Section S Tumor Viruses and Oncogenes S1 Oncogenes Found in Tumor Viruses S2 Categories of Oncogenes S3 Tumor Suppressor Genes

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Section S Tumor Viruses
and Oncogenes S1 Oncogenes Found in Tumor
Viruses S2 Categories of Oncogenes S3
Tumor Suppressor Genes
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S1 Oncogenes Foundin Tumor Viruses

• Cancer
• Oncogenic retroviruses
• Oncogenes
• Isolation of oncogenes

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Cancer

• Definition Cancer is a disease that results from
  the breakdown of the regulations and controls of
  normal cell growth.
• Evidence It has long been recognized that cancer
  is a disease with a genetic element.
• The tendency to develop certain types of cancer
  may be inherited
• In some types of cancer the tumor cells possess
  characteristically abnormal chromosomes
• There is a close correlation between the ability
  of agents to cause cancer and their ability to
  cause mutations.

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Oncogenic retroviruses

• Oncogenic viruses The basic concepts of
  oncogenes were come from the studies on oncogenic
  retroviruses. The retroviruses were found to
  contain an extra gene, not present in closely
  related but non-oncogenic viruses. This extra
  gene was shown to be an oncogene by transfecting
  it into non-cancerous cells which then became
  tumorigenic.
• Function Oncogenic viruses are important cause
  of cancer in animals, although only a few rare
  forms of human cancer have been linked to
  viruses.

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Oncogenes

• Definition Oncogenes are genes whose expression
  causes cells to become cancerous.
• Tumorigenic mechanism The normal version of the
  gene (termed a proto-oncogene) becomes mutated so
  that it is overactive. Because of their
  overactivity, oncogenes are genetically dominant
  over proto-oncogenes, that is only one copy of an
  oncogene is sufficient to cause a change in the
  cell's behavior.

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Oncogenes

• Relationship between virus oncogene and
  proto-oncogenes
• V-onc virus oncogene, v-onc
• P-onc proto-oncogenes, p-onc
• Finding The first oncogenes to be isolated were
  those present in oncogenic retroviruses. When
  these had been cloned and were used as
  hybridization probes, a discovery was made The
  genes with DNA sequences homologous to retroviral
  oncogenes were present in the DNA of normal
  cells.
• Extrapolate It was then realized that retroviral
  oncogenes must have originated as proto-oncogenes
  in normal cells and been incorporated into the
  viral genome when the pro-virus integrated itself
  nearby in the cellular genome. Subsequently,
  similar oncogenes were isolated from non-virally
  caused cancers.

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Oncogenes

• The differences between onc and normal proto-onc
  
• Quantitative differences The coding function of
  the gene may be unaltered but, for example, it is
  under the control of a viral promoter/enhancer or
  it has been trans-located to a new site in the
  genome, it is transcribed at a higher rate. This
  results in overproduction of a normal gene
  product.
• For example int-2 is a such kind of oncogene.

H
Int-2 Low Expression
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Oncogenes

• Qualitative differences The coding sequence may
  be altered, for example by deletion or by point
  mutation, so that the protein product is
  functionally different, usually hyperactive.
• For example the erbB oncogene codes for a
  truncated growth factor receptor. Because the
  missing region is responsible for binding the
  growth factor, the oncogene version is
  constitutively active, permanently sending
  signals to the nucleus instructing the cell to
  'divide'.

truncated erbB
Normal erbB
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Oncogenes
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Isolation of oncogenes

• Advantages of this assay are
• It is a cell culture rather than a whole animal
  test and so particularly suitable for screening
  large numbers of samples
• Results are obtained much more quickly than with
  in vivo tests
• The NIH-3T3 cells are good at taking up and
  expressing foreign DNA
• It is a technically simple procedure compared
  with in vivo tests.

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Isolation of oncogenes

• However, extensive use has revealed some
  drawbacks, both real and potential
• Some oncogenes may be specific for particular
  cell types and so may not be detected with mouse
  flbroblasts
• Large genes may be missed because they are less
  likely to be transfected intact
• The NIH-3T3 cells are not 'normal' cells since
  they are a permanent cell line and genes involved
  in early stages of carcinogenesis may therefore
  be missed
• The assay depends upon the transfected gene
  acting in a genetically dominant manner and so
  will not detect tumor suppressor genes.

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S2 Categories of Oncogenes

• Categories of Oncogenes
• Oncogenes and growth factors
• Nuclear oncogenes
• Co-operation between oncogenes

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Categories of oncogenes
Onc of coding protein like growth factors
Onc of coding protein like transcription factors
Onc
Onc of coding protein like membrane receptors
GF
Onc of coding protein like signal molecules
mRNA
mRNA
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Categories of oncogenes
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Oncogenes (growth factors)

• The sis oncogene which codes for a subunit
  (p28sis) of platelet-derived growth factor.
  Over-production of this growth factor
  auto-stimulates the growth of the cancer cell, if
  it has receptors for PDGF

v-sis
p28sis
mRNA
Over expression
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Oncogenes (membrane receptor)

• The fms oncogene which codes for a mutated
  version of the receptor for colony-stimulating
  factor-1 (CSF-1). The growth factor stimulates
  bone marrow cells during blood cell formation.
  The 40 ammo acids at the carboxyl terminus of the
  normal CSF-1 receptor are replaced by 11
  unrelated amino acids in the Fms protein. As a
  result, Fms protein is constitutively active
  regardless of the presence or absence of CSF-1

v-fms
Fms
CSF-1 receptor
mRNA
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Oncogenes (signal molecules)

• The various ras oncogenes which code for members
  of the G-protein family of plasma membrane
  proteins that transmit stimulation from many cell
  surface receptors to enzymes that produce second
  messengers. Normal G-proteins bind GTP when
  activated and are inactivated by their own GTPase
  activity, ras oncogenes possess point mutations
  which inhibit their GTPase activity so that they
  remain activated for longer than normal.

v-ras
mRNA
GDP?GTP
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Oncogenes (transcription factors)

• Another group of oncogenes codes for nuclear
  DNA-binding proteins that act as transcription
  factors regulating the expression of other genes.
  
