Download 2) Inactivation of tumour suppressor genes

BB20023:DNA: make, break, disease
Dr. MV Hejmadi, 2004-05
Oncogenesis
Oncogenesis due to an imbalance between activity of
1) Oncogenes: Genes known as proto-oncogenes
code for proteins that stimulate cell division; mutated
forms, called oncogenes, cause stimulatory proteins to
be overactive, with the result that cells proliferate
excessively.
2) Tumor suppressor genes (TSGs) code for
proteins that inhibit cell division. Mutations can
cause the proteins to be inactivated and may thus
deprive cells of needed restraints on
proliferation.
Oncogenes and TSGs in cell signalling:
One of the ways cell behaviour is
controlled is through the effects of
polypeptide growth factors which
interact
with
membrane-bound
glycoprotein receptors that transduce
the message via a series of
intracellular signals that promote or
inhibit the expression of specific
genes. Cancer cells often show
alterations in the signal transduction
pathways that lead to proliferation in
response to external signals. E.g many
growth
factor
receptors,
their
membranes, cytoplasmic and nuclear
downstream effectors have been
identified as oncogenes or tumour
suppressor genes. It is the activation of
protooncogenes and/or the inactivation
of tumour suppressor genes that lead to oncogenic transformation
1) Activation of proto-oncogenes (transformation)
Transformations brought about in several ways
 point mutations e.g. c-ras human bladder carcinoma
 Altered regulatory sequences high normal protein expression e.g. v-fos
 Loss of degradation signals
 Chromosomal rearrangements e.g. Burkitts lymphoma t(8:14) CML t(9:22)
 Gene amplification e.g. rat neuroblastomas show a T-A mutation resulting in a constitutively active
growth factor receptor
 Viral insertion: Cancer induced by viral infections e.g. Rous Sarcoma Virus (RSV) has v-src
BB20023:DNA: make, break, disease
Dr. MV Hejmadi, 2004-05
2) Inactivation of tumour suppressor genes
Retinoblastoma (Rb )
The retinoblastoma gene, RB, was the first tumoursuppressor gene to be identified in children with
hereditary retinoblastoma and is correlated with a
loss of heterozygosity (LOH) at chromosome
13q14.2.
Its protein product, RB (~110kD), was subsequently
found to be functionally inactivated in several other
human tumour types, both hereditary & sporadic.
RB is critical for normal development and normally
inhibits proliferation in conjunction with p53. RB
has >10 phosphorylation sites and its function is
regulated by phosphorylation in a cell-cycle specific
manner.
RB controls transcription by interacting with other
proteins like transcription factors. These include
E2F, TF111 and UBF. RB was also identified as a
cellular target for viral oncoproteins.
p53
Most frequently mutated gene in cancer (50% mutation
rate in most cancers & 90% rate in SCC). It encodes a
stress-regulated transcription factor that co-ordinately
induces or represses sets of gene products in response to
changes in the cellular microenvironment. It is involved
in multiple functions in regulating cell cycle control via
apoptotis or cycle arrest, differentiation, DNA
replication, repair and angiogenesis. Mutation in p53
leads to loss of DNA binding capacity, cell cycle arrest
& also increase mutation rate
Structure: 4 main functional domains encoded by p53
1) Transcriptional activation: stimulates transcription
indirectly by binding to other nuclear proteins
e.g.Mdm2, GADD45, Cyclin G, BAX, IGF –BP3
2) Sequence specific DNA binding: Certain genes have
a p53 response element that specifically binds to the
p53 tetramer e.g. BAX, p21
3) Oligomerisation (tetramer formation)
4) Autoinhibitory domain: Causes transcriptional
repression e.g. JUN, FOS, PCNA, MYC, BCL2
Mutational hotspots in the p53 gene. The domain
structure of p53 is indicated and includes the transactivation domain, the DNA-binding domain, and the Cterminal regulatory domain. The C-terminus has two functions,
1) Negative regulatory domain: It can destabilize the folding of the DNA-binding domain by
phosphorylation of the C-terminus, which relieves the inhibition by the C-terminus and activates DNAbinding
2) Positive regulatory domain: Acetylation of the C-terminus of DNA-bound p53 stabilizes p300-binding
and is required for p300-coactivated p53-driven transcription. Hot-spot mutations map to the core-DNA
binding domain, as indicated
Each tumour is characterised by its own array of genetic lesions making it difficult to predict a
treatment outcome
BB20023:DNA: make, break, disease
ONCOGENES
Dr. MV Hejmadi, 2004-05
Genes for growth factors or their receptors
PDGF Codes for platelet-derived growth factor. Involved in glioma (brain cancer)
erb-B
Codes for the receptor for epidermal growth factor. Involved in
glioblastoma (brain cancer) and breast cancer
erb-B2 Also called HER-2 or neu. Codes for a growth factor receptor. Involved in
breast, salivary gland and ovarian cancers
RET
Codes for a growth factor receptor. Involved in thyroid cancer
Genes for cytoplasmic relays in stimulatory signaling pathways
Ki-ras
Involved in lung, ovarian, colon and pancreatic cancers
N-ras
Involved in leukemias
Genes for transcription factors that activate growth promoting genes
c-myc
Involved in leukemias and breast, stomach and lung cancers
N-myc Involved in neuroblastoma (a nerve cell cancer) and glioblastoma
L-myc Involved in lung cancer
Genes for other molecules
Bcl-2
Codes for a protein that normally blocks apoptosis. Involved in follicular B
cell lymphoma
Bcl-1
Also called PRAD1. Codes for cyclin D1, a stimulatory component of the
cell cycle clock. Involved in breast, head and neck cancers
MDM2
Codes for an antagonist of the p53 tumor suppressor protein. Involved
sarcomas (connective tissue cancers) and other cancers
TUMOUR
Genes for cytoplasmic proteins
Involved in colon & stomach cancers
SUPPRESSOR APC
DPC4
Involved in pancreatic cancers. Codes for signalling molecule involved in
GENES
inhibition of cell division
NF-1
Codes for inhibitor of ras protein. Involved in neurofibroma0,
pheochromocytoma (peripheral nervous system) & myeloid leukaemia
NF-2
Involved in meningioma & ependynoma (brain) & schwannoma (shwann
cells surrounding the neuron)
Genes for nuclear proteins
MTS1
Codes for the p16 protein, a braking component of the cell cycle clock
Involved in a wide range of cancers.
Rb
Codes for the pRB protein, a master brake of the cell cycle. Involved in
retinoblastoma and bone, bladder, small cell lung and breast cancer
P53
Codes for the p53 protein, which can halt cell division and/or induce
apoptosis. Involved in a wide range of cancers (50%)
WT1
Codes for transcription factor WT1, Involved in Wilms' tumor of the
kidney
Genes with unclear cellular locations
BRCA1/2 Implicated in cell signalling pathways. Involved in breast/ovarian
cancers
VHL
Involved in renal cell cancer
References:
1) Cancer Biology (2nd edition) by RJB King: chapter 6
2) Molecular biology of Cancer by F McDonald and CHJ Ford Chapters 2, 3.
3) Scientific American (1996) September pgs 32-47
Optional reading:
1) Drug Discovery Today (15 April 2003) Vol 8, Issue 8, Pages 329-373 Pages 347-355 Drug
discovery and p53 by David P. Lane and Ted R. Hupp
2) Cancer by MR Alison (www.els.net)