Transcript MOLECULAR GENETIC OF CANCER PART II

Theories of cancer genesis
1- Standard Dogma
•Proto-oncogenes (Ras – melanoma)
•Tumor suppressor genes (p53 – various cancers)
2- Modified Dogma
•Mutation in a DNA repair gene leads to the accumulation of
unrepaired mutations (xeroderma pigmentosum)
3- Early-Instability Theory
•Master genes required for adequate cell reproduction are
disabled, resulting in aneuploidy (Philadelphia chromosome)
Dr MOHAMED FAKHRY
1
CHROMOSOMAL REARRANGEMENTS OR TRANSLOCATIONS
Neoplasm
Translocation
Proto-oncogene
Burkitt lymphoma
t(8;14) 80% of cases
t(8;22) 15% of cases
t(2;8)
5% of cases
c-myc1
Chronic myelogenous
leukemia
t(9;22) 90-95% of cases
bcr-abl2
Acute lymphocytic
Leukemia
t(9;22) 10-15% of cases
bcr-abl2
1c-myc
is translocated to the IgG locus, which results in its activated expression
2bcr-abl
2
fusion protein is produced, which results in a constitutively active abl kinase
Dr MOHAMED FAKHRY
GENE AMPLIFICATION
Oncogene
Amplification
Source of tumor
c-myc
~20-fold
leukemia and lung carcinoma
N-myc
5-1,000-fold
neuroblastoma
retinoblastoma
c-abl
~5-fold
chronic myoloid leukemia
K-ras
4-20-fold
30-60-fold
colon carcinoma
adrenocortical carcinoma
3
Dr MOHAMED FAKHRY
Tumor suppressor genes

Normal function - inhibit cell proliferation

Tumor suppressor genes are genes that, when mutated, fail
to repress cell division.