• They can divided into 5 classes
• bZIP fos, jun
• bHLH myc, N-myc, L-myc, lyl-1, fal, scl
• ZF myl/RARA, erbA, evi-1, gli-1
• HD pbx, Hox-2,4
• Others myb,rel, est-1, est-2, spi-1, ski.

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Nuclear oncogenes (bZIP-fos, jun)

• The fos and jun oncogenes code for subunits
  of a normal transcription factor, AP-1. In normal
  cells, expression of fos and jun occurs only
  transiently, immediately after mitogenic (????)
  stimulation. The normal cellular concentrations
  of the fos and jun gene products are regulated
  not only by the rate of gene transcription but
  also by the stability of their mRNA. In cancer
  cells, both processes may be increased.

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Nuclear oncogenes (bHLH-myc)

• The expression of the myc gene in normal
  cells is induced by a variety of mitogens (agents
  that stimulate cells to divide), including PDGF.
  The myc-encoded protein binds to specific DNA
  sequences and probably stimulates the
  transcription of genes required for cell
  division.

Influence of a viral enhancer
Translocation of the coding sequence from its
normal site on chromosome 8 to a site on
chromosome 14
Over-expression of myc
In cancer cells
Deletion of 5'-noncoding sequence of the mRNA,
which increases the life time of the mRNA
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Nuclear oncogenes (ZF-erbA)

• The erbA oncogene is a second oncogene (besides
  erbB) found in the avian erythroblastosis virus.
  It codes for a truncated version of the nuclear
  receptor for thyroid hormone. Thyroid hormone
  receptors act as transcription factors regulating
  the expression of specific genes, when they are
  activated by binding the hormone.
• The ErbA protein lacks the carboxyl-terminal
  region of the normal receptor so that it cannot
  bind the hormone and cannot stimulate gene
  transcription. However, it can still bind to the
  same sites on the DNA and appears to act as an
  antagonist of the normal thyroid hormone
  receptor.

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Co-operation between oncogenes

• Transformation The transformation of a normal
  cell into a fully malignant cancer cell is (1) a
  multi step process (2) involving alterations in
  the expression of several genes.
• For example, when cultures is from normal rat
  fibroblasts. Neither the ras nor the myc oncogene
  on its own is able to induce full transformation
  in the normal cells, but simultaneous
  introduction of both oncogenes does achieve fully
  malignant.
• Pairs of oncogenes A variety of other pairs of
  oncogenes are able to achieve together what
  neither can achieve singly, in normal rat
  fibroblasts.
• Co-operation Interestingly, to be effective, a
  pair must include (1) one growth factor-related
  onc and (2) one nuclear oncogene. It seems that
  any one activated oncogene is only capable of
  producing a subset of the total range of changes
  necessary to convert a completely normal cell
  into a fully malignant

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S3 Tumor Suppressor Genes

• Overview
• Evidence for tumor suppressor genes
• RB1 gene
• p53 gene

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Tumor suppressor Overview

• Definition A tumor suppressor gene acts, in a
  normal cell, to restrain the rate of cell
  division. Tumor suppressor genes cause cells to
  become cancerous when they are mutated to become
  inactive.
• Function Tumor suppressor genes act in a
  fundamentally different way from oncogenes

Proto-oncogenes Suppressor genes
Function promote cell division inhibit
cell division
Cancerous by mutated active by
mutated inactive
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p53 gene (As an oncogene)

• Features The gene for p53 is located on the
  short arm of chromosome 17, and deletions of this
  gene have been associated with nearly 50 of
  human cancers. Its mRNA codes for a 52 kDa
  nuclear protein. The protein is found at a low
  level in most cell types and has a very short
  half-life (6-20 min).
• Functions Confusingly, p53 has some of the
  properties of both oncogenes and of tumor
  suppressor genes
• As an oncogene ? many mutations (point
  mutations, deletions, insertions) have been shown
  to occur in the p53 gene, when co-transfected
  with the ras onc, they will transform normal rat
  fibrobiasts. ? In cancer cells, p53 has an
  extended half-life (4-8 h), resulting in elevated
  levels of the protein. ? All this seems to
  suggest that p53 is an oncogene.

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p53 gene (As a tumor suppressor gene)

• As a tumor suppressor gene A consistent deletion
  of the short arm of chromosome 17 has been seen
  in many tumors. In brain, breast, lung and colon
  tumors, where a p53 gene was deleted, the
  remaining allele was mutated. This suggests that
  p53 is a tumor suppressor gene
• The explanation is the dominant-negative effect.

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• Thats all for Section S