Scientific American

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Scientific American October 2003
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Oncolytic Viruses
Onco cancer Lytic killing
An innovative cancer therapy that seeks to
harness the natural properties of viruses to aid
in the fight against cancer. These non-human
viruses replicate in cancer cells and leave
normal cells largely unaffected. Clinical data
indicates that oncolytic viruses have a very high
therapeutic index, in some instances 1000001.
This therapeutic index is significantly higher
than the index commonly seen in chemotherapy -
61- and may result in greater efficacy with
fewer side effects.
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Human Inmmunodeficiency virus (HIV) and Acquired
Immune Deficiency Syndrome (AIDS)
AIDS is the first great pandemic of the second
half of the 20th century. First described in
1981, is the result of an infection by the
HIV virus, a lentivirus within the family of the
Retroviridae.
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Distribution of AIDS as of 1993.
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AIDS probably began in central Africa as early as
the 1950s. A virus similar to the HIV has been
isolated from green monkeys and it is believed
to be the ancestor of the AIDS causing
virus. Somehow, the simian virus entered humans
and mutated to the current one through
intermediate forms. Once established,
HIV-1 spread to the Carribbean and then to the
USA and Europe.
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Epidemiologically, AIDS occurs worldwide. More
than 40 million people are currently
infected. By the year 2000, 10 million of
children were estimated to be infected. The
case-fatality rate was estimated to be 92 for
those adults diagnosed with AIDS before 1987.
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AIDS is caused primarily by the HIV-1 virus, and
rarely by the HIV-2. The virus is a retrovirus,
an enveloped lentivirus with a cylindrical core
inside its capsid. The core contains two copies
of its single stranded RNA genome plus several
enzymes. Thus far, 10 virus specific proteins
have been discovered. One of them the gp120
envelope protein, participates in the attachment
of the HIV-1 virus to the T-helper cells of the
host.
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Diagram of the HIV-virus
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Lentiviruses are remarkably complex, compared to
the other retroviruses. Most retroviruses
contain only three genes gag, pol, and env. Gag
and env encode the core nucleocapsid proteins and
the surface coat proteins, respectively, whereas
the pol gene codes for the viral reverse
transcriptase gene and other enzymes. The HIV-1
virus contains in its 9Kb genome at least 6
additional genes. The distinct and concerted
action of these genes underlie the profound
pathogenicity of HIV-1.
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Table Modes of transmission.
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...

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1. There are at least two receptors on
T-lymphocytes to which the human immunodeficiency
virus (HIV) sticks. The primary receptor is
called "CD4". But a second receptor that loops
through the cell membrane 7 times is critical for
infection to occur. HIV infection of a
lymphocyte requires attachment of the virus to
the cell membrane through both of these
"ligand-receptor" links. In cells whose
"7-transmembrane receptor" is different, the HIV
"key" no longer matches the lymphocyte "lock" and
attachment is incomplete. Those cells may avoid
infection by HIV.

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Tight attachment of the viral particle to
receptors on the lymphocyte membrane enables
fusion with the cell membrane. The viral
contents, including viral RNA (shown in yellow)
then empty into the cell's cytoplasm.   Like
other viruses that infect human cells, HIV
commandeers the host's machinery to make multiple
copies of itself.  
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2. Reverse Transcription An enzyme (protein)
that's part of the HIV reads the sequence of
viral RNA that have entered the host cell and
transcribes the sequence into a complementary DNA
sequence. That enzyme is called "reverse
transcriptase" . Without reverse transcriptase,
the viral genome couldn't become incorporated
into the host cell, and couldn't reproduce
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Reverse transcriptase sometimes makes mistakes
reading the RNA sequence. The result is that not
all viruses produced in a single infected cell
are alike. Instead, they end up with a variety of
subtle molecular differences in their surface
coat and enzymes. Vaccines, which induce the
production of antibodies that recognize and bind
to very specific viral surface molecules, are an
unlikely player in fighting HIV, because
throughout infection, HIV surface molecules are
continually changing.
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The first major class of drugs found useful in
slowing HIV infections are collectively called
"reverse transcriptase inhibitors". These include
AZT, 3TC, d4T, ddc, and ddl that act by blocking
the recoding of viral RNA into DNA. The
chameleon-like nature of HIV, however, limits
their continued effectiveness.
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3. Integration. Once the viral RNA has been
reverse-transcribed into a strand of DNA, the DNA
can then be integrated (inserted) into the DNA of
the lymphocyte. The virus has its own enzyme
called "integrase" that facilitates incorporation
of the viral DNA into the host cells DNA. The
integrated DNA is called a provirus.
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4. Translation. As long as the lymphocyte is not
activated or "turned-on", nothing happens to the
viral DNA. But if the lymphocyte is activated,
transcription of the viral DNA begins, resulting
in the production of multiple copies of viral
RNA. This RNA codes for the production of the
viral proteins and enzymes (translation) and will
also be packaged later as new viruses.
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5. Protein maturation. When viral RNA is
translated into a polypeptide sequence, that
sequence is assembled in a long chain that
includes several individual proteins (reverse
transcriptase, protease, integrase). Before
these enzymes become functional, they must be cut
from the longer polypeptide chain. Viral protease
cuts the long chain into its individual enzyme
components which then facilitate the production
of new viruses.
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Inhibitors of this viral protease can be used to
fight HIV infection. By blocking the ability of
the protease to cleave the viral polypeptide into
functional enzymes, protease inhibitors interfere
with continued infection. Mutations enable HIV
to avoid treatments that involve only one drug,
so there is growing use of multiple-drug
therapies in which both a protease inhibitor AND
a reverse transcript inhibitor are combined.
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6. Budding. Finally, viral RNA and associated
proteins are packaged and released from the
lymphocyte surface, taking with them a swatch of
lymphocyte membrane containing viral surface
proteins. These proteins will then bind to the
receptors on other immune cells facilitating
continued infection. Budding viruses are often
exactly like the original particle that initially
infected the host. In the case of HIV, however,
the resulting viruses exhibit a range of
variations which makes treatment difficult.
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• Summary
• HIV entry
• Reverse Transcription
• DNA Integration
• Translation
• Protein Maturation
• Budding and infection

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• Possible targets for HIV therapy
• Reverse transcriptase
• Viral proteases

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• After HIV infection, four different types of
  pathological changes may occur.
• Mild form of AIDS (fever, headache, rash, weight
  loss, lymph node enlargement, oral candidiasis).
  Presence of antibodies against HIV-1. Occur in
  the first few months after infection, last 1-3
  wks, and recur. AIDS-related complex or ARC.
• A true case of AIDS occurs upon infection.
  Usually, 8-10 years pass between infection and
  AIDS manifestation, but its highly variable. At
  first, antibodies are produced but not in
  sufficient number to stop the viral attack. The
  virus becomes established in the CD4 T-helper
  cells that proliferate abnormally in the lymphoid
  organs.

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Thereafter, the lymph nodes collapse do to viral
replication. This leads to decline in number of
lymphocytes, selective depletion of CD4 T-cells
that is critical for propagation of the entire
T-cell pool. Consequently, Interleukin 2
production also declines and in turn production
of T-cells in general declines. This leaves the
person open to opportunistic infections, because
the immune system is defective. Dendritic cells
(present the antigen to T cells)are disabled or
destroyed by the virus. HIV mutates rapidly so it
could evade and eventually overwhelm the immune
system.
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3. The third type of disease involves the central
nervous system since the virus-infected
macrophages can cross the blood-brain barrier.
Classic symptoms headache, fever, subtle
cognitive changes, abnormal reflexes, and
irregular muscle action. Dementia, and severe
sense and muscle response can develop, as well as
brain tumors, and cerebrovascular diseases. All
neurological changes are correlated to high
levels of HIV antigens and/or HIV genome in the
CNS.
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4.Cancer. HIV-infected persons have increased
risk of three types of cancer Kaposis sarcoma,
carcinomas of mouth and rectum, and B-cell
lymphoma. It seems likely that the depression of
the initial immune response enables secondary
tumor-causing agents to initiate the cancers.
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It has been recently shown that if a person
infected with HIV-1 is also infected with HHV-6
(Human Herpesvirus), the development of AIDS is
greatly enhanced. Clear evidence that a co-factor
is involved in AIDS. How does it work.
Ordinarily, the immune system takes care of the
herpes virus. However, if the immune system is
suppressed by HIV-1 , the herpes virus will start
to replicate, unleashing a cycle of
destruction. First, it kills T-helper cells, then
infects AIDS-virus infected cells and awakens the
virus from its latent form, and they both kill
CD4 T-helper cells. Genital herpes is also a
cofactor for HIV1 and 2.
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Primary treatment is directed to the symptoms
caused by HIV. The four drugs available inhibit
the reverse transcriptase as it synthesizes DNA.
Structurally, the drugs resemble thymidine, but
they lack a hydroxy group necessary for the
attachment of another nucleotide and so they
serve as a terminator. These drugs slow the
progress of the disease, but they do not cure it.
Another possible treatment is with IL-2( It
increases the CD4 cell count).
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Current research is the development of a
vaccine -stimulate the production of
neutralizing antibodies -promote the destruction
of those cells already infected. Major problem
the envelope proteins of the virus continually
change their antigenic properties.
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Prevention and control of AIDS Screening of
blood and heat treatment of blood
products Education and protected sexual behavior
and practices. Education of IV drug users
(sharing of needles and syringes).
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RNAi provides promising technique for therapeutic
treatment of AIDS. Short dsRNAs have been
designed against the CD4 protein, which is the
main target of the HIV-1 virus Against the env
protein ..Problem how to deliver these sdRNA
and avoid degradation by the host cell
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New approach Pack the dsRNA into a viral vector,
similar to HIV, infect the host target cells
(T-cells) and obtain release of the
dsRNA. Possible side effects integration of the
viral DNA into critical genes of the host,
possible development of tumors such as leukemia.