Absence/inactivation of inhibitor --> cancer

Both gene copies must be defective
4
Dr MOHAMED FAKHRY 2015
Knudson’s Two-Hit Hypothesis
 When tumor suppressor genes are mutated, a
predisposition to develop cancer often follows a
dominant pattern of inheritance.
 The mutation is usually a loss-of-function mutation in the
tumor suppressor gene.
 Cancer develops only if a second mutation in somatic
cells knocks out the function of the wild-type allele.
KNUDSON’S TWO-HIT HYPOTHESIS IN FAMILIAL CASES
Familial RB (%30)
rb
RB
RB
LOH
Tumor cells
rb
RB
Normal cells
rb
Inactivation of a tumor suppressor
gene requires two mutations, inherited
mutation and somatic mutation.
Normal cells
KNUDSON’S TWO-HIT HYPOTHESIS IN SPORADIC CASES
Normal
Cells
RB
RB
RB
Mutation
8
RB
RB
RB
LOH
Tumor cells
Inactivation of a tumor
suppressor gene requires two
somatic mutations.
TUMOR SUPPRESSOR GENES
Disorders in which gene is affected
Gene (locus)
Function
Familial
Sporadic
DCC (18q)
cell surface
interactions
unknown
colorectal
cancer
WT1 (11p)
transcription
Wilm’s tumor
lung cancer
Rb1 (13q)
transcription
retinoblastoma
small-cell lung
carcinoma
p53 (17p)
transcription
Li-Fraumeni
syndrome
breast, colon,
& lung cancer
BRCA1(17q)
transcriptional
breast cancer
breast/ovarian
tumors
BRCA2 (13q)
regulator/DNA repair
9
Rb gene
 Rb protein controls cell cycle moving past G1 checkpoint
 Rb protein binds regulatory transcription factor E2F
 E2F required for synthesis of replication enzymes
 E2F - Rb bound = no transcription/replication
 Growth factor --> Ras pathway
--> G1Cdk-cyclin synthesized
 Active G1 Cdk-cyclin kinase phosphorylates Rb
 Phosphorylated Rb cannot bind E2F --> S phase
 Disruption/deletion of Rb gene
 Inactivation of Rb protein
--> uncontrolled cell proliferation --> cancer
10
Verification of the Two-Hit Hypothesis for Retinoblastoma
 Several cases of retinoblastoma are associated with a small
deletion in chromosome 13q. Mapping refined the RB locus
to 13q14.2.
 Positional cloning was used to isolate a candidate RB gene
that encodes a protein that interact with transcription factors
that regulate the cell cycle.
 In retinoblastoma cells, both copies of this gene were
inactivated.
 In cell culture, expression of a wild-type RB allele could
revert the phenotype of cancer cells.
Cellular Roles of Tumor Suppressor Proteins
The proteins encoded by tumor suppressor genes are involved
in:
 cell division,
 cell differentiation,
 programmed cell death,
 DNA repair.
p53
 Phosphyorylated p53 activates transcription of p21 gene
 p21 Cdk inhibitor (binds Cdk-cyclin complex --> inhibits kinase activity)
 Cell cycle arrested to allow
DNA to be repaired
 If damage cannot be repaired
--> cell death (apoptosis)
 Disruption/deletion of p53 gene
 Inactivation of p53 protein
--> uncorrected DNA damage
--> uncontrolled cell proliferation --> cancer
13
The Cellular Function of p53
 Expression of p53 is very low in normal cells.
 Expression of p53 increases in response to DNA damage due to a
decrease in degradation.
 p53 can inhibit cell division or induce apoptosis.
[ increase ]
p-p53
Theories of cancer genesis
1- Standard Dogma
•Proto-oncogenes (Ras – melanoma)
•Tumor suppressor genes (p53 – various cancers)
2- Modified Dogma
•Mutation in a DNA repair gene leads to the accumulation of
unrepaired mutations (xeroderma pigmentosum)
3- Early-Instability Theory
•Master genes required for adequate cell reproduction are
disabled, resulting in aneuploidy (Philadelphia chromosome)
16
Dr MOHAMED FAKHRY
DNA REPAIR GENES
These are genes that ensure each strand of genetic
information is accurately copied during cell division of the
cell cycle.
Mutations in DNA repair genes lead to an increase in the
frequency of mutations in other genes, such as protooncogenes and tumor suppressor genes.
i.e. Breast cancer susceptibility genes (BRCA1 and BRCA2)
Hereditary non-polyposis colon cancer susceptibility genes
(MSH2, MLH1, PMS1, PMS2) have DNA repair functions.
Their mutation will cause tumorigenesis.
17
IMPORTANCE OF DNA REPAIR
18
pBRCA1 and pBRCA2 regulate DNA repair
 Mutations in the tumor suppressor genes BRCA1 and BRCA2 have
been implicated in hereditary breast and ovarian cancer.
 Both genes encode proteins that are localized in the nucleus and
have transcriptional activation domains.
 pBRCA1 and pBRCA2 may be involved in DNA repair in human
cells.
Theories of cancer genesis
1- Standard Dogma
•Proto-oncogenes (Ras – melanoma)
•Tumor suppressor genes (p53 – various cancers)
2- Modified Dogma
•Mutation in a DNA repair gene leads to the accumulation of
unrepaired mutations (xeroderma pigmentosum)
3- Early-Instability Theory
•Master genes required for adequate cell reproduction are
disabled, resulting in aneuploidy (Philadelphia chromosome)
20
Dr MOHAMED FAKHRY
Chromosome Rearrangements: The Philadelphia
Chromosome
 The Philadelphia chromosome is the result of a reciprocal translocation
between chromosomes 9 and 22 with breakpoints in the c-abl gene on
chromosome 9 and the c-bcr gene on chromosome 22.
 The fusion gene created by this rearrangement encodes a tyrosine kinase that
promotes cancer in white blood cells.
Translocation and Bcr-Abl fusion in CML
22
The Philadelphia Chromosome
Smart bullet STI-571 lockes itself to the target molecule
STI-571
24
Chromosomal Rearrangements:Burkitt’s Lymphoma
 Burkitt’s lymphoma is associated with reciprocal
translocations involving chromosome 8 and a chromosome
carrying an immunoglobulin gene (2, 14, or 22).
 The translocations juxtapose c-myc to the genes for the
immunoglobulin genes, causing overexpression of c-myc in B cells.
 The c-myc gene encodes a transcription factor that activates genes
for cell division.
A Reciprocal Translocation Involved in Burkitt’s Lymphoma
8p21.1
Genetic Pathways to Cancer
 Cancers develop through an accumulation of somatic (not a
single) mutations
suppressor genes.
27
Dr MOHAMED FAKHRY
in
proto-oncogenes
and
tumor
Multiple Mutations in Cancer
 Most malignant tumors cannot be attributed to mutation of a
single gene.
 Tumor formation, growth, and metastasis depend on the
accumulation of mutations in several different genes.
 The genetic pathways to cancer are diverse and complex.
Tumor Progression
Cellular
Multiple mutations lead to colon cancer
Genetic changes --> tumor changes
29
Hallmarks of Pathways to Malignant Cancer
1.
Cancer cells have self-sufficiency in the signaling
processes that stimulate division and growth.
2.
Cancer cells are abnormally insensitive to inhibitory
signals of growth.
3.
Cancer cells can escape programmed cell death
(apoptosis).
4.
Cancer cells have not limit of replication potential.
4.
Cancer cells develop ways to grow themselves.
5.
Cancer cells acquire the ability to invade other tissues
and colonize them.