transcribed and are stably inherited with each cell division so
they are passed on as the malignant cell divides. The most
important mechanisms are methylation of cytosine residues in
DNA and enzymatic alterations such as acetylation or
methylation of the histone protein that package DNA.
Prostate
Cancer
• Prostate Cancer is the most common cancer among men (esp.
>65 yrs, African - Americans) & 2nd in cancer deaths in men.
• Risk factors include increasing age, race, family history, fat diet,
male hormones over many years.
• Adenocarcinoma occurs in periphery of prostate gland.
• Metastasises to lungs, bones (bone pain often first symptom as
early stage of primary tumor may be asymptomatic).
• Tumors are graded from 1 (well differentiated cells) to 5 (least
differentiated cells, high malignancy).
• Manifestations (some similar to Benign Prostatic Hyperplasia):
- changes in voiding pattern, dysuria, hematuria,
– from metastasis low back pain from bone, wt loss, anemia, &
shortness of breath.
• Screening (important for early asymptomatic cancers) includes:
- digital rectal exam (palpation of prostate by DRE detects
nodular lump)
- transrectal ultrasonography (measures prostate vol: more
sensitive than DRE)
- PSA blood test
PSA Test for Prostate
Cancer
• Tumor cells express abnormal genes so form abnormal
proteins (antigens) so can act as tumor markers.
• Prostate
Specific
Antigen
(PSA) a glycoprotein
released by prostate gland into the blood identified as a
marker of prostate cancer in 1980.
• PSA is highly specific to prostate gland but not specific to
prostate cancer. Elevated blood PSA can also occur with
non cancerous conditions (i.e. false positives occur with
prostatitis or with benign prostatic hyperplasia
- BPH)
• The need to treat stage 1 cancers detected by PSA test is
controversial: stage 1 tumors are asymptomatic, not detected
by digital exam & present in 80% of men over 80 yrs.
• Treatment
of Prostate
Cancer includes surgery,
radiation & hormonal manipulation (e.g. androgen inhibitors)
expected survival >10yrs, otherwise “watchful waiting” is
preferred in elderly patients.
• PSA test is used to assess treatment (correlates with
prostate size & cancer stage).
if
Etiology
of Breast Cancer
Breast cancer is most common cancer in women & 2nd most
common in cancer deaths in women (after lung cancer).
Risk Factors for Breast Cancer
• Prolonged exposures to estrogens (early menarche & late
menopause). Breast cancers that are estrogen receptor +ve are
treated with drugs (e.g. tamoxifen) that bind to these receptors.
• Late Childbearing (having first child after age 30)
• Breasts with a high proportion of lobular (milk producing)
and ductal tissue density.
• Not breast feeding babies increases post menopausal BC.
• Exposure to radiation.
• High alcohol consumption.
• Family History of BC & Genetic Predisposition in 5-20% of
cases (inheriting mutated breast cancer susceptibility
genes, BRCA-1 or BRCA-2).
Genetics of Breast Cancer
• 5 - 20% of breast cancers are Familial .
• Most involve mutations in 2 Tumor Suppressor genes
involved in DNA repair so are used as genetic markers.
• Both genes also increase the risk of ovarian cancer.
- Breast Cancer Susceptibility Gene 1 (BRCA1) on
chromosome 17
- Breast Cancer Susceptibility Gene 2 (BRCA2) on
chromosome 13.
• Mutated HER-2/neu Gene (Human Epidermal Growth
Factor Receptor 2 gene is an Oncogene for a protein that
stimulates cell division & occurs in 25- 30% of Breast
Cancers.
• Her-2/neu breast cancers strike early in adulthood & spread
quickly.
• Herceptin is a monoclonal antibody based drug that binds
to Her-2/neu receptors & blocks cell division in tumors.
Pathogenesis
of Breast
Cancer
Tumor (usually
adenocarcinoma
in milk ducts)
Campbell, & Reece,
Biology, fig. 12.19
Glandular
tissue
Note: lumpectomy
a possible treatment
A tumor grows from a
single cancer cell.
in early stages
Cancer cells invade
neighboring tissue.
Pathogenesis
of Breast
Cancer
-2
Lymph
vessel
Blood
vessel
Cancer cell
• Cancer cells spread via
lymph & blood to other
parts of the body.
• “Sentinel lymph node ”
biopsy determines if
cancer has spread & if
further lymph node
removal required.
Metastatic
tumor
Small percentage
of metastisised cancer
cells may survive and
establish a new tumor in
another part of the body.
Diagnostic methods used to study malignant cells
1- Karyotype analysis:
It is a direct morphological analysis of
chromosomes from tumor cells under the microscope.
2- Fluorescent in situ hybridization analysis
FISH analysis involves the use of fluorescent- labelled genetic probes which
hybridize to specific parts of the genome. This can detect extra copies of
genetic material or reveal chromosomal translocation.
3- Southern blot analysis: It
involves extraction of DNA from
leukaemic cells followed by restriction enzyme digestion, gel
electrophoresis and transfer by blotting to a suitable membrane. The DNA is
then hybridized to a probe complementary to the gene of interest.
4- Polymerase chain reaction:
Can be performed on blood or
bone marrow for a number of specific translocations such as t(9; 22) and
t(15; 17). It is very sensitive and can detect one abnormal cell in one million
normal cells. It is of great value to diagnose minimal residual disease.
5- DNA microarray:Allows rapid and comprehensive analysis of
cellular transcription by hybridizing labelled cellular mRNA to DNA
probes which are immobilized on a solid support. This approach can
rapidly determine mRNA expression from a large number of genes and
may be used to determine the mRNA expression pattern of different
leukaemia or lymphoma subtypes.
6- Flow cytometry:
Normal cells each have a characteristic
profile but malignant cells often express an aberrant phenotype that
can be useful in allowing their detection.
7- Immunohistochemistry: Antibodies can be used to stain
tissue sections with fluorescent markers.
Value of using these methods:
a- Initial diagnosis.
b- Establishing treatment protocol.
c- Monitoring response to therapy.
Cytogenetics
• Cytogenetics
is the original cancer genetic test
used to identify abnormal
mutated
chromosomes
by karyotype
analysis.
• Cytogenetics
identified the Philadelphia
chromosome
resulting from a translocation
error in chromosome 22 forming an oncogene
for chronic myelogenous
leukemia (CML) in
1960.
• Karyotype
Analysis is done by culturing tumor
cells, arresting them in metaphase & spreading
chromosomes via use of hypotonic solutions.
• Chromosomes
are stained and interpreted by a
cytogeneticist.
• Process may take weeks (as many tumors do not
grow in vitro).
Thank You
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