Transcript 7-Tumor Suppressor genes, Oncogenes and Development The

8 TH W E E K
Gihan E-H Gawish, MSc, PhD
Ass. Professor
Molecular Genetics and Clinical Biochemistry
KSU
Development of Cancer
1. Defective chick points mechanisms allows errors in
the cell duplication process to persist into the next
generation and can lead to and regulated proliferation
and the development of cancer
2. Two different types of mutations contribute to cancer
formation: inactivating mutations in tumor
suppressor genes and activating mutations in protooncogene.
Tumor Suppressor Genes
Inhibit Proliferation
Promote Differentiation
Stimulate Apoptosis
Oncogenes
Stimulate Proliferation
Inhibit Differentiation
Inhibit Apoptosis
Tumor Suppressor Genes
• Tumor
Suppressor genes: are genes that act to
inhibit
cell
proliferation
and
tumor
development.
If Tumor Suppressor Gene is
Mutated
Inactivated
It will lead to cell transformation
Normal
transformed
Oncogenes = gain of function
Tumor suppressor = loss of function
• First discovered in 1960s by Henry Harris.
• Harris fused tumor cells with normal cells
and discovered some of the hybrid cells
were normal.
• Harris hypothesized that the normal cells
contained gene products that suppressed
uncontrolled cell proliferation.
Sporadic retinoblastoma
– 60% of retinoblastoma cases.
– Develops in children with no family history.
– Occurs in one eye.
Hereditary retinoblastoma
– 40% of retinoblastoma cases.
Retinoblastoma (Rb) caused by
mutated Rb gene
– Onset typically is earlier than sporadic cases.
– Multiple tumors involving both eyes.
Two mutations are required for the development of retinoblastoma.
• Sporadic retinoblastoma
– Child starts with two wild type alleles (RB+/RB+).
– Both alleles must mutate to produce the disease (RB/RB).
– Probability of both mutations occurring in the same cell is low; only
one tumor forms (e.g., one eye).
• Hereditary retinoblastoma
– Child starts with heterozygous alleles (RB/RB+).
– Only one mutation is required to produce disease (RB/RB).
– Mutations resulting in loss of heterozygosity (LOH) are more
probable in rapidly dividing cells, and multiple tumors occur (e.g.,
both eyes).
•Tumor suppressors must be inactivated
• This means both copies must be lost/mutated
Hereditary cancer is caused by the inheritance of
one copy of a defective tumor suppressor
tumor-suppressor gene
Normal
growthinhibiting
protein
Cell division
under control
Mutated tumor-suppressor gene
Defective,
nonfunctioning
protein
Cell division not
under control
Functions of Tumor Suppressor genes
1. Antagonize the action of oncogenes
– eg. p53 is activated by oncogenes. p53 protects against
cancer by inducing cell cycle arrest and/or apoptosis
myc
p53
cell growth
Functions of Tumor Suppressor genes
2. Block proliferation:
– Cell cycle inhibitors: eg. Rb – blocks entry into S phase by binding to
and inhibiting RB. INK-4 gene: that produces P16 that inhibits
cdk4/cyclin D action ( to phosphorylate Rb gene to inactivate it’s
action)
– Repressor transcription factors: e.g.; WT1 is a repressor that appears
to suppress transcription factor ( Insulin like growth factor) which will
contribute in the development of tumor.
– Activator transcription factors: e.g.; SMAD family that are activated
by TGF-β, leading to inhibition of cell proliferation . P53: that
produces P21 that has the same action of P16 in inhibiting the action
of cdk/cyclin.
Functions of Tumor Suppressor genes
3. Induce apoptosis:
– Form of cell suicide. A cell which has lost growth control will
often undergo apoptosis.
– Cell damage or ‘stress’ can also lead to apoptosis.
– p53 is a critical regulator of apoptosis. Transcription factor
which activates pro-apoptotic molecules
p53 Mutations
• Most commonly mutated gene in cancers (50% of total).
• When p53 is mutated, DNA-damaged cells are not
arrested in G1 and DNA repair does not take place.
• This failure to arrest DNA-damaged cells will be repeated
in subsequent cell cycles permitting other mutations to
accumulate, culminating in neoplastic transformation...
tumor formation and cancer.
p53 – the guardian of
the genome
Regulation of the cell cycle
Functions of Tumor Suppressor genes
4. DNA Repair
– DNA repair prevents the accumulation of mutations
– Defects in DNA repair genes leads to genomic instability
– Accelerates the activation of oncogenes and the loss of
tumor suppressors
– Many cancer prone syndromes associated with defects in
DNA repair, BRCA1, ATM, MRE11, NBS,
Breast Cancer Tumor Suppressors
• A small proportion of breast cancer is heritable.
Two genes are associated with predisposition to breast
cancer.
– BRCA1 on chromosome 17
– BRCA2 on chromosome 13
• Normal function of both is in repair of ds DNA breaks.
• Tumor suppressor genes
– inhibit oncogenes
– suppress proliferation
– Induce cell death
– repair DNA – prevent mutation
• These are “loss of function” or recessive mutations.
• Responsible for hereditary forms of cancer
• Being heterozygous enhances the probability of cancer but this will
require a mutation in the corresponding other allele. e.g., it need to
be homozygous for the gene.
Oncogenes
• Gene that can cause cancer. It is a sequence of DNA that has
been altered or mutated from its original form, the protooncogene.
• Proto-oncogenes promote the specialization and division of
normal cells. A change in their genetic sequence can result
in uncontrolled cell growth.
• Dominant mutation: one copy is sufficient to cause cancer.
In humans, proto-oncogenes can be
transformed into oncogenes in three ways:
• point mutation (alteration of a single nucleotide
base pair)
• translocation (in which a segment of the
chromosome breaks off and attaches to another
chromosome),
• or amplification (increase in the number of copies of
the proto-oncogene)..
Inherited mutations of oncogenes
• A few cancer syndromes are caused by inherited mutations
of proto-oncogenes
• Multiple endocrine neoplasia type 2 is caused by an inherited
mutation in the gene called RET
medullary cancer of the
thyroid.
• Inherited mutations in the gene called KIT cause hereditary
gastrointestinal stromal tumors (GIST).
• Inherited mutations in the gene called MET cause hereditary
papillary renal cancer.
Acquired mutations of oncogenes
• Most cancer causing mutations involving oncogenes are
acquired, not inherited.
• They generally activate oncogenes by chromosome
rearrangements, gene duplication, or mutation. For
example, a chromosome rearrangement leads to formation
of the gene called BCR-ABL. This leads to the disease
chronic myeloid leukemia (CML).
Chromosomal Translocation that creates Philadelphia
Chromosome
BCR-ABL Oncogene: Breaks in ABL Gene of Chromosome 9 and
BCR Gene of Chromosome 22
Fusion Protein causes Chronic Myelogenous Leukemia
History of oncogene
•
Oncogenes were first discovered in certain retroviruses and were later
identified as cancer-causing agents in many animals
•
First link between viruses and cancer proposed by Francis Peyton Rous in
1910 (Nobel Prize, 1966): cell-free extracts from chicken tumors injected
into healthy chickens caused new tumors.
Rous Sarcoma Virus (RSV)
•
Discovered by Harold Varmus and Bishop, 1975-76 (Nobel Prize, 1989).
•
A transforming retrovirus: a cancer-causing single-stranded RNA virus that
uses reverse transcriptase enzyme to make ssDNA, then ds DNA, which
integrates into host DNA.
•
Note: not all oncogenes caused by viruses.
•
100’s of oncogenes now known.
•
Human T-cell leukemia virus (HTLV) is a human RV; codes a TF.
Southern Blots Probed with viral src Gene
Revealed Cellular Origin of Oncogenes
Infected chicken
Uninfected chicken
(Negative Control)
v-src
c-src
Proto-oncogene
SURPRISE!
Origin of Transforming Retroviruses
Capsid protein Reverse Transcriptase Envelope Protein
Mutation creates oncogene
Ras Proto-oncogene
• Mutated in 30% of all cancers.
• A “molecular switch” in the signal transduction pathway
leading from growth factors to gene expression
controlling cell proliferation: GF  receptor   Ras 
  TF  target genes  growth.
• A single amino acid change in Ras protein can cause
constant stimulation of the pathway, even in the absence
of growth factors.
Cancers Usually Result from a Series of
Mutations in a Single Cell
• Colon Cancer:
oncogene
oncogene
Tumor suppressors
Tumor Progression: Evolution at the Cellular
Level
Benign tumor (polyp in epithelial cells) is
confined by basal lamina; then additional
mutation occurs.
Malignant tumor (carcinoma in epithelial cells)
grows very fast, becomes invasive, and
metastasizes.
Some of the more important oncogenes
include:
• ras (a signal transduction molecule),
• myc (a transcription factor),
• src (a protein tyrosine kinase),
• HER-2/neu, also called erbB-2 (a growth factor receptor),
• Bcl-2 (a membrane associated protein that prevents apoptosis